How do I fix cracks in a concrete driveway?

Most driveway cracks can be sealed in an afternoon with a tube of crack sealant, a chisel and a wire brush. Sealing does not make the slab stronger. What it does is keep water, sand and weeds out of the crack, so the crack is not pried wider every winter. The repair only lasts if the crack has stopped moving, so start by working out what kind of crack you have.

First, read the crack

Narrow and level. A narrow crack with both sides at the same height is the kind crack sealants are made for. A narrow crack that has been there since the driveway was young is the usual one to seal yourself. A crack that is new in an old driveway, or that keeps growing between checks, deserves a closer look first; the Federal Highway Administration names coarse aggregate that is susceptible to freeze-thaw damage (D-cracking) among the factors behind deterioration along joints.^[5]

One side lower than the other. Lay a straightedge or a level across the crack. If one edge has dropped, the ground under the slab has probably settled. Sealant fills the gap but does nothing about the sinking. See whether to repair or replace cracked concrete before you spend money on it.

Rust stains or flaking along the crack. A brown stain or a chip lifting off along a crack often means steel inside the slab is corroding. The National Park Service explains that rust takes up more space than the steel did, that the resulting expansive forces cause cracking and spalling of the adjacent concrete, and that rust staining is often a precursor to spalling.^[6] That is a job for a pro, not a sealant.

A web of cracks with broken pieces. A driveway like the one below, broken into loose chunks with weeds in every gap, has gone past crack filling.

Choose the filler

There are three common kinds of driveway crack filler, and the label tells you which crack each one is made for.

Pourable crack seal. Quikrete's Concrete Crack Seal is a pourable latex filler for cracks in horizontal concrete such as sidewalks, steps and patios.^[2] It is not meant for cracks wider than 1/2 inch.^[2] If the crack is deeper than 1/4 inch, Quikrete says to fill it with backer rod or sand to within 1/4 inch of the surface.^[2] Use it between 45 and 100 F, not when rain is expected within 24 hours, and add a second coat after 24 hours if needed.^[2] For planning, one quart fills a crack 1/4 inch wide and 1/2 inch deep for about 38 feet.^[2]

Flexible polymer sealant. For a driveway crack that opens and closes with the seasons, a sealant that stretches is the safer bet. Quikrete's Advanced Polymer Concrete Crack Sealant is a flexible sealant for cracks and joints up to 1 inch wide,^[3] rated for joint movement of plus or minus 25%.^[3] Quikrete calls for a sealant bead no deeper than 1/2 inch and no thinner than 1/4 inch, about twice as wide as it is deep.^[3] Apply it between 40 and 95 F, never to a frozen surface, and keep it free of traffic for at least 3 days.^[3] Cooler temperatures and low humidity lengthen the cure.^[3] A 10 oz tube fills about 12 feet of a joint 1/2 inch wide and 1/4 inch deep.^[3]

Caulk-type concrete repair. A gray repair caulk in a tube is easy to tool flush with the surface. Quikrete notes its Concrete Repair caulk starts to skin over in about 20 to 40 minutes,^[1] so smooth it right after you lay the bead.

If you plan to resurface later, choose the patch with that in mind. Quikrete's Re-Cap resurfacer calls for cracks to be widened, cleaned and filled with the resurfacer mixed to a trowelable consistency,^[4] and says it should not go over acrylic or polyurethane crack fillers.^[4]

Preparing the crack

The prep is most of the job. Quikrete calls for widening the crack to a minimum of 1/4 inch and breaking away deteriorating concrete, so the edges are vertical or beveled in an inverted V.^[1] A cold chisel and a hand sledge are enough for most cracks. Then brush and vacuum out every bit of dust, because sealant bonds to clean concrete, not to grit.

Depth matters as much as width. Quikrete says for cracks or joints over 1/4 inch, put backer rod in before the self-leveling polyurethane sealant.^[1] Backer rod is a round foam cord sold next to the sealant; buy it a little wider than the crack so it stays put when you push it in with a putty knife.

After the repair

A sealed crack in a moving slab may open again. Quikrete warns that old cracks can reappear when the base concrete moves.^[4] Check your repairs each spring, reseal any bead that has split, and fix the drainage if water runs across the driveway or pools along one edge. How to keep concrete from cracking in winter covers deicers, sealing and drainage, and why new concrete cracks explains joints and early cracking.

Stop and call a pro if

• One side of the crack is higher than the other, or a whole section has sunk or tilted. Raising a slab is specialist work; see slab lifting and leveling.
• The crack is wider than the sealant is made for, or keeps getting wider between checks.
• You see rust stains, exposed steel or chunks lifting off along the crack.
• The cracks run into the garage floor, the house or the steps, or water is getting into the basement nearby.
• The driveway is broken into loose pieces, where patching will not last.

Getting help in Virginia

Loclis lists DPOR-licensed concrete businesses with their licence number and class. Look up concrete contractors in Blacksburg, browse concrete repair, or see all concrete and hardscaping answers.

Describe the crack once and licensed local concrete businesses quote the repair.