Cracks in Concerete

8/10/2019 Cracks in Concerete

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INTRODUCTION

The Assyrians and Babylonians used clay as cement in their concrete. The

Egyptians used lime and gypsum cement. In the Roman Empire, concrete made from

Quicklime, pozzolani ash pozzolana and an aggregate made from pumice !as "ery

similar to modern #ortland cement concrete. In $%&', the British engineer (ohn

)meaton pioneered the use of #ortland cement in concrete, using pebbles and

po!dered brick as aggregate. In the modern day, the use of recycled materials as

concrete ingredients is gaining popularity because of increasingly stringent

en"ironmental legislation. The most conspicuous of these is fly ash, a by*product of

coal po!er plants. This has a significant impact in reducing the amount of +uarrying

and landfill space re+uired.

The properties of concrete ha"e been altered since Roman and Egyptian times,

!hen it !as disco"ered that adding "olcanic ash to the mi allo!ed it to set under

!ater. )imilarly, the Romans kne! that adding horse hair made concrete less liable to

shrink !hile it hardened, and adding blood made it more frost*resistant. In modern

times, researchers ha"e added other materials to create concrete that is etremely

strong, and e"en concrete that can conduct the -omposition.


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-/#)ITI0

The composition of concrete is determined initially during miing and finally

during placing of fresh concrete. The type of structure being built as !ell as the

method of construction determines ho! the concrete is placed and therefore the

composition of the concrete mi 1the mi design2.

-ement

#ortland cement is the most common type of cement in general usage, as it is a

basic ingredient of concrete, mortar and plaster. An English engineer named (oseph

Aspdin patented #ortland cement in $345, and it !as named after the limestone cliffson the Isle of #ortland in England because of the similarity of its color to the stone

+uarried from #ortland. It consists of a miture of oides of calcium, silicon and

aluminium. #ortland cement and similar materials are made by heating limestone 1a

source of calcium2 !ith clay and grinding this product 1called clinker2, !ith a source

of sulfate 1most commonly gypsum2. The resulting po!der, !hen mied !ith !ater,

!ill become a hydrated solid o"er time.

6igh*temperature applications, such as masonry o"ens and the like, generally

re+uire the use of refractory cement7 concretes based on #ortland cement can be

damaged or destroyed by high heat, !hereas refractory concretes can absorb the heat

better !ith less degradation.

Water

8ater suitable for human or animal consumption can be used for the

manufacture of concrete. The !c ratio 1mass ratio of !ater to cement2 is the key

factor that determines the strength of concrete. A lo!er !c ratio !ill yield a concrete

!hich is stronger, a higher !c ratio yields a concrete !ith a lo!er strength. -ement

paste is the material formed by combination of !ater and cementitious materials * that

part of the concrete !hich is not aggregate or reinforcing. The !orkability or

consistency is affected by the !ater content, the amount of cement paste in the o"erall


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mi and the physical characteristics 1maimum size, shape and grading2 of the

aggregates.

Aggregates

The !ater and cement paste hardens and de"elops strength o"er time. In order

to ensure an economical and practical solution, both fine and coarse aggregates are

utilized to make up the bulk of the concrete miture. )and, natural gra"el and crushed

stone are mainly used for this purpose. 6o!e"er, it is increasingly common for

recycled aggregates 1from construction, demolition and eca"ation !aste2 to be used

as partial replacements of natural aggregates, !hilst a number of manufactured

aggregates, including air*cooled blastfurance slag and bottom ash are also permitted.

9ecorati"e stones such as +uartzite, small ri"er stones or crushed glass are

sometimes added to the surface of concrete for a decorati"e :eposed aggregate:

finish, popular among landscape designers.

Admixtures

Admitures are materials in the form of po!der or fluids that are added to the

concrete to gi"e it certain characteristics not obtainable !ith plain concrete mies. In

normal use, admiture dosages are less than &; by mass of cement, and are added to

the concrete at the time of batching miing. The most common types of admitures

are$; to .$;, depending on a

number of factors listed belo!. )ince most concrete structures are not free to shrink,

but are restrained at the ends, the results of the tendency to shrink is to de"elop tensile

stresses in the concrete, !hich cause the de"elopment of cracks, approimately

spaced at some inter"al "arying from about & feet to about 4> feet. Although it is not

possible to control the tendency of the concrete to shrink, the size and se"erity of the

cracks can be controlled by the addition of reinforcement. )hrinkage reinforcement

must be continuous and uniformly distributed throughout the structure.

