Shrinkage of concrete from Drying
The common cause of cracking in concrete is shrinkage due to drying. This type of shrinkage is caused by the loss of moisture from the cement paste constituent, which can shrink by as much as 1% per unit length. Unfortunately, aggregate provides internal restraint that reduces the magnitude of this volume change to about 0.05%. Upon wetting, concrete tends to expand.
These moisture-induced volume changes are a characteristic of concrete. If the shrinkage of concrete could take place without any restraint, the concrete would not crack. It is the combination of shrinkage and restraint, which is usually provided by another part of the structure or by the subgrade that causes tensile stresses to develop. When the tensile stresses of concrete are exceeded, it will crack. Cracks may propagate at much lower stresses than are required to cause crack initiation.
In massive concrete structure elements, tensile stresses are caused by differential shrinkage between the surface and the interior concrete. The larger shrinkage at the surface causes cracks to develop that may, with time, penetrate more deeply into the concrete. Magnitude of the tensile stresses is influenced by a combination of factors:
- Amount of shrinkage
- Degree of restraint
- Modulus of elasticity
- Amount of creep
The amount of drying shrinkage is influenced mainly by the amount and type of aggregate and the water content of the mix. The greater the amount of aggregate is, the smaller is the amount of shrinkage. The higher the stiffness of the aggregate is, the more effective it is in reducing the shrinkage of the concrete. This means that concrete containing sandstone aggregate has a higher shrinkage rate—about twice that of concrete containing granite, basalt, or limestone. The higher the water content is, the greater is the amount of shrinkage from drying.
Surface crazing on walls and slabs is an excellent example of shrinkage from drying on a small scale. Crazing usually occurs when the surface layer of the concrete has higher water content than that of the interior concrete. The result is a series of shallow, closely spaced, fine cracks.
REDUCING DRYING SHRINKAGE IN CONCRETE
Drying shrinkage can be reduced by using the maximum practical amount of aggregate in the mix. The lowest water-to-cement ratio is important to avoid this type of shrinkage. A procedure that will help reduce settlement cracking, as well as drying shrinkage in walls, is to reduce the water content in concrete as the wall is placed from the bottom to the top. Using this procedure, bleed water from the lower portions of the wall will tend to equalize the water content within the wall. To be successful, this procedure needs careful control and proper consolidation.
Cracking due to shrinkage can be controlled by using properly spaced contraction joints and proper steel detailing. It may also be controlled by using shrinkage compensating cement.