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Transcript of DESIGN OF CONCRETE BRIDGES FOR …digitool.library.mcgill.ca/thesisfile106496.pdfi ABSTRACT...
DESIGN OF CONCRETE
BRIDGES FOR
SUSTAINABILITY AND DURABILITY
Juan Manuel Macía
Department of Civil Engineering and Applied Machanics
McGill University
Montréal, Canada
July 2011
A Thesis submitted to the Graduate and Postdoctoral Studies Office in partial fulfilment of the requirements of the degree of Master of Engineering (Thesis Option)
Copyright © by Juan Manuel Macía (2011)
To My Wife Maria Luisa
i
ABSTRACT
Sustainable and durable infrastructure facilities, including bridges, require optimum use of all
resources involving reduction of energy and water consumption during all project phases,
including planning, design, construction, maintenance, operations, repair and rehabilitation, and
finally decommissioning and disposal of the debris at the end of its service life. Design of a
sustainable and durable bridge structure requires consideration of a few feasible alternatives to
develop an optimum option to fulfill all of the relevant limit states, with the best life-cycle
performance and with the lowest life-cycle costs.
The current national standards do not account for the observed increases in operating loads and
the increasing deterioration of bridge structures over their service life. While these standards
emphasize quality control in choice of materials, design and construction, they do not provide
guidance and scientific tools to design and maintain a bridge structure for durability over its
service life, and include only prescriptive tools for preventing some deterioration modes.
This research program integrates sustainability and durability in the design of a conventional
bridge structure in a cold climate country, subjected to the various mechanical natural and man-
made loads and an aggressive environment, and considers the performance of the various
materials and structural components over the design service life. The latest available models of
the relevant deterioration modes have been incorporated in the life-cycle performance and design
considerations. The basic procedure adopts a multiple protection strategy for all deterioration
modes, resulting from the relevant aggressive actions, and integrates durability considerations
with structural calculations for the final design and defines maintenance strategies and any
needed supplementary protection techniques. The design-for-durability procedure is illustrated in
a worked out bridge design example.
ii
RÉSUMÉ
Les infrastructures durables, incluant les ponts, ont besoin de l’utilisation optimale de ressources
naturelles, en considérant une réduction de la consommation d’énergie, des matériaux et d’eau
pendant toutes les phases du projet, tels que la conception, la construction, l’entretien, l’opération,
la réfection, le renouvellement et finalement le démantèlement à la fin de la vie de service. La
conception de ponts sous le principe du développement durable demande la mise en
considération de quelques possibles solutions qui répondent aux différents états limites à
respecter, avec la meilleure performance et les coûts les plus bas pendant la vie de service de
l’ouvrage.
Les normes de conception de ponts au niveau national ne considèrent pas l’augmentation des
charges d’opération ni l’accroissement de la détérioration des ouvrages d’art pendant leur vie de
service. Bien que ces normes mettent l’accent sur le contrôle de la qualité pendant la sélection
des matériaux de construction, la conception et la construction, elles ne fournissent pas de
directives ni des outils scientifiques pour faire la conception et l’entretien des structures pour
atteindre une durabilité spécifique selon la vie de service requise. Ces normes incluent seulement
des outils prescriptifs pour prévenir quelques modes de détérioration.
Ce programme de recherche fait l’intégration des principes du développement durable avec la
conception classique des structures de ponts dans un pays de climat froid, soumis à différents
charges mécaniques et environnementales d’origine naturelle et artificielle. Il considère aussi la
performance des différents matériaux de construction et composants structuraux pendant la vie
de service du pont. Les plus récents modèles disponibles concernant les modes de détérioration
de matériaux de construction, ont été incorporés dans les considérations de la vie de service et la
conception de la structure. La procédure de base adopte une stratégie de protection multiple
contre tous les modes de détérioration qui résultent des actions environnementales agressives.
Elle intègre les considérations de durabilité avec les calculs de conception structurale d’une
manière itérative jusqu’à l’identification de la conception définitive. Cette procédure inclut
l’utilisation des mesures de protection supplémentaire, ainsi que la définition des stratégies
d’entretien. La procédure de conception pour la durabilité est illustrée à travers un exemple
détaillé de conception d’un pont.
iii
ACKNOWLEDGEMENTS
The author would like to express his deepest gratitude to Professor M.S. Mirza, McGill University,
for his guidance and advice throughout the preparation, development, improvement and
publication of this thesis. Professor Mirza has been a source of valuable encouragement, support,
constructive criticism and inspiration since the early stages of the research program. The author
would like to acknowledge the enthusiasm, encouragement and interest of Professor Mirza to
make public the progress and results obtained from this thesis.
The author would like to sincerely thank Professor Andrew, J. Boyd, McGill University, for his
advice and discussions related to the durability of materials aspects of this research work. The
author would like to thank the Civil Engineering and Applied Mechanics Department Staff for their
help and assistance during the research program.
The author has a profound gratitude to Mr. Guy Maurel, Mr. Santiago Saenz, and the rest of the
Bridge Engineering Design Team at AECOM, for their support and encouragement throughout
the development of this research work. The author would like to express his sincere appreciation
and gratitude to Pierre Nadon, François Charbonneau and the rest of the Bridge Engineering
Division at GENIVAR, for their constant support and the opportunity given to the author to
continue his career in the company as a Civil Engineer.
The author would also like to thank Professors Jorge I. Segura Franco and Carlos Iván Gutiérrez,
Universidad Nacional de Colombia, for their advice and encouragement since the beginning of his
career as a Civil Engineer.
The author would like to express his deepest love and sincere appreciation to his family. He
would like to express his heartfelt thanks to his parents Carlos Macia and Martha Niño, who
taught him that training, hard work and persistence are the key elements to accomplish all the
goals and projects that are fixed in life. Above all, the author is deeply grateful to his wife Maria
Luisa Reyes. Her unconditional love, patience, encouragement and support made possible the
completion of this project. The author affectionately dedicates this thesis to his wife and
companion.
iv
TABLE OF CONTENTS
ABSTRACT i
RÉSUMÉ ii
ACKNOWLEDGEMENTS iii
TABLE OF CONTENTS iv
LIST OF FIGURES ix
LIST OF TABLES xii
1. INTRODUCTION 1
2. GENERAL OVERVIEW OF THE BRIDGE PROJECT 2
2.1. Description of the structure 2
2.2. Structural system 5
2.3. Foundation system 6
3. PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN 7
3.1. Sustainable Development in Bridge Engineering 7
3.2. Durability Concerns about Reinforced Concrete Bridges 8
3.3. Durability of Construction Materials 9
3.4. Review of the Design and Construction Practice of Bridge Structures 10
3.5. Lessons from Previous Experiences 12
3.6. Durability Design Approach 14
3.7. Service Life Definition of Reinforced Concrete Bridges 17
3.8. Life Cycle Cost Analysis for Evaluating the Sustainability of Reinforced Concrete Bridges 18
4. SERVICE LIFE OF THE STRUCTURE 20
4.1. Required service life 20
4.2. Durability design formulation 20
4.3. Determination of lifetime safety factor 21
4.4. Design service life 24
5. BRIDGE DESIGN FOR DURABILITY 26
v
5.1. Durability design aspects 26
5.1.1. Identification of macro-climatic conditions 26
5.1.2. Identification of micro-climatic conditions 29
5.1.3. Environmentally induced mechanisms of deterioration 32
5.1.3.1. Frost attack 32
5.1.3.2. Abrasion of concrete by ice 33
5.1.3.3. Surface deterioration 34
5.1.3.4. Chloride-induced corrosion 35
5.1.3.5. Carbonation-induced corrosion 41
5.1.4. Minimum required conditions of the main construction materials 42
5.1.4.1. Minimum concrete cover 50
5.1.4.2. Type of steel 51
5.2. Concrete mixture design for durability 52
5.2.1. Concrete mixture requirements 52
5.2.2. Base materials specifications 53
5.2.3. Concrete mixture proportions 54
5.3. Concrete Handling, Placing and Curing 56
5.3.1. Concrete handling 56
5.3.2. Placing and finishing of concrete 56
5.3.3. Curing 56
6. STRUCTURAL DESIGN FOR DURABILITY 58
6.1. Materials properties 58
6.2. Superstructure design 59
6.2.1. Prestressed concrete girder design 60
6.2.1.1. Bridge deck parameters 62
6.2.1.2. Composite section 62
6.2.1.3. Diaphragms 63
6.2.1.4. Wearing surface 63
vi
6.2.1.5. Barriers 63
6.2.1.6. Load analysis 63
6.2.1.7. Prestressing steel 67
6.2.1.8. Ultimate limit states 72
6.2.1.9. Serviceability limit states 74
6.2.1.10. Girder design for the required service life 76
6.2.1.11. Girder performance with time 79
6.2.1.12. Supplementary protection measures 84
6.2.2. Reinforced concrete design for durability 85
6.2.2.1. General design parameters 85
6.2.2.2. Flexural design for durability 87
6.2.2.3. Check for shearing resistance 88
6.2.2.4. Deflection analysis 89
6.2.3. Deck slab design 92
6.2.4. Transverse bending moments in the bridge deck 92
6.2.4.1. Load analysis 92
6.2.4.2. Durability Parameters 94
6.2.4.3. Initial Conditions of the Bridge Deck Slab 94
6.2.4.4. Assumption for steel reinforcement and performance with time 94
6.2.5. Transverse bending moments in the cantilever overhang 96
6.2.5.1. Load analysis 96
6.2.5.2. Durability Parameters 98
6.2.5.3. Initial Conditions of the Bridge Deck Slab 98
6.2.5.4. Assumption for steel reinforcement and performance with time 98
6.2.5.5. Final design details 99
6.2.6. Transverse vertical shear 99
6.2.6.1. Load analysis 100
6.2.6.2. Durability Parameters 101
vii
6.2.6.3. Initial Conditions of the Bridge Deck Slab 101
6.2.6.4. Shearing resistance performance with time 101
6.2.7. Analysis of deflection with time 102
6.2.8. Design of the semi-continuity of the bridge deck slab 103
6.2.8.1. Load analysis 104
6.2.8.2. Durability parameters 105
6.2.8.3. Initial conditions of semi-continuity of the bridge deck 105
6.2.8.4. Semi-continuity performance of the bridge deck with time 107
6.2.9. Supplementary protective measures 108
6.3. Substructure design 108
6.3.1. Pier column design 109
6.3.1.1. Load analysis 109
6.3.1.2. Durability parameters 121
6.3.1.3. Initial conditions of the pier columns 121
6.3.1.4. Assumptions for reinforcement and performance with time 121
6.3.1.5. Supplementary protective measures 124
7. MAINTENANCE STRATEGIES 125
7.1. General principles for maintenance strategies 125
7.2. Design for maintainability 125
7.3. Preventive maintenance 125
7.3.1. Bridge inspection 126
7.3.1.1. Routine inspection 126
7.3.1.2. Detailed inspection 127
7.4. Corrective maintenance 129
8. SUMMARY AND RECOMMENDATIONS 130
8.1. Bridge design for durability 130
8.2. Holistic design approach 131
8.3. Basic conclusions and recommendations 132
viii
REFERENCES 134
APPENDIX 1 – DURABILITY CALCULATIONS 136
APPENDIX 2 – GIRDER DESIGN CALCULATIONS 140
APPENDIX 3 – MASS LOSS OF STEEL CAUSED BY CORROSION 141
ix
LIST OF FIGURES
2.1. Bridge cross-section A-A.
2.2. Plan view of the bridge.
2.3. Elevation view of the bridge.
2.4. Typical detail of the semi-continuous girders at the pier support.
2.5. Overview of the foundation system of the bridge at the piers.
3.1. a) Semi-probabilistic modelling of resistance and load effects. b) Increase of failure probability in time.
3.2. Current and proposed engineering practices.
3.3. Flow chart of the durability design procedure.
3.4. Qualitative cost flow diagram for the present worth of a bridge.
4.1. Relationship between mean service life and target service life.
4.2. Lifetime safety factor in terms of degradation process.
5.1. Montreal climate graph (Altitude: 57m).
5.2. Ice accretion in the Montreal metropolitan area.
5.3. Frost Index for southern regions of Quebec.
5.4. Microclimates on the bridge deck.
5.5. Microclimates of the bridge at the intermediate pier.
5.6. Diagram of the two limit states of corrosion in reinforced concrete elements.
5.7. Mass loss of reinforcement vs. time due to chloride-induced corrosion.
5.8. Response for deterioration mechanisms for: a) Bridge deck slab, b) Barriers.
5.9. Response for deterioration mechanisms for: a) Piercap, b) Abutments.
5.10. Response for deterioration mechanisms for: a) Pier columns, b) Caissons.
5.11. Response for deterioration mechanisms for: a) Edge girders, b) Internal girders.
5.12. Chloride-induced corrosion progress in the different members of the bridge over a stipulated service life of 150 tears.
5.13. Progress of carbonation front in the bridge.
6.1. NEBT girder characteristics.
6.2. Geometrical parameters of the composite section.
x
6.3. a) CL-625 design truck clearance envelope. b) CL-625 and CL-W design truck loads. c) CL-625 and CL-W design lane loads.
6.4. Definition of the vehicle edge distance, Dve.
6.5. Non-prestressed reinforcement of the bridge girder.
6.6. Prestressed reinforcement of the bridge girder.
6.7. Localized deterioration on the lower flange of a bridge girder.
6.8. Representation of the degradation of the composite slab-girder section.
6.9. Variation of internal compressive stresses at the top of the composite section vs. time.
6.10. Variation of internal tensile stresses at the bottom of the composite section vs. time.
6.11. Loss of flexural resistance of the composite section vs. time.
6.12. Loss of shearing resistance of the composite section vs. time.
6.13. Increment of the static deflections of the composite section caused by the live load.
6.14. Increment of the stress variation on the prestressing strands of the girder.
6.15. Reinforced concrete section parameters for the structural and durability design.
6.16. Cracked transformed concrete section for flexural analysis.
6.17. Representation of the compatibility of stresses and deformations.
6.18. Bridge deck slab resistance for positive bending moments vs. time.
6.19. Bridge deck slab resistance for negative bending moments vs. time.
6.20. Detail of the deck slab reinforcement.
6.21. Notation for cantilever moments.
6.22. Bridge deck slab flexural resistance at the cantilevers overhang.
6.23. Reinforcement details for the bridge deck slab overhangs.
6.24. Axle loads moving across the deck lanes.
6.25. a) Influence line for shear forces in the slab at the intermediate support. b) Shear force diagram for the loading location that generates the largest shear force at the intermediate support.
6.26. Bridge deck slab shearing resistance vs. time.
6.27. Deflections on the cantilever overhang of the bridge deck slab.
6.28. Bending moments diagram for the bridge deck under permanent loads.
xi
6.29. a) Influence line for bending moments at the firs intermediate support of the bridge deck and location of the truck loads that generates the maximum negative bending moment at the support. b) Bending moments diagram for the bridge deck under the truck loading condition.
6.30. Detail of the bridge deck reinforcement for the semi-continuity condition at the intermediate supports.
6.31. Bridge deck bearing capacity for negative bending moments with time.
6.32. Reinforcement of the pier column.
6.33. Interaction curves for pier column for abrasion by ice.
6.34. Interaction curves for pier column for frost attack.
xii
LIST OF TABLES
4.1. Lifetime safety factors determined by a normally distributed degradation function.
5.1. Depth of frozen soil in terms of frost index.
5.2. Microclimates, exposure conditions and mechanisms of deterioration of the different structural members of the bridge.
5.3. Environmental coefficient values for frost attack.
5.4. Environmental coefficient values for surface deterioration.
5.5. Rate of corrosion at anodic areas in carbonated and
5.6. Rate of corrosion at anodic areas in carbonated and chloride-contaminated concrete (Tuutti, 1982).
5.7. Corrosion rate found by Andrade et al (1994).
5.8. Required development lengths for corroding steel reinforcement in various concrete mixtures.
5.9. Environmental coefficient for carbonation-induced corrosion.
5.10. Air content coefficient for carbonation-induced corrosion.
5.11. Parameters a and b.
5.12. Durability parameters for the various bridge components according to the different modes of deterioration.
5.13. Rates of deterioration, initiation time for corrosion and carbonation coefficients for the various bridge components.
5.14. Concrete cover thicknesses for the analyzed members of the bridge.
5.15. Detailed proportions for the concrete mixture designs.
5.16. Summary of the concrete mixture proportions for the bridge members.
6.1. Material properties for the elements of the bridge.
6.2. Conditions for use of the simplified method of analysis for dead loads.
6.3. Conditions for use of the simplified method of analysis for live loads.
6.4. Bending moments on the girder produced by the different load cases.
6.5. Induced internal actions caused by prestressing.
6.6. Compressive stresses of concrete in the girder at transfer.
6.7. Compressive stresses of concrete in the girder at SLS.
xiii
6.8. Stress levels inside composite section.
6.9. Shearing forces cause by the different load cases.
6.10. Shear reinforcement for the girder.
6.11. Final deflection caused by permanent loads.
6.12. Geometrical properties of the NEBT1600 girder of different steps of deterioration.
6.13. Durability design parameters of the bridge deck slab.
6.14. Initial conditions of the bridge deck slab.
6.15. Maximum cantilever moments due to unfactored CL-625 truck wheel loads including the DLA (kNm/m).
6.16. Initial conditions of the bridge deck at the intermediate supports.
6.17. Durability design parameters for the pier columns.
6.18. Initial conditions of the reinforced concrete section of the pier columns.
INTRODUCTION
1
1. INTRODUCTION
The main objective of this research is to establish the necessary steps to carry out a concrete
bridge design for durability based on the integration of durability and sustainability principles to
the standard structural design practices. The bridge design example will involve the detailed
design procedure for some key elements of the bridge structure such as bridge girders, bridge
deck slab and intermediate foundation units.
The purpose of this durability design is to establish how the construction materials and the
various bridge components will behave over time under certain and specific environmental
conditions, and thereby determining the overall performance of the structure over its service life.
This design procedure takes into account a rational understanding of how the construction
materials deteriorate by different mechanisms. It integrates different fields of engineering that
concern the durability of a reinforced concrete structure, including construction materials
engineering, structural design, construction practices, durability and sustainability concepts, life-
cycle costing, bridge management and maintenance strategies. It is necessary to recognise that
durability can only be achieved through a holistic design approach.
The durability design of the structure incorporates multiple protection strategies against the
different actions that may cause deterioration on the structure. The key steps that are considered
within the design-for-durability procedure include: determination of the service life of the structure,
analysis of the environmental effects, identification of the mechanisms of deterioration, selection
of adequate calculation models and determination of durability parameters, design and selection
of good-quality construction materials, integration between durability parameters and structural
calculations to define the final design, the identification of supplementary protection measures,
and the definition of the maintenance strategies.
This concrete bridge design for durability is proposed to guide the engineer to implement the
essential durability and sustainability principles into the current bridge engineering practice. The
design procedure is based on the fact that higher initial investments during the design and
construction of a bridge project will require lower maintenance, renovation and reparation costs
during its future operations. In addition, higher levels of maintenance will represent more durable
and hence, more sustainable structures.
GENERAL OVERVIEW OF THE PROJECT
2
2. GENERAL OVERVIEW OF THE BRIDGE PROJECT
2.1. Description of the structure
The proposed bridge structure crosses a river and connects an intermediate town with a major
highway system. The town is located near the metropolitan region of Montreal, Quebec, Canada.
The total width of the bridge is 15.07m including four traffic lanes, two lanes for each direction,
two shoulders of 1m each, and one concrete barrier placed on both sides. The bridge has three
spans of 26m, with intermediate semi-continuous supports configured by the pier and bridge deck
arrangements. The bearing axes are parallel to each other and perpendicular to the axis of the
bridge.
The general overview of the bridge project is shown in Figures 1.1, 1.2 and 1.3.
Figure 2.1: Bridge cross-section A-A .
GENERAL OVERVIEW OF THE PROJECT
3
Figure 2.2: Plan view of the bridge.
GENERAL OVERVIEW OF THE PROJECT
4
Figure 2.3: Elevation view of the bridge.
GENERAL OVERVIEW OF THE PROJECT
5
This bridge is an important link between the town and the highway system. It represents a vital
corridor not only for the regular transportation of residents and visitors, but also for goods,
services and merchandise. This bridge is a lifeline structure and a part of the emergency system
which must remain operational during and after a mayor event, such as an earthquake.
Once the bridge is constructed and open to service, it is estimated an immediate traffic between
1000 and 4000 daily vehicles per lane using the bridge; this which is equivalent to a traffic
between 250 and 1000 daily trucks per lane. However, it is expected that the previous numbers of
daily vehicles and trucks will be exceeded within the bridge service life. For this reason, the
bridge will be designed for a daily traffic of more than 4000 vehicles, or 1000 trucks. Accordingly,
it is possible to classify this bridge to be part of a highway Class A (Clause 1.4.2.2)1.
The river that passes under the bridge is wide but shallow, and only small boats use it. However,
all bridge elements prone to vessel collision will be designed for such forces. The bridge must
remain functional and open to traffic after the incidence of abnormally unspecified loads such as
earthquakes, vessel collisions, floods, fires, and wind loads (among others). Accordingly, the
bridge can be classified as a lifeline bridge (clause 4.4.1)1.
2.2. Structural system
The structure consists of three sets of seven simply-supported girders, arranged in three spans.
The intermediate supports at the piers are designed to provide the necessary negative
reinforcement at the top fibres as well as a continuity of the bottom “fibres”.
Figure 2.4: Typical detail of the semi-continuous girders at the pier support.
GENERAL OVERVIEW OF THE PROJECT
6
Cast-in-place diaphragms are provided at the foundation units (girder ends) and at intermediate
location along the girder spans to ensure satisfactory transverse distribution of applied loads.
Additionally, some high-stress strength dowels are embedded at the middle of the pier along the
pier axis to provide lateral restraint to improve the continuity of the girders and to ensure the
condition of semi-continuity over the piers (see Figure 1.4).
The foundation units are basically defined by two intermediate rigid-frame piers 2, and two open-
end 3, cantilever abutments 4. These foundation units are reinforced concrete elements.
2.3. Foundation system
The foundation units are supported by a series of circular caissons that transfer the loads to the
bedrock. The number and the diameter of these shafts, as well as their arrangement are
designed for each one of the foundation units. This kind of foundation is chosen because of the
nature and characteristics of the bridge, being able to sustain effectively large compressive and
lateral forces. Additionally, they represent a feasible solution for this structure that overpasses a
river, where the caissons can be bored down through the soil strata to a depth where the bedrock
can be reached. The condition of end-bearing caissons involves a distribution of pressures at the
bearing stratum that provides a good resistance against overturning moments.
Figure 2.5: Overview of the foundation system of the bridge at the piers.
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
7
3. PRINCIPLES OF DURABLE AND SUSTAINABLE
CONCRETE BRIDGE DESIGN
3.1. Sustainable Development in Bridge Engineering
The concept of sustainability can be related to reasonable use of natural resources to provide
important benefits to the society without compromising their supply for future generations.
Sustainable development in civil engineering is focused on the efficient use of natural resources
and energy at the planning, design, construction and operation stages of a project. This efficiency
is dependant on the positive impact of the structure on the community 7. This goal can be
achieved by emphasizing conservation measures, use of renewable resources, waste reduction,
recycling of used materials and elements, and the preparation of more complete environmental
and economic assessments, using valuable tools, such as life-cycle cost analysis.
A complete analysis of the life-cycle costs of the project is essential to ensure safety and
serviceability of the bridge during its design life. This analysis involves not only the initial costs of
design and construction, but also the future costs of maintenance, repair, rehabilitation, and
decommission of the structure at the end of its service life. The design of a durable structure,
complying with the principles of sustainability requires optimization of the design alternatives to
identify the option with the lowest life-cycle costs.8
As long as the bridge is well-designed and performs correctly during its desired service life, the
positive impact on society will be attained. The design of a bridge must be engineered to ensure
that the structure doesn’t attain any of the possible limit states (ultimate, serviceability, functional,
economic, environmental, and other limit states) during the specified service life of the structure.
Past experience shows that, when the performance of the bridge declines early, additional
amounts of resources are required for repair and rehabilitation of the system to upgrade its
performance to an acceptable level. In this case, the concept of sustainability is normally not
respected because of the excessive use of natural resources. According to this, it is possible to
establish that the sustainable performance of a bridge is proportional to its durability.
The inclusion of durability aspects in the structural design process of a bridge requires the
consideration of different models of the possible mechanisms of deterioration that may be
initiated by the environmental conditions (microclimate and macroclimate) present at the bridge.
The concepts of reducing, reusing and recycling (the basic three R’s in sustainable development)
8 must be present in all phases of the project. Reducing involves a reduction of the consumption
of natural resources and a reduction of waste generation. This implies that the construction of
new structures must be undertaken only when it is strictly necessary. Reusing implies that any
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
8
new element on the structure may be useful and applicable for the functions in other structures
with other characteristics. For instance, it is possible to mention the reuse of some of the
“composing elements” of the bridge, such as precast girders, precast panels for reinforced-earth
retaining walls, expansion joints, and bearings mechanisms (among others). Recycling means
that some of the elements of the bridge structure can be transformed or processed to create new
elements or components with different characteristics.
Only recent colossal transport infrastructure projects like the Confederation Bridge in Canada, the
Second Gateway Bridge in Australia, and the Great Belt Link in Denmark have been developed
following a coherent synchronization between the structural design procedure and the principles
of durability and sustainability with the purpose of attaining an impressive service life of more that
100 years. These projects have had major impacts in their countries. The synchronization of
criteria in these projects necessitate proper selection and design of the construction materials, the
use of high quality construction practices, the identification and detailing of critical parts, the
definition and implementation of an effective maintenance program and the monitoring of the
performance of the structural components.
It is essential to integrate sustainability and durability principles in the practice of structural design.
