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Immediately after placing the concrete, solid particles tends to settle down by gravity action and water rises to the surface. This process is known as bleeding in concrete. The bleeding process produces a layer of water at the surface and this process continues till concrete has set.
During construction, concrete cracks are occurs due to plastic shrinkage on the surface of freshly laid concrete before it has set.
When the concrete surface loses water faster than the bleeding action, initial shrinkage of top layer takes place. Since the concrete is still in the plastic state (not completely stiffened) can’t resist any tension and concrete cracks develop on the surface. If there is a continuous layer of water at the surface then shrinkage does not occur.
Plastic Shrinkage Cracks in Concrete:
Plastic shrinkage cracks are common in slabs. In slabs due to rapid drying, the surface concrete turns very stiff to flow and then it will possess no strength to withstand the tensile stresses caused by restrained shrinkage. This causes concrete to develop cracks on surfaces.
Courtesy – Gujarat Ambuja Cements ltd.
Moreover, the most notable feature of plastic shrinkage cracks is that they do not normally extend to the edge of the slab because this is free to shrink without restraint.
Plastic shrinkage concrete cracks should not be confused with ordinary drying shrinkage cracks which occur only after a considerable time. Plastic shrinkage cracks form within the first 3 hours of casting although they are often not noticed until the following day. They are not directly affected by the reinforcement in the concrete.
(It can provide you with an overview of all the types of cracks, which can affect the structure either in short-time or long-time.)