Is rapid-hardening concrete dangerous?

Answers to your questions about the news.
May 11 2006 6:14 PM

Is Rapid-Hardening Concrete Dangerous?

If you get stuck, just add sugar.

Download the MP3 audio version of this story here, or sign up for The Explainer's free daily podcast on iTunes.

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.

Got a question about today's news? Ask the Explainer.

Explainer thanks Terry Collins of the Portland Cement Association.

TODAY IN SLATE

Sports Nut

Grandmaster Clash

One of the most amazing feats in chess history just happened, and no one noticed.

The Extraordinary Amicus Brief That Attempts to Explain the Wu-Tang Clan to the Supreme Court Justices

Amazon Is Officially a Gadget Company. Here Are Its Six New Devices.

Do the Celebrities Whose Nude Photos Were Stolen Have a Case Against Apple?

The NFL Explains How It Sees “the Role of the Female”

Future Tense

Amazon Is Now a Gadget Company

Food

How to Order Chinese Food

First, stop thinking of it as “Chinese food.”

Scotland Is Inspiring Secessionists Across America

The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant

The XX Factor
Sept. 18 2014 11:40 AM The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant
Moneybox
Sept. 17 2014 5:10 PM The Most Awkward Scenario in Which a Man Can Hold a Door for a Woman
  News & Politics
The World
Sept. 18 2014 1:34 PM Americans Fault Obama for Giving Them Exactly the Anti-ISIS Strategy They Want
  Business
Moneybox
Sept. 18 2014 2:18 PM The NFL Is Not a Nonprofit So why does it get to act like one?
  Life
Doonan
Sept. 18 2014 2:00 PM On the Death of My Homophobic Dog I named him Liberace, but I couldn’t have chosen a less appropriate namesake for this coarse, emotionally withholding Norwich terrier.
  Double X
The XX Factor
Sept. 18 2014 12:03 PM The NFL Opines on “the Role of the Female”
  Slate Plus
Behind the Scenes
Sept. 18 2014 1:23 PM “It’s Not Everyday That You Can Beat the World Champion” An exclusive interview with chess grandmaster Fabiano Caruana.
  Arts
Brow Beat
Sept. 18 2014 2:03 PM Ryan Adams’ New Song Is a Reminder That He’s One of His Generation’s Best Songwriters
  Technology
Future Tense
Sept. 18 2014 1:24 PM Can the Celebrities Whose Photos Were Stolen Really Sue Apple? It may be harder to prove “harm” than it seems.
  Health & Science
Bad Astronomy
Sept. 18 2014 7:30 AM Red and Green Ghosts Haunt the Stormy Night
  Sports
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.