The question we get on every pour day is the same: when can I walk on it, when can I drive on it, and when is it done? The answer is in three windows, and behind them is a strength curve that explains why the windows are what they are. Here's the short version, and then the detail.
The three windows
| Time after pour | What you can do | Strength (% of full) |
|---|---|---|
| 24 – 48 hours | Walk on it (foot traffic) | ~20 – 30% |
| 7 days | Drive on it (cars, light vehicles) | ~70% |
| 28 days | Full strength — build on it, load it | 100% |
These are the standard windows for residential concrete in normal Melbourne conditions. The concrete isn't 'dry' at any of these points — it's reached a percentage of its ultimate strength, and the percentage is what decides what it can carry.
The strength curve
Concrete doesn't gain strength in a straight line. It gains most of its strength in the first week — about 70% by day 7 — and then the curve flattens, reaching full (design) strength at 28 days. That's why the 7-day mark is the rule for vehicle traffic: the slab is strong enough to carry a car at 70%, even though it's still gaining strength. The last 30% takes three more weeks.
Curing is chemistry, not drying — the #1 misconception
Curing is not drying. Concrete doesn't harden because the water evaporates out of it — it hardens because the water reacts with the cement (hydration) and forms the crystals that give it strength. If the water evaporates too fast, the reaction stops early and the concrete never reaches full strength. That's why we keep concrete wet and covered while it cures.
This is the thing that trips people up. A slab left to bake in the sun on a hot Melbourne day with no curing will 'dry' fast and feel hard to the touch — but it'll be weak, dusty on the surface, and prone to cracking for the rest of its life. The water has to stay in the slab for the chemistry to finish. That's curing.
Melbourne weather adjustments
- Hot, dry, windy days — The slab loses water fast. We use evaporation retarders, shade, and we may pour early in the morning to avoid the worst of the heat. We keep the surface moist.
- Cold days (under 5°C) — The hydration reaction slows. We use cold-weather accelerators and early covers to keep the heat in the slab. The curing windows stretch out — 7 days for cars might become 8 or 9.
- Rain — We don't pour into water. If rain is due on pour day, we postpone. A slab poured into water is a weak slab.
- Frost — We never pour onto frozen ground. The ground moves when it thaws and the slab goes with it.
What breaks curing
- Driving on it too early — A car on a 3-day slab can crack it. The slab isn't at 70% yet. Wait the 7 days.
- Letting it dry out — A slab that bakes uncovered on a hot day loses its water and its strength. Keep it moist or covered.
- Heavy point loads early — A motorcycle stand, a skip bin, a pallet of bricks on a young slab can punch through. Spread the load or wait.
Our cure-and-seal practice
On pour day, we apply a cure-and-seal compound to the surface. It does two things at once: it traps the water in the slab so the hydration reaction runs to completion (curing), and it leaves a protective film on the surface (sealing). It's the single best thing you can do for a new slab — it's the difference between a slab that reaches full strength and one that doesn't. We do it on every pour, included in the quote.