Concreter › Curing and cracking: why concrete cracks and how to prevent it
Curing and cracking: why concrete cracks and how to prevent it
Nearly all concrete cracks a little. The aim is to make it crack in places you chose, and to avoid the wide, uneven cracks that mean something went wrong.
Curing is not drying
Concrete hardens through a chemical reaction with water. If it dries out too fast, the surface is weak and cracks. One UK ready-mix supplier advises that concrete needs about four weeks to reach full strength and is most prone to cracking in that period. Slow, protected loss of moisture helps. Decide on a curing method before you pour: a sprayed curing compound, wet hessian sheets, or polythene held down at the edges. BestBuilders suggests covering with polythene for seven days and sealing after 28 days, which is a sensible rule of thumb.
What causes cracking
- Weather. Rain, heat and wind all raise the risk. Pour in mild, calm conditions when possible and shelter the slab if not.
- The ground. Soft or uneven ground, or poor compaction, lets the slab move. Dry sub-base can also pull water out of the mix, so soak it before the pour.
- No joints. The supplier says "large areas of concrete laid without joints will almost always crack". Plan joint positions before pouring. They can be formed during placing or saw cut soon after the concrete sets.
- Too much water. Extra water makes the mix easier to work and the concrete weaker.
- Too thin. A slab that is too shallow for its load will crack under the weight.
- Tree roots and drains. They move the ground over time.
- Fibres and mesh. Fibres can reduce early cracking in flat work; mesh holds cracks tight but does not stop them.
Joints: where you want it to crack
Control joints are shallow cuts or formed grooves that give cracks a straight, neat path. BestBuilders mentions joints every 3 to 4 m on drives. Expansion joints are gaps against walls or other slabs to allow movement. Ask your contractor where they will put joints and see the layout before the pour. On a stamped finish, joints can be hidden in the pattern. See imprinted concrete.
Is this crack a problem?
| What you see | Usually |
|---|---|
| Fine hairline cracks in a few days or weeks | Common (shrinkage). Cosmetic. Keep an eye on them. |
| Cracks along the joints | Working as designed. |
| Random cracks across a big slab | Missing joints or rapid drying. Cosmetic but may grow. |
| Cracks wide enough to catch a coin, or where one side is higher | Movement in the ground or a thin slab. Get it assessed. |
| Crumbling or flaking surface | Often frost or salt damage, or too much water in the mix. See resurfacing. |
This table is general guidance, not a diagnosis. Have a contractor or engineer look at anything that is widening.
Your curing checklist
- Check the weather. Avoid frost, heavy rain and very hot, windy days.
- Keep people, pets and rain off the fresh surface for the first day.
- Cover or spray as agreed and keep it in place for several days.
- Keep vehicles off for as long as your contractor says; the full strength takes around four weeks.
- Seal when told to, not before.
Who is responsible for cracks?
Ask before work starts what the guarantee covers, and get it in writing. A contractor can reasonably say hairline cracks are normal, so a good one tells you where cracks are likely and explains the joint plan. See questions to ask a concreter.
Frequently asked questions
How long should concrete cure?
A UK supplier says around four weeks to reach full strength, and the first period is when it is most prone to cracking. Protect the surface for at least the first week.
Is it normal for new concrete to crack?
Fine cracks are common. Wide, stepped or widening cracks are not and need assessing.
Can I water concrete while it cures?
Keeping it damp or covered helps it cure slowly. Ask your contractor which method they are using.