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Repair, lift or replace: reading what a failed slab is telling you
Once a slab begins cracking, dropping or flaking, ripping it out can seem like the obvious move, and occasionally it is. Often, though, the slab failed because of something underneath or beside it, and a new slab poured in the same spot will fail the same way. In the Tallahassee area the usual culprits are settling fill, clay that softens in wet seasons, sand washed out by runoff, tree roots and, now and then, a sinkhole forming in the limestone below. Reading the damage first leads to the right repair.
Reading the pattern of the damage
Cracks and movement form patterns, and each pattern points toward a cause. Before calling anyone, walk the slab and note what you see, where it is, and whether it is getting worse. Dated photos taken a season apart reveal whether anything is still moving.
- Hairline cracks in a random web across the surface often come from rapid drying during the first hours after the pour. They are usually cosmetic.
- Straight cracks running across a slab between joints suggest joints were spaced too far apart or cut too late.
- A corner or edge that has dropped points to soft soil, washed-out sand or settling fill under that spot.
- A section pushed up, often with a crack near a tree, suggests root pressure.
- Flaking or pitting on the surface, with the slab otherwise level, indicates a weak surface layer or chemical wear.
- A depression that appears suddenly or keeps growing, especially with soil dropping nearby, needs attention quickly.
Settlement over backfill and utility trenches
The most common failure near homes is a slab sinking where it sits over disturbed soil. Ground beside a foundation is excavated during construction and pushed back in. Trenches for water, sewer and electrical lines cross yards and driveways. Unless that soil was packed down in thin passes, it continues to consolidate for years, and the concrete on top sinks right along with it.
The telltale sign is a slab that tips toward the house, or a crack running parallel to the foundation a few feet out. Water then flows toward the house instead of away, which makes the problem worse. When a settled section is otherwise intact, lifting may be an option. When it is broken into pieces, replacement with proper compaction underneath is usually the answer.
Washout on sandy lots
In southern Leon County, Wakulla County and other sandy areas, heavy rain running along slab edges can carry sand away. The slab ends up spanning a hollow near its edge. For a while it holds, then it cracks and the edge drops. You can sometimes see the gap by looking under the edge or pushing a stick beneath it.
Replacing the concrete without dealing with the water will repeat the problem. Redirecting downspouts, regrading soil against the edges, and adding drainage where runoff concentrates usually come first. Where washout is limited, filling the void underneath may stabilize the slab.
Root lift near big trees
Live oaks and other large trees with shallow, spreading roots gradually push up slabs as roots thicken. The result is a raised panel, often with a crack at the high point and a lip at a joint. This is not fixable by leveling alone, since the root keeps growing.
Options include cutting or rerouting the slab around the tree, grinding down small lips, or rebuilding the section with changes that give the root room. Removing large roots can harm or destabilize a tree, so get a tree professional's opinion before that decision, and check local tree rules, particularly on canopy lots and near public right-of-way.
Sinkholes and karst depressions
Much of the Big Bend sits on limestone that slowly dissolves, and sinkholes are part of the local landscape. Most sagging slabs are caused by ordinary settlement, not sinkholes, but a depression that appears suddenly, grows over weeks, or comes with cracks in nearby walls, doors that stick or soil dropping around it deserves a closer look before any concrete is replaced.
A concrete contractor can describe what they see, but a sinkhole question belongs with a geotechnical professional. If you are concerned, keep people and vehicles away from the area and get it evaluated first. Pouring new concrete over an active depression does not solve it.
Surface wear, scaling and spalling
Flaking or pitting on a slab that is otherwise flat is surface damage. Common causes include too much water added during finishing, overworking the surface, finishing while bleed water was still present, or poor curing in hot weather. Road salt is rarely the cause here, since it is seldom used in Tallahassee.
Heat plays a part too. A slab poured on a blazing, windy afternoon without careful curing can end up with a soft top layer that dusts, wears in traffic paths and shows fine map cracks within a few seasons, even though the concrete below is sound.
