Although most experienced contractors know how to place concrete in ambient conditions, some conditions are better than others. Jerry Holland, director of design services for Structural Services, Dallas, defines the “reference volume” of a slab in relation to the conditions under which it was placed. The ideal ambient temperature is when the day of placement is closest to the yearly average for your location. At that temperature the concrete will experience the least amount of expansion and contraction or volume change at the coldest or hottest time of the year. For most regions of the country that best time is either in the spring or fall.
There are at least four ways that temperature can impact concrete: the temperature of the concrete itself, the ambient air temperature at the time of placement, the ground temperature, and the effect of the sun on the finishing process.
When concrete is too hot
The ideal concrete temperature for flatwork is about 70° F with initial set occurring about three hours after it is batched. When concrete approaches 90° F ready-mix producers typically add up to 10% more water to achieve the same slump. This combination of higher water content and higher concrete temperature results in increased shrinkage, shorter set times, a reduction in air entrainment, rapid slump loss resulting in less time for placement, and a reduction in concrete strength.
Cooling the aggregates or chilling the mix water are standard ways to reduce the concrete temperature. To cool the aggregate, ready-mix producers often sprinkle their aggregate piles with water to take advantage of evaporative cooling. Because aggregate is such a large percentage of the concrete's mass, cooling it has a significant influence on concrete temperature and doesn't cost much.
Mixing the concrete using chilled water can also control its temperature. Batch plants in hot areas of the country, where concrete temperatures are frequently high, often use ice-making equipment to cool their mixes. Cooling concrete by injecting liquid nitrogen is another good technique, but it is expensive and used only in extreme situations.
Replacing some of the portland cement in a mix with either fly ash or slag cement is a good way to increase the time between mixing and initial set when conditions are hot. Retarding admixtures also provide additional time for placement, but when concrete temperatures exceed 90° F the effect of retarding agents wears off very quickly so that when they “let go” there is little time for finishing.
When concrete is too cold
When the weather turns cold, the problem for concrete contractors is achieving initial set in a timely manner and preserving enough heat in the concrete for adequate strength development. When concrete temperatures go below 50° F, it takes considerably longer for initial set to occur, and at 40° F, hydration virtually stops. To achieve adequate strength development, your ready-mix producer can switch to Type III or Type HE cement, if available, heat the mix water or aggregates or both, increase the amount of cement in the concrete, and add accelerating admixtures. On the jobsite your challenge is to maintain ambient and ground temperature.
Few things reduce profit more quickly than flatwork that is so cold it delays set time. After cold concrete is placed there tends to be excessive bleeding also, compromising the finish. In response, by a certain date each year, most ready-mix plants switch to hot water. The goal, according to John Al-binger, president of Davidson Ready Mix, Frankfort, Ill., is to batch concrete so that it leaves the plant at 65° F to 70° F. If this temperature can be maintained on the jobsite, normal three-hour setting times can occur. Chloride and nonchloride accelerating admixtures (NCA) are commonly used to decrease initial set time. Calcium chloride is the most commonly used admixture because of its low price but its effect diminishes when concrete temperatures approach 30° F. Albinger says that a 2% dose of calcium chloride (by the weight of cement) is needed to get the best result. But, chloride admixtures should not be used when steel reinforcement is present in a slab.
When there is steel reinforcement, NCA admixtures should be used. They won't promote corrosion of steel reinforcement and one classification of NCA can reduce the time to initial set for concrete placed in ambient temperatures as low as 20° F. These admixtures, however, still require additional heat for the curing period in order for ultimate strengths to be obtained. The downside of NCA is its cost.
With the exception of metakaolin, it's best not to use pozzolans, such as fly ash or slag cement, in cold temperature placements unless you are willing to pay for accelerators to counter their set-retarding effects or use methods to keep the concrete warm—such as heated enclosures or heating blankets.