Durability of RC Structures
-
Upload
macovei-alin -
Category
Documents
-
view
227 -
download
0
Transcript of Durability of RC Structures
-
7/26/2019 Durability of RC Structures
1/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DUR BILITY OF
REINFORCED
CONCRETE
STRUCTURES
-
7/26/2019 Durability of RC Structures
2/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Durability is the ability of constructions to preserve their performances on
the entire duration of service.
The performances represent quantitative expression of user requirements
on the characteristics required in service (strength, stability, safety in
service, fire safety, economy, acoustic & visual comfort, etc.)
The durability of concrete is its property to resist on any deterioration
processes during service (mechanical action, chemical actions, climate
actions, abrasion etc.). A durable concrete is one that keep, with minimal
maintenance costs, initial form, features and functionality throughout theservice.
The degradation of a material or construction element is defined as any
negative change of the physical and / or chemical properties, who affect the
performance criteria of the building.
BUILDING DEGRADATIONS. CAUSES
-
7/26/2019 Durability of RC Structures
3/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Degradations can be visible or hidden.
Degradations can appear both on structural and non-structural elements.
Designing and execution deficiencies can be considered initial degradations
of the building.
The level of degradation for a building is inversely proportional to its safety
level.
-
7/26/2019 Durability of RC Structures
4/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
5/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Actions are defined as causes of any kind capable of generating
degradation in a building. Actions can be classified as follows:
mechanical actions which produce mechanical stresses being represented
usually by force systems;
physical actionsthat produce changes in the integrity of materials and / or
components but without altering the chemical structure;
chemical and biological actions that produce changes in the chemical
structure of materials.
In the design codes (Eurocode 1), the term action is usually used for
mechanical actions.
Physical, chemical and biological actions are also named corrosive actions.
-
7/26/2019 Durability of RC Structures
6/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
CLASSIFICATION OF MECHANICAL ACTIONS
-
7/26/2019 Durability of RC Structures
7/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
A. Actions from construction:
Loads (dead loads, live loads)
Corrosive actions:
Alkali-aggregate reaction
Expansive cements
B. Actions from natural environment:
Climatic actions:
Humidity
Temperature variations
Freeze-thaw
Snow
Wind
-
7/26/2019 Durability of RC Structures
8/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Corrosive actions:
Air
Water
Marine environment
Aging (creep, shrinkage)
Biological actions
Exceptionally actions:
Earthquake, fire, hurricanes, flooding
Soil failure
C. Actions from industrial environment:
Corrosive actions
Polluted air
Polluted water
-
7/26/2019 Durability of RC Structures
9/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Exceptionally actions:
Fires
Shock
D. Actions resulted in designing/execution process
Accepted risk
Insufficient knowledge
Ignorance
Mistakes
Blunders
Malevolence
-
7/26/2019 Durability of RC Structures
10/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
BUILDING/ENVIRONMENT RELATIONSHIP
-
7/26/2019 Durability of RC Structures
11/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
NATURAL AIR
-
7/26/2019 Durability of RC Structures
12/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
13/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
MARINE ENVIRONMENT
Gases in sea water reveal the ways in which a variety of physical, chemical,
and biological processes interact in the oceans and coastal environments. A
series of reactive trace gases found in sea water include methane, carbon
monoxide, nitrous oxide, hydrogen sulfide, and hydrogen. These gases are both
produced by and consumed by various types of organisms. The marine
environment can be a source of these gases to the atmosphere.
Action of marine air is complex due to the large number of aggressive ions,
but less intense than if these ions were acting on simple solutions.
O O O
-
7/26/2019 Durability of RC Structures
14/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Gas Chemical Symbol Percentage in AirPercentage in Sea
Water
Nitrogen N 2 78.08 62.6
Oxygen O 2 20.95 34.3
Argon Ar 0.934 1.6
Carbon Dioxide CO 2 0.033 1.4
Neon Ne 0.0018 0.00097
Helium He 0.00052 0.00023
Methane CH 4 0.00020 0.00038
Krypton Kr 0.00011 0.00038
Carbon Monoxide CO 0.000015 0.000017
Nitrous Oxide N 2 O 0.000050 0.0015
Xenon Xe 0.0000087 0.000054
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
15/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
16/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
POLLUTED AIR FROM INDUSTRIAL
ENVIRONMENT
Due to industrial activity, in the atmosphere is discharged a large amounts
of pollutants from the various industries.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
17/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
18/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
AGGRESSIVE WATER FROM SOIL
Soil water is aggressive because of the dissolution of substancescontained in the soil and acid rain. Another important source is the infiltration
of polluted wastewater.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
19/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
20/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
AGGRESSIVE WATER FROM SURFACE
Surface waters, both from the seas, lakes or rivers contain dissolved
substances that have chemical aggressivity.
In addition to chemical corrosion, water has physical effects on concrete
(erosion by abrasion).
