Understanding Concrete Maturity for Curing Cold Weather Concrete Pours

The forming system has minimal impact on the early-age strength gain of concrete during cold weather.
The forming system has minimal impact on the early-age strength gain of concrete during cold weather.

Question: The onset of cold weather has complicated our project schedules tremendously. One of the municipalities we work in is requiring us to leave forms on the wall for three days and telling us to rent wood forms instead of using our aluminum forms when the temperature is below 32-degrees Fahrenheidt. Are these arbitrary conservative precautions or are their concerns valid? Our customer schedules are suffering due to these requirements.

Answer: Cold weather is certainly the time of the year that becomes most concerning to the consumer and the inspector in the construction industry. The simple act of having to add layers to the body translates to an immediate concern for the placement of concrete and what might happen to a process that involves water, which obviously must freeze. ACI 306 is largely considered to be the most appropriate resource for determining a position for concrete construction and yet, this document that is a guide or a compilation of recommended practices, is perhaps the most misunderstood of technical references.

ACI 306 has the challenge of representing an entire industry, all variables and conditions and delivering practicable methods for protecting the ultimate goal, concrete maturity. So then, what is the market to do when it does not have the time to investigate the specific conditions realistic to the application? This is where the selection of overly conservative parameters occurs and often from resources that are built upon other conservative decisions and not research or science.

My recommendations are to build a conversation and an approach based on the research completed specifically for residential foundations, research that demonstrates a rational approach to cold weather concrete maturity. CFA’s Cold Weather Research Report 2004 describes the impact of sub-freezing laboratory and field temperature conditions on concrete maturity. This report is a reference included in both ACI 306 and the residential concrete code document, ACI 332 and provides a much more effective set of guidelines for your market.

Fact: The freezing point of concrete is not an air temperature of 32-degrees Fahrenheidt. This is supported in part because concrete is a physical composition of more than just water. However, the primary support for this fact is that once a concrete mix is made, it immediately begins a chemical reaction with a by-product of heat. The generated heat elevates the material temperature of the mix and continues to do so as the chemical process continues. This is a counter balance to the chilling effects of the ambient air condition and a benefit to the construction process.

Fact: A concrete mix can achieve sufficient strength for protection and self-support under cold weather conditions in 24 hours or less. The primary benefit of the CFA’s research is the recognition of mix design performance for delivering varying levels of strength or concrete maturity based on temperature conditions, time durations and mix economies.

ACI 347, "Guide to Formwork for Concrete" states in section 3:
"Determination of the time of form removal should be based on the resulting effect on the concrete. When forms are stripped there should be no excessive deflection or distortion and no evidence of damage to the concrete due to either removal of support or to the stripping operation (Fig. 3.5). When forms are removed before the specified curing is completed, measures should be taken to continue the curing and provide adequate thermal protection for the concrete.

So the contractor has the duty of removing forms when the concrete can be self-supporting. That is a measure of concrete maturity and research can be used to demonstrate or correlate the performance of a selected mix design to the targeted anticipated temperature ranges. Furthermore, research further supports the nature of a concrete mix achieving strength to withstand the impact of freezing without impacting durability and return to strength gain once freezing conditions subside or supplemental curing conditions are established."

Fact: A bigger risk for residential foundations is the open exposure of the excavated grade to freezing conditions than the exposure of the concrete. Soil temperature is critical to the protection from frost and heave. Excavations that remain open for significant time schedules will experience abnormal levels of frost development and heaving or pressure from the expansion of naturally occurring moisture. Therefore, the concrete contractor must work with the ready mix producer to have a mix capable of reaching self-supporting strength (a strength just beyond final set or 500 psi) so that forms can be removed, the wall braced or floor deck installed and the backfill process initiated. Leaving the excavation open for days with forms still on the walls only increases the true risk to the foundation created by abnormal soil pressures due to freezing.

Fact: Short-term protection is beneficial. CFA’s research shows the contractor can find some great benefits in making practical decisions early in the construction process. These include pouring earlier in the day; using blankets along the top of the wall in the 20s and full blankets in lower ranges; and maintaining a strict adherence to the w/cm. These all impact the behavior of the concrete mix designs toward the goals of developed maturity.

Fact: The forming system will have little impact on the concrete temperature. Although insulated concrete forms will bring a measure of blanketed protection to the curing condition of concrete, all other forming systems have little impact on the practical concrete maturity. A concrete mix design targeted to permit forms to be removed in one-day means that concrete temperatures will remain elevated beyond the impact of surface temperature exposure. Wood, aluminum and steel forming systems are not capable of delivering insulating properties significant enough to perform as insulators for the temperature conditions being considered here. Therefore, basing curing protection on the decision of one of these over another only falsifies the protection condition being assumed.

The best solution for cold weather concrete work is to know your mix design performance, become informed of the research that supports the prediction and monitoring of concrete performance and understanding the options for temperature ranges that ensure the best balance of maturity, schedule and economy. The CFA offers education in the form of webinars and regional classes that can help concrete professionals and municipalities come together to provide assurance for quality concrete.

Ed. Note: Jim Baty is the Executive Director for the Concrete Foundations Association after having served as Technical Director since 2001. He is currently chair of ACI 332 and a voting member for ACI 306 with priorities of establishing better guidance and structure for residential concrete construction. For more information on this topic, contact Jim Baty at jbaty@cfawalls.org. Find out more at www.cfawalls.org.