This involves being able to visualize how construction materials will perform over time under
specific and aggressive environmental conditions, and consequently defining the overall
performance of the structure over its design service life. The new approach to bridge design
requires significant advanced planning, rethinking and retroactive improvement of possible
solutions before the project is defined for execution.
3.2. Durability Concerns about Reinforced Concrete Bridges
Durability and sustainability issues have not been taken into account seriously in bridge design
practice over the last decades in North America and in many other regions of the world. This has
led to premature deterioration of bridges, which has become a serious problem for all nations
around the globe, creating immense social, economic and environmental impacts. This early
deterioration of infrastructure is directly associated to some aspects, like deferred maintenance,
employment and conception of inadequate construction materials, ignorance of the environmental
conditions that may affect the bridge, bad construction practices, and poor workmanship during
the different phases of the bridge project.
In the past, bridge engineering practice was focused mostly on mechanical and structural
behaviour. Structural design and material selecion were developed in separate ways until the last
few years, leading to the appearance of flaws in the real-life performance of bridge structures.
From the structural design point of view, all considerations about the required quality of the
construction materials have been mostly prescriptive, based on a series of recommendations
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
9
found in different bridge design codes and manuals. On the other hand, considerable research
has been undertaken on construction materials in such a way that the results and conditions of
the test are not representative of the field conditions of the “actual” bridge structures; this has
resulted in inaccurate recommendations for some bridge designs.
Presently, adequate knowledge is available for the different modes of deterioration that may
occur on bridge structures. However, the provisions adopted by different national codes about the
necessary actions to ensure durability of construction materials against the degradation imposed
by the aggressive environmental conditions vary significantly. Nonetheless, most of the provisions
in the codes concur in two major aspects, the identification of the local exposure conditions
(microclimate), and the necessity of low concrete permeability, a property that can be reached
following the concept of the four C’s for each type of environmental exposure: constituents of
concrete mixture, good-quality concrete cover thickness, compaction, and curing.8
3.3. Durability of Construction Materials
Satisfactory performance of reinforced concrete structures can be attained by a thorough
evaluation during the design procedure of all possible scenarios involving all possible loading
cases and environmental conditions that may affect the structure, excellent quality control during
construction, use of protective measures for the most critical structural elements against the
exposure to severe environmental conditions, planning and implementation of effective
maintenance strategies, and as an essential component, the design and employment of high-
quality construction materials. This means the use of construction materials and techniques that
require relatively low maintenance while they provide the required strength properties and
durability characteristics over the design service life of the structure. In the case of reinforced
concrete bridges, the level of performance of the structure depends directly on the quality control
exercised in design, construction and maintenance of the structure during operation.
When a reinforced concrete element is exposed to severe environmental conditions, such as
constant moisture, direct contact with chloride solutions, high temperature ranges, and wetting
and drying cycles, it is essential to provide and protect reinforcing steel to withstand these
conditions without generating active corrosion at early ages of the structure, or at least, being
able to develop corrosion rates slow enough to ensure that this degradation mechanism will not
affect the required performance during the design service life. Nowadays, some of the most
feasible solutions generally used in the construction industry is the use of galvanized steel and
epoxy-coated steel, provided that they are properly detailed and specified in the detailed
drawings and technical recommendations of the project. The use of stainless steel, although it is
more expensive, has also proved to be effective in mitigating or considerably delaying corrosion
of steel embedded in concrete. Additionally, there is a growing interest and research in the use of
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
10
fibre-reinforced composites to replace reinforcing steel; however, special care must be taken
during the design process to ensure the fulfillment of the different required limit states during its
service life.
The quality and level of performance of concrete depend strongly on the microstructure of the
hydrated cement paste (hcp), which involves the hydration products, the pore structure, and the
free and bound water on the pore walls. After the initial set, the concrete quality is governed
mostly by hcp, which is the one that controls low capillarity (low permeability), bonding with the
aggregates, high pH (around 13), production of calcium-silicate-hydrate gel, and the resistance to
the variations in humidity and temperature conditions. These properties are developed during the
various stages of concrete preparation and placing, and are controlled strongly by curing and
other on-site conditions. Under appropriate conditions, the water-tightness and strength of
concrete will increase due to the continuous hydration of the un-hydrated cement particles in the
concrete mixture. When concrete is fresh, the aggregates help to define some characteristic,
such as workability and segregation during placing. In addition, when concrete has hardened, the
aggregates help to increase the stiffness and stability of concrete. The concrete mixture needs to
be carefully designed for all of the above factors or characteristics, and mixed, placed,
consolidated and cured using high quality control through on-site supervision 8, and follow-up with
an excellent maintenance program through its service life.
3.4. Review of the Design and Construction Practice of Bridge Structures
It is important to take review of the past and current construction practices to understand, control
and mitigate the principal causes of premature deterioration in present reinforced concrete
infrastructure facilities.
Several mechanisms of deterioration are closely related to the ability of aggressive elements and
substances to penetrate into reinforced concrete elements. Therefore, deterioration is greatly
associated with permeability, which depends on factors, such as pore system interconnectivity
and cracking. Above certain crack widths (0.15 to 0.18mm) 9, aggressive substances can easily
ingress into the concrete structure, interacting with different elements and activating different
modes of deterioration. There are many different reasons for cracking in concrete; one
predominant factor which influences cracking at early age is the use of high-early strength
cements and concrete mixtures necessary to facilitate the high speed of modern construction.9
During the first decades of the twentieth century, concrete was produced using coarsely ground
cement, which caused the hydration process to be very slow, generating low heat of hydration,
and enabling concrete to develop strength at a relatively small rate. Consequently, thermal
cracking of concrete elements was not a problem. After the 1930’s, the increased fineness of
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
11
cement introduced new problems due to an increment of the rate of hydration and heat
generation, making it harder to adequately cure the concrete, and promoting early age cracking.
After the 1950’s, significant changes were introduced in concrete construction practices, including
the development of the ready-mixed concrete industry, the placement of concrete by pumping,
and the vibration procedures, using mechanical vibrators. During this period, achieving high
strength at early ages and providing more fluidity and versatility to concrete, engineers decided to
augment water content in the concrete mixture, increasing cement fineness and C3S content,
which is the component of cement responsible for early strength, normal setting, temperature rise
during hydration, and major contribution to creep and shrinkage. As a result of these changes,
concrete tended to be more permeable and consequently less durable.
During the 1970’s, a wide range of water-reducing admixtures were introduced. Having identified
the high w/c ratio as a clear factor responsible for the low durability of concrete, it was believed
that reducing the water content in the concrete mixture could improve the durability of concrete.
However, it must be emphasized that this factor is not the only one that determines the
performance of hardened concrete. The type of cements that were developed and used to
achieve the fast schedules of the construction industry produced concretes with high thermal and
drying shrinkage, low creep, and high elastic modulus at early ages. These concrete mixtures
were prone to cracking and prone to lack of durability.
Since the 1980’s, an increased use of water-reducing admixtures and the introduction of mineral
admixtures like puzzolanic materials, produced concrete mixtures able to develop high workability
at very low water-cementitious materials (w/cm) ratio, generating high compressive strengths and
high elastic modulus at early ages, and very low permeability in laboratory specimens. These
kinds of concretes were termed “high-performance concretes”. However, these very high
strengths and moduli were accompanied by a significantly reduced creep potential and high
brittleness, resulting in significant cracking at different ages of concrete, especially for those
elements subjected to important cyclic loading conditions, such as bridge decks.
From the structural design point of view, most of the current bridge design standards are based
on semi-probabilistic considerations of mechanical loads and resistances, considering various
limit states that the bridge structure fulfill during its service life (Figure 3.1a). This consideration
does not take into account for the future increase of loads and the deterioration of the structure
expressed in terms of loss of resistance of the construction materials. Consequently, the
probability of failure increases considerably over the service life of the structure, resulting in
premature deterioration with huge impacts on the surrounding environment (Figure 3.1b). 8
The Canadian standards provide detailed guidance to address the design of safe and serviceable
concrete structures in terms of structural mechanics, putting a special emphasis on certain
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
12
phenomena that are not as well understood, such as shear and torsion among others. However,
the different aspects related to durability and sustainability are not explained or introduced using
the same scientific and rational approach as used for structural analysis and design. The codes
deal with these subjects by providing some prescriptive requirements for durability of construction
materials, including the use of de-icing chemicals, minimum concrete covers, type of reinforcing
steel, chloride ion content, sulphate attack, and freezing and thawing cycles, among others. The
Canadian standards emphasize the need to employ high-quality concrete and steel. Additionally,
they indicate a minimum service life of 75 years that the bridge structures must provide. However,
the codes do not show clearly how to attain this service life by assuring enough durability of a
properly designed, constructed and maintained structure.
Figure 3.1: a) Semi-probabilistic modelling of resistance and load effects. 10
b) Increase of failure probability in time.5
3.5. Lessons from Previous Experiences
The experiences related to the evolution of bridge engineering practice throughout the 20th
century and the early years of the 21st century can be highlighted as follows:
As soon as the gain of strength became a priority in the construction industry, new deterioration
problems were introduced in bridge structures. Higher elastic moduli and resistances at early
ages, lower creep and higher thermal and drying shrinkages made modern and high performance
concrete more prone to cracking. This significant cracking of concrete at early ages was closely
connected to premature deterioration of reinforced concrete elements exposed to severe
aggressive environments.
Significant advances in the research about durability of construction materials have resulted from
some excellent laboratory work. Nevertheless, some conditions of the tests that simulate the
degradation mechanisms do not reflect the “actual” conditions on “real” bridge structures.
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
13
Therefore, some of the prescriptive measures adopted by the codes were ineffective when used
in the design and construction procedures.
These facts highlight the real deficiencies in the procedures and considerations regarding
durability and sustainability aspects in design and construction practices. Additionally, there was
a strong need to better study and analyze how the construction materials deteriorate by different
mechanisms. The most important change needed was to integrate the different fields of
engineering that concern the durability of a reinforced concrete structure, including construction
materials, structural engineering and construction quality. It is necessary to recognize that
durability can be achieved only through a holistic design approach.
A reduction of the speed of construction is needed to accomplish the goal of building durable and
sustainable structures. Additionally, some changes are required in the approaches adopted by
owners, builders, and designers in their professional practices. Any bridge project must seriously
take into account durability, economic and environmental considerations from the early stages of
the project, during the construction of the bridge, and maintained diligently throughout the entire
service life. Figure 3.2 shows the current engineering practices and how emphasis on some
related issues must change to attain sustainable and durable structures.
Figure 3.2: Current and proposed engineering practices.11
Some construction materials, such as steel, concrete and wood, are currently being produced at
a great cost to the environment; consequently, the effective use and the conservation of materials
must become a new priority in the construction industry. Accordingly, standard specifications
need to be changed from prescriptive to performance-based recommendations for materials. 9
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
14
3.6. Durability Design Approach
A new design procedure for bridge structures must achieve safety, serviceability, durability, and
general socio-economic and environmental benefits for the surrounding communities throughout
its specified service life. The basic steps involved in durability design are summarized in Figure
3.3.
Figure 3.3: Flow chart of the durability design procedure.12
During analysis and design stages, it is essential to consider the different load cases that may
affect the structure during its service life, including not only the conventional mechanical actions,
but also all possible environmental effects that may be developed globally and locally on the
structure. A semi-probabilistic analysis has been use in the various national standards and codes
to derive specific deterministic safety factors in terms of materials and load cases to ensure that
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
15
the specified limit sates are not exceeded. Moreover, there is a need to define a lifetime safety
factor for the bridge considering all the parameters involved on the definition of the design service
life and the target service life. The bridge is considered to attain its service life when the
maximum permissible level of deterioration and damage on the structure is reached, or when the
bridge is unable to provide service with an adequate level of safety and serviceability.
The durability design of the structure can be conceived by following required protection strategies
against the different actions that may cause deterioration of the structure. One of them can be
avoiding all possible degradation modes, another one may include the provision of a single
barrier of protection, and the other one involves the use of several shields of protection.
An essential part of the durability design process must be based on a life cycle cost analysis
approach, which must involve all costs of the project at its different life stages, including planning,
design, construction, maintenance, repair, rehabilitation and decommissioning. Additionally, the
possible socio-economic and environmental costs that correspond to the each one of these
stages during service life must be analyzed and weighted with the purpose of optimizing and
identifying the best design option that minimizes the overall life cycle cost of the bridge project.
Despite the fact that the bridge design for durability can be carried out by integrating the results of
ordinary mechanical design and durability design performed separately, the design approach can
be established in a more efficient way by combining the traditional structural design with the
durability parameters, to avoid overdesign, and unnecessary overuse of precious construction
materials. The key steps that need to be considered within the design-for-durability procedure of
a reinforced concrete bridge are briefly reviewed. 12
- Determination of the target service life and design service life
The target service life for the structure must be determined following the requirements
provided in the current regulations, standards and codes, according to the type of the
structure and the special needs of the client. The definition of the service life for the bridge
structure which is the subject of this work is discussed in Chapter 4.
- Analysis of the environmental effects
It is essential to analyze the environmental effects that define global and local conditions
acting on the different members of the structure, which create the macro- and microclimates
responsible for generating the different modes of degradation of the construction materials
and the bridge elements. It is essential to identify climatic conditions, such as temperature
and moisture fluctuations, rain, moisture condensation, freezing, solar radiation, and pollution.
It is also necessary to identify some geological conditions such as the presence of ground
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
16
water, sea water and contaminated soil. Additionally, all man-made activities that generate
specific microclimates on the structure, like the use of de-icing salts and abrasion by traffic,
need to be considered, along with any man-made hazards, such as terrorist activities aimed
at destroying the structure.
- Identification of the mechanisms of deterioration
A sound knowledge of probable environmental effects will enable identifying the different
mechanisms of deterioration that may take place in different parts of the structure. The
principal mechanisms of deterioration which may produce long-term effects on the bearing
capacity of the structure include chloride-induced corrosion, carbonation-induced corrosion,
mechanical abrasion, salt weathering, surface deterioration, and frost attack, among others.
These mechanisms of deterioration affect either concrete, or reinforcing steel, or both. They
destroy the material progressively from the outside towards the interior, causing a reduction
of the cross-sectional area of concrete and steel, loss of bond between the concrete and the
steel, splitting and spalling of concrete cover due to progressive corrosion of reinforcement,
and a generalized loss of bearing capacity of reinforced concrete elements.
- Selection of adequate calculation models and determination of durability parameters
The bridge engineer must make a careful selection of the various degradation models that
describe the mechanism of deterioration that affect the structure. It might be necessary to
make a preliminary evaluation of the rates of degradation for the different mechanisms to
accurately assess the service life of the structure. It should be noted that these models can
be readily substituted by newer models, developed using semi-probabilistic approaches.
Using the selected calculation models, it is possible to determine some durability parameters,
such as concrete cover thickness, durability of materials, detailing and supplementary
protective measures.
- Integration between durability parameters and mechanical calculations to define the
final design.
By integrating durability and mechanical calculations, it is possible to visualize the rate of the
loss of bearing capacity of the various components and the structure due to the degradation
caused by the various defined factors. Accordingly, an iterative improvement of the structural
characteristics of the bridge can be made, defining at the final stage, the minimum required
geometry for the structural members, the best reinforcement distribution, the quality of the
construction materials of the bridge, and the provision of supplementary shields of protection
for the critical members.
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
17
- Definition of the maintenance strategies
When the structural design for durability is completed, the engineer must conceive a
complete maintenance strategy that has to be followed during the entire service life of the
structure, executing some preventive, corrective and improvement activities in a periodic
manner. A complete description of the maintenance strategies for this bridge is presented in
Chapter 7.
3.7. Service Life Definition of Reinforced Concrete Bridges
The service life of a bridge and its components must be defined to respond to future needs or to
calculate life-cycle costs. In the past, service life was estimated by approximate projections
enhanced by judgement. Statistical analysis techniques of empirical data for durability of
materials are being used to develop mathematical models of the degradation modes. These
models are used to estimate deterioration projections that could be used to approximate service
life.13
Presently, to implement a durable and sustainable bridge design, engineers must explore outside
the current codes to evaluate possible environmental effects and ensure adequate material and
structural performance over the design service life. This requires an extrapolation of current
knowledge about climate, material properties as well as a projection of the models that describe
the different mechanisms of deterioration. The design, construction and maintenance teams are
required to go beyond the prescriptive durability requirements in the codes, to explore new ways
of attaining a required service life and to achieve higher levels of performance.7
Figure 3.4: Deterioration vs. time in terms of levels of maintenance expressed in terms of percentage of investment of the initial costs. 11
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
18
The economics of service life for civil infrastructure are based on the fact that higher initial costs
invested during the design and construction of a bridge project will generally involve lower
maintenance, renovation and reparation costs during its future operation. In addition to this,
higher levels of maintenance will represent longer periods of time for the structure to attain the
maximum acceptable levels of deterioration (Figure 3.4). For many cases, especially
infrastructure located in very congested urban areas, any replacement operation could represent
large costs, even many times higher than the initial costs, considering all possible socioeconomic
and environmental impacts, and great amounts of additional energy and natural resources,
thereby contradicting the principles of sustainable development.
3.8. Life-Cycle Cost Analysis for Evaluating the Sustainability of Reinforced Concrete
Bridges
The life cycle of a transportation infrastructure asset like a bridge regroups all the sequence of
events and actions involved during the life of the structure. These actions include the initial design
and construction, operation, monitoring and inspection, maintenance procedures, repairs, retrofit,
replacement, decommissioning and disposal. The life-cycle cost analysis of the project is a
process where the total economic worth of the bridge is evaluated by analyzing all initial costs,
and discounted future expenses related to the previously mentioned events and actions. The
present worth of the bridge project can be estimated using the following equation:
SpwfUCFRCICMCpwfFCPWnt
t
0
(3-1)
where:
FC = First (initial) cost
t = Time period of analysis (must be the same for all alternatives analyzed)
MC = Maintenance costs
IC = Inspection costs
FRC = Future rehabilitation costs
UC = Users costs (time delays, fuel consumption, user’s discomfort, vehicle operating costs,
accidents)
S = Salvage values costs (difference between salvage and decommissioning costs)
i = Discount rate
n = Number of years when cost (benefit ) will occur
pwf = Present worth factor = ni11
PRINCIPLES OF DURABLE AND SUSTAINABLE CONCRETE BRIDGE DESIGN
19
This can be clearly seen in the following cost flow diagram:
Figure 3.5: Qualitative cost flow diagram for the present worth of a bridge.
A detailed estimation of service life is essential to accurately assess the life cycle costs and
impacts of a bridge structure, and to evaluate the advantages that are brought by new
technologies and advances in bridge design. This estimate must include schedules of repairs,
rehabilitation, or additional costs between initial construction and decommissioning of the
structure. This evaluation of service life and maintenance are essential when determining future
impacts of construction repairs produced by increases in traffic flow and financial discounting
during the operation of the infrastructure asset. Additionally it is a very useful tool to compare
different alternatives for the design of a bridge structure, including the type of structural system,
the type of bridge, the materials to be used, and the general dimensioning of geometric design for
the project.
SERVICE LIFE OF THE STRUCTURE
20
4. SERVICE LIFE OF THE STRUCTURE
4.1. Required Service Life
According to the Canadian Highway Bridge Design Code, the required target service life of new
structures must be 75 years (Clause 1.4.2.3) 1. This minimum overall service life of the bridge of
75 years implies that the most critical elements of the structure, such as foundations (abutments
and piers) and girders must perform satisfactorily over this period of time and must fulfill all of the
required limit states for the structure.
Other components less critical to the integrity of the structure may have a shorter service life,
being necessary to replace them as part of the maintenance strategy. Some of these components
may be the asphalt concrete pavement, waterproofing membranes, protective coatings for
concrete and steel elements, expansion joints, bearings, barriers, drains, slope protection
systems and others.
The durability design of this project aims the fulfillment of the service life using a scientific
procedure that combines durability models with the current bridge structural design practice. This
procedure allows the visualization of the performance of the structure over time.
4.2. Durability Design Formulation
The service life of a structure is affected by the macro- and micro-environmental conditions that
determine the type and severity of the degradation mechanisms of the materials that compose
the different structural members of the bridge. Additionally, the quality of the construction
materials and the degree of exposure of the different members to the aggressive conditions may
vary significantly. Therefore, the performance and service life of this kind of structure should be
preferably treated stochastically. This stochastic procedure takes into account the real nature of
structural performance to produce a reliable structural design. 5
The various formulae for load, resistance and service life are quite complex; since many
degradation factors affect the performance of the structure, the application of this methodology
become even more complex. Therefore, it is preferable to apply the lifetime safety factor method.
Despite the fact that this method is based on the theory of safety and reliability, the formulation of
the procedure results in a deterministic form. The design service life is determined by multiplying
the target service life by a lifetime safety factor (Sarja and Vesikari, 1996)5.
gtd tt (4-1)
SERVICE LIFE OF THE STRUCTURE
21
where:
td = design service life.
tg = target service life.
t = lifetime safety factor.
4.3. Determination of lifetime safety factor
The lifetime safety factor is the relationship between the mean service life obtained from a
distribution of probable service life values of a structure, and the target service life that is required
for the design purposes of the project.
g
Lt t
t (4-2)
where:
t = lifetime safety factor.
(tL) = mean service life.
tg = target service life.
Figure 4.1: Relationship between mean service life and target service life.5
Figure 4.1 represents a distribution of service life values and the relationship between the target
service life, failure probability and mean service life. The mean service life can be identified by the
point of intersection of the degradation curve with the limit state of durability. To ensure that the
mean service life is greater than the target service life, the lifetime safety factor t must be grater
than one. The lifetime safety factor depends on the maximum allowable failure probability. Low
SERVICE LIFE OF THE STRUCTURE
22
values of maximum allowable failure probabilities require larger values of lifetime safety factor,
which also depends on the form of the service life distribution. 5
The durability of a bridge can be expressed in terms of the degradation process that is present in
the structural system due to the degrading effect of the environmental loads that act on the
different structural members (Figure 4.2).
Figure 4.2: Lifetime safety factor in terms of degradation process.5
It is evident from Figure 4.2 that the point where the degradation curve intersects the maximum
degradation at the design service life, a time that must be longer than the target service life by a
factor represented by the lifetime safety factor t. The range Dmax – D(t) in Figure 4.2 represents
the safety margin. 5
The lifetime safety factor can be determined by a stochastic method assuming a normally
distributed degradation around the mean value, and a standard deviation of this degradation
being proportional to the mean degradation. Following these assumptions, it is possible to obtain
the following expression for the lifetime safety factor (Sarja and Vesikari, 1996)5:
nDt
11 (4-3)
where:
t = lifetime safety factor.
= reliability index.
D = coefficient of variation of degradation.
n = degradation rate exponent.
SERVICE LIFE OF THE STRUCTURE
23
The Canadian Highway Bridge Design Code establishes that for the design of new bridges,
components must not fail suddenly and abrupt collapse of the structure must be avoided.
Accordingly, the lifetime target of the reliability index can be defined as 3.75 for most bridge
structures for the ultimate limit state (ULS). For the serviceability limit state (SLS), the target value
of the safety index can be selected as zero for an appropriate period (Clause C1.4.2.1) 6.
The required reliability index and the probability of failure Pf for ordinary design determined by
Eurocode 1 are: 5
For the ultimate limit state (ULS):
= 3.8 (serious consequences of a durability failure). → Pf = 7.2 x 10-5.
= 3.1 (no serious consequences of a durability failure). → Pf = 9.7 x 10-4.
For the serviceability limit state (SLS):
= 2.5 (serious consequences of a durability failure). → Pf = 6.2 x 10-3.
= 1.5 (no serious consequences of a durability failure). → Pf = 6.7 x 10-2.
Table 4.1: Lifetime safety factors determined by a normally distributed degradation function.
SERVICE LIFE OF THE STRUCTURE
24
The degradation rate exponent n will affect the determination of the lifetime safety (Equation 4-3)
in the following way: 5
n = 1 → represents a linear degradation.
n = 0.5 → represents a retarding degradation.
n = 2 → represents an accelerating degradation.
The calculation of different values of the lifetime safety factors for different values of the
parameters previously mentioned are shown in table 4.1.
4.4. Design service life
Aggressive environments can dramatically deteriorate the integrity and performance of different
members of a bridge structure. This is the case of the chloride-induced corrosion, which once it
starts the active phase, the deterioration occurs quite rapidly.
Chloride- induced corrosion is perhaps one of the most critical deterioration mechanisms that can
be generated in most bridge structures of North-America where de-icing salts are usually
employed for traction during the winter seasons. Accordingly, the use of a degradation rate
exponent of the order of 2 could closely represent the behaviour of a reinforced concrete element
subjected to such aggressive environments.
Because a series of widely varied micro-climates can be developed at different parts of members
of the bridge, depending on the degree of exposure against certain environmental conditions, the
variation of the deterioration mechanisms can be considered to be elevated. For this reason, high
values of D can be adopted for the determination of the lifetime safety factor. Additionally, it
should be emphasized that poor durability performance of the structure may lead into serious
resistance and serviceability consequences, which must be considered rationally in the durability
design process.
Considering that any premature failure of the structure would represent serious consequences on
the surrounding environment, a reliability index = 3.8 is adopted for ULS. Accordingly, the
lifetime safety factor at ULS for this structure can be determined as:
201.218.08.3 21 t
Consequently, the design service life for the ultimate limit state results:
SERVICE LIFE OF THE STRUCTURE
25
752ULSdt years 150 years
Considering that any premature serviceability failure of the structure would represent serious
consequences on the use of the structure, a reliability index = 2.5 is adopted for SLS.
