New slabs-on-ground crack when tensile stresses from restrained shrinkage exceed the concrete's strength. This type of early-age cracking occurs during the first few days of the slab's life and is caused by early concrete volume changes related to dry shrinkage and thermal contraction. Sawcut contraction joints are the most common method of controlling early-age cracking — when properly installed, cracking should occur in the joints.
Sawcut contraction joints do not prevent cracking but control the location of the cracking. To ensure cracking occurs in the sawcut contraction joints, joints must be installed before shrinkage stresses exceed the tensile strength of the concrete, must be the proper depth and must have the proper spacing. Otherwise, out-of-joint cracking may occur. These unwanted cracks are commonly called "random cracks" or "uncontrolled cracks" and typically create costly repairs and tear outs for contractors.
To avoid random slab cracking, contractors must understand the mechanisms that cause early-age shrinkage cracking so they can take the necessary actions to avoid these unsightly and costly cracks.
Shrinkage and restraints
Concrete shrinks and expands due to moisture and temperature changes. Because of moisture loss, a 100-foot-long slab-on-ground can shrink from 0.48 to 0.96 inches over several months. If this slab also has a temperature drop of 50° F after casting, then it can shrink another 0.21 to 0.53 inches because of thermal contraction, yielding a total shrinkage of 0.69 to 1.49 inches. Of course, not all this shrinkage will occur during the first couple of days, but sufficient shrinkage does occur to create early-age cracks that will continue to grow in width as the concrete dries.
For a hypothetical slab that is free to shrink as shown in Figure 1, tensile stresses and cracks do not occur. However, tensile stresses and subsequent cracking occurs when concrete shrinkage is restrained by the subbase or other elements that prevent the concrete from freely shrinking. The number and width of the cracks depends primarily on the magnitude of the tensile stresses created by the concrete shrinkage and restraints. Hence, the cracking potential of slabs can be significantly reduced by minimizing the concrete shrinkage and slab restraints or elements that restrain shrinkage.
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.