Minor surface wear may be cleaned and sealed. Wider damage may be resurfaced with an overlay if the slab below is sound. Deep spalling or exposed reinforcement usually means the section should be replaced.
Patch, lift, resurface or replace
Weighing the options
Patching and crack filling suit narrow cracks and small damaged spots on a slab that is still level and well supported. They keep water out and slow further damage, though patches rarely match in color.
Lifting, by pumping foam or a grout mixture under the slab, suits sections that have settled as one piece. It restores level and fills voids, but it works poorly on slabs broken into many pieces or over ground that is still moving.
Resurfacing adds a thin layer over a sound slab with a worn surface. It does not fix settlement or structural cracks.
Replacement removes the old concrete and starts again. It is the right choice when the slab is broken, badly tipped or deteriorated, and it is the chance to fix the base, drainage and joints. Partial replacement of individual panels is often possible.
What a replacement quote should cover
- The contractor's explanation of why the old slab failed.
- What will be done about the cause: drainage, base, roots or soil.
- Removal and haul-off of the old concrete.
- Base repair, material and compaction.
- Thickness, reinforcement and joint layout of the new slab.
- How new sections meet any concrete that stays in place.
- Finish and whether color matching to existing concrete is expected.
Repair, lift or replace — common questions
Is it worth fixing a cracked driveway or should I replace it?
It depends on the cracks and what caused them. Narrow cracks in a slab that is still level and well supported can often be sealed. Sections that have settled in one piece may be lifted. When the slab is broken into many pieces, badly tipped, or crumbling, replacement usually makes more sense. A contractor can look at the pattern and the ground and tell you which applies.
How can I tell if a sunken spot might be a sinkhole?
Most sinking concrete is ordinary settlement of loose soil. Warning signs that deserve more attention include a depression that appears suddenly, grows over days or weeks, or comes with cracks in the house walls, doors or windows that start sticking, or soil dropping around it. In that case, keep people off the area and contact a geotechnical professional before anyone replaces the concrete.
Will new concrete match my existing slab?
Rarely right away. Fresh concrete looks paler and more even, while the old surface has weathered, darkened and picked up stains. The difference usually softens over time. Replacing a whole section between joints, rather than a patch in the middle, looks cleaner. If appearance matters, ask whether replacing an entire panel or area makes sense, or whether a sealer or stain could help blend them.
Can mudjacking or foam lifting fix a sunken patio?
Often, if the patio settled as a single piece and is not badly cracked. Material is pumped under the slab through small holes to raise it and fill the voids. It is less disruptive than replacement. It does not work well on slabs broken into many pieces, and if water keeps washing soil out, the problem can return unless the drainage is fixed too.
Why does my concrete keep cracking in the same place?
Repeated cracking means the cause is still there. Common reasons include a root growing underneath, water running under one edge, soft fill that keeps settling, or a missing joint that forces the slab to relieve stress at that spot. Filling the crack treats the symptom. A contractor should look for the cause and address it, whether that means drainage, base work, a new joint or partial replacement.
What happens to the old concrete when it is removed?
It is typically broken up and hauled away. Concrete debris is heavy, and disposal is a real part of the job, so the quote should state whether removal and haul-off are included. In some cases broken concrete can be recycled as fill or base elsewhere. Ask the contractor how they handle it, and whether any cleanup of the surrounding yard is part of the scope.
Should I replace a whole driveway or just the damaged panels?
When only a handful of panels are damaged and the remainder is solid, flat and shedding water properly, replacing just those panels can make sense. If most of the driveway is cracked, sinking or deteriorating, full replacement may be better, since it lets the base and drainage be corrected throughout. The contractor can look at the overall condition and explain the tradeoffs for each approach.
Talk through your project
Start with the address, what you want poured or replaced, rough dimensions, and a few photos of the area and any existing concrete.
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