Erosive action of water usually appear to hydraulic structures and is not
the subject to this lecture.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
21/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Characteristic degradation of a concrete bridge pylon (Pimpama River,
Queensland, Australia), occurs as the sulfuric acid from acid sulfate soils
attacks the carbonate in the concrete.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
22/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
RHEOLOGICAL PHENOMENAThe shrinkage represents a time dependent deformation which reduces
the volume of concrete, without the impact of external forces. The time flow
and the final values of shrinkage are influenced by numerous factors:temperature and humidity, dimensions of elements, the type and quantity of
cement, w/c factor, aggregates, concrete strength, method of workability and
curing, concrete age at the end of curing and many other factors.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
23/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Concrete creep is the tendency to deform under the influence of
mechanical stresses. Creep is a deformation mechanism that may or may not
constitute a failure mode. For example, moderate creep in concrete is
sometimes welcomed because it relieves tensile stresses that might otherwise
lead to cracking.
Unlike brittle fracture, creep deformation is a result of long-term stress.
Therefore, creep is a "time-dependent" deformation.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
24/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
CAUSES OF CONCRETE DEGRADATION
Degradation of concrete can appear from internal causes (due to
internal processes), or external causes (which are due to environmentalfeatures).
Internal causes are able to initiate concrete destruction by chemical
or physical processes occurring in concrete mass. They can be grouped as
follows:
expansion of harmful chemical components that are in excess
on cement, such as calcium and magnesium oxides or sulfur
trioxide;
differentiated mechanical stresses that can be caused by large
variations in temperature (particularly between the outer
surfaces and the interior of massive elements);
aggregates with the thermal coefficient of expansion different
from that one of the cement stone (the changes in volume
under the action of temperature provoke internal stresses);
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
25/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
alkaliaggregate reaction who is a term mainly referring to a reaction
which occurs over time in concrete between the highly alkaline cement
paste and non-crystalline silicon dioxide, which is found in many common
aggregates; this reaction can cause expansion of the altered aggregate,
leading to spalling and loss of strength of the concrete.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
26/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
External causes can be: mechanical, physical, chemical, and
biochemical.
Mechanical causes are composed by static and dynamic loads
(short or long term), fatigue, etc.
Physical degradation of the concrete under the influence of
mechanical action mainly consist of cracking process.
Cracks causes many problems in reinforced concrete elements
because they fragments the internal structure, reduce the stiffness and the
active area of concrete. At the same time, cracks allow the penetration of
aggressive substances into the element, which affects not only the
concrete but also the embedded reinforcement.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
27/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Physical degradation of concrete under repeated freeze-thaw
action is a problem of wet concrete. The degradation of concrete occurs
due to the pressure that arises in its mass, due to the increasing volume of
frozen water contained in the structure.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
28/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Chemical and biochemical factors induce degradation by
decalcification of cement stone and / or expansion processes.
Concrete deterioration is rarely produced by a single cause. In
practice, the concrete degradation occurs due to simultaneous action of
several factors.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
29/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Carbonation occurs in concrete because the calcium bearing phases
present are attacked by carbon dioxide of the air and converted to calcium
carbonate.
The carbon dioxide along with water, forming a solution of carbonic
acid which, although weak and unstable, under favorable circumstances can
react with cement paste. In the presence of high humidity, CO2 is chemically
aggressive, destroying any cement.
The carbonation process requires the presence of water because CO2
dissolves in water forming H2CO3. If the concrete is too dry (RH 90%) CO2cannot enter the concrete and the concrete will not carbonate.
Optimal conditions for carbonation occur at a RH of 50%...60% (range 40-90%).
CARBONATION OF CONCRETE
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
30/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
31/175
The carbonation of concrete will not affect plain concrete. Carbonation
will increase mechanical strength, modulus of elasticity. The areas of carbonated
concrete are usually small and the increased resistance is generally low and the
benefits are irrelevant in practice.
Cement paste contains 25-50 % calcium hydroxide, which mean that
the pH of the fresh cement paste is at least 12.5. The pH of a fully carbonated
concrete is about 8. Under these conditions, concrete can not provide the
necessary protection for reinforcements, which are susceptible to corrosion
phenomenon.
Concrete carbonation does not automatically produce corrosion of
reinforcement. If you meet certain conditions of moisture, even if the concrete is
carbonated reinforcement can not corrode.
The detection of the carbonated concrete can be made using a
phenolphthalein solution.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
32/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
33/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
34/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
35/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
36/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
37/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
38/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
39/175
CHLORIDE ATTACK ON CONCRETE
Chloride attack is one of the most important aspects for
consideration when we deal with the durability of concrete. Chloride attack is
particularly important because it primarily causes corrosion of reinforcement.
Statistics have indicated that over 40% of failure of structures is due to
corrosion of reinforcement.
The destructive effect of the dry chlorine gas is relatively weak on
concrete, but the wet gas is extremely harmful and it manifested as an
aggressive gas both to concrete and steel reinforcement.
Chloride ions diffuse through the concrete without change the pH of
concrete.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
40/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
41/175
The depth of penetration of chloride ions depends on their
concentration at the surface of the concrete and the variations of moisture. In
periods of high humidity, large amounts of chlorine enters into concrete. When
the humidity decrease, the water evaporates but the chlorine ions remain.
