There are always brief stretches of weather during any season when it is not feasible or advisable to place concrete. Fortunately in Wisconsin, these usually don't last too long. A little bit of common sense, communication between all the parties involved, talent, and care will produce a quality concrete product.
Adverse conditions do not have to mean the end of a building project. However, it does force the builders, contractors, and architects to make additional decisions based upon the working environment. One area frequently overlooked, or misunderstood, is the relationship between temperature and concrete.
Difficult weather does not have to mean the end of placing concrete. Wisconsin, with its warm and gentle summers and mild autumns, often provide the perfect weather for placing concrete. Temperatures frequently range between 50° F and 70° F and do not create problems for the concrete.
However, high heat, driving winds or marginal temperatures provide trying conditions for the placing of concrete. Depending on the weather, preparation, and finishing, temperature may affect the strength, appearance, and price of concrete — factors important to the decision making process. Yet, with a basic understanding of sound concrete practices, concrete may be placed successfully in less than ideal situations.
If the temperature of fresh concrete is 55° F or greater, and if the concrete is maintained at a temperature of 55° F or higher, then winter concreteing should be trouble-free. Approximate set time for concrete at 70° F is six hours. Set time jumps to just over 14 hours if the concrete temperature drops to 40° F. If it drops below this point and the concrete actually freezes early in the process, loss of strength, up to 50%, increases permeability and a lower resistance to weather may result.
The key is to start with warm concrete and keep it warm. The internal heat of the concrete mix may be raised by heating the materials, using extra or special cements, or by addition of accelerators. The environment may also be altered by suing enclosures and moist heat, applying insulating blankets, polystyrene sheets and leaving the forms in place.
Caution needs to be applied when placing concrete in hot weather. Without the proper care, concrete may have reduced strength and will be very prone to cracking due to rapid drying. It also may stiffen quickly making finishing quite difficult.
At some point usually between 75° F and 100° F, hot weather problems for concrete may begin. The combination usually causing the most problems is low relative humidity and high wind velocity. These conditions, when added to sun and high temperatures, create a very high potential for problems.
There are several methods of cooling concrete. The most efficient way is to cool the aggregates, which may be done as simply as sprinkling them with water and allowing the evaporation process to cool them. Other methods of cooling the concrete include using ice or injecting liquid nitrogen into the mixer. However, both methods add cost to the concrete. The contractor should also be prepared with sunshades, windbreaks and other means to prevent rapid drying.
For further information, reference PCA's Design and Control of Concrete Mixtures, Chapter 11
There are always brief stretches of weather during any season when it is not feasible or advisable to place concrete. Fortunately in Wisconsin, these usually don't last too long. A little bit of common sense, communication between all the parties involved, talent and care will produce a quality concrete product.