Defects in Construction

8/16/2019 Defects in Construction

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DEFECTS IN CONSTRUCTION

CHAPTER # 5


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DEFECTS IN BRICK WORK

Common defects occurring in brick work are• Sulphate attack on mortars

• Unsound materials

•

Frost action• Corrosion of iron and steel

• Crystallization of salts

•

Linear changes resulting from variation in moisturecontent


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SULPHATE ATTACK ON MORTARS• Sulphate attack leads to expansion of mortar, thereby, causing

cracking of brickwork, spalling of brick edges, deterioration of

mortar, wide horizontal and vertical cracks in the plaster and

falling of the plastered surface• !he cause of this attack is the chemical action between the

sulphate salts in bricks and constituents of "ortland cement

• !his action is rapid in the presence of water and hence

wherever moisture penetrates, excessive dampness occurs• !his type of defect may be prevented by preventing moisture

penetration

• #t will avoid the defect to a large extent $ricks of low sulphate

content and the sulphate resisting cement should be used.


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UNSOUND MATERIALS• Unsound materials cause the formation of small pits at

the mortar %oints

• &eneral expansion and cracking of brickwork is visible

• Unsoundness in lime is caused by the presence of un'

slaked particles of lime, present in the mortar Similarly

un'slaked lime particles may be present in the bricks

alsoFROST ACTION

• (efects due to frost action would cause cracking in

brickwork "revention of water accumulation would

prevent this defect


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CORROSION OF METALS• $rickwork may get opened or cracked or stained due to

corrosion of metals lying ad%acent to it Unprotected iron

and steel are liable to get corroded when acted upon by

moisture and they increase in bulk, thereby causing

cracks in masonry

• "rotecting the metal surface with cement mortar up to a

layer of ) to * cm thick is essential to prevent corrosion• "artially embedded steel or iron members should be

surrounded with bituminous compound for portions not

embedded in mortar


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CRYSTALLIZATION OF SALTS (OR EFFLORESCENCE)• !his is a prominent defect in brick masonry

• #n moist climate, in damp places, like basements or under

leaky gutters, masonry often gets disfigured by the formation

of a white deposit called efflorescence• deposit originates from the mortar and fre+uently spreads over

a part or entire face of the wall

• bsorbed water dissolves the salts of sodium, potassium and

magnesium contained in the mortar and while evaporating,forms a crystalline deposit on the surface

• #n addition to unsightly appearance, the crystallization of salts

in the pores of the bricks or mortar may cause disruptive

expansion resulting in disintegration due to cracking


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-./.(0• voiding the use of porous bricks in contact with limestone

• "rotection of brickwork against contamination of salt'bearing

materials during building operations

• From being thoroughly soaked during construction• Correct design of ("C

SHRINKAGE EFFECTS

• $rickwork may crack due to the shrinkage movements arising from

changes in moisture content

• !his defect is more common with concrete and lime mortars

-./.(0• &ood +uality bricks should be used in dry condition

• ll the work should be protected from rain


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DAMPNESS IN OLD STRUCTURES

•

$efore applying any remedial measure to a damp wall thereshould be free escape for any water that has already entered

in the wall

• Silica solution is transparent and very effective in resisting

dampness• #nternal treatment of affected walls would consist of

removing the old plaster, applying a slurry coat of neat

cement with a water proofing compound and then cement

rendering with a dense mortar, of )1 * with integral water proofer added

• nother internal treatment for damp walls is the application

of an impervious coating of some material or a coating of

bitumen or tar followed by blinding with sand and plastering


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DAMPNESS IN OLD STRUCTURES•

!he following methods are also used for preventingdampness in walls

a2 !wo parts by weight of coal tar and one part by weight of

pitch are put in a vessel, heated and stirred, until the mixture

is sufficiently li+uid #t is then applied on walls, and is foundto keep out dampness very wall

b2 Spray or paint the walls with a solution of sodium silicate,

followed by a solution of calcium chloride, which forms an

insoluble silicate• #f dampness is confined to one position near ground floor

level above the ("C, it may be due to a hole or crack in

("C through which moisture can pass into the wall

alone


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DAMPNESS IN OLD STRUCTURES

•

(ampness below ground level may be due to lack of sub'soil drainage, absence of vertical ("C or leaking drains

-.".LL#3& /!.-#LS

4ater repelling compounds are mixed in cement to make

it water repellant .xamples are1

• Chemically active ''''''''''''''''' Sodium and potassiumsoaps

• Chemically inactive ''''''''''''' Calcium soaps and resin

oils


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DEFECTS IN PLASTER WORK

Cracks in plaster work are chiefly due to1

• Structural defects in buildings and discontinuity of

surface

• "lastering on very wet background

• 5ld surfaces not being properly prepared

• 5ver'rapid drying

• "itting and blowing are due to partly slaking andhydration of the lime particles in plaster


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DEFECTS IN PLASTER WORK

Falling out of plaster is chiefly due to1

• Lack of adhesion for not having formed a proper 6key7

in the back ground

• .xcessive moisture in background

• .xcessive thermal changes either in background or in

plaster

• -apid drying

• #nsufficient drying between each coat of plaster


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DEFECTS IN CONCRETE CONSTRUCTION

• !here are three basic defects in a concrete structure, cracking,

spalling, and disintegration

• !hese defects may be defined as a general decay of the

surface involving loss of cement paste and loosening of the

particles of coarse aggregates!he following defects are associated with concrete construction1

• Settlement of foundation or soil

• /ovement of formwork or disturbance of concrete during

setting• #nternal settlement of concrete

• Setting shrinkage

• Corrosion of reinforcement


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S.!!L./.3! 5F F5U3(!#53 5- S5#L#f there are local soft particles in the soil on which the

concrete is placed, or if there are any air pockets, there

will be a local settlement of the concrete due to weightof the plastic mass #f this settlement occurs after

finishing of the concrete surface, cracks will appear

-./.(0

!he occurrence may be prevented by giving proper

attention to compacting and draining the soil below


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/58./.3! 5F F5-/45-9 5- (#S!U-$3C. 5F

C53C-.!. (U-#3& S.!!#3&• ny movement of the formwork, which occurs between the times that the

concrete begins to lose its fluidity and the time that it has fully set, will

cause cracks to appear in the structure

• !hese cracks may be internal and visible by surface inspection s such,they form a water pocket in the concrete mass, which upon freezing :in

very cold climate2 will expand the concrete surface /oreover, corrosion

of the reinforcement can occur from such water pockets

-./.(0• "revention of such concrete re+uires that surfaces of the formwork in

contact with the concrete be coated with suitable mould oil to prevent

absorption of moisture and conse+uent swelling

• Formwork should be properly designed particularly with respect to the

details and deflection considerations


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#3!.-3L S.!!L./.3! 5F C53C-.!.

• Fluid concrete before attaining initial set, is sub%ected to

settlement of the heavier particles through the fluid matrix

• Since the surface concrete hardens first, if such settlement is not

prevented, local cracking will occur

• Still worse condition may be where the reinforcement consists of

a heavy material of closely spaced bars #nstead of surface

cracks, a plane of general separation may be formed

-./.(0

• Surface cracks can be closed by delayed finishing of concrete

surface Commencing the curing operation as soon as possible,

after placement of concrete, is also beneficial #t reduces the

amount of differential settlement between the surface and interior

volume of the suspension "roper vibratory compaction is a must


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S.!!#3& S;-#39&.• 8olume changes during the initial setting of concrete tend to

cause the formation of shallow surface cracks

• Setting shrinkage is normally considered after *< hours

• Shrinkage is mainly due to migration of moisture in theconcrete

• Chemical action due to sulphates may also cause shrinkage

•

soft sandstone aggregates are more sub%ected to shrinkage-./.(0

• "roviding the movement of %oints :such as expansion %oints2

will reduce the effect of cracking due to shrinkage

• "rovide an ade+uate +uantity of distribution reinforcement


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C5--5S#53 5F -.#3F5-C./.3!

• !he reinforcement is normally placed within a few inches

of the surface

• #f the reinforcement is exposed to air and water, it will

corrode• !he volume of the oxide produced by corrosion is about =

times that of the parent metal, and the result is that the

concrete cover is cracked and spalled

-./.(0• !he steel be kept away from contact with water in the

presence of oxygen !his can best be accomplished by

encasing the bars in a dense concrete mass and providing

ade+uate cover


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•

9eep weep holes open and see that roofs, balconies, pro%ections and similar features drip :dropping of water2

clear of the lower structure

• Calcium and /agnesium Chlorides, used to accelerate

the setting of concrete, will also promote corrosion, if present in excess amount

• #f used, their +uantity must not exceed *>

•

&ood concrete has *> absorption of water, however, it is permissible up to )?> $ut exceeding this >age will

increase the corrosion effect


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C;./#CL (.!.-#5-!#53 5F C53C-.!.

• Concrete is chemically stable, but deteriorates if sub%ected to

aggressive chemical agents, such as Sulphate, acids

• Sulphate solutions react with the constituents of cement,

forming calcium'sulpho'aluminate hydrate !his reaction isaccompanied by a substantial expansion and causes cracking

and disruption of the concrete mass.

-./.(0

• ;igh alumina cement is a remedy for sulphate attack

• #n acid environments, the use of limestone aggregate

will somewhat prevent attack by neutralizing a portion

of the acid


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• more positive answer however is, to form a protective

coating $ituminous coatings work nicely for exposure

to sulphate or weak acid solutions

• facing of ceramic tile set in acid'proof mortar is an

excellent precaution• .ven a little thing, like fre+uent cleaning for the concrete

surface with water is a big help

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Defects in Construction