Concrete Temperature Monitoring In Cold Weather
If you work in construction, you know that pouring concrete in cold weather creates additional challenges. Careful consideration must be taken for proper form preparation, monitoring of temperatures during the pour, and detailed care during the curing stage. Construction does not need to stop when the temperature drops, but it is imperative for the project team to have clear communication, be properly educated, and thoroughly prepared for the challenges with placing concrete in cold weather.
Concrete temperature monitoring technology that promotes real-time connectivity provides engineers and contractors accurate data on concrete and ambient temperature, helping them foresee issues and proactively manage timely completion of the concrete curing process.
Understanding Cold Weather Concreting
According to the ACI’s (American Concrete Institute) Guide to Cold Weather Concreting, cold weather concreting conditions exist when the air temperature falls below 40° F (4° C) during the concrete’s protection period. The protection period is defined as the period in which the concrete could be adversely affected by the cold weather. This period usually does not exceed 48 hours. In Canada, the CSA (Canadian Standards Association) states that material and equipment needed for cold weather protection and curing shall be available and ready for use if the temperature is expected to fall below 41° F (5° C) within 24 hours of placing the concrete.
Although the critical period to avoid freezing is usually not more than 48 hours, additional care with the structure is required beyond this initial period to promote adequate strength development. It is important to minimize rapid temperature changes when removing forms to avoid excessive thermal stresses in the structure.
Furthermore, if the mix was not designed to undergo freeze/thaw conditions (no air entrainment), consideration should be given to the potential of freezing during construction.
The project schedule and economic implications of pouring concrete in cold weather also need to be considered. The extra costs from building and heating enclosures, placement of insulated blankets, and other labor and material costs can be substantial. If the project schedule is demanding and the critical path is driven by the concrete work, the higher costs associated with accelerating the form’s cycle rate can be justified, allowing the crew to move more quickly from one pour to the next.
Concrete Protection – Why It Is Important
Cold weather concreting procedures are provided for the following reasons:
Prevent Early Age Freezing
Ensure the concrete develops the required strength for form removal
Limit rapid internal temperature changes and excessive thermal stresses
Prevent long-term durability or serviceability issues
At a basic level, the curing of concrete is a function of temperature and time. The hydration of the cement results in the hardening of the concrete. When lower temperatures are present, strength development takes place at a slower rate, which increases the amount of time needed before forms can be removed. It also increases the possibility of damaging corners and edges during form removal. In general, hydration becomes dormant when the temperature of the concrete drops below 40º F (4º C). Without proper planning, this can result in delays to the project schedule. In worst case scenarios, the effects of the cold weather can lead to irreversible damage to the concrete, resulting in potential rework and substantial monetary and schedule setbacks.
Minimum Compressive Strength prior to freezing
If the water in the concrete freezes prior to gaining the required tensile strength to resist the water expansion pressure, irreversible damage can occur. ACI recommends that concrete needs to attain a minimum compressive strength of 500 psi (3.5 MPa) prior to freezing, otherwise a single cycle of freezing can cause irreparable damage. Until the concrete reaches this state, absolute protection from freezing is critical. The CSA’s recommendation is more conservative, requiring the concrete to reach 1,000 psi (7 MPa) prior to the first cycle of freezing.
Note that this applies to a single freezing event and does not apply when dealing with repeated freeze/thaw cycles. For cases in which the concrete will be exposed to multiple cycles of freezing, it is recommended that the concrete achieves a minimum compressive strength of 3,500 psi (24 MPa). An effective way to monitor the concrete during this initial and critical stage is with temperature monitoring devices. Additionally, the maturity method is strongly recommended to determine the in-place strength of the concrete, as discussed in further detail in a subsequent section.
Temperature Differentials and Thermal Stresses
Another risk of cold weather concreting is excessive temperature differentials. Thermal cracking can occur if there is a large temperature difference between the surface and inner core of the structure. It is more prone to occur on larger elements or mass concrete where the inner core is generating a greater amount of heat, and the surface is rapidly cooling from being exposed to a low air temperature. The larger the difference between the concrete and ambient temperature, the higher the rate of heat loss. Unless there is a large amount of data on the mix design’s performance, most thermal control plans limit the maximum temperature differential to 35° F (20° C). Exceptional care should be taken during the removal of forms, as this can lead to a rapid drop in temperature at the surface of the element. Therefore, contractors often remove forms during the warmest part of the day; and if necessary, cover and heat the element before the ambient temperatures drop.