Through this process, the content of chloride ions may increase significantly.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
42/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
43/175
Given the action of chloride attack, the density of concrete
becomes an important influencing factor on the rate of its deterioration:concrete with smaller pores and lower pore connectivity will absorb less
water or vapour and inhibit its transport thus slowing down the ingress of
chlorides into the structure.
The physical condition of surface concrete plays an important role
in the rate of deterioration. Where there is existing surface damage
particularly in the form of abrasions, cavities or other impact damage the
resultant cracks serve to speed up the transportation of moisture and ionsto the steel which amplifies the rate of corrosion. Freeze thaw cycles can
then exacerbate the process further.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
44/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
45/175
SULPHUR ATTACK ON CONCRETE
Corrosion by sulphur compounds is usually founded in the
industry.
The action of the sulphur compounds consist of replacement of
the OH ion (from the Ca(OH)2) by the SO4 ions.
Sulfuric acid reacts first with the calcium hydroxide and form thegypsum. In the first stage, the gypsum formed with volume expansion,
filling the pores of the concrete and compacting it into a layer of variable
thickness.
If the corrosive action continues, other constituents of cement
stone (tricalcium aluminate) reacts with the sulphate ions, forming
ettringite.
Both products (gypsum and ettringite) have volume expansion in
concrete (the ettringite expands by 2.5 times) and the concrete is peeling.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
46/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
47/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
48/175
Concentrated sulfuric acid has no influence on the
reinforcements but diluted sulfuric acid heavily corroded them.
Sulfuric acid react with steel only at high humidity (over 75%) and
initially form ferrous sulphate. Ferrous sulphate is converted in rust and
the sulfuric acid is released at the end of the reaction. Released sulfuric
acid attacks a new zone of steel and the process will continue. A massive
corrosion of the reinforcement can be produced even under the action of a
small amount of sulfuric acid.
If the reinforcement is already corroded (due to other causes) and
the sulfur dioxide is absorbed by existing rust, a ferrous sulphate results
from the reaction. The ferrous sulfate is converted in sulfuric acid, and the
reaction will continues.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
49/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
50/175
The most active compounds of the nitrogen are nitric acid, ammonia
and ammonium nitrate.
In the first phase, nitric acid react with surface layer of concrete by
forming calcium nitrate that is soluble and peeling.
The reinforcement is not attacked by concentrated nitric acid but is
corroded by the diluted one. The action of the ammonium nitrate is manifested
both in corrosion and by embrittlement and breaking of the crystalline structure.
NITROGEN ATTACK ON CONCRETE
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
51/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
52/175
PARTICULARITIES REGARDING CORROSION OF
PRESTRESSING REINFORCEMENTS
Hydrogen embrittlement is the process by which high-strength steel,
become brittle and fracture following exposure to hydrogen. Hydrogen
embrittlement is the result of the corrosive process and, at the same time, a big
static load.
The mechanism starts with a locally corrosion of the reinforcement thatform a fragile layer on the surface of the metal oxide, accompanied by the
release of hydrogen. The hydrogen atoms diffusing through the metal and, when
they re-combine in minuscule voids of the metal matrix to form hydrogen
molecules, they create pressure from inside the cavity where they are located.
This pressure can increase to levels where the metal has reduced ductility and
tensile strength, up to the point where it cracks open.
A new corrosive process occurs in the crack accompanied by the release
of hydrogen ions and the process will continue until failure.
-
7/26/2019 Durability of RC Structures
53/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
54/175
The failure is brittle and occurs without warning.
The hydrogen embrittlement appear only when two conditions are met
simultaneously: mechanical stress in reinforcement and corrosive attack by a
specific agent.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
55/175
CORROZIVE ACTIONS ON CONCRETE
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
56/175
-
7/26/2019 Durability of RC Structures
57/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
58/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
59/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
60/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
61/175
ACCIDENTAL ACTIONS
-
7/26/2019 Durability of RC Structures
62/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
63/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
64/175
Windsor Tower, Madrid, Spain
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
65/175
A typical floor was two-way
spanning 280mm deep waffle slabsupported by the concrete core,
internal RC columns with additional
360mm deep steel I-beams and
steel perimeter columns.
The Windsor Tower was a 32-
storey concrete building with a
reinforced concrete central
core.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
66/175
Wi
ndsor
Tower
Madrid
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
67/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
68/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
69/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
70/175
ASSESSMENT METHODS OF MECHANICAL
CHARACTERISTICS OF CONCRETE AND
REINFORCEMENTS
Investigations must be performed with an adequate complexity to
assess the safety level of the structure with satisfactory accuracy. At the
same time, the assessment cost must justify the proposed solution.
