Types of Cracks in Concrete
Updated
Most cracks in concrete are about shrinkage and workmanship, not structural failure — but a few are serious, and telling them apart is the whole point. Cracks form either before the concrete has hardened (plastic-stage) or after (hardened-stage). This guide covers the common types, what causes each, and how to prevent them — because nearly all of them come down to water, curing and detailing.
Plastic & hardened
The two stages
Water & curing
The usual cause
Pattern tells the cause
How to diagnose
Plastic-stage cracks (first few hours)
| Type | Looks like | Cause |
|---|---|---|
| Plastic shrinkage | Short, random surface cracks on slabs | Fast surface drying in sun/wind before setting |
| Plastic settlement | Cracks over the top bars | Fresh concrete settling around reinforcement |
| Crazing | Fine map-like hairline network | Surface over-trowelling / rapid drying |
These form while the concrete is still soft. Prevent them by protecting fresh surfaces from sun and wind, not adding water to the top, and starting curing early.
Hardened-stage cracks
| Type | Looks like | Cause |
|---|---|---|
| Drying shrinkage | Straight cracks at regular spacing | Concrete shrinking as it dries; too much water |
| Thermal | Cracks in thick/large pours | Heat of hydration, then cooling and contraction |
| Structural (flexural/shear) | Cracks following the load pattern | Overload, under-design, settlement |
| Corrosion (rust staining) | Cracks along the bars, with rust | Steel corroding and expanding; low cover |
The two that actually matter
Most cracks are cosmetic shrinkage. Watch for two kinds:
- Structural cracks — wide, following the bending or shear pattern (mid-span soffit, near supports, diagonal). These need an engineer.
- Corrosion cracks — running along the reinforcement with rust stains, caused by too little cover or chloride ingress. These shorten the structure's life and must be treated.
How to prevent cracks
- Keep the water–cement ratio low — excess water is the biggest single cause.
- Start curing early and keep it up for at least 7 days.
- Provide correct cover to protect the steel.
- Protect fresh concrete from sun and wind; provide movement/construction joints in large pours.
Frequently asked questions
What are the main types of cracks in concrete? Plastic shrinkage, plastic settlement and crazing form before the concrete hardens; drying shrinkage, thermal, structural and corrosion cracks form after. Most are shrinkage-related and cosmetic.
Which concrete cracks are dangerous? Structural cracks that follow the load pattern (bending or shear) and corrosion cracks running along the reinforcement with rust staining are the serious ones. Fine surface shrinkage cracks usually are not.
What causes cracks in a concrete slab? The common causes are too much water in the mix, fast surface drying before curing, poor curing, heat of hydration in thick pours, and inadequate cover to the steel.
How do I prevent shrinkage cracks in concrete? Keep the water–cement ratio low, cure early and for at least seven days, protect the fresh surface from sun and wind, and provide joints in large areas.
Are hairline cracks in concrete normal? Fine hairline shrinkage or crazing cracks are common and usually cosmetic. They matter only if they let water reach the steel; wide or patterned cracks should be assessed by an engineer.
CivilSite Editorial Team✓ Engineer reviewed
Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.