Factors that contribute to the formation of cracks in cement concrete are discussed below.
8.11.1 Plastic Shrinkage Cracks
Evaporation of water from fresh concrete is due to the absorption by formwork and during the hydration process. The surface of concrete dries up when the loss of water from the surface of the concrete is faster than the migration of water from the interior to the surface. This creates a moisture gradient which results in surface cracking while the concrete is stiff in a plastic condition. The magnitude of the crack depends upon the rate of evaporation of water from the surface of the concrete.
8.11.2 Settlement Cracks
Plastic concrete settles when vibrated. There will be no cracks if the concrete settles uniformly. Some cracks are bound to occur if there is any obstruction to uniform settlement due to reinforcement or larger pieces of aggregates. Such cracks are called settlement cracks. Such cracks are formed generally in deep beams.
8.11.3 Cracks Due to Bleeding
Water being light in weight moves upward in concrete, which leads to bleeding. This water evaporates to make the top surface porous having very little abrasion resistance. Masons often float the concrete when the bleeding water is still standing on the surface. Too much of working on the exposed surface leads to pressing the coarse aggregate down and bringing up fine particles of cement and water. Such a condition develops cracks on the surface.
8.11.4 Cracks Due to Delayed Concrete Curing
During the initial stages of curing, uninterrupted hydration is a must. Due to the hot sun, winds and lower relative humidity, young concrete dries faster leading to plastic shrinkage cracks.
8.11.5 Cracks Due to Construction Effects
Improper formwork with less rigidity may lead to sinking, bending, etc., and the wet concrete may cause cracks or deformation after compaction which may go unnoticed.
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