Post on 11-Jul-2016
description
UNIT-2
SERVICEABILITY AND DURABILITY OF CONCRETE
QUALITY ASSURANCE FOR CONCRETE CONSTRUCTION
INTRODUCTION
• Quality assurance scheme is management system
• Increase confidence that a material,product(or)service will confirm to special requirement policies designed responsibilities and
requirements the owner Controlling to pre determined requirements actives which influence quality
Quality assurance for concrete structures
• Types of structures guidelines as to permissible loading
• Periodic inspections maintenance cannot be ignored ,possible anticipated durability serviceability (or) functional efficiency
• Detail at time at final inspection and use of the structures
Need for quality appearance
• Construction and use of a concrete structures• Client requires• Designer depends• Material producer• Building contactor• Finally the user is rewards by a functional
efficient structure a good appearance
Causes of design faults
• Mis interpretation of the client needs• Lack of good communication between
members• Production ,inadequate and impressive
speciation's• Design standard (or) codes of practices• Use of incorrect or out of data
Causes of design faults in construction
• Mis interpretation of design drawings or speciation's
• Lack of effective communication with suppliers and sub contractors
• Inefficient co-ordination of sub contracted work
• Inadequate on site supervision• Poor workmanship
ACI committee
• Freezing • Aggressive chemical exposure• Abrasion• Corrosion of steel and other embedded
material• Chemical reactions of aggregate
PROPERTIES OF CONCRETE
Strength of concrete
• The strength of the concrete is resistance and rupture
• It may be measured in a number of way such as strength in compression strength in tension strength in shear in flexureAll these indicate strength with reference
particular method of testing
Permeability of concrete
• Volume change and cracking of concrete and durability of concrete structures
• Primary responsible for affecting durability ,permeability of concrete
• Concrete resists external attack• Aggregate of low permeability into cement paste at reducing
permeability of the system• Large size of aggregate increase the permeability• Air entrainment makes the concrete process upto 6%makes of
concrete• Crushed silica decrease the permeability
Thermal properties
• To study of thermal properties of concrete is an dealing with durability of concrete
• Concrete material used all climatic regions all land of structures
• Properties of concrete are thermal conductivity thermal diffusivity specific heat coefficient of thermal expansion
• Thermal conductivity material conduct heat it is measured joule per second per square meter values of
conductivity of concrete made with different aggregate limestone – 3.2 sandstone -2.9Thermal diffusitivity it represent the rate which temperatureChange concrete massDiffusitivity =conductivity/cpWhere c =specific heat p =density of concrete range of diffusitivity =0.002 to 0.006m^2/h
• Specific heat quality of heat required to raise the
temperature unit mass of a material by one degree centigrade ranges of concrete 840 to 1170
Coefficient of thermal expansion ratio between change in unit length per
degree change in temperature
Cracking
• Cracking will occur the tensional strain which occur exceeds the tensional strain capacity of concrete
• Tensional strain capacity of concrete varies the age of rate of application of strain
Effects of cracking
• Immediate structural distresslong run to a reduction of through corrosion of steel
• Mal function of the structure as evidence • Cracks leading structural failure• Cracks causing corrosion• Cracks affecting function• Cracks affecting appearance
Cracks in concrete`
Before hardening• Drying plastic shrinkage settlement shrinkage bleeding curingConstructional cracks formwork excess vibration finishingEarly first damage
After hardening• Un sand material• Long term drying shrinkage• Thermal• Moisture movement• Tensile zone• Structural design
deficiencies• Chemical• Corrosion of reinforcement
Types of crack
• Delayed curing• Formwork movement• Excess vibration• Sub grade settlement• Finishing• Early frost damage• Unsound materials\
Cracking in plastic concrete• The removal of water from
the top surface by evaporation exceeds the rate at which bleed water is coming to the surface
• Early shrinkage of concrete
Cracking in hardened concrete• Structural response to
applied loads and external displacement
• The interstice nature of the concrete and is constituent materials
EFFECT DUE TO CLIMATE
Concrete can be protected from freeze-thaw by the
entertainment of appropriate quantities of air distributed
through the cement paste, with spacing between bubbles
of not more than about 0.4mm
For effective protection air entraining agents must be
added to the mix
The amount of air required is between 4-8% depending
on size of aggregate
FREEZING AND THAWING
If the concrete has been made more dense without any
capillary pores , the resistance to freeze and thaw will exist
without any air entrainment.
