CONTRACTION JOINTS 101
When I had a new concrete driveway poured several years ago, the contractor told me he could guarantee me of two things: it would be grey, and it would crack. When he saw the disappointed look on my face about the cracking, he explained I shouldn’t worry because he would use contraction joints (he called them control joints) to make it crack where he wanted it to.
He was describing a simple fact of concrete—it shrinks as it dries and that contraction causes cracks. The contractor’s job, then, is to give it a place to crack by creating weakened lines for the crack to follow when the joint “activates” without destroying the integrity or esthetics of the pour.
On smaller jobs like sidewalks, those control lines are usually made by hand with grooving tools. Larger slabs often call for a concrete saw. Either way, the timing, depth and spacing of the joints are crucial. Wait too long and the concrete will crack on its own; make the joints too shallow or too far apart and it will, too.
Conventional wet or dry concrete saws can be used once the concrete has set to 500 psi (4-12 hours, depending on the weather) to prevent raveling of the edges, but early-entry saws and blades like the Husqvarna Soff-Cut® series let you cut joints within 1-4 hours of pouring, virtually eliminating the chance the concrete will have a mind of its own. Regardless, the joint should be cut to at least ¼ of the slab’s thickness.
For coarse aggregate that’s ¾” or smaller, the rule of thumb is to space the joints twice the thickness of the slab apart in feet. In other words, a 5” slab would want joints no more than ten feet apart (5x2 converted to feet). With larger coarse aggregate, you can stretch the distance to 2½ times the slab thickness. And of course you can place the joints closer and incorporate them as part of a decorative design. A regular dry-cut cut-off saw can be used for decorative work, but only with a respirator to guard against airborne silica along with eye and hearing protection. Now there are even early-entry saws made specially for decorative work. The jointing pattern may be specified on a job, but don’t hesitate to chime in if you’re uncomfortable with their functionality.
For an in-depth look at contraction joints, take a look at this article on ConcreteNetwork.com.
Always using the right tools and techniques ensures a safe and professional job with results you’ll be proud to show off. Whatever your concrete cutting, coring, and finishing requirements may be, equipment and tools from Ace Cutting Equipment are your choice for top-quality results.