Report on cracks in buildings

7/17/2019 Report on cracks in buildings

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CRACKS

IN

BUILDING(Causes and Prevention)

--Jatin George 1PI12CV049


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Sl no TOPIC Pg no

1 Introduction 3

2 Thermal o!ement 4

3 "la#tic $e%ormation &

4 'oundation mo!ement and #ettlement o% #oil (

) $ue to Chemical *eaction 10

& Sul+hate ,ttac 10

. Car/onation 11

( Corro#ion o% rein%orcement in concrete and

/ricor

11

9 ,lali aggregate reaction 12

10 Cree+ 1211 Groth o% Vegetation 13

12 Crac# due to #oil characteri#tic# 14

T," O' COT"T


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IT*O$CTIO

Occurrence of various crack patterns in the building during construction, aftercompletion when it is subjected to super imposed load or during the service life, is acommon phenomenon. A building component develops cracks whenever the stress inthe components exceeds its strength. Stress in the building component could be causedby externally applied forces, such as dead, live, wind or seismic loads, foundationsettlement etc. or it could be induced internally due to thermal movements, moisturechanges, elastic deformation, chemical action etc.

Cracks in buildings could be broadly classified as structural and nonstructural cracks.

Structural Crac# 5 !hese occur due to incorrect design, faulty construction oroverloading and these may endanger the safety of a building. e6g6 "xtensive cracking ofan #CC beam.

on #tructural Crac#5 !hese are mostly due to internally induced stresses in buildingsmaterials and do not endanger safety of a building but may look unsightly, or may createan impression of faulty work or may give a feeling of instability. $n some situations due topenetration of moisture through them non structural cracks may spoil the internalfinishes thus adding to the cost of maintenance, or corrode the reinforcement, therebyadversely affecting the stability of the Structure in long run. e6g6 %ertical crack in a longcompound wall due to shrinkage or thermal movement.

Cracks may appreciably vary in width from very thin hair crack barely visible to nakedeye to gaping crack. &epending upon the crack width cracks are classified as '

7a8 Thin Crac - less than ( mm in width,7/8 edium Crac - ( to ) mm in width,

7c8 ide Crac - more than ) mm in width.

7d8 Cra:ing - Occurrence of closely spaced fine cracks at the surface of a material iscalled cra*ing.

Cracks may of uniform width throughout or may be narrow at one end

gradually widening at the other. Crack may be straight, toothed, stepped,

map pattern or of random type and may be vertical, horizontal or diagonal.

Cracks may be only at surface or may extend to more than one layer of

material. Cracks due to dierent causes have varying characteristics and bythe careful observations of these characteristics, one can diagnose the cause

of cracking for adopting the appropriate remedial measures.


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Thermal Movement

+ The most potent causes of cracking in buildings.

+ All materials more or less expand on heating and contract on

cooling.

+ The extent of thermal movement depends upon

Ambient temperature variation

Co-efcient o thermal expansion! "xpansion of cement

mortar # concrete is almost twice of the bricks and brick work.

$ovement in brickwork in vertical direction is %&' more than

in horizontal direction.

Dimensions o components

+ The coe(cient of thermal expansion of brickwork in the vertical

direction is )fty percent greater than that in the horizontal direction,

because there is no restraint to movement in the vertical direction

+ The coe(cient of thermal expansion of brickwork in the vertical

direction is )fty percent greater than that in the horizontal direction,

because there is no restraint to movement in the vertical direction

Remedial Measures

+ Thermal movement cracks can be avoided by

introducing expansion *oints, control *oints and slip

*oints.

; +n structures having rigid frames or shell roofs

where provision of movement *oints is not

structurally feasible, thermal stresses have to be

taken into account in the structural design itself to

enable the structure to withstand thermal stresses

without developing any undesirable cracks.


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Elastic Deormation

tructural components of a building undergo elastic deformation

due to dead and the super imposed live loads, in accordance with

hook law. The amount of deformation depends upon elastic

modulus, magnitude of loading and the dimension of the

component.

This elastic deformation under certain circumstances causes

cracking in the building as under

+ -hen walls are unevenly loaded with wide variations in stress in

dierent parts, excessive shear stress is developed which causes

cracking in walls.

+ -hen a beam or slab of large span undergoes excessive deection

and there is not much vertical load above the supports /as in the


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case of roof slab0, ends of beam 1slab curl up causing cracks in

supporting masonry.

+ -hen two materials, having widely dierent elastic properties, are

built side by side, under the eect of load, shear stresses are set up

at the interface of the two materials, resulting in cracks at the

*unction. uch a situation is commonly encountered in the

construction of 2CC framed structure and brick masonry panel

/external0 and partition /internal0 walls.

Foundation movement and settlement

o soil

+ hear cracks in buildings occurs when there is largedierential settlement of foundation due to one of the

following cause.

/a0 3ne4ual bearing pressure under dierent parts of the

structure.

/b0 5earing pressure being in excess of safe bearing

strength of the soil.

/c0 6ow factor of safety in the design of foundations.

