Cool Weather Concrete
Hot Weather. Cold Weather. What about cool weather concreting?
from Ken Hover, Ph.D., FACI, guest writer for Concrete News

Congratulations! You survived a summer that, in many parts of the US, set records for heat, rain, or lack of rain.

In the process, your successful field crews mastered the art of hot weather concreting. In some cases it was necessary to lower the initial temperature of the concrete with chilled water or ice. In tougher situations you cooled the concrete with liquid nitrogen. For other mixes, you reduced the heat generated by the hydration of the cement by finding ways to lower cement content (such as water reducers and/or more favorable combined aggregate grading), or you substituted fly ash, ground slag, or other pozzolans for some of the cement.

You controlled aggregate temperature by using shaded stockpiles or by sprinkling the stockpiles with water and allowing evaporation to cool the sand and stone. You not only used set retarding admixtures to delay setting for flatwork, but you also made sure that you had enough finishers and finishing machines on site to get on the slab as soon as it was ready. For higher-performance mixes with a low rate of bleeding, you used a fog spray as an initial cure to slow the rate of evaporation—or you may have properly used an evaporation retarding liquid to accomplish the same thing, making sure that you did not finish these high-water content solutions into the slab surface. Finally, you made sure that your test cylinders were kept from overheating on site, since a hot test cylinder gives artificially low 28-day strengths in return for artificially high one-day test results.

Cooler days call for changes of strategy. The combination of cooler air and reduced solar heating of aggregates, formwork, and rebar will help keep concrete temperatures cooler. Compared to hotter weather, this will slow down the rate of slump loss and setting. The finishing crew will notice the longer setting time right away, especially given that a concrete temperature drop of 20°F will double the setting time. This can be a relief if the finishers have been pushed for time all summer, but can also lead to longer days and overtime hours waiting for the concrete to set. Close coordination with the concrete supplier is required to modify doses of set retarders and set accelerators as the seasons change, and this can be tricky on fall days that are cool in the morning but like summer in the afternoon.

Don't expect that cooler weather means slower drying of the concrete surface, either. The rate of evaporation of water from lakes, reservoirs, or concrete surfaces is usually greater in cool weather than it is in hot weather. Fall can increase drying rate for three reasons.

The first is a generally lower humidity in the air. Second, drying rate increases as the temperature of the air drops relative to the temperature of the concrete, and fall air temperature can be 20° to 40°F cooler than summer air temperature. But the initial concrete temperature in October might only be 10°F or so lower than the initial concrete temperature in July. (In fact, the most rapid drying conditions occur when warm, heated concrete is cast into cold winter air.) Finally, increased wind speed increases drying, because the wind moves the evaporated water vapor away from the concrete surface. In many parts of the US fall means faster wind speeds.

When you put these factors together, you realize that it is great to be on the job site on a cool, crisp, breezy fall day, but such a day can bring rapid drying and the risk of plastic shrinkage and early age cracking. (This is why we buy more lip balm and skin moisturizer in cold weather than in hot weather.)

Once the concrete is placed, finished, and protected from rapid drying, one payoff of cooler concrete is a nice boost in the 28-day strength of cylinders compared to summertime cylinders that were left in the hot sun. But the early-age cylinder strength will decrease because of the cooler concrete, and perhaps more importantly, the early-age in-place strength of the concrete in the structure will be lower as well. This could dictate a change in schedule for pulling pre-stressing tendons, or removing formwork or shoring. It can be a dangerous mistake to continue summertime placing, stressing, and shore-removal schedules into cooler weather, especially when the concrete quality control is based on standard laboratory tests. It is a good idea to do some in-place testing as the seasons change to find out what is really happening to the concrete.

Concrete temperature is a serious concern with thick or massive concrete, and this problem does not go away with the arrival of cool weather. This is because the interior temperature of thick concrete is not affected much by the outside weather. For walls 18 inches or more thick (or footings in the range of 2 to 4 feet thick) the temperature at the center of the concrete is influenced more by heat of hydration than by air or ground temperature, at least over the first few days after casting.

The temperature at the center of these members can rise more than 100°F above placing temperature in the first 24 hours or so. The good news is that this accelerates hydration of the cement. The bad news is that thermally accelerated hydration generally leads to a weaker and more porous microstructure of the hardened cement paste. (This is the reason that hot cylinders have a reduced 28-day strength). Compared to the same concrete hydrated at a cooler temperature, faster hydration increases strength for ages up to about three days. At later ages we observe that concrete that hydrates more slowly at cooler temperatures yields higher strength (see Figure 1 below). This means that cooling a concrete mix for thick placements can be just about as important in fall as in summer.

A related problem, however, is that for thick concrete the surface and edges start to cool to the surrounding air temperature a lot sooner than at the center. This leads to a thermal contraction of the cooler exterior concrete at the same time that there is a thermal expansion in the warmer interior. This leads to thermal stress that can produce a full-depth crack. To reduce this risk, it is a good idea to either cool the inside or insulate the outside to keep the maximum temperature difference at no more than about 30°F. The thicker the concrete and the cooler the air temperature, the more challenging this becomes. For example, casting a thick wall in winter is more likely to cause a thermal crack that casting the same wall in summer, since it is temperature difference that causes the stress.

"Cool weather concrete" usually gets lost in the shuffle somewhere between the well-publicized specifications and recommended practices for installing concrete in hot and cold weather. When the weather is brutally hot and dry, or when it is freezing cold, it is obvious to nearly everyone from the concrete producer to the finishers that special precautions need to be taken. Unfortunately, several of the most costly and deadly temperature-related concrete construction accidents in the US have occurred in fall and spring—right between the obvious need for attention to either hot or cold weather conditions.


About the Author
Ken Hover, Professor of Structural Engineering, Cornell University.

Ken Hover, Ph.D., FACI
Earned Ph.D. at Cornell University. Faculty member in Cornell's School of Civil & Environmental Engineering for 18 years, teaching courses in reinforced concrete design, concrete materials, and construction management. Research concentrated in concrete durability and the impact of construction operations on concrete quality. Earned the top teaching awards at departmental, college, and university levels. Appointed to rank of Full Professor in 1992. Undergraduate Dean of College of Engineering, 1996-1999.

Received ACI's National Educators Award (the Kelly Award) in 2001. ASCE Best Basic Research Award, 1992.

Current member of ACI 305-Hot Weather, 308-Curing (Past Chairman), 309-Consolidation, 318A-General Concrete and Construction, Elected to ACI Board of Directors, 1999. Chair of 308 Subcommittee for new "Guide to Curing Concrete." Fellow since 1992.

Prepares and presents seminars and short courses nationally and internationally. Developed Federal Highway's short courses on Concrete Materials and on Concrete Mix Design and Proportioning. Rated top technical speaker at World of Concrete every year since 1994.

Licensed Professional Engineer since 1974.

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© 2006 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2005/2006.

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