A crack is your concrete telling you what the soil did.

Cracks are handwriting, and the pattern names the author. Curing shrinkage writes thin, uniform hairlines, often in a faint map or grid, with both sides perfectly flush; they appear early in a slab's life and then stop changing. Settlement writes differently: cracks that run diagonally across a slab or track its low side, that widen season over season, and above all that show offset, one side sitting higher than the other. Offset is the single most diagnostic feature a crack can have, because shrinkage cannot produce it; only movement can. Expansive clay adds a seasonal rhythm, cracks that open in the dry months and tighten after rain, while a washout void tends to crack the slab in a line over the void's edge, where the supported concrete meets the bridge.

Tree roots write their own signature: cracks following the root path near a mature tree, often with a heaved panel nearby. The full field guide lives on our cracks in concrete page, but the practical summary is short: flush and stable means cosmetic, offset or growing means the soil is involved, and the two get entirely different repairs.

Plenty of cracks need nothing but a good seal, and we would rather tell you that than invent a project. A shrinkage hairline with no offset and no elevation change is structurally meaningless; the only risk it carries is admission, the water it lets through to the soil below. Cleaning it and sealing it closes that door for a fraction of the cost of any structural work, and pairing it with surface sealing where the slab is worn buys protection for the whole surface, not just the crack.

The discipline is in confirming the crack qualifies. Flush edges today do not prove the slab never moves, so the evaluation checks elevations on both sides and looks for the settlement signatures before pronouncing a crack cosmetic. If the measurements say stable, the written findings say seal it and stop worrying, and that is the whole invoice. If they say the slab is moving, sealing alone would just buy a season, and you deserve to know that before spending anything.

A crack with offset is a symptom wearing a disguise. The real problem is the void or soft soil that let one side of the slab drop, and any filler placed in that crack while the slab is still moving becomes part of the wreckage next season. So the order of operations is fixed: stabilize first, seal second. The low side is lifted with polyurethane foam injected beneath it, which fills the void, densifies the disturbed soil, and brings the two crack faces back into plane in controlled stages.

Only then does the crack itself get treated, and by then it has usually narrowed, because closing the elevation gap closes the crack with it. What remains is cleaned, filled, and sealed like any stable crack. The lift portion of that work is polyurethane concrete leveling, and its warranty terms arrive in writing with the proposal, which is exactly the assurance a formerly moving crack needs standing behind it.

An open crack is a drain, and what it drains into is the one place water does the most damage: the soil supporting the slab. Every rain and every sprinkler cycle sends water through the opening, where it either erodes the base out from under the crack or soaks the clay and swells it. Both responses move the slab, and a moving slab widens its cracks, which admit more water. That loop is the whole reason a crack you first noticed at pencil width holds a coin two years later, and it is why crack sealing is cheap insurance rather than cosmetics.

The loop has a second entrance most owners never suspect: the expansion joints. Joint filler in our sun fails years before the concrete does, and a failed joint admits water exactly like a crack, just in a straight line. When we seal a slab's cracks, the joints get inspected and resealed in the same visit, because closing one door and leaving the other open is half a repair.

Crack filling is not one product, and the wrong choice fails politely within a year. A narrow, stable crack in a driveway wants a self leveling sealant that flows to the crack's bottom and bonds both walls while staying flexible through temperature swings. A wider break gets cleaned back to sound edges and may need a backer material first, so the sealant cures as a proper joint rather than an unsupported plug. Where the slab endures traffic, the fill has to carry wheel loads without tearing; where it lives beside a pool, it has to shrug off chlorinated splash and sun.

Exposure decides as much as width does. UV is the great destroyer of sealants in Arizona and inland California, so exterior work gets UV stable products, applied to clean, dry crack walls, because preparation is most of the lifespan. None of this needs a brand pitch; it needs a match between the material's flexibility, the crack's movement range, and the exposure it will live in. That match is part of the written scope, so you know what is going into your slab and why.