Accordingly, the lifetime safety factor at ULS for this structure can be determined as:
73.118.05.2 21 t
Therefore, the design service life for the serviceability limits state results:
7573.1 SLSdt years 13075.129 years
DURABILITY PARAMETERS
26
5. DURABILITY PARAMETERS
5.1. Durability Design Aspects
The environmental conditions around the bridge induce certain physical, chemical and biological
processes responsible for the different mechanism of deterioration that can occur and affect the
different composing elements, depending on their degree of exposure to these specific
environmental conditions.
5.1.1. Identification of Macro-climatic Conditions
The bridge project is situated in the southwest of the province of Quebec (Canada), near the city
of Montreal, with an approximate longitude of 73º 35’ west of the Greenwich meridian, and a
latitude of 45º 30’ north of the Equator.
Figure 5.1: Montreal climate graph (Altitude: 57m).15
The environment of this zone is greatly influenced by the confluence of several climatic regions
generating a climate classified as humid continental or hemiboreal, according to the Köppen
climate classification14. The average temperature is 7ºC, with a highest monthly average high
DURABILITY PARAMETERS
27
temperature of 26ºC in July and a lowest monthly average low temperature of -13ºC in January.
The range of average monthly temperatures is 31ºC. However, on the hottest and coldest days,
temperatures can go up to 37ºC and -40ºC respectively (Figure 5.1).15
This region receives an average of 1047mm of precipitation per year, and 87mm per month.
During the driest conditions in February, it is possible to have an average precipitation of 76mm
during 15 days. However, during the wet season in summer, it is possible to receive an average
precipitation of 98mm during 13 days. There is an average annual precipitation of 218cm of
snowfall, which occurs from November through March. Thunderstorms are common; they begin in
the late spring and usually finish in the early fall season. Tropical storm remnants can bring heavy
rains during the hurricane season at the Atlantic and the Pacific oceans. The average annual
relative humidity is 77.4% and the average monthly relative humidity varies from 71% in May to
83% in September. 14, 15
Figure 5.2: Ice accretion in the Montreal metropolitan area.1
During winter season, important snow and ice accretions take place in the rural and urban areas
of all regions in Canada. The degree of accumulation varies according to the geographic location.
In the case of the metropolitan region of Montreal, there exists an extreme possibility of
accumulation of ice, generating important mechanical and environmental loads that must be
DURABILITY PARAMETERS
28
considered (Figure 5.2). Additionally, due to the extreme cold temperatures during this period of
the year, saturated soil can be subjected to freezing up to a certain depth below the ground level.
The frozen depth of soil depends on different factors, where the most important is the cold
temperature. This factor can be estimated by the frost index, which is calculated by adding all the
mean daily temperatures of the air below 0ºC during the year. The depth of frozen soil is a
function of the frost index (Figure 5.3). According to the Ministry of Transportation of Quebec
(MTQ), the depth of frozen soil can be more than 1m. This factor must be considered when
designing the foundations of the bridge, placing the foundation elements at adequate depth to
protect the footings, shafts, piles and pile-caps against the effects of freezing and thawing cycles.
Consequently, the minimum required protection of the foundation units of a bridge has been
established by the MTQ considering the frost index and the classification of the road way (Table
5.1).
Figure 5.3: Frost Index for southern regions of Quebec.16
Frost index (ºC * days)
Depth of the frozen soil (m)
National Highway Regional and
Collector Road Local Road
< 1200 2.00 1.80 1.60 1200 – 1700 2.25 2.00 1.80
< 1700 2.50 2.25 2.00
Table 5.1: Depth of frozen soil in terms of frost index.16
DURABILITY PARAMETERS
29
The average wind speed around the Montreal’s metropolitan area is relatively constant
throughout the year, showing more intensity during the winter season. According to the Canadian
Highway Bridge Design Code, an hourly mean wind pressure of 461Pa can be estimated for the
analysis of wind loads for bridge structures located in the metropolitan area of Montreal for a
return period of 150 years. 1
The bridge project is meant to cross a river, 75m in width and 6m in depth at its deepest point.
This is generally calm and unaffected by tides; however, there is some fluctuation between high
water and low water levels during the driest season during the summer and the wet seasons from
fall to spring. During winter, the river does not get frozen; however, it carries ice blocks of large
sizes that may affect the foundations of the structure.
5.1.2. Identification of Micro-climatic Conditions
The macroclimate surrounding the bridge can create certain micro-climatic or local conditions at
different locations of the structure, depending on the degree of exposure of the structural
elements to the specific environmental conditions.
The different microclimate zones that can be identified in a typical cross-section of the bridge
deck are shown in the Figure 5.4.
Figure 5.4: Microclimates on the bridge deck.
It is possible to identify a wider range of microclimates at the foundation units, such as the
intermediate pier, where its different components can experience varying conditions and can
develop different mechanisms of deterioration. The possible microclimates that can be identified
at the central pier are displayed in the Figure 5.5.
A complete description of the microclimates, exposure conditions and mechanisms of
deterioration that take place on the different structural members of the bridge, is presented in the
Table 5.2.
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30
Figure 5.5: Microclimates of the bridge at the intermediate pier.
Microclimate Bridge Element Exposure Conditions Deterioration Mechanisms
A Upper side of the deck slab
Rain and wind exposure; accumulation of de-icing salts; abrasive deterioration mechanisms due to traffic; freezing and thawing cycles; exposure to CO2; wetting and drying cycles; exposure to sunshine and daily temperature changes.
Frost attack; surface deterioration; carbonation and chloride-induced corrosion.
B Reinforced concrete barrier
Rain and wind exposure; accumulation of de-icing salts; splashing of chloride solutions and water from continuous traffic; freezing and thawing cycles; exposure to CO2; wetting and drying cycles; exposure to sunshine and daily temperature changes.
Frost attack; surface deterioration; carbonation and chloride-induced corrosion.
DURABILITY PARAMETERS
31
C
Reinforced concrete barrier; exterior girders; external faces of the piercap.
Exposure to rain; wind abrasion; accumulation of de-icing salts; wetting and drying cycles; freezing and thawing cycles; exposure to CO2; rundown of water carrying aggressive agents; exposure to sunshine and daily temperature changes.
Frost attack; surface deterioration; carbonation and chloride-induced corrosion.
D Lower face of the deck slab
Possible cracking and leakage of water carrying aggressive agents causing efflorescence; wetting and drying cycles; freezing and thawing cycles and frost attack.
Frost attack; surface deterioration and chloride-induced corrosion.
E Interior girders Exposure to wind; wetting and drying cycles; freezing and thawing cycles; frost attack.
Frost attack; surface deterioration and reinforcing steel corrosion.
F Upper parts of the pier columns and abutments.
Exposure to rain and wind; wetting and drying cycles; freezing and thawing cycles; frost attack.
Frost attack; surface deterioration and reinforcing steel corrosion.
G
Internal parts of the piercap; zone between the columns.
Exposure to wind; wetting and drying cycles; freezing and thawing cycles; frost attack.
Frost attack; surface deterioration and reinforcing steel corrosion.
H Middle part of the pier columns (spray zone).
Exposure to rain and wind; exposure to spray from the river; exposure to cyclical splashing; wetting and drying cycles; frost attack.
Frost attack; surface deterioration and reinforcing steel corrosion.
I
Lower part of the pier columns at the transition zone between high tide and low tide.
Frequent wetting - drying and freezing - thawing cycles; exposure to rain and wind; ice abrasion and ice impact.
Frost attack; surface deterioration and reinforcing steel corrosion; abrasion of concrete by ice.
J Lower part of the pier columns in the submerged zone.
Water and ice abrasion; ice impact; absence of air and free oxygen.
Surface deterioration; abrasion of concrete by ice.
K
Foundation of the pier columns through the sedimentary deposit of the river bed.
Embedded zones into the sedimentary stratum; absence of air. There is no significant potential for chemical attack due to the interaction with aggressive soil deposits.
No major deterioration is expected.
L Embedded zone of the pier columns into the bedrock.
Embedded zones into the bedrock; absence of air. There is no significant potential for chemical attack due to the interaction with aggressive soil deposits.
No major deterioration is expected.
Table 5.2: Microclimates, exposure conditions and mechanisms of deterioration of the different structural
members of the bridge.
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32
5.1.3. Environmentally induced mechanisms of deterioration
It is possible to identify the main mechanisms of deterioration from all possible modes of
deterioration in a component, that may affect the integrity of the construction materials and hence,
the bearing capacity of the structural members of the bridge. The mechanisms are described in
the following sections.
5.1.3.1. Frost Attack
Frost attack causes a gradual disintegration of concrete surfaces as a result of repeated freezing
and thawing cycles. In environments with moist and freezing conditions, a decrease in concrete
strength takes place, generating an eventual disintegration and complete loss of material near the
surface after a prolonged period of exposure.
The main reason for the disintegration of concrete is the effect of water freezing inside the
capillary pore system. When water freezes, it expands by about 9% of its volume, inducing some
internal pressures inside the pore system that eventually are dissipated through the generation of
cracking. The rate of this mechanism of deterioration depends not only on the quality of the
concrete (strength, low permeability, air content, etc.) but also on the aggressiveness of the
environmental conditions. Additionally, the presence of de-icing salts worsens even more the
consequences of this mechanism by adding the effects of concrete flaking or scaling caused by
the crystallization of salts, solar heating and water migration towards the surface (Litvan’s
theory).10
The gradual weakening or disintegration of the concrete cover surface, resulting from repeated
freezing and thawing processes can be modeled by the equation: 5
4.17.0 cmagecurenv facccr (5-1)
where:
r = the rate of disintegration [mm/year].
cenv = the environmental coefficient.
ccur = the curing coefficient.
cage = the ageing coefficient.
a = the air content [%].
fcm = the mean cubic compressive strength of concrete [MPa]; fcm= (f’c/0.80).
The curing and ageing coefficients can be expressed by the following equations:
DURABILITY PARAMETERS
33
dccur
10log17.085.0
1
(5-2)
flslsfage ppp
c001.0008.0045.01
1
(5-3)
where:
d = the curing time [days].
psf = the proportion of silica fume with respect to the total weight of binding agent [%].
psl = the proportion of blast furnace slag with respect to the total weight of binding agent [%].
pfl = the proportion of fly ash with respect to the total weight of binding agent [%].
Different environmental coefficients (Cenv) have been evaluated and presented by Sarja and
Vesikari 5, depending on the macro-climate conditions of different latitudes around world. These
coefficients are shown in Table 5.3.
Class Conditions Cenv
1
Very Hard: Frost, snow, ice, numerous freezing and thawing cycles, salt water or de-icing salts, temperature and moisture variations. Latitudes 60º ± 5º.
80 - 160
2
Hard: Frost, snow, ice, numerous freezing and thawing, constant contact with water (no chlorides), temperature and moisture variation. Latitudes 60º ± 10º.
40 - 80
3 Moderate: Normal outdoor conditions, freezing and thawing effects. Latitudes: 60º ± 10º.
20 – 40
4 Favourable: No freezing and thawing effect.
< 20
Table 5.3: Environmental coefficient values for frost attack.
5
5.1.3.2. Abrasion of concrete by ice
The reinforced concrete columns of the intermediate pier can suffer chemical changes from the
permanent exposure to the flow of the river. Additionally, some physical changes can also occur
due to wetting and drying cycles at the transition zone between the high water level and the low
water level, and due to freezing and thawing cycles. However, the final cause for the detachment
of concrete is the impact and friction of ice blocks or ice sheets against the concrete surface.
There exist many models that are intended to represent the degradation of concrete caused by
ice abrasion. However, in structural design of reinforced concrete structures the following
DURABILITY PARAMETERS
34
approximations can be used for obtaining the rate of abrasion expressed in the loss of concrete in
time. 5
When the aggregate stones are not loosening due to frost attack, the rate of abrasion can be
estimated as:
vf
pr
ck
'3 (5-4)
When the aggregate stones are loosening due to frost attack, the rate of abrasion can be
estimated as:
vfp
rck
'
3 (5-5)
where:
= the movement of the ice blocks or ice sheets [km/year].
p’ = the total proportional volume of cement stone in concrete including aggregates up to =
4mm. (Approximately, p’ can vary between 0.4 and 0.6).
fck = the characteristic (cubic) compressive strength of concrete [MPa] 8 cmf
5.1.3.3. Surface Deterioration
This mode of deterioration represents different types of weathering mechanisms due to the
exposure of the structure to the different microclimates. The model for surface deterioration
(Equation 5-6) includes temperature and moisture fluctuations, leaching and efflorescence effects
on the internal structure of the concrete cover, and physical deterioration due to the accumulation
of salts.
Temperature cycles can cause gradual cracking in weak zones of concrete elements like corners
and edge zones. Wetting and drying cycles with climatic moisture changes can induce slight
cracking and small changes in the permeability of the concrete cover. The generation of cracking
and the increase of the concrete cover permeability can produce an increment in the ingress of
water. The flow of water through certain zones of concrete can generate leaching of concrete
minerals, affecting the durability properties of the affected regions. Salt weathering and
efflorescence is associated with the crystallization of salts and minerals in the pores of concrete
while water vaporises. An associated mechanism is the expansion and shrinkage of these
crystals as a result of hydration and dehydration leading to cracking and disintegration of the
concrete. This mechanism can be represented by the following equation: 5
DURABILITY PARAMETERS
35
3.3 ckcurenv fccr (5-6)
where:
r = the rate of disintegration [mm/year].
cenv = the environmental coefficient.
ccur = the curing coefficient.
Some environmental coefficients (Cenv) for the surface deterioration model have been evaluated
and presented by Sarja and Vesikari 5 depending on the macro-climate conditions found at
different latitudes around the world. These coefficients are shown in Table 5.4.
Class Conditions Cenv
1
Very Hard: “Gulf conditions” Marine structures or structures within the capillary rise of saline ground water, temperature and moisture variations. Latitudes 20º ± 10º.
100000 - 500000
2
Hard: Marine structures or structures within the capillary rise of saline ground water, temperature and moisture variations. Latitudes 40º ± 10º.
10000 - 100000
3 Normal: Normal outdoor conditions, small climatic changes. Latitudes 40º ± 10º.
1000 - 10000
4 Favourable: Air continuously dry, no sunshine.
< 1000
Table 5.4: Environmental coefficient values for surface deterioration.
5
5.1.3.4. Chloride-induced Corrosion
Chlorides present in de-icing salts can penetrate into the reinforced concrete elements
developing a gradient near the concrete surface. The time required for the chloride threshold
(critical level of chlorides) to be reached and destruction of the passive layer is normally defined
as the initiation time for corrosion.
The modelling of corrosion of reinforcement can be established by defining two main limit states,
the initiation period and the propagation of active corrosion (Figure 5.6). The first period
corresponds to the progression of the chloride ions into the concrete, moving towards the
reinforcing steel and increasing in concentration until the attainment of the chloride content
threshold that marks the end of the initiation period and the start of active corrosion.
The initiation period must be designed to be as long as possible to impede, or at worst diminish
the occurrence of active corrosion.
DURABILITY PARAMETERS
36
Figure 5.6: Diagram of the two limit states of corrosion in reinforced concrete elements.
In the case of prestressed concrete elements, the service life must corresponds to the initiation
period because once the prestressed strands get depassivated, localized pitting corrosion occurs
which can lead to brittle failures in prestressing steel subjected to high stress levels. In this
situation, and other cases where corrosion cannot be allowed, service life can be defined as:
0ttd (5-7)
where:
td = design service life
t0 = time to initiation of corrosion
In other cases, some corrosion may be allowed, which could generate some cracking in the
concrete cover due to the rust products that result from the corrosion reactions. In these cases,
the service life includes some of the propagation period of corrosion, which produces a
progressive decrease of the cross-section area of the reinforcing steel, a loss of bond between
steel and concrete, and a reduction of the effective cross-sectional area of the reinforced
concrete elements due to cracking and spalling of the concrete cover.5
In this case, the service life can be defined according to the following formula:
10 tttd (5-8)
where:
t1 = time for propagation of corrosion.
The propagation of active corrosion ends when a maximum allowable loss of cross-sectional area,
loss of bond, or crack width is attained.
DURABILITY PARAMETERS
37
The initiation period of corrosion can be determined based on the Fick’s second law of diffusion,
which describes the gradient of chloride content in the concrete cover as: 5
tD
xerfCC
c
sx2
1 (5-9)
where:
xerf error function x
t dte0
22
.
cx = the chloride content at depth x.
cs = the chloride concentration at the concrete surface.
x = the depth from the surface of the structure.
Dc = the diffusion coefficient of chlorides.
t = time
The initiation period of corrosion can be derived from the following equation: 5
2
210
112
1
s
thc
CC
c
Dt (5-10)
where:
cth = the critical chloride content.
c = concrete cover thickness.
t0 = time to initiation of corrosion.
Once the initiation period is terminated, active corrosion starts to deteriorate the reinforcing steel
bars. The length of the propagation period (t1) depends at first on the design criteria, whether
active corrosion is allowed or not within the design service life. If corrosion occurs during the
service life, the end of the propagation period will be determined when a critical reduction of the
reinforcing steel cross-section area and a maximum allowable reduction of the bearing capacity of
the reinforced concrete element are reached, attaining consequently the defined limit states of the
structure.
The length of this period depends on the rate of corrosion, which is controlled by different aspects,
such as ambient temperature, relative humidity, chloride content, w/cm ratio, type of cement,
degree of exposure to severe and aggressive micro-climates, and the presence of supplementary
protective measures, among other factors. It has been discovered that the rate of corrosion slows
DURABILITY PARAMETERS
38
gradually with time; however, in absence of detailed information about this phenomenon,
corrosion rate can be conservatively considered to be constant for structural design for durability.
Some researchers have determined the values of corrosion rate on-site and in laboratories
around the world. According to the experimental data reported by Tuutti (1982)5, some
approximate averages of rates for carbonation- and chloride-induced corrosion are presented in
Table 5.5.
Relative Humidity(%)
Carbonated Concrete(m/year)
Chloride-contaminated Concrete (m/year)
99 2 34 95 50 122 90 12 98 85 3 78 80 1 61 75 0.1 47 70 0 36 65 0 27 60 0 19 55 0 14 50 0 9
Table 5.5: Rate of corrosion at anodic areas in carbonated and
chloride-contaminated concrete (Tuutti, 1982). 5
The values in Table 5.5, as well as the factors that affect the rate of corrosion, are specified for a
temperature of +20ºC, and some corrections can be applied to obtain the actual rates of corrosion
at different temperatures. This is expressed in the following equation: 5
0rcr T (5-11)
where: r = the rate of corrosion.
cT = temperature coefficient.
r0 = the rate of corrosion at +20ºC.
Some values of temperature coefficients determined by Tuutti (1982), and average daily
temperatures for some cities in the northern hemisphere (Europe) are shown in the Table 5.6.
Some other values of corrosion rates were found in association with the relative humidity and the
nature of the mechanism of corrosion by Andrade et al. in 1994. These values are presented in
Table 5.7.
DURABILITY PARAMETERS
39
City
Rate of corrosion (m/year)
Exposed to rain Sheltered from
rain Sodankylä
Temperature coefficient,
CT
0.21 11 2.5 Helsinki 0.32 16 4
Amsterdam 0.47 24 6 Madrid 0.73 37 9
Doniphan County, Kansas, USA
Condition of concrete
Uncracked 1.35 -
Mission Creek, Shawnee County,
Kansas, USA
Uncracked 3.64 -
Cracked 9.37 -
Table 5.6: Temperature coefficients, condition of concrete and evaluated rates of corrosion for some cities in Europe. 5 , 27, 28
Relative Humidity(%)
Rate of Corrosionm/year)
Only aggressive action is carbonation 90 - 98 5 - 10
< 85 ≤ 2 Chloride-contaminated environments
100 ≤ 10 80 - 95 50 - 100
< 70 ≤ 2
Table 5.7: Corrosion rate found by Andrade et al (1994). 5
A major consequence of steel corrosion is the loss of bond between concrete and the steel
reinforcing bars. This loss of bond is closely related to the mass loss of the reinforcing bars.
Amleh (2000) 17 developed bond strength equations as a function of mass loss of reinforcing bars,
and other factors. These equations are shown in the Table 5.8.
Concrete Mixture Required Development Length
(Ld design)
Point Tupper fly ash concrete
(W/CM = 0.32) MLfd
c
dfL
cb
bydesignd
34.0'597.116.04.0
25.0
(5-12)
Thunder Bay fly ash concrete
(W/CM = 0.32) MLfd
c
dfL
cb
bydesignd
31.0'205.116.04.0
25.0
(5-13)
DURABILITY PARAMETERS
40
Sundance fly ash concrete
(W/CM = 0.32) MLfd
c
dfL
cb
bydesignd
31.0'337.116.04.0
25.0
(5-14)
Normal Portland cement concrete
(W/C = 0.32) MLf
d
c
dfL
cb
bydesignd
31.0'284.116.04.0
25.0
(5-15)
Normal Portland cement concrete
(W/C = 0.42) MLf
d
c
dfL
cb
bydesignd
31.0'425.116.04.0
25.0
(5-16)
Table 5.8: Required development lengths for corroding steel reinforcement in various concrete mixtures. 17
where: Ld = the required development length for the reinforcing bar under corrosion [mm].
c = the concrete cover thickness [mm].
ML = the mass loss of reinforcing bars [%].
db = the diameter of the reinforcing bar [mm].
fc = the concrete compressive strength [MPa].
fy =the reinforcing steel yield strength [MPa].
Figure 5.7: Mass loss of reinforcement vs. time due to chloride-induced corrosion. 17
DURABILITY PARAMETERS
41
To use these equations, it is necessary to determine the increment of mass loss of the reinforcing
bars in time as a result of the corrosion process. Mirza et al (2005) presented some charts
relating the progress of mass loss of reinforcing steel with time depending on the concrete cover
thickness and other factors. A typical chart is shown in Figure 5.7. 17
The rest of the charts are presented in the Appendix 3, for different chloride concentrations at the
concrete surface (Cs), ranging from 1% to 6%.
5.1.3.5. Carbonation-induced Corrosion
Carbon dioxide present in the air penetrates concrete, causing shrinkage of the hydrated cement
paste that leads to cracking, a reduction of the wearing resistance of the surface, and a decrease
of alkalinity of the pore solution of the concrete microstructure that creates a carbonation front
that progressively penetrates into the reinforced concrete element. When this carbonation front
reaches the reinforcement, the passive protective layer of the steel dissolves, allowing corrosion
to occur. The initiation time of corrosion is defined as the time required for the complete
carbonation of the concrete cover. The model that represents the progress of the carbonation
front from the external surface of the concrete cover towards the interior of the reinforced
concrete elements is represented by the equations 5-17 and 5-18. 5
tKd c (5-17)
bcmairenvc faccK
(5-18)
where:
d = the depth of carbonation at a time t [mm].
Kc = the carbonation coefficient.
t = age [years].
cenv = the environmental coefficient.
cair = the air content coefficient.
a, b = parameters depending on the binding agent.
Two distinctive environmental coefficients (Cenv) are evaluated and presented by Häkkinen
(1993)5 according to some specific micro-climatic conditions. These coefficients are presented in
Table 5.9.
Environment Cenv
Structures sheltered from rain 1 Structures exposed to rain 0.5
Table 5.9: Environmental coefficient for carbonation-induced corrosion.
5
DURABILITY PARAMETERS
42
Häkkinen et al. have presented some values for the air content coefficient (Cair). The typical
values are presented in Table 5.10.
Air porosity Cair No entrained air 1
Entrained air 0.7
Table 5.10: Air content coefficient for carbonation-induced corrosion.5
The same authors published some values of parameters a and b depending on the type of
cementing material present in the concrete mixture. These values are shown in Table 5.11.
Binder a b Portland cement 1800 -1.7
Portland cement + fly ash 28% 360 -1.2 Portland cement + silica fume 9% 400 -1.2
Portland cement + blast furnace slag 70% 360 -1.2
Table 5.11: Parameters a and b. 5
Accordingly, the initiation period of carbonated-induced corrosion can be calculated using the
equation:
2
0
cK
dt (5-19)
5.1.4. Minimum Required Conditions of the Main Construction Materials
The quality and thickness of the concrete cover as well as the type of reinforcing steel are two
key parameters controlling the durability of reinforced concrete members. These aspects are
some of the multiple measures of protection against aggressive agents present in the different
microclimates. The different modes of deterioration need to be evaluated for each member of the
bridge, depending on the specific environmental conditions of exposure.
After having followed an iterative process of analysis for the different modes of the deterioration, it
was possible to identify the durability requirements for the construction materials with the purpose
of attaining an appropriate performance during the service life of the bridge. The input parameters
are established according to the microclimatic conditions present in the different zones and
elements of the structure that were described earlier. These parameters are summarized in the
table 5.12. According to this information, it was possible to determine the different rates of
deterioration of the concrete members of the bridge described earlier, which were produced by
the various mechanisms of deterioration developed according to the microclimatic conditions.
These rates are described in the Table 5.13.
DURABILITY PARAMETERS
43
Table 5.12: Durability parameters for the various bridge components according to the different modes of deterioration.
DURABILITY PARAMETERS
44
As expected, the elements exposed to the most severe environmental conditions would
experience the greatest rates of deterioration. These elements are the bridge deck slab, barriers
and edge girders.
Corrosion initiates relatively soon in the girders, because the have smaller concrete cover
thicknesses among all of the elements of the bridge (Table 5.14). Active corrosion occurs at the
lowest rate for the interior girders since they are sheltered from direct exposure to the chlorides
and moisture. However, the edge girders present a significantly high rate of active corrosion. For
these elements, it is essential to establish appropriate additional protective measures to impede
the direct exposure of the member to chlorides and moisture, or at least to reduce it. These
measures could be membranes, coatings or even cornices that act as shields against the
exposure to rain and rundown of water carrying de-icing salts.
Table 5.13: Rates of deterioration, initiation time for corrosion and carbonation coefficients for the various bridge components.
