Before you can fix a cracked concrete driveway, you have to read the crack. Most driveway cracks around here are shrinkage cracks, and they are cosmetic. A smaller number are structural, and no filler or sealer will fix those. Telling the two apart takes about five minutes and saves people thousands of dollars every year in both directions.
I run Skinner Customs out of Rigby. Every spring I walk driveways in Rigby, Idaho Falls, Ammon, and Rexburg that came through a winter looking rough, and the honest answer is often that the slab is fine and the owner should leave it alone.
Read the crack first
Four things tell you what you are looking at.
Width. Under about an eighth of an inch is hairline. An eighth to a quarter is worth sealing. Wider than a quarter inch is a real movement crack.
Offset. Put your foot across the crack. If one side sits higher than the other, the slab has moved, which means the ground under it moved. That is a base problem, not a concrete problem.
Pattern. A single crack running from a corner or between two joints is shrinkage doing what shrinkage does. A network of cracks across the middle of a panel means the slab is failing structurally.
Movement over time. Mark both ends with a pencil and a date. A crack that has not grown in a year is stable. One that lengthens every season is telling you something is still moving underneath.
What causes them in the first place
The National Ready Mixed Concrete Association’s bulletin CIP 4, Cracking Concrete Surfaces puts the usual causes plainly: omitted isolation and contraction joints and improper jointing practices, high slump concrete, poor finishing, rapid moisture loss at the surface, and inadequate curing. It also notes that most early random cracks are unsightly rather than structural, with two exceptions worth knowing: D-cracking, which is freeze-thaw deterioration, and alkali-aggregate reaction, which both cause long-term structural damage.
Its jointing numbers are the ones I work to: contraction joints cut about a quarter to a third of the slab thickness deep, spaced 24 to 36 times the slab thickness with a maximum around 15 feet, and panels no longer than about one and a half times their width. Most cracked driveways I look at around here were jointed at 20 feet or more, or in long skinny panels, or not jointed at all.
Add our climate on top. Concrete that was salted its first winter, or poured thin over uncompacted fill, does not need much help to crack.
Cracks you can handle yourself
If the crack is narrow, stable, has no offset, and the slab around it is sound, it is maintenance and not a repair job.
- Clean it out. A wire brush, a screwdriver or utility knife to dig out debris, and a shop vac or a hose. Anything loose in the crack keeps the filler from bonding.
- Let it dry completely. A damp crack fails within a season.
- Fill it. A gray polyurethane or self-leveling concrete crack sealant from any hardware store works for cracks up to about a half inch. Backer rod first if the crack is deep. Overfill slightly; these products settle.
- Stay off it until the product’s cure time is up, and keep it out of standing water that first week.
Do not use a thin cementitious patching compound on a moving crack. It has no flex, and it will crack again in the same line by spring.
The real reason to seal is not looks. It is keeping water out. Water in a crack freezes, expands, and widens the crack every cycle. Around here that runs dozens of times a winter.
Cracks that need concrete work
Call somebody when you see any of these:
- Vertical offset you can catch a toe or a plow blade on
- Cracks wider than about a quarter inch that are still growing
- A panel broken into pieces smaller than roughly three feet across
- Surface scaling or spalling over a large area, where the top layer flakes off and leaves aggregate exposed
- A corner or edge that has settled and holds water against the garage or the house
The first four are the slab. The fifth is usually drainage, and fixing the drainage matters more than fixing the concrete, because if water is still getting under the slab, the new concrete has the same future as the old.
When it is one or two bad panels and the rest of the drive is sound, cutting those panels out at the joints and repouring them costs a fraction of replacing everything. Expect a visible color difference between the new panels and 20-year-old concrete. It fades but never disappears.
When the whole slab is broken up, scaling widely, or too thin for what you park on it, replacement is the honest answer. Replacement in this area runs $10 to $17 per square foot with the old slab removed and hauled, of which tear-out is $2 to $4.
Do not let the new one go the same way
Whether you are repairing or replacing, the reasons driveways fail here do not change:
- 4,000 PSI air-entrained mix. Non-negotiable in freeze-thaw country.
- #3 or #4 rebar on an 18-inch grid. Not wire mesh, which sinks to the bottom of the pour and does nothing.
- At least 4 inches of compacted gravel base, placed in lifts, more on soft or irrigated ground.
- Joints planned before the pour, cut on time, at the spacing above.
- Real curing and no deicers the first winter. Sand for traction instead. CIP 4 calls for keeping the surface moist at least three days.
Frequently asked questions
Are hairline cracks in a concrete driveway normal?
Yes. Concrete shrinks as it cures and it cracks somewhere. Joints are there to control where. Hairline cracks with no offset are not a defect.
Should I seal cracks before winter?
Yes. Sealing in fall keeps water out through the freeze-thaw season, which is when a small crack becomes a big one.
What is the best filler for a concrete driveway crack?
For cracks up to about half an inch, a self-leveling polyurethane crack sealant. Rigid patching compounds crack again because they cannot move with the slab.
Can a cracked driveway be resurfaced instead of replaced?
An overlay only works over a slab that is structurally sound. Over a slab that is cracked through or moving, an overlay cracks along the same lines within a year or two.
Why did my driveway crack the first year?
Usually jointing that was too far apart or cut too late, a wet mix, or moisture lost from the surface before curing started. All four are placement issues, not homeowner issues.
Does repairing a crack stop it from spreading?
Sealing stops water from driving it. If the cause is settlement under the slab, sealing buys time and nothing more.
Not sure which one you have
Send photos of the driveway, a close-up of the worst crack with something in frame for scale, and a note on whether one side sits higher than the other. I will tell you whether it is maintenance, a couple of panels, or a replacement. If it is maintenance, I will tell you that too.
Need concrete work in Eastern Idaho? Call Zach at Skinner Customs at (208) 390-5794 or start at skinner-customs.com.
Content produced in partnership with S4 Media, marketing partner of SwagerBuilds LLC.
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