Engineering and Construction

There can be many different reasons for the development of excessive concrete shrinkage. For example, excessive shrinkage cracking can develop in concrete if the mix was inadequately prepared with a water-cementitious materials ratio that is too high (i.e., too much water in the mix). Another possibility is that water may be added to the concrete at the job site in order to make the concrete easier to place and consolidate, in which case the extra water will cause excessive drying shrinkage. Other possibilities are that the concrete slab-on-grade is too thin (i.e., low volume-to-surface ratio) or it is not adequately cured. Figures 12.11 and 12.12 show two examples of excessive concrete shrinkage cracks in a slab-on-grade foundation.

There are three basic methods that can be used to limit or prevent the development of shrinkage
cracks in concrete slab-on-grade foundations, as follows (PCA, 1994):

1. Post-tensioned slabs. A concrete slab-on-grade foundation can be built with post-tensioning cables, which basically consist of steel strands in ducts that are tensioned after the concrete hardens. By tensioning the cables, the concrete is subjected to a compressive stress that will counteract the development of tensile stress due to concrete shrinkage. If the compressive stress is high enough, an essentially crack-free foundation can be created. Especially for a long slab-on-grade foundation, special effort may be needed to reduce subgrade friction.

2. Steel reinforcement. It has been stated that steel reinforcement equal to 0.5 percent of the cross-sectional area of the slab-on-grade foundation will be sufficient to essentially eliminate visible shrinkage cracking.

3. Expansive cements. Expansive cement is defined as  hydraulic cement that expands slightly during the early hardening period after setting. Concrete that contains expansive cement can be used to offset the amount of anticipated drying shrinkage. However, steel reinforcement is still needed in order to produce compressive stresses during and after the expansion period of the cement.