Quick Concrete Fix: Crack Injection

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Jan 01, 2011
Fixing a Hole

Photo of two guys doing a DIY crack injection project on a cracked driveway by slgckgc/Flickr.


Conventional products for repairing concrete cracks are mastic—essentially a caulk, not unlike the silicone goo that's sprouting mold in your shower. Because mastics keep dirt and water (and thus ice formation) out of the cracks, these product do help, but sealing the crack is really all they do. They don't bond the concrete or add any strength. Crack injection epoxy is different. It acts more like a caulk mixed with ultra-turbo super glue. That means it seals AND bonds the concrete. And unlike real super glue, which seems to work about 0.01% of the time, injection epoxy sticks, even if the concrete remains dry, damp or completely wet after the repair. It also works on vertical and horizontal surfaces, so you can use it to repair driveway slabs and basement walls, plus curbs, steps, swimming pools and, yes, garden gnomes.


How Crack Injection Works

Remember fixing stuff with two-part epoxy in the old days? You bought a kit with two little tubes that looked like ointment containers. After carefully squirting a dollop from each tube onto a paper plate, you mixed the two globs together with a Popsicle stick. This imprecise, messy mixing process left you with about 5 minutes of time to apply the epoxy and hope you mixed the right proportions; if you used too much resin and not enough hardener your glue wasn't so strong. These days, all-purpose epoxy is available in a special two-chamber syringe that dispenses equal amounts of hardener and resin. You still have to do the mixing but at least you know the proportions are right.


Concrete injection products take the fool-proofing one big step further. The mixing is done right in the applicator, which looks and works just like a tube of calk. The resin is in the tube, and the hardener is in a long nozzle that attaches to the tube. Once the nozzle is secure, you turn a knob that lets the hardener flow down into the resin. Give the applicator a good shake, pop it in your caulking gun and you're ready to get crackin'.


The bond of concrete epoxy is chemical; that's why it lasts and why it's strong, stronger in some ways than the concrete itself. Fully cured epoxy is resistant to water and many common acids and solvents. It can also be chiseled, grinded or sanded, as well as painted (good news for your beloved garden ornaments).


Application Methods

Injection epoxy is applied with a caulking gun, but the material is much less viscous than caulk so it can flow into small cracks. For repairs on horizontal surfaces, you simply fill the crack until the epoxy is flush with the surrounding surface, then let it cure.


Vertical application is quite a bit more involved, and this is where the "injection" comes into play. You start by epoxying plastic injection ports to the wall, every 8 inches or so along the length of the crack (in this case, you're back to portioning and mixing the epoxy the old-fashioned way). Then, you seal over the crack with a swath of the hand-mixed epoxy; this creates a continuous dam or seal that keeps the thin repair epoxy from oozing out during the injection process. You fill the crack using a caulking gun and the same epoxy tube and nozzle used for horizontal repairs but with the addition of a hose that attaches to the injection ports. Start at the bottom and work your way up the crack, filling it full and closing the ports as you go. After the epoxy cures for about a day, you knock off the ports with a hammer then grind excess epoxy from the wall before painting (if desired). The ports and hand-mix epoxy are sold in all-inclusive kits by the same manufacturers of injection epoxy.


Important Considerations

Check the product specifications regarding the size of crack the product will fill. One major brand sets the range at 1/64 inch to 1/4 inch wide. Follow the manufacturer's preparation and application instructions to the letter. Things like temperature—of the epoxy, the air and the concrete—are important, as are the specific prep steps for each application. For the best bond with epoxy, you really get only one chance, so take your time to do it right, and wait for warmer (or cooler) weather if necessary.


Injection epoxy is designed for solid concrete, not concrete block (cinder block), brick or other porous masonry materials. The thinness of the epoxy can be a complicating factor: It's nice for filling small cracks, but if a crack runs all the way through a wall or slab, the epoxy can too. If there's access to both sides of a wall you can seal over the crack on both sides with the hand-mix epoxy, as described, but of course this option isn't applicable to slabs on grade (on the ground) or subgrade (underground) portions of foundation walls; in these cases, the earth or subbase materials should provide some backing, but you may end up using a lot more epoxy than anticipated.


Don't expect crack injection to save a failing concrete structure. If your foundation walls are moving (you can test this with some pencil marks, a ruler and periodic measuring), get some advice from a qualified builder, concrete pro or structural engineer before attempting repairs with epoxy or any other product. Injection epoxy can't be applied in cracks that are admitting flowing water; you have to stop the water before making the repair. For large gaps or cracks, such as an oversize or chipped out hole around a drain pipe running through a basement wall, hydraulic cement is a more appropriate repair product than epoxy.


Philip Schmidt writes for Networx.com.

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