When placing in cold weather, extra precautions must be taken to ensure concrete sets properly and gains adequate strength. The Transtec Group, Texas-based concrete pavement practitioner, notes how the first few days after placement are the most important because that is when cement hydration reaction proceeds the fastest and concrete gains most of its compressive strength. However, cold conditions slow the hydration process, meaning that concrete placed in low temperatures gains less strength than the same mix in warmer temperatures during the same length of time. If the water in the concrete mix freezes, the hydration process stops completely. Even if it thaws, strength gain may not recover.
|Command Center hardware and software equip users to optimize cold weather concrete thermal control and curing methods.|
Therefore, regulating the temperature of concrete in cold weather is essential to its proper strength development. In ACI 306R-16 Guide to Cold Weather Concreting, the American Concrete Institute cites targets based on the air temperature. It recommends minimum temperatures during mixing and placement, plus maximum allowable gradual temperature drop within 24 hours after the end of slab or structure protection from the cold weather. Such recommendations are often mirrored as requirements in job specifications.
To ensure the internal temperature does not drop below freezing and is within specified requirements per ACI and/or project specifications, sensors can be embedded in the concrete to continuously monitor and record temperature at regular intervals. This data informs the user of the concrete’s temperature history, so he or she can verify compliance with project specifications.
Contractors in cold weather concrete placements measure temperatures for three primary reasons: Continuous monitoring for potential early-age freezing and thermal shock; optimizing curing procedure; and, improving safety of structure. Monitoring can aid the user in keeping fresh and early-age concrete within the project-specified range of temperatures during the curing process to ensure that freezing and thermal shock do not occur.
As previously mentioned, freezing of the water within the matrix would halt the concrete’s strength gain and prevent it from reaching the necessary level per construction specs. Therefore, concrete in the form or protected by blanket or other cover must remain above freezing temperature to properly gain strength while curing.
When the curing protection is removed from a newly placed slab or structure, the concrete is at risk of thermal shock. It occurs when the curing is removed too suddenly and there is a large differential between the concrete and the ambient temperature, leading to the surface cooling significantly quicker than the center. The cold surface contracts more than the warmer center, causing tensile stress and cracking. Thermal shock can be prevented by using a monitoring system to ensure that the temperature differential between the concrete and the ambient remains within specifications.
Temperature monitoring systems can often also act as concrete maturity gauges. Temperature history can indicate slab or structure maturity, the calculation of which can be used to estimate in-place concrete strength. Knowing both the temperature and estimated strength of the concrete placement can allow the user to optimize curing.
Using temperature data, the user can determine when to modify conditions by increasing or decreasing thermal control (using heaters, tarps, insulated blankets, etc.). By capturing maturity data, the user can know when the concrete has gained enough early-age strength to remove the curing protection altogether. Monitoring both temperature and maturity allows the user to adequately cure concrete at job-specified temperatures to the necessary strength without wasting time or money on unneeded curing.
SAFETY, PSI ASSURANCE
Tracking the early-age concrete strength development through temperature and maturity monitoring is essential to the safety of the final structure. Because cold weather slows the strength development process, using a maturity system to estimate the in-place progress of the concrete’s strength gain can be good practice to verify that the concrete has reached adequate strength for proceeding in the construction process, reducing the risk for premature structural failure.
Continuous temperature monitoring is essential to cold weather concreting. Using a non-destructive temperature and strength monitoring system such as Command Center is among the best choices for users to know, throughout the curing process, if their concrete is meeting a project’s thermal specifications.
The Command Center system, notes Transtec Group, is the optimal choice for monitoring because it is affordable, easy to use, and has been proven to be reliable in the field. The system includes self-powered sensors and robust software for viewing, analyzing, and reporting data. Sensors are pre-programmed and do not require initialization in the field, and the same sensors that collect temperature data can also be used to measure maturity. Command Center software is free and allows the user to toggle between Celsius and Fahrenheit as well as view and analyze temperature data for individual sensors or compare data from multiple sensors at once—ideal for monitoring the temperature differential between the concrete and ambient to prevent thermal shock.
Temperature and maturity monitoring is required in most cold-weather concreting applications. It becomes essential for preventing freezing and thermal shock, optimizing curing, and ensuring safety of a structure. Temperature and maturity monitoring systems like Command Center provide a user-friendly, affordable option for this purpose. By continuously collecting temperature data, users can optimize cold-weather curing and thermal control procedures based on project specifications to ensure structural safety and save money on curing methods. — Transtec Group, Command Center, Austin, Texas; www.thetranstecgroup.com, www.commandcenterconcrete.com