/d0 6ocal variations in the nature of supporting soil, which

remained undetected and could not be taken care of in the

foundation design at the time of construction.

/e0 7oundation resting in active zone on expensive soil.


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$ue to Chemical *eaction

+ Certain chemical reactions in building materials result in

appreciable increase in volume of materials, due to which

internal stresses are setup which may result in outward

thrust and formation of cracks. !he common instances of

chemical reaction are'

Sul+hate ,ttac

+ Soluble sulphates which are sometimes present in soil ,

groundwater or clay bricks reacts with tri calcium

aluminate content of cement and hydraulic lime in the

presence of moisture and form products which occupies

much larger volume than the original constituents . !hisexpansive reaction causes weakening of masonary ,

concrete and plaster and formation of cracks. or above

reaction it is necessary that soluble sulphate , tri calcium

aluminate and moisture , all three are present. $t takes

about two or more years before the effect of this reaction

becomes apparent. !he severity of sulphate attack

depends upon '

+ Amount of soluble sulphates present

+ -ermeability of concrete and mortar

+ Content of tri calcium aluminate in the cement used for

concrete and mortar

+ &uration for which the building components remain damp


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+ !he building components which are liable to sulphate

attack are concrete and masonry in foundation and plinth,

and masonry and plaster in super structure.

Car/onation

+ &uring hardening of concrete some calcium hydroxide is

liberated in the process of hydration of cement . $n course

of time , free hydroxide in concrete reacts with

atmospheric carbon dioxide forming calcium carbonateresulting in shrinkage cracks , since calcium carbonate

occupies lesser volume than calcium hydroxide . !his

phenomenon is known as carbonation .

+ $n good dense concrete carbonation is confined to mainly

to surface layer and depth of carbonation normally does

not exceed )mm in /years.

Corrosion of reinforcement in concrete and

brickwork

+ 0ormally concrete provides good protection to steel reinforcement

embedded in it . 1owever , when the reinforcements get corroded ,

it increases in volume with setting up internal stress in concrete . $ncourse of time it first causes cracks in the line with the direction of

reinforcement , later on causing spalling of concrete thus seriously

damaging the structure .

actors which contribute to corrosion of reinforcement in concrete are

+ -ermeability in concrete

+ Carbonation


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+ -resence of moisture

+ $nade2uacy of cover to reinforcement

+ -resence of soluble sulphates

+ Alkali aggregate reaction

Alkali aggregate reaction

+ $n ordinary portland cement , sodium oxide and potassium

oxide are present to some extent . !hese alkalischemically react , with certain siliceous mineral

constituents of aggregate and causes expansion ,

cracking and disintegration of concrete.

+ $n rcc members , it also causes corrosion of reinforcement

+ !his takes a number of years for the cracks to develop.

Creep

0ormally used building materials such as concrete, brickwork,

mortar, timber etc. when subjected to to sustained load not only

under go instantaneous elastic deformation but also under go a

gradual and slow time dependent deformation know as creep or

plastic strain.

!he factors on which the creep in concrete depends on are'

+ 3ater and Cement content

+ 3aterCement #atio

+ !emperature and 1umidity

+ 4se of Admixture and -o**olonas.

+ Age5Strength of Concrete at the time of loading


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+ Si*e and Shape of the component.

+ 3ater 6 Cement #atio ' Creep increases with the increase

in the water content, cement content, and the water

cement ratio.

+ !emperature and 1umidity ' Creep increases with the

increase in the humidity of the surroundings and the

age5strength of the material at the time of loading.

+ Admixtures and -o**olonas ' 4se of -o**olonas and

Admixtures increases the creep. Creep also increases

with the surface to volume ratio of the component.

$n 7rickwork'

+ !he creep depends upon the stress5strength ratio

therefore the creep in brickwork with weak mortar is

generally higher.

+ Creep in brickwork $ is approximately ) 8 6 )/8 that ofconcrete. $n brickwork it ceases after 9 months while in

concrete it may be continued upto a year or so.

+ :ost of the creep takes place in the first month and there

after its pace slows down.

"ffects of Creep

+ !he major effect of creep in concrete is the substantial

increase in the deformation of structural members, which

may be to the extent of ) to ; times the initial elastic

deformation.

+ !his deformation sometimes causes cracks in brick

masonry and load bearing structures.


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+ =a>' ' $n areas where old trees had been cut of to make way

for building consrtuction roots had dehydrated the soil.

On receiving moisture from sources, such as rain etc. the

soil swells up and causes an upward thrust on

!he following precautions should be taken while removing or

growing trees in close vicinity of the building.

(. &o not let trees grow too close to building, compound

walls etc. =within a distace of expected height of trees> if

any sapling of trees start growing in fissures of walls, etc.

remove them at the earliest.

). $f some large trees exist close to the building and these

are not causing any problem as far as possible, do not

disturb these trees if soil under the foundation happens tobe shrinkable.

;. $f removal of old trees within a distance of height of the

tree from the building is unavoidable then these should not

be removed all at once in operation, rather removal should

be done in stages.


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Report on cracks in buildings

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