Many of the technical issues reported on in the residential concrete foundation industry are the result of questions or situations brought to the technical staff of the Concrete Foundations Association.  The original content of this article is one such example, updated with additional information as you prepare for the 2023-24 winter season.

Situation:  During a winter placement a couple of weeks ago, freezing temperatures were forecast so we requested the addition of 2% calcium chloride to the mixture at the batch plant for our early morning placement. It was very difficult for our crews to pour and put extreme pressure on the conveyor, the workers and the ready mix company. We had additional labor on the job, extra vibrators, and we still ended up with significant honeycombing.

Thoughts:  Over the more than thirty years I’ve been working with concrete, the use of Calcium Chloride (CaCl2) has perhaps been the most common decision made by contractors and producers to get concrete to an early age maturity capable of withstanding freezing temperatures.  After all, why shouldn’t this admixture be considered as, like Portland cement, calcium chloride reacts with water in a chemical reaction known as exothermal, which means the resulting byproduct is heat.  Goals are set to elevate or simply maintain the concrete temperature with decisions that impact the mixture, the timing or external systems, countering the radiant loss of heat to the surrounding air.

As air temperatures drop, protecting the internal temperature of the concrete is your primary responsibility.  In the traditional cold weather regions, contractors and producers begin with heating aggregate and using hot water.  The next decision, and often the first in markets where heating systems are not common or available, is the addition of an accelerating admixture.  Historically, when this decision has been made in the concrete foundation industry, calcium chloride has been the go-to product.  As reported in CFA’s Cold Weather Research Report from 2004, “consider the use of small doses as it gives you the best bang for your buck.”  However, in today’s market where all concrete walls are required to have at least temperature and continuity reinforcement, unless replacing an alternative to steel, calcium chloride is significantly restricted by code.  The use of non-chloride accelerators have become the preferred solution, yet calcium chloride remains in the conversation.

This project exposes one of the most common problems in the use of calcium chloride, the impact of heated water in combination with the accelerating admixture.  A minimum concrete delivery temperature is normally specified during cold weather placements.  When considering the addition of an accelerator, the starting temperature point of the concrete batch is critical to monitor, particularly at the start of the day. Early in the day is readily known as the best time to place concrete during cold weather. You gain the added benefit of the entire day’s solar exposure to bring extra energy to the concrete mass as well as likely realizing the maximum daily temperatures. However, early in the morning, the aggregate, cement, and water temperatures at the plant are at their highest.  They have been heated overnight with systems prepared for heating incremental use throughout the day and it is not surprising to see mix components much higher than the target batch temperatures.  As the calcium chloride reacts to already elevated batch temperatures, the concrete temperature elevates in a semi-controlled setting resulting in accelerating the set and consuming water.

Further complicating the issue is the requirement for calcium chloride to be mae water-soluble before mixing.  Assuming a target batch temperature of say 85 degrees F, adding the water-soluble calcium chloride to the mix immediately kicks the accelerator into action. The mix is then loaded into the drum and transported to the site. Drive times in most markets today are not uncommon between 30 to 60 minutes or more and at those temperatures, the concrete will begin rapidly hydrating. During this initial start of hydration, significant moisture is consumed resulting in a placement slump far below the expectation of the team. Knowing that an addition of water to reconstitute the slump beyond the designed w/cm is not possible, this leaves the contractor with few options.   When determined to go through with it without adding more cost for a water-reducing admixture, the impact is most often realized in the finished concrete surface in the form of honeycombing as well as the very challenging placement efforts described by the contractor.

One the best alternative decision the contractor might consider is the jobsite addition of the calcium chloride. This product must be converted from its bulk flake to water-soluble in buckets before addition to the mix and this can be planned easily at the site. Once added and mixed thoroughly through the required resolutions of the drum, the concrete can be placed at a slump much closer to anticipated and the result will be far less risk of a stiff mix and resulting aesthetic imperfections.

Contractors need to be proactive and not reactive on this topic and evaluate the performance of your current mixes. As recommended in that same cold weather report, another great decision is investing in a maturity meter system as it is the fastest and most effective way to evaluate. Real time concrete temperatures are recorded and reported and when a drop in performance due to lower temperatures is realized, small changes can be made to tweak performance, one change at a time.

It is worth repeating that when using steel reinforcement, a proper reaction to this issue is to abstain from using calcium chloride altogether. While it is certainly stated in both ACI 318 and ACI 332 that calcium chloride is not to be used for structurally reinforced concrete that is not dry in service, its use in all other concrete applications is certainly permitted. The alternative would be a non-chloride accelerator (NCA) or a combination of higher strength mixes with Type III cement. Caution needs to be applied to both of these selections as there is research out there suggesting much lower hydration temperatures for NCAs than anticipated and Type III cements are often difficult to find in markets as well as having other use problems.

The best solution is to know your admixtures, develop a good relationship with your selected producer and plan the timing of the products to maximize their performance and prevent adverse problems.

For more information on this topic, contact the Concrete Foundations Association at info@cfaconcretepros.org or visit our websites, www.cfaconcretepros.org and www.concretefactsmagazine.com for more information.