A patio slab in St Albans had three cracks I’d been asked to fix twice already — once with a caulk-grade sealant that lasted a summer, once with a patch that a previous crew had troweled in without routing the crack first, which meant it never bonded and popped out within a year. By the time I got there for a third look, the owner had spent more on repeated patching than a partial tear-out would have cost. That is the pattern I see most often with cracked and settled flatwork: not one bad decision, but three or four small ones that add up past what a real fix would have cost the first time.
Reading the crack itself
A hairline crack under 1/8 inch wide with no vertical offset between the two sides is usually a shrinkage crack and often a cosmetic issue, not a structural one. Once a crack opens past 1/4 inch, or one side sits noticeably higher or lower than the other — even a difference you can just barely catch with a fingertip — something below the slab has moved, and that has to be understood before any patch goes in. A crack monitor, which is a cheap plastic gauge glued across the crack, or even a set of pencil marks checked over six to eight weeks, tells you whether the movement has stopped or is ongoing. Patching an actively moving crack is patching over a live problem; the patch material has no chance against ground that is still settling underneath it.
Rocking panels and hollow ground
Stand at one edge of a slab section and rock your weight onto it. If it moves — even slightly — there is a void or soft zone underneath that panel, and no surface repair addresses that. Tapping the slab with a hammer or a length of rebar and listening for a hollow, drummy sound versus a solid thud is a rough but useful test for the same thing. A rocking panel next to panels that are solid usually means a localized void, which can sometimes be corrected by lifting rather than replacing — but a slab with multiple rocking panels across its width is telling you the base under the whole thing failed, not just one spot.
How much damage is too much to patch
Corner spalling — small triangular chips breaking off where cracks intersect near a slab edge — is common and patchable in small amounts. Once spalling shows up at more than roughly 10 to 15 percent of a slab’s total edge length, or on more than a couple of panels in a driveway, patch-and-repeat starts costing more than it saves. Exposed rebar that has rusted and expanded, cracking concrete outward around it, is a different category entirely: once corrosion has jacked concrete loose, patching over rusted steel just buries a problem that keeps swelling. That steel needs to be exposed, cleaned or replaced, and protected before any new concrete goes over it, per the guidance the American Concrete Institute publishes on repair of corrosion-damaged concrete.
The math on repairing three times
A single crack repair on a residential driveway panel might run $150 to $400 depending on the routing, filler, and finish work involved. That looks cheap next to a $2,500 to $6,000 panel replacement — until it has to be done three times because the underlying cause was never addressed. Three repairs at $300 each is $900, plus the aggravation of doing it again each spring, against one repair that lasts because it dealt with the base. The break-even point usually arrives faster than homeowners expect, especially once you count the cosmetic mismatch of patched concrete next to original finish, which rarely disappears even with a good color match.
When replacing just the bad panels is the right call
Residential driveways are typically poured with control joints separating individual panels, and that jointing is exactly what makes partial replacement possible. If one or two panels out of six are cracked, rocking, or badly spalled, and the panels around them test solid with no hollow spots, sawing out and replacing just the failed sections is a legitimate, lower-cost path — as long as the subgrade under the failed panels gets corrected before new concrete goes back in, not just poured over the same soft spot that caused the failure. Skipping that step is how a partial replacement turns into the same three-times-the-cost pattern a year or two later.
Frequently asked questions
How wide does a crack have to be before it needs more than filler? Past about 1/4 inch, or with any vertical offset between the two sides, filler alone is treating a symptom rather than the movement causing it.
Is a rocking panel always a full replacement? Not always — an isolated rocking panel with a void underneath can sometimes be corrected by lifting rather than removal; see slabjacking and polyurethane lifting for when that applies and when it does not.
Can you just replace the cracked panel and leave the rest? Yes, if the adjoining panels test solid and the subgrade under the failed panel gets corrected before the new pour, not simply covered over again.