Shrinkage is generally due to drying. Like creep, shrinkage is also time dependent process. In fact shrinkage and creep are not independent process but for safe calculation they are understood separately. The factors which effect creep also effect shrinkage. For designing, it is taken as 0.003 mm/mm. Rate of shrinkage decreases with time, almost 50% of total shrinkage takes place in 1 month and 75% of total shrinkage is taken taken place in 6 month.
Shrinkage in concrete is of following types:
a. Plastic shrinkage
it initiate as soon as concrete is placed in the formwork when concrete is in plastic stage. This shrinkage is absorbed due to loss of water by evaporation or absorption of water over the surface of aggregate. To avoid this shrinkage either aluminium is used or expanding powder is used.
b. Drying Shrinkage
It is everlasting process and occurs mainly due to loss of water held in gel pores of cement when concrete is kept in drying condition. The finer is gel, more is drying shrinkage. Harder aggregate leads to reduce shrinkage. shrinkage decreases with increase size of member. Concrete made with smaller aggregates gives higher shrinkage. If grade of concrete is increased then shrinkage reduces.
c. Autogenous Shrinkage
It is type of shrinkage which take place where there is no movement of moisture. It takes place due to utilization of water by constituent of concrete for hydration. It is minor importance. This shrinkage increases with the increase of grade of concrete.
d. Carbonation Shrinkage
When CO2 present in the atmosphere reacts with the product of hydration i.e. ca(oH)2 in the presence of water and forms CaCO3 ppt & water. This is accompanied by increase in weight of concrete reduction of permeability and increase in strength.
Reduces permeability due to CaCO3 and increase in strength is due to release of water which participates in hydration process.
