Most slabs-on-ground are unreinforced or nominally reinforced for crack-width control. When positioned in the upper or top portion of the slab thickness, steel reinforcement limits the widths of random cracks that may occur because of concrete shrinkage and temperature restraints, subbase settlement, applied loads or other issues. This type of reinforcement is commonly called shrinkage and temperature reinforcement.
Shrinkage and temperature reinforcement is different than structural reinforcement. Structural reinforcement is typically placed in the bottom portion of the slab thickness for the purpose of increasing the slab's load capacity. Most structural slabs-on-ground have both top and bottom layers of reinforcement for controlling crack-widths and increasing load capacities. Because of constructability issues and costs associated with two layers of reinforcement, structural slabs-on-ground are not as common as nonstructural slabs.
While there are several reinforcing options for nonstructural slabs-on-ground, this article focuses on steel reinforcing bars and welded wire reinforcement for crack width control.
Steel reinforcing bars and welded wire reinforcement will not prevent cracking. Reinforcement is basically dormant until the concrete cracks. After cracking, it becomes active and controls crack widths by restricting crack growth.
If slabs are placed on high quality subbases with uniform support and consist of low shrinkage concrete with joints properly installed with spacings of 15 feet or less, reinforcement is generally unnecessary. Most likely, there will be few random or out-of-joint cracking. If random cracks do occur, they should remain fairly tight because of the limited joint spacing and low concrete shrinkage thereby limiting future serviceability or maintenance issues.
When slabs are placed on problematic subbases with risks of non-uniform support or consist of moderate to high shrinkage concrete or joint spacings exceed 15 feet, then reinforcement is necessary to limit widths of cracks should they occur. As crack widths grow and approach about 35 mils (0.035 inches), the efficiency of load transfer through aggregate interlock diminishes and differential vertical movements across cracks or slab "rocking" can occur. When this happens, crack edges become exposed and edge spalling will likely occur, especially if the slab is exposed to wheeled traffic and especially hard-wheeled lift trucks. Once spalling starts, crack widths at the surface become wider and slab deterioration along cracks increase significantly.
When contraction joints are unacceptable and not installed, shrinkage and temperature reinforcement is required. This design approach is sometimes referred to as continuously reinforced or joint-less slabs and allows numerous, closely spaced (3 to 6 feet), fine cracks to occur throughout the slab.
Crack control options
In general, there are two options for controlling cracks in slabs-on-ground: 1) control the location of cracking by installing contraction joints (does not control crack widths) or 2) control crack widths by installing reinforcement (does not control crack location).
With Option 1, we tell the slab where to crack and widths of contraction joints or cracks in the joints are largely controlled by the joint spacing and concrete shrinkage. As joint spacings and concrete shrinkage increases, joint widths increase. Similar to cracks, if joint widths approach about 35 mils, the efficiency of the aggregate interlock to transfer loads and prevent differential vertical movements across joints can be significantly reduced. For this reason, many designers use load-transfer devices including steel dowels, plates or continuous reinforcement through contraction joints to ensure positive load transfer and to restrict differential vertical movements across joints.