Environmental factors like hot and cold weather affect concrete properties and the construction operations of mixing, transporting, and placing of the concrete materials.
By understanding how these factors affect the placing and curing of concrete, producers can adjust mix designs and compensate in a variety of other ways to maintain high quality standards and avoid issues with the finished product.
Jack Holley, a concrete quality control veteran of 45 years, shares his expertise and recommends best practices for hot and cold weather conditions in the two-part webinar series.
Jack Holley Webinar Series
This blog post will summarize the main points from the Best Practices for Cold Weather Concreting webinar. If you missed the summary of the Best Practices for Hot Weather Concreting, you can read it here.
What is Cold Weather Concrete?
The American Concrete Institute (ACI) defines cold weather concreting as, “a period when the average daily ambient temperature is below (or expected to fall below) 40°F (5°C) for more than 3 successive days. The average daily temperature is the average of the highest and lowest temperature during the period from midnight to midnight. When temperatures above 50°F (10°C) occur during more than half of any 24-hour duration, the period shall no longer be regarded as cold weather.”
Cold weather concreting is when the average daily ambient temperature is below 40°F (5°C) for more than 3 successive days.
Common Mistakes with Cold Weather Concrete
According to Jack, special precautions must be taken in cold weather to prevent damage to concrete. The necessary degree of protection increases as the ambient temperature decreases. Common mistakes include:
1. Letting concrete freeze
Concrete temperatures should be maintained between 50°F (10°C) until the expected strengths have been reached after the finishing process. If concrete freezes prior to reaching an initial strength of 500 psi (3.5 MPa), it will not achieve its intended strength. Exterior concrete should be air entrained and at the minimum required strength prior to exposure to freezing and thawing cycles.
2. Placing concrete on frozen ground
Contractors sometimes make the mistake of placing concrete on cold or frozen ground. Frozen ground can settle when thawed, causing the concrete to crack. Further, the fresh concrete closest to the ground will cure slower than the surface, meaning the top of your slab will set while the bottom stays soft, resulting in surface defects like cracking or crusting.
3. Sending concrete out at improper temperatures
The need for proper planning in cold weather cannot be overstated. According to Jack, the concrete should arrive at the site on time and, from a production standpoint, the water and aggregates should be heated to maintain proper temperatures. One of the best ways to do this is with blends of hot water, warming the sand, and even blending the sand in colder areas.
4. Not monitoring temperatures constantly
Mechanical and personnel resources should be on site to monitor the temperatures at all times. One of the best ways to do it is with a temperature calculator from ACI:
- Take the temperature of all of the materials
- Calculate the output temperature of the concrete
- Target your water temperatures and make other adjustments
There are temperature sensors available today that can be easily installed in the concrete as it is placed. These sensors can send data wirelessly so you can monitor and foresee any potential issues or changes in set times.
5. Improper placing and finishing
Removing heating and hoarding too quickly after the curing period has ended will cause a rapid cooling of the outer surfaces and create a thermal differential within the placement. There are recommended time limits that can be referenced through the Portland Cement Association, ACI, and other industry bodies. If those limits are exceeded, there's a very high possibility that the structure will crack due to the differential in temperature.
A good rule of thumb is to:
- Turn the heat down first but leave the hoarding up
- Continue to monitor the internal and surface temperatures
- Consider using insulated blankets
- Continue to monitor until the differentials are equalized or at a small enough number that it won’t create stresses within the placement
For colder climates, if you have a water curer, it's best to remove it a day prior to removing the heat to prevent freezing in a saturated situation, depending on how mature the process concrete is.
Mix Design Adjustments in Cold Weather
Jack recommends examining mix designs and making adjustments as you come into the Fall season to help with cold weather concreting. The following are some common adjustments:
1. Increase cement
Some producers will add 100 lbs per cubic yard (50-60 kg per cubic meter) of Type I or Type II cement; others will add Type III—a high early strength cement that is ground finer and reacts faster so the early compressive strength gains are greater. However, none of these options are very efficient or cost-effective for producers.
2. Reduce Supplementary Cementitious Materials (SCMs)
Cutting back on the percentage of SCMs is a common practice but it will depend on the application. For flatwork, you need to be a little more cautious and attentive with your initial sets.
3. Adjust admixtures
If the normal water reducer had been elevated to near the max limits for the summer months, this should be moved back. Decreasing the amount of water-reducing retarder is another common adjustment in cold weather. However, you don't want to decrease water-reducing retarder past the point of its moderate reduction or you'll start to have workability problems.
4. Use accelerators
There are two types of accelerators—chloride-based and non-chloride-based. The chloride-based accelerators are best but they are limited due to the corrosion potential of the reinforcing steel. Non-chloride accelerators work well if the manufacturer's recommendations for dosages are followed.
CarbonCure and Cold Weather
CarbonCure has been used for many years in cold weather conditions. It won't contribute to any of the issues described above, nor will it prevent any of them. While CO2 is a cooling material, the CarbonCure technology only uses a very small amount of CO2—not significant enough to alter the temperature of the concrete.
While all of these adjustments can be considered, at the end of the day Jack stressed that there is no standard mix for cold weather—each cold weather scenario should be analyzed individually by qualified personnel, who should find the optimum mix of quality, practicability, and cost-efficiency. Mix designs should be adjusted regularly as temperatures change.
Interested in this topic? Watch our recent webinar on this topic featuring CarbonCure’s resident expert, Jack Holley.