Expanded from Q&A Article seen in Concrete Contractor magazine.

The Concrete Foundations Association explains how commonly and effectively residential concrete foundations are constructed during the winter.

Question: Our foundations scheduled for installation these next few months will be affected by projected temperatures in the mid 30s to mid 40s.  What advice is there to ensure my customers and our building inspectors that the walls will perform as designed? – Concrete Contractor (Wisconsin).

Answer: There is no denying that the vast majority of basement markets in the U.S. are turning colder, colder to the point of concern from customers, code officials and building inspectors as to what will happen to the concrete during these placements.  A wealth of information has been generated in the past decade substantiating the recommended procedures necessary to produce quality foundation concrete during these conditions.  Documents such as the CFA Cold Weather Research Report, ACI 332R-06 Guide to Residential Concrete, ACI 332-10 Residential Code Requirements for Structural Concrete, and ACI 306 Guide to Cold Weather Concrete, all establish consensus for the contractor to proceed with both caution and confidence under cold weather conditions.

In order for cold weather foundation installations to be successful, there are some hard rules to recognize and follow.  Most of these are related to the supporting soil condition, but some do affect the concrete and form preparation.

1. The excavation must be not be frozen and must also be free of frost.  A foundation is designed to transfer the building load to the ground.  This most often involves a foundation wall and a footing, the latter contacting the soil condition.  The foundation system is designed based on the strength or rather that bearing capacity of the supporting soil.  When ground becomes frozen, its volume expands from the creation of ice particles and therefore changes its support condition.  Once frozen ground has received a structure, that structure is falsely supported by the expanded grade and will experience settlement as the ground thaws.  Similarly, ground with frost introduces both volume change and additional moisture to the grade condition for the concrete placement. While settlement may not be as likely here, the moisture present in the frost condition is moisture beyond what the mix design considers and may impact the water-cementitious material ratio that is rather important to strength development.  This consideration is documented in ACI 306:  6.3—Subgrade conditionConcrete should not be placed on frozen subgrade. Remove all frost before placing the concrete and recompact thawed soil disturbed by frost. Placement of insulation over the subgrade, or provision of heat, is required to remove any frost in the soil and to raise the subgrade temperature above 32°F (0°C). An appropriate provision for heat should be selected based on the amount of frost depth and temperature difference with the air. Once thawed and recompacted, insulation should be placed directly over the subgrade to protect it from refreezing.
   

   Test cylinders taken by Purinton Builders during cold weather testing demonstrate the effects of early-age freezing versus successful maturity protection.

2. Concrete must be protected from freezing until it has reached 500 psi.  It must be protected from multiple freeze/thaw cycles until it has reached 1,500 psi.  ACI 332-10 provides the following requirement for residential concrete:6.6.1        During anticipated ambient temperature conditions of 35°F or less, concrete temperature shall be maintained above a frozen state until a concrete compressive strength of 500 psi has been reached.The freezing point of concrete is in the neighborhood of 27°F, depending on the concrete mix.  Both ACI 332 and ACI 306 establish a benchmark ambient or air temperature that contractors should consider targeting their monitoring or application of protection for freshly placed concrete.  Concrete has remarkable strength gain characteristics due to the natural hydration process.  Chemical processes result in a by-product and for hydration of cement the by-product produced is heat.  This is a significant benefit to cold weather concrete, particularly for the construction of foundations.  Foundation walls are most commonly created in vertical forms, set below the surrounding grade in an excavation…a protected environment.  The forms are wood, steel, aluminum or in some cases insulation.  All of these forming systems protect the concrete from air movement across the largest surface areas and therefore prevent convention from accelerating temperature and moisture loss.  Significant research conducted on full-scale wall elements by the CFA has resulted in a more thorough understanding of this performance.  So much so that with confidence, contractors have escalated the amount of cold weather foundations they install and have decreased the average cost of protection systems.  The CFA Cold Weather Research Report offers:

   Full analysis of the temperature histories of the wall segments and ambient conditions reveals that the mass of the curing concrete kept the concrete above ambient for at least the first 24 hours after placement, even in the leanest of mixtures.  It is also observed that the cement content and type played a very large role in how long the concrete stayed above ambient temperatures.

