WHEN TO USE HOT WEATHER CONCRETING METHODS

Most everyone at one time or another have used concrete for pond constructions. I have done concrete in CA,NV,AZ and in some of hottest weather for these area's. I would like to share an article that is from the "Concrete Network" talking about tips when working with "Mud"  on a hot day.

The obvious first step is to use a knowledgeable ready mix producer with whom you have developed a good relationship. The objective is to get concrete that is as cool as possible. Most references don't give a maximum allowable temperature, but any concrete warmer than about 80° could be a problem.

Your ready mix producer can do several things to keep the concrete cool, starting with the mix ingredients. Since there's more aggregate than anything else in concrete, aggregate temperature has the greatest effect on concrete temperature. Shading of aggregate piles is ideal, although not always possible. Using cool water is another way to get cool concrete. Ready mix producers in hot climates use chilled water or ice to lower the concrete temperature. "We have an evaporative cooler that turns on at 2:00 a.m.," says Frank Kozeliski, president of Gallup Sand & Gravel, a ready-mixed concrete producer in Gallup, N.M. "We can get our water temp down to 40° without turning the chiller on. Very seldom in the summer do we have concrete over 80°. We can't really keep our aggregate out of the sun, but a lot of people keep their coarse aggregate wet and if you can get the wind to blow through it the evaporation will lower the concrete temperature 10° with no problem."

Retarding admixtures are another tool to control concrete in warm weather. Retarder can be added at the plant or on the job site to delay concrete setting time, which can be very quick when the concrete is hot. Retarders give you extra time but they also give the concrete more time to dry out, so curing is critical. Retarders come as straight retarders or as water-reducing and retarding admixtures. Mid-range water reducers often also retard the set and both retarders and water reducers can increase the air content of the concrete. Also, if too much retarder is added to concrete used for a slab it can lead to crusting, where the surface sets but the concrete below is still soft. This can really reduce your flatness and even lead to delamination of the surface. To learn more, check out ACI 212.3, Chemical Admixtures for Concrete, and talk with your admixture manufacturer.

In extreme situations, or when concrete has a long trip to the job site, hydration controlling admixtures are available-find Concrete Admixture Suppliers. Ready mix producers can add this material to delay set by up to 5 hours. Kozeliski routinely uses this to ship concrete as far as 200 miles. "On long haul concrete, we limit mixing to 1 revolution per mile, and we leave the plant at about a 5 or 5½ inch slump so we've reduced the friction between the particles and then it doesn't generate heat in transit."

Another way to slow set times is by using fly ash for part of the cementitious material, although this will change the color, so test it in advance and don't alter the percentage between batches. However, with hot concrete, substituting fly ash may not be very effective. "I don't see that fly ash or slag affects the set time if the concrete temperature is above 75 degrees," says Bob Harris from the Decorative Concrete Institute. "At lower temperatures, it does have a dramatic effect but over about 75° or 80° I don't see it."