The repair method for a concrete crack is determined by what caused it, not just how wide it is. A 1/8 inch (3 mm) crack from alkali-silica reaction needs a different fix than an identical-width crack from shrinkage — one is stable, the other is active and will re-open any rigid filler within months.
Before buying materials, use the Concrete Crack Repair Calculator to estimate filler volume by crack length, width, and depth. Underestimating means a second trip; overestimating means wasted product that has a limited shelf life once opened.
How to diagnose your crack type before repairing it
Crack width alone does not tell you what you are dealing with. These four tests take under 10 minutes and determine both cause and correct repair approach.
Chalk line test (active vs dormant): Draw a chalk line across the crack perpendicular to its length. Check again in 2 weeks. If the line has shifted or the crack has widened, it is an active crack under ongoing movement — rigid fillers will fail. If it is unchanged, the crack is dormant and can be filled with any compatible product.
Depth check: Push a thin wire into the crack. Hairline cracks less than 1/4 inch (6 mm) deep are surface-level. Cracks penetrating to the full depth of the slab (typically 4 inches / 100 mm for residential slabs) affect structural integrity.
Staining pattern: Rust-coloured staining along a crack indicates corroding rebar — the steel has expanded and split the concrete. This is not a surface repair issue. The rebar must be exposed, treated for corrosion, and the concrete restored with a structural epoxy mortar or full section replacement.
Pattern recognition: Map cracks (fine random network, also called crazing or craze cracking) are surface-only and caused by rapid surface drying during curing. They look alarming but are cosmetic. A single long diagonal crack at a slab corner typically indicates differential settlement. Parallel cracks running the length of a driveway panel suggest poor or missing expansion joints.
Crack types, causes, and correct repair methods
| Crack Type | Width | Cause | Repair Method |
| Hairline / map cracking | < 1/16 in / < 1.5 mm | Rapid surface drying, over-trowelling | Concrete sealer or thin overlay |
| Shrinkage crack | 1/16–1/4 in / 1.5–6 mm | Normal drying shrinkage, poor joint placement | Polyurethane sealant (flexible) |
| Settlement crack | > 1/8 in / > 3 mm, stepped | Soil movement, poor compaction | Mudjacking or foam lifting + epoxy fill |
| Structural crack | Variable, often > 1/4 in / 6 mm | Overload, rebar corrosion, freeze-thaw | Epoxy injection or section replacement |
| Expansion joint failure | Gap at joint face | Joint filler degraded | Backer rod + polyurethane or polysulfide sealant |
Hairline and map cracking
These are the most common and least urgent cracks. No structural action is needed. If sealing for aesthetics, apply a penetrating silane/siloxane sealer that does not film-form over the cracks — acrylic film-forming sealers can highlight crazing by creating a sheen that shows the crack network. For a cleaner finish, a thin polymer-modified overlay at 1/4 inch (6 mm) bonds well to sound concrete and hides the pattern entirely.
Shrinkage and working cracks
These need a flexible filler, not a rigid one. Concrete slabs move — thermally, seasonally, and under load. A rigid epoxy filler in a moving crack will debond at the edges within one freeze-thaw cycle in northern climates (Canada, northern US, UK Scotland, Scandinavia). Self-levelling polyurethane sealant (e.g. NP1, Sikaflex 1a) bonds to concrete, remains flexible from -40°F to 200°F (-40°C to 93°C), and accepts foot and vehicle traffic once cured (24–48 hours).
Prep is non-negotiable: rout the crack to a minimum 1/4 inch (6 mm) wide × 1/4 inch (6 mm) deep U-shaped profile using an angle grinder with a crack chasing blade or a dedicated crack router. Blow out dust and debris. Install backer rod (closed-cell polyethylene foam) to control fill depth — the sealant should fill to 1/4 inch (6 mm) below the surface, not flush. Flush fill causes three-sided adhesion, which prevents proper joint movement and leads to cohesive failure in the sealant.
Step-by-step repair for the most common crack scenarios
Dormant hairline crack (< 1/8 in / 3 mm), no movement: Clean with a wire brush and compressed air. Apply a low-viscosity epoxy crack filler (water-thin consistency) that wicks into the crack by capillary action. Spread kiln-dried sand over wet epoxy to match the surrounding surface texture. Cure time: 12–24 hours before foot traffic, 72 hours before vehicle traffic.