The complexity of the investigations is established according with:
type and characteristics of the structure;
causes and spreading of damages;
the importance class of the building;
technical equipment available for investigations;
availability of standard and norms.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
71/175
The investigations can be achieved
by:
simple tests on site,
performed by a trained
person with simple tools;
complex investigations
that are carried out by
qualified personnel using
special equipment;
complex investigations
can be made on site or in
the laboratory.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
72/175
SIMPLE TESTS
Visual examination
Acoustic impact
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
73/175
Reinforcement inspection
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
74/175
Testing of alkalinity (with phenolphthalein)
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
75/175
Testing of chloride ions (with silver nitrate)
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
76/175
Complex investigations are carried out by qualified personnel using
special equipment. Complex investigations can be made on site or in the
laboratory.
COMPLEX INVESTIGATIONS
VISUAL INSPECTION OF INACCESSIBLE AREAS
A borescope is an
optical device consisting of a
rigid or flexible tube with an
eyepiece on one end, an
objective lens on the other
linked together by a relay
optical system in between.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
77/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
78/175
The assessment of mechanical characteristics of concrete and
reinforcements can be made with:
direct methods, who establish directly the strength of the
material;
indirect methods, who establish other properties of the material
(such as hardness) and the strength can be estimated lateraccording with these properties.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
INDIRECT METHODS FOR STRENGTH ASSESSMENT
-
7/26/2019 Durability of RC Structures
79/175
ULTRASONIC PULSE VELOCITY TEST
Ultrasonic Pulse Velocity (UPV)
testing of concrete is based on the pulse
velocity method to provide information on
the uniformity of concrete, cavities, cracks
and defects. The pulse velocity in a material
depends on its density and its elastic
properties which in turn are related to the
quality and the compressive strength of the
concrete. It is therefore possible to obtain
information about the properties of
components by sonic investigations.
INDIRECT METHODS FOR STRENGTH ASSESSMENT
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
80/175
Ultrasonic pulse velocity (UPV)
test instrument to examine the quality of
concrete and other materials such as rock,
wood and ceramics.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
81/175
IMPACT ECHO TEST
Impact Echo is a method for
nondestructive evaluation of concrete andmasonry. It is based on the use of impact-
generated compression waves that travel
through the structure and are reflected by
internal flaws and external surfaces.Impact Echo can be used to measure the
thickness of slabs, plates, columns and
beams, and hollow cylinders. It can also be
used to determine the location and extentof flaws such as cracks, delaminations,
voids, honeycombing and debonding in
plain, reinforced and post-tensioned
concrete structures.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
82/175
SURFACE WAVE TEST
This method appliesthe mechanical surface waves,
to investigate the medium.
Surface waves penetrate the
medium from the surface down
to a depth of approximately one
wave length. This means that by
using different wave lengths itwill be possible to investigate
the medium to different depths
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
83/175
SCLEROMETER TESTS ON CONCRETE
The sclerometer tests are NDTs that allow the estimation of the
concrete quality on-site. The tests use a Schmidt sclerometer that
measures the superficial hardness of the concrete from the recoil of an
incident mass after the collision with the surface being tested. This recoil is
then converted to a value of compression resistance through an abacus.
The test must be carried out on homogeneous concrete surfaces and it
usually involves the removal of the carbonated superficial layer by scraping
before.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
84/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
85/175
WINDSOR PROBE TEST
The Windsor probe test
is used to evaluate the
compressive strength of in-place
concrete. This non-destructive
test can be used on fresh or
mature concrete with equal
effectiveness. The system
features an electronic measuring
device for accuracy and
efficiency.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
86/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
87/175
COMPLEX INVESTIGATIONS
DIRECT ON SITE METHODS
FOR STRENGTH ASSESSMENT
Direct methods can establish directly the strength of the material by
measuring the force required to provoke failure of a small area of the element.
Because the concrete has a good behavior in compression, usually the
direct methods establish the strength of concrete in tension.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DIRECT METHODS FOR STRENGTH ASSESSMENT
-
7/26/2019 Durability of RC Structures
88/175
DIRECT METHODS FOR STRENGTH ASSESSMENT
PULL OUT TEST
A pullout test measures the force
required to pull a specially shaped steel rod
out of the hardened concrete into which it
has been cast. Because of its shape, thesteel rod is pulled out with a cone of
concrete whose surface slope is
approximately 450 to the vertical. A hollow
tension ram bearing on the concrete
surface exerts the necessary pull on the
steel rod, with power supplied by a hand-
operated hydraulic pump.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
89/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
90/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
91/175
The pull-off test is a near-to-
surface method in which a circular
steel disc is glued to the surface of
the concrete with an epoxy or
polyester resin. The force required to
pull this disc out from the surface,
together with an attached layer of
concrete, is measured. Simple
mechanical hand-operated loading
equipment has been developed for
this purpose.
PULL OFF TEST
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
92/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
BREAK-OFF TEST
-
7/26/2019 Durability of RC Structures
93/175
A direct assessment on
strength can be made by core
sampling and testing. Cores are
usually cut by means of a rotary
cutting tool with diamond bits. In this
manner, a cylindrical specimen is
obtained usually with its ends being
uneven, parallel and square and
sometimes with embedded pieces of
reinforcement.
