LESSONS LEARNED

For builders who need a foundation poured in midwinter, the CFA research offers a basis for making informed choices. The major lessons can be easily summed up:

  • Use an appropriate mix. The CFA offers 44 standard mix designs, but it's wise for local ready-mix suppliers to do their own maturity testing of their own batches, so that differences in cement or aggregates can be taken into account.
  • Be prepared to cover walls with blankets. If the ambient temperature is above about 30°F, says Anderson, “You don't need to blanket anything.” (Water in fresh concrete freezes at a temperature slightly below the freezing point of pure water.) Between 20°F and 30°F, covering all or part of the wall is a judgment call, based on knowledge of the behavior of the mix; but covering the whole wall can't hurt, and it's good insurance.
  • Brace walls before backfilling, and backfill with extra care. Damage to walls from “abusive backfilling,” notes Anderson, is far more common than freezing damage. “The code now requires all walls to be braced before backfilling.” His advice: In warm weather, a brace every 12 feet between wall corners is sufficient, but walls gain strength more slowly in cold weather and spacing of braces should be reduced accordingly. Taller walls also require better bracing, says Anderson: “A 10-foot wall should have double braces, with one brace centered vertically and a second brace about 2 feet from the top of the wall.” Walls should reach at least 1,750 psi before backfilling starts, advises Anderson.

    • FACING THE FROST: Right: On an icy January day, crews set up aluminum forms at the ready-mix plant of Osborne-Medina in Medina, Ohio. Top: The test panels sit at dusk after the completed pour (only the tops of the walls were blanketed). Above: A worker drills a core sample for strength testing after the forms are stripped off.

    FACING THE FROST: Right: On an icy January day, crews set up aluminum forms at the ready-mix plant of Osborne-Medina in Medina, Ohio. Top: The test panels sit at dusk after the completed pour (only the tops of the walls were blanketed). Above: A worker drills a core sample for strength testing after the forms are stripped off.

  • Do not add water to the mix on site. Maturity testing data is only valid for the mix as designed, and additional water will change the mix properties. If a contractor needs to use extra water on site, the maturity curves should be re-created first using the actual water-cement ratio the concrete will have when placed.
  • Monitor concrete delivery temperatures. Concrete should be placed at a minimum temperature of 60°F. Says Gnaedinger, “At the end of a cold day, ready-mix plants may have a hard time keeping the water and aggregate hot enough—sometimes their boilers just can't keep up. And a five-degree difference in delivery temperature can be the difference between freezing or not freezing that concrete overnight.” Says CFA technical director Baty, “One of our members has an infrared, point-and-shoot thermometer on each job, and he takes a reading from the bottom of the drum. If it reads colder than 60°F, he rejects the load.”

    • For advice and support with cold weather concrete work, says Baty, foundation contractors are welcome to contact CFA. And he's asking them for feedback: Says the CFA report, “Please document and report any problems you see to the CFA. The research in this report is not finished; we will continue to study the effects of cold weather on concrete mixes as they pertain to residential concrete foundation walls.”

    TO LEARN MORE ABOUT COLD WEATHER CONCRETE, VISIT OUR WEB SITE AT WWW.BUILDER ONLINE.COM, CLICK ON “THE MAGAZINE” TAB, AND THEN CLICK ON “BUILDER ARTICLE LINKS.”