Concrete Crack Repair: Foundations, Slabs, and Driveways

Reading the Crack

Hairline cracks (less than 1/16 inch wide) are normal shrinkage cracking. They appear within the first year as concrete cures and loses moisture. These are not structural. Seal them to prevent water infiltration, but do not panic about them. Nearly every poured concrete foundation develops a few hairline cracks during the curing process.

Vertical cracks in foundation walls are usually settlement cracking or continued shrinkage. If both sides of the crack are flush (not offset from each other), the wall has not moved differentially. Seal these and monitor them over time, but they rarely indicate a structural problem. Vertical cracks wider than 1/8 inch deserve closer monitoring.

Horizontal cracks in foundation walls are the ones that concern structural engineers. Horizontal cracking indicates lateral soil pressure pushing the wall inward. If the wall is bowed inward above or below the crack, this is a structural issue that requires professional assessment. Do not fill a horizontal crack and assume the problem is solved.

Stair-step cracks in block foundation walls follow the mortar joints in a step pattern. These indicate differential settlement, meaning one corner of the foundation is sinking more than another. Monitor the crack width over time. If the crack is widening month over month, consult a structural engineer.

Wide cracks (over 1/4 inch) or cracks where one side is offset vertically from the other indicate that structural movement has occurred. Do not just fill these. Determine the cause before spending money on surface repairs that will fail once the underlying movement continues.

Monitoring Cracks

Before repairing any crack, determine if it is active (still moving) or dormant (stabilized). Place a pencil mark at each end of the crack and draw a mark across the crack at its widest point. Date the marks. Check monthly for at least three months, preferably through a full seasonal cycle.

If the marks move apart, the crack is active. Repair with a flexible material that can accommodate ongoing movement. Applying a rigid epoxy repair to an active crack results in the epoxy re-cracking within one season, wasting your time and materials.

Cracks that only appear or widen seasonally (wider in winter, tighter in summer) are responding to frost heave or thermal expansion in the surrounding soil. This is common in northern climates and not necessarily structural, as long as the total seasonal movement stays under 1/8 inch. Monitor through a full year before deciding on a repair strategy.

Filling Hairline and Small Cracks (Under 1/4 Inch)

For slabs and driveways: clean the crack thoroughly with a wire brush and compressed air to remove loose debris and dust. Apply a concrete-specific caulk (polyurethane or silicone formulated for masonry) and tool the surface smooth with a wet finger or a caulk finishing tool. Sashco Slab and Quikrete Polyurethane Concrete Crack Sealant ($6 to $10 per tube) are both rated for exterior concrete and remain flexible through seasonal temperature changes.

For foundation walls: apply a coat of hydraulic cement or concrete patching compound. Press the material into the crack with a putty knife, building up thin layers. Feather the edges smooth to blend with the surrounding wall surface. For cosmetic repairs on dormant cracks, this approach is sufficient and lasts years.

For actively leaking cracks in foundation walls: use hydraulic cement, which sets in 3 to 5 minutes even with water flowing through the crack. Quikrete Hydraulic Water-Stop ($8 to $12 per container) is the most widely available product for this application. Mix a small batch, roll it into a rope shape, and press it firmly into the crack starting at the top. Hold each section in place until the cement grabs. Work quickly because the set time is short.

Epoxy Injection for Structural Cracks

Epoxy injection fills the full depth of a crack in poured concrete walls, bonding the two sides together at near-original strength. This is the professional-grade repair method for structural cracks in poured foundations where the crack is dormant and the cause of movement has been resolved.

The process involves three steps. First, seal the surface of the crack with paste epoxy to prevent the injection material from leaking out. Second, install injection ports every 6 to 8 inches along the crack length. Third, inject liquid epoxy starting from the lowest port, filling until it flows out the next port up. Plug the finished port and move to the next one. The epoxy fills the full crack depth by gravity and injection pressure.

DIY kits are available from Simpson Strong-Tie (Crack-Pac, roughly $40 to $60) and Emecole (roughly $50 to $80 depending on crack length). The technique takes practice. Over-pressuring the injection blows out the surface seal. Under-pressuring leaves voids inside the crack that can hold water. For a first-time repair on a critical foundation crack, professional installation at $300 to $600 per crack is worth the cost and the warranty.