Other elements, such as the bridge deck slab and the barriers have higher initiation times for
corrosion between 20 and 30 years, due to thicker concrete covers. Nevertheless, the rates of
active corrosion are the highest in the bridge, caused by the direct exposure of the required
components for corrosion which are moisture, oxygen and above all, high concentration of
chloride ions. The abutments, piercap and columns are the elements that present highest
initiation times of corrosion. Among these elements, columns which are more protected against
rain and high concentration of chloride ions are the ones that present the lowest rate of active
corrosion. Caissons and the submerged part of the columns present a significantly delayed
initiation time of corrosion of 74 years. Additionally, they experience the lowest active corrosion
rate in the bridge due to the absence of free oxygen under water.
DURABILITY PARAMETERS
45
The responses for the different mechanisms of deterioration for each of the bridge members are
presented in Figures 5.8 to 5.11 for a service life of 150 years.
a)
b)
Figure 5.8: Response for deterioration mechanisms for: a) Bridge deck slab, b) Barriers.
DURABILITY PARAMETERS
46
a)
b)
Figure 5.9: Response for deterioration mechanisms for: a) Piercap, b) Abutments.
DURABILITY PARAMETERS
47
a)
b)
Figure 5.10: Response for deterioration mechanisms for: a) Pier columns, b) Caissons.
DURABILITY PARAMETERS
48
a)
b)
Figure 5.11: Response for deterioration mechanisms for: a) Edge girders, b) Internal girders.
DURABILITY PARAMETERS
49
It is evident in Figure 5.10 that the abrasion of concrete by ice on columns and caissons results to
be a very aggressive mechanism that may be very difficult to control only with an adequate
design of concrete for these elements. In this case, it is essential to configure an additional
protective measure by means of ice barriers upstream, protective islands around the piers, and
the use of steel shields attached to the concrete sections, among other solutions.
The initiation and development of corrosion of reinforcing steel for the different elements of the
bridge are shown in Figure 5.12.
Figure 5.12: Chloride-induced corrosion progress in the different members of the bridge over a stipulated service life of 150 tears.
The progress of the carbonation front for the various bridge members is shown in Figure 5.13. In
this figure it is possible to identify three groups of elements that experience the same rate of
carbonation progress into concrete.
The elements, which are exposed to rainy conditions, present lower progress of carbonation,
because of the higher moisture saturation in these elements, which impede or make more difficult
the diffusion of CO2 into the concrete. This is the case of the bridge deck slab, barriers and edge
girders, which are normally quite saturated with moisture.
Other elements like the piercap, abutments and caissons, which are more protected against rain,
and therefore have a lower moisture content, demonstrate a higher progress of the carbonation
DURABILITY PARAMETERS
50
front. Also, the elements that are sheltered from rain, such as the internal girders and columns,
also experience the most rapid progress of the carbonation front.
Figure 5.13: Progress of carbonation front in the bridge.
5.1.4.1. Minimum Concrete Cover
From Figures 5.8 to 5.13, and the results of the rates of deterioration of concrete, it is possible to
determine the minimum concrete cover thickness necessary to ensure satisfactory performance
of the bridge members during the established service life.
For instance, the necessary concrete cover thickness for the top surface of the bridge deck slab
can be established by identifying the deterioration of the concrete cover that take place due to the
most severe mechanism of deterioration, which in this case is the surface deterioration. After a
design service life of 150 years, a depth of 62mm of concrete will be affected. However,
considering a simultaneous degradation process caused by carbonation, an additional 11 mm will
be necessary to protect the reinforcing steel. Therefore, the minimum concrete cover thickness
should be 62mm + 11mm = 73mm, which is larger than the minimum specified concrete cover
thickness in the different standards. CSA-S6-06 specifies for this element a concrete cover
thickness of 70 ± 20mm1, CSA A23.1 specifies 60mm18, and the Ministry of Transportation of
Quebec (MTQ) establishes a minimum concrete cover thickness of 60mm. Following the
principles of design for durability, the adopted concrete cover thickness for the top slab of the
bridge deck slab is 75mm. This is a considerable thick concrete cover. For this reason, especial
care will be taken for the design of this element, especially in terms of cracking control. The
concrete mixture for the slab will have microfilament polypropylene fibres to reinforce the
DURABILITY PARAMETERS
51
concrete cover. Additionally, a galvanized thin wire mesh can be provided at the middle of the
concrete cover to improve crack control in case of thick concrete covers.
Similarly, the concrete covers for other bridge members can be determined, following the same
procedure. Table 5.14 summarizes the minimum required and adopted concrete cover
thicknesses for the different bridge elements.
Table 5.14: Concrete cover thicknesses for the analyzed members of the bridge.
For design purposes, in the cases where the minimum required concrete cover thickness is
smaller than the minimum specified by the standards, the previous one will be adopted for the
element to respect the code provisions. For instance, in the case of barriers, the minimum
required concrete cover thickness is 56mm; however, the MTQ code establishes a minimum of
75mm for these elements, therefore the concrete cover adapted is 75mm.
5.1.4.2. Type of Steel
Development and progress of corrosion in reinforcing steel has been analyzed for this example,
using regular uncoated construction steel. However, following a multiple-stage protection strategy
for durability design, the recommended type of steel to be used in the analyzed member of the
bridge is galvanized steel or epoxy-coated steel. This kind of steel will increase the corrosion
initiation time or even better, under optimal conditions, it could impede this mechanism of
deterioration.
Hot-dip galvanized rebars can be used for large bridge members like abutments, transition slabs,
bridge deck slab, diaphragms, and piers. This kind of steel allows the use of cathodic protection
as another supplementary protection measure in the future when it is necessary. For other
members, such as barriers and girders, epoxy-coated steel can be used, following adequate
procedures of assembling and coating the reinforcing bars.
Especial care must be taken when using epoxy-coated steel in bridge construction. During
handling, bending, attaching and placing the reinforcement, the epoxy coating around the bar can
get damaged, leaving some unprotected spots and pinholes, where localized corrosion could start
and the spread along the bar. A good solution has been implemented for some important bridge
DURABILITY PARAMETERS
52
projects around the world using a technique called “the fluidized technique for epoxy coating”.
This technique consists of preparing welded reinforcement cages, which are blast cleaned,
heated to 160ºC, dipped into unheated fluidized powder of epoxy, and then cured and stored. The
epoxy powder fuses onto the reinforcing bars providing an even coating practically without any
pinholes, and covers all deformations as well as welding points in a reliable manner.19
5.2. Concrete Mixture Design for Durability
Because the concrete in different bridge elements is going to be exposed to diverse and specific
micro-climatic conditions, it must be able to cope with these different conditions, and therefore, its
quality can vary from element to element. Consequently, special concrete mixture designs need
to be prepared for the various elements of the structure, following certain durability requirements
and taking into consideration the precise environmental exposure of each bridge member.
5.2.1. Concrete Mixture Requirements
The microclimatic conditions described in Table 5.2 can be associated with the types of exposure
indicated in the Table 1 of the CSA Standard A23.118. The types of exposure conditions in this
table that concern the various bridge elements are:
C-XL: Reinforced concrete exposed to chlorides or other aggressive agents, affected or not
exposed to freezing and thawing cycles, and expecting higher durability performance exposures
classes C-1, A-1, and S-1.
C-1: Reinforced concrete exposed to chlorides and affected or not exposed to freezing and
thawing cycles. Examples: Bridge deck slabs, slabs and ramps for parking structures, coastal
structures located at the tidal and splashing zones, concrete structures exposed to splashing and
spraying of sea water or salted water from reservoirs.
C-3: Concrete permanently submerged, exposed to chlorides but not exposed to freezing and
thawing cycles. Example: submerged parts of coastal structures.
F-1: Concrete exposed to freezing and thawing cycles in saturated conditions but not exposed to
chlorides.
Considering the exposure condition of the bridge members, supplementary cementing materials
can be used in the concrete mixture to improve workability, enhance durability, decrease paste
porosity, decrease the heat of hydration and its rate of generation. The supplementary cementing
materials that are used tor the concrete mixtures are silica fume, blast furnace slag and fly ash.
Because these materials are added to the concrete mixture, prolonged and adequate curing
processes need to be implemented.
DURABILITY PARAMETERS
53
For protection against frost attack and freezing-and-thawing cycles, it is essential to use entrained
air for improved performance of the concrete in these bridge elements. A good microstructure of
concrete in terms of void size distribution and connectivity (permeability) is enhanced with the use
of low water to cementitious materials ratios. At lower W/CM ratios there is lower capillary
porosity in the concrete microstructure. Adequate slumps of concrete are required for some
bridge elements that may contain congested reinforcement, especially in the bridge deck slab and
the precast and prestressed girders. This fluidity of concrete must be attained without increasing
the W/CM ratio, which is a key factor to the durability of concrete. The use of plasticizers may be
necessary and recommended in some cases.
Corrosion protection of the reinforced concrete elements can be planned at the concrete mixture
design stage by using corrosion inhibitor admixtures. These admixtures contains anodic inhibitors
such as nitrites that block the corrosion reaction of the chloride ions by chemically reinforcing and
stabilizing the passive protective film on the steel, which is created by the high pH environment in
concrete. In effect, the chloride ions are prevented from penetrating the passive film and making
contact with the steel when this kind of admixture is added to the concrete.
To minimize cracking in the bridge members, especially those that have large surfaces,
shrinkage-reducing admixtures can be used. Additionally, as it has been mentioned before,
because some elements have thick concrete covers, the use of microfibers will help to control
crack generation. The use of fibres in concrete helps to control plastic shrinkage and settlement
cracking. Additionally, these fibres can improve impact, shatter and abrasion resistance of
concrete; they enhance durability and toughness of concrete, and can also help to reduce its
bleeding. Microfilament polypropylene fibres are recommended instead of steel fibres to avoid
corrosion problems that may initiate cracking.
5.2.2. Base Materials Specifications
The base materials for the concrete mixture designs, available at the various concrete mixing
plants in Montreal, are described as follows:
Cement: Type 20, relative density 3.14.
Silica Fume: Relative density 2.25.
Blast Furnace Slag: Relative density 2.90.
Fly Ash: Relative density 2.60.
Coarse Aggregate: Well-graded crushed rock with an oven-dry relative density of 2.68, absorption
of 0.5%, and oven-dry density of 1600kg/m3. The laboratory sample has a moisture content of 2%.
DURABILITY PARAMETERS
54
Fine Aggregate: Natural sand with some crushed particles with an oven-dry relative density of
2.64 and an absorption of 0.7%. The laboratory sample has a moisture content of 6%.
5.2.3. Concrete Mixture Proportions
The concrete mixture proportions are summarized in detail in the Table 5.15 for each bridge member.
Table 5.15: Detailed proportions for the concrete mixture designs.
DURABILITY PARAMETERS
55
The summary of the concrete mixtures design is shown in Table 5.16.
Table 5.16: Summary of the concrete mixture proportions for the bridge members.
DURABILITY PARAMETERS
56
5.3. Concrete Handling, Placing and Curing
5.3.1. Concrete Handling
The handling and transporting of concrete must be planned carefully to avoid the occurrence of
certain situations that could seriously affect the quality of the finished concrete. These situations
are delays, early stiffening and drying out, and segregation.
Additionally, it is essential to provide the use of the proper equipment for producing, transporting,
placing and finishing concrete in the most effective and efficient way. Considering some of the
construction characteristics for some elements, the use of the concrete pump is essential. As a
result, the concrete mixture has to be designed such that it will have adequate fluidity without
increasing the W/CM ratio; this is the case for the deck slab, abutments, columns and caissons.
For other elements, such as barriers and the piercap, concrete can be easily cast using the crane
and bucket system.
5.3.2. Placing and Finishing of Concrete
The placement and casting of concrete at the construction site or at the precast facility must be
performed carefully, following appropriate standards and procedures. The adequate placement
and handling of concrete will determine the quality of concrete. All concrete casting operations
need to be performed to avoid segregation that results in problems, such as rock pockets and
honeycombs, which results in increased ingress of aggressive agents into the concrete elements.
Consolidation is an essential process for a well-finished concrete element. It is related to vibration,
which promotes the uniformity of concrete and the removal of entrapped air. Nevertheless,
prolonged vibration must be avoided to prevent segregation of concrete and removal of entrained
air; it also influences the structure of internal voids and increasing W/CM ratios on the periphery
of the concrete elements inside the formwork and in the upper surfaces (concrete covers).
5.3.3. Curing
For the concrete mixtures described before, curing must start immediately after finishing. A
minimum curing period of seven days under moist conditions must be implemented for all cast-in-
place concrete elements. To lower the permeability of concrete surfaces exposed to the most
aggressive conditions, including the concrete slab and barriers, the curing period should be
increased to 14 days under moist conditions.
All recently placed and finished concrete surfaces must be cured and protected from drying, from
extreme changes in temperature, and from damage by subsequent construction operations and
traffic.
DURABILITY PARAMETERS
57
Certain limitations of the proportions of pozzolanic materials added to concrete should be
respected to ensure the effectiveness of the addition of these materials. Additionally, for
concretes with high dosages of supplementary cementing materials, the slow rate of pozzolanic
reactions requires prolonged periods of moist curing to achieve the benefits of the pozzolans;
otherwise, the addition of these materials become non-cementitious fillers. Pozzolans react with
Calcium Hydroxide and water to produce more Calcium Silicate Hydrate, which adds to the
strength and enhances the durability characteristics of the concrete. The extent of the pozzolanic
reactions is determined by the consumption of Calcium Hydroxide and moisture in the concrete.
Special care must be exercised during design and proportioning of the concrete mixture limiting
the dosages of the supplementary cementing materials. Elevated contents of silica fume can
make concrete highly cohesive with low aggregate segregation or bleeding. Low levels of bleed
water at the concrete surface contribute to the appearance of plastic cracking due to rapid water
evaporation at the surface, especially on hot and windy conditions. Special precautions are
needed when concrete is placed and finished during periods with adverse weather. During cold
weather, some arrangements, such as heating, covering, enclosing and insulating of concrete
must be provided to avoid freezing of concrete or a significant decrease in temperature. By
contrast, during hot-weather conditions, special precautions against rapid evaporation and drying
need to be taken.
The curing methods that maintain the presence of mixing water in concrete during the early
hardening period are recommended, especially for bridge deck slab, approach slabs, foundation
units and certain parts of piers and abutments. These methods include ponding, fogging, spraying
and employing saturated coverings.20
Other methods that involve the use of impervious membranes and that reduce the loss of mixing
water from the surface of concrete are recommended for most of the bridge members, including
the soffit of the bridge deck slab, diaphragms, girders, abutments, piers, and barriers. Steam
curing may be used for the girders in adequate facilities, considering that these elements are
normally precast, cured, and then transported to the construction site and installed.
Steam curing can enhance the gain of concrete strength at early ages; however, this is not the
main priority in this study. What is more important is to perform a proper curing process that will
ensure an adequate concrete performance, enhancing the mechanical properties, the durability
characteristics, the reduction of volume changes due to shrinkage, and an optimum pore size
distribution and structure that minimizes capillary porosity, and hence the permeability of concrete.
STRUCTURAL DESIGN FOR DURABILITY
58
6. STRUCTURAL DESIGN FOR DURABILITY
The structural design of the different bridge members is performed according to the guidelines
and specifications provided by the Canadian Highway Bridge Design Code (CHBDC) 1.
6.1. Materials Properties
Based on the durability requirements for the construction materials described earlier (Chapter 5)
as well as the necessary bridge performance requirements during the service life of the structure,
the summary of the material properties for the different elements of the structure is presented in
Table 6.1.
Material Properties
Properties Slab Barriers Piercap Columns Abutment Caissons Girders
Reinforced Concrete Elements
Unit weight, c (kgf/m³) 2284 2203 2272 2285 2337 2300 2273
Compressive strength at 28 days, f 'c (MPa)
45 45 35 35 35 35 50
Compressive strength at transfer, f 'ci (MPa)
- - - - - - 40
Modulus of elasticity, Ec (MPa) 28729 27216 25946 26158 27057 26417 29712
Reinforcing Steel
Yield strength, fy (MPa) 400 400 400 400 400 400 400
Modulus of elasticity, Es (MPa) 200000 200000 200000 200000 200000 200000 200000
Prestressing steel, high tensile 7 wire, low relaxation (stabilized) strands
Nominal strand diameter, dps (mm)
- - - - - - 12.7
Area of prestressing strand, Aps (mm²)
- - - - - - 98.7
Ultimate strength, fpu (MPa) - - - - - - 1860
Strand braking strength (kN) - - - - - - 183.58
Modulus of elasticity, Ep (MPa) - - - - - - 200000
Factor kp of steel strands 0.30
Table 6.1: Material properties for the elements of the bridge.
The edge girders will be designed, because of their exposure to a more aggressive microclimate,
therefore, only the material properties for the exterior girders are presented in Table 6.1 along
with the properties of materials for other structural elements.
STRUCTURAL DESIGN FOR DURABILITY
59
6.2. Superstructure Design
The superstructure of the bridge can be analyzed using a simplified method for dead and live
loads. However, certain conditions must be respected according to Clauses 5.6.1.1 and 5.7.1.1 of
the Canadian Highway Bridge Design Code1, these are summarized in Tables 6.2 and 6.3.
Article 5.6.1.1 of CHBDC
Conditions for use Bridge properties Condition satisfied
(a) Constant width Constant bridge width = 15.07m
Yes
(b) Supports conditions are closely equivalent to line supports at ends and intermediate supports
Aligned simple supports at end and intermediate supports
Yes
(c) For slab-on-girder bridges, the skew parameter must be less than 1/18
The bridge is straight and rectangular. The skew angle is 0º, consequently, the skew parameter is 0
Yes
(d) Restrictions described in Clause A5.1.3.2 for bridges that are curved in plan and built with shored construction
The bridge is straight in plan. Therefore, the condition does not apply.
Yes
(e) Uniform slab cross section Uniform slab thickness = 225mm
Yes
(f) For a bridge with longitudinal girders and an overhanging deck slab, the overhang must not be greater than 1.8m and must not exceed 60% of the spacing of the two outermost adjacent webs.
Bridge deck overhang = 1.25m 60% spacing = 0.6 x 2.095m = 1.257m
1.25m < 1.8 1.25m < 1.257m
Yes
Table 6.2: Conditions for use of the simplified method of analysis for dead loads.
Article 5.7.1.1 of CHBDC
Conditions for use Bridge properties Condition satisfied
(a) Constant width Constant bridge width = 15.07m Yes (b) Supports conditions are closely equivalent to line supports at ends and intermediate supports
Aligned simple supports at end and intermediate supports
Yes
(c) For slab-on-girder bridges, the skew parameter must be less than 1/18
The bridge is straight and rectangular. The skew angle is 0º, consequently, the skew parameter is 0
Yes
(d) Restrictions described in Clause A5.1.3.2 for bridges that are curved in plan and built with shored construction
The bridge is straight in plan. Therefore, the condition does not apply.
Yes
(e) Uniform slab cross section Uniform slab thickness = 225mm
Yes
(f) At least three longitudinal girders of equal flexural rigidity and equal spacing, or with variations from the mean in rigidity and spacing of not more than 10% in each case.
There are seven identical girders distributed with the same spacing of 2095mm
Yes
(g) Requirements for box-girder bridges
For this structure, which is a slab-on-girder bridge, these requirements are not applicable.
N/A
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(h) For a bridge with longitudinal girders and an overhanging deck slab, the overhang must not be greater than 1.8m and does not exceed 60% of the spacing of the two outermost adjacent webs.
Bridge deck overhang = 1.25m 60% spacing = 0.6 x 2.095m = 1.257m
1.25m < 1.8 1.25m < 1.257m
Yes
(i) Requirements for continuous spans given by the Clause A5.2.1.2
The girders are designed and built as simply supported beams. A condition of semi-continuity will be introduced in order to avoid the use of expansion joint at the intermediate supports.
N/A
(j) Requirements for box-girder bridges
For this structure, which is a slab-on-girder bridge, these requirements are not applicable.
N/A
(k) Requirements for box-girder bridges
For this structure, which is a slab-on-girder bridge, these requirements are not applicable.
N/A
Table 6.3: Conditions for use of the simplified method of analysis for live loads.
The Clause C5.7.1.1 of the Commentary on the Canadian Highway Bridge design Code 21
describes that for a deck-on-girder bridge with equally spaced girders, the most desired condition
of the cantilever overhang should be nearly 50% of the girder spacing. According to this, each
longitudinal girder can be associated to equal tributary areas and thus, a uniformly distributed
load over the entire deck area would then result in girders supporting equal external actions in
terms of mechanical loads. However, a maximum overhang of 60% of the girder spacing is
permitted, where the outer girders would be required to resist higher bending moments and shear
forces than the interior ones; nevertheless the assumption for uniformly distributed loads is still
acceptable.
In the present case, the cantilever overhang of the bridge deck is 1.25m / 2.095m = 0.60.
According to this, the geometric characteristics of the bridge deck of the structure respect all of
the previously mentioned requirements, thereby permitting the use of the simplified structural
analysis of the superstructure of the bridge.
6.2.1. Prestressed Concrete Girder Design
The design of the edge girders is presented in detail here, because these elements are more
critical than the intermediate girders, in terms of structural and environmental performance.
Following the standards determined by the Ministry of Transportation of Quebec (MTQ) and most
of the transportation agencies of the East Coast of North America, the sections that are
considered in this design exercise are the New England Bulb-Tee (NEBT) precast girders (Figure
6.1). A complete structural analysis is performed to determine the required structural parameters
STRUCTURAL DESIGN FOR DURABILITY
61
for the girders in to withstand the external loading conditions. After several iterations, the girder
section, NEBT1600, was the one that revealed adequate bearing capacity immediately after
construction, and sufficient capacity in reserve to endure the external actions throughout the
entire service life. During this iterative process, the material properties and the general
configuration of the bridge were modified and improved to adjust the durability and mechanical
characteristics of the girders.
Figure 6.1: NEBT girder characteristics. 22, 16
STRUCTURAL DESIGN FOR DURABILITY
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6.2.1.1. Bridge Deck Parameters
The bridge deck parameters for the girder design are listed as follows:
Girder span (m) = 26 Center-to-center girder spacing (m) = 2.095
Width of the design lane (m) = 3.05 Shoulder width (m) = 1
Number of shoulders = 2 Number of lanes = 4
Girder NEBT1600 at year = 0 Number of girders = 7
Height (mm) = 1600 Width of the upper flange (mm) = 1200
Area, Ag (mm² x 10³) = 589 Centroid to bottom, Yb (mm) = 760.9
Moment of inertia, Ig (mm^4 x 10^9) = 204.8 Section modulus from the top, Stg (mm³ x 10^6) = 244.07
Section modulus from the bottom, Sbg (mm³ x 10^6) = 269.16 Weigth (kN/m) = 13.13
Web thickness, bv (m) = 0.18 6.2.1.2. Composite Section
According to Clause 5.8.2.1 of the CHBDC 1, the parameters of the composite section are shown
in Figure 6.2.
Figure 6.2: Geometrical parameters of the composite section.
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63
6.2.1.3. Diaphragms
The number of diaphragms and their distribution are established according to the Clause 8.18.5.
Accordingly there is a total of five diaphragm distributed along each span. These diaphragms are
1.10m in height and 0.30m in width.
6.2.1.4. Wearing Surface
The wearing surface is meant to be composed by a waterproofing membrane welded on top of
the deck slab and an asphalt concrete layer of 65mm in thickness. The unit weight of the asphalt
concrete is considered as 23.5kN/m3. This pavement is placed over a width of 14.2m along the
three spans of the bridge.
6.2.1.5. Barriers
There are cast-in-place reinforced concrete barriers at each edge of the bridge deck. According to
the MTQ Standards 22, these barriers correspond to a barrier type 301. These are selected and
designed to have a level of performance PL-3 in accordance to the CHBDC 1. The cross-section
area of each of these barriers is 0.35m2.
6.2.1.6. Load Analysis
The load analysis is performed per metre length of the bridge deck. The results from this analysis
are presented in the following sub-sections.
6.2.1.6.1. Dead Loads
DLslab (kN/m) = 11.90 DLgusset (kN/m) = 1.34 DLgirder (kN/m) = 13.13
DLdiaphragms (kN/m) = 2.98 6.2.1.6.2. Superimposed Loads
DLwearing surface (kN/m) = 3.10 DLbarriers (kN/m) = 2.16
Total (kN/m) = 5.26 6.2.1.6.3. Live Loads
The design truck CL-625 described in the CHBDC is implemented for the design of the
superstructure (Figure 6.3). The design truck is placed centrally in a space of 3m within each
lane. Alternatively a CL-W lane load will be analyzed in order to determine the possible most
critical condition.
STRUCTURAL DESIGN FOR DURABILITY
64
a)
b)
c)
Figure 6.3: a) CL-625 design truck clearance envelope. 1
b) CL-625 and CL-W design truck loads. 1
c) CL-625 and CL-W design lane loads.1
6.2.1.6.4. Acting ending Moments
The bending moments due to dead and live loads along a typical simply supported span of the
bridge are presented in Table 6.4.
STRUCTURAL DESIGN FOR DURABILITY
65
Point Distance/L
Distance (m)
Bending moments (kNm) Mg Ms Md Msd Mt
0 0 0.00 0.00 0.00 0.00 0.00 0.1 2.6 399.46 362.01 90.58 186.64 881.50 0.2 5.2 710.16 643.58 161.03 331.80 1638.20 0.3 7.8 932.08 844.70 211.35 435.48 2165.70 0.4 10.4 1065.24 965.37 241.54 497.70 2267.20 0.5 13 1109.62 1005.59 251.60 518.43 2368.70 0.6 15.6 1065.24 965.37 241.54 497.70 2208.40 0.7 18.2 932.08 844.70 211.35 435.48 1854.90 0.8 20.8 710.16 643.58 161.03 331.80 1487.00 0.9 23.4 399.46 362.01 90.58 186.64 743.50 1 26 0.00 0.00 0.00 0.00 0.00
Table 6.4: Bending moments on the girder produced by the different load cases.
where:
Mg = Bending moments caused by the self weight of the girder.