Active shrinkage crack (any width, confirmed movement): Rout to U-profile. Clean. Install backer rod sized 25% wider than the crack (it compresses to fit and holds position). Apply polyurethane sealant by gun, tooling to a concave finish. Do not paint over within 48 hours — most polyurethane sealants need full cure before coating.
Settlement crack with vertical displacement (stepped crack): Routing and filling the crack alone does not address the cause — one side of the slab has moved and will continue to move unless the substrate is stabilised. Options: polyurethane foam injection (slabjacking) for slabs with void beneath; mudjacking with a cementitious slurry for larger areas. After levelling, seal the filled crack with a flexible sealant to accommodate residual movement.
Common mistakes that make the repair fail
Filling without routing. Pouring filler into a raw crack leaves a V-shaped void. The filler bonds only at the surface, with no mechanical key and minimal contact area. It falls out under traffic within weeks. Every crack wider than 1/16 inch (1.5 mm) must be routed or chiselled to a uniform U-profile before filling.
Using rigid epoxy on an active crack. Epoxy has zero flexibility after cure — typical elongation at break is 2–5% versus 150–300% for polyurethane. Applying epoxy to a crack that still moves (seasonal slab movement in climates with a 40°F / 22°C temperature range is typically 1/16–1/8 inch / 1.5–3 mm across a 10 ft / 3 m slab) guarantees re-cracking at the fill edges within one year.
Skipping backer rod. Without backer rod, sealant fills the full depth of the crack. This creates a thick bead that is too rigid to flex without tearing, and it wastes expensive sealant. Backer rod controls the depth of sealant to a 1:1 width-to-depth ratio, which is the optimal geometry for joint movement.
Repairing in cold weather without precautions. Epoxy and polyurethane both have minimum application temperatures: most require 40–45°F (4–7°C) minimum substrate temperature. Applying below this threshold results in poor cure, soft filler, and adhesion failure. In cold conditions, warm the crack with a heat gun immediately before application and tent the area for at least 6 hours during cure.
Related calculators you might need
If the crack resulted from an improperly placed or missing joint, the Concrete Expansion Joint Spacing Calculator will give you the correct joint spacing for your slab dimensions and thickness going forward. For a full cost picture — including whether repair vs replacement makes more financial sense — use the Concrete Demolition and Removal Cost Estimator alongside the Concrete Cost Calculator. If you are dealing with settlement and are assessing whether to pour a replacement slab, the Concrete Slab Calculator gives you volume and bag count for any slab dimensions.
Frequently asked questions
How do you fix cracks in concrete? The method depends on crack type and activity. Dormant hairline cracks under 1/8 inch (3 mm) are filled with low-viscosity epoxy. Active or wide cracks need routing to a uniform U-profile, backer rod installation, and flexible polyurethane sealant. Structural cracks with vertical displacement require substrate stabilisation before any surface fill.
Can concrete cracks be permanently fixed? Dormant cracks in stable substrates can be permanently filled — epoxy injections used in structural repair have bond strengths exceeding the tensile strength of the surrounding concrete. Active cracks cannot be “permanently” filled with a rigid product; flexible sealants that accommodate movement are the correct long-term solution and will need replacement every 5–10 years.
What is the best product for filling concrete cracks? For dormant hairline cracks: low-viscosity epoxy. For active or wide cracks: self-levelling polyurethane sealant (NP1, Sikaflex, or equivalent). For structural cracks requiring load transfer: two-component epoxy injection system. There is no single best product — the crack type dictates the chemistry required.
How wide does a concrete crack have to be before it is structural? There is no single threshold, but cracks wider than 1/4 inch (6 mm), cracks with vertical displacement (one side higher than the other), and cracks associated with rebar staining all warrant engineering assessment before repair. Width alone is a poor indicator — a narrow crack caused by rebar corrosion is a structural issue regardless of its opening width.
How long does concrete crack repair last? Epoxy fills in dormant cracks: 10–20 years if properly prepared. Polyurethane sealants in active joints: 5–10 years, depending on movement cycles and UV exposure. Repairs that fail within 1–2 years almost always trace back to inadequate surface preparation or wrong product selection. Use the Concrete Crack Repair Calculator to estimate material quantities before starting.
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