BREAK OFF TEST
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
94/175
-
7/26/2019 Durability of RC Structures
95/175
-
7/26/2019 Durability of RC Structures
96/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
METHODS TO DETECT REINFORCEMENTS
-
7/26/2019 Durability of RC Structures
97/175
ELECTROMAGNETIC METHOD
The profometer is an advanced
cover meter for the precise and non
destructive measurement of
concrete cover and rebar diameters
and the detection of rebar locations
using the eddy current principle with
pulse induction as the measuring
method.
METHODS TO DETECT REINFORCEMENTS
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
98/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
RADIOGRAPHIC TESTING
-
7/26/2019 Durability of RC Structures
99/175
RADIOGRAPHIC TESTING
Radiography can be used to
obtain permanent image of surface and
sub-surface (embedded) discontinuities.
With concrete radiography, precise
locations of rebar, cable and metal
conduit can be detected prior to core
drilling or saw cutting. This can verify
size and spacing of reinforcement in
concrete.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
100/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
101/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
102/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
103/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
RADAR TESTING
-
7/26/2019 Durability of RC Structures
104/175
Radar test is a high-frequency electromagnetic method that can
be commonly applies to a number of engineering problems associated
with both new and aging concrete structures.
A GPR system radiates short pulses of high-frequency EM energy
into the concrete from a transmitting antenna. This EM wave propagates
in the concrete at a velocity that is primarily a function of the relative
dielectric permittivity of subsurface materials. When this wave
encounters the interface of two materials having different dielectric
properties, a portion of the energy is reflected back to the surface, where
it is detected by a receiver antenna and transmitted to a control unit for
processing and display.
RADAR TESTING
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
105/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
106/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
107/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
INFRARED THERMOGRAPHY
-
7/26/2019 Durability of RC Structures
108/175
INFRARED THERMOGRAPHY
Infrared thermography, a nondestructive, remote sensing
technique, has proved to be an effective, convenient, and economical
method of testing concrete. It can detect internal voids, delaminations,
and cracks in concrete structures. As a testing technique, some of its most
important qualities are that it is accurate, it need not inconvenience the
public and it is economical.
An infrared thermographic scanning system can measure and
view temperature patterns based upon temperature differences as small
as a few hundredths of a degree Celsius. Infrared thermographic testing
may be performed during day or night, depending on environmental
conditions and the desired results.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
109/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
110/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
111/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
HALF-CELL POTENTIAL TEST
-
7/26/2019 Durability of RC Structures
112/175
The method measures the electrochemical potential of
reinforcement against a reference electrode placed on the concrete surface.A number of reference electrodes may be used, including copper/copper
sulphate or silver/silver chloride.
The evaluation of the results is normally performed by means of a
personal computer.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
113/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ASTM C876 87 provides a classification for assessing the results of the
-
7/26/2019 Durability of RC Structures
114/175
ASTM C876-87 provides a classification for assessing the results of the
half-cell potential mapping:
a potential bigger than 350 mV, indicate an active corrosive process;
for a potential between 200 and 350 mV the result is inconclusive
(probability of 50 % for a corrosive process);
for a potential under 200 mV the corrosive process is not present on
the reinforcement.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ELECTRICAL RESISTIVITY TEST
-
7/26/2019 Durability of RC Structures
115/175
The electrical resistivity test is used to estimate the speed of the
corrosive process in the reinforcements.
Concrete electrical resistivity can be obtained by applying a current
into the concrete and measuring the response voltage. There are different
methods for measuring concrete resistivity:
with two electrodes (contact resistance can significantly add to the
measured resistance causing inaccuracy);
with four electrodes when the problem of contact resistance is overcome
(the two end electrodes are used to inject current as before, but the
voltage is measured between the two inner electrodes).
ELECTRICAL RESISTIVITY TEST
-
7/26/2019 Durability of RC Structures
116/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Th l b l ifi d i
-
7/26/2019 Durability of RC Structures
117/175
The results can be classified in:
for resistivity bigger than 20 kcm, the corrosive speed is
negligible;
for resistivity between 10 and 20 kcm, the corrosive speed is
small;
for resistivity between 5 and 10 kcm, the corrosive speed is
high;
for resistivity lower than 5 kcm, the corrosive speed is very
high.
-
7/26/2019 Durability of RC Structures
118/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
119/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
MICROWAVE METHOD
-
7/26/2019 Durability of RC Structures
120/175
FOR ASSESSMENT OF HUMIDITY
In the microwave methods two components are considered, the
attenuation of penetrating microwave bean and the phase change. It has
been found that measurements of the attenuation and phase change can
often be combined to obtain a measure of moisture content.
Microwave attenuation measurements use a transmitter on one side
of a sample and a receiver on the opposite side. In such a case the sampled
volume can be measured with reasonable certainty. However, attenuation
increases at higher frequencies and at high moisture contents in the sample,
hence, higher frequencies and high moisture contents should be avoid for
measurements using microwave attenuation.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
NEUTRON SCATTERING
-
7/26/2019 Durability of RC Structures
121/175
FOR ASSESSMENT OF HUMIDITY
When fast neutrons are emitted from a radioactive source they
penetrate into concrete and collide with the nuclei of atoms composing
the concrete. The velocity reduction is greatest for collisions with nuclei
that have mass comparable to that of the neutron. After a series of
collisions the slow neutrons can be monitored using a detector that
incorporates a slow neutron absorber.
