Types of Shrinkage in Concrete
The defects in concrete due to presence of cracks, particularly in floors and pavements happens due to the volume change on account of inherent properties of concrete which is called shrinkage. It is difficult to make concrete which does not shrink and crack. It is the magnitude which is estimated.
Plastic Shrinkage
Plastic shrinkage is contraction in volume due to water movement from the concrete while still in the plastic state, or before it sets. This movement of water can be during the hydration process or from the environmental conditions leading to evaporation of water that resides on the surface on the wet concrete. So, the more the concrete bleeds, the greater the plastic shrinkage should be.
This can be reduced by preventing the rapid loss of water from surface.
Drying Shrinkage
The loss of free water contained in hardened concrete, does not result in any appreciable dimension change. It is the loss of water held in gel pores that causes the change in the volume. Under drying conditions, the gel water is lost progressively over a long time, as long as the concrete is kept in drying conditions.
Effect of Drying shrinkage
Autogeneous Shrinkage
It is the shrinkage due to chemical reactions between cement with water, known as hydration, and do not include environmental effects such as temperature and moisture changes. Its magnitude is usually ignored in concretes with W/C more than 0.40.
Carbonation Shrinkage
Carbonation shrinkage is probably caused by the dissolution of crystals of calcium hydroxide and deposition of calcium carbonate in its place. As the new product is less in volume than the product replaced, shrinkage takes place.
