Protecting concrete in cold weather is a continuous challenge for concrete contractors and site supervisors. Placing concrete in the cold weather condition requires special preparation and protection. All necessary precautions should be taken in order to to alleviate the negative impacts of cold weather. Special curing and protection is required in most cases. In my previous article, we reviewed what is considered cold for concrete construction, and what should be done prior to placing concrete. In this article, I will review some of the widely used protection techniques and strategies and the challenges in protecting the concrete from extreme cold. But first, let’s see what cold means for concrete:
Why Cold Temperature is critical
The hydration of cement is a chemical reaction. Extremely low temperatures as well as freezing can significantly slow down the reactions, thus, affecting the strength growth. In fact, freezing temperatures within the first 24 hours (or when concrete is still in plastic state), can reduce the strength by more than 50%.
Protecting Concrete in Cold Weather
The CSA A23.1 specifies that protection shall be provided by means of: 1) Heated enclosures, 2) Coverings and 3)Insulation. The question is, how to select the most affordable, most efficient method!
Covering – Curing Blankets w/o Insulation
Covering with curing blanks is widely used in construction sites during the cold season. The heat generated from the hydration of cement is normally sufficient for many cases, should blankets are used properly. Blankets should remain for a couple of days. The required insulating value depends on the thickness of concrete, the amount of cement, and anticipated cold temperature. Consult ACI 306 chapter 7 for details on the insulation.
Site managers, and engineers are responsible to assess if concrete has reached desired strength. Temperature monitoring using infrared thermography from concrete surface or maturity method can be used for better decision making.
The heat generated from hydration process should suffice in most cases, if appropriate insulating blankets of polyethylene sheets are used. Additional source of heat might be required based on area and temperature.
When covering with blankets, special attention should be given to the corners and edges of the slabs. These area often require further insulating layers. If covering can not keep the concrete temperature at desired levels, external heat source such as electric heating blankets, or hydronic heating pipes should be used.
If blankets do not provide enough protection, or if the weather is extremely cold (even prior to placing concrete), then heated enclosures should be used. This technique includes enclosing the construction site (for example the storey under construction), and heating the space. Certain challenges should be addressed:
1- Carbon Dioxide-Carbonation
One common challenge with heated enclosure is the problem of carbonation. The carbon-dioxide produced by some of commercially available heaters increases the chance of carbonation of freshly placed concrete. This can lead to formation of a weak concrete layer which is often unacceptable. It is recommended to use heating systems that exhaust to the outside of the enclosure.
2- Rapid Drying / Uneven Heating
The use of heaters can result in very rapid drying of concrete, which will increase the chance of plastic shrinkage, and might lead to poor quality concrete (if water required for hydration process evaporates). It is recommended to move the location and direction of heat source for a more uniform heating pattern.
Special attention should be paid to the heaters that use propane gas.
When Is It Safe to Stop Protection
The protection should be continued until required structural properties such as strength is achieved. The minimum strength before exposing concrete to extreme cold is 500 psi (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing. Traditionally, cast-in-place punch out cylinders are used to estimate the strength at certain intervals. The maturity method is gaining popularity with the recent advancement in wireless sensor technology. Wet curing during this period should be avoided.