| Crack Type | Likely Cause | How to Fix or Prevent |
|---|---|---|
| Hairline surface cracks | Shrinkage or fast drying | Keep concrete damp during curing, add fibre additives to the mix |
| Random cracks across slab | No control joints | Saw-cut joints 24–48 hours after pour, space joints every 2–3m |
| Cracks along the edges | Poor sub-base or weak edges | Use compacted MOT Type 1 and support edges with a haunch or formwork |
| Wide/deep structural cracks | Heavy load or no reinforcement | Use steel mesh, thicker slab, or rebar depending on use case |
| Spiderweb/map cracking | Overworked surface in hot weather | Finish earlier, avoid too much trowelling, use a curing agent |
Every solid slab starts with solid ground. Remove any soft or loose topsoil, then lay MOT Type 1 or similar crushed stone in layers. Each layer should be no more than 75mm deep and compacted thoroughly using a plate compactor. This ensures the slab won’t shift or sink later — a major cause of cracking. Don’t cut corners here. If the base is soft, consider adding a geotextile fabric to separate the stone from the soil. A flat, firm base helps distribute the slab’s weight evenly and avoids dips that stress the concrete over time.
Choose a mix that matches the job. For light-use patios, C20 may do — but for driveways, sheds, or heavy loads, you want C25 to C35. Avoid over-watering the mix — too much water weakens the structure and makes shrinkage cracks more likely. Consider adding fibres to help bind the concrete and reduce surface cracking. If the slab will carry weight, mesh reinforcement or rebar may also be needed to control deeper stress. Don't guess — use the right mix for the conditions and intended use.
Concrete wants to crack — so give it a place to do it cleanly. Control joints should be saw-cut or trowelled in 24–48 hours after pouring. Space them every 2–3 metres depending on the slab size. These joints help relieve internal stress from drying and shrinkage. Without them, the slab will crack randomly and often in the worst spots. Make the joints about a quarter of the slab’s depth — so for a 100mm thick slab, aim for 25mm deep joints.
Most cracks form in the first few days after pouring — often from drying out too quickly. Keep the slab damp for at least 5–7 days. Use a curing compound, water spray, or plastic sheeting to lock in moisture. This helps the slab set evenly and gain strength gradually, reducing shrinkage stress. Don’t let hot sun or wind dry it too fast. On warm days, curing is critical. Even fibre-reinforced mixes need this care to prevent spiderweb or surface cracking.
Once the concrete has been poured and screeded level, avoid excessive trowelling or finishing — especially on hot or windy days. Overworking can bring water and cement to the surface, weakening the top layer and leading to crazing or spider cracks. Let bleed water rise and evaporate before floating or smoothing. If the finish is rushed or polished too early, it traps moisture underneath and encourages surface tension. Less is more — aim for a smooth, even finish without forcing it.
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