Particular form of frost action is known as D-cracking or
Durability Cracking
Caused by the coarse aggregate and air-entrainment, which
does not therefore provide protection
Carbon dioxide from air penetrates into concrete an reacts
with calcium hydroxide to form calcium carbonates
One of the reason for corrosion of reinforcement
Carbonated concrete is less permeable
Alkalinity of the concrete reduced when carbonation takes
place therefore protection against corrosion gets reduced
So its important to ensure quality and thickness of concrete
cover
CARBONATION
Factors affecting Rate of Carbonation
The level of pore water
Grade of concrete
Permeability of concrete
Whether the concrete is protected or not
Depth of cover
Time
Highest rate of carbonation occurs at
relative humidity of between 50-70%
Effects Due to Temperature
Removal of evaporable water
Removal of combined water
Alteration of cement paste
Disruption from disparity of expansion and resulting
thermal stress
Alteration of aggregate
Change of the bond between aggregate and paste
Effect due to Chemical Attack
High porosity and hence permeability and absorption
Improper choice of cement type for the conditions of
exposure
Inadequate curing prior to exposure
Exposure to alternate cycles of wetting and drying and to a
lesser extent heating and cooling, with allowance for the fact
that higher temperatures increases reactivity
Increased fluid velocity
Expansive reactions of any sort which may cause
cracking and any other physical phenomena, which
lead to greater exposure of reactant surfaces
Suction forces
Unsatisfactory choice of shape and surface to
volume ratios of concrete sections
• A concrete is said to be durable if it withstands the conditions for which it has been designed, without deterioration over a period of years
• Resistance of a concrete to variety of physical or chemical attacks due to external causes or by the internal causes
Durability of a structures
CORROSION
Definition of Corrosion
Reinforced concrete – weakness
Characteristic of concrete
Stages of corrosion
Corrosion of steel
reinforcement
Principles and mechanism of
Corrosion
Causes of corrosion in reinforced concrete
Carbonation
Chloride attack
Factors Influencing Corrosion
Effects due to
corrosionDamages induced by corrosion
Preventive methods
corrosion rate
monitoring
Corrosion is the deterioration of materials by chemical interaction with their environment.
The term corrosion is sometimes also applied to the degradation of plastics, concrete and wood, but generally refers to metals.
Definition of Corrosion
Reinforced concrete - weakness
• Statics indicates that 40% of structure fails due to corrosion of reinforcement
• susceptible to environmental attack– deterioration accelerates under
humid conditions– corrosion leads to spalling of
concrete– reinforcement is exposed– bond reduces– strength reduces
Characteristic of Concreteconcrete provides chemical protection to the
steel with the formation of highly alkaline pore solution.
The highly alkaline in the concrete promotes the formation of an adhering passive film in the surface of the steel, which protects it from corrosion.
Characteristic of concrete
Corrosion
Stages of corrosion
• Rust staining • Bulging of cover (collection of rust products)• Micro cracks• Macro cracks• Spalling of cover concrete
Corrosion of steel reinforcement
Corrosion of steel reinforcement Penetration of unfavourable chemical species
can destroy the protective environment and eventually cause corrosion of the steel.
Significant chemical species which can break down the passive film on the steel are carbon dioxide and chlorides.
• Concrete is a high alkalinity material. • The pH of new concrete is 12 to 13.• In this range of alkalinity, embedded steel is protected
from corrosion by a passivating film ( gamma iron oxide film) bonded to the reinforcement surface.
• However when the passivating film is disrupted, corrosion may take place.
• Reinforcement corrosion can get initiated through one or more effects from the environment.
