Shrinkage Of Concrete

When concrete structures are not permanently water-saturated, it starts to lose moisture and starts to shrink from the time when the process of curing ceases.

The 7 primary structural effects of shrinkage are as follow.

1. Joints

Shrinkage of concrete between movement joints causes joints to open or makes it wider. Therefore joints must be designed to accommodate the widening caused by shrinkage.

2. Other materials

Where other materials, such as ceramic tiles, are fixed on top of concrete surface, shrinkage of the concrete causes relative movement between the different materials. The resulting stresses can cause failure at the interface.

3. Cracking

If shrinkage is restrained, the concrete is put into tension and when tensile stress becomes equal to tensile strength, the concrete cracks.

Structural designers can design structures to minimize restraint, prestress the concrete to prevent tensile stress, or use reinforcement to control cracking.

4. Loss of Prestressing Force

Shrinkage causes a reduction in prestressing force. When calculating prestresing forces, designers take into account to ensure that residual stress is structurally adequate.

5. Repairs

If concrete is used to fill a cavity in old concrete, shrinkage of the new concrete is restrained by the surrounding old concrete. Repair concretes and mortars must be specially formulated (by incorporating a polymer material) to prevent cracking caused by this restraint.

6. Bond Strength

Shrinkage of the concrete causes the concrete to grip reinforcing bars more tightly. This increases friction between concrete and steel and so improves bond strength, especially for plain bars.

7. Deflections

The deflection of flexural members is increased by shrinkage. This is because the lightly reinforced compression zone is free to shrink more than heavily reinforced tension zone.