Three construction workers in Bal Harbour, Fla., died last weekend when a support frame in the building they were working on gave way. The men fell and were buried in 3 feet of rapid-hardening concrete. Co-workers tried to rescue them, but the concrete had already set. How long does it take for rapid-hardening concrete to harden?
About 15 to 45 minutes. (Some rapid-hardening concrete might set in just a couple minutes, but it's used only for small repair work.) Typical concrete—a blend of cement, minerals (usually gravel and sand), and water—requires anywhere from 90 to 150 minutes to solidify. Contractors and manufacturers have a few different methods for shortening this time span, though. One technique is to increase the cement-to-water ratio, thickening the mixture. Grinding the cement into smaller particles also speeds up setting. Finally, adding chemicals called accelerators moves things along by increasing the rate of the exothermic reaction in the liquid. This chemical process initiates the "strength gain" phase, when the water hydrates the cement and coats the minerals, causing them to bond into a stonelike consistency.
Because it puts workers in a time crunch, rapid-hardening concrete is considered somewhat dangerous and has only two main uses. The first is high-rise construction, because it allows workers to progress upward, adding new floors without much delay. It's also used for road repairs, since cars can drive over it soon after it's been poured.
Accidents involving rapid-hardening concrete are rare, but they do happen occasionally. When a worker becomes stuck, there is a relatively simple antidote: sugar and stirring. Mixing plain white sugar in concrete prevents the cement from joining with the water and slows the hardening of the minerals. Sugar-based solutions are sometimes added to the mixture when workers need to keep concrete moist (e.g., paving a very large surface).
Both standard-issue and rapid-hardening concrete set more quickly in tropical climates like that of South Florida. For that reason, construction crews in hot-weather areas are more likely to use those sugar-based solutions.
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Explainer thanks Terry Collins of the Portland Cement Association.