Every concrete house cracks somewhere; concrete shrinks and moves. The skill is telling harmless cracks from the few that signal a problem, and preventing the avoidable ones.
Types of Cracks
| Type | When it appears | What it looks like | Concern |
|---|---|---|---|
| Plastic shrinkage | Hours after pouring | Short, random, shallow cracks on slab surface | Low; prevent with early curing |
| Drying shrinkage | Weeks to months | Fine cracks, often at re-entrant corners | Low |
| Thermal | Seasonal | Cracks in roof slabs and parapets, at junctions | Low to moderate |
| Settlement | Months to years | Diagonal cracks in walls, stepped cracks | Moderate to high |
| Corrosion | Years | Cracks along bars, rust stains, spalling | High |
| Structural (flexural/shear) | Under load | Cracks at mid-span bottom of beams, diagonal near supports | High |
Preventing the Common Ones
- Plastic shrinkage: shade and wind-break fresh concrete; start curing early; avoid pouring in the hottest hours.
- Drying shrinkage: control water content; cure for the full period; provide joints in long slabs and screeds.
- Thermal: insulate or coat the roof; allow movement at parapets.
- Corrosion: adequate cover, dense concrete, good curing.
- Settlement: soil test and proper foundation design.
When to Call an Engineer
- Cracks in columns, or diagonal cracks near beam ends.
- Cracks wider than a few millimetres, or growing over weeks.
- One side of a crack displaced relative to the other.
- Rust stains or spalling.
- Doors and windows sticking, floors sloping.
Monitor a doubtful crack by marking its ends and width with dates; growth is the most important sign.
Repair Approach
| Crack | Repair |
|---|---|
| Hairline, dormant | Clean and fill with crack filler or sealant |
| Wider, dormant | Polymer-modified mortar, or epoxy injection where structural bond matters |
| Moving | Flexible sealant; address the cause |
| Corrosion-related | Expose and treat steel, then repair concrete |
| Structural | Engineer's design — strengthening if needed |
The Short Version
Fine, random, stable cracks are usually shrinkage. Wide, growing, diagonal, displaced or rust-stained cracks are not — call an engineer. Cure well, keep cover, and design the foundation properly to avoid most of them.