Hydrogen in water molecules is the dominant source of light
nuclei that causes the production of slow neutrons in concrete. Thus, in
the absence of organic material and other sources of hydrogen, the slow
neutron count is primarily a measure of the total water content.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ASSESSMENT OF PERMEABILITY ON WATER
-
7/26/2019 Durability of RC Structures
122/175
This method is intended to determine the susceptibility of an
unsaturated concrete to the penetration of water. In general, the rate of
absorption of concrete at the surface differs from the rate of absorption
of a sample taken from the interior.
The exterior surface is often subjected to less than intended
curing and is exposed to the most potentially adverse conditions. This
test method is used to measure the water absorption rate of both the
concrete surface and interior concrete.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The equipment consist of a
-
7/26/2019 Durability of RC Structures
123/175
pressure chamber that containing a
watertight gasket. The chamber is
secured tightly to the surface by two
anchored clamping pliers or by
means of a suction plate.
The chamber is filled with water and the water is allowed to be
absorbed by the test surface for 10 minutes. The filling valve is closed,
and the top cap of the chamber is turned until a desired water pressure is
displayed on the gauge. As water permeates into the concrete, the
selected pressure is maintained by means of a micrometer gauge pushing
a piston into the chamber. The piston movement compensates for the
volume of water penetrating into the material. The travel of the piston as
a function time is recorded and the speed the piston travel in m/s is
used to characterize the permeation of the test surface.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ASSESSMENT OF PERMEABILITY ON AIR
-
7/26/2019 Durability of RC Structures
124/175
The on site permeability
test permits a rapid analysis of the
air permeability of the cover
concrete by a non destructive
method. The essential features of
the permeability test method are a
two-chamber vacuum cell and a
pressure regulator which ensures an
air flow at right angles to the surface
into the inner chamber. The
permeability test takes only 2 12
minutes.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Drilled-hole tests in concrete
-
7/26/2019 Durability of RC Structures
125/175
The measurement of transport to or from
a drilled hole is the alternative to surface
measurements for in situ assessment of
air permeability. A convenient way of
carrying out these tests is to drill a hole,
seal the top of it, evacuate the space
below the seal and measure the time
taken for the vacuum to decay.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
METHODS FOR ESTABLISHING MECHANICAL,
-
7/26/2019 Durability of RC Structures
126/175
PHYSICAL AND CHEMICAL PROPERTIES OF
MATERIALS IN THE LABORATORY
A direct assessment on
mechanical, physical and chemical
properties can be made in the
laboratory on:
disengaged material;
core samples;
elements extracted from
structure.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
MECHANICAL PROPERTIES
-
7/26/2019 Durability of RC Structures
127/175
Compressive Strength
Tensile Strength (splitting)
Modulus of Elasticity (static modulus of elasticity or dynamic
modulus)
PHYSICAL PROPERTIES
Density
Permeability
Freeze-thaw
Shrinkage
Thermal movement
Microscopically examination
DURABILITY OF REINFORCED CONCRETE STRUCTURES
CHEMICAL PROPERTIES
-
7/26/2019 Durability of RC Structures
128/175
Chemical analyzes performed on concrete and reinforcement
should elucidate the following:
identify corrosive agent;
determining the depth of penetration;
concentration of corrosive agent;
changes in the chemical composition of the concrete and
reinforcement.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
STATISTICAL ANALYSIS OF EXPERIMENTAL DATA
-
7/26/2019 Durability of RC Structures
129/175
STATISTICAL ANALYSIS OF EXPERIMENTAL DATA
Statistical analysis of experimental data must cover two aspects,
namely:
establishing the boundaries (intervals) for the studied
characteristic; differentiation between the real values on degraded material
and the results affected by measurement errors and / or
interpretation.
Degraded areas can be identified by imposing standard deviation
relative to mean value.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
SIMPLE STATISTICAL ANALYSIS
-
7/26/2019 Durability of RC Structures
130/175
Mean value
Standard deviation
n
XX
i
1n
XXs i
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
131/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ESTABLISHING THE NUMBER OF MEASUREMENTS
-
7/26/2019 Durability of RC Structures
132/175
F - is a number that corresponds to a very low probability that the
difference between the results of testing and result considering all
the characteristics and operating conditions of the item, not
greater than E (a probability of 4.5%, F = 2)Mean value
0- standard deviation
E - the maximum imposed error
n FE
o
2
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ADVANCED STATISTICAL ANALYSIS
-
7/26/2019 Durability of RC Structures
133/175
Advanced statistical analyzes can be performed when has the
same characteristics determinations made by different methods.
