The old joke in northern climates was that there were two seasons: winter and construction. Highway departments would plan their work in stages they could complete “before the snow flies”. Nowadays, while we may have less activity in winter than in summer, it’s more accurate to think of construction seasons.
Supplementary cementitious materials such as fly ash and slag cement hydrate more slowly and evolve less heat than portland cement. Interground limestone in Type IL cement doesn’t really hydrate at all. In hot weather, these characteristics can be beneficial; in cold weather they probably won’t be. If your project spans several construction seasons, you may want to develop more than one concrete mixture to get the properties you need when you need them. Test them under the conditions they’ll encounter in the field.
Summer
Summer is peak construction season. As the climate warms, the period when hot weather prevails starts earlier in the year and ends later. We’ve discussed hot-weather concreting in previous blogs. Some key points to remember:
- ASTM C31, Standard Practice for Making and Curing Concrete Test Specimens in the Field, specifies the temperatures for concrete cylinders for acceptance testing. Exceeding the maximum allowable temperature will yield artificially low test results. Controlling the temperature is difficult in summer, but essential if you don’t want to spend a lot of time investigating apparent failures due to bad testing.
- Air entrainment can be harder to control in hot weather. Be prepared to adjust the admixture dosage as necessary to maintain the air content.
- Water demand can increase. Don’t exceed the allowable w/cm. Use water reducers and/or retarders to maintain workability.
- Slump loss and setting time are faster in hot weather. Make sure you have enough people on site so the concrete doesn’t get ahead of you.
- Plastic- and drying shrinkage can cause cracking. Shade, wind breaks, and misting can all help slow evaporation. Once you’ve finished the surface, apply the curing compound or wet burlap as soon as possible. You may want to place flatwork at night.
- Type IL cement is much finer than Type I cement. Accordingly, Type IL cement concretes bleed very little, so the surface can dry out much faster. Expect to use more air-entraining and water-reducing admixtures. You may want to use a retarder or hydration control admixture as well.
- Workers are more vulnerable to dehydration and fatigue in hot weather, so provide plenty of water and frequent breaks.
Winter
Winter has become one of the construction seasons. Concreting in cold weather requires special measures to prevent two undesirable outcomes: (1) permanent damage due to premature freezing and (2) failure to achieve the desired strength in time. The American Concrete Institute’s “Guide to Cold Weather Concreting” (ACI PRC306-16) summarizes what to do and why. Here are some key points to keep in mind.
- Never place concrete on frozen ground. Even if it’s just cold, you may want to heat the subgrade.
- Cylinders cool off faster than the concrete in the forms unless you protect them. If they freeze prematurely, they’ll never gain strength. If they cure at temperatures below the allowable range in ASTM C31, they’ll be weak when you transport them to the lab. You may damage them in transport or when removing the molds. Either way, they may fail the strength requirement even if the concrete in place is fine.
- Maturity in accordance with ASTM C1074 will give you the most accurate information about the strength of the concrete in place. Concrete containing Type IL cement or supplementary cementitious materials is sensitive to low curing temperatures. It will gain strength slowly. With maturity data, you’ll know when it’s safe to remove forms, posttension, or open pavement to traffic.
- Thermal modeling lets you predict and control the timing of strength gain. You can try out “what if” scenarios to see the effects of, say, more insulation, weather changes, or additional heat. That way you can get your crews to the job at the right time and stay on schedule. You can also avoid thermal cracking.
- Plastic- and drying shrinkage can be a problem even in winter. Cold, dry air over warm, moist concrete can cause rapid evaporation.
Fall and Spring
The transitions between winter and summer pose their own challenges. Here in the Upper Midwest, it’s not uncommon to see diurnal temperature changes of 30 F. If you place flatwork on a warm day, the temperature drop overnight could cause cracking due to thermal contraction. You may want to place it early in the morning to let it expand and gain strength during the day.
As in winter, maturity data and thermal modeling can inform your decisions about protecting the concrete. Make sure you have plenty of insulating blankets available. You may want to add or remove one layer at a time. You’ll also need to keep an eye on the weather, as a sudden downpour could damage newly placed concrete.
Understanding how concrete works under different conditions will enable you to do good work in all of the construction seasons.