Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage.
Respected Alex Mrema sir,
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Concrete cracks mainly due to the following reasons.
1. Lack of proper curing of the cast concrete. Curing of the cast concrete must take place right after casting, and should include the following:
- Protection against heat, to avoid excessive drying.
- Protection against wind, to avoid excessive dehydration.
- Protection against very low temperatures(in temperate climate), to avoid damage by freezing water in pores or cracks if the cooling rate is high-higher temperature difference between surface and core.
- Protection against flowing water, to prevent washing out of concrete
- Protection against chemical attack
- Protection against vibrations
Curing is essential for all concrete members the surface of which has to be resistant against water, chemical attack, frost and other effects. Long, extended construction units, e.g. bridges and roads, should especially be prevented from gaining any heat other than the minimum possible. They should be allowed to cool slowly and should not be allowed to dry out suddenly, in order to prevent tensile stresses in hardened concrete due to temperature and shrinkage.
In order to ensure a sufficient moisture content the surface of the concrete should be kept wet, normally for 7 days, by spraying it with water or covering it with either plastic sheets, wet session sacks or straw.
2. The other possible causes of cracking could be due to
- design errors
- poor construction work
- inadequate supervision and checking of design, construction and material quality
- departure of material and structural behavior from the usually assumed laws of Hook and Navier
- etc.
Construction joints are normally provided in structures to allow for free expansion due to temperature changes, swelling etc.
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