Epoxy is rigid when cured. Only use it on dormant cracks that have stopped moving. For active cracks that are still experiencing seasonal movement, use polyurethane foam injection instead. Polyurethane remains flexible after curing.

Polyurethane Foam Injection

Polyurethane foam expands as it cures, filling voids and conforming to irregular crack geometry. It remains flexible after curing and accommodates minor ongoing movement. It also reacts with water to expand and seal, making it the preferred method for actively leaking cracks where water is present during the repair.

The injection process is similar to epoxy injection, but the material behaves differently. Polyurethane foam expands 10 to 20 times its injected volume. Use significantly less material than you think is necessary. Over-injection creates hydraulic pressure inside the crack that can actually widen it, making the problem worse.

Polyurethane does not bond the concrete together like epoxy does. It fills and seals but does not restore structural strength. For cracks where both sealing and structural repair are needed, some professionals inject polyurethane first to stop water infiltration, then follow with epoxy injection to restore the structural bond. This two-stage approach is more expensive but addresses both issues.

Slab and Driveway Crack Repair

Clean the crack with a pressure washer or wire brush and remove all loose material from inside the crack. For cracks deeper than 1/2 inch, fill the lower portion with backer rod (a closed-cell foam rope available in various diameters at hardware stores for $3 to $8 per roll) to within 1/4 inch of the surface. The backer rod provides a base for the sealant and prevents the sealant from bonding to the bottom of the crack, which allows the sealant to flex properly.

Apply self-leveling concrete crack filler for narrow cracks. Products in caulk-tube format like Quikrete Self-Leveling Polyurethane Sealant ($7 to $10) work well for cracks up to 1/2 inch wide. For wider cracks, use a concrete patching compound mixed to a pourable consistency and troweled smooth.

For a cosmetic match on a driveway or patio where individual crack repairs are visible, resurfacing the entire slab with a thin concrete overlay hides the repairs and gives a uniform appearance. Overlays are 1/8 to 1/4 inch thick and bond to the existing slab with a polymer adhesive. Quikrete Re-Cap and Sakrete Top 'N Bond are both available at home improvement stores for around $15 to $25 per 40-lb bag, which covers roughly 8 to 10 square feet at 1/4-inch thickness.

When to Call a Professional

Call a structural engineer (not a foundation repair company) if you see any of the following: horizontal cracks in foundation walls with visible inward bowing, stair-step cracks in block walls wider than 1/4 inch or widening over time, any crack where one side is offset vertically from the other indicating shear movement, or cracks accompanied by doors and windows that no longer close properly or floors that slope noticeably.

A structural engineer provides an independent assessment of whether the crack is cosmetic or structural and recommends the appropriate repair. Foundation repair companies have a financial incentive to recommend expensive solutions like wall anchors, carbon fiber straps, or piering systems. Getting an independent engineer assessment first ($300 to $500 for a residential inspection) is money well spent before committing to a $5,000 to $15,000 foundation repair project.

Frequently Asked Questions

Can I Use Regular Caulk to Fill Concrete Cracks?

No. Standard silicone or latex caulk does not bond to concrete and peels out within one season of weather exposure. Use concrete-specific caulk (polyurethane or silicone formulated for masonry) or concrete patching compound. These products are formulated to bond to concrete surfaces and withstand the thermal cycling and moisture exposure that destroy general-purpose caulk.

A Foundation Crack Is Leaking Water. How Urgent Is This?

Address it promptly, but it is not typically an emergency unless the water volume is significant. Most foundation leaks start as seeps through hairline cracks during heavy rain. Hydraulic cement stops the immediate leak from inside. Then investigate the exterior: poor grading, clogged gutters, or a failed drain tile system are usually the root cause pushing water against the foundation. Fix the water source, not just the symptom. See our French drain installation guide for exterior water management.

Does Filling a Crack Prevent It from Spreading?

Rigid repairs (epoxy, hydraulic cement) can prevent further movement on dormant cracks where the underlying cause has stabilized. Flexible repairs (polyurethane, caulk) accommodate ongoing movement but do not resist it. Neither type of repair addresses the root cause. If soil pressure, settlement, or frost heave is driving the crack, the repair will fail eventually unless the cause is also addressed.

Related Reading

• Driveway Repair Guide
• Concrete Form Building
• French Drain Installation