Another variable that is often overlooked in cold weather is the concrete’s evaporation rate, as it is a bigger issue in hot weather concreting. The concrete’s evaporation rate is a function of the wind speed, relative humidity, concrete temperature, and ambient temperature. As the difference between the concrete temperature and the ambient temperature increases, so does the evaporation rate. Because concrete temperatures are primarily driven by the aggregate temperatures, there is less volatility in concrete temperatures compared to ambient temperatures, which can be problematic when there is a large drop in ambient temperature. This is often overlooked, but on a late fall day with high wind speeds, low relative humidity, and a substantial drop in ambient temperatures from one day to the next, the evaporation rate will often be high enough to result in shrinkage cracking. This is a larger concern on slabs and bridge decks compared to foundation walls, columns, or piers.
Best practices for cold weather concreting:
Prior to the pour, the contractor should ensure that the forms, rebar, and the base are free of ice and snow. Although specifications vary on the preparation of the forms and rebar, if the ambient temperature during the pour is below 40º F (4° C), it is recommended to pre-heat the interior surfaces of the forms, adjacent concrete, rebar, steel piling, and any other embedments to a minimum of 32° F (0° C), although 40º F (4° C) is preferred. The larger the embedment, the more critical it is to minimize the temperature difference between the embedment and the concrete. The engineer and the contractor should discuss and agree on the procedures and method for checking the temperature of these elements prior to the pour.
ACI 306 provides guidance for the minimum concrete temperature during batching and at point of placement. The required temperature varies depending on the section size of the element being poured as well as the ambient temperature. If the air temperature falls below 40° F (4° C), many engineers and specifications require a minimum temperature of 50° F (10° C) at point of placement. To avoid unnecessary surprises, this should be discussed in the pre-pour planning as it may require the producer to take measures for heating the concrete.
After placement, exposed concrete should be protected with polyethylene sheeting, insulated curing blankets or other materials approved by the engineer. The ambient temperature, size of the element, and project schedule will dictate the level of post-pour protection. In some cases, a layer of polyethylene is sufficient, where in other cases, the situation requires building a heated enclosure and installation of multiple layers of insulated blankets. Care should be taken to provide adequate ventilation and avoid the concrete’s direct contact with carbon dioxide. Otherwise, carbonation can occur, leading to durability issues such as a weak surface that is prone to scratching and dusting.
After placement, the temperature of the concrete should be closely monitored to ensure compliance with the project specifications. Although a high/low thermometer provides records of the concrete’s temperature, it is inherently a reactive approach. If the thermometer identifies the concrete’s temperature is out of specification, it is too late to remediate the situation. Newer technology allows for a more proactive approach.
Digital temperature sensors that provide real-time data to any connected device (i.e., tablet or smartphone) are replacing old thermocouples. With these devices, engineers, superintendents, and project managers are provided with accurate data on concrete and ambient temperature, helping them foresee issues and manage timely completion of the concrete curing process. The temperature data help them to be proactive in assessing the condition of the concrete. For instance, is the temperature drop rate too high? Does the concrete require additional heating? Is the heater running out of fuel overnight and on weekends? Is the temperature differential between the surface and the core increasing at a problematic rate? These devices are designed to send notifications and alarms based on the project criteria.
The contractor should also consider using the maturity method in lieu of field cured test cylinders. Determining concrete strength using the maturity method is based on the relationship between the curing time and concrete temperature. Because the temperature of the concrete in the forms will vary to a higher degree than the cylinders, the maturity method will provide a more accurate prediction of the concrete’s in-place strength. The advancements in concrete monitoring technology allow for easy and accurate use of this method. It is strongly recommended that the contractor and engineer agree ahead of time on the location of the temperature sensors for the target strength to be accurately represented.
This article provides an overview of the challenges and recommended procedures with cold weather concreting. For further and more detailed information, please refer to ACI’s Guide to Cold Weather Concreting. There are unique environmental challenges with construction in the winter months. However, with proper education, team planning, and use of real-time connectivity, high quality concrete can be placed in even the harshest conditions.
About the author:
Eric Van Dixhorn spent many years managing infrastructure projects during the cold winter months in Wisconsin. He recalls that on one particular project, a bridge widening across the Henry River in Beloit, he spent a significant amount of time climbing in and out of cofferdams to check concrete temperatures on the bridge’s substructure. Having real-time connectivity to those temperatures, instead of climbing up and down icy ladders and hooking up wire leads to the reader, would have led to a safer and more productive approach with managing the cold weather concrete operations.