Contributing Factors

igh !ater"ce#ent ratio

The larger the proportion of !ater in the concrete, the greater the "olume

change on drying, and the greater the tendency to shrink. As stated abo"e, a

!atercement ratio of about .4& is chemically sufficient for hydration of the cement,

but additional !ater must be added to make the concrete !orkable. Darge amounts of

ecess !ater are undesirable from the point of "ie! of concrete strength, and

dimensional stability, but do impro"e the !orkability and the economy of the

concrete.


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Te#perature extre#es

Temperature etremes, especially ust after placement of the concrete, may

promote more rapid drying and hasten the de"elopment of shrinkage cracks.

$ack o% ade&uate rein%orce#ent

Reinforcement of the concrete cannot pre"ent shrinkage cracking, but can

control the se"erity and etent of the de"elopment of cracks.

$ack o% ade&uate curing o% concrete at initial place#ent

-uring concrete properly means maintaining the material in a moist condition as it

gains its early strength. If drying and subse+uent shrinkage de"elop early, the

material has much lo!er tensile strength and is much more susceptible to the

de"elopment of cracks.

Results

The result of shrinkage cracking, like structural cracking in properly designed

concrete is simply a "isual problem< o!ners and building users often find cracks in

concrete to be unsightly, or e"en threatening. )hrinkage cracks also form a path!ay

for moisture and air to reach the reinforcement, and may hasten the corrosion of the

reinforcement 1although this is the topic of a "igorous debate in the engineering

literature2. They do allo! penetration of moisture and promotion of freeze*tha!

effects, and can e"entually de"elop into larger cracks and spalled or damaged areas.


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)e"ere shrinkage cracking of a concrete patch< the patching concrete mi !as

probably o"er!atered andor insufficiently cured.

Remedies

sually, this is a condition that does not re+uire remedies. 8here the cracking

becomes unsightly, or admits too much moisture to the interior of the structure, some

sort of repair may be !arranted. If the structure is deficient, eternal reinforcement,

or eternal post tensioning may be undertaken. If the problem is superficial, the

cracks may be repaired by epoy inection.

Investigations of Srinkage Cracking

In"estigations of shrinkage cracking may include in"estigations of the

conditions under !hich the concrete !as placed**hot and dry !eather promote early

age shrinkage cracking7 the addition of !ater to the concrete during placement makes

the material more susceptible to shrinkage cracking. In"estigations should also be

undertaken to discern shrinkage cracking from structural cracking.


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'ree(e"Tha! E%%ects


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Process

-oncrete is a porous material and !ill absorb !ater, either into pores, !hich

al!ays eist !ithin the cement matri, or into pre"iously formed structural or

shrinkage cracks. As is !ell*kno!n, the "olume of !ater increases as it freezes, and

freezing !ater contained !ithin the concrete can cause stresses to de"elop in the

concrete. 8hen these stresses eceed the tensile capacity of the concrete, they may

cause a number of effects< spalling of the concrete, de"elopment of further cracks, and

GpopoutsH of the surface of the concrete

Contributing Factors

Free!e"ta# c$cles

The number of freeze*tha! cycles in a !inter season is an important factor in

producing damage to concrete. This +uantity "aries not only !ith the coldness of the

!inter climate, but also !ith the daily "ariations in temperature. The masonry

industry defines a !eathering inde as the product of the a"erage annual number of

freezing cycles times the a"erage annual !inter rainfall. The !eathering inde

contours are sho!n belo!.


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)oisture path!ays

The pores in concrete, during freezing, must be nearly saturated !ith !ater

1more than > percent of saturation2 1Bureau of Reclamation $%2.

igh !ater"ce#ent ratio

A higher than necessary !ater*cement ratio in the initial concrete placement

contributes to freeze*tha! problems in t!o !ays. =irst, more !ater in the mi

reduces the strength of the concrete, and so reduces its resistance to the stresses

produced by freezing !ater. The reduced strength also makes the concrete more

susceptible to structural, shrinkage and thermal cracking. )econd, ecess !ater in the

concrete mi dries e"entually on aging of the concrete and results in "oids in the

micro*structure of the concrete. These "oids admit !ater readily, and if the !ater

freezes, damage to the concrete may result.

