Concrete reaches roughly 70 percent of its design strength in about 7 days, which is when framing or steel typically starts per the engineer's schedule, and its full rated strength at 28 days. Post-tensioned slabs stress their cables in the first week or so per the design.
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Curing is a chemical reaction on a known curve, and the construction schedule is built around it rather than around impatience. The first 7 days do the heavy lifting: with moisture retained (curing compound or wet cure), the slab climbs to roughly 70 percent of design strength, and that is the point where most engineers allow framing, steel erection, or masonry to begin. The rated strength on the plans (3,000 to 4,000 psi for most residential and shop work) is a 28-day number, which is why heavy loads like concrete trucks, loaded goosenecks, and equipment stay off new slabs and approaches for the full month. Post-tensioned foundations, the common engineered answer on expansive clay here, add one step: the cables are stressed once the concrete reaches the strength the design specifies, typically within 3 to 10 days of the pour, and the stressing record joins the project documentation. Weather bends the curve at both ends. Texas summer pours start at first light because hot, windy air pulls water out of fresh concrete faster than the reaction can use it, and cure protection is what prevents a weak, dusty surface; rare winter cold snaps slow the reaction, and pours schedule around freezing nights. The written scope states the cure method and the hold points, so the framing crew, the steel erector, and the concrete all agree on the calendar.