A reader wants to fix a gap that has appeared in a sidewalk. (Reader Photo)

Q: The first block of the sidewalk in front of my 50-year-old home has sunk about two inches and opened a one-inch gap between it and the second block. What is the best way to fill this gap and make the two sections level?

Clover Hill, Md.

A: Whether it’s a sidewalk or part of a patio, when one piece of concrete sinks and its neighbor does not, the uneven surface looks unsightly and causes a significant trip hazard.

Assuming the sidewalk is on your property (or at least is your responsibility, not your city’s), you’ll need to figure out whether the problem really is a section that sank or a neighboring section that jutted upward. If it’s not obvious, use a carpenter’s level and a straight board long enough to span a whole section of the sidewalk and extend at least partway onto both neighboring sections. Tree roots can push up a section of a sidewalk, but when a section sinks, it’s usually because moisture eroded soil under the slab.

If tree roots are the issue, call an arborist. You might need to rip out and replace part of the sidewalk. Or, at least as a temporary measure, you could grind off the higher edge where the two sections meet. This would expose gravel embedded in the concrete, giving the treated area a terrazzo look. And grinding releases silica dust, which causes permanent lung damage if inhaled. Pros have tools that cut wet, eliminating the hazard, but if you opt to do it yourself with a right-angled grinder, be sure to wear a respirator and eye protection.

If a section has sunk, see whether there is an obvious water source, such as a gutter downspout, aimed at the sidewalk. Redirect the water. Sometimes soil just washes out bit by bit over the years, especially on a hillside, and there’s nothing you can do except try to make the sidewalk level. You have three options: coat the sunken section with a sand-and-cement mixture to make the surface higher, raise the sunken section using a process called mudjacking, or raise the sunken section using expanding polyurethane foam.

Patching fixes the safety issue without costing much, but the patch is sure to show. To help make sure the patch sticks, use a patching product that matches the job. If you need to raise the entire section, use a sand-and-cement product, such as Sakrete Sand Mix ($7.47 for 60 pounds at Home Depot). It’s suitable for layers of half an inch to two inches thick. But if the sidewalk section is tipped and you need to add two inches along one edge but feather the patch to virtually nothing toward the other side, use a product such as Quikrete Vinyl Concrete Patch ($14.20 for 40 pounds at Home Depot). It contains an acrylic resin that helps the patch bond better, which is especially important along a thin edge, said Steve Witowich, a technical adviser for Quikrete. If part of the patch needs to be deeper than two inches, mix in a little pea gravel for that area, he suggested.

If you want to avoid a patched look, you’ll need to raise the sunken section and fill in underneath either via mudjacking or with polyurethane foam. Mudjacking, which is also known as pressure grouting, has been done for years. Installers drill one or more holes (typically two inches in diameter) into the sunken section and then inject a slurry of sand, cement and water with enough pressure to lift the concrete. The slurry then hardens in place.

Using polyurethane foam is a newer method, available for about the past 10 years. Installers need to drill holes only about one-third to half an inch diameter, and because the foam hardens very quickly, the repaired sections are safe to walk on and even drive a car across in just 90 minutes, rather than the many hours needed for mudjacking mixes to cure. The foam “verges on being a miracle product,” said Jim Wiederaenders, a design technician at Matvey Foundation Repair in Seattle (206-207-0540; matveyconstruction.com), which uses polyurethane foam from PolyLevel (888-310-4467; polylevel.com), an Omaha-based company with dealers across the country. Polyurethane foam creates its lifting force through the chemical reaction released when the two components of the product mix.

Structural polyurethane foam is a cousin of the expanding, closed-cell foam that builders use to seal gaps and add insulation to buildings. Could someone buy the insulation-type foam and use it to lift a sidewalk section? The foam itself might work, Wiederaenders said, but he cringed at the challenge of getting enough of the liquid ingredients under the slab. “When we install it, it goes in under some pressure,” he said. “You have to get enough of it under the slab.” Without the right injecting rig, he said, “I think it would spray back in your face.”

With both mudjacking and polyurethane foam, you’ll probably face a hefty minimum charge that reimburses the company for bringing a truck with a rig to your lot. Matvey’s minimum is around $1,500 to $2,000, which would cover the cost of lifting up to 150 square feet, far more than you need. If some of your neighbors also have sidewalks that need leveling, you might be able to team up and get everyone’s jobs done for the same price as doing just yours.