You've paid good money for a new driveway, alfresco or polished slab, and a few weeks later you notice a fine web of cracks across the surface, a bit like a dried-up riverbed. That's crazing. It won't bring the house down, but it's avoidable, and knowing what causes it helps you ask the right questions before the concrete truck turns up.
C6 - Post Pour Slab Inspection Checklist
$59.00 AUD
Checklist Type: Minor
Form Factor: Fillable, print-ready PDF (A4, 10 pages)
When to use: After the concrete pour, before the frame goes up
A 62-point checklist to inspect your slab after the pour: finish, penetrations, termite protection, curing and site clean-up, while repairs are still easy to arrange.
What is crazing?
Crazing is a network of very fine, shallow cracks in the top few millimetres of the surface. The cracks usually form small irregular hexagons, often 10 to 40mm across. They're often hardest to see when the concrete is dry and most obvious just after it's been wet, as the surface dries out.
Crazing is cosmetic. It doesn't weaken the slab. It can hold dirt, though, and it can show through some sealers and coatings, which matters on exposed aggregate, coloured or polished finishes.
Crazing is not plastic shrinkage cracking
These two get mixed up a lot.
- Crazing is a fine, shallow surface pattern that appears days or weeks after the pour.
- Plastic shrinkage cracks form while the concrete is still plastic, before it has set, when the surface loses water faster than bleed water can replace it. They're deeper, longer and often roughly parallel, and they can run a fair way into the slab.
Both come down to the surface drying out too fast, but plastic shrinkage cracks are the more serious of the two and are worth raising with your builder.
What causes crazing?
The surface layer shrinks faster than the concrete underneath it, and it cracks. The usual culprits:
- Rapid drying. Hot sun, wind and low humidity pull moisture out of the surface. A Queensland summer afternoon is a perfect recipe.
- Finishing while bleed water is on the surface. Working that water back in leaves a weak, cement-rich top layer.
- Over-trowelling, especially with a steel trowel too early, which draws fine particles and water to the top.
- Sprinkling dry cement or adding water to the surface to make it easier to finish.
- Late or no curing.
How concrete should be cured
Curing means keeping the concrete moist so it can keep gaining strength, particularly at the surface. The Cement Concrete and Aggregates Australia (CCAA) guidance is to keep it moist for at least 3 days and ideally 7.
Common methods on houses:
- Water curing: keeping the surface wet with sprinklers or a hose, or ponding water on the slab.
- Plastic sheeting: laid over the slab and held down at the edges to trap moisture.
- Curing compounds: sprayed on after finishing to slow moisture loss. They don't speed up curing, they just stop the water leaving.
You'll sometimes hear "concrete takes 28 days to cure". Twenty-eight days is when the concrete's design strength is tested, not how long you have to wait to use it. As a rule of thumb, you can walk on a slab after a day or so, but stay off a new driveway with cars for about 7 days.
Questions to ask your concreter
- Will you use an evaporation retarder on hot or windy days?
- How will you cure it, and for how long?
- Who is responsible for keeping it wet over the weekend?
- If it's a hot day, can we pour early in the morning?
Our C6 Post Pour Slab Inspection Checklist covers what to look at once the slab is down.
Can crazing be fixed?
Because it's surface-only, crazing usually doesn't need structural repair. The options:
- Leave it. On a plain driveway or under floor coverings it's often the best call.
- Clean and seal. A penetrating sealer can reduce dirt pick-up. Test a small area first, since some sealers make the pattern more obvious.
- Resurface or grind. For decorative or polished finishes, a topping or grind can remove the affected layer, though matching colour and texture is never guaranteed.
If the cracks are wider than a hairline, run in straight lines or show any height difference across the crack, that's not crazing. Get your builder or an engineer to look at it.
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