How to Fix Every Type of Concrete Crack: Hairline, Structural, Spalling

Matching the right repair to the right crack type is the most important step. The table below maps every common crack type to its cause and repair method. For a deep dive into root causes, see why concrete cracks.

Crack Type Quick Reference

Before buying patching material, use the concrete calculator to estimate the volume you need for larger sections.

Hairline Cracks: Normal Shrinkage, Low Risk

Hairline cracks form as concrete loses moisture and shrinks in the first weeks after placement. They are less than 1/16 inch wide and typically follow a random map or spider-web pattern across the surface.

What to look for:

• Width less than 1/16 inch (thinner than a credit card edge)
• Random branching pattern over a wide area
• No vertical offset — both sides of the crack at the same height
• Stable over time, not growing

Root causes:

• Normal drying shrinkage
• Inadequate curing (surface dried too fast)
• Insufficient crack-control joint spacing

Repair: In most climates, hairline cracks require no action beyond sealing. Apply a penetrating silane/siloxane sealer across the entire surface — this protects all hairline cracks simultaneously without patching each one individually. In freeze-thaw climates, seal before the first winter to prevent water from entering and expanding inside the crack.

For new concrete, read new concrete cracking: normal vs. problem to confirm your cracks fall within the expected range.

Wide Shrinkage Cracks: Control Joint Failures

Wide shrinkage cracks are 1/16 to 1/8 inch across and typically run in a straight line across the middle of a slab bay. They form when crack-control joints were spaced too far apart or cut too late.

What to look for:

• Width 1/16 to 1/8 inch
• Runs across the middle of a slab panel, parallel to the longer dimension
• Both sides at the same height
• Stable after the first year

Root causes:

• Control joints spaced more than 10–12 feet in a 4-inch slab
• Joints tooled or saw-cut too late after placement
• High water-to-cement ratio producing excess shrinkage

Repair:

1. Clean the crack with a wire brush and compressed air
2. For cracks under 1/4 inch, apply self-leveling polyurethane caulk directly
3. For cracks 1/8 to 1/4 inch, insert a foam backer rod first to control fill depth
4. Smooth flush and allow 24 hours to cure before traffic

Do not use rigid epoxy for shrinkage cracks — the slab still moves seasonally and epoxy will re-crack within a cycle or two.

Settlement Cracks: Check the Subbase First

Settlement cracks form when the soil beneath a slab shifts or washes out. They are wider than shrinkage cracks and almost always show vertical displacement — one side sits higher than the other.

What to look for:

• Width 1/8 inch or more
• One side higher than the other (step or offset along the crack)
• Often runs diagonal, from corner to corner of a slab panel
• May be accompanied by a hollow sound when tapping nearby

Root causes:

• Poorly compacted fill under the slab
• Soil erosion from water (downspouts, poor drainage)
• Tree root displacement
• Expansive clay soils shrinking and swelling seasonally

Repair: Patching the surface of a settlement crack without addressing the subbase will fail — the slab keeps moving. Options:

• Slab lifting (mudjacking or foam injection): Fills voids beneath the slab and lifts settled sections before crack repair. Best for slabs that dropped but are otherwise intact.
• Epoxy injection after leveling: Stabilizes the crack once the slab is re-leveled.
• Replacement: For sections with severe settlement, crumbling edges, or failed prior repairs, replacement is often more cost-effective.

See concrete settlement repair options for a detailed decision guide.

Structural Cracks: Get Professional Eyes First

Structural cracks run through load-bearing elements — walls, beams, columns, or foundation slabs — and can compromise the building if unaddressed.

What to look for:

• Width 1/4 inch or more
• Step cracking through masonry coursing
• Crack runs through the full thickness of the element
• Crack is progressive — measure, mark, and date it; check again in 60 days
• New crack in a structure less than 2 years old

Root causes:

• Overloading beyond design capacity
• Foundation movement or differential settlement
• Thermal expansion in long runs without adequate expansion joints
• Insufficient reinforcement for the applied loads

Do not attempt structural crack repair without professional assessment. Document the crack with photos and measurements. An engineer can determine whether the cause is still active — an active crack will re-crack any repair applied to it. For help evaluating severity, use the concrete damage assessment guide.

