By Kayla Hanson, P.E.
As fall comes to a close, many of us across the country have started preparing for winter by dressing in warmer layers or getting thick coats out of storage. We’re also more conscious of the thermostat both at home and in the precast concrete plant. It’s imperative throughout the year, and especially during times of seasonal transition, to remember how temperature change affects concrete and the materials used to produce it.
What is considered cold weather?
Section 4.4.7 of the NPCA Quality Control Manual for Precast Concrete Plants outlines cold weather concreting precautions. For the purposes of the manual, and in accordance with guidelines outlined in ACI 306, “Guide to Cold Weather Concreting,” cold weather is defined as a period when the ambient air temperature of the casting environment meets the following conditions for more than three consecutive days:
1. The average daily air temperature is less than 40 F.
2. The air temperature is not greater than 50 F for more than one-half of any 24-hour period.
However, it is a best practice to begin using cold weather concreting precautions when the ambient temperature is expected to drop to 50 F or lower at any point during production or curing. Precautions can be taken at each step of the manufacturing process including raw material preparation, form preparation, mixing, placing, finishing and curing.
Why use cold weather concreting practices?
ACI 306 outlines five primary objectives for using cold weather concreting practices:
1. Prevent damage to concrete due to early-age freezing. Cement hydration reactions occur at a slower rate in cooler temperatures, which results in slower development of hydration products that give concrete its inherent strength, density and durability. It is imperative for fresh concrete to be placed at an appropriate temperature and protected from adverse temperature fluctuations throughout the production process, especially during curing. Fresh concrete with a measured temperature below 45 F at the time of pouring should be discarded.
Dry ambient air and low humidity from cold weather can increase and expedite evaporation of mix water from fresh concrete. Maintaining an adequate concrete moisture level and ambient relative humidity is important for proper curing. A critical degree of saturation is described as the point when a single cycle of freezing can cause damage to the concrete, which generally occurs when concrete reaches a compressive strength of 500 psi. Additionally, concrete allowed to freeze before reaching a compressive strength of 500 psi will never reach its full compressive strength potential.
In fact, if concrete freezes prior to reaching 500 psi and is then cured in ideal conditions from that point forward, the concrete will not reach its design strength and its durability, resistance to freeze-thaw cycles, density and other attributes will be severely compromised. As a result, it is advised to discard any concrete that froze prior to reaching at least 500 psi. Additionally, any fresh concrete with a measured temperature before 45 F at the time of pouring should be discarded.
2. Ensure concrete develops the strength necessary for stripping the formwork and lifting and handling the product. Because concrete strength development slows at colder temperatures, a form stripped after 18 hours in the summer or fall may need 24 hours or longer to cure before stripping in the winter.
Precast concrete products are engineered for the conditions they will experience in service, but their design must also account for stresses the product will endure during stripping operations as well as lifting and handling at the plant and on the job site. In some cases, loads imparted on products during lifting and handling far exceed the most extreme conditions endured while in service. Therefore, it is critical to ensure concrete is allowed to reach the necessary stripping strength before removing formwork.
During colder months, allow additional time to cure in place before handling, if needed. Handling a product and inducing stresses too early can damage the structure, resulting in lifting inserts pulling out and introducing serious safety concerns.
3. Maintain curing conditions that promote strength development without exceeding the recommended concrete temperatures and without using water curing. Skillful curing can bring out the best features of concrete. This includes enabling the concrete to reach its full potential in terms of strength, density and durability, and ensuring the product performs as designed. To help reduce the impact of low temperatures on cement hydration, concrete curing and concrete strength development, certain methods may be used to raise fresh concrete’s temperature as well as the ambient curing temperature. However, there are tradeoffs as well as precautions that must be followed.
Some of the most common strategies to raise the temperature of a fresh concrete mix or concrete that was just placed include:
- Heating mix water. If warming the water to greater than 140 F, consider the batching order and adjust. It may be beneficial to mix the coarse aggregate and water prior to introducing cement to prevent the cement from flash setting. Mix water should not be heated to more than 180 F.
