By: Joe Czaszynski
Winter temperatures can delay, or even stop a masonry restoration project. These delays can add costs to the budget, lengthen the anticipated completion time, and inconvenience building residents or occupants. Fortunately, measures can be taken to prevent these delays.
Freezing temperatures deleteriously affect concrete and mortar. If the water within concrete freezes during the curing process, the chemicals in the mixture cannot properly bond, and the target pounds per square inch (PSI) cannot be attained. This results in cracking, scaling and pop-outs in the concrete surface. Visible cracks of various lengths and widths are typically the first sign of improperly cured concrete. Wide spans of the concrete surface deteriorating and detaching from the slab is known as spalling. Pop-outs are caused by the freezing moisture within the concrete expanding through the surface. This condition manifests as holes in the concrete.
While these issues negatively affect the aesthetics of the concrete, the more serious threat is to the strength of the concrete. Depending on where concrete is being installed and the loads it must withstand, concrete must reach a specified PSI. Concrete installed for bridge supports or multilevel garage slabs must meet or exceed specific load ratings to safely support their intended loads. One can imagine the possible catastrophic failure that could result if the proper PSI is not attained due to concrete installation being performed during freezing temperatures without implementation of proper curing methods.
Concrete installers need to pay special attention to forecasted weather conditions, specifically the overnight low temperatures. Even though concrete can be installed during 50°F daytime temperatures, an overnight drop to 20°F can freeze the moisture within that newly installed concrete, preventing it from reaching the proper strength. According to the American Concrete Institute (ACI), if exterior temperatures drop below 40°F, protective measures must be put in place. ACI also states that concrete must reach 500 PSI before it can be exposed to freezing temperatures. During warm weather (approximately 70°F), concrete can take up to 24 hours to reach 500 PSI. The curing time to reach 500 PSI is reduced considerably during colder temperatures.
Protecting against cold temperatures begins with preparing the area where concrete will be installed. If directly on or below grade, any frost, snow or ice must first be removed. The area must then be warmed to above 32°F prior to installation. Any steel reinforcement or support structures must be similarly prepared. Protection and supplemental heat in in conjunction with thermal blankets and tents should be used to retain warmer temperatures in any areas of concrete installation.
Alterations to the mix design of concrete can also be made. A high-early mix design allows the concrete to cure faster, and more quickly attain the minimum required PSI. Hot water should be used to mix the concrete. Water temperatures can range from 40°F to 120°F, but not exceed 140° F. Other components such as aggregate and bag mix concrete should be stored at or above 40°F prior to mixing. In freezing temperatures, the mixing of the concrete should take place in an area protected with a supplemental heat source.
Protection must be placed over newly installed concrete to prevent freezing temperatures from affecting these areas. Thermal blankets can be placed over horizontal repairs/installation, or secured with fasteners over vertical repair/installation areas. If temperatures in a tented work area drop to freezing or below, a supplemental heat source, such as a gas-fired torpedo heater or kerosene space heater, is required. When a supplemental overnight heat source is needed, a fire watch is required. This can add a substantial cost to the project that should be accounted for in the construction budget. Regardless of the overnight protection/heating options utilized, thermometers must be installed within the tents or under the thermal blankets so that low temperatures over the curing period can be recorded and documented. Depending on the intended use of the concrete and the exterior temperatures during the curing time, above freezing temperatures must be maintained for a period of 24 to 72 hours.
Required cold weather concrete installation protection adds cost to the project. Also, the necessary implementation of these additional measures can lead to a larger margin for error. The additional costs associated with a more diligent and experienced workforce being more carefully monitored by design and consulting professionals, should be discussed with the client and weighed against the overall goal of the project. While most projects shut down for the winter to avoid possible cold weather application issues, work stoppage is not always an option. If the measures discussed above and defined by ACI are followed, this type of work can be performed with little to no effect on the project outcome, other than a financial one. Sullivan Engineering has aided many clients in measuring the pros and cons of performing concrete installation, in cold weather. We have also provided construction oversight when cold weather installations have been approved, documenting all procedures, methods and temperatures to ensure the quality of the work performed meets the project specifications.