Why has my new concrete cracked?

Concrete shrinks as it cures, by a small but unavoidable amount. Whether that shrinkage appears as a crack depends almost entirely on whether it was given somewhere to go and whether it was allowed to cure slowly.

What you'll need

• concrete repair mortar (optional)
• polythene sheet or curing membrane
• crack monitoring marks (optional)

Step by step

1. Look at the pattern.Fine map-like crazing over the surface is plastic shrinkage. Straight cracks running across the slab at intervals are drying shrinkage. Cracks with a step or a level difference indicate ground movement or subsidence.
2. Measure and monitor.Mark the ends with a pencil and date them. Hairline cracks that stop moving are cosmetic; ones that keep widening need investigating.
3. Recognise the too-much-water cause.Extra water makes concrete easier to place and dramatically weaker and more shrink-prone. It is the single most common domestic error and it causes both surface dusting and cracking.
4. Understand plastic shrinkage cracking.If the surface dries faster than the concrete can bleed water up to it — in sun, wind or heat — the surface shrinks against the still-plastic body beneath and crazes.
5. Cure it properly.Cover with polythene or a curing compound and keep it damp for at least seven days. Concrete gains most of its strength in that period and it does so by hydration, not by drying.
6. Put in movement joints.Slabs need joints to control where cracking happens. Cutting or forming joints at regular intervals gives the shrinkage a planned line rather than a random one.
7. Reinforce where appropriate.Mesh does not stop concrete cracking; it holds the crack tight so it does not open or step. Cover matters — mesh too near the surface rusts and spalls the concrete off.
8. Prepare the base properly.A well-compacted sub-base is what stops structural cracking. Soft spots and organic material under a slab cause cracks that no mix design can prevent.
9. Repair cosmetic cracks with an appropriate product.Fine cracks can be filled with a concrete repair mortar or a resin injection. Ordinary filler and cement slurry both fail quickly in a floor.
10. Get moving structural cracks assessed.A crack that is widening, stepping, or accompanied by settlement elsewhere is a ground or structural problem. Filling it hides the evidence.

Tips

• Every extra litre of water makes the concrete easier to place and measurably weaker. Add plasticiser rather than water for workability.
• Cover new concrete with polythene for a week. It is the cheapest and most effective thing you can do for its strength and its appearance.
• Formed or cut movement joints do not prevent cracking — they decide where it happens.

Common mistakes

• Adding water to make the mix easier to work — Strength falls sharply with increasing water content and shrinkage rises. A sloppy mix is easier to place and produces a weaker, dustier, more crack-prone slab.
• Letting new concrete dry in the sun or wind — Concrete gains strength by hydration, which needs water. Drying it out early stops that process and causes surface crazing at the same time.

If it doesn't work

Questions people ask