How to fix cracks in concrete (© This Old House)

Concrete consists primarily of cement, sand, gravel and water. As the water in the slurry evaporates, the remaining ingredients cure into a hard, monolithic slab. Unfortunately, the curing process causes the concrete to shrink slightly, often resulting in hairline cracks. Larger stress cracks occur when a house settles or the ground beneath it shifts. These types of cracks typically don't threaten the structural integrity of the house, but they do create an entry point for groundwater, insects and radon gas. (Bing: How can you tell if a foundation crack is serious?)

Here, we'll show you a simple, effective way to patch cracks in poured-concrete walls.

If you notice large, recurring cracks or bulging walls at your house, don't try to fix them yourself. The cracks might indicate a more serious structural problem; call in a structural engineer for an in-depth evaluation.

Repairing the crack
Several masonry-patching products, such as hydraulic cement, do an adequate job of filling cracks in concrete walls. However, these products can fail after a few years if your foundation or retaining wall continues to move slightly, leaving you with little choice but to chip them out and start all over again.

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We repaired a foundation wall that had an 8-foot-long crack that leaked water into the basement during heavy rain. To patch the crack permanently, we used an epoxy-injection system from Polygem, called the Liquid Concrete Repair Kit. Each kit contains a two-part epoxy crack sealer, two 10-ounce tubes of Liquid Concrete Repair (LCR), a viscous epoxy that comes in a caulk-type cartridge, and seven plastic injection ports that deliver the LCR deep into the crack. There's enough material in each kit to repair a crack that is one-sixteenth inch wide, 8 inches deep and 8 feet long.

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Before you start, check to make sure the crack is dry. If the crack is slightly damp, dry it with a blow dryer, then wait 15 minutes. If it remains dry, proceed with the repair. If the dampness returns, water is still seeping into the crack, and you'll have to wait for it to dry out on its own.

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First, scrub the crack clean of any loose concrete, paint or old filler using a wire brush. Remove all dust and debris with a shop vacuum. Tap 3-inch (10d) finishing nails partway into the crack, spaced 12 inches apart. You'll use them to align the injection ports with the crack.

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Open up the two containers of epoxy crack sealer and scoop out equal amounts of Part A and Part B; use two separate sticks to avoid contamination. Mix the two parts on a scrap board using a clean putty knife; blend until you achieve a uniform gray color. Spread some sealer onto the base of one of the plastic injection ports, being careful not to plug up its hole. Slide the port over one of the nails sticking out of the crack and press it to the wall. Install the remaining ports in a similar manner.

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Next, mix up a slightly larger batch of epoxy sealer and apply it to the entire crack using a 1.5-inch-wide putty knife or margin trowel. Spread the sealer about one-eighth inch thick and 1 inch on either side of the crack. Also, cover the entire flange of each injection port with crack sealer, leaving only the extended neck portion showing. Smooth out the sealer and feather its edges with a paintbrush dipped in mineral spirits.

If the other side of the wall is accessible, see if the crack goes clean through. If it does, seal it up with crack sealer, too. Allow the sealer to cure for six to 10 hours before injecting the epoxy. Thoroughly mix the LCR epoxy using the plunger rod that comes with the kit. Place the LCR cartridge into a caulk gun. Starting at the lowest injection port, dispense the epoxy into the crack. Continue squeezing the trigger until epoxy begins to ooze out of the port directly above. Remove the gun and plug up the port you just filled. Now insert the cartridge tip into the port that's oozing and squeeze the trigger to dispense the epoxy. Repeat this procedure for the remaining ports; plug up each one before moving on to the next.

Allow the LCR to cure for five days, then cut off the necks of the ports with a hacksaw. If desired, you can patch the severed ports with a little crack sealer.

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