Ms = Bending moments caused by the self weight of the deck slab.
Md = Bending moments caused by the self weight of the diaphragms.
Msd = Bending moments caused by the superimposed dead loads.
Mt = Bending moments caused by the deign truck.
6.2.1.6.5. Analysis of the Load Effects
The conditions of the Clause 5.7.1.1 of the CHBDC are satisfied and the simplified live load
analysis can be applied. According to the Clauses 3.8.4.5.1 and 3.8.4.5.3, the dynamic load
allowance (DLA) must be equal to 0.25. Accordingly, the truck load incremented by the DLA
produces the following maximum bending moment:
MTruckDLA (kNm) = 3014.75 The lane load without DLA will cause the following maximum bending moment:
MLane (kNm) = 2684.45 According to the Clause 3.8.9, the maximum bending moment caused by the truck load on the
shoulders is:
PShoulder (kN/m²) = 4.00 MShoulder (kNm) = 676.00
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66
According to Clause 3.5.1, the load factors for the Ultimate Limit State (ULS) and the
Serviceability Limit State (SLS) are 1.7 and 0.9 respectively.
A. Design Bending Moment for ULS
According to Clauses 3.8.4.4 and 3.8.4.9, the average positive design bending moment is:
Mgavg (ULS) (kNm) = 2181.37 The ULS design bending moment Mg is obtained multiplying Mgavg by an amplification factor (Fm)
which accounts for transverse variation in the longitudinal moment intensity. The modification
factor is determined as follows:
Lane width modification factor, = -0.42
Width dimension for the loading distribution, F Internal part (m) = 10.35 External psrt (m) = 9.81 ULS, SLS
Percentage correction factor, Cf (%) = 9.04 Amplification factor Fm Internal part = 1.26
External part = 1.33
Design bending moment, Mg (ULS) Internal girders (kNm) = 3210.56
External girders (kNm) = 3389.34 B. Design Bending Moment for SLS
According to Clause 3.8.4.2, the average positive design bending moment is:
Mgavg (SLS) (kNm) = 1395.40
Design bending moment, Mg (ULS) Internal girders (kNm) = 2053.76
External girders (kNm) = 2168.13 C. Design Bending Moment for FLS
According to Clause 5.7.1.2.2.2, the bending moment for the fatigue limit state (FLS) is calculated
for one design truck on the span, and affected by the DLA. The average positive bending moment
results:
Mgavg (FLS) (kNm) = 430.68 The FLS design bending moment Mg is obtained by multiplying Mgavg by an amplification factor
(Fm) which accounts for transverse variation in the longitudinal moment intensity. It is necessary
to define the vehicle edge distance according to Figure 6.4.
STRUCTURAL DESIGN FOR DURABILITY
67
Figure 6.4: Definition of the vehicle edge distance, Dve. 1
The modification factor is determined as follows:
Lane width modification factor, = -0.42 Width dimension for the loading distribution, F Internal part (m) = 5.09
External psrt (m) = 3.91 Percentage correction factor, Cf (%) = 0
Vehicle edge distance, Dve (m) = 2.06 FLSCorrection factor for vehicle edge distance, Ce Internal (%) = 0
External (%) = 34.89 Amplification factor Fm Internal part = 2.88
External part = 2.78
Design bending moment, Mg (FLS) Internal girders (kNm) = 1240.64
External girders (kNm) = 1196.20 6.2.1.7. Prestressing Steel
The prestressing strands used for the reinforcement of the girder are low-relaxation, grade 1860
(Table 6.1). The distribution and stress losses of these strands are:
Number of straight strands = 42 Number of inclined strands = 14
Total number of strands = 56
The prestress loss at transfer, fst (MPa) = 130
The final loss, fs (MPa) = 440 According to Clause 8.7.1, the stress in the prestressing strands at transfer is:
fst (MPa) = 1246.40 The prestressing force per strand at transfer is (kN) = 123.02
The total prestressing force at transfer (kN) = 6889.10
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The prestressing stress at SLS, fse (MPa) = 936.40 The prestressing force per strand at SLS is (kN) = 92.42
The total prestressing stress at SLS (kN) = 5175.67 6.2.1.7.1. Prestressing Strands Profile
The prestressing strands profile is determined according to the following considerations:
The prestressing strands are inclined symmetrically at 0.4L (m) = 10.4 Concrete cover of the girders (m) = 0.045
Rebar diameter for girder stirrups (m) = 0.016 Concrete cover for the strands (m) = 0.061
Centre of gravity of the strands, Ycg: at midspan (m) = 0.153 at end (m) = 0.443
Strand eccentricity (e) from the girder centroid: at midspan (m) = 0.608 at end (m) = 0.318
The eccentricities and moments due to the initial and effective prestressing forces are presented
in Table 6.5.
Fraction of L
Distance (m)
e (m)
Initial prestressing
force, Pi (kN)
Bending moment due to Pi,
Mpsi (kNm)
Effective prestressing
force, Pf
(kN)
Bending moment
due to Pf, Mpsf (kNm)
0 0 0.318 6889.10 2189.60 5175.67 1645.01 0.1 2.60 0.390 6889.10 2689.06 5175.67 2020.25 0.2 5.20 0.463 6889.10 3188.52 5175.67 2395.48 0.3 7.80 0.535 6889.10 3687.98 5175.67 2770.72 0.4 10.40 0.608 6889.10 4187.44 5175.67 3145.96 0.5 13.00 0.608 6889.10 4187.44 5175.67 3145.96 0.6 15.60 0.608 6889.10 4187.44 5175.67 3145.96 0.7 18.20 0.535 6889.10 3687.98 5175.67 2770.72 0.8 20.80 0.463 6889.10 3188.52 5175.67 2395.48 0.9 23.40 0.390 6889.10 2689.06 5175.67 2020.25 1 26.00 0.318 6889.10 2189.60 5175.67 1645.01
Table 6.5: Induced internal actions caused by prestressing.
The angle between the centre of gravity of the strands at the end and at midspan of the girder
results to be 0.63º. The horizontal factor for the horizontal component of the prestressing is
0.9999. Accordingly, the horizontal component of the prestressing force is considered to be
constant.
6.2.1.7.2. Stress Limitations
According to Clauses 8.8.4.6 and 8.4.1.8, the maximum permissible concrete stresses in
compression and tension are:
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At transfer: Compressive strength (MPa) = 24 Cracking strength, fcri (MPa) = 2.53
At SLS: Compressive strength (MPa) = 20
Cracking strength, fcr (MPa) = 2.83 6.2.1.7.3. Stress Conditions of the Girder
A. Initial Compressive Stresses
The concrete compressive stresses in the girder at transfer (initial prestress) at the top and
bottom fibres are:
Fraction of L
Distance (m)
ftop (MPa) fbottom (MPa)
Condition
0 0 2.73 19.83 OK 0.1 2.60 2.32 20.20 OK 0.2 5.20 1.54 20.90 OK 0.3 7.80 0.40 21.94 OK 0.4 10.40 -1.10 23.30 OK 0.5 13.00 -0.91 23.13 OK 0.6 15.60 -1.10 23.30 OK 0.7 18.20 0.40 21.94 OK 0.8 20.80 1.54 20.90 OK 0.9 23.40 2.32 20.20 OK 1 26.00 2.73 19.83 OK
Table 6.6: Compressive stresses of concrete in the girder at transfer.
B. Compressive Stresses at SLS
The concrete compression stresses in the girder at SLS (final prestress) at the top and bottom
fibres are:
Fraction of L
Distance (m)
ftop (MPa) fbottom (MPa)
Condition
0 0 2.05 14.90 OK 0.1 2.60 3.63 13.46 OK 0.2 5.20 4.52 12.66 OK 0.3 7.80 4.71 12.48 OK 0.4 10.40 4.22 12.93 OK 0.5 13.00 4.56 12.62 OK 0.6 15.60 4.22 12.93 OK 0.7 18.20 4.71 12.48 OK 0.8 20.80 4.52 12.66 OK 0.9 23.40 3.63 13.46 OK 1 26.00 2.05 14.90 OK
Table 6.7: Compressive stresses of concrete in the girder at SLS.
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C. Stress Conditions of the Composite Section
Once the concrete of the deck slab has hardened, a composite section is formed. The stresses of
concrete in the composite section are shown in Table 6.8.
Section modulus of the composite section at top fibres, Stc (m³) = 0.629 Section modulus of the composite section at bottom fibres, Sbc (m³) = 0.374
The section modulus of the transformed composite section at the interface between the concrete
deck and the top fibre of the precast girder results:
Sic (m³)= 1.037
Section Distance
(m) Concrete Deck Concrete Girder
Condition ftc (MPa) fbottom (MPa) ftop (MPa) fbottom (MPa)
0 0 0.00 0.00 2.05 13.19 OK 0.1 2.60 1.60 0.97 8.25 7.01 OK 0.2 5.20 2.96 1.79 13.09 2.05 OK 0.3 7.80 3.90 2.37 16.04 -1.14 OK 0.4 10.40 4.14 2.51 16.11 -1.50 OK 0.5 13.00 4.33 2.62 16.98 -2.32 OK 0.6 15.60 4.05 2.46 15.81 -1.19 OK 0.7 18.20 3.43 2.08 14.46 0.52 OK 0.8 20.80 2.72 1.65 12.32 2.86 OK 0.9 23.40 1.39 0.84 7.55 7.75 OK 1 26.00 0.00 0.00 2.05 13.19 OK
Table 6.8: Stress levels inside composite section.
D. Prestress losses
It is necessary to evaluate different prestress losses that take place on the girder, like anchorage
slip and friction, elastic shortening of concrete, relaxation of tendons, creep of concrete,
shrinkage and other special conditions.
i. Elastic Shortening
Modulus of elasticity of the steel strands, Ep (MPa) = 200000 Modulus of elasticity of the concrete at transfer, Eci (MPa) = 27291.32
Concrete stress at the tendon centre of gravity at Mmax, fcir (MPa) = 18.12 Prestress loss due to elastic shortening, ES (MPa) = 132.82
ii. Relaxation of Tendons at Initial Prestressing
Age of concrete after casting, t (days) = 0.75 Stress in the prestressing strands at jacking, fsj (MPa) = 1450.8
Yield strength of prestressing steel, fpy (MPa) = 1674 Relaxation of tendons at initial prestressing, REL1 (MPa) = 12.82
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iii. Prestress Loss at Transfer (Short Term)
Prestress loss at transfer (elastic shortening-short term), fs1 (MPa) = 145.64 iv. Losses at Transfer
a) Prestress Loss due to Creep
Mean annual relative humidity, RH (%) = 77.4 Factor of prestress loss due to creep of pre-tensioned concrete, Kcr = 2
Modulus of elasticity of prestressing steel, Ep (MPa) = 200000 Modulus of elasticity of the girder concrete, Ec (MPa) = 29712.26
Concrete stress at the tendons centre of gravity at transfer, fcir (MPa) = 18.12 Stress at the girder top fibre (loads-dead) at transfer, ftg (MPa) = 1.64
Stress at the girder bottom fibre (loads-dead) at transfer, fbg (MPa) = 4.55 Neutral axis at transfer, y (m) = 1.18
Concrete stress at the tendons centre of gravity (loads-dead) at transfer, fcds (MPa) = 3.96 Prestress loss due to creep, CR (MPa) = 246.82
b) Prestress Loss due to Shrinkage
Prestress loss of pretensioned members due to shrinkage, SH (MPa) = 35.73 c) Relaxation of Tendons at Transfer
Relaxation of tendons at transfer, REL2 (MPa) = 16.27 d) Prestress Loss After Transfer
Prestress loss after transfer (long term), fs2 (MPa) = 301.76
fs = fs1 + fs2 (MPa) = 447.40 Diference with respect to the initial assumed prestress loss (%) = 1.65
The prestress loss is about the same as the earlier assumed value at the beginning of Section
6.2.1.7.
e) Differential Shrinkage Between Cast-in-place Deck and Girder
Differential shrinkage strain, sh = 1.00E-04 Modulus of elasticity of the concrete deck slab, Esc (MPa) = 28729.32
Cross-sectional area of the concrete deck slab (m²) = 0.47 Force caused by the differential shrinkage (SLS), F (kN) = 338.56
Additional stress at the top of the girder (composite section) (MPa) = 1.42 Additional stress at the bottom of the girder (composite section) (MPa) = -0.19
Total stress of the girder at top fibres (composite section) (MPa) = 17.55 Stress limitation at top fibres = OK
Total stress of the girder at bottom fibres (composite section) (MPa) = -2.40 Stress limitation at bottom fibres = OK
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6.2.1.8. Ultimate Limit States
Concrete resistance factor, c = 0.75
Prestressing steel strands resistance factor, p = 0.95 Resultant compressive and tensile force in the girder (kN) = 9766.56
Depth of the compressive block, a (m) = 0.146
The neutral axis is located within the concrete slab
Distance from the extreme compressive fibre to the C.G. of strands, dp (m) = 1.72
Depth ratio of rectangular compressive block to depth to the Cu, 1 = 0.845 Neutral axis location at ULS, Cu (m) = 0.173
Cu/dp = 0.101
The reinforced section is under-reinforced
The condition of the girder is satisfactory
The stress in the prestressing tendons at ULS, fps (MPa) = 1804 Factored flexural resistance of the section, Mr (kNm) = 15617.61
Maximum bending moment at ULS, Mu (kNm) = 7289.25 Mu < Mr OK
Cracking moment, Mcr (kNm) = 3407.54 Mr > 1.2Mcr OK
6.2.1.8.2. Anchorage Zone Reinforcement
Reinforcing bars resistance factor, s = 0.9 Area of stirrups for achorage zone, (mm²) = 23.15
Anchorage zone length, (m) = 0.4 Bar type = 10M
Selection of rebar size = OK Number of stirrups = 4 Stirrup spacing (m) = 0.13
6.2.1.8.3. Longitudinal Reinforcement at the Exterior Support
Yield strength of prestressing tendons, fpy (MPa) = 1674.00 Prestressing stress at SLS, fpe (MPa) = 1004.40
Compression factor, kp = 0.28 Distance from the extreme compression fibre to the neutral axis, c (m) = 0.70
Effective shear depth at ends, dv (m) = 1.35 Distance from the extreme compression fibre to the centroid of tendons, dp (m) = 1.47
c/dp = 0.48 Stress in prestressing tendons at factored resistance, fpr (MPa) = 1612.29
Development length of pretensioned strands, ld (m) = 1.73 Prestressing force at SLS, Pf (kN) = 5175.67
Angle between the C.G. of strands at end and at midspan (°) = 0.63
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Horizontal component of Pf (kN) = 5175.36 Vertical component of Pf, Vp (kN) = 56.52
Bearing distance (m) = 0.5 Available resistance at the face of the bearing for all strands, Pfa (kN) = 1065.85
Design bending moment at distance dv from the bearing, Mf (kNm) = 1216.56
Fraction of L
Distance (m)
Shearing forces (kN) Vg Vs Vd Vsd Vt
0 0 170.71 154.71 38.71 68.36 339.04 0.1 2.6 136.57 123.77 30.97 54.69 339.04 0.2 5.2 102.43 92.82 23.22 41.02 289.04 0.3 7.8 68.28 61.88 15.48 27.35 39.04 0.4 10.4 34.14 30.94 7.74 13.67 39.04 0.5 13 0.00 0.00 0.00 0.00 39.04 0.6 15.6 -34.14 -30.94 -7.74 -13.67 -135.96 0.7 18.2 -68.28 -61.88 -15.48 -27.35 -135.96 0.8 20.8 -102.43 -92.82 -23.22 -41.02 -285.96 0.9 23.4 -136.57 -123.77 -30.97 -54.69 -285.96 1 26 -170.71 -154.71 -38.71 -68.36 -285.96
Table 6.9: Shearing forces cause by the different load cases.
Design shearing force at distance dv from the bearing, Vf (kN) = 1033.80
Longitudinal strain, x = 0.0000
Shear resistance factor of cracked concrete, = 0.221
Angle of the principal diagonal compressive stress to the horizontal axis, (º) = 41 Shear resistance of concrete, Vc (kN) = 285.16
Shear resistance provided by the sturrups, Vs (kN) = 694.95 The force at the face of the bearing, Flt (kN) = 1621.09
The resulting axial force (kN) = 555.23 Required area of longitudinal reinforcement (mm²) = 1388.08
Bar size for longitudinal reinforcement = 15M Number of bars required= 6.94
Adjusted number of logitudinal bars = 8 6.2.1.8.4. Shear Resistance
The shear reinforcement calculations for the girder are summarized in Table 6.10.
6.2.1.8.5. Interface Shear
A cohesion resistance for the interface shear is assumed as c = 0.5MPa, considering that fresh
concrete of the deck slab is placed against hardened concrete of the girders, with the surface
clean and free of laitance, and intentionally roughened to a full amplitude of about 5mm and a
spacing of about 15mm.
Parameter dependent on the density of concrete, 1 = 1 Type of concrete = Normal-density concrete
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Friction coefficient, = 1 Unfactored permanent load normal to the interface area, N (kN) = 0.00
Area of shear-friction reinforcement, Avf (m²) = 0.028 Area of concrete resisting shear transfer, Acv (m²) = 31.2
The ratio Avf / Acv, = 0.0008974
Compressive stress across a shear-friction plane, (Mpa) = 0.36 Shear resistance at the plane, v (MPa) = 0.64
Average factored horizontal shear stress from end to midspan, vfv (MPa) = 0.31 The provision of vertical stirrups that go into the slab is satisfactory
Section of
L Distance
(m) Ycg (m) d (m) dv (m) Mf (kNm) Vf (kN)
0 0.0 0.443 1.56 1.35 0.00 1078.28 dvg 1.2 0.411 1.53 1.35 1216.56 1033.80 0.1 2.6 0.371 1.48 1.34 2673.04 977.90 0.2 5.2 0.298 1.41 1.27 4872.92 792.52 0.3 7.8 0.226 1.34 1.21 6422.16 267.13 0.4 10.4 0.153 1.27 1.14 6986.21 166.75 0.5 13.0 0.153 1.27 1.14 7289.25 66.36 0.6 15.6 0.153 1.27 1.14 6886.25 -331.52 0.7 18.2 0.226 1.34 1.21 5893.80 -431.90 0.8 20.8 0.298 1.41 1.27 4615.88 -787.29 0.9 23.4 0.371 1.48 1.34 2438.44 -887.67
L-dvg 24.8 0.411 1.53 1.35 1112.62 -943.58 1 26.0 0.443 1.56 1.35 0.00 -988.05
Vp (kN) Vc (kN) Vs (kN) Rebar size s (m) smax (m) sadopted (m) 56.52 285.16 739.42 15M 0.30 0.3 0.3 56.52 285.16 694.95 15M 0.32 0.3 0.3 56.52 282.24 641.96 15M 0.34 0.3 0.3 56.52 268.46 470.36 15M 0.44 0.6 0.4 56.52 254.68 0.00 15M - 0.6 0.6 56.52 240.90 0.00 15M - 0.6 0.6 0.00 240.90 0.00 15M - 0.6 0.6
56.52 240.90 36.92 15M 5.08 0.6 0.6 56.52 254.68 123.53 15M 1.61 0.6 0.6 56.52 268.46 465.13 15M 0.45 0.6 0.3 56.52 282.24 551.73 15M 0.40 0.6 0.4 56.52 285.16 604.72 15M 0.37 0.6 0.3 56.52 285.16 649.20 15M 0.34 0.3 0.3
Table 6.10: Shear reinforcement for the girder.
6.2.1.9. Serviceability Limit States
6.2.1.9.1. Deflections
Prestressing force at transfer, Pi (kN) = 6889.10 Prestressing force at SLS, Pf (kN) = 5175.67
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Modulus of elasticity at transfer, Eci (MPa) = 27291.32 Second moment of area of the girder about its neutral axis, Ig (m^4) = 0.2048
Moment due to prestressing force at transfer, Mo (kNm)
at ends = 2189.60 at 0.4L = 4187.44
The deflections of the girder are calculated using the conjugate-beam deflection method. The
deflections of the girder at the different construction stages are determined as follows:
Vertical deflection due to the initial prestress, p (m) = 0.057 ↑
Downward deflection due to selfweight of the girder, g (m) = -0.014 ↓
Net camber at transfer, net (m) = 0.043 ↑
Deflection due to fresh concrete slab, s (m) = -0.013 ↓
Deflection due to diaphragms load, diaph (m) = -0.002 ↓
Deflection due to superimposed dead load, sdl (m) = -0.002 ↓ The deflections of the girder during the construction stages and during service are summarized in
Table 6.11.
Time Stage Deflection
factor Deflection,
(m) Direction
18 hours Transfer 1 0.043 ↑
2 months Deck
casting 1.7 0.058 ↑
3 months Hardened
deck 1.7 0.047 ↑
1 year Deck
surfacing 1.8 0.047 ↑
5 years Service 2 0.050 ↑
Table 6.11: Final deflection caused by permanent loads.
6.2.1.9.2. Superstructure Vibrations
According to the Clause 3.4.4, the verification of the structure vibrations for the Serviceability
Limit State is made. Accordingly, the acting loads on the deck are amplified by the load factors for
SLS, the DLA, and the amplification factors. The results are presented as follows:
Distribution factor, Fu = 0.439
Deflection, (m) = 0.006 Weight of the bridge per unit width, Wd (kN/m per metre width) = 16.52
Fundamental flexural frequency, FFF (Hz) = 6.47 Static deflection (m) = 0.012
The dynamic response of the bridge is satisfactory
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6.2.1.9.3. Fatigue
In non-prestressed and fully prestressed members, the change in the stresses of the rebars and
strands due to repetitive live loads is usually not critical. But in partially prestressed members,
repetitive loads can cause fatigue damage to these parts of the structure. The allowable stress
range in straight rebars and straight strands is 125MPa (Clause 8.5.3.1 and 8.5.3.2). The
calculations for the variation in tensile stresses in the strands are presented as follows:
Variation in the tensile stress in the prestressing steel, fs (MPa) = 157.03 The stress range in straight steel bars and strands is (MPa) = 125
The stress range for steel reinforcement is exceeded Additional non-prestressing steel is required
Number of bars added at between the two lower layers of strands = 9 Size of rebar = 30M
Resultant compressive and tensile force in the girder (kN) = 2520 Depth of the compressive block, a (m) = 0.038
Effective depth, d (m) = 1.78 Incremented flexural resiatance at ULS, Mr (kNm) = 19617.86
New variation in the tensile stress in the strands, fs (MPa) = 114.08 The response of the girder for FLS is satisfactory
The final step concludes the design of the precast and prestressed concrete girder. It also
represents the performance of the girder immediately after construction. A general overview of
the girder’s reinforcement is presented in the Figures 6.5 and 6.6.
6.2.1.10. Girder Design for the Required Service Life
Considering the durability parameters described earlier in this chapter, it is possible to identify
that the controlling mechanism of deterioration for the composite section of the girders along the
service life is the surface deterioration (Section 5.1.3.3). This mechanism acts with a rate of
degradation of 0.41mm per year for the upper face of the deck slab, and 0.17mm per year for the
perimeter of the girder. Based on several in-situ inspections of actual bridges under active
deterioration, the degradation of the surface of concrete on the girders is focused around the
lower flange, which is the zone where most of the aggressive agents are concentrated (Figure
6.7). The carbonation front acts simultaneously with the surface deterioration, with a rate that
varies proportionally to the square root of time, affected by a carbonation coefficient which is 0.92
and 0.90 for the bridge deck slab and the exterior girders, respectively (Table 5.13). The
integration of these rates of deterioration cannot be made directly in the formulation of the
different parameters involved in the structural design of a prestressed concrete bridge girder.
However, the input information for these parameters can be affected by the rates of deterioration
at different time intervals, establishing an iterative process that simulates the progressive
deterioration of the bridge girder and its corresponding loss of bearing capacity and serviceability.
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Figure 6.5: Non-prestressed reinforcement of the bridge girder.
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Figure 6.6: Prestressed reinforcement of the bridge girder.
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Figure 6.7: Localized deterioration on the lower flange of a bridge girder.
6.2.1.11. Girder Performance with Time
The surface deterioration and the progress of the carbonation front were calculated for time
intervals of five years, starting from the time t = 0 years when the bridge is put to service, until t =
150 years, which represents the end of the design service life. At every time interval, the
geometrical properties of the composite slab-girder section were evaluated, along with the effects
on the reinforcement. Once these basic parameters were established, a structural analysis and
design iteration was performed, including verifications of all entities calculated at time interval, t =
0 years presented previously. In Figure 6.8, the progress of deterioration of the composite slab-
girder section is represented as a grey-coloured zone that moves into the girder and the slab with
time according to the rate of deterioration acting on the bridge deck.
Figure 6.8: Representation of the degradation of the composite slab-girder section.
It is important to note that after 150 years, the progress of deterioration, including the carbonation
front, did not reach the level of the steel reinforcement, or the prestressing steel. This
corresponds to the appropriate design of the quality and thickness of the concrete covers for the
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girders and the deck slab. However, some supplementary protection measures must be used for
these elements to diminish the deterioration in the element. These protective measures are
described in the Section 6.2.1.1.2.
From each one of the structural analysis and design iterations, the basic results with respect of
the limit states were identified and plotted, to prepare a graphic representation of the loss of
bearing capacity and serviceability of the bridge girders during the design service life. The
evaluated limit states for the bridge deck included the ultimate limit state (ULS), the serviceability
limit state (SLS) and the fatigue limit state (FLS).
