Make your concrete repairs rock-solid

Concrete repairs, when completed correctly, can bring like-new life back to an old surface.

Concrete-to-concrete or epoxy-to-concrete repairs can mend cracked or failed concrete. It is a relatively straightforward process, but must be done correctly, says Al Browning, construction products sales manager at Quikrete. He shares the steps needed to make solid concrete-to-concrete repairs.

Remove failed concrete and prepare the surface.
“Old concrete must have a rough, clean surface so the new concrete can bond to it,” he says. Any loose or failing concrete must be removed and it’s best to cut out the old area squarely.

“Irregular shapes and feathered edges can cause problems because concrete shrinks and expands. Use a concrete saw to get a straight and square repair area,” he says.

The repair surface must also be free of oil and the surface needs to be scarified to about 1/16" or more for good adhesion.

Mix properly.
The water-to-cement ratio drives the strength and quality of concrete, so it’s important to follow the mixing directions on the concrete bag. “The water helps place the concrete where it belongs and is key in bonding the concrete and aggregate,” he says.

Work the concrete as 
little as possible to get it in place.
  “The more you work concrete, the less water you have in the mix for proper hydration,” he says. Edge or joint spalls are often caused by overworking the edges when finishing concrete.

Cure it correctly.
Concrete continues to harden as long as it is hydrated. If the concrete dries out, the hardening process stops. Rewetting the concrete will start the process again, but won’t proceed as efficiently or as completely when compared with concrete that remains hydrated throughout the curing process.

“You can use wet burlap, polyethylene or a spray-on polymer to keep concrete from drying while curing,” he says.

Stay off of the concrete until adequately cured.
“Follow the label directions for the amount of time the concrete must cure before it is put into service,” he says.

Epoxy injection another option
Cracks can be repaired with epoxy injection compounds.

Like concrete-to-concrete repairs, proper surface preparation is key to a successful repair. “The crack should be clean and dry and the surrounding surface should be free of any bond-inhibiting contaminants,” says Craig Schumann, a former restoration contractor and technical sales representative at Simpson Strong-Tie.

Epoxies are usually injected with a high-volume/high-pressure metering pump or low-volume/low-pressure (cartridge) dispensing system. The high-volume/high-pressure systems are used in direct port-to-port repairs; the low-volume/low-pressure systems rely on gravity to fill the crack with the epoxy.

Slab-on-grade structural concrete cracks, vertical concrete structures and cracked elevated slabs should be repaired using direct port-to-port filling.

Shorter cracks or cracks that are scattered over a large area may be best filled with low-volume/low-pressure (100 max. psi) cartridge dispensing systems.

Epoxy selection can affect success. Select an epoxy’s viscosity based on crack width and injection method. Use lower-viscosity epoxies for gravity-fill patches; port-to-port injection method should use higher viscosities.

In port-to-port repairs, the crack must be patched over with an epoxy paste, then filled with epoxy injected through ports placed in the patch.

“Ports should be installed using enough epoxy paste to completely cover the ports and crack at least 1/8" thick and 1" on either side of the crack,” says Schumann.

Non-structural cracks in on-grade slabs may be best repaired by gravity-filling epoxies if the crack is wide enough and the epoxy’s viscosity is low enough for 100 percent crack-fill. 

“Routing the crack to a v-shape or square cut helps achieve 100 percent crack-fill. Silica sand can be used to dam the bottom of the crack to keep the low-viscosity epoxy from flowing into the subgrade,” says Schumann.

A crack that appears to be 1/8" wide on the top of a 6" slab may only be a hairline crack at grade level. When the port-to-port method of injection is used to repair this type of crack, epoxy may be forced out of the adjacent port before the epoxy totally fills the crack. Crews must be properly trained to ensure success.

Cracks in concrete are often incorrectly diagnosed as structural problems. “This often happens with cracks in water-retaining structures. Given the large amount of steel reinforcement in these structures, it is highly unlikely the crack is structural in nature. These are waterproofing repairs and can usually be repaired by injecting an expandable urethane grout into the crack,” he says.

If a water-retaining reinforced concrete structure needs a structural repair, the structure must be taken out of service before repairs can begin. The cracked area must be dry before epoxy injection to get the proper concrete-epoxy bond.

Published in the May/June 2002 issue of Contractor Tools and Supplies magazine.

back to top