Walk the edge of almost any driveway or sidewalk in Camden, Burlington, or Gloucester County that is more than fifteen years old and you will find it: one slab sitting an inch lower than its neighbor, a corner of the patio that has tipped toward the house, a section of walkway pushed up into a trip hazard. The concrete itself may look fine. It is the ground underneath that gave way. Here is why that happens around here, what the repair options really are, and how to tell when a sunken slab is worth saving.
Why Concrete Sinks and Heaves in South Jersey
Concrete is only as stable as what it sits on. A slab that was poured over a properly compacted gravel base will stay put for decades. A slab poured over loose fill, or over soil that later changed, will move. In our area three things do most of the damage.
- Clay soil that swells and shrinks. Much of South Jersey sits on clay-heavy ground that swells when it is wet and shrinks when it dries out. Every wet spring and dry August moves the soil a little, and the slab rides along.
- Water washing out the base. A downspout dumping next to the driveway, a gutter that overflows onto the walkway, or a patio with no slope will slowly carry the gravel and fine soil out from under the slab. Once there is a void, the concrete has nothing to rest on and cracks or drops into it.
- Freeze-thaw heaving. When water gets under a slab and freezes, it expands and lifts the concrete. In the spring the ice melts and the slab drops, but rarely back to exactly where it started. Twenty winters of that will leave a sidewalk looking like a set of stairs.
Tree roots and poorly compacted fill from old construction cause their share as well. Whatever the reason, the pattern is the same. The slab did not fail. The ground under it did, and any repair that ignores that will not last.
What Sunken Concrete Looks Like
- One slab noticeably lower or higher than the one beside it, especially at sidewalk joints
- A patio or walkway that now slopes toward the house instead of away from it
- Water pooling in one spot after rain where it used to drain
- Steps or a stoop pulling away from the house, with a widening gap at the wall
- Cracks that follow a low spot, or a slab that rocks when you step on the edge
- A hollow sound when you tap the surface, which means there is a void underneath
A lip of a quarter inch or more at a joint is a trip hazard, and on a public sidewalk in many New Jersey towns that is the homeowner’s problem to fix. A slab tilting toward the foundation is worse than a cosmetic issue because it is steering water at your basement.
Your Repair Options, Explained Honestly
There are three ways to deal with a sunken slab, and the right one depends on how far it has moved, how sound the concrete is, and what is going on underneath.
Lifting the slab back into place. You may have heard of mudjacking or foam leveling. Both work the same way: holes are drilled through the slab, and a material is pumped underneath to fill the void and push the concrete back up. Mudjacking uses a cement slurry and typically runs a few dollars per square foot. Polyurethane foam is lighter and cures in minutes but costs several times more. Lifting is a fair option when the slab is thick, sound, and has dropped only an inch or two. It is not a cure. If the base washed out once, it can wash out again, and a slab that is cracked in several places will not lift as one piece. We do not offer slab lifting, and we will tell you plainly when it is the right call so you can look into it.
Repairing and resurfacing. When the movement is small and the slab is still basically level, grinding down a raised edge or building up a low spot with a bonded overlay can remove the trip hazard and restore drainage without replacing anything. This works best on sidewalks and patios where the slab is structurally fine and the problem is a lip at one joint. It does not work on a slab that is still moving.
Removing and repouring. When a slab has dropped several inches, broken into pieces, or tilted toward the house, replacement is usually the honest answer. The old concrete comes out, the base is dug out, compacted, and rebuilt with proper gravel, the drainage problem that caused the sinking is corrected, and new concrete goes in with control joints and reinforcement. It costs more up front than lifting, but you get a slab on a base that will not move, and a new surface with decades ahead of it.
When Lifting Makes Sense and When It Does Not
A good rule of thumb: lifting is worth considering when the concrete is in good shape and the only thing wrong is that it sank. Replacement is the better investment when any of the following is true.
- The slab is cracked through in more than one place
- The surface is already flaking, pitting, or crumbling
- The drop is more than two or three inches
- The slab is thin or was poured without a real base, which is common on older walkways
- It is a driveway section that takes vehicle weight every day
- The concrete is old enough that you would be replacing it in a few years anyway
Lifting a slab that meets any of those conditions means paying for a repair to concrete that is on its way out. We would rather tell you that now than have you pay twice.
Fix the Water First
Whichever route you choose, the sinking will come back unless the cause is dealt with. Before or during the repair, look at where water goes when it rains. Extend downspouts so they discharge well away from the slab. Make sure the ground next to the concrete slopes away from it. If a low corner of the yard is always soggy, the fix might be regrading or a drain, not just new concrete. This is the part most quick fixes skip, and it is why so many lifted slabs sink again within a few years.
What It Costs
For a rough sense of scale: lifting a sunken section typically runs a few hundred to a couple thousand dollars depending on the method and the area. Grinding or overlaying a single joint is usually the cheapest option. Removing and replacing a section of sidewalk or patio in South Jersey generally falls in the range of ten to fifteen dollars per square foot, with driveways somewhat higher because of thickness and base work. Every job is different, which is why we look at it in person before quoting a number.
Got a slab that is sinking, tilting, or turning into a trip hazard?
Almand Bros has been repairing and replacing settled concrete across South Jersey since 1985. We will tell you whether it is a small fix, a lift job worth looking into, or time to repour, and we will fix the drainage that caused it so it stays fixed.
Call Almand Bros Concrete at (856) 310-1390 for a free estimate. We serve Camden, Burlington, and Gloucester counties since 1985.