Why They’re There
Simply put, concrete shrinks as it hardens or cures, according to Scott Tarr and Peter Craig at Concrete Construction. There’s two reasons for this: cooling and drying.
Concrete shrinks because of a chemical reaction between the concrete mix and water that gives off heat as it cures, and anything that radiates heat will eventually cool. Concrete also loses excess water as it cures. Anything that cools or dries tends to shrink, and that applies to concrete as well as anything else.
The problem, according to Tarr and Craig, is that, as concrete cures, it shrinks before it gains enough strength to resist this shrinkage. Because of this, it cracks.
There’s not much that can be done to prevent cracking. According to the Portland Cement Association, contractors can try reinforcing the concrete with steel, but this reinforcement can actually increase random hairline cracks inthe surface. As a result, most contractors opt to control cracking by cutting control joints.
How They Work
Control joints are a brilliantly simple way to control cracking in curing concrete.
According to the U. S. Federal Highway Administration, a sufficiently deep groove weakens a concrete slab along that joint and encourages cracks to develop in and underneath it, instead of elsewhere on the slab. This groove, needless to say, is called a control joint.
The good news, according to Concrete Network, is that you can plan your joints so that they run underneath walls and carpets, where they won’t be seen. You can’t just place them anywhere, though, or cut them anytime you want.
How to Make Them
The Portland Cement Association recommends cutting control joints to produce panels that are as square as possible, with a length-to-width ratio of no more than 3:2. Concrete Network adds that joints should be set no more than two to three times in feet the slab thickness in inches. The example they give is a four-inch slab; this slab should have joints no more than eight to 12 feet apart.
The joint also has to be deep enough, otherwise it won’t encourage cracking. Joints should be at least one-quarter the thickness of the slab. For the four-inch slab above, that means control joints that are at least one inch deep.
Then there’s the timing.
You have narrow window in which to cut your control joints. They must be cut before the slab cures, typically within 12 hours after it’s been laid. However, many factors can affect concrete curing. According to the Portland Cement Association, cracking can begin within six to 12 hours in hot weather, and control joints are useless once cracking begins.
How to Make Early Entry Joints
One way to address early cracking is to cut “early entry” joints as soon as the concrete is hard enough to be walked on, usually within one to four hours of being laid. This is a relatively new development in joint cutting, according to Martin McGovern at Portland Cement Association’s Concrete Technology Today, with Soff-Cut, the first commercially-available early entry saw model, being introduced by concrete contractor Ed Chiuminatta only 35 years ago.
Early entry saws differ from other joint cutting saws in several ways:
- Lighter: Early entry saws are typically lighter than conventional joint cutting saws.
- Dry-cutting: Early entry blades blades are designed to cut without water, unlike other joint cutting saws.
- Up-cutting: Early entry saws have cutting edges that move upward during operation, which helps sweep debris out of the joint.
- Skid Plate: Some saws, like Soff-cut, have “skid plates” that prevent the joint edges from raveling during cutting.
This advantages allow early entry saws to cut joints sooner in the curing process without ruining the slab’s finish and without the necessity of a water feed or cleaning up slurry after the joints are cut.
According to Tarr and Craig, the joints are also cut when the concrete is softer, resulting in faster cuts. Early entry joints can also theoretically be shallower than conventional cuts because of how concrete shrinks when it’s very young – that is, from the surface down – although it’s still a good idea to make early entry joints as deep as conventional joints.
Several studies cited by the Federal Highway Administration found that early entry cuts are at least as good as conventional cuts. However, according to one contractor in McGovern’s article, “Sometimes the quality of the cut isn’t as good as a wet cut. If the skid plate isn’t flat, or if you get on the slab too soon, or if the blade is worn, you can get raveling.” Tarr and Craig add that the high tolerances between blade and skid plate can mean frequent replacements to minimize raveling during cuts.
“Be Active in Deciding Where Control Joints Will Be Placed.” Concrete Network. N.p., n.d. Web. 06 Aug. 2013. < http://www.concretenetwork.com/concrete/slabs/controljoints.htm >.
“Contraction/Control Joints.” Portland Cement Association. N.p., n.d. Web. 06 Aug. 2013. < http://www.cement.org/tech/basics_joints.asp >.
“Early-Entry Sawing of Portland Cement Concrete Pavements.” U. S. Department of Transportation Federal Highway Administration. N.p., June 2007. Web. 6 Aug. 2013. < http://www.fhwa.dot.gov/pavement/concrete/pubs/07031/07031.pdf >.
McGovern, Martin. “The Latest on Early-Entry Sawing.” Portland Cement Association. Concrete Technology Today, Nov. 2002. Web. 6 Aug. 2013. < http://www.cement.org/tech/pdfs/CT023Saw.pdf >.
Tarr, Scott, and Peter Craig. “Early-Entry Joint Sawcutting Provides Long-Term Benefits.”Concrete Construction. N.p., 5 Aug. 2008. Web. 06 Aug. 2013. < http://www.concreteconstruction.net/concrete-construction/early-entry-joint-sawcutting-provides-long-term-b_2.aspx?printerfriendly=true >.