Concrete cracks. It shrinks as it cures, it moves with freeze and thaw, and it gives way under tree roots and heavy trucks. Most driveway cracks are cosmetic — ugly, but not structural — and a weekend of cleaning and filling stops water from getting in and making them worse. The trick is knowing which crack you have, because the filler that fixes a hairline will not fix a slab that is sinking.
Hairline cracks under an eighth of an inch are shrinkage cracks: normal, cosmetic, and easy to fill. Cracks from an eighth to a quarter inch are the common repair — they need filler, but the slab is still sound. Wider than a quarter inch, or with one side higher than the other, means the slab is moving, and filler will not hold it. Spiderweb patterns on the surface are crazing — shallow, cosmetic, and best handled by sealing. And a crack that follows a control joint is the joint doing its job; fill it, do not fight it.
Clean the crack out. Sweep loose debris out, then clean the crack with a wire brush, a pressure washer, or a leaf blower. Pull out any weeds growing in it — the weed guide covers keeping them from coming back — and get the dirt and old filler out so the new filler bonds to concrete, not to dust.
Let it dry. Filler needs dry concrete to bond. Wait for a dry day and let the crack dry fully after washing — a day in the sun is usually enough. Filling a wet crack is the most common reason a repair pops out by spring.
Put a backer rod in wide cracks. For cracks wider than a quarter inch, press a foam backer rod into the crack so it sits about half an inch below the surface. It gives the filler something to sit on and keeps you from pouring filler into a bottomless void. Skip it for hairlines — they are too narrow to need it.
Fill it. Hairlines get concrete caulk from a tube. Wider cracks get a concrete crack filler or a self-leveling sealant on flat slabs. Overfill slightly — the filler settles as it cures. For spalled, flaking areas around a crack, a vinyl concrete patch spread thin works better than caulk.
Tool it smooth. Smooth the filler with a putty knife or a wet finger before it skins over, and feather the edges into the surrounding concrete. A neat, level repair looks better and wears longer than a proud bead that tires and feet catch on.
Let it cure. Keep cars and heavy traffic off for 24–48 hours, longer in cold weather. Most fillers cure in a day; concrete patches need longer. Mark the spot with a cone or a chair so nobody drives over it before it is ready.
Seal the driveway. Once the repair has cured, seal the whole driveway — or at least the repaired area — with a concrete sealer. Sealing keeps water out of the next hairline before it becomes a crack, and it is the same idea as fixing sunken pavers: the sunken paver guide lifts and levels the slab so water drains off it instead of pooling on it.
Call a pro when the slab is heaving or sinking, when a crack is wider than half an inch, when water pools on the driveway after rain, or when a tree root is lifting the slab. Those are ground problems, not surface problems, and they need mudjacking, replacement, or root work — not a tube of filler. Several slabs moving together is also a pro job. One crack in an otherwise sound driveway is yours.
Seal the driveway every two to three years, keep heavy trucks off it where you can, and keep water moving away from the slab — the same grading that protects a basement protects a driveway. Go easy on de-icing salts: they speed up surface damage, and sand gives grip without the chemical wear. A driveway that is sealed, drained, and kept clear of weeds lasts far longer than one that is only fixed when it cracks.
Not well. Most fillers need temperatures above 40°F to cure, and a wet, freezing crack will not bond. Clean the crack in winter and fill it in spring — a few months of waiting beats a repair that pops out.
A good repair lasts two to five years, depending on the weather and the traffic. Cracks move with the seasons, so expect to touch them up — sealing the driveway after filling is what stretches the repair toward the long end.
After. Fill first, let the filler cure, then seal over it. Sealing first traps dirt in the crack and keeps the filler from bonding to clean concrete.
Concrete shrinks as it cures, and the control joints are cut to give it a place to crack on purpose. Hairline cracks in the first year are normal. Wide cracks or heaving in a new slab mean a base or drainage problem, and that is a call to the contractor.