Why Concrete Cracks: 7 Causes & How to Fix Them
Match your crack to its cause, then fix the right problem. Most concrete cracks are explainable — and many are preventable.
Quick diagnosis
Start here: hairline surface cracks that appeared within days of the pour are usually shrinkage (causes 1–2) and mostly cosmetic. Cracks that show up months or years later, especially ones that are wide, uneven, or growing, point to movement, overload, or weather (causes 3–7) — those deserve a closer look.
1. Plastic shrinkage — surface dried too fast
What it looks like: shallow, map-like (crazing) cracks appearing within hours of finishing, often on hot, windy days.
Why: the surface loses moisture faster than bleed water rises to replace it, so the top shrinks while the concrete below is still plastic.
Fix & prevention: usually cosmetic — seal the surface to keep water out. Prevent with windbreaks, sunshades, fog spraying, and prompt curing (cover or cure compound immediately after finishing).
2. Drying shrinkage — the most common cause
What it looks like: thin cracks appearing weeks to months after the pour, often in long unjointed stretches.
Why: concrete shrinks slightly as excess mix water evaporates over time. Restraint (from the subgrade or adjacent structures) turns that shrinkage into tension — and concrete is weak in tension.
Fix & prevention: control joints give shrinkage a planned place to happen — they should be cut within 24 hours at the right spacing. Existing hairline cracks can be filled with concrete caulk; wide ones (>1/4 in) may need epoxy injection or panel replacement.
3. Thermal cracking — temperature swings
What it looks like: cracks that open in cold weather and close in heat, often near restrained edges.
Why: concrete expands and contracts with temperature. Large pours and restrained slabs build up stress that releases as cracks.
Fix & prevention: expansion/isolation joints around columns, walls, and existing structures absorb the movement. Keep joints sealed so water can't enter and worsen the cycle.
4. Settlement — the ground moved
What it looks like: one side of the crack higher than the other (differential settlement), often near downspouts, backfilled trenches, or soft spots.
Why: poorly compacted subgrade, eroding soil, or water undermining the base lets sections of slab sink unevenly.
Fix & prevention: proper subgrade compaction and drainage before the pour prevent most of this. For settled slabs, mudjacking or polyurethane foam injection can lift sections back to level; severely undermined slabs need replacement. Fix the water problem first or it will happen again.
5. Overloading — too thin for the load
What it looks like: cracks under wheel paths or where heavy equipment sits, on slabs poured at 3–4 inches for loads that needed 6.
Why: the slab flexes under loads it wasn't designed for and cracks in tension on the underside, which propagates upward.
Fix & prevention: match thickness to use — 4 in for patios and walkways, 5–6 in for driveways with trucks or RVs, with reinforcement for heavy loads. An overloaded cracked slab usually needs replacement at the right thickness, not just patching. Use our concrete slab calculator to work out quantities for the replacement.
6. Freeze-thaw damage
What it looks like: surface scaling, flaking, and spalling that worsens each winter, especially where de-icing salts are used.
Why: water trapped in the concrete's pores expands about 9% when it freezes, prying the surface apart cycle after cycle.
Fix & prevention: air-entrained concrete (microscopic air bubbles give freezing water room to expand) is the standard defense in cold climates, plus proper drainage and sealed joints. Scaled surfaces can be resurfaced if the base is sound; badly deteriorated slabs get replaced.
7. Tree roots, chemicals & rebar corrosion
What it looks like: slabs lifted or cracked in lines radiating from nearby trees; rust stains weeping from cracks; surface deterioration near chemical exposure.
Why: roots grow under slabs and lever them up; water reaching steel reinforcement rusts it, and rust expands, splitting the concrete from inside; certain chemicals (sulfates, some de-icers) attack the cement itself.
Fix & prevention: root barriers and proper setback distances for new plantings; adequate concrete cover over rebar per code; the right mix design for the exposure. Root-lifted panels are usually removed, the root managed, and the panel re-poured.
When to worry — and when not to
- Usually fine: hairline (<1/16 in) shrinkage cracks, stable over time, both sides level.
- Watch it: cracks wider than 1/8 in, or any crack that keeps growing — mark the ends with a pencil and check monthly.
- Call a professional: cracks wider than 1/4 in, differential settlement (one side higher), cracks that leak water, or any cracking in foundations, retaining walls, or structural elements.
FAQ
Are cracks in concrete normal?
Hairline shrinkage cracks are extremely common — concrete shrinks as it cures, and some cracking is expected even in well-built work. Control joints exist precisely to decide where that cracking happens. What's not normal is wide, growing, or uneven cracking.
Can cracked concrete be repaired, or does it need replacing?
Hairline and stable cracks: fill and seal. Wide but stable cracks: epoxy injection or routing and sealing. Settled, heaved, or structurally overloaded slabs: lifting (mudjacking/foam) or full replacement. Match the repair to the cause — sealing a settlement crack without fixing the soil just buys time.
How do I prevent concrete from cracking?
The big five: proper subgrade compaction and drainage, the right thickness and reinforcement for the load, control joints cut promptly at correct spacing, proper curing (keep it moist for the first week), and the right mix for your climate (air-entrained where it freezes). See our driveway cost guide for what proper installation typically costs.