Concrete is hard stuff, but don't let that intimidate you. With the proper tools and techniques we show in our photo series, even a novice can make a durable patch, first try. Sawing concrete with a special masonry blade (opening photo) may be new to you, but if you've handled a circular saw, you'll quickly get the hang of it. It's less hazardous than sawing wood. However, the blade kicks up an incredibly thick cloud of abrasive dust, so be sure to wear goggles to protect your eyes, ear protection, gloves and a dust mask, as well as old clothes.
Size up the job first. Before beginning any repair, assess the general condition of the concrete slab. (See “Patch or Replace? ” below) Sometimes the best strategy is to break out an entire section and repour it with new concrete rather than patch it. Patching works best for local damage in otherwise sound concrete.
We won't deal with the other common problem, cracks. You can repair them exactly as we show here, but they'll most likely return unless you can stabilize the concrete slab to prevent the movement that caused the cracks in the first place.
If this is your first concrete repair project, allow about a half day to pick up materials and complete two to three patches. It took us about four hours from start to finish to complete the two repairs we show here.
Complete the job during comfortable working conditions, ideally in dry weather with a temperature between 50 and 80 degrees F. Both you and fresh concrete happen to agree on this one. Fresh concrete is easiest to handle and hardens best (a process technically called “setting” and “curing”) in this temperature range. Colder weather lengthens the setting time; freezing temperatures can ruin the concrete. Hotter weather causes faster setting and drying; the slab may harden before you can smooth it. Or the surface can dry too fast and not harden properly, eventually causing it to spall. In hot weather, work in the cooler mornings or in the shade.
Patch or Replace?
Should you patch your old concrete or completely tear it out and repour it? While there's no hard and fast rule, here are some tips to guide your decision:
- Assess the severity of the damage. If your driveway is full of spalled areas and broken edges, the surface is probably severely weakened. It'll continue to deteriorate, and chances are the patches won't last.
- Call in a concrete contractor (Yellow Pages under “Concrete Contractors”) to help you assess the situation and ask for a price on complete replacement. But keep in mind that contractors are in the business of selling concrete. With their labor costs, it's usually cheaper for them to replace than repair. We had trouble finding a contractor who would even do patching.
- Is appearance important? A patch will be lighter-colored than the old concrete. Even after weathering for a few years, the new patch will probably still stand out. One way to hide the patch is to stain the entire surface to blend the old with the new. But you'll have to renew the stain periodically.
- How much are you willing to spend? Material costs for a repair are low. Pros would want to completely replace the slab to insure a high quality result. The cost would be substantial.
“Spalling” is the mason's term for concrete that's pitted or chipped, as in Photo 1. The key to a lasting repair is to make a saw cut around the perimeter of the damaged area, cutting back to solid concrete. The cut should be at least 3/8 in. deep: Most repairs that fail do so because the patch is too thin at the edges and breaks off. Set a masonry blade (see above) at a 5-degree angle so the cutout is slightly wider at the bottom than at the top. This helps “lock in” the patch (Photo 1). Slowly guide the saw through the concrete. The masonry blade grinds a groove, so don't put a lot of pressure on the saw; let the blade do the work. Cut about 1/4 in. on each pass. A diamond grit blade can cut the concrete about five times faster than a masonry blade, and it won't wear out as fast. But it also costs more. The extra cost is worth it if you have more than about 10 ft. of concrete to cut. Or save money—and time—by renting a diamond blade (about a day, plus wear fee), or a concrete saw with diamond blade.
Tip: You'll raise an impressive dust cloud when sawing, so close up nearby windows and doors. Otherwise, you'll be housecleaning, too!
Once you've cut and thoroughly cleaned loose concrete from the repair area (Photos 2 and 3), moisten the area with a wet sponge before packing in the concrete mix. Don't leave standing water in the patch area; use just enough to dampen the old concrete and help it bond to the new.
For your patching mix, use either a prepackaged sand mix or concrete mix, depending on the depth of the patch. (See “Use the Correct Patching Mix, ” below.) Each 60-lb. bag makes about 1/2 cu. ft. of concrete, enough for a 2-in. thick patch about 1 ft. wide by 3 ft. long. Estimate the volume of patching material by multiplying approximate length, width and thickness (in feet) to arrive at cubic feet, and buy a bit more than you think you'll need. Better to have too much than to fall a few scoops short!
Mix it with water and acrylic fortifier, following the mixing directions on the package. The fortifier strengthens the new concrete and helps it bond better to the old concrete. The mix should be just wet enough to hold together when it's troweled into the repair. Don't add too much water. It'll result in weaker concrete.
We used a normal sand mix, which sets hard enough to walk on in about a day or two. It continues to cure and harden for weeks. If you have to use the area right away, you can buy a special fast-setting concrete, which hardens in about an hour. It costs about twice as much and you won't have as long to spread and smooth it, so stick to the regular mix when possible. Most building supply dealers that carry regular concrete also carry the fast-setting type. After mixing the concrete, pack it firmly into the repair area using a wood float (Photo 4), the tighter the better. Mound the mix so it's slightly higher than the old concrete. Then immediately level it even with the old concrete using a straight board (Photo 5).
Begin finish trowel work when the surface moisture starts to evaporate and the patch begins to harden. It may only be 10 minutes on a warm day or perhaps an hour on a cool day. One sign of evaporation is the loss of some of the surface gloss. When the patch appears to be getting stiff, test it with your thumb (Photo 6). Once the surface is about as stiff as the skin of a grapefruit, begin the finish work. We used a sponge float (Photo 7) to match the slightly rough texture of the old concrete. A sponge float has a rough rubber surface. If you want a smooth surface, use a steel trowel; for a lightly grooved appearance, drag a stiff-bristle broom across the surface.
After using a float on the surface, cover the patch with plastic for two days. The plastic helps the concrete retain the moisture, which the concrete needs to cure and strengthen (Photo 8).
Use the Correct Patching Mix
Concrete mix consists of sand, gravel and Portland cement. The gravel reduces shrinkage, so it won't crack as readily when laid in thick. However it may not bond well in thin layers. Sand mix (sometimes called topping mix) consists of sand and Portland cement. It's easier to trowel in thinner layers (to about 1/4 in. minimum) for thinner patches or patches with thinner edges. However, if you apply it thicker than 2 in., its natural shrinkage might cause it to crack or break the bond with the old concrete. In a nutshell: Use a sand mix for repairs less than 2 in. deep. Use a concrete mix for repairs deeper than 2 in. Replace 50 to 80 percent of the water with the acrylic fortifier in either mix to improve the patch's bonding strength.
Wet concrete is highly alkaline and can cause severe burns to bare skin. Immediately wash off any that gets on your skin with cool water.
Use the same basic patching technique for a broken corner as for spalled areas, but add three key steps.
- Create a ledge for the patch to make a stronger bond with the old concrete (Photos 9 and 10). Don't be afraid to make the cutout well beyond the edges of the original damaged area. Smaller patches simply won't hold as well as larger ones.
- Prop a wood form tightly against the vertical portion of the step to hold the concrete patch in place (Photo 10). Block the form against the step with bricks or other heavy objects. Tip: Coat the wood form with motor oil or WD-40 to keep the concrete from sticking to the wood.
- When the concrete begins to set, use the edging tool to create a slightly rounded edge to match the step (Photo 11). Then remove the form board (carefully, because the concrete is still soft) and finish all exposed surfaces with a float or trowel to match the texture of the old concrete. Cover the patch with plastic and let it cure for at least two days before walking on it.