Concrete sets in hours at moderate temperatures, but the bonds that make concrete strong continue to mature over days to years. However, for replacement concrete slabs on highways, it is crucial that concrete develop enough strength within a few hours so that the lane under repair can be reopened for traffic. The Florida Department of Transportation (FDOT) specifies that concrete in this application have compressive strength of at least 2,200 pounds per square inch (psi) before reopening and 3,000 psi at 24 hours. The earlier that engineers know the concrete will meet its maturity, the better. There is a tradeoff between opening too early and damaging the concrete or waiting and incurring additional costs.

US Transportation Collection