Cracks in Concrete and Plaster: Causes, Types and How to Prevent Them
Not every crack is dangerous, but every crack has a cause. Learn to read the common types and stop most of them before they start.
Most cracks in concrete and plaster come from plastic shrinkage (surface drying within hours), drying shrinkage (over weeks), thermal movement, settlement of fresh concrete, reinforcement corrosion, and junctions between different materials. Prevent them with a low water-cement ratio, proper compaction, timely and adequate curing, correct cover, mesh at concrete-brick junctions, appropriate cement (PPC/OPC 43 for plaster) and joints where needed. Wide, growing or diagonal cracks in structural members need an engineer.
Common crack types
| Crack type | When it appears | Typical look | Main cause |
|---|---|---|---|
| Plastic shrinkage | Within hours | Short, parallel or map cracks on slab surface | Rapid surface drying |
| Plastic settlement | Within hours | Cracks above top bars | Concrete settling around steel |
| Drying shrinkage | Weeks to months | Fine cracks, map pattern in plaster | Loss of moisture, rich or wet mixes |
| Crazing | Days | Fine hexagonal network | Over-trowelling, rich surface |
| Thermal | Days to years | Straight cracks across members | Heat of hydration or seasonal movement |
| Junction cracks | Months | Along column/beam–brick joints | Different materials moving differently |
| Corrosion cracks | Years | Along bar lines, rust stains | Inadequate cover, chlorides, carbonation |
| Structural | Any time | Diagonal, wide or growing | Overload, settlement, design or construction defects |
Prevention: concrete
- Keep water-cement ratio low — see water-cement ratio.
- Compact thoroughly with vibrators.
- Protect and cure early, especially in heat and wind — see curing and hot weather concreting.
- Provide correct cover using proper cover blocks.
- Choose suitable cement — PPC or PSC for massive sections to reduce heat.
- Provide joints in large slabs and long walls as designed.
Prevention: plaster
- Use PPC or OPC 43 — avoid OPC 53 for plaster. See OPC 53 vs 43.
- Use clean, well-graded sand — silty sand increases shrinkage. See silt content test.
- Do not over-rich the mix — follow 1:4 to 1:6 as specified.
- Wet the masonry before plastering (not soaked, not dry).
- Fix chicken mesh across concrete-brick junctions.
- Apply thick plaster in two coats, letting the first stiffen.
- Do not over-trowel.
- Cure for at least 7 days.
When to call an engineer
- Cracks wider than about 1–2 mm in structural members
- Diagonal cracks near openings or corners
- Cracks that are growing (mark ends and date them to monitor)
- Rust stains or spalling along cracks
- Doors or windows that start to jam
- Floors that slope or sink
Repairing minor cracks
- Hairline plaster cracks: fill with flexible crack filler or acrylic putty before painting.
- Non-structural concrete cracks: clean, then seal with suitable polymer-modified mortar or sealant.
- Leaking cracks on roofs: treat as part of waterproofing.
Never just paint over a growing crack.
Key takeaways
- Most cracks come from drying, heat, settlement and junctions.
- Low water, good compaction, cover and curing prevent most of them.
- Get wide, diagonal or growing cracks checked by an engineer.
Frequently asked questions
Why do cracks appear in new plaster?
Common causes are rich mixes, silty sand, over-trowelling, applying plaster on dry walls, thick single coats and poor curing.
Are hairline cracks in walls dangerous?
Hairline cracks in plaster are usually shrinkage cracks and are not structural. Wide, diagonal or growing cracks should be checked by an engineer.
How can I prevent cracks between brick walls and columns?
Fix galvanised chicken mesh across the junction before plastering so the plaster bridges the two materials.
Need materials for this work?
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