Causes of cracks in slab
Why are there cracks in the slabs next day of casting?
Following are some of the reasons
for cracks in slab: -
1)
Casting time temperature: -
According to the IS code, the casting time temperature of the
environment should be between (5-40) degrees. There will be plastic shrinkage
due to the high temperature of environment while casting. Due to high temperature, the water in the concrete gets evaporated faster than it should. So, in the summer
season, it is better to start casting of slabs in the evening when the temperature
gets normal.
2)
Steel and surface heat: -
Steel and surface heat also effects concrete so
we make ta steel and surface wet before concreting.
3)
Silt in Sand: -
If there is a high amount of silt in the sand then it causes cracks in concrete. Silt is a particle which effects the
joining of the cement and aggregates. If the silt in the sand is greater than
5% by weight then the sand must not be used. If needed to check-in the site if
there is silt in the sand or not then take a hand full of the sand and rub it
properly in the hand. Then if we can see soil or silt in the hands then sand is
not good to use.
4)
Old stock cement: -
Use of old cement also causes
cracks in the concrete because old cement might have lost its strength after
being in stock for a long time. To prevent this supervisor and storekeeper should
be careful when choosing the cement.
5)
Concrete mix: -
Concrete mix should be carefully
prepared in the site with the proper proportion of sand, cement, and aggregate. It is
better to calculate and prepare a design mix rather than nominal mix because
the quantity of material depends upon the quality of material. So good
calculation should be done for preparing the mix to get the desired strength.
6)
Less cover provided: -
Not providing enough cover is also
one of the causes for the cracks in the concrete. Hence proper cover blocks
should be provided before concreting.
7)
Wrong Admixtures: -
Whenever we use admixture then we
must be sure that how much quantity of the admixture should be used and which
admixture needs to be used. And must be sure about the expiry date of the admixture.
Hence, we need to be careful during the use of admixture else cracks might be seen
in the concrete.
8)
Water cement ratio: -
The Water-cement ratio must be
maintained according to the design of concrete. If water-cement ratio is very
low then the workability of the concrete decreases and the compaction of concrete is also less. If the water-cement ratio is higher then it does decrease the
strength of the concrete. If there is high amount of water then there might be
shrinkage of concrete and this may cause cracks in concrete.
9)
Concrete pour within 1 hour: -
It is better to use the concrete
within 1 hour of its batching. The initial setting time of cement is 30 minutes and the final setting time is 10 hours.
10)
Staging Settlement: -
Sometime while casting slab
concrete due to the weight of the concrete there may be a settlement of the formwork
or the plywood might break. These things might also bring cracks in the concrete
due to uneven casting.
Hence these are the reasons for
cracks in slabs that occur a day after casting. Among these, the main reason
is the temperature effect. Most of the contractors avoid this factor but this one
of the major reason for the cracks in concrete slabs in the building.
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