Hot and Cold Weather ConcretingJun 27 2016 · 0 comments · Concrete, NISHKIAN MONKS ·0
By Serena Gilles, PE
Pouring concrete in hot and cold weather conditions requires special attention in order to achieve desirable strength and quality.
Cold weather concreting
ACI 318-11 (Building Code Requirements for Structural Concrete) directs the user to ACI 306R which serves as a guide cold weather concreting. It defines cold weather as, “a period when for more than three successive days the average daily air temperature drops below 40 degrees Fahrenheit and stays below 50 degrees for more than one-half of any 24 hour period.” The main issue with pouring concrete in cold weather is that the concrete sets much slower and actually stops curing below 40 degrees. The concrete is unable to reach its design strength if it isn’t allowed to cure properly. Reductions in ultimate strength can be up to 50 percent due to early freezing. If the concrete is allowed to reach 500 psi, typically within 24 hours, it will continue to gain strength as designed, so the concrete must be protected until then.
How to prevent concrete from freezing during cold weather
- Heat materials: heating water and aggregates has been shown effective in increasing concrete temperature
- Accelerate set time: with the use of accelerating chemical admixtures
- Insulate the concrete: use insulation blankets to retain the heat from hydration (heat generated during the curing process)
- Use heated enclosures: expensive but effective way to increase concrete temperature
Hot weather concreting
ACI 318-11 states, “During hot weather, proper attention shall be given…..to prevent excessive concrete temperatures or water evaporation that could impair required strength or serviceability of the member or structure.” Ambient temperatures above 90 degrees can negatively affect the quality of concrete. Recommendations for methods and practices are given in ACI 305R. ACI 305R notes that consideration isn’t only required for hot weather, but for other causes of water evaporation including low humidity and high wind.
How to protect concrete from excessive water evaporation
- Precool ingredients: Moisten subgrade, steel reinforcement, and formwork prior to concrete placement
- Protect concrete surface with plastic during curing.
- Install temporary wind breaks and sunshades to limit concrete exposure.
- Consider morning or night concrete pours at cooler temperatures.
If precautions are taken when pouring concrete during cold and hot weather, high quality durable concrete is attainable. The upfront cost is minimal compared to having improperly cured concrete that may require replacement.
ACI Committee 305, “Hot Weather Concreting (ACI 305R-10),” American Concrete Institute, Farmington Hills, MI, 2010.
ACI Committee 306, “Cold Weather Concreting (ACI 306R-10),” American Concrete Institute, Farmington Hills, MI, 2010.
ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary,” American Concrete Institute, Farmington Hills, MI, 2011.
“CIP 12 – Hot Weather Concreting,” National Ready Mixed Concrete Association, Silver Spring, MD, 2014.
“Things to Consider about Hot Weather Concreting,” Portland Cement Association, Skokie, IL, 2015. <http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-construction/hot-weather-concreting>
“What Happens When Concrete Freezes?” Portland Cement Association, Skokie, IL, 2015. <http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-construction/cold-weather-concreting>