2
apr
apr21
2
apr
RR
2
1exp)(2)Rf(
2
apr
r
1i2
Si
n
1i2
Si
2
Si
ii2apr
r
1i2
Si
apr
apo
1/1
1/)v|x(E1/R
R
r
1i2
Si
2
apr
r
1i2
Si
apr
2
apo
1/1
1/
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ASSESSMENT OF SAFETY LEVEL FOR EXISTING BUILDINGS
-
7/26/2019 Durability of RC Structures
134/175
Evaluation of existing buildings involves two fundamental
aspects, namely:
assessment of bearing capacity of elements/structure;
estimation of service life for various hypothesis (with or
without interventions).Evaluation of the safety level of existing buildings is done in
three distinct steps:
preliminary data collection
investigation of degradation
evaluation of bearing capacity of the structure and service
life prediction.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ESTABLISHING OF BEARING CAPACITY
-
7/26/2019 Durability of RC Structures
135/175
The evaluation process include:
checking of compliance requirements;
investigation of degradations;
evaluation of the bearing capacity.
Based on all results the building vulnerability is established
according with four seismic risk classes:
Rs I (high risk of collapse);
Rs Class II (low risk of collapse but high risk of structural
damages); Rs III (low risk of structural damages but high risk of non-
structural damages);
Class IV (the behavior is similar with new buildings).
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ASSESSMENT OF COMPLIANCE REQUIREMENTS
-
7/26/2019 Durability of RC Structures
136/175
Assessment of compliance requirements must establish if
compliance rules provided in current norms are satisfied by the analyzed
building. The main components of this qualitative assessment are:
a. verification of the loads path
b. verification of the redundancy
c. verification of the geometrical configuration
d. verification of interaction with other buildings near by
e. verification of the infrastructure and foundation soil
f. verification of the structural details
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
137/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
138/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Assessment of seismic risk class from compliance requirements point of
-
7/26/2019 Durability of RC Structures
139/175
view is made according with the table below.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ASSESSMENT OF STRUCTURAL DEGRADATION
-
7/26/2019 Durability of RC Structures
140/175
The structural damage degree express the structural degradation
produced by the seismic action and other causes.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
141/175
-
7/26/2019 Durability of RC Structures
142/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ASSESSMENT OF SEISMIC SAFETY DEGREE
Th i i f t d l d th t th d
-
7/26/2019 Durability of RC Structures
143/175
The seismic safety degree revealed the strength and
deformability capacity of the structure. Three methodologies can be used
for the assessment of the seismic safety degree.
Methodology 1
Seismic safety degree is determined in terms of strength:
3 =
where:
capable shear stress
shear stress determined according with seismic norm
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Methodology 2
-
7/26/2019 Durability of RC Structures
144/175
Individual values were determined for each of the structural
elements:
3 =
where:
capable internal force, injelement
design internal force, injelement
behavior factor forjelement
Global value of the seismic safety degree is:
3 = ,,/where:
,capable shear force forjelement
,design shear force forjelement
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Methodology 3
-
7/26/2019 Durability of RC Structures
145/175
Seismic safety degree is determined in terms of displacements:
3 =
where:
ultimate lateral displacement
imposed lateral displacement
DURABILITY OF REINFORCED CONCRETE STRUCTURES
ESTIMATION OF SERVICE LIFE
l f f b ld h h ll h b ld
-
7/26/2019 Durability of RC Structures
146/175
Service life of a building is the time where all the building
properties are maintained at an acceptable levels under the conditions ofcurrent maintenance.
Because reinforced concrete degradation is a complex
phenomenon with many variables, service life may be affected by large
errors.
In the estimation of the service life of an element or construction
must be taken into account:
physical, chemical and biological actions from theenvironment
characteristics of the materials
the influence of structural damages
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The service life assessment can be achieved by the following
th d
-
7/26/2019 Durability of RC Structures
147/175
methods:
assessment based on experience
evaluating by comparisons with similar situations
assessment with accelerated tests
assessment by mathematical modeling of the degradation
process
evaluation by stochastic analysis
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DURABILITY REQUIREMENTS FOR NEW BUILDINGS
-
7/26/2019 Durability of RC Structures
148/175
The requirement of an adequately durable structure is met if,
throughout its required life, a structure fulfils its function with respect to
serviceability, strength and stability without significant loss of utility or
excessive unforeseen maintenance.
To provide the required overall durability, the intended use of
the structure shall be established, together with the load specifications
to be considered. The required life of the structure and the maintenance
programme shall also be considered, in assessing the level of protection
required.
Durability may be affected both by direct actions and also by
consequential indirect effects inherent in the performance of the
structure (e.g. deformations, cracking, water absorption, etc.).
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Actions shall be assessed in accordance with the definitions given
in Eurocode 1 and based on values given in appropriate national codes. In
-
7/26/2019 Durability of RC Structures
149/175
g pp p
special cases, it may be necessary to consider modification of these values
to meet particular durability requirements.
Environmental influence means those chemical and physical
actions, to which the structure as a whole, the individual elements, and
the concrete itself is exposed, and which results in effects not included in
the loading conditions considered in structural design.