$ack o% entrained air Entrained air is introduced into concrete by means of a chemical admiture

that produces small air bubbles in the concrete matri that pro"ide space for !ater

epansion during freezing. If the proper air entraining admiture 1AEA2, at the

correct concentration, is properly mied into high +uality concrete, there should be

"ery little damage resulting from cyclic freezing and tha!ing ecept in "ery se"ere

climates. 1Bureau of Reclamation $'2 The mechanism of entrained air@s

contribution to resistance to freeze*tha! cycles appear to be to pro"ide a reliefpath!ay for the epansion of the !ater due to freezing. The use of AEA@s in eterior

eposed concrete did not begin until the mid*$5>@s and !as not !idespread in the

building industry until !ell into the $'>@s.


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*oorly consolidated concrete

#oorly consolidated concrete produces "oids in the concrete that cannot

control freeze tha! action as air entraining does, admits moisture into the concrete,

and also !eakens the concrete.

Results

=reeze*tha! damage may manifest itself as cracking, delamination, spalling,

or popouts. -racking may de"elop or become "isible as a result of the enlargement of

eisting hairline cracks by freeze*tha! action. 9elamination refers specifically to the

co"er o"er the concrete reinforcement losing connection to the concrete belo! the

reinforcement. Jones of delamination are identified by sounding !ith a hammer or a

chain drag. 8hen concrete spalls, the corners, or the concrete co"er o"er the

reinforcement lose their connection to the main body of the concrete member by the

de"elopment of !idespread internal cracking or delamination. #opouts of a concrete

surface usually ha"e a further underlying cause, such as o"er!orking during finishing,

or improper curing procedures.

Remedies

=reeze*tha! damage to concrete is not generally repairable, ecept by remo"al and

replacement of the affected part of the material. =reeze*tha! action can be arrested

by denying access to moisture. In the case of concrete under a roof membrane !hich

has been !etted and frozen, this may be accomplished by replacement of the roofing

membrane !ith a more suitable material. ften, simple impro"ements in drainage

can direct !ater a!ay from the affected zones. The application of sealers to historic

eposed concrete is not generally recommended, as it may alter the appearance of the

concrete, or it may entrap mositure !ithin the concrete and cause further problems

1-oney, undated2. 8hen the source of the moisture has been remo"ed or controlled,

repairs to the cracks or spalls may be undertaken by the methods outlined belo!.

Investigations of Free!e"Ta# Cracking

Kisual inspection can locate areas of damaged concrete and make an initial

determination that the freeze*tha! mechanism is the source of the damage by


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in"estigation of moisture path!ays, and the pattern of cracking The resistance of

concrete to freezing and tha!ing by etracting a core of the concrete under

in"estigation and subect the specimen to cycles of !etting drying and freezing

according to A)T/ -'''. This test does not gi"e any absolute measure of the

resistance of the concrete, but does gi"e a relati"e measure for comparison !ith other

cases. #etrographic analysis by A)T/ -3&' can also be "ery useful< the presence or

absence of entrained air can be detected by eamination of the concrete.

Rein%orce#ent Corrosion

Process

-orrosion of embedded steel in concrete, including reinforcement, is acomple electrochemical process that can result in "ery se"ere damage to a concrete

structure. In order to corrode, the reinforcement must ha"e access to moisture,

oygen, and electrolyte. Because concrete is a porous material, permeable to air and

!ater, these three elements are nearly al!ays a"ailable in concrete. 6o!e"er, the

alkalinity of the en"ironment !ithin a concrete member tends to suppress the

corrosion reaction, and other conditions are necessary for the de"elopment of

damaging corrosion of reinforcement.