Time (years)
Height (mm)
Cross-sectional
area (mm² x 10³)
Centroid to
bottom, Yb
(mm)
Second moment of
inertia (mm4 x 109)
Section modulus for the top fibre (mm³ x 106)
Section Modulus for the
bottom fibre (mm³ x 106)
Weight (kN/m)
Web thickness
bv (m)
Width of the upper
flange (mm)
0 1600 589.0 760.9 204.8 244.071 269.155 13.13 0.18 1200 5 1599 587.4 762.0 204.0 243.728 267.717 13.10 0.18 1200
10 1598 585.3 763.1 203.2 243.382 266.282 13.05 0.18 1200 15 1597 583.4 764.2 202.4 243.036 264.852 13.01 0.18 1200 20 1597 583.4 764.2 202.4 243.036 264.852 13.01 0.18 1200 25 1596 581.4 765.4 201.6 242.716 263.392 12.96 0.18 1200 30 1595 579.4 766.5 200.8 242.366 261.970 12.92 0.18 1200 35 1594 577.5 767.6 199.9 241.893 260.422 12.88 0.18 1200 40 1593 575.5 768.7 199.1 241.538 259.009 12.83 0.18 1200 45 1592 573.6 769.9 198.3 241.212 257.566 12.79 0.18 1200 50 1591 571.7 771.0 197.5 240.854 256.161 12.75 0.18 1200 55 1590 569.8 772.2 196.7 240.523 254.727 12.70 0.18 1200 60 1590 569.8 772.2 196.7 240.523 254.727 12.70 0.18 1200 65 1589 567.9 773.3 195.8 240.039 253.201 12.66 0.18 1200 70 1588 566.0 774.5 195.0 239.705 251.775 12.62 0.18 1200 75 1587 564.2 775.6 194.2 239.339 250.387 12.58 0.18 1200 80 1586 562.3 776.8 193.4 239.001 248.970 12.54 0.18 1200 85 1585 560.4 778.0 192.5 238.538 247.429 12.49 0.18 1200 90 1584 558.6 779.2 191.7 238.196 246.022 12.45 0.18 1200 95 1583 556.8 780.3 190.9 237.822 244.649 12.41 0.18 1200
100 1583 556.8 780.3 190.9 237.822 244.649 12.41 0.18 1200 105 1582 554.9 781.5 190.0 237.352 243.122 12.37 0.18 1200 110 1581 553.1 782.7 189.2 237.004 241.727 12.33 0.18 1200 115 1580 551.3 783.9 188.4 236.654 240.337 12.29 0.18 1200 120 1579 549.5 785.1 187.5 236.176 238.823 12.25 0.18 1200 125 1578 547.7 786.3 186.7 235.822 237.441 12.21 0.18 1200 130 1577 545.9 787.5 185.8 235.339 235.937 12.17 0.18 1200 135 1577 545.9 787.5 185.8 235.339 235.937 12.17 0.18 1200 140 1576 544.2 788.7 185.0 234.980 234.563 12.13 0.18 1200 145 1575 542.4 789.9 184.2 234.620 233.194 12.09 0.18 1200 150 1574 540.6 791.2 183.3 234.159 231.673 12.05 0.18 1200
Table 6.12: Geometrical properties of the NEBT1600 girder of different steps of deterioration.
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The internal stress levels in the girder at the ULS for different steps of degradation of the bridge
deck are shown in Figures 6.9 and 6.10.
Figure 6.9: Variation of internal compressive stresses at the top of the composite section vs. time.
Figure 6.10: Variation of internal tensile stresses at the bottom of the composite section vs. time.
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With time, compressive and tensile stresses at the girder extremes increase as a result of the
loss of cross-section area of the composite slab-girder section. The tensile stresses increased
more than the compressive stresses during the service life. However, none of these internal
stresses attained the maximum allowable levels.
The gradual loss of flexural and shearing resistances (ULS) is shown in the Figures 6.11 and 6.12.
Figure 6.11: Loss of flexural resistance of the composite section vs. time.
Figure 6.12: Loss of shearing resistance of the composite section vs. time.
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The losses of flexural and shearing resistances vs. time are described by parabolic curves. The
loss of shearing resistance was more pronounced than the loss of flexural capacity. However,
none of these ultimate limit states were attained by the composite section.
The increment of the static deflections with time that are accounted for the analysis of vibrations
produced by the action of the truck loads (SLS) is presented in Figure 6.13. Again, the
serviceability limit state of deflection is fulfilled throughout the service life of the bridge.
Figure 6.13: Increment of the static deflections of the composite section caused by the live load.
To complete the analysis and design for durability of the prestressed girders, the performance of
the composite girder in terms of the FLS vs. time is presented in the Figure 6.14. The increase of
static vibration is again represented by a second-degree curve, similar to the increments of the
internal stresses in the composite section. As for the other limit states, the serviceability limit state
for vibration is fulfilled throughout the service life of the bridge. The stress variation started at
103MPa at the beginning of the service life, and reached a value of 109MPa at the end of the 150
years of the design service life, and was within the maximum allowable strand stress range of
125MPa.
As a general conclusion, the selection of the type of girder, as well as the careful design and
selection of the construction materials for the deck elements produce a satisfactory performance
of the composite slab-girder section during the design service life within the different limit states
considered in the design of the bridge superstructure for durability.
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Figure 6.14: Increment of the stress variation on the prestressing strands of the girder.
6.2.1.12. Supplementary Protective Measures
Some additional measures can be implemented to enhance the performance of the girders and
to reduce the effects of the degradation mechanisms on the construction materials. One of
these measures is the waterproofing of the girder surfaces, which may be exposed to various
microclimates. This waterproofing can be established by means of sealing treatments, coatings
and/or membranes on the surfaces.
Further to the previous measures and the fact that the concrete of the girders has been
carefully designed to be as impermeable as possible, it is important to improve the protection of
the prestressing strands against corrosion. For this reason, regular steel for the passive
reinforcement of the girders is implemented, with the purpose of reducing the electrochemical
potential for corrosion of the prestressing strands.
The installation of long cornices at the edges of the bridge deck can provide additional
protection to the edge girder from the direct exposure to moisture, de‐icing salts, wetting and
drying and freezing and thawing cycles. These cornices can be designed on precast concrete, or
as a synthetic material resistant enough to withstand the effects of the microclimates in these
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zones. Additionally, these cornices must be designed such that their installation, maintenance
and replacement can be executed easily.
6.2.2. Reinforced Concrete Design for Durability
Reinforced concrete elements, such as the bridge deck slabs, the concrete barriers, seismic
shear keys, and the foundation units, allow a more direct integration of the durability parameters
into the structural design equations. The definition of a model that integrates all of these
parameters is presented in the following sections.
6.2.2.1. General Design Parameters
A reinforced concrete section is assumed to be affected by a progressive deterioration of its
construction materials. The concrete will deteriorate depending on the controlling mechanism of
deterioration that affects the element. At the same time, the carbonation and chloride ingress
fronts move into the reinforced concrete section and initiate the active corrosion process at a
moment defined by the corrosion initiation time (t0) described in the previous sections.
Figure 6.15: Reinforced concrete section parameters for the structural and durability design.
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The basic parameters that define the resistance of the reinforced concrete section are identified
in the Figure 6.15, where it is clear how these parameters are affected by the rates of
deterioration of the construction materials.
Figure 6.15 (a) presents the reinforced concrete section to perform an analysis per unit width of
the element (deck slab, barrier, beam, etc.), where the concrete covers on both sides are
subjected to different mechanisms of deterioration. This will be the case for a bridge deck slab
which is subjected to two microclimates with specific rates of deterioration taking place differently
at the top and bottom of the element. Figure 6.15 (b) represents the situation where the concrete
cover is completely deteriorated and then the reinforcing steel begins to corrode. The active
corrosion process starts once the initiation period for corrosion is over, and then it continues
according to the rates of corrosion and the other durability parameters that have been defined in
the Sections 5.1.3 and 5.1.4. Figure 6.15 (c) shows that corrosion first starts at the most exposed
point of the reinforcing bar and the corrosion will continue around the perimeter of the bar
according to the rate of corrosion 17. Once the bar corrosion has occurred over the entire
perimeter, corrosion will continue towards the inside of the rebar, reducing the cross section of
the bar in proportion to the rate of corrosion (Figure 6.15 (d)).
The parameters involved in the resistance of this reinforced concrete section are:
b = width of the reinforced concrete section.
h0 = initial height of the section.
d0 = initial effective height of the section.
c01 = initial concrete cover at the internal face of the section.
c02 = initial concrete cover at the external face of the section.
rc1 = depth of deterioration of concrete that takes place at the internal face of the section.
rc2 = depth of deterioration of concrete that takes place at the external face of the section.
ht = height of the section being affected by the rates of deterioration rc1 and rc2.
dt1 = effective height of the section being affected by the rate of deterioration rc1.
dt2 = effective height of the section being affected by the rate of deterioration rc2.
ct1 = concrete cover of the section affected by the rate of deterioration rc1.
ct2 = concrete cover of the section affected by the rate of deterioration rc2.
STRUCTURAL DESIGN FOR DURABILITY
87
db1 = initial diameter of the reinforcing bars near the internal face of the section.
db2 = initial diameter of the reinforcing bars near the external face of the section.
dbt1 = diameter of the reinforcing bars affected by the rate of corrosion rs1.
dbt2 = diameter of the reinforcing bars affected by the rate of corrosion rs2.
rs1 = depth of corrosion of the reinforcing steel at the internal layer of reinforcement.
rs2 = depth of corrosion of the reinforcing steel at the external layer of reinforcement.
The suffixes 1 and 2 make reference to the side of the element being analyzed, such as the
internal or external faces of the reinforced concrete element, or the internal or external layers of
reinforcing steel. The general equations of the reinforced concrete section resistance do not
include these suffixes; however, they will be adjusted for every specific case of analysis and
design in terms of the durability and structural parameters corresponding to each one of the sides
of the element.
6.2.2.2. Flexural Design for Durability
The flexural resistance of a reinforced concrete section can be determined using the conventional
analysis for ultimate bending resistance based on strain compatibility and equilibrium using
material resistance factors and material properties. According to this the flexural resistance can
be expressed in terms of the following equation:
2'
121 t
cc
ystystn bd
f
ffM
(6-1)
where:
t = ratio of non-prestressed tension reinforcement = tst bdA .
Ast = area of reinforcing steel =
2
2
btd
.
fy = yield strength of non-prestressed reinforcing steel.
f’c = compressive strength of concrete.
s = resistance factor for non-prestressed reinforcing steel = 0.9. 1
STRUCTURAL DESIGN FOR DURABILITY
88
c = resistance factor for concrete = 0.75. 1
1 = ratio of average stress in the rectangular compression block.
1 = 0.85 – 0.0015 f’c ≥ 0.67. 1, 23
According to the Clause 8.8.4.3, the flexural reinforcement must be designed such that the
resulting factored flexural resistance, Mn of the section is at least 1.20 times the cracking
moment, Mcr. However, this minimum reinforcement could be lowered if the factored flexural
resistance provided is at least one-third greater than the resistance required according to the
factored loads on the member. In accordance to Clause 8.8.4.4, the cracking moment is the one
that induces tensile cracking stresses, fcr in the concrete section. The cracking stress is
calculated according to the equation: 1
'4.0 ccr ff (6-2)
The cracking moment is the given by:
ct
gcrcr y
IfM ;
ct
gtcrcrt y
IfM (6-3)
where:
fcr = cracking strength of concrete , according to Clause 8.4.1.8.
Igt = second moment of area of the gross concrete section about its centroidal axis, neglecting the
reinforcing steel bars. The geometrical property of the concrete section changes with time
because of the reduction of the cross-section area of the slab as a result of the degradation
induced by the mechanisms of deterioration.
yct = the distance from the centroidal axis of the gross section to the extreme concrete fibre in
tension, neglecting the reinforcing steel bars. For the present case of the bridge deck slab,
222210 cct
ct
rrhhhy
(6-4)
6.2.2.3. Check for Shearing Resistance
Despite the fact that the CHBDC provides an empirical method for the design of the
reinforcement of the deck slab, a detailed analysis of the shearing resistance is performed. A
STRUCTURAL DESIGN FOR DURABILITY
89
sectional design model is assumed to determine the shearing resistance of the reinforced
concrete section. A beam action with a critical section extending in a plane across the design
width and located at a distance, d, from the face of the reaction area, or from any change in slab
thickness is analyzed. Since no shear reinforcement is planned to be distributed in the slab, the
shearing resistance provided by the concrete itself must be adequate to withstand the external
factored shearing actions. According to Clause 8.9.3.4, the shearing resistance of concrete is
determined according to the following equation:
vcrcc bdfV 5.2 (6-5)
where:
dv = effective shear depth. According to Clause 8.9.1.5, the effective shear depth must be
considered as the larger of the values of 0.72h and 0.9d. Using the durability parameters defined
earlier, the effective shear depth is given by:
21072.072.0 cct rrhh (6-6)
101 9.09.0 ct rdd ; 202 9.09.0 ct rdd (6-7)
= a factor accounting for the shear resistance of cracked concrete. According to Clause 8.9.3.6,
for reinforced concrete sections not containing transverse reinforcement but having a specified
nominal maximum size of coarse aggregate not less than 20 mm, β shall be equal to:
vd1000
230 (6-8)
6.2.2.4. Deflection Analysis
The deflections of the deck slab caused by the external actions will increase with time due to the
ongoing degradation of the construction materials, according to the specific microclimates that
are developed in this part of the bridge. For this reason the derivation of the deflection formulation
will include the rates of deterioration of steel and concrete, and will result in an equation, which is
a function of time.
For a rectangular concrete section, the second moment of inertia can be calculated according to
the following equation:
12
3tt
gt
hbI (6-9)
STRUCTURAL DESIGN FOR DURABILITY
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The parameters for the second moment of area of the cracked transformed section, Icrt are
defined in Figure 6.16.
Figure 6.16: Cracked transformed concrete section for flexural analysis.
The transformed section is based on an additional equivalent area of concrete that is equivalent
to the reinforcing steel area in tension induced by flexure. The equivalent additional concrete
section is obtained by multiplying the reinforcing steel area by the modular ratio, n, which is
expressed by the following equation:
c
s
E
En (6-10)
where:
Es = the modulus of elasticity of the reinforcing steel = 200000MPa.
Ec = the modulus of elasticity of concrete. This parameter can be calculated in terms of the
compressive strength, f’c and the unit weight of concrete according to Clause 8.4.1.7 as:
5.1
'
230069003000
c
cc fE
(6-11)
Figure 6.17: Representation of the compatibility of stresses and deformations.
STRUCTURAL DESIGN FOR DURABILITY
91
In accordance with the compatibility of deformations of the concrete section in the elastic range
(Figure 6.17), it is possible to formulate the required parameters for the analysis of deflections of
a reinforced concrete element under flexure.
In Figure 6.17,
cctct E ; sstst E
TC ; ststttct Abc 2
1
stttstc AcdEbcE 2
2
1 ; stttstc AcdEbcE 2
2
1
stttc
st Acd
E
Ebc 2
2
1 ; stttt Acdnbc 2
2
1 ; 02
1 2 tsttstt dnAcnAbc
b
dnAbnAnAc
tsttstst
t
2
142
(6-12)
Multiplying equation (6-12) by dt / dt at both sides of the equation, gives:
22
22 2
t
tststt
t
tstt
db
dbnAAnd
bd
dnAc
(6-13)
Since t
stt bd
A , we obtain:
ttttt nnndc 222 (6-14)
The second moment of area of the cracked transformed section is defined the by:
223
212 ttstt
tt
crt cdNAc
bcbc
I
; 2
3
3 ttstt
crt cdNAbc
I (6-15)
The deflections and rotations of the reinforced concrete section can be calculated using the
effective second moment of area, Iet, according to Clause 8.13.3.3.
STRUCTURAL DESIGN FOR DURABILITY
92
gtA
crtcrtgtcrtet I
M
MIIII
3
(6-16)
Here, MA corresponds to the maximum bending moment in the concrete section at a load stage
for which the deflection is calculated. At this moment, it is possible to calculate the deflections by
means of the conventional structural analysis for deflection, using the effective second moment of
area, Iet, of the concrete section in the relevant deflection equations.
6.2.3. Deck Slab Design
The bridge deck slab has to be analyzed for positive and negative bending moments resulting
from loads applied on the different panels of the slabs. Moreover, the structural analysis has to
consider the bending moments induced in the longitudinal direction, resulting from the longitudinal
overhangs at the expansion joints of the abutments, and the condition of semi-continuity of the
bridge deck established at the intermediate support over the pier. The cantilevered parts of the
deck slab are analyzed for transverse negative bending moments that are generated as a result
of selfweight of the reinforced concrete elements and truck loads applied to the cantilever
portions, as well as the horizontal loads applied to the barriers.
6.2.4. Transverse Bending Moments in the Bridge Deck
The design of the internal segments of the bridge deck slab is carried out considering some basic
steps that are listed and explained as follows.
6.2.4.1. Load Analysis
To determine the bending moments in the slab, it is necessary to identify the load cases that may
generate these actions. These load cases can be permanent loads, such as the self weight,
transitory loads, such as the truck loads, the strains and deformation effects, wind loads,
differential settlement; or exceptional loads, such as earthquake loads, stream forces, ice
accretion, and collision forces. However, the most relevant load cases for the analysis and design
of these elements are the dead loads and the truck loads.
According to Clause 5.7.1.7.1, concrete deck slabs that are supported on longitudinal girders may
be analyzed for transverse bending moments using the simplified elastic method, where the
maximum unfactored transverse negative and positive bending moments caused by the design
truck CL-625 in the portion of the slab between the outer girders may be determined by the
equation:
STRUCTURAL DESIGN FOR DURABILITY
93
DLAPS
M eCL
110
6.08.0625 (6-17)
where:
M = the negative and positive transverse bending moment [kNm/m].
Se = the effective transverse span, considered to be the smaller of the centre-to-centre spacing of
the girder webs and the clear span between girder webs plus the deck thickness [m].The centre-
to-centre spacing of the girder webs = 2.095m, and the clear span between girder webs plus the
deck thickness = 1.915m + 0.225m = 2.175m. Therefore, Se = 2.095m.
P = the maximum wheel load of the CL-625 truck, which is 87.5kN.
DLA = dynamic load allowance, which is 0.4 because only one axle of the CL-625 truck is used
for the analysis.
Having defined the required parameters, the transverse bending moment caused by the truck
load is:
kNmkNm
M CL 41.264.0110
5.876.0095.28.0625
The dead load effects can be determined in terms of the self weight of the slab. The permanent
loads corresponding to the bridge deck slab are:
- Selfweight of the reinforced concrete slab: (0.225m)(24kN/m3)(1m) = 5.4kN/m.
- Waterproofing membrane and asphalt concrete: (0.065m)(23.5kN/m3)(1m) = 1.53kN/m.
The distributed load on a one-metre-wide slab strip results: w = 5.4kN/m+1.53kN/m = 6.93kN/m.
For the internal segments of a multi-span bending element, the resulting bending moments at the
supports and at the midspan can be calculated as:
- A the supports:
kNmmmkNwS
M eD 53.2
12
095.293.6
12
22
- At midspan:
kNmmmkNwS
M eD 27.1
24
095.293.6
24
22
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94
According to Clause 3.5, the design bending moment for this element must be calculated
considering some load factors, which are 1.2 and 1.7 for the dead load and the live load
respectively, giving as a result the following design bending moment:
- At the supports: kNmkNmkNmM u 93.4741.267.153.22.1
At midspan: kNmkNmkNmM u 42.4641.267.127.12.1
6.2.4.2. Durability Parameters
The durability design parameters were determined in the Section 5.1.4. A summary of these
parameters for the bridge deck slab are listed in Table 6.13.
Element Rates of
deteriorarion (mm/year)
Cocrete Top 0.411
Bottom 0.131 Reinforcing
steel Top 0.031
Bottom 0.019 Carbonation coefficient Location Kc
Top 0.92 Bottom 1.17
Initiation time for corrosion
Location (years)
Top 18 Bottom 25
Table 6.13: Durability design parameters of the bridge deck slab.
6.2.4.3. Initial Conditions of the Bridge Deck Slab
The general conditions of bridge deck need to be assumed initially and then improved after
several iterations during the design for durability process in the same way that was used for the
prestressed concrete girders. The conditions of the bridge deck slab immediately after
construction are summarized in Table 6.14.
6.2.4.4. Assumption for Steel Reinforcement and Performance with Time
Starting with the minimum reinforcement provisions for flexure, a preliminary reinforcement is
assumed for the slab. Then, after having performed several iterations with the structural and
durability models for this element, it is possible to adjust and improve the rebar distribution for its
adequacy.
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Table 6.14: Initial conditions of the bridge deck slab. The reinforcement of the slab is established as 20M @ 0.1m in the top layer, and 20M @ 0.15 in
the bottom layer, for the transverse (principal) reinforcement.
Figure 6.18: Bridge deck slab resistance for positive bending moments vs. time.
Figure 6.19: Bridge deck slab resistance for negative bending moments vs. time.
STRUCTURAL DESIGN FOR DURABILITY
96
As for the longitudinal reinforcement, it is assumed to be two thirds of the transverse
reinforcement, in accordance to Clause 5.7.1.7.1, and Clause 8.2.2.2 of the MTQ Manual 22. This
means that the longitudinal reinforcement is 20M @ 0.15m at the top and bottom layers.
Considering the durability parameters, initial slab properties, and reinforcement distribution, the
performance of the bridge deck slab with time is shown in Figures 6.18 and 6.19.
It is evident from Figures 6.18 and 6.19 that with the assumed reinforcement distribution, the
adequate selection of materials, and the careful design of the concrete mixture, the slab presents
an adequate performance throughout the service life of the bridge.
6.2.4.5. Final Design and Details
The summary of the deck slab reinforcement for the internal sections is shown in Figure 6.20.
Figure 6.20: Detail of the deck slab reinforcement.
6.2.5. Transverse Bending Moments in the Cantilever Overhang
The design of the cantilever overhang of the bridge deck slab is carried out similarly considering
some basic steps as follows.
6.2.5.1. Load Analysis
According to Clause 5.7.1.6.1.1, it is possible to calculate the design moment intensity due to the
CL-625 truck for a cantilever slab of constant or linearly varying thickness, using Table 6.15,
which is accompanied by Figure 6.21, and explains the basic parameters that determine the
bending moments generated in the deck slab overhang.
According to the bridge deck characteristics defined in Chapter 2, the transverse distance from
the longitudinal external edge of the bridge deck to the supported edge of cantilevered slabs
located at the outside face of the web of the external girder, Sc is equal to 1.16m. Considering
that the thickness of the slab remains constant across the bridge deck, and a cast-in-place barrier
STRUCTURAL DESIGN FOR DURABILITY
97
is provided at the edge of the slab; the maximum cantilever moments due to the unfactored
CL-625 truck wheel loads results in a bending moment of 34kNm/m, including the dynamic load
allowance.
Table 6.15: Maximum cantilever moments due to unfactored CL-625 truck wheel loads including the DLA (kNm/m). 1
Figure 6.21: Notation for cantilever moments. 1
The dead load effects can be determined in terms of the self weight of the slab and the concrete
barrier placed at the edge of the deck. The permanent loads corresponding to the bridge deck
slab are:
- Selfweight of the reinforced concrete slab: (0.225m)(24kN/m3)(1m) = 5.4kN/m.
- Selfweight of the concrete barrier: (0.35m2)(1m)(24kN/m3) = 8.4kN.
- Waterproofing membrane and asphalt concrete: (0.065m)(23.5kN/m3)(1m) = 1.53kN/m.
The distributed load on a one-metre-wide slab strip results: w = 5.4kN/m+1.53kN/m = 6.93kN/m.
The resulting bending moments at the supported edge of the cantilever overhang of the slab can
be calculated as:
STRUCTURAL DESIGN FOR DURABILITY
98
2
435.0
2
435.0 2 mSP
mSwM c
cD (6-18)
Therefore,
kNmm
mkNmmmkN
M D 74.92
435.016.14.8
2
435.016.193.6 2
According to Clause 3.5, the design bending moment for the slab overhangs must be calculated
considering some load factors, which are 1.2 and 1.7 for the dead load and the live load
respectively, giving the design bending moment as:
kNmkNmkNmM u 49.69347.174.92.1
6.2.5.2. Durability Parameters
For the cantilever part of the deck slab, the durability design parameters remain the same as the
ones described previously for the internal panels of the slab in Section 6.2.4.2.
6.2.5.3. Initial Conditions of the Bridge Deck Slab
Similarly, the initial physical conditions of the cantilever overhang of the bridge deck slab
correspond exactly to those described for the rest of the slab in Section 6.2.4.3.
6.2.5.4. Assumption for Steel Reinforcement and Performance with Time
The calculated factored design bending moment can be used to determine the minimum required
reinforcement. Then, this reinforcement provision can be refined by using the integration of the
structural and durability models, following an iterative process.
Adequate reinforcement for the slab at the cantilever overhangs works out to be 20M @ 0.10m in
the top layer, alternating the rebars with the previous bar distribution of 20M @ 0.10m that was
provided for the upper layer of the internal panels. This results in a rebar distribution of 20M @
0.05m for the cantilever part of the slab. The lower layer and the longitudinal reinforcement
remains the same as the one established for the rest of the slab. This reinforcement in the bridge
deck slab results in the performance with time as shown in Figure 6.22.
It is evident from Figure 6.22 that the proposed design for the slab performs adequately with time,
ensuring adequate bearing capacity throughout the design service life. At the end of the 150
years of design service life, the element still has some residual resistance of about 15kNm.
STRUCTURAL DESIGN FOR DURABILITY
99
Figure 6.22: Bridge deck slab flexural resistance at the cantilevers overhang. Following the same multi-stage protection strategy that has been used for the rest of the bridge
elements, additional protective measures will help to reduce the impact of the deterioration
mechanisms on the most vulnerable parts of the bridge deck slab. These supplementary
protection measures are described in Section 6.2.9.
6.2.5.5. Final Design and Details
The summary of the deck slab reinforcement for the internal sections is shown in Figure 6.23.
Figure 6.23: Reinforcement details for the bridge deck slab overhangs. 6.2.6. Transverse Vertical Shear
The verification of shearing resistance is necessary to ensure that the bridge deck slab can
withstand the induced shearing forces without any shearing reinforcement during the required
service life.
STRUCTURAL DESIGN FOR DURABILITY
100
6.2.6.1. Load Analysis
The shear stresses in the slab are calculated considering the permanent loads and the truck
loads that act on the bridge deck. The dead or permanent load generates the following vertical
shear:
- At the internal sections of the slab: kNmmkNVD 27.13915.1/93.6 .
- At the cantilever overhangs: 16.44kN16.1/93.64.8 mmkNkNVD .
The transverse vertical shear on the bridge deck slab that is produced by the truck loads is
determined by the effects of the wheel loads of the heaviest axle moving across the bridge lanes.
This loading condition is shown in Figure 6.24.
Figure 6.24: Axle loads moving across the deck lanes. Using the influence line theory, it is possible to determine the position of loads that generates the
maximum internal shear forces in the slab. The analysis is aimed of determining the design
shearing force at the intermediate support.
The relevant influence line is presented in the figure 6.25 (a). The loading location and the
consequent shear diagram required by the influence line in Figure 6.25 (a), is shown in figure
6.25 (b). From these diagrams, it is possible to identify the maximum shear force,
VTruck = 89.04kN, induced in the slab due to the truck loads.
This load must be increased by the dynamic load allowance of 0.4 considering that a single axle
is considered in this analysis. The maximum factored shearing force induced in the slab is
determined considering the applicable load factors as:
kNkNkNVu 49.25096.964.017.144.162.1
STRUCTURAL DESIGN FOR DURABILITY
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Figure 6.25: a) Influence line for shear forces in the slab at the intermediate support. b) Shear force diagram for the loading location that generates the largest shear force at the intermediate support.
6.2.6.2. Durability Parameters
The durability design parameters remain the same as the ones described previously for the
flexural design of the slab in Section 6.2.4.2.
6.2.6.3. Initial Conditions of the Bridge Deck Slab
The initial physical conditions of the slab correspond exactly to those described in Section 6.2.4.3.
6.2.6.4. Shearing Resistance Performance with Time
The shearing resistance of concrete is a property that varies with time due to the progressive
deterioration of the slab caused by the relevant mechanism of deterioration, and hence causing a
progressive reduction of the slab effective height that accounts for the shearing resistance. The
performance of the bridge deck slab shearing resistance with time is presented in Figure 6.26.
STRUCTURAL DESIGN FOR DURABILITY
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Figure 6.26: Bridge deck slab shearing resistance vs. time.
It is clear from Figure 6.26 that the bridge deck slab performs satisfactorily in shear throughout
the service life. As for the rest of the bridge elements, some supplementary protection measures
are adopted to decrease the rates of deterioration and the loss of slab shear resistance.
6.2.7. Analysis of Deflections with Time
The analysis of deflections is based on the principles outlined in Section 6.2.2.4. According to
these design assumptions, the deflections produced by the dead and live loads will increase with
time due to the progressive deterioration of the section, with a reduction of the height of the
reinforced concrete slab and the corrosion of the reinforcing steel bars, which decreases the
rigidity of the of the concrete element, and consequently its stiffness against deflection. The slab
deflections are evaluated at the cantilever overhangs, which is the part of the slab that presents
the maximum values of deflections produced by the acting loads. The maximum allowable
deflections is considered to be (2Sc) / 240 which is the limit value established for slabs supporting
non-structural elements (reinforced concrete barriers) not likely to be damaged by large
deflections, according to the CSA A23.3 Standard 23.
The performance of the slab for the serviceability limit states in terms of deflections is presented
in Figure 6.27, which shows that after 135 years of service life the bridge deck slab will reach the
limit of its serviceability limit state in terms of deflections at the cantilever overhangs. However, an
increase of the slab section in this part of the deck is not required. Considering that some
supplementary protection measures, such as waterproofing coatings and membranes are being
STRUCTURAL DESIGN FOR DURABILITY
103
adopted, the degree of deterioration over the slab will be reduced, extending the service life of
this bridge element.
Figure 6.27: Deflections on the cantilever overhang of the bridge deck slab.
6.2.8. Design of the Semi-continuity of the Bridge Deck Slab
The bridge superstructure is designed as a series of precast, prestressed concrete girders which
will resist their own dead load over a simple span. However, continuity will be established at
intermediate supports after the girders have been installed. These three-span continuous
systems will then resist the live loads (trucks) and any other superimposed loads as a continuous
girder with negative bending moments over the intermediate piers. This continuity is implemented
by providing a specially designed diaphragm over the intermediate piers along with high-strength
steel dowels. Additional reinforcement is arranged at the top fibres of the deck slab to provide the
necessary resistance for negative moments due to live loads. Moreover, reinforcement is
provided at the lower part of the girder ends to provide continuity at the bottom fibres at the
intermediary support.
For this bridge, the girders have been designed to withstand not only the dead loads, but also the
truck loads, following the philosophy of a durable design under the principles of multi-protection
approach design. This design assumption accounts for the event of the loss of continuity at the
intermediate supports under the event of deterioration, repair, modification or renovation of the
bridge deck. The main purpose for this semi-continuity setup is to eliminate the deck joint at the
pier supports and hence, eliminate the source of important mechanisms of deterioration that
would occur during the bridge service life if some expansion joints are installed at these locations.
STRUCTURAL DESIGN FOR DURABILITY
104
These expansion joints over the piers have been a source of serious ingress of aggressive
elements and significant deterioration of girder ends, bearings, pier caps and piers; eliminating
these joints would cut off the ingress of aggressive elements and the resulting extensive
deterioration. The reinforcement for negative bending moments must be enough to provide at
least an appropriate crack control of the slab. However, the negative moment at the supports is
calculated based on the assumption of full structural continuity, following the recommendations of
Clause 8.19.4.3 1.
6.2.8.1. Load Analysis
The maximum bending moment caused by the permanent loads and the truck can be determined
considering a three-span bridge continuously supported over the intermediate supports at the pier.
The permanent loads can be considered as uniformly-distributed loads that cause flexural
moments in the bridge deck: These uniformly distributed loads are:
- Slab: mkNmkNmm 38.812407.15225.0 3
- Gussets: mkNmkNmm 08.1072405.02.1 3
- Diaphragms:
mkNm
mkNmm81.8
263
562419.330.0 32
- Wearing surface: mkNmkNkNmm 69.215.23)20.14065.0 3
- Barriers: 332 80.162435.02 mkNmkNm
Therefore, the distributed permanent loads are wD = 138.76 kN/m. The maximum negative
bending moment produced by the permanent loads results -9377.49kNm, according to the
bending moment diagram presented at Figure 6.28.
Figure 6.28: Bending moments diagram for the bridge deck under permanent loads. The maximum bending moment caused by the truck live loads can be determined using influence
lines for the moments at the first intermediate support (Figure 6.29). This analysis allows the
identification of the precise location of the truck loads that generates the worst-case scenario for
STRUCTURAL DESIGN FOR DURABILITY
105
the maximum negative bending moment at this support. Accordingly, the maximum negative
bending moment caused by the truck loads is -2233.85kNm.
Figure 6.29: a) Influence line for bending moments at the firs intermediate support of the bridge deck and location of the truck loads that generates the maximum negative bending moment at the support. b) Bending
moments diagram for the bridge deck under the truck loading condition. The design bending moment, considering the DLA and the load factors described in the CHBDC 1
results:
kNmkNmkNmM u 92.1599925.0185.22337.149.93772.1
6.2.8.2. Durability Parameters
The durability design parameters remain the same as the ones described previously for the
flexural design of the slab in the Section 6.2.4.2.
6.2.8.3. Initial Conditions of Semi-continuity of the Bridge Deck
The initial physical conditions of the bridge deck at the support correspond to the condition set by
the integration of the deck slab, the end-diaphragms and the girders of the bridge deck at the
intermediate supports. These initial conditions will be affected by the presence of the degradation
mechanisms over the bridge deck at the previously mentioned locations. The most aggressive
degradation mechanisms will take place at the top of the deck slab due to the action of the
surface deterioration, which is the controlling mechanism that determines the durability
parameters for the design of this part of the bridge deck. The degradation of the bridge deck at
STRUCTURAL DESIGN FOR DURABILITY
106
the top of the slab will produce a loss of effective height and hence, a loss of bearing capacity for
negative bending moments at the support.
The conditions of the bridge deck at the intermediate supports immediately after construction are
described in Table 6.16.
Table 6.16: Initial conditions of the bridge deck at the intermediate supports. The determination of the bridge deck reinforcement for the negative bending moments under the
semi-continuity condition at the intermediate supports is determined after an iterative process
following the guidelines for a structural design for durability.
After several iterations, the adequate deck reinforcement for negative bending moments at the
intermediate supports results in a distribution of 25M rebars @ 0.15m (Figure 6.30). This
reinforcement at the top of the deck is extended up to the point where no negative reinforcement
is required. After having analyzed the bending moment diagrams for permanent and truck loads,
it can be established that the negative moment reinforcement must be extended 7m on each side
of the deck from the axis of the pier, then it will be overlapped with the top longitudinal
reinforcement of the slab for the intermediate portion of the deck.
Figure 6.30: Detail of the bridge deck reinforcement for the semi-continuity condition at the intermediate supports.
STRUCTURAL DESIGN FOR DURABILITY
107
The positive moment reinforcement over the supports is proportioned for structural continuity to
resist the moments due to creep, shrinkage, temperature change, and live load in farther spans.
According to Clause 8.19.4.2, a minimum reinforcement for positive bending moments cannot be
lower than (1.5 x 1600) mm2 at the location of every girder. This reinforcement corresponds to
2400mm2 that can be distributed among 4 u-shaped 20M rebars that are adequately embedded
in the bottom flange of the girders beyond the strand transfer length and anchored into the
diaphragm over the continuity supports.
6.2.8.4. Semi-continuity Performance of the Bridge Deck with Time
The performance of the bridge deck reinforcement resistance for negative bending moments with
time is presented in Figure 6.31.
Figure 6.31: Bridge deck bearing capacity for negative bending moments with time.
Figure 6.31 shows that the semi-continuity of the bridge deck is satisfactorily available until about
140 years of service life. There is a slight gain of bearing capacity for negative bending moments
over the first 20 years of use, which is caused by the loss of the concrete cover in top of the
bridge deck slab that generates a loss of weight and hence, a reduction of dead load at this
location of the bridge. However, after the year 20, the initiation of active corrosion begins and
then, a reduction of the rebar cross-sections occurs, continuously reducing the flexural resistance
of the deck section for negative bending moments. The service life of the semi-continuity
condition of the bridge deck can be increased by applying additional protection measures that are
described in Section 6.2.9, and by ensuring an adequate maintenance strategy.
STRUCTURAL DESIGN FOR DURABILITY
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6.2.9. Supplementary Protective Measures
The performance of the bridge deck slab design for durability as detailed and reviewed in the
previous sections confirms satisfactory performance of the deck throughout the design service life
of the bridge. However, following the principle of a multi-front protection strategy, some
supplementary protection measurements are adopted, including the use of galvanized steel,
which may reduce significantly the rates of corrosion, extending the satisfactory behaviour of the
structural elements beyond the required service life.
The installation of waterproofing coatings and membranes attached on top of the concrete slab
are required for the purpose of reducing the direct exposure to wetting and drying cycles, and
freezing-and-thawing effects, preserving the top concrete surface for longer time. This will
diminish the ingress of moisture and solutions carrying de-icing salts, delaying the initiation time
for active corrosion to occur.
Additionally, it is planned to use microfilament polypropylene fibres in the concrete mixture of the
bridge deck slab; these fibres help to control plastic shrinkage and settlement cracking.
Additionally, they may improve impact, shatter and abrasion resistance. They enhance durability
and toughness of concrete, and may also help to reduce bleeding.
6.3. Substructure Design
The structural design of the substructure elements for durability is focused on the intermediate
piers. The piercaps will be subjected to different load cases that generate different flexural
conditions depending on the way that these loads are applied. The design loading conditions may
include different load cases, such as dead loads, live loads, earthquake effects, wind loads, wind
effects over the vehicles, braking forces from the traffic, and the effects caused by strains,
deformations and displacements suffered by the bearings placed on top of the piercap, caused by
temperature change and temperature differentials, concrete shrinkage, differential shrinkage and
creep. The flexural conditions on the piercaps may be generated in different planes of the
element, resulting in a condition of biaxial flexure. For these reasons, adequate reinforcement
must be provided for this part of the piers, considering as well the durability considerations
required for these elements over the service life of the bridge. The previously developed
procedure for flexural design of reinforced concrete for durability can be used for this purpose.
The lower part of the piers composed by the pair of columns can be subjected to flexural and
axial loads, caused by the combination of the previously described load cases. For the purpose of
the present bridge design, the substructure design will be focused on durability of the pier
columns.
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6.3.1. Pier Column Design
The structural design for durability of pier columns is presented using a different approach due to
the complexity of the model of reinforced concrete columns subjected to flexural and axial loads
simultaneously. This approach is addressed by analyzing in an iterative manner, the different
reinforced concrete sections of the columns that are being deteriorated by the various
degradation mechanisms, identifying their bearing capacity against the applied loads by using the
interaction diagrams of the different column sections for flexure and axial loads.
6.3.1.1. Load Analysis
A detailed description and analysis of the different load cases considered for the design of the
pier column of the bridge is presented in the following sections.
6.3.1.1.1. Dead Loads (D)
The dead loads that affect the foundation units comes from the self weight of the bridge elements
of the bridge that are involved in the tributary area that accounts for each one of the intermediate
piers. These loads are:
- Self weight of the slab: kNmkNmmm 83.2115242607.15225.0 3
- Self weight of the gussets: kNmkNmmm 06.259142485.122.105.0 3
- Selfweight of the barriers: kNmkNmm 8.4362242635.0 32
- Self weight of diaphragms:
Intermediate: kNmkNmmm 71.2333624915.125.013.1 3
End: kNmkNmmmm 41.748224730.0589.0675.019.3 322
- Selfweight of the girders: kNmmkN 81.24982785.1289.13
- Selfweight of the wearing surface: kNmkNmmm 95.5635.232620.14065.0 3
- Selfweight of the pedestals: kNmkNmmm 21.537243.29.0153.0 3
- Selfweight of the piercap: kNmkNmm 6.1407243.250.25 32
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- Selfweight of the pier columns: kNmkNmm 78.90422461 32
The total dead load of the structure acting on each one of the intermediate piers is D =
9222.15kN.
6.3.1.1.2. Live Loads (L)
The live loads considered for the design of the pier columns correspond to the case where a
maximum number of trucks can be located near an intermediate pier within the tributary area of
the foundation unit. In this case, it is assumed that both of the traffic lanes are occupied with
trucks traveling on both directions of the bridge. According to the geometry of the bridge deck, it
is possible to arrange one complete design truck CL-625, plus the first and the last load axles
from the other trucks located in front and behind of the truck considered. This loading condition
can be found on each one of the four traffic lanes of the bridge. Therefore, the maximum live load
acting on the intermediate piers is:
kNkNkNkNkNkNkNkNL 330045015017512512550150
6.3.1.1.3. Wind Loads (W)
According to the recommendations in he CHBDC 1, the wind loads can be calculated according to
the following equations:
hgeh CCqCF ; vgev CCqCF (6-19)
where:
Fh = the horizontal wind pressure on the structure.
Fv = the vertical wind pressure on the structure.
q = the hourly mean reference pressure of 400Pa for a return period of 50 years.
Ce = the wind exposure coefficient of 1.0, according to the height of the superstructure.
Cg = the gust effect coefficient of 2.0, according to the span length of the bridge.
Ch = the horizontal wind load coefficient of 2.0.
Cv = the vertical wind load coefficient of 1.0.
Therefore, the resulting vertical and horizontal wind pressures are:
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kPaPaPaFh 6.116000.20.20.1400
kPaPaPaFv 8.08000.10.20.1400
These wind pressures are applied horizontally and vertically over the exposed areas of the bridge
in a perpendicular direction to the longitudinal axis of the bridge. The vertical and horizontal wind
loads are considered to act simultaneously. The vertical wind load is considered to be applied
downwards or upwards, resulting in two different load cases. To consider a possible eccentric
effect, the vertical load resultant is applied at the quarter point of the transverse bridge deck width.
According to the geometrical characteristics of the bridge deck and the eccentricity for vertical
wind load is 3.77m. The intermediate piers are designed for wind-induced loads transmitted by
the bridge deck, and for wind loads acting directly on them. The foundation units are designed for
directly applied horizontal wind forces calculated with a horizontal wind load coefficient (Ch) of 0.7
for circular pier columns, according to Clause 3.10.3.3.
Considering the vertical and horizontal surfaces of the bridge involved in the tributary areas of the
intermediate piers, the resulting wind loads are:
- Horizontal wind force on the bridge deck:
kNmmkPaW deckh 8.1242636.1 (Load case: WHD)
- Vertical wind force on the bridge deck:
kNmmkPaW deckv 46.3132607.158.0 (Load case: WVD)
- Lateral wind force on the pier:
kNmkPaW pierh 85.702.147.00.20.14.0 2 (Load case: WHP)
- Frontal wind force on the pier:
kNmkPaW pierhf 17.2595.447.00.20.14.0 2 (Load case: WFP)
6.3.1.1.4. Wind Loads on Traffic (V)
According to the CHBDC 1, the wind effects on the moving vehicles are calculated from the
applied wind pressure, which is determined using a horizontal wind load coefficient (Ch) of 1.2.
This wind pressure is applied over the exposed surface of the design truck. The resulting wind
force over the vehicles is:
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mkNmkPaW vehiclesh 88.232.10.20.14.0
This load should be applied over the entire length of the superstructure. According to this, the
resulting wind force on the intermediate piers resulting from the traffic over the bridge is:
kNVpier 37.82
6.3.1.1.5. Stream Pressure Loads (F)
Following the recommendations of the CHBDC1, the load acting longitudinally on the intermediate
piers of the bridge can be calculated according to the following equation:
2
2AvCP D (6-20)
where:
P = the total load due to flowing water acting on the pier columns in the direction of the
longitudinal axis of the piers [N].
CD = the longitudinal drag coefficient of 0.7 for circular pier columns.
= the density of water of 1000kgf/m3
A = the area of a pier column exposed to the flowing water, projected parallel of the longitudinal
axis of the pier. This area is 6m2.
v = water velocity of the design flow. It is taken to be 0.55m/s.24
The lateral load on the pier columns, generated from the water flow depends on the angle
between the water flow and the longitudinal axis of the piers. This force can be determined by the
following expression:1
2
2HLvCP L
p
(6-21)
where:
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Pp = the total load due to flowing water acting on the pier columns in the horizontal direction
perpendicular to the longitudinal axis of the piers [N].
CL = the lateral load coefficient of 0.7 for an angle between the water flow and the longitudinal
axis of the piers of 8.5º.
H = depth of the flowing water at the pier equal to 3.5m.
L = Length of the pier along the longitudinal axis. This length will be considered as 2m for each
column.
Having defined these parameters, the stream pressure loads acting on the pier columns are:
kNN
smmmkgfP 635.025.635
2
55.0610007.0 223
(Load case: FH)
kNN
smmmmkgfPp 741.013.741
2
55.025.310007.0 23
(Load case: FF)
6.3.1.1.6. Ice Loads
According to the location of the bridge crossing a river, the foundation units placed in the water
can be subjected to significant forces due to ice effects during the winter season. The ice effects
can be related with the pressures developed on the piers due to moving ice, ice impact forces, ice
jams, ice adhesion and ice accretion. The loading cases are described in the following
subsections.
A. Pressures Due to Moving Ice (F)
Following the recommendations of the CHBDC 1, the resulting horizontal force caused by moving
ice can be determined by the following expressions:
The lesser of bF or cF → for 0.6tw
F (6-22)
cF → for 0.6tw
where: 2ptCF nb ; ptwCF ac
where:
Fb = the horizontal ice load caused when ice floes fail by flexure [kN].
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Fc = the horizontal ice load caused when ice floes fail by crushing [kN].
Cn = the coefficient of pier nose inclination. This coefficient is equal to 0.133.1
Ca = the coefficient allowing for the ratio of the pier width to ice thickness when the ice fails by
crushing. This coefficient is equal to 1.41.1, 24
p = the effective crushing strength of ice of 1500kPa, when ice movement occurs at temperatures
below the melting point.1
t = the ice thickness expected to make contact with the piers. This is equal to 0.4m.24
w = the frontal pier width at the level of ice action where ice is split or crushed, perpendicular to
the direction of the ice motion. For the analyzed pier columns, this value is equal to 2m.
Therefore, the ice load on the pier columns caused by moving ice is:
Fm
m
t
w 5
4.0
2is the lower value between bF and cF
kNmkPaFb 92.314.01500133.0 2
kNF 92.31
kNmmkPaFc 169724.0150041.1
B. Ice Impact Forces (FII)
To account for ice impact forces, the following cases are analyzed:
- Case 1: A longitudinal force F (load case: FIIH) with a transverse force equal to 0.15F (load
case: FIIF).1
- Case 2: A longitudinal force 0.5F (load case: FIIH) with a transverse force equal to 0.34F
(load case: FIIF).1
C. Ice Jams (FIJ)
Considering the span lengths of the bridge of 26m, some floating ice accumulations can be
developed generating some pressures of the order of 10kPa 1 on the exposed pier columns, and
applied longitudinally and laterally to the pier orientation above the level of still water for the
expected thickness of ice jam. For the actual conditions of the bridge, ice jams are expected to be
developed on a thickness around 0.75m 25 around the pier columns. According to this, the ice
force that can be generated on the foundation units results to be:
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kNmmkPaFIceJams 1575.0210 (Load cases: FIJH and FIJF)
D. Ice Adhesion (FIA)
According to the CHBDC1, when there exist fluctuations of the water levels, the forces generated
on circular pier columns frozen to an ice formation can be calculated using the following equation:
75.02 13.005.11250 tRtFv (6-23)
where:
Fv = the vertical force on a bridge pier caused by to ice adhesion [kN].
t = the ice thickness expected to make contact with the piers equal to 0.4m.24
R = radius of the circular pier column. This is equal to 2m.
Therefore, the ice adhesion force generated on each pier columns is:
kNm
mmFv 39.313
4.0
213.005.14.01250
75.0
2
(Load case: FIA)
E. Ice Accretion (A)
The ice accretion loads can be developed on all exposed surfaces of the bridge deck. The
thickness of the ice accretion depends on the geographical location of the bridge. For this
purpose, it is possible to obtain the ice thickness accretion from Figure 5.2.1 According Figure 5.2,
the ice accretion thickness for the Montreal region corresponds to 31mm.
Considering the exposed surfaces of the bridge as the sum of the surfaces of the bridge deck
slab, the concrete barriers and the external faces of the edge girder, the resulting ice accretion
load applied vertically on the bridge deck is:
kNmkNmmF AccretionIce 62.651807.9031.057.2143 32
6.3.1.1.7. Earthquake Loads (EQ)
Based on the considerations for earthquake load analysis for multi-span bridges presented in the
CHBDC 1, the equivalent uniformly distributed static seismic loading can be determined from the
following equation:
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L
WCP sm
e (6-24)
where:
W = the effective weight of the bridge. According to the Section 6.3.1.1.1, the weight of the bridge
is 29978.86kN, say 30000kN.
L = the total length of the bridge, which is 78m.
Csm = the elastic seismic response coefficient. This coefficient can be determined using the
equation: 1
AI
T
SACsm 5.2
2.132
(6-25)
where:
A = the zonal acceleration ratio, which is considered to be 0.20 for the Montreal metropolitan area.
S = the site coefficient. This parameter depends on the type of soil where the foundation of the
bridge structure is built. For the current bridge design, the foundation stratum for the substructure
corresponds to profile of stiff clays, sands and gravels with a depth varying from 70m to 85m.
According to Clause 4.4.6.1, the foundation soil corresponds to a soil profile Type I, which is
related to a site coefficient S = 1.2.
I = the importance factor, which for a lifeline bridge (Section 2.1), has a value of 3.0.
T = the natural period of the structure [s]. This period can be calculated according to the
equation:1
gK
WT 2 (6-26)
where:
g = the acceleration due to gravity equal to 9.807m/s2.
K = the lateral stiffness of the bridge. This parameter can be calculated from the following
equation: 1
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max,s
o
V
LpK (6-27)
where:
po = an arbitrary uniform lateral load applied on the structure.
Vs,max = the maximum static displacement of the bridge due to the arbitrary load po.
The lateral stiffness of the bridges results to be:
mkN
m
mmkNK 4875000
0016.0
78100
Therefore, the natural period of the bridge structure is:
smkNsm
kNT 157.0
4875000807.9
86.299782
2
Therefore, the elastic seismic response coefficient is:
686.0157.0
2.12.02.132
sCsm ; 5.10.32.05.25.2 AI
Thus, 686.0smC
Finally, the uniformly distributed static seismic load is:
mkN
m
kNPe 75.263
78
86.29978686.0 (Load cases: EQH and EQF)
According to the provisions of Clause 4.4.9.2, the elastic seismic effects on the principal axes of
the piers resulting from analyses in the two perpendicular and principal horizontal directions must
be combined in such a way that 100% of the static seismic resulting force is applied in one of the
principal horizontal direction, simultaneously with the application of 30% of the resulting seismic
force in the other perpendicular principal direction. The load cases consider the application of
100% of the resulting seismic force in each one of the principal directions of the piers, in the
different senses, and in each one of the considered directions. This is reflected in the load
combination list that is presented in Section 6.3.1.1.9.
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6.3.1.1.8. Vessel Collision Forces (H)
The bridge must remain open to all traffic after a vessel collision (Section 2.1). This consideration
classifies this structure as a Bridge Class I, according to the CHBDC 1.
According to the CHBDC, the vessel collision forces on the bridge can be divided in two groups:
vessel collision forces on the substructure, and vessel collision forces on the superstructure. The
ship collision force on the pier columns can be calculated using the equation: 1
4.85.0 VDWTPs (6-28)
where:
Ps = the equivalent static vessel collision force [MN].
DWT = the dead weight of the vessel [t].
V = design collision velocity [m/s].
For the current design case, the dead weight of the vessel is assumed to be equal to 2650lbf
(1.20t), which corresponds to the upper limit of the small-vessel category. The collision impact is
considered to be to the order of 45mi/h = 20.11m/s, according to the selected type of vessel.
Therefore, the static vessel collision force applied on the pier columns is:
kNMNsmtPs 59.262762.24.8/11.2020.1 5.0 (Load cases: HFH and HFF)
The analysis of the loads resulting from a ship collision with the superstructure can be made
based on the following collision scenarios: 1
- Collision with bow: 1 SBHBH PRP (6-29)
where:
PBH = the ship collision force [MN].
RBH = the ratio of exposed superstructure depth to the total bow depth. According to the
characteristics of the type of boat assumed for design, there are no potential exposed surfaces.
For the small-vessel category, the bow height of a typical boat is much smaller than the bridge
clearance for navigation.
For this reason, the collision bow force is not going to be considered.
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- Collision with deck house: 1 SDHDH PRP (6-30)
where:
PDH = the vessel deck collision force.
RDH = the reduction factor calculated using the following equation:
2.010.0100000
2.12.010.0
1000002.0
DWTRDH
Therefore, the vessel deck collision force is:
kNkNPDH 52.52559.26272.0 (Load case: HDH)
- Collision with mast: 1 DHMT PP 10.0
kNkNPMT 55.5252.52510.0 (Load case: HMT)
For pier design, the collision forces are applied as equivalent static forces, considering 100% of
the force acting in the direction parallel to the alignment of the centreline of the navigable channel,
and simultaneously, 50% of the force acting normally to this direction.
The superstructure must design for an equivalent static impact force applied perpendicularly to
the bridge deck elements vulnerable for collision, according to Clause A3.3.8.2.
6.3.1.1.9. Load Combinations
The load factors and load combinations for the design of the bridge structure are the ones
described in Clause 3.5 of the CHBDC 1. These combinations are reproduced here for
completeness.
- Fatigue Limit State:
1. D + L
- Serviceability Limit States
1. D + 0.9L 2. 0.9D
- Ultimate Limit State:
1. 1.2D + 1.7L
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2. 1.2D + 1.6L 3. 1.2D + 1.4L + 0.5WHD + 0.5WVD + 0.5WHP + 0.5WFP + 0.5 4. 1.2D + 1.4L - 0.5WHD - 0.5WVD - 0.5WHP - 0.5WFP - 0.5 5. 1.2D + 1.4L + 0.5WHD - 0.5WVD + 0.5WHP - 0.5WFP + 0.5 6. 1.2D + 1.4L - 0.5WHD + 0.5WVD - 0.5WHP + 0.5WFP - 0.5 7. 1.2D + 1.4L + 1.65WHD + 1.65WVD + 1.65WHP + 1.65WFP + 1.65 8. 1.2D + 1.4L - 1.65WHD - 1.65WVD - 1.65WHP - 1.65WFP – 1.65 9. 1.2D + 1.4L + 1.65WHD - 1.65WVD + 1.65WHP - 1.65WFP + 1.65 10. 1.2D + 1.4L - 1.65WHD + 1.65WVD - 1.65WHP + 1.65WFP - 1.65 11. 1.25D + EQH + 0.3EQF 12. 1.25D + 0.3EQH + EQF 13. 1.25D - EQH - 0.3EQF 14. 1.25D - EQH + 0.3EQF 15. 1.25D + EQH - 0.3EQF 16. 1.25D - 0.3EQH - EQF 17. 1.25D - 0.3EQH + EQF 18. 1.25D + 0.3EQH - EQF 19. 1.2D + 1.3FH + 1.3FF 20. 1.2D - 1.3FH + 1.3FF 21. 1.2D + 1.3FH - 1.3FF 22. 1.2D - 1.3FH - 1.3FF 23. 1.2D + 0.9WHD + 0.9WVD + 0.9WHP + 0.9WFP + 0.9V + 1.3A 24. 1.2D - 0.9WHD - 0.9WVD - 0.9WHP - 0.9WFP - 0.9V - 1.3A 25. 1.2D + 0.9WHD - 0.9WVD + 0.9WHP - 0.9WFP + 0.9V - 1.3A 26. 1.2D - 0.9WHD + 0.9WVD - 0.9WHP + 0.9WFP - 0.9V + 1.3A 27. 1.2D + 1.3FI 28. 1.2D – 1.3FI 29. 1.2D + 1.3FIIH + 1.3FIIF 30. 1.2D - 1.3FIIH + 1.3FIIF 31. 1.2D + 1.3FIIH - 1.3FIIF 32. 1.2D - 1.3FIIH - 1.3FIIF 33. 1.2D + 0.65FIIH + 2.95FIIF 34. 1.2D - 0.65FIIH + 2.95FIIF 35. 1.2D + 0.65FIIH - 2.95FIIF 36. 1.2D - 0.65FIIH - 2.95FIIF 37. 1.2D + 1.3FIJH + 1.3FIJF 38. 1.2D - 1.3FIJH + 1.3FIJF 39. 1.2D + 1.3FIJH - 1.3FIJF 40. 1.2D - 1.3FIJH - 1.3FIJF 41. 1.2D + 1.3FIA 42. 1.2D - 1.3FIA 43. 1.2D + HFH + HFF 44. 1.2D + HFH – HFF 45. 1.2D - HFH + HFF
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46. 1.2D - HFH – HFF 47. 1.2D + HDH 48. 1.2D – HDH 49. 1.2D + HMT 50. 1.2D – HMT 51. 1.35D
After analyzing the bridge structure according to the above load combinations, it is possible to
identify the maximum factored axial loads and bending moments acting in the principal directions
of the pier columns. These factored actions are:
kNPu 22.12998 ; kNmM xxu 75.6965 ; kNmM yyu 30.4598
These actions induced by the different load cases acting on the structure need to be evaluated in
terms of the bearing capacity of the columns, which is reflected in the interaction diagrams for
flexural compression of the columns.
6.3.1.2. Durability Parameters
The durability design parameters have been determined in Section 5.1.4. The controlling
mechanisms of deterioration are frost attack and abrasion by ice. A summary of the durability
parameters for the pier columns is presented in the following table:
Rates of deterioration (mm/year)
Concrete Ice abrasion 2.3835
Frost attack 0.1024
Reinforcing steel 0.002
Carbonation coefficient, Kc 2.36
Initiation time for corrosion 28 years
Table 6.17: Durability design parameters for the pier columns.
6.3.1.3. Initial Conditions of the Pier Columns
The initial conditions of the pier columns are assumed at first and then, after several iterations of
analysis, making the integration of the structural and durability parameters, it is possible to
identify the proper characteristics of the reinforce concrete sections. A summary of the general
initial conditions of the pier columns is presented in Table 6.18.
6.3.1.4. Assumptions for Reinforcement and Performance with Time
Initially, a reinforcement ratio of 1% of the cross-sectional area of the columns is assumed, which
results in 48 30M rebars that can be arranged in such a way that 36 bars are distributed on an
STRUCTURAL DESIGN FOR DURABILITY
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outer reinforcement layer and the remaining 12 rebars are arranged in an inner layer, as shown in
Figure 6.32. The performance of this reinforced concrete section under the conditions of flexural
compression and deterioration mechanisms is analyzed initially for abrasion by ice as the
controlling mechanism of deterioration. This performance is shown in the Figure 6.33, which
shows that the abrasion by ice on the pier columns produces a significant deterioration of the
reinforced concrete section, up to the point of inducing failure of the column at the ULS after 145
years of service life. This failure condition is related to flexural compression of the column with
respect to an axis direction perpendicular to the longitudinal axis of the pier (Mux-x, Pu).
Diameter Concrete
cover Material properties
D0 (m) c (mm) fy (MPa) f'c (MPa) c s2 75 400 35 0.75 0.9
Table 6.18: Initial conditions of the reinforced concrete section of the pier columns.
Figure 6.32.: Reinforcement of the pier column.
The piers of a bridge play a critical role in the integrity of the bridge structure, but at the same
time they are very difficult and costly to replace. Therefore, it is very important as a minimum
requirement, that these elements attain the required design service life of the bridge. For this
reason, it is required to limit the aggressive action of ice abrasion on the pier column by
implementing additional protection measures represented by the installation and construction of
protective islands around the piers, made of protective rock layers placed and arranged to
diminish the flow around the pier columns.
Once these protective islands are installed, the controlling mechanism of deterioration acting on
the pier columns is frost attack. The performance of the pier column over the design service life,
X X
Y
Y
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considering the effect of frost attack, is presented in the Figure 6.34, which shows that the
performance of the pier column over the design service life is satisfactory after having analyzed
and integrated the structural resistance and durability considerations. At the end of the service life
there is enough resistance of the column against the ultimate flexural compression conditions
developed on the two principal directions of the columns (Mux-x, Pu; Muy-y, Pu).
Figure 6.33: Interaction curves for pier column for abrasion by ice.
Figure 6.34: Interaction curves for pier column for frost attack.
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6.3.1.5. Supplementary Protective Measures
Some additional measures can be incorporated to protect the pier columns and to enhance their
service life by using a multi-layer protection system. As mentioned previously, protective islands
were created around the piers to diminish the effects of abrasion by ice. At the same time, these
islands can help to reduce the risk of vessel impact against the piers, due to the difficulty of
navigating near these elements.
If the construction of these protective islands results to be inconvenient from the point of view of
cost or severe hydraulic disruption of the flow, another possibility could be to install some steel-
plate lining around the columns covering the zone that may be affected by ice abrasion and
vessel impacts. However, if this approach is adopted, it is recommended that the use of the steel
lining be extended along the entire length of the pier column. As a matter of fact, it would be
useful to leave the steel caisson driven up to the foundation stratum, and then fill it with reinforced
concrete. This can not only serve to protect the columns, but also significantly improves the
columns resistance besides enhancing its durability.
Rock protection of the river bed (Rip-rap) near the location of the bridge will also help to ensure
satisfactory river flow, thereby avoiding any hydraulically-related problems that may affect the
performance of the foundation units.
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7. MAINTENANCE STRATEGIES
7.1. General Principles for Maintenance Strategies
The service life commences as soon as the bridge construction is completed. At this time, the
external actions start to affect the bridge structure, and the construction materials begin to suffer
progressive degradation. These external actions involve mechanical loads and environmental
actions.
The structural design for durability must consider these kinds of external actions that interact with
the structure and produce an ongoing deterioration in each one of the structural elements. The
results from a durable structural design approach are related to a defined service life for the
bridge. The maintenance procedures must be addressed to ensure the attainment of the design
service life for each bridge element, from the foundation to the superstructure.
The maintenance strategy must consider different kinds of maintenance procedures to be
performed on the bridge during its service life, including its maintainability, and the needed
preventive and corrective maintenance.
7.2. Design for Maintainability
This stage of maintenance strategy concerns all the aspects considered during the design
process to facilitate all maintenance operations to be performed in a practical and efficient
manner. These aspects involve, for example, the definition of a simple geometry of the structure
that provides easy access for inspection of all structural elements, access to hidden places of the
structure, such as the space between the diaphragms and the front wall of the abutments; and
the installation of monitoring mechanisms 29 that could provide valuable information on the future
behaviour and performance of the structure.
7.3. Preventive Maintenance
This stage of maintenance strategy concerns all of the actions that have to be performed to
preserve the original conditions of the bridge when it was opened to traffic. The preventive
maintenance procedures must consider cleaning activities for the different elements of the
structure, specially the most affected ones because of regular use. These cleaning actions must
involve:
- Cleaning of the draining system to ensure its proper operation, and to avoid the generation of
degradation sources (microclimates) caused by its malfunctioning.
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126
- Cleaning of the expansion joints at the abutments to clear the accumulation of debris, dust
and water carrying different aggressive agents.
- Removing the accumulated debris, dust and water carrying aggressive substances
underneath the expansion joints, especially at the abutments seats.
- Cleaning the bottom flanges and the webs of the girders where several deleterious
substances may accumulate, especially at the extremities supported on the abutment seats.
- Cleaning and maintaining the proper conditions of the fixed and mobile bearings to ensure
their proper performance.
- Performing a thorough washing of the entire deck, especially in the splash zones at the
barriers, at the expansion joints, and any other critical zones conductive to accumulation of
salts and any other aggressive agents.
These procedures must be planned to be performed periodically. For this reason, it is necessary
to configure a guideline for seasonal cleaning procedures, especially in the spring, when the use
of de-icing salts is over, with the purpose of eliminating the maximum amount of chlorides
accumulated in the structure, and reducing the aggressiveness of the microclimates acting on the
structural elements, especially during the most severe conditions of temperature and relative
humidity that can be found during the summer season.
7.3.1. Bridge Inspection
As a part of the maintenance strategies, it is important to establish a plan for regular inspections
on the structure during preventive maintenance. The purpose of these inspections is to monitor
the state of the construction materials and the various bridge components over time.
These inspections must be performed by qualified personnel with the necessary training to
develop adequate criteria when making decisions after analysis of the information collected from
the inspections.
As for the cleaning procedures, these inspections must be performed periodically. However, the
degree of detail of each inspection may vary. For these reasons, different kinds of inspections
need to be considered, such as routine inspections, detailed inspections, and special inspections.
7.3.1.1. Routine Inspection
Routine inspections need to be performed more frequently, say about once a month. The
purpose of this kind of inspections is to determine the overall health of the bridge, and to identify
the obvious flaws developing in the various structural elements, which could lead to future
MAINTENANCE STRATEGIES
127
deficiencies in the quality of the materials and performance of the components of the bridge. This
inspection must to cover the structural elements of the superstructure, such as the concrete deck
slab, expansion joints, barriers, cornices, girders, diaphragms and the approach slabs.
The inspection of the different elements of the substructure must involve the abutments,
pedestals, bearings, front-walls, wing-walls, piers, piercaps, dowels, and shear-keys. All visible
and accessible parts must be inspected, including the visible parts of the footings and caissons,
the embankments, and the approaches to the bridge.
Under-water inspections are required to assess the condition of the submerged zones of the
foundations. These inspections cannot be as frequent as the routine inspection; however, they
are useful in establishing any deterioration in the foundation units and scour in the river bed which
may affect the stability of the foundations and the structure.
7.3.1.2. Detailed Inspection
This inspection is undertaken based on the findings from the routine inspection. Depending on
the importance and degree of precision of the required detailed inspection, two main groups are
established: general inspection and major inspection.
7.3.1.2.1. General inspection
The general inspection must be performed annually. Their purpose is to verify the detailed
condition of each element of the bridge, identifying all possible flaws. The extent of the flaws
must be determined by on-field measurements, using the necessary equipment and standard
instruments. All of the flaws found on the different structural elements must be recorded
pictorially, and on suitable forms and sketches prepared before the inspection. Immediately after
this process, a written report on the conditions and performance of the materials and the
structural elements of the bridge must be prepared.
7.3.1.2.2. Major Inspection
The major inspection may be required based on the information acquired from the general
inspection. These inspections demand a detailed examination of any of the previously identified
affected, or deteriorated structural elements. These kinds of inspections may be necessary after a
period of time of about 5 years or less, depending of the degree of deterioration found on
previous inspections.
7.3.1.3. Special Inspection
After a major event like an earthquake, a flood, a hurricane, a tornado, an important overload, or
a major accident, such as an explosion, a complete evaluation of the bridge performance must be
MAINTENANCE STRATEGIES
128
carried out. This must include testing of materials and a detailed analysis of the entire bridge
structure. These actions must be specially focused on the most critical elements that constitute
the basic structural system of the bridge, including girders, piercaps, piers, abutments and
foundations.
Figure 6.1: Framework for decision-making in bridge management.26
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129
Immediately after the analysis is completed, a detailed report must be prepared, showing the
conclusions on all possible distress and the loss of load bearing capacity of the structure. The
report must also include all of the recommendations that must be implemented in terms of repairs
and strengthening, to restore the structural capacity of the bridge, and to provide additional
service life.
7.4. Corrective Maintenance
Any needed corrective maintenance must be defined as a result of the observations from the
different inspections performed on the structure. In this stage of maintenance strategies, it is
essential to define a framework based on the results of the evaluation of the quality of the
different construction materials and the performance of the various structural elements. A
tentative framework for decision making is shown in Figure 6.1.
Corrective maintenance involves repair, rehabilitation, and strengthening. The rehabilitation
process engages all necessary repairs that must be performed with the goal of restoring the
service levels, safety and serviceability of the bridge as closely as possible to the original
conditions.
Strengthening also involves the improvement of the load bearing capacity of the bridge at a
certain time, by increasing the strength and the stiffness of the relevant components of the
structural system, as closely as possible to the original capacity of the bridge.
The corrective maintenance procedure will normally be directed to different kinds of elements in
the structure. These elements are:
- Short-life elements, including pavement, waterproofing membranes and coatings, expansion
joints, elastomeric joint seals, membranes and coatings for the concrete elements exposed to
the most aggressive conditions, including barriers, cornices, drainage meshes, and pipes.
- Medium-life elements, including concrete deck slab, girders, diaphragms, bearings,
embankments, embankment protections, and protective islands.
- Long-life elements, including piercaps, piers, abutments, wing-walls, retaining walls,
abutments, and foundation units.
SUMMARY AND RECOMMENDATIONS
130
8. SUMMARY AND RECOMMENDATIONS
8.1. Bridge Design for Durability
To design a bridge structure for durability, it is essential that all the possible loads that could
affect the structure considered in the analysis, and general performance studies performed as an
initial stage of the design process. Consequently, it will be necessary to select construction
materials with sufficient resistance against all anticipated loads and other actions.
One important step during the design process is the definition of the mechanisms of deterioration
developed as the action of each microclimate that can be generated in different parts of the
structure due to the interaction of the macroclimates and the characteristics of the different
elements of the structure.
The various microclimates acting on the various structural elements can generate different
mechanisms of deterioration that can affect the bridge structure. These include freezing and
thawing cycles, carbonation, chloride ingress, chemical attacks (sulphate attack and acid attack),
alkali-aggregate reactions, biological attacks and steel corrosion. Normally, the mechanisms that
affect concrete act as the initiation phase of the deterioration of a reinforced concrete element.
The mechanism of corrosion of steel reinforcement takes place more rapidly, and consequently it
can be considered to be the propagation phase of deterioration.
The rates of these mechanisms of deterioration depend on how the ingress of moisture, air and
aggressive substances occurs into the concrete by different mechanism of transport, such as
capillary suction, diffusion and permeability. Wetting and drying cycles, and cracking of concrete
produced before or after hardening, can accelerate the rate at which these substances can
penetrate the concrete. There exist some mathematical models to describe the rate of the
mechanism of deterioration with time. Some models are more accurate than others and they
have been developed through extensive experimental and field experiences, and theoretical
studies and deductions from the different physical and chemical laws. Finally, these models allow
the prediction of the amount of deterioration with time, and hence these models have to be
evaluated while determining the concrete cover thickness. Additionally, these models should be
integrated in the different formulations of the traditional structural design principles and methods.
According to the information available after the analysis using these models, one important
objective of the structural design for durability is to provide the structure with appropriates
concrete cover thicknesses for the different elements, by ensuring a good quality of the concrete
(low permeability, high resistance to chemical attacks), a sufficient concrete cover thickness
(while also evaluating the risk of cracking depending on the thickness of this cover), and
SUMMARY AND RECOMMENDATIONS
131
controlling the executions process of concrete placing, compaction, vibration and curing during
construction; these must be implemented using a very high quality control.
The geometrical forms and shapes of the structure may affect the rate of deterioration, because
these forms can make some elements more sensitive to accumulation of aggressive agents and
increased deterioration. In these cases, different kinds of imperfections and low-quality levels
during construction can be generated, resulting in increased ingress of aggressive substances at
early stages of the service life of the structure. Additionally, the cost of construction, execution
and maintenance procedures could increase significantly.
It is necessary to acquire all necessary information and parameters to work with the different
models of deterioration. The precision of these parameters will determine the accuracy of the
estimation of the service life of the bridge.
According to the evaluation of the required design service life and the evolution of different types
of degradation in the structure, the design process could be oriented towards two different
philosophies: to avoid the degradation processes on the structure by changing the environment,
selecting non-reactive materials for concrete and steel, and by inhibiting the reactions by using
protective measures, such as cathodic protection for example. However, it is very difficult to
ensure that all protection measures will perform adequately during the service life. For this
reason, the second type of philosophy is based on considering the existence of the degradation
of the materials and therefore, selecting optimal material compositions and protective measures
to allow the structure to resist the degradation processes that take place during its service life.
For this second approach, the integration between the degradation models and the traditional
design techniques is necessary.
In conclusion, a design procedure for durability must consider: the identification of the
microclimates and mechanisms of deterioration, the definition of the geometry of the structure,
the determination of the composition of concrete, the concrete cover thickness, the type of
reinforcement to use, the definition of protective measures for concrete surfaces and steel bars,
the necessity of the limitation of crack development according to the concrete cover thickness of
the bridge elements, and the determination of the maintenance strategies during the service life
of the structure.
8.2. Holistic design approach
Civil engineers must follow a holistic durability design process to be able to design and build
sustainable and durable bridges. However, the holistic principle involves the interaction of many
considerations that can contribute to the creation of not only durable structures, but also
sustainable practices and procedures of design and construction, seeking a balance between the
SUMMARY AND RECOMMENDATIONS
132
economic aspects and the environmental impact. Sustainable development is not an option
anymore; it is a must to find a balance between the economical development and the protection
of the environment.
It is time now to define strength through durability rather than durability through strength in the
present and future bridge design practice. The durability of materials cannot be assured only by
focusing on the achievement of a specific strength. The extreme damage and early deterioration
caused by the present practice is manifested in many infrastructure assets around the world, just
because the environmental effects and the deterioration processes that take place on the
structures due to the environment were not considered in their design, construction and
maintenance.
It is important to consider the durability aspects as essential steps during the design process, with
the purpose of incorporating concrete with low permeability, low heat of hydration, low w/c ratio,
enough entrained air, and non-reactive and strong aggregates. This concentration on the
production of durable materials can be guided towards the production of the required strength
concrete for a specific bridge project. The design of the concrete mixtures involves the choice of
proper materials and other ingredients used for the preparing the concrete mixture, including
aggregates, water, type of cement, air, pozzolanic materials and other admixtures. Since the
design process should be holistic, adequate protections and the type of the reinforcement steel
should be considered as well, to be able to build durable reinforced concrete structures.
The holistic design of structures for durability must involve an integrated design procedure,
involving the resistance of materials, their degradation with time, principles of sustainability, life
cycle costing, and environmental protection. If these concepts are implemented in the
engineering and construction practices around the world, there will be a reduction of the amount
of waste of materials, and hence a lower consumption of the natural resources of the planet;
there will be a more effective and efficient use of the construction materials, which can be
reflected in benefits for the present society, and also for the future generations.
8.3. Basic Conclusions and Recommendations
The following basic conclusions can be drawn from this research work:
- A complete structural design for durability is essential to ensure the attainment of the required
service life.
- A holistic design approach is necessary to define an adequate durable design.
- Different interdisciplinary studies need to be coordinated within the holistic approach.
SUMMARY AND RECOMMENDATIONS
133
- The feasibility of a bridge project must be based on a life-cycle performance and cost
analysis, converting all future operation and maintenance costs into a present value.
- A thorough understanding of the macroclimatic and microclimatic conditions that surround the
bridge is necessary to simulate the decline of the bearing capacity of the bridge, and to
formulate the necessary supplementary protective measures.
- The Integration between structural design and materials engineering is basic to define the
time-dependant performance of the bridge structure.
- The structural design must be accompanied by a maintenance strategy for the owner or the
operator to ensure satisfactory performance of the bridge throughout its service life and
beyond.
134
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APPENDIX 1 – DURABILITY CALCULATIONS
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APPENDIX 1 – DURABILITY CALCULATIONS
The effects of the various mechanisms of deterioration for each bridge element are presented as
follows:
APPENDIX 1 – DURABILITY CALCULATIONS
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APPENDIX 1 – DURABILITY CALCULATIONS
138
APPENDIX 1 – DURABILITY CALCULATIONS
139
APPENDIX 2 – GIRDER DESIGN CALCULATIONS
140
APPENDIX 2 – GIRDER DESIGN CALCULATIONS
The different verifications of the various limit states considered for the girder design for durability
are presented for time intervals of five years.
APPENDIX 3 – MASS LOSS OF STEEL CAUSED BY CORROSION
141
APPENDIX 3 – MASS LOSS OF STEEL CAUSED BY CORROSION
The following Figures summarize the reinforcement mass loss due to chloride-induced corrosion
over a period of 100-year service life, for concrete cover thicknesses ranging from 25 to 70mm in
thickness, chloride concentrations at the concrete surface ranging from 1% to 6 %, a critical
chloride threshold of 0.4%, and a chloride diffusion coefficient of 1.5x10-12m2/s.17
APPENDIX 3 – MASS LOSS OF STEEL CAUSED BY CORROSION
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APPENDIX 3 – MASS LOSS OF STEEL CAUSED BY CORROSION
143