- Heating aggregates. In most cases if the mix water is sufficiently warm, heating coarse aggregate beyond 60 F and fine aggregate beyond 105 F is unnecessary. Be mindful that rapid heating could cause some moist aggregates to explode if the moisture within the aggregate’s pores increases in temperature too quickly. Heating aggregates could also cause the cement paste immediately surrounding the aggregate to set rapidly and inconsistently. Avoid using frozen aggregates or aggregates with ice or snow accumulation. Not only will frozen aggregates introduce additional moisture to the fresh concrete mix, but they will also drastically lower the temperature of the mix which could slow or delay hydration. When possible, consider protecting aggregate storage areas from inclement winter weather with, for example, a permanent awning or temporary covering on outdoor aggregate hoppers, or store a portion of aggregate underground or indoors.
- Creating a tent over formwork with tarps or sheeting and placing heaters inside the tent. Gas-fired heaters are an effective way to warm the air surrounding curing products but must not be used to directly heat exposed or unformed concrete surfaces. The extreme heat can cause severe concrete carbonation or possibly delayed ettringite formation (DEF). Drastically increased air temperature supplied by a heater tends to reduce humidity and accelerates evaporation of mix water. Since the corners and edges of a product are susceptible to freezing, extra care must be taken to maintain these areas of a curing product at an appropriate temperature.
- Heating formwork and embedded items. All forms, reinforcement and embedded items must be at least 32 F at the time of concrete placement. These items must also remain within a 25-degree range of the fresh concrete temperature at time of placement – specifically, forms, reinforcement and embedded items must not be more than 10 F cooler or 15 F warmer than the fresh concrete. Plants may use heated enclosures, electric blankets or other heating systems to warm these items to the necessary temperature.
No matter the approach a plant chooses to warm its mix, concrete shall not exceed a temperature of 90 F at time of placement. ACI 306 states, “Concrete placed at lower temperatures [40-to-50 F], protected against freezing, and properly cured for a sufficient length of time, has the potential to develop higher ultimate strength and greater durability than concrete placed at higher temperatures.”
4. Limit rapid temperature changes, particularly before concrete has developed sufficient strength to withstand thermal stresses. When using steam curing, particular attention must be given to the ambient temperature inside the steam chamber as well as to the concrete itself. Extreme temperature fluctuations can shock the concrete and cause irreversible damage.
While working to prevent concrete from freezing before reaching a compressive strength of at least 500 psi, know that concrete should also not be exposed to accelerated curing methods using heat and moisture until after attaining initial set in accordance with ASTM C403, “Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance.” Section 4.5.3 of the NPCA QC Manual states plants must establish an ambient curing cycle that ensures the ambient curing temperature does not exceed 150 F unless measures to prevent DEF are used. The rise in ambient curing temperature must be limited to a maximum of 40 F per hour.
Additionally, rapid cooling of a concrete structure’s surface or a drastic temperature differential between the product’s surface and interior could cause cracking. A gradual cooling process is ideal.
5. Provide protection consistent with the durability of the structure during its design life. Precast concrete products are specified and designed with long service lives in mind. A high compressive strength is only one factor in ensuring a structure will endure its design life with resilience.
If certain measures are taken early in the curing process to expedite strength gain, other concrete characteristics could be compromised. For instance, calcium chloride works as an accelerator which may be beneficial in cold months, but use of chloride-containing admixtures in steel-reinforced concrete is not advised as they can cause issues with corrosion. Heaters help maintain a warmer ambient temperature, but can cause rapid drying of concrete if not used properly, resulting in cracks and decreased durability. Curing with steam can provide excellent results, yet careful attention must be given to the ambient and concrete temperatures throughout the curing process or the concrete could be subjected to thermal shock.
Cold weather concreting in your plant
Proper curing is not a one-size-fits-all process. The ideal curing time, temperature and moisture strategy will not only vary from season-to-season, but it can also vary day-to-day. Short-term production convenience should not be prioritized at the cost of long-term strength or durability. Now is an ideal time to review cold weather concreting procedures in your plant and potentially increase the frequency of tests to ensure quality and consistency in raw materials, concrete and final products.
Kayla Hanson, P.E. is NPCA’s director of technical services.