A full corrosion cell consists of t!o components, a cathode, !here free

electrons combine !ith oygen and !ater to form hydroide 162L ions and an anode,

!here iron ionizes by the loss of electrons, and combines !ith the hydroide ions to

form products of corrosion, commonly kno!n as rust. The electrons migrate from the

anode to the cathode through the steel, !hile the negati"ely charged hydroide ions

migrate through a medium, !hich consists of !ater and dissol"ed ions, or

electrolytes. )o, the reactions in the corrosion process are

Anode 1oidation2 =e M =eNNN 4eL

OP=e1624N 64M iron oide products of corrosion 1rust2

-athode 1reduction2 4N 464 N 5eLM 4162L


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?enerally, because of the high p6 1lo! acidity2 !ithin the concrete en"ironment, the

corrosion reaction is suppressed, and the reinforcement does not corrode. 6o!e"er,

certain conditions can cause the concrete to become acti"e, either by changing the p6

of the en"ironment in the concrete, or by changing the en"ironment. Eamples of

these conditions are deg. -elcius by keeping full section of concrete cool in

dams and bridges.

Alkali"Aggregate Reactivit$

-ertain types of sulfate*containing aggregates, !hen !etted, react !ith the

alkaline elements in concrete, causing large "olume changes around the aggregate.

This process produces large and !idespread tensile stresses in the affected zones of

the concrete. Because in this condition, practically the entire "olume of the concrete

is affected, it is practically incurable, and usually calls for remo"al and replacement of

the affected concrete.


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Figure *+ T$,ical ma,",attern cracking due to alkali"silica reaction in a retaining

#all in )tta#a- )ntario

T$,es of Reaction

There are t!o types of alkali*aggregate reaction years of research, the mechanism of the reaction is still not !ell

understood but it is kno!n the alteration of dolomite to calcite is in"ol"ed and clay

minerals may also ha"e a role in the reaction. The reaction results in cracks in the

concrete 1=igure 42 similar to those caused by alkali*silica reaction. It should be noted


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that limestone aggregates may be susceptible either to alkali*silica reaction, or alkali

carbonate reaction, or a combination of the t!o.

&"Cracking

8hereas most freeze*tha! cracking of normal !eight concrete occurs in the

cement matri, !hen freeze*tha! epansion and damage occurs in porous aggregate,

it produces a characteristic pattern of roughly parallel cracks euding calcite. These

cracks most fre+uently occur at eposed corners and edges in the concrete. The main

defense against this condition is ensuring that the concrete element is not subected to

periodic !etting. ther!ise, remo"al and replacement of the concrete may be

!arranted.

Efflorescence and cracking pattern characteristic of 9*cracking


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CASE ST3&IES

*+ I('ESTI4ATI)(S )F CRAC0S I( C)(CRETE

STR3CT3RES-Prof+%+&+A,te+

)5SER'ATI)(S

6e had an occasion to in"estigate collapse of an under construction factory

building etension at Aurangabad 1/aharashtra2.The design of the structure and steel

reinforcement ,fiing the reinforcement as !ell as the super"ision of the !ork !ere

found to be contributed to the collapse. )ince the original building !as also

constructed by design engineers ten years ago, the o!ners re+uested to check the old

building as !ell. The assembly hall !as frame!ork of beams and columns supporting

the roof slab. 6e sa! large cracks at the unctions of many columns, beams and beambodies. The cracks !ere !ide and appeared to be deeper than the thin plastic layer at

the top, suspected the hollo!ness in the members and approached in 09Ttesting in

/umbai. )ome of the beams and columns !ere tested by the ltrasonic #ulse

Kelocity tester and compared !ith that of cubes. The !ere found to ha"e their

densities less than the design one by more than C>; This confirmed that beam

contained lot of hollo!s. This !as filled !ith epoy grout at "arious places by a

pump. Its density !as checked after 45 hours and !as found to be better.


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C)(C63SI)(

If a !ide crack appears on the body of a member, chances are that member is hollo!.

.+ CRAC0I(4 A(& REPAIR )F E('IR)(%E(TA6

C)(CRETE ST3CT3RES-%r &ov 0aminetsk$

En"ironmental concrete structures often contain long concrete tanks !ith continuous!alls. In some proects these !alls etend to a length of >m upto$&>m.

nfortunately cracks are obser"ed in those long tanks. The cracks are spaced bet!een

$.&mto 4.5m apart and are essentially "ertical.

These cracks are caused by restraint to "olume changes due to concrete

shrinkage and temperature "ariation of !alls, !hich are pre"ented from shortening

by do!el connections to the base of the tank. The intensity and fre+uency of these

cracks depend upon the horizontal reinforcement in these !alls, the thickness of the

!alls, and the strength of the concrete.

