Curved Joint Placement
by Jack Innis
Most professionals agree that nearly all concrete slabs will, over the course of time, crack. The reasons are widely known.
It shrinks and expands with changes in ambient temperatures. Unfortunately, due to friction and other forces, concrete slabs rarely shrink and expand as solid monolithic units. Barring extraordinary efforts such as post-tensioning, cracks usually appear. Although they generally do not affect the integrity of the slab, irregular concrete cracks are unsightly, difficult to maintain, and often pose aesthetic problems for those in the decorative concrete business.
Disheartening as the concept of inevitable cracking may seem to home or business owners, decorative concrete professionals have a few tricks up their collective sleeves to make cracks seem all but invisible.
“If you can’t beat them, join them,” could be the motto of seasoned concrete professionals. Rather than stressing out over stress cracks, industry professionals have long relied on contraction joints (see sidebar, “The Rules of Contraction”) to control cracking. Placed on the surface of concrete slabs and created by forming, tooling or sawing, these joints weaken the slab to provide attractive avenues in which cracks form. Cracks formed inside these joints are relatively easy to disguise.
Placing contraction joints in concrete is not exactly rocket science. While care must be taken in the joints’ design and creation, three fundamentals come into play. First, maximum joint spacing should be 24 to 36 times the thickness of the slab, with a maximum spacing of 15 feet. Second, the minimum depth of a joint should be a fourth of the thickness of the concrete. Third, all panels created by contraction joints should be as square as possible.
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