SHRINKAGE

The shortening in length of a member of contraction of the concrete per unit length due to drying when concrete sets is known as shrinkage.
Shrinkage can be classified as:
(a). Plastic Shrinkage 
(b). Drying Shrinkage

Plastic Shrinkage

This type of shrinkage menifests itself soon after the concrete is placed in the form while the concrete is still in plastic state. Loss of water by evaporation from the surface of concrete or by absorption by aggregate or subgrade is believed to be the reason for plastic shrinkage.

As aggregate and steel restrain this effect cracks appear at the surface or internally around aggregate. This shrinkage is prevented by using aluminium powder and expanding cement.

Drying Shrinkage

1. It is an ever lasting process and occurs mainly due to loss of water held in the gel pores when concrete is kept in drying condition.
2. The finer the gel, the more is the shrinkage.
3. Harder aggregate leads to lower shrinkage but higher shrinkage stress. reverse is true for softer aggregate.
4. Shrinkage decrease with increase in size of member.
5. Concrete made with smaller size aggregate shrinkage more than concrete made with larger size aggregate.
6. Shrinkage produces tensile cracks in any member which is restrained. 
7. For a given environment, the total shrinkage depend more on the total amount of water present in concrete at the time of mixing and to a lesser extent on the cement content.
8. In the absence of test data, an approx value of the total shrinkage strain for design may be taken as 0.0003.
9. Shrinkage decreases with increase in relative humidity.
10. 1/2 of the total shrinkage assumed in 1st month and 3/4th in 1st 6 months.