There is at least one thing all concrete installers agree on: Concrete cracks. They differ, however, on how to approach repairing those cracks.
The American Concrete Institute report "Causes, Evaluation, and Repair of Cracks in Concrete Structures," prepared by ACI Committee 224, lists a dozen different ways to repair cracks, so it is no surprise that installers use different methods and approaches. According to the report, "cracks need to be repaired if they reduce the strength, stiffness, or durability of the structure to an unacceptable level, or if the function of the structure is seriously impaired ... In addition, repairs that improve the appearance of the surface of a concrete structure may be desired."
Cracks often appear because not enough joints have been cut in the concrete, notes Scott Metzger of Metzger/McGuire. "Once the crack has been induced, you already have a plane of weakness."
So when it comes to crack repair, where to start? John Heidmann, general sales manager for Concrete Polishing Solutions, says the first thing to do is to clean the crack. "You use a crack-chaser blade, which is specifically designed to even the crack out to a specific depth," he explains. "This will allow you to have a clean, even edge on either side of the crack and then come back and fill it with a joint filler." A semirigid material is used to fill the crack because it has some flexibility to it, he says.
It is important to establish why the crack is being repaired, says Chris Sullivan, vice president of sales and marketing with ChemSystems Inc. "Is this an aesthetic repair, where the repair needs to look as good as possible, or is it is a repair that is going to be covered by something, like an overlay or something like that? That will determine what materials you use, how careful you need to be, and what it needs to look like."
Also, cracks should be reviewed to determine whether they are plastic shrinkage cracks, structural cracks or settlement cracks before any repair technique is considered. Because most repairs aren't aesthetic and most crack repair is done on floors, Sullivan prefers what is known as bridging or knitting a crack. "You fill the crack and then put some kind of bridge across the surface so it's convex," he explains. "You take the impression out so whatever you put on the surface doesn't (settle) into the opening."
Bob Harris of the Decorative Concrete Institute uses crack stitching. "We take a 4-inch diamond grinder," he says, "and we'll chase the crack for depth, following the contour of the crack." Perpendicular cuts are made across the crack, and block ladder wire is embedded into those cuts. The crack is then overfilled, and when it is dry, they grind it back flush. "It looks like Frankenstein's monster," Harris admits.
One of the more popular ways to fix cracks that are structural is with crack injection. Holes are drilled into the crack at intervals that equal the thickness of the slab. Epoxy is injected into the first hole until it overflows out of the second one, and so on, Harris says.
A specialized repair product like Roadware 10-Minute Concrete Mender is another option for crack repair. "We put Mender into the crack and it has an attraction to the rock," explains Richard King, vice president of sales and distribution for Roadware Inc. "It's thin enough to get into the concrete and it pulls to the aggregate." Sandblast sand and Concrete Mender in a crack combine to create "polymer concrete," in which a polymer does the work of portland cement. "Once it cures, it will never get harder than the original slab," King adds.
Repairing static cracks in stamped concrete or countertops is much more difficult, says Sullivan. "You are blending color and texture, and there is a limited number of materials that are going to allow you to create something that is exactly the same or to blend in," he says. "You can use colored caulks or grouts, but trying to blend those in can make it look worse. I tell people that it is sometimes better to make it a contrasting color and make it look like the crack is there for a reason."
Sometimes not fixing a crack is okay, says Bob Harris. "You can make it part of the design," he says. "We do that often by doing things like staining it a different color or saw-cutting leaves on the other side of it to make it look like a vine. Every situation is different."
When should a crack be repaired? Sullivan suggests repairs in the following circumstances:
- The crack has become a safety hazard.
- The performance of the concrete is compromised.
- The appearance of the crack takes away from the look of the concrete.
According to Harris, a moving or working crack that is more than the width of a credit card or a crack that is elevated should be repaired. Also, if the crack could cause an unhygienic situation, as a crack in a countertop or on a kitchen floor might, it should be repaired to prevent bacterial growth.
"But if it is a tiny, paper-thin crack and is static, I'd leave it," says Sullivan.
Rarely is replacing the slab preferable to fixing the crack. The cost of replacement can be prohibitive, costing up to five times as much as the original slab. "There are repair materials to address just about any situation," says Heidmann, "so I don't know if the crack itself will dictate busting up and replacing the slab. I think it comes to down to whether the slab is going to be polished as a final flooring or if something is going to be put on top of it."
Although concrete experts understand that cracks will happen, their customers aren't always so knowledgeable. In fact, Metzger says, most customers view cracks as a flaw. Therefore, managing customer expectations is very important, the experts agree, and it is vital to have good communication from the very beginning of project discussions.
"It's very important not to overpromise," says Heidmann. "Concrete turns gray. It gets hard. It cracks. And there is no way to guarantee that it won't crack, so you have to be up front that it will happen, and that any kind of repair will likely stand out."