Concrete professionals often refer to two separate points in the concrete pouring process. The concrete is set to have set when it has stiffened to the point it supports some pressure without damage. A common example is walking on the slab without leaving footprints. Concrete is cured when it reaches its full strength. At this point the material is as solid and strong as it ever will be and should support whatever weight it was designed to accommodate. Temperature plays a part in both processes.
Concrete can set in less than two hours or can take as long as nearly 20 hours depending on conditions. Hot conditions provide the shortest setting times with a 100-degree day producing setting in about 100 minutes. Concrete exposed to temperatures just above freezing takes the longest at about 19 hours. Concrete exposed to temperatures below freezing may not set at all.
Contractors add chemicals to the concrete to effect the setting time. For example, on a hot day the concrete may "set up" to quickly to be adequately leveled and troweled to produce the desired surface. Cooler weather calls for chemicals to speed the setting process. Ready mix operators add these chemicals during the mixing process according to the instructions from the contractor.
Ultimate curing takes several weeks although strength tests for concrete are performed after 28 days. Temperature and humidity play a part in this process as well. Warm weather that dries the concrete reduces or slows the curing process. The strongest concrete develops when the material is kept moist and allowed to cure naturally over four weeks.
The curing process of concrete is a chemical process rather than a drying process. This chemical process requires moisture to work. If the concrete becomes too dry, the material may shrink before the concrete cures enough to establish strength. This leads to cracks in the newly poured slab.