Q: Concrete contractors in our part of the Southwest use diesel fuel to cure concrete. Is this an acceptable way to cure concrete in the desert? It seems to evaporate in a matter of a few hours in extreme temperatures in the summer, but seems effective in the Cooper months, or is this just an illusion? What would you suggest for the best cure, other than water or a cover, in this area?

A: It seems unlikely that diesel fuel would be satisfactory in the summer if it evaporates in a few hours. ASTM Standard Specification C 309, "Liquid Membrane Forming Compounds for Curing Concrete," requires that the concrete lose no more than 0.055 grams of water per square centimeter of surface in a period of 72 hours when tested in accordance with ASTM C 156, "Water Retention by Concrete Curing Materials." This test is performed in a curing cabinet at 100ºF on a specimen coated with a standard amount of liquid membrane curing compound.

If diesel fuel evaporates within a few hours it certainly cannot retain moisture for the length of time required by ASTM C 309. Although diesel fuel may perform better at low than at high temperatures there does not seem to be a good way of evaluating its performance under such conditons.

Concrete itself cures more slowly at low temperatures and therefore needs the benefit of the curing medium longer, thus cancelling out at least part of the benefit of slower evaporation of diesel fuel at low temperatures.

It seems likely that if diesel fuel were a good and acceptable curing agent someone would be marketing it under a trade name.If water or a cover, such as polyethylene or waterproof curing paper, cannot be used the best curing is normally to use a curing compound that passes the requirements of ASTM C 309. If there is any question about its effectiveness at extreme temperatures in the desert in the summertime the curing compound could be applied more thickly