Crack Sealing vs Slab Replacement

I filled a crack in a 900 square foot shop floor outside Rockrimmon on a Tuesday, coated it Thursday, and got the phone call fourteen months later. The crack was back, dead straight through my flake, the full width of the bay. I had routed it and filled it with rigid polyurea because it read static: tight, no faulting, edges not spalled. It was not static. It had become that slab’s working joint after a previous crew filled the real control joints solid with mortar.

That rework cost me two days and about $1,900 in material and labor I never billed. It bought me a habit: before I quote a coating, I decide whether the concrete underneath is going to keep moving. That is the same question sitting behind every seal-versus-replace argument.

Static or active is the only sorting question

Everything follows from it. A static crack has stopped opening and closing; it happened once, from drying shrinkage or a curing problem, and the slab has settled. An active crack is still responding to something — seasonal movement in the subgrade, a joint never cut deep enough, a void under one panel, or load the slab was never built for.

You do not have to guess. Tape a crack monitor across it and read it monthly for a season. Run a straightedge over it and measure faulting — if one side sits more than about 1/8 inch below the other, something under the slab moved, and filling the top will not put it back. Then tap along both sides with a hammer. A hollow, drummy sound means a void, and a slab bridging a void will crack again right beside your repair. NRMCA’s Concrete in Practice series covers why slabs crack in the first place, and it is worth ten minutes before anyone spends money.

What sealing actually buys you

Sealing is a water strategy, not a structural one. Keep water out and you slow the two things that turn a crack into a hole: corrosion of the steel down there, and freeze-thaw wedging every time the temperature crosses 32°F. In a climate with 100-plus freeze-thaw cycles a year, that alone can buy a slab another decade.

The material has to match the movement:

Rigid epoxy or rigid polyurea, Shore D 60 to 80, for genuinely static cracks and for control joints under hard wheels. Route to roughly 1/4 inch by 1/4 inch, blow it clean, fill proud, shave flush next day.
Semi-rigid polyurea filler, Shore A 80 to 90, for interior joints that still creep a little. It supports the joint edge under a pallet jack without tearing.
Flexible urethane over backer rod for exterior expansion joints and anything still moving seasonally. It stretches instead of splitting, and it never disappears visually.
Hot-applied rubberized sealant on drives and lots, where the goal is water exclusion over hundreds of linear feet, not a pretty line.

Bond is where most of these go wrong. Vacuuming the slot is not optional, and filler dumped into a dusty saw kerf releases in a season — our notes on patching compounds that actually bond cover that side of it.

When the slab has stopped being repairable

I stop writing repair numbers and start writing replacement numbers when I see any of these:

Panels that rock underfoot or tick under a vehicle — the base is gone, not the concrete.
Faulting over 1/4 inch, or panels tipped enough to pond water.
Cracking on more than about 25 to 30 percent of the area, or one panel broken into four or more pieces.
Exposed, rusted rebar with concrete delaminating around it. Once steel is corroding and expanding, patching the surface just moves the failure a few inches over.
Scaling deeper than about 1/4 inch over a large area, which usually means the concrete was finished wrong or never had the entrained air it needed.

There is a middle option people forget: replace the worst one or three panels and leave the rest. Sawcut full depth on the joint lines, remove, fix the base, repour to match. Not cosmetically perfect, and a fraction of a full tear-out.

The money, roughly

Routing and filling cracks runs about $4 to $12 per linear foot with a real interior filler, less for hot-pour on a driveway. Replacing a single 10 by 12 foot panel — sawcut, demo, haul-off, base repair, 5 to 6 inch pour — is roughly $1,200 to $2,600. A full 600 to 800 square foot driveway is $6,000 to $14,000. Slabjacking a settled but sound panel is $600 to $1,800 and is worth pricing before you condemn anything.

My rule of thumb: if repair runs past about 40 percent of replacement, replacement is the cheaper decade.

Frequently asked questions

Can you coat over a repaired crack and have it disappear? Only if the crack is genuinely static. Routed, filled, ground flush and covered with a flake system, it is invisible. Anything still moving telegraphs back through the film in one to three years.

How long does crack sealing last? Interior polyurea filler in a clean, dry slot commonly holds 8 to 15 years. Exterior sealant in a freeze-thaw climate is more like 3 to 7 years and should be checked each fall, because it is doing the hardest job — stretching, shrinking and taking ultraviolet all year. Anything sitting on the slab shortens that interval — a Tarrytown HVAC contractor sees condenser pads work joints loose years before the rest of the surface goes.

Are hairline cracks in a new slab a defect? Usually not. Drying shrinkage cracks under about 1/16 inch are normal in slabs on grade and are why control joints exist. Worth a conversation: a crack that crosses a control joint, widens over the first year, or faults. Those point at the base or the jointing layout.

Code reference: American Concrete Institute.