For the design of regular buildings, environmental conditions
should be classified in accordance with Eurocode 2, to establish the
overall level of protection.
In addition, it may be necessary to consider certain forms of
aggressive or indirect action individually.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
150/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
151/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
152/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
153/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
154/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
155/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
156/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DETERMINATION OF CONCRETE COVER
-
7/26/2019 Durability of RC Structures
157/175
The concrete cover is the distance between the outer surface of
the reinforcement (including links and stirrups) and the nearest concrete
surface. A minimum concrete cover shall be provided in order to ensure:
the safe transmission of bond forces;
that spalling will not occur;
an adequate fire resistance;
the protection of the steel against corrosion.
The protection of reinforcement against corrosion depends
upon the continuing presence of a surrounding alkaline environment
provided by an adequate thickness of good quality, well-cured concrete.
The thickness of cover required depends both upon the exposure
conditions and on the concrete quality.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The minimum concrete cover required shall first be determined.
-
7/26/2019 Durability of RC Structures
158/175
This shall be increased by an allowance for tolerances () , which is
dependent on the type and size of structural element, the type of
construction, standards of workmanship and quality control, and
detailing practice.
The result is the required nominal cover which shall be specified
on the drawings.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
159/175
cmin bminimal cover from bond requirements;
cmin durminimal cover from durability requirements;
Dcdur,
additional safety value;
Dcdur,streduction of the minimum cover for stainless steel reinforcement;
Dcdur,add reduction of the minimum cover for reinforcements with
supplementary protection against corrosion.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
To transmit bond forces safely, and to ensure adequate
-
7/26/2019 Durability of RC Structures
160/175
compaction, the concrete cover, to the bar or strand being considered,
should never be less than:
orn
or (+ 5 mm) or (n
+ 5 mm) if dg> 32 mm
where:
is the diameter of the bar, diameter of a strand or of the
duct (post-tensioning);
n
is the equivalent diameter for a bundle;
dgis the largest nominal maximum aggregate size.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
161/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
162/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
163/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
164/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The allowance for tolerances () will usually be in the range
-
7/26/2019 Durability of RC Structures
165/175
of 0 mm and 5 mm, for precast elements, if production control can
guarantee these values and if this is verified by quality control.
The allowance for tolerances () will be in the range of 5
mm and 10 mm for in situ reinforced concrete construction.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
DURABILITY OF CONCRETE ELEMENTS FIRE ACTION
The fire triangle or combustion
-
7/26/2019 Durability of RC Structures
166/175
The fire triangle or combustion
triangle is a simple model for understandingthe necessary ingredients for most fires. The
triangle illustrates the three elements a fire
needs to ignite: heat, fuel, and an oxidizing
agent (usually oxygen). A fire can be
prevented or extinguished by removing any
one of the elements in the fire triangle.
The fire tetrahedron represents the addition of a component, thechemical chain reaction. Once a fire has started, the resulting exothermic
chain reaction sustains the fire and allows it to continue until or unless at
least one of the elements of the fire is blocked.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
Oxygen is an essential element for the maintenance of fire.
Burning speed is influenced by the amount of fresh air.
-
7/26/2019 Durability of RC Structures
167/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The simplified calculation method o fire determines the ultimate
load bearing capacity of a heated cross section. The method is applicable
-
7/26/2019 Durability of RC Structures
168/175
to structures subjected to a standard fire exposure. The procedure is also
applicable for the calculation of the ultimate resistance at a specified time
for any other fire exposure, if the temperature profiles corresponding to
that exposure are known or calculated, and correct data for material
properties corresponding to it are used.
The procedure is to first determine the temperature profile of
the cross section, reduce the strength and the short term modulus of
elasticity of concrete and reinforcement, reduce the concrete cross
section, and then calculate the ultimate load bearing capacity of the
construction and to compare the capacity with the relevant combination
of actions.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
In a real fire, the temperature may have different values.
Numerous experimental and theoretical studies has determined the
-
7/26/2019 Durability of RC Structures
169/175
time-temperature curve and led to the establishment of a standardized
curves.
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The fire damaged cross-section is represented by a reduced section
by ignoring a damaged zone of thickness at the fire exposed surfaces.
-
7/26/2019 Durability of RC Structures
170/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The reduced values for strength are determined by the relation:
-
7/26/2019 Durability of RC Structures
171/175
where:
Xk
strength at normal temperature (for concrete and
reinforcement);
k()reduction factor due to temperature;
temperature value due to fire;
M,fisafety factor.
fiMkfid XkX ,, /)(
DURABILITY OF REINFORCED CONCRETE STRUCTURES
The reduced values for the modulus of elasticity are determined
by the relation:
-
7/26/2019 Durability of RC Structures
172/175
by the relation:
where:
Eckmodulus of elasticity at normal temperature;
k()reduction factor due to temperature;
temperature value due to fire;
M,fisafety factor.
2
, ,( ) /
d fi ck M fiX k E
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
173/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
174/175
DURABILITY OF REINFORCED CONCRETE STRUCTURES
-
7/26/2019 Durability of RC Structures
175/175