Concretecuretime,cracksandcontroljoints

The 7-day and 28-day milestones

Concrete reaches roughly 70% of its strength at 7 days and 100% (design strength) at 28 days. The two milestones that matter:

• 7 days, no driving. Foot traffic is fine from day 2. Cars off the slab until day 7.
• 28 days, no heavy vehicles. No skip bins, no trade utes carrying tonnage. The concrete keeps gaining strength up to month 6, but for residential use 28 days is the live-on-it date.

Shrinkage cracks vs structural cracks

Control joint cracks are intentional. The slab shrinks 0.5 to 0.6mm per metre as it cures. Control joints, cut every 3 to 4 metres (or 24 to 30 times the slab thickness), give the shrinkage a designated weak point so it cracks where you choose, not at random.

A hairline crack along a control joint at 14 days is normal. A 1mm crack on the surface in random directions is shrinkage and cosmetic. A 3mm crack running diagonally across the slab is structural and signals settlement or load failure.

Why control joint spacing matters

Joints further apart than 4 metres (or 30 times the slab thickness) shrink uncontrolled and crack in random patterns. Decorative concrete sometimes hides joints inside the stamp pattern; the joints are still there, just disguised.

Why is my new Adelaide driveway cracking already?

Likely causes ranked by frequency:

1. Control joints cut too late or too far apart. Common, cosmetic, fixable by filling.
2. Slab too thin. Common, structural in the long term, may need replacement.
3. Reactive soil (Class M or higher) without proper footings. Suburb-dependent. Structural, often requires rectification.
4. Pour-day weather extreme (very hot, very cold). Cosmetic surface cracking, sometimes structural.
5. Sub-grade preparation skipped. Structural, may show as settlement.

When do I worry?

Any crack wider than 1mm or running diagonally needs a structural assessment. Cracks at control joints are not a worry. Cracks at the corners of a slab indicate settlement.