Concrete slabs contract slightly while the concrete mix dries and cures. Temperature changes also affect concrete. Warmer ambient air causes concrete slabs to expand, while cooler air results in a slight contraction of the poured and set slabs. Concrete floors are commonly laid between immovable structures such as interior walls. In order to deter concrete movement into fixed structure, expansion or movement joints must be included. In complicated applications, a structural engineer may need to be consulted to properly design a concrete floor.
The American Concrete Institute's 2008 building code ACI 318-08 mandates that a non-reinforced concrete slab requires movement joints at intervals equal to 30 times the thickness of the poured concrete. For a 4-inch thick slab there should be some type of joint every 120 inches (4 times 30), or one joint every 10 feet. Aesthetics may allow for slight joint positioning alterations in order to avoid unsightly gaps in highly visible areas. Control joints may be used in those situations to minimize movement damage.
The Expansion Joint
An expansion joint prevents the forces of expansion and contraction, naturally found in concrete floors, from harming nearby stationary structure such as framed or bricked walls. With an expansion joint, there is usually a visible gap between concrete slabs. The gap is filled using one of several materials that keep the slabs from moving laterally. Correct joint placement minimizes pressure cracks within the slabs or damage to perimeter walls due to slab movement.
Joint Material Options
Expansion joints may be filled with a fibrous material designed to compress as concrete expands. Flexible board is commonly available in precut strips measuring either 1/2 inch, 3/4 inch or 1 inch in thickness. According to ACI 318-08, expansion joints should not exceed 1 1/4 inches. Industrial flooring often uses 1/4-inch diameter steel dowels approximately 20 inches in length that are fitted laterally between slabs to allow for movement. Additional cement joint sealing compound is used to finish the process.
In addition to expansion joints, many contractors employ contraction or control joints. These partially cut joints are controlled recesses that allow for straight-line cracking in designated areas, thereby avoiding random damage. ACI 318-08 mandates having control joints placed at twice the slab thickness for aggregate sizes of 3/4 inch or less. For aggregates greater than 3/4 inch, spacing should increase to 2 1/2 times the slab thickness. Therefore, a 5-inch slab of 3/4-inch aggregate would need a control joint placed every 10 feet.