Our concrete walk has pits and seems to be crumbling. How can we fix it?
—Jace Laakso, Missoula, Mont.
Tom Silva replies: Spalling, the pits that form in concrete, happen when too much water is used in a concrete mix, weakening it.
In the past, when a concrete patio or driveway started to show serious signs of aging, using a jackhammer or bringing in a bulldozer were the only repair options. Today there are more practical alternatives.
As long as your walkway isn't cracked all the way through, you can coat the slab with a concrete resurfacer, a no-shrink blend of Portland cement, sand, and polymer additives that fills divots and makes a uniform finish. Its natural color is dark gray, so buy enough to cover your entire walk. One 40-pound bag coats about 35 square feet. If you want a different color, just add a concrete tint.
What Temperature Should You Pour Concrete?
Before resurfacing, strip off any paint or sealers from your concrete surface and watch the weather. Temperatures should remain above 50 degrees F for 8 hours after the pour and above freezing for 24 hours after that. For optimum results, work on a day with low humidity, no rain and an air temperature ranging between 70° and 75°F. The surface temperature of the existing concrete must be at least 50°F.
How Long Does Resurfacing Take?
Resurfacing takes most of the day, but your walkway will be back in pristine shape once you're done.
How to Resurface Worn Concrete
Step 1: Clean the Concrete
- For surface preparation, wear heavy boots and protective glasses and blast off surface dirt with a gas-powered pressure washer that can reach at least 3,500 psi.
- Fit the wand with a 25-degree fan tip, and hold it 6 to 8 inches from the surface as you clean the concrete with slow, even sweeps.
- To remove mildew or algae, use a concrete wash.
Step 2: Patch Spalled Areas
- While the concrete is damp, mix up enough resurfacer to fill any divots. Combine 1 part water with 7 parts resurfacer in a 5-gallon bucket; blend with a paddle mixer attached to a corded drill.
- Fill the recesses with resurfacer, and smooth with a finish trowel.
- Go to the next step when you can stand on the patch without indenting it, in about 2 to 5 hours.
Step 3: Spread the Resurfacer
- Fill the walkway joints with peel-and-stick foam weatherstripping, and dampen the concrete if necessary.
- The resurfacer is workable for about 20 minutes, so mix just half a bag of powder with 5½ cups of water at a time. That's enough to cover two sidewalk slabs, or about 17 square feet.
- Pour it on the walk.
Step 4: Trowel Out the Resurfacer
- Spread the resurfacer until it's between 1/8 and ¼ inch thick.
- For a slip-resistant finish, wait 5 minutes, then sweep a nylon-bristle concrete broom across it, 90 degrees to the foot traffic.
- Remove the weatherstripping after 20 minutes. Wait 6 hours before walking on it. Cover only if rain threatens in the meantime.