Durability of RC Structures

7/26/2019 Durability of RC Structures

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DURABILITY OF REINFORCED CONCRETE STRUCTURES

DUR BILITY OF

REINFORCED

CONCRETE

STRUCTURES


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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


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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.


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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.


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CLASSIFICATION OF MECHANICAL ACTIONS


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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


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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


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Exceptionally actions:

Fires

Shock

D. Actions resulted in designing/execution process

Accepted risk

Insufficient knowledge

Ignorance

Mistakes

Blunders

Malevolence


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BUILDING/ENVIRONMENT RELATIONSHIP


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NATURAL AIR


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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


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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



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POLLUTED AIR FROM INDUSTRIAL

ENVIRONMENT

Due to industrial activity, in the atmosphere is discharged a large amounts

of pollutants from the various industries.



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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.



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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.



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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.



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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.



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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.



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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);



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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.



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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.



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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.



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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.



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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



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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.



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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.



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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.



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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.



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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.



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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.



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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



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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.


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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.



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CORROZIVE ACTIONS ON CONCRETE



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ACCIDENTAL ACTIONS


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Windsor Tower, Madrid, Spain



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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.



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Wi

ndsor

Tower

Madrid



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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.



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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.



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SIMPLE TESTS

Visual examination

Acoustic impact



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Reinforcement inspection



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Testing of alkalinity (with phenolphthalein)



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Testing of chloride ions (with silver nitrate)



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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.



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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.


INDIRECT METHODS FOR STRENGTH ASSESSMENT


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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



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Ultrasonic pulse velocity (UPV)

test instrument to examine the quality of

concrete and other materials such as rock,

wood and ceramics.



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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.



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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



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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.



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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.



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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.


DIRECT METHODS FOR STRENGTH ASSESSMENT


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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.



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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



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BREAK-OFF TEST


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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



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METHODS TO DETECT REINFORCEMENTS


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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



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RADIOGRAPHIC TESTING


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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.



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RADAR TESTING


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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



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INFRARED THERMOGRAPHY


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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.



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HALF-CELL POTENTIAL TEST


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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.



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ASTM C876 87 provides a classification for assessing the results of the


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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.


ELECTRICAL RESISTIVITY TEST


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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


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Th l b l ifi d i


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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.


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MICROWAVE METHOD


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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.


NEUTRON SCATTERING


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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.


ASSESSMENT OF PERMEABILITY ON WATER


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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.


The equipment consist of a


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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.


ASSESSMENT OF PERMEABILITY ON AIR


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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.


Drilled-hole tests in concrete


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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.


METHODS FOR ESTABLISHING MECHANICAL,


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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.


MECHANICAL PROPERTIES


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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


CHEMICAL PROPERTIES


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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.


STATISTICAL ANALYSIS OF EXPERIMENTAL DATA


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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.


SIMPLE STATISTICAL ANALYSIS


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Mean value

Standard deviation

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1n

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ESTABLISHING THE NUMBER OF MEASUREMENTS


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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


ADVANCED STATISTICAL ANALYSIS


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Advanced statistical analyzes can be performed when has the

same characteristics determinations made by different methods.

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ASSESSMENT OF SAFETY LEVEL FOR EXISTING BUILDINGS


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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.


ESTABLISHING OF BEARING CAPACITY


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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).


ASSESSMENT OF COMPLIANCE REQUIREMENTS


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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



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Assessment of seismic risk class from compliance requirements point of


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view is made according with the table below.


ASSESSMENT OF STRUCTURAL DEGRADATION


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The structural damage degree express the structural degradation

produced by the seismic action and other causes.



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ASSESSMENT OF SEISMIC SAFETY DEGREE

Th i i f t d l d th t th d


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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


Methodology 2


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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


Methodology 3


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Seismic safety degree is determined in terms of displacements:

3 =

where:

ultimate lateral displacement

imposed lateral displacement


ESTIMATION OF SERVICE LIFE

l f f b ld h h ll h b ld


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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


The service life assessment can be achieved by the following

th d


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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 REQUIREMENTS FOR NEW BUILDINGS


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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.).


Actions shall be assessed in accordance with the definitions given

in Eurocode 1 and based on values given in appropriate national codes. In


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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.



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DETERMINATION OF CONCRETE COVER


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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.


The minimum concrete cover required shall first be determined.


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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.



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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.


To transmit bond forces safely, and to ensure adequate


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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.



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The allowance for tolerances () will usually be in the range


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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 CONCRETE ELEMENTS FIRE ACTION

The fire triangle or combustion


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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.


Oxygen is an essential element for the maintenance of fire.

Burning speed is influenced by the amount of fresh air.


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The simplified calculation method o fire determines the ultimate

load bearing capacity of a heated cross section. The method is applicable


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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.


In a real fire, the temperature may have different values.

Numerous experimental and theoretical studies has determined the


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time-temperature curve and led to the establishment of a standardized

curves.


The fire damaged cross-section is represented by a reduced section

by ignoring a damaged zone of thickness at the fire exposed surfaces.


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The reduced values for strength are determined by the relation:


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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 ,, /)(


The reduced values for the modulus of elasticity are determined

by the relation:


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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



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Durability of RC Structures

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