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To reduce concrete dry shrinkage, only use enough water to produce the required workability for placing, consolidating and finishing concrete. As the water content of the freshly mixed concrete increases, so does the potential for dry shrinkage. Concrete suppliers can minimize the water content by adjusting the cementitious material content and combining aggregate gradation sizes to achieve a uniform aggregate distribution. Also, minimize the addition of water on site when adjusting slumps because additional water increases dry shrinkage of the concrete.

The risk of cracking can be appreciably reduced by reducing the number and severity of restraints. The biggest restraint preventing concrete shrinkage or slab shortening is the subbase. Always place concrete on flat, hard subbases that are free of ruts and holes. If necessary, use a thin layer of fine material to fill in the surface voids of rough subbases so the bottom of slabs are free to slip or move relative to the subbase. Other slab restraints can be eliminated by isolating slabs from footings, walls, columns and other elements such as drains, manholes and sumps by inserting preformed joint fillers between slabs and adjacent elements. Do not connect slabs to other elements with steel reinforcement or tie bars.

Concrete strength and shrinkage stresses

As concrete hardens, it gains considerable compressive strength but little tensile strength. Tensile strength of concrete is about one-tenth of its compressive strength. Also, concrete is a brittle material — when stretched, it cracks.

When shrinkage stresses due to dry shrinkage and thermal contraction exceed the tensile strength of the concrete, cracking occurs. Contraction joints must already be in place to control the location of the cracks. When joints are spaced close enough, cracking in the joints relieves the shrinkage stresses between joints so that out-of-joint cracking is unlikely.

Sawed contraction joints are merely continuous sawed slots in tops of slabs that provide weakened or thinner sections of concrete so cracking occurs at these locations, as shown in Figure 1.