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Concrete Shrinkage Issues

When concrete is mixed, there is much more water added into the mix for workability than is needed to hydrate the cement. This is done so that it easily combines with the cementitious materials in the mix. This is called excess or free water. Eventually, this water evaporates from the concrete leaving microscopic voids in the concrete matrix. The evaporation causes the concrete to lose mass and shrink. Cracks can occur in framed floor slabs and slabs on grade as a result of the concrete dragging (friction) on the support material as the concrete volume reduces. Curling can also occur when the top (exposed) surface of a slab changes volume at a rate different than that of the slab surface on the (bottom) supporting material.