    

   

   Full-scale wall segments tested during CFA Cold Weather Research project in 2003.

   During temperature periods beginning at 35°F and dropping significantly lower, concrete has the proven ability to self-sustain internal temperature permitting strength gain beyond the critical benchmark of 500 psi.  This does not mean that concrete walls will not freeze, as they will and they may below the 500 psi benchmark.  Contractors must also understand the behavior of even colder ambient temperatures that may flash freeze concrete or accelerate the temperature drop so that they can plan protection such as blankets and in the most severe cases, auxiliary heat sufficient to meet the strength gain goals.

3. Mix design must be considered a top priority for successful cold weather concrete.  Although the research statement above suggests that even lean concrete mixes perform well, it is important to realize that there are four important cold weather mix design decisions that set cold weather concrete up for success.  The first is the water-cementitious material ratio.  Water is required for the hydration process and is consumed by it.  However, water is also the culprit for freezing.  The higher the water content, the more susceptible the concrete is to early freezing.  The CFA Cold Weather Report identifies:

There is a significant impact resulting from adding excessive water to concrete under cold weather conditions—the greater the water content, the greater the affect those freezing conditions will have on the concrete.  A 6-inch slump may be acceptable, but a high water to cement ratio is not.

Cement type is another important decision.  Most markets will offer Type III cement with characteristics of faster strength gain.  While Type I cement is proven to be successful in cold weather conditions, Type III can give extra control and protection from the aspect of higher internal temperatures resulting in accelerated strength gain.  Another aspect of the decision process for cold weather concrete is the use of admixtures.  Calcium chloride has been the predominant cold-weather admixture for the residential concrete industry with decades of success.  CaCl₂, however, does increase the corrosion potential of reinforcement present in the foundation wall and therefore may not be the best solution.  There are plenty of additional admixture options with non-chloride chemistry (NCAs) that research evidences as every bit as successful.  The limits to amount of calcium chloride combined with the consideration for reinforcement compared to the cost of the non-chloride admixtures makes cost-effectiveness and control important to the contractor.  The final, mix decision is temperature at production and delivery.  This is a very important consideration for both producer and contractor.  The higher the concrete temperature at delivery time, the more likely 500 psi will be attained in very cold conditions before freezing.  The vast majority of residential foundations do not have enough mass present in reinforcement and forming systems to cause concern for a large difference between higher concrete delivery temperatures and the in-place low temperatures of those materials.  While this can be a concern when such mass is high in that it may cause a portion of the concrete to drop in temperature more rapidly than other areas, residential concrete foundations do not pose this issue.  These practice decisions are described in greater detail in the CFA Cold Weather Report such as:

Contractors should work with their local ready-mixed concrete producer to design concrete mixes that will perform well based on the expected variables for a placement.  The mix designs used in this research provide a sound basis for your own mix development but should be used after localized testing.

CFA member, Dennis Purinton of Purinton Builders in East Granby, Conn. was asked about his thoughts on cold weather concrete foundations.  “The four most important things to remember in cold weather concreting are 1) mix design, 2) concrete temperature, 3) CONCRETE TEMPERATURE and 4) the correct balance of accelerator to concrete temperature.”  Dennis is a CFA Board member as well as a voting member on ACI 306 along with CFA Managing Director, Jim Baty.  Dennis is also the head of a new task force CFA has formed to research cold weather performance of residential concrete slabs.

A wealth of recommendations is available in the resources this industry has at its disposal.  ACI 332-10, ACI 332R-06 and ACI 306 are all available through the bookstore at www.concrete.org.  The CFA Cold Weather Report is likewise available through the online order system at www.cfaconcretepros.org.  CFA also offers its members annual free webinars on cold weather concrete preparation as well as a task force that members can access for discussion and experience.  The most important aspect of cold weather concrete foundations is to understand that one solution does not fit every contractor, market or project condition and certainly all temperature ranges.  There is great economy and performance that can be attained by identifying the variables that contractor and producer together can control.  Most contractors develop a range of three to four mixes to use each season, dependent on the target ambient temperatures for their placements.  Want to know more?   Contact CFA Managing Director, Jim Baty at 319-895-6940 or by email at jbaty@cfaconcretepros.org.