Shrinkage Cracking

The most important structures that experience shrinkage-induced cracks are floors, slabs, and pavements. Typical cases of dry shrinkage can be seen in figure 1. Both cases depend on the balance of time dependent stresses and the material properties. The amount and rate of shrinkage depends on the type of concrete and the surrounding temperatures. In Figure 1-a, the single crack has a crack width proportional to the dry shrinkage strain. In Figure 1-b, the concrete cracks are in a distributed pattern due to the effect of the frictional bond between the slab and the sub-base. The most severe initial cracks, such as the one seen in Figure 1-a, are randomly located throughout the concrete, and filters to the surface from low external stress. These cracks are separate of each other and called primary cracks. However, under high external stress, additional cracks form between the primary cracks. These cracks are called secondary cracks. Furthermore, when the axial stresses increase and no more primary cracks form, the secondary cracks begin to widen. The contribution of drying shrinkage to concrete cracking can be controlled by calculating a proper mix design, proportioning the concrete member to minimize differential shrinking stresses, optimizing curing procedures, and proper use and application of joints.

Figure 1: Typical cases of internally loaded and time dependent strains caused by shrinkage