You caught your own toe on it carrying groceries in, and now you can't stop seeing it. One slab of the walk has come up an inch or so above the next one, or it has sunk down by the porch step, and what keeps running through your head is somebody's grandmother, or a line of trick-or-treaters, coming up that walk in the dark next month.
The good news is that this is one of the more fixable problems in concrete, and there are three or four ways to handle it depending on what moved and why. Before any of that, though, there's a question that has nothing to do with concrete.
First Figure Out Whose Sidewalk It Is
In Charlotte this matters more than people expect, because the answer runs the other way from a lot of cities.
The strip of sidewalk along the street in front of your house is usually public right of way, even though it sits in your yard. Inside Charlotte city limits, the Charlotte Department of Transportation inspects those public walks and, in the city's own words, repairs unsafe conditions "at the City's expense." You report the problem through 311. At a house, that includes the stretch of driveway pad the sidewalk runs across. Don't be surprised if a crew patches the spot with asphalt first, just to get rid of the trip hazard. They come back and pour concrete later, when a crew is free. One resident summed it up well: the landowner is responsible for reporting broken sidewalk, but not for fixing it.
So if the lifted slab is on the public sidewalk, don't hire anybody to touch it. Report it and hang on to a dated photo of the spot. If you're in Matthews, Huntersville, Mint Hill or anywhere else outside city limits, call that town instead, since each one sets its own rules.
The walk that runs from your driveway or the street up to your front door is yours, and that's where the choices below come in.
Why Walkway Slabs Move Around Here
A front walk is about the thinnest, lightest concrete on the property, usually four inches thick and three or four feet wide. It has no weight to push back with, so when the ground under it moves, the walk goes along for the ride.
Our red clay is most of the story. It swells when it's soaked and shrinks when it dries, and a walk poured near a downspout or across a low spot in the yard rides that cycle every single year. After enough of them one slab has dropped against the next or tipped toward the house. If you noticed the gap opening up after a wet spring or one of our late summer storm weeks, that's the clay at work.
The rest is tree roots. Willow oaks and the other big street trees are a large part of why Charlotte's older neighborhoods look the way they do, and they're also why so many walks under them have a slab kicked up at a joint. A root thickening underneath will lift concrete slowly for years.
Knowing which one you have matters, because a sunken slab and a heaved slab need opposite fixes.
Grinding Down the Lip
When one slab sits a little higher than its neighbor at the joint and the concrete is otherwise solid, grinding the high edge down is often the quickest answer. A diamond grinder shaves the lip into a gentle ramp so a toe or a stroller wheel rolls over it instead of catching.
The accessibility standard for public walkways treats a change in height of more than about a quarter inch as a trip hazard, and grinding works well from there up to roughly an inch. Past that you're grinding away a real share of the slab's thickness, and a four inch walk doesn't have much to give.
People ask whether the result will be a smooth finish or a crumbled mess. It looks ground. The cut exposes the stone inside the concrete and won't match the broom finish beside it, but on sound concrete it's flat and it's safe, and on a front entry where looks matter that's a trade worth thinking through.
On a small lip you can do it yourself with an angle grinder and a diamond cup wheel, slowly. Keep the concrete wet or use a shroud with a vacuum on it, because concrete dust is silica, and wear a proper respirator either way.
Grinding does nothing about the root that caused the lip, so on a heaved slab it buys you a few years rather than settling the matter. It's also not something we do. We lift sunken slabs and pour new concrete, so for a grind you'd call a company that handles trip hazard repair, or take on a small lip yourself.
Lifting a Sunken Slab Back Up
When a section has dropped, like the classic walk that sank three inches and turned the bottom porch step into a doozy, lifting it is usually the best value. Concrete leveling, which you'll also hear called mudjacking or slabjacking, means drilling a few small holes through the sunken slab and pumping a cement slurry underneath to fill the void and raise it back into place. The holes get patched and the walk is usable the same day, and it typically costs 50 to 70 percent less than tearing the slab out.
You will see the patched holes. They're small, about an inch and a half across, and most people are glad to trade a few circles for not having a jackhammer in the front yard.
Two things have to be true for a lift to last. The slab has to be sound, without a web of cracks running through it, and the ground under it has to be finished moving. If a downspout is still washing out the base, the slab will sink again in a couple of years, for the same reasons water is so hard on driveways here, so fixing the drainage belongs in the same visit.
And a lift can't help a slab that heaved up. There's no pumping a slab back down.
When Replacing the Section Is the Right Call
Sometimes the right answer is to cut out the bad slab and pour a new one. That's usually where you end up when a root has pushed a slab up more than an inch and is still pushing, when the slab is cracked through in more than one place, or when it sank and tilted so far that lifting would crack it anyway.
Replacing one section is a smaller job than most people picture. The bad slab comes out at its joints, the base gets compacted properly and the new section is poured flush with its neighbors. If a root caused it, the root gets dealt with before the pour. If it's a city tree or one you want to keep, talk to an arborist before anybody cuts a large root.
If the whole walk is tired, pouring a new one runs about $8 to $15 a square foot installed around Charlotte, so a typical three foot by thirty foot front walk lands somewhere around $720 to $1,350 before any decorative finish. Deciding between one section and the whole run is the same weighing we go through for repair versus replacement on a driveway, just on a smaller scale.
Questions Charlotte Homeowners Ask About Lifted Sidewalks
My sidewalk has lifted and someone could trip, what are my options?
If it's the public sidewalk along the street inside Charlotte city limits, report it to 311, because the city repairs those. On your own walk, a small lip can be ground down, a sunken slab can be lifted back into place with mudjacking, and a slab that a root has heaved or that is cracked through should be cut out and replaced.
Grinding down the lip where my driveway meets the sidewalk, should I expect a smooth finish or crumbled mess?
You should expect a flat, safe surface that looks ground rather than finished. Grinding exposes the stone in the concrete, so it won't match the broom texture around it, but on sound concrete it won't crumble.
Who fixes the sidewalk in front of my house in Charlotte, the city or the homeowner?
Inside city limits, the Charlotte Department of Transportation repairs damaged public sidewalks, including ones lifted by street tree roots, and you report them through 311. The walk from your driveway to your door is private and is yours to fix.
Until It's Fixed
Make the lip easy to see. A stripe of bright orange marking paint along the raised edge costs a few dollars and does more than you'd think, and through the fall it helps to leave the porch light on and keep the leaves swept off that stretch.
If you can't tell whether you're looking at a grind, a lift or a replacement, that's quick to settle in person, usually about ten minutes standing on it. A lifted joint is also often one of the first signs that the ground is moving somewhere else on the property, so we look at the rest while we're there. We handle walkways and sidewalks all over Charlotte, and we'll tell you straight which fix your slab needs and what it costs. Give us a call at (704) 387-3064 and we will come take a look at it.