Date: July 1, 2009

News - Tips for Dealing with Hot Weather on the Job Site

  • In very hot weather, place the concrete early in the morning or in the evening. Be prepared! Make sure your crew and equipment are ready to go when the concrete arrives so you can get the concrete out of the truck quickly—it can heat up just sitting in the mixer waiting to be placed. Friction within the concrete during mixing can generate enough heat to raise the concrete temperature 5° in 30 minutes.
  • If slump loss becomes a problem, rather than adding water, use a superplasticizer (high-range water reducer). These admixtures increase slump without affecting the concrete's final strength, water-to-cement ratio or appearance. 
  • Keep the sun off the concrete surface if possible—for interior slabs, try to place the concrete after the building has been closed in. Also, keep your tools and equipment out of the sun, especially things like pump hoses that come in direct contact with the concrete.
  • For exterior concrete, wet down the subgrade and forms with cool water prior to placing the concrete, and be sure that the moisture seeps through the entire subgrade layer. This prevents the subgrade from absorbing the concrete’s moisture, causing it to crack. Too often the contractor only wets the top 1/16 inch of the subgrade and it sucks the water out of the concrete.
  • As soon as you bull float the concrete, apply a monomolecular film, or evaporation retarder. These materials stop the evaporation of surface water, preventing plastic shrinkage cracking and surface crusting.
  • Curing is critical any time, but especially in hot, dry weather. Curing should start as soon as finishing operations are completed. With plain gray concrete, use a white pigmented curing compound that will help reflect sunshine. Likewise, if you are using curing blankets, use white ones.
  • If you use a curing compound, make sure it works—often the concrete is blamed for problems when it’s actually the curing compound that didn’t perform properly. I advise running this test: put a three-foot-square piece of plastic on the slab and put sand around the edges. Come back later and if there's moisture under the plastic, the curing compound isn't working.

Other news articles from July 2009