by JAMES R. BATY II 

As most contractors realize, it is quite normal for walls to still be formed on footings the next day (or two) during the colder months of December to March. While concrete does gain strength at a slower rate during this time of the year, the footing concrete placed with just 24 hours of strength gain can be shown as still more than substantial enough to handle the forming operation that takes place next. On Day 2, and then by the end of Day 3, forming and concrete placement does not bring a significant load over the area of contact (Baty 2016)¹.

The Concrete Foundations Association (CFA) Cold Weather Research Report documents the work conducted from 2002-04 to establish significant evidence of the ability for concrete temperature to maintain maturity despite sub-freezing ambient air temperatures.

Forty-four different mixes and more than 800 cylinders along with more than 100 core samples from multiple full-scale walls demonstrate that maturity occurs more aggressively than ambient temperature predicts.  

In figure 1, the strength gained for forty of the research mix designs based on a 30 deg F ambient air temperature condition is shown ranked based on the green segment representing 500 psi.  ACI 332² requires concrete to be kept above a frozen state until this strength is achieved.  It is believed that this early strength level is both sufficient for the concrete to be self-supporting as well as the surfaces strong enough to resist the damage from any expansion due to ice lenses in the interstitial spaces. In just two days, most of these mix designs were well over 1,000 psi and some as great as over 2,000 psi with near design strength or more by the third day.  

Keep in mind that the load of an 8-ft wall of concrete is 150 lb/cu ft x 8 ft x 8in./12in. = 800 lb/lin. ft. over the 8-in. thickness.  Divide this by an area of 8 in. x 12 in. and you only have a load of 8.33 lb. per sq.in. on the footing.  At the very lowest of 1-day strengths for a plain, 2,500 psi mix, you have a capacity 46 times that.

Figure 2 shows the rapid increase of strength ensured by hydration despite a below-freezing curing temperature.  By the end of a third day, with this basic plain mix, you have over 2,000 psi.  This approaches the required strength for the foundation wall itself to withstand the design backfill pressures when supported or connected properly.

Figure 3 demonstrates the principle of why concrete temperature is the true predictor of strength gain and how cold weather is overcome by it.  Here a graph of the ambient temperature profile for a three-day stretch (dark blue) is compared to the measured temperature of the concrete at two key locations, top of wall and center of wall.  While a footing is protected even further from the effects of heat loss by radiation, the top of wall condition can be a conservative application as shown here.  Once a wall is cast on the footing, the footing curing condition is much closer to the center of wall condition.  This is due to the placement of fresh concrete at a warmer delivery temperature to support further hydration and therefore maturity.

While colder air temperatures slow the maturity of concrete and prolonged freezing air temperatures can temporarily halt it, concrete footings will reach sufficient strength for the entire construction process making the forming process elementary provided proper mix designs are selected and manageable precautions employed.  For more, visit www.cfaconcretepros.org/toolbox. 

References:

“Why Wait to Form Walls on Footings?”, August 16, 2016 by Concrete Contractor magazine | www.forconstructionpros.com

Residential Code Requirements for Structural Concrete (ACI 332-20) and Commentary published by the American Concrete Institute, 38800 Country Club Drive, Farmington Hills, MI 48331 | Phone: 248-848-3700 | www.concrete.org