-oncrete shrinkage is a phenomenon !hich occurs !ithin the first se"eral years of

the life of the structure. Then rate of shrinkage gradually reduces o"er a period of time

and the chances of subse+uent cracking due to this phenomenon are minimal.

6o!e"er these shrinkage cracks do mo"e as a result of temperature "ariations.

)tucturally, the long tank !alls act as cantile"ered plates fied at their bottom. This

restraint is the cause of high concentrated tensile stresses leading to the "ertical

cracks. 9uring the design of the proect, epansion oints are introduced !ith the

intension of reducing the lengths of the !alls, thereby reducing the effects of linear

epansion or conteaction.9uring the hardening process, the !all is restrained by the

bottom do!el bars at the base and the friction along the contact bet!een the bottom of

the !all and the base.


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nce shrinkage effects are minimized by the limitation of distance bet!een oints, the

minimum temperature reinforcement and ade+ate curing, the number of cracks !ill be

minimised.Another factor that must be considered is the minimum cross*sectional

areas of the horizontal reinforcement in the !all !hich controls the !idth of the

cracks and keeps them farther apart.A-IC&>$recommends that the minimum area

of the reinforcement be based on the distance bet!een shrinkage dissipating oints.

C)(C63SI)(

If the cracks are not acti"e leaks 1say, the hydraulic structure is lined !ith !ater

proofing membrane on the interior2,cracks repair treatments such as routing and

caulking can be considered.

The cracks !hich sho! leakage may be repaired by inection of lo! "elocity

epoy1or chemical grout2Do! modulus epoy is recommended for repair of cracks in

such tanks to permit minimal mo"ements in future !ithout damage to the repaired

cracks.


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23A6IT7 C)(TR)6

#erform tests re+uired to confirm structural integrity. -racks must be ninety percent

filled to bond strength of approimately ',&>> psi.

S3%%AR7

nce any construction has been completed and occupied for its intended purposes, the

en"ironmental factors are seasonal climatic aspects do affect the condition of the

structure. )o the structure constructed must be inspected periodically to kno! the

etent of deterioration that has set in it.

The cracks appear on the concrete surfaces as a routine feature.8hen a

structure is plastered in cement mortar, it is apparent unless altering is done

continuously and properly the plaster surface !ill de"elop hair cracks to start !ith. As

a remedial measure that portion of the cracks should be remo"ed and redone !ith duecare.

The patterns of cracks that !ill be found on inspection gi"e help to eperienced

eye to assess the reasons for the cracks. This !ill help the engineer to pro"ide suitable

measures to make the cracks ineffecti"e in deteriorating the struture.The measures of

repair and maintainance in time !ill ensure that the durability of the structure is

maintained.


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C)(C63SI)(

A good deal of cracking problem in other!ise !ell designed concrete

structures be a"oided if correct construction practices are follo!ed like using dust

Sclay free7 not highly absorpti"e dry !ell graded !ashed aggregates, using rust free

reinforcement bars, proper batching of aggregates, cement and !ater, ade+uate

miing, transporting, placing, compacting and curing of concrete, rigid form !ork

!orking at right !eather conditions, ade+uate +uantity of !ater and cement combined

!ith ade+uate plasticizers, retardersors, accelerators as re+uired.

The cracks, !hich sho! leakage, may be repaired by inection of lo! "iscosity

epoy 1or chemical grout2.Do! modulus epoy is recommended for repair of cracks

to permit minimal mo"ements in the future !ithout damage to repaired cracks.

9ue to heterogeneous nature G/icro -racksH do appear in concrete, immaterial

!hether loaded or not. )ome micro cracks get healed partly or completely during thecontinued hydration of cement. Any micro cracks that occur before hardening of

concrete may pro"e to be a source of cracking distress in the ser"ice life of the

structures.


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REFERE(CES8

$. -oncrete Technology by /.).)hetty.

4. #roperties of concrete by 0e"ille A.//

C. The Indian -oncrete ournal, (anuary 4>>$.

5. The Indian -oncrete ournal, =ebruary 4>>>.

&. -onstruction Engineering S-onstruction Re"ie!, (une 4>>4.

'. 0B/S-8 9ecember 4>>&.

%. 8ebsites

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Cracks in Concerete