Spalling-Related Cracks: Surface Deterioration with Chunks

Spalling cracks are different from through-cracks — the surface is breaking away in irregular chunks, often near rebar lines or where repeated freeze-thaw damage has weakened the surface layer.

What to look for:

• Rust stains near the crack (sign of corroding reinforcement below)
• Loose concrete chips or chunks around the crack edges
• Damage worsening each spring after winter
• Crack is shallow and irregular, not a clean through-crack

Root causes:

• Rebar too close to the surface corroding and pushing concrete off from underneath
• Freeze-thaw damage to non-air-entrained concrete
• Deicing salt damage in the first winter after placement

Repair: For spalling cracks, surface patching without addressing the root cause fails quickly. See our full guide on concrete spalling: causes, prevention, and repair products for step-by-step repair matched to each cause.

Active and Moving Cracks: Never Use Rigid Epoxy

Active cracks continue to grow or move seasonally. Applying a rigid filler to an active crack will cause the repair to re-crack — typically within one freeze-thaw cycle or one full season.

How to confirm a crack is active:

• Mark the ends with a pencil line and date it
• Check again in 60–90 days
• If the mark is no longer at the crack tip, the crack is growing

Repair:

• Use flexible polyurethane caulk for all active cracks — it stretches with movement
• Never use rigid epoxy on a crack that is still growing or moving
• If the crack has been active for more than 2 years without stabilizing, the underlying cause (settlement, overload, thermal cycling) needs to be addressed — not just the symptom

Repair Method by Crack Width

Under 1/8 Inch

1. Clean with wire brush and compressed air
2. Apply penetrating sealer across the whole surface (hairline) or self-leveling polyurethane caulk (shrinkage)
3. No backer rod needed at this width

1/8 to 1/2 Inch

1. Clean and dry the crack completely
2. Install foam backer rod to fill the lower 2/3 of the crack depth
3. Apply flexible polyurethane caulk over the rod
4. Smooth flush and feather edges with a putty knife

Wider than 1/2 Inch

1. Chip out all loose concrete to solid, clean vertical edges
2. Dampen the crack interior (damp but no standing water)
3. Pack with polymer-modified repair mortar in lifts no deeper than 3/4 inch
4. Overfill slightly, then screed flush
5. Cure for 48 hours before traffic

For full DIY repair walkthroughs with product recommendations, see how to repair concrete cracks.

Prevention: Stop Cracks Before They Start

Most surface cracks are preventable at the design and placement stage:

• Space control joints at 8–12 feet for a 4-inch slab. More joints mean smaller shrinkage sections and narrower cracks.
• Cut or tool joints within 24 hours of placement. Waiting too long means cracks have already formed.
• Cure for a minimum of 7 days. Cover with plastic sheeting or apply a curing compound immediately after finishing.
• Compact the subbase. Most settlement cracks trace directly to inadequate subbase compaction. Compact in 4-inch lifts and test density before placing concrete.
• Keep the water-to-cement ratio as low as workability allows. Extra water increases shrinkage.

Key Takeaways

• Identify the crack type before buying repair products — wrong repair fails fast
• Hairline cracks are cosmetic; seal in freeze-thaw climates but otherwise leave them
• Wide shrinkage cracks need self-leveling polyurethane, not rigid epoxy
• Settlement cracks require subbase repair, not just surface patching
• Structural cracks need a professional assessment before any repair work begins
• Spalling cracks near rust stains indicate corroding rebar — a structural issue, not just cosmetic
• Active cracks always get flexible polyurethane; rigid epoxy will re-crack within a season

Browse all concrete guides for help at every stage of your project.