The 28-day cure myth - what concrete actually does between Day 1 and Day 28
"The concrete needs 28 days to cure" is the line most concreters give when asked when something can be used. It is correct in the sense that 28 days is the standard design-strength reference. It is misleading in the sense that customers often interpret it as "do not touch for 28 days" - and that is not what the chemistry actually says.
What is happening chemically
Concrete cures by a process called hydration. Water reacts with the cement to form crystalline structures that bind the aggregate together. It is not a drying process - if it were, you could speed it up with a fan. It is a chemical reaction, and it needs water available to keep happening.
The reaction proceeds quickly at first and slows down asymptotically:
- Day 1: ~16% of design strength
- Day 3: ~40%
- Day 7: ~65-70%
- Day 14: ~85%
- Day 28: ~99% (this is the "design strength" reference)
- Year 1: ~115% (yes, concrete keeps getting stronger for years)
What you can do at each stage
Day 1 (first 24 hours): stay off it. The surface has only just set hard enough to walk on without leaving footprints. Pets, kids, dragging anything across it - all bad ideas.
Day 2-3: light foot traffic is fine. Sitting a chair on it is fine. Don't move heavy furniture across it.
Day 4-7: normal foot traffic, bicycles, light tools. Still no vehicle traffic on driveways.
Day 7-14: car traffic on a driveway is acceptable. Avoid turning the wheels while stationary (this concentrates load and can scuff the surface), and keep heavy vehicles off (concrete trucks, loaded trailers).
Day 14-28: normal use. Still avoid extreme point loads (jack stands without spreader plates, etc.).
Day 28+: full design strength. Treat the slab as you would any concrete.
Why does the contractor keep coming back to wet it down?
Because hydration needs water. In Brisbane summers, the surface of a fresh slab dries out within hours - if there's no water available, the top millimetre stops curing properly and you get surface scaling later. Hence: curing compound (a temporary plastic-like film), or wet hessian, or repeated misting for the first 3-7 days. After that, the slab is hard enough that internal moisture sustains the reaction.
The practical takeaway
If your concreter says "drive on it after 7 days" that is correct, not a corner-cut. They are working from the same hydration curve everyone in the industry uses. The "28 day" reference is for engineers specifying design strength - it is not a "do not touch" period.
That said: the closer to Day 1 you put load on, the more risk of damage. If you can wait 14 days before parking, you will. If you can wait 28, even better. But there's nothing magic about Day 28 - just a familiar reference point on a smooth curve.