Monday, March 17, 2014

How to prevent cracks in building

Cracks in building are a common occurrence and it is important to understand the causes and the measures to be taken for their prevention.Materials such as bricks,mortar and concrete, which contain considerable quantity of water at the time of construction, dry out subsequently and under go major contraction.



GENERAL MEASURES FOR PROTECTION OF CRACKS..

The measures for prevention of cracks could be broadly grouped under the following main sub heads :-
a) Selection of materials:-
Good quality of building materials according to the specification if used that will minimize the cracks in the building.
b)Specifications for mortar and concrete:-
The ratio of the mortar and concrete should be according to the prescribed design. Care should be taken that the coarse aggregate, fine aggregate should be free from silt suspended impurities and the mix should be homogeneous, well mixed and contain no excessive of water.
c) Drawing and Design of the building:-
While doing the construction work care should be taken that foundation of the building should not be laid on loose /muddy soil. The mortar, M S steel reinforcement should be according to the design.
d) Good Construction practices:-
It is necessary that work proceed uniformly in all part of building. The workmanship should be according to the prescribed norms and best practice in the building construction.
e)Weather Effect:-
The construction work should be avoided in very hot and dry weather, and during very low temperature also.
In general quality of water, sand ,Bajri , mixing of mortar ,quantity of water in mix , socking of bricks and workmanship have great effect on construction work. If possible the subsequent items of construction should be done after the drying of work previously done.

1)Diagonal cracks in bricks wall:-

Causes :-
a) Differential settlement of foundation .
b) Shrinkage of clayey soil below foundation. This some time happen from the absorption of moisture by the tree roots in the vicinity.
PREVENTIVE MEASURES:-
a) Building should be constructed over soil strata have in uniform characteristics.
b) Adequate plinth protection and drainage arrangements should be made around the building to minimize water entry in the foundation.
c) Avoid construction of walls on filled up soil
d)Do not let trees grow too close to buildings an d compound wall specially if the soil happen to be shrinkage soil/clay.

2)Vertical cracks in long compound wall:-

Causes :-
Do not leaving expansion/contraction joints at proper interval , in cold weather when the wall tense to contract due to temperature drop but being not allowed to contract freely. It develop tensile stress and when the tensile stress exceed the strength, the wall cracks. Similarly in hot weather wall tends to expend and if expansion is not allowed to occur freely compressive stresses will develop in the wall and when they exceed the strength , wall will break.
PREVENTIVE MEASURES :-
Leave expansion/contraction joints at regular interval specially in long stresses of wall.

3)Cracks in load bearing masonry wall below RCC slab:-

Causes :-
Absence of slip joints between RCC slab and wall does not allow RCC slab to move freely over wall and leads to cracking in the wall. Some times the movement of the slab may also cause cracking in masonry at lintel and window sill level because here is the masonry is weak. These cracks are observed mainly on the top most story of the building because roof is more exposed to temperature variation 
PREVENTIVE MEASURES :-
Before casting RCC slabs over brick wall , smooth bearing plaster should be done over brick walls coupled with white wash / bitumen coating . over it. It allowed slab to move freely over wall .


4)Vertical Formation of cracks at the base of parapet wall:-

Causes :-
An instant of very frequent occurrences of thermal cracks in building is the formation of horizontal cracks at the sport of a brick parapet wall. Factors which contribute to this type of cracking are :-
a) Thermal coefficient of concrete is twice that of brick work and thus differential expansion and contraction causes a horizontal shear stress at the junction of the two materials.
b) Drying shrinkage of concrete is three to four time that of brick masonry.
c) Parapet are generally built of concrete slab before the latter has undergone its drying shrinkage fully . Parapet or railing does not have much self weight to resist horizontal shear force at it sports caused by differential thermal movement and differential drying shrinkage
PREVENTIVE MEASURES :-
Construction of masonry over the slab should be deferred as much as possible ( at least one month ) so that concrete under goes some drying shrinkage prior to construction of parapet. Mortar of parapet masonry should be rich and a good bond should be ensured between masonry and concrete. Plastering on masonry and RCC work should be deferred as much as possible and made discontinuous at the junction by Providing V groove in plaster. This way the cracks if occur , will get concealed behind the groove and not be conspicuous. In case of iron railing , cracks could be avoided by substituting the brick work with a RCC wall, sporting RCC railing.


5)Vertical cracks at junction of main wall and cross walls:-

Causes :-
Improper bonding of cross walls with main walls by not leaving proper keys in the main wall.
PREVENTIVE MEASURES :-
Main and cross walls should be properly bonded by proper toothing.


6)Vertical and horizontal cracks in panel walls in RCC framed structure:-

Causes :-
Panels wall are built very tightly to the RCC beams so that RCC beams are not free to deflect, load is transmitted to the wall and vertical and horizontal cracks are produced in the wall depending upon the direction in which wall is more slender ( i..e. Along height or length ) .
PREVENTIVE MEASURES :-
Some gap should be left between top of panel wall and bottam of the beam/slab so that beam can deflect freely. This gap can be filled by weak mortar. If lateral support is required to be given to wall beam/slab then some special lateral connections can be made in such a way that free deflection of beam is not hindered in vertical direction.


7)Horizontal cracks in the top most story below slab:-

Causes :-
Cracks are due to deflection of slab and lifting up of edge of the slab. Cracks are mostly confined to the top most story because of light vertical load of parapet wall on slab due to which end of the slab lifts. These cracks normally occur in roof slabs of large span.
PREVENTIVE MEASURES :-
a) Bearing plaster should be done on top of wall below RCC slab with a coat of white wash or bitumen.
b) Avoid large spans of roof slab , if possible provide beams to reduce the length of the slab.


8)Diagonal cracks in panel walls in RCC framed structure:-

Causes :-
When RCC frame moves/deflects due to loads, temp,variation etc. it causes diagonal cracks in walls which are located parallel to the movement.
PREVENTIVE MEASURES :-
Walls should not be build tightly to the columns. For taking lateral supports from column, special connections should be made between wall and column which provide lateral supports to wall but does not tie it too tightly to the column .


9 )Vertical cracks below openings in line with window jambs:-

Causes :-
These cracks are caused due to vertical shear caused by differential strain in the lightly loaded masonry below the opening and heavily loaded portion of wall having no opening.
PREVENTIVE MEASURES :-
A) Too much disparity in stress in different walls or parts of a wall should be avoided.
B) Portion of wall acting as pillars and having too much concentrated loads should be replaced by R R C pillars., if possible.


10)Cracking due to arching and up heaving of a floor:-

Causes :-
a) Due to sulphate action in base concrete.
b) Due to expansion of clayey soil below by absorbing moisture
PREVENTIVE MEASURES :-
a) Take all measures to prevent contact of sulphates with cement or use sulphate resistant cement.
b) If clayey soil exists below building , measures should be taken to prevent water entry under the ground by constructing suitable plinth protection and drains etc.


11)Arching up and cracking of coping above retaining/boundary wall:-

Causes :-
Due to not leaving expansion joints wall & coping.
PREVENTIVE MEASURES :-
Leave proper expansion joints in walls. Avoid long stretches of wall.

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