What a week old slab is actually doing
The forms are off, the surface is hard to the touch, and it looks finished. It is not. Concrete keeps reacting chemically with the water in the mix long after it stops looking wet, and that reaction is what is still building strength under a surface that already looks done.
A week in, most of that reaction has not finished. The slab is strong enough for people to walk on and for normal household use, but it has not reached the number that was specified for it, and it will keep climbing toward that number for weeks yet.
Why the gain does not stop at seven days
Strength comes from cement paste bonding to itself and to the aggregate, and that bonding keeps happening as long as there is water and cement left to react. It happens fast in the first week, slower after that, and it continues in smaller amounts for a long time after the slab is in daily use.
That is also why the number a mix is designed to hit gets tested at a fixed point rather than the day the forms come off. Judging a slab's final strength off how it feels at one week is judging a race by where the runners are at the first turn.
What warm weather and cold weather do to it
Warm weather speeds the reaction along. Cold weather slows it, sometimes to a crawl, which is part of why a cold weather pour gets handled differently on the mix and the cure side. How the slab was cured matters too. A slab given a wet cure, kept damp in the first days, gains strength more evenly than one left to dry out fast in the sun.
- Warm temperatures: faster gain
- Cold temperatures: slower gain
- A wet cure in the first days: steadier gain
What loading it too soon actually looks like
The risk at one week is not the slab collapsing under a car. It is the smaller damage that comes from treating a young slab like a finished one. Turning the wheel while sitting still on new concrete scuffs the surface. The outside edge of any slab, the first several inches in from the joint, is the most vulnerable part of it, and heavy tires riding that edge before it has caught up in strength is how it chips or cracks first.
That is a texture problem more than a structural one at one week out, but it is avoidable, and it is the kind of thing that is worth knowing before the first vehicle rolls onto a new pour rather than after.
Why we do not give one number for every slab
The exact strength a given slab has hit at one week depends on the mix that was ordered, the weather it cured in, and how it was protected. Two pours a week apart in different weather do not sit at the same point on that curve. That is why the real number for your slab and your weather is worth a phone call rather than a rule of thumb applied blind.