Experts agree that the best temperature to pour concrete is between 50-60°F. The necessary chemical reactions that set and strengthen concrete slow significantly below 50°F and are almost non-existent below 40°F. Even when daytime temperatures are within the satisfactory range, winter concrete setting creates risks that could result in weak, inadequate concrete. If nighttime temperatures are below freezing, the water in the concrete will freeze and expand, causing cracks. Additionally, if temperatures reach below 40°F (but not freezing) during set time, concrete will take much longer to reach required strength. However, if the correct measures are taken, concrete can still be successfully placed during even the coldest months of the year.
Before embarking on a cold weather concrete project, it’s important to determine any special strength requirements or considerations. This will help as you schedule your pouring and determine which strategies you will use to keep your surroundings and materials warm. The predominant challenge you will face during a winter concrete project is ensuring that the concrete sets before it is exposed to freezing temperatures. You might take the following suggestions into consideration as you plan your upcoming project:
- Use heaters to thaw frozen ground, snow or ice.
- Use hot water to mix cement.
- Keep dry materials in a dry, warm location.
- Use products designed to set quickly. During cold weather, these products will not set as quickly as the instructions may indicate, but will set faster than conventional materials.
- Use additives that accelerate set time. Use caution; if additives contain calcium chloride, any rebar or metal wire mesh in concrete will rust and cause concrete to crack.
- Use extra cement (typically 100 lb/cubic yard) to make the reaction hotter and cause concrete to hydrate more rapidly.
- Remember that you still need to wait for bleed water to evaporate. Incorporating the water into the surface during finishing will weaken the surface. Bleeding starts later and takes longer during cold weather; you can use squeegees or a vacuum to remove water quickly.
- Wait until concrete has reached desired strength to remove any framework. If the framework is removed too early, the concrete will be damaged and the surface could collapse.
After implementing the above suggestions, It’s important to consider how you will keep concrete at the correct temperature during the curing process. Concrete must maintain a temperature above 50°F for approximately 48 hours for the correct chemical reactions to take place, allowing the concrete to eventually settle at 4000 psi. Two popular options used during cold weather concrete curing are heated enclosures and insulated blankets. If using an enclosure, ensure that the structure is both wind and waterproof. Additionally, ensure that there is proper ventilation for the space heater. Heaters cause an increase in carbon dioxide that could cause carbonation in the surface of the concrete, not to mention a work hazard for employees.
Powerblanket® concrete blankets are an extremely effective option for attaining and maintaining the correct temperatures for concrete pouring and setting. Powerblanket® concrete blankets can be used to thaw ground before pouring concrete and again after finishing to keep concrete from freezing. Utilizing Powerblanket concrete blankets will ensure that concrete is kept at the correct temperature for the necessary reactions to happen quickly and the desired strength to be reached. With Powerblanket, it’s almost never too cold to pour concrete!
Temperature makes all the difference when pouring concrete. Check out our article on how temperature affects concrete that is still curing inside the Hoover Dam!