Principles and mechanism of Corrosion
Causes of corrosion in reinforced concrete
• Corrosion Due to Carbonation• Corrosion Due to Chloride Ion Penetration
Carbonation CO2 from air penetrates into concrete In the absence of moisture CO2 is non reactive In the presence of moisture CO2 diluted carbonic acid it reacts with Ca(OH)2 to form CaCO3
shrinkage It also reduces alkalinity of concrete detoriates passivating film corrosion of steel reinforcement
chloride attack
• Sulphate attacks the concrete, chloride attacks the steel
• Sources of chloridecementwaterAggregateSometimes from Admixtureproviding from marine atmosphere
Chloride attack
• Amount of chloride required to initiate corrosion depends on the pH of pore water
• pH < 11.5 corrosion occur without chloride• pH > 11.5 chloride is needed for corrosion• Concrete contains chloride
In insoluble form (chlorialuminates)does not cause corrosion
In soluble form causes corrosion
• Electrochemical process (transfer of electrons as a result of chemical Reactions)
Corrosion = Iron + Oxygen + Moisture• Difference in electrical potential along steel
reinforcement in concrete, an electrochemical cell is set up
• One part of steel anode• Another part of steel cathode• pore water Electrolyte
Electrochemical corrosion in the presence of chloride
Reactions in electrode
• The anode is the site of the oxidation of the steel. The reaction at the anode is expressed as:
Fe → Fe2++ 2e-
• The cathode is the site of the reduction reaction. The liberated electrons are used at the cathode
• Positively charged ferrous ions at anode passes into the solution
• Negatively charged free electrons passes through steel into cathode
Corrosion in steel
• Anode Reaction Fe → Fe2++ 2e-
• Cathode Reaction 2e- + H2O + ½ O2 → 2 OH-
H2O H2OO2 O2
Cl- Cl-
OH- OH-Cathode Cathode
Anodee- e-
Fe2+
rust rustCl-
Cl-
Cl-
Cl-
Cl-Cl-
Cl-
• These electrons are absorbed by electrolyte and react with water and oxygen to form hydroxyl ion Fe2+ + 2OH- → Fe(OH)2 {Ferrous hydroxide}
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3 {ferric oxide} 2Fe(OH)3 → Fe2O3•H2O + 2H2O
Fe2O3•H2O {hydrated ferric oxide}
Patch Accelerated Corrosion
Chloride-Free Patch
Potential Difference Between Patch and Chloride Contaminated Concrete Results in Accelerated Corrosion
Chloride Contaminated Concrete
© Vector
Effect due to cover thickness
• Cover thickness is the physical production• If larger cover thickness is provided then the time
taken for the penetration of chemicals and other agent is high
• If thickness of cover is less the penetration will occur in short period
As per IS
S.No Exposure conditions Nominal concrete cover in mm not less than
1 Mild 20
2 Moderate 30
3 Severe 45
4 Very severe 50
5 Extreme 75
As per IS 456-2000nominal cover to meet durability requirement
Factors Influencing Corrosion
• Cover thickness• Quality of concrete in the cover region• Environmental conditions• pH value of concrete • Chloride level in concrete• Presence of cracks
• the cross sectional area of the reinforcement is reduced, which in turn reduces the load carrying capacity of the steel
• products of the corrosion, iron oxide (rust), occupy a volume about 6 times the volume of the original steel depending upon the oxidation rate.
• This increase in volume leads to cracking and ultimately spalling of the concrete.
• produces internal stresses in concrete
Effects due to corrosion
cracking
Damages induced by corrosion
Spalling
delamination
Prevention of Metal Corrosion
• Use of concrete with low permeability• Use of properly proportioned concrete having low
W/C ratio• Use of good workmanship in placing the concrete• Curing the concrete properly• Providing adequate cover to the steel reinforcement• Limiting chlorides in the concrete mix• Providing good drainage to prevent water from
standing on the concrete
Preventive methodsTo Prevent Corrosion Following methods areadopted Metallurgical method Corrosion inhibitors Coatings to reinforcement Coatings to concrete Cathodic protection Design & Detailing
• Steel is made to be more corrosion resistant by altering its structure through metallurgical process
• This can improve mechanical and corrosion resistant property of steel
• Stainless steel can be used
Metallurgical method
• This is chemical method• Chemical inhibitors
Nitrites (calcium nitrite)PhosphatesBenzoates
• Inhibitors are mixed with concrete during mixing
corrosion inhibitor
Corrosion Inhibitors
• In the oil production and processing industries, inhibitors have always been considered to be the first line of defense against corrosion.
• Rules, equations or theories to guide inhibitor development or use are very limited.
• Reducing the movement or diffusion of ions to the metallic surface
• Increasing the electrical resistance of the metallic surface.
• Types:Cathodic Inhibitors - Amine based salts (NH2)Anodic Inhibitors - Nitrite based salts (NO2)passivating InhibitorsOrganic InhibitorsPrecipitation InhibitorsVolatile Inhibitors
types of inhibitors
Advantages:• Initial lab testing showed promise for reducing
corrosion potentials.
Disadvantages:• Field testing has shown low bond strengths, no
reduction in corrosion potential. • High cost to apply
advantage and disadvantage
Coating to reinforcement•Coating should withstand pouring of concrete and compaction by vibrating needle
•to provide a durable barrier to aggressive materials
• The principle involved in the cathodic protection is to force the metal to behave like a cathode.
• It consist of application of impressed current to an concrete above steel reinforcement.
• This serves as anode and steel serves as cathode• Steel is connected to negative terminal of DC source• When current is applied external anode will get corroded• So the cathode is prodected from corrosion
Cathodic production
• In this method the negatively charged chloride ions which are causing damages to the passivating film are drawn away from the vicinity of steel towards anode
• In anode chloride ions are converted to chlorine gas because of oxidation process
• The environment around the steel reinforcement reverts back to alkaline condition which products the steel
• Cathodic protection is a technique by which the electrical potential of the steel is increased to a level at which corrosion cannot take place.
Cathodic protection is used to protect :
• Pipelines
• Storage tank bases
• Harbour structures
• Steel sheet, tubular and foundation pilings
• Offshore platforms, floating and sub sea structures
• Ship’s tanks
Applications of Cathodic production
• Design errors may be divided into two general types:
those resulting from inadequate structural design.
those resulting from lack of attention to relatively minor design details.
design errors
• Mechanism:The concrete is exposed to greater stress than it is capable of
carrying or it sustains greater strain than its strain capacity.• Symptoms:
Visual examinations of failures resulting from inadequate structural design will usually show one or two symptoms.
1) Errors in design resulting in excessively high compressive stresses will result in spalling. Similarly, high torsion or shear stresses may also result in spalling or cracking.
2) High tensile stresses will result in cracking. To identify inadequate design as a cause of damage, the locations of the damage should be compared to the type of stresses that should be present in the concrete.
In adequate structural design
Inadequate design is best prevented by thorough and careful review of all design calculations.
Prevention
• While the structure may be adequately designed to meet loadings and other overall requirements, poor detailing may result in localized concentrations of high stresses in concrete.
• These high stress may result in cracking that allows water or chemicals access to the concrete.
• In other case, poor design detailing may simply allow water to pond on a structure resulting in saturated concrete.
Poor design detailing
• Generally, poor detailing does not lead directly to concrete failure, rather it contributes to the action of one of the other causes of concrete deterioration.
Abrupt change in section:It may be cause stress concentrations that
may result in cracking.Insufficient reinforcement at reentrant corners and
openings:The best prevention is to provide additional
reinforcement in areas where stress concentrations are expected to occur.
Inadequate Provision for Deflection:Deflection in excess of those anticipated may
result in loading of members or selection beyond the capacities for which they are designed.
Typically, these loadings will be induced in walls or partitions resulting in cracking.
Inadequate Provision for Drainage:Poor attention to the details of draining a
structure may result in the ponding of water. It results in leakage or saturation of concrete.
Insufficient travel in expansion joints:Inadequately designed expansion joints may
result in spalling of concrete adjacent to the joints.
In compatibility of materials:The use of materials with different
properties adjacent to one another may result in cracking or spalling.
Failure to follow the specified procedures and good practice or outright carelessness may lead to a number of conditions that may be grouped together as construction errors.
Typically, most of these errors don't lead directly to failure or deterioration of concrete. Instead, they enhance the adverse impacts of other mechanisms.
Construction errors
• Adding water to concrete• Improper alignment of formwork• Improper consolidation• Improper curing• Improper location of reinforcing steel• Premature removal of shores or reshores• Settling of the concrete• Setting of the sub grade• Vibration of freshly placed concrete• Improper finishing of flatwork
corrosion rate monitoring
Non-destructive corrosion rate monitoring
• Electrochemical methods1. Static measurements
– Half-cell potential measurements– Hand held equipment– Embedded reference electrodes Corrosion macro cell current
measuring Electrochemical noise
2. Polarization measurements– Linear polarization method– Hand held equipment– Embedded linear polarization sensors– Electrochemical impedance spectroscopy– Localized electrochemical impedance spectroscopy– Galvanostatic pulse method– Scanning reference electrode method
• Nonelectrochemical methods1. Visual inspection2. Seismic method3. Infrared thermography4. Acoustic emission5. Radiography and radiometry Radar6. Electrical resistivity of concrete7. Electrical resistance method8. Optical fibber sensors Magnetic technique9. Microwave based Thermoreflectometry
POLARIZATION TEST SET-UP
Specimens are ready for polarization test
Polarization Test Setup
Specimens shows after crack initiation and propagation under polarization test
Experimental set up
Regulated DC supply
Fibre reinforced tub
RCC Beam
Salt solutionStainless steel plate
Steel bar
Half Cell Potential (HCP)
Schematic showing the half cell potential measurement technique
THANK YOU