Q.: Job specifications require us to water-cure columns. However, the columns are made with white cement, and water-curing is causing discoloration and staining. It seems to me that I have read that curing at 80 to 85 percent relative humidity is satisfactory for columns. Wouldn't it be satisfactory for us not to cure the columns at all, since the weather has been fairly damp and rainy?
A.: The following comment comes from Section 2.8 of ACI 308.81, Standard Practice for Curing Concrete. "During rainy or foggy weather little or no effort is needed to achieve curing, although protection of the surface against washout or erosion in heavy rainfall may be needed. For a very low humidity environment particular care should be taken to prevent excessive moisture loss from the concrete." Section 2.10 says "Natural curing from rain, mist, high humidity, low temperature, moist backfill, etc., may be regarded as sufficient to provide ample curing when its effect is at least the equivalent of keeping the concrete moist for the first 14 days if made with Type II cement, 