Cracking lines on the surface of the concrete may be found along which it has split without breaking apart. These lines are referred to as concrete cracks.
Concrete is prone to cracking due to several reasons. The reasons for cracking of concrete are as follows:
Let us see each one of them in detail below.
Shrinkage cracks are often seen in large slabs of concrete, relatively straight parallel with the span of floors. They are mostly seen and occurred due to inadequate provision of distribution steel in concrete.
IS: 456 – 2000, the Indian standard code for plain and reinforced concrete, suggests that the minimum reinforcement in slabs in either direction shall not be less than 0.15 percent in case of mild steel reinforcement and 0.12 percent in case of high strength deformed bars, of the total cross-sectional area, to avoid shrinkage cracks.
Shrinkage cracks occur when subjected to a very rapid loss of moisture due to various factors such as the temperature of air and concrete, relative humidity, and wind velocity at the surface of concrete. Especially when the terrace slab develops shrinkage cracks, it starts creating problems such as water leakage from the cracked portion of the slab.
We know that some of the materials expand on heating and contract on cooling. It is a well known principle of science. This phenomenon is also applicable to concrete and steel.
When a concrete structure or a part of it loses heat of hydration at different rates, the temperature differences are seen on the concrete. The difference in temperature results in differential volume changes. This causes the development of cracks in concrete structures.
Drying shrinkage is also a common cause of development of cracks in a concrete structure. Drying shrinkage mainly occurs due to loss of moisture from the cement paste constituents, which can probably shrink by as much as 1 percent.
Change in volume of concrete due to moisture is a characteristic of concrete. The combination of shrinkage and restraint causes the development of tensile stress in concrete. The tensile stress, when exceeds than the maximum permissible tensile stress, it starts developing cracks on the concrete structure.
Harmful chemical reactions may occur in concrete, causing cracks in it. The chemical reactions may be due to the materials which are used in making concrete or the materials that come into contact with the concrete after it has hardened and matured properly.
Cracks in concrete may occur with time as the result of slowly developing expansive reactions between aggregates containing active silica and alkalies derived from cement hydration, admixtures, or external sources.
The reaction results in the formation of a swelling gel, which tends to draw water from other parts of the concrete. This causes local expansion and accompanying tensile stresses, and mass eventually results in the complete deterioration of the structure.
Cracks in concrete may also develop due to weathering processes such as freezing and thawing, wetting and drying, and heating and cooling too. Concrete may develop cracks due to the freezing of water or moisture in the concrete paste. Damage in hardened concrete from freezing is caused due to the movement of water to freezing sites and by hydraulic pressure generated by the growth of ice crystals.
Poor construction practices may be the result of cracked concrete sometimes. Some of the poor practices performed on site due to which the concrete may crack are as follows:
– Adding excess water to concrete to improve its workability.
– Improper and insufficient curing.
– Lack of sufficient formwork support.
– Inadequate compaction of concrete.
– Placement of construction joints at point of high stress.
1. There are many ways to prevent cracks in concrete. They are as follows:-
2. Curing should be done properly to the concrete structure. To keep the moisture intact for a longer period, gunny bags are used.
3. Addition of water as per the requirements of the concrete, according to the water-cement ratio. Excess water may damage the concrete.
4. Use of good quality materials for the preparation of concrete and preparing according to the proper mix design.
5. Proper vibration should be done while concreting.
6. Proper placement of concrete from a suitable height, not more than 1m, if the concrete is to be laid from a height.
7. To avoid formation of cracks in concrete, shrinkage-reducing admixtures can be used.
8. Providing proper control joints in concrete.
9. Avoid using the calcium chloride admixtures in the concrete mix.
There are various materials that are used to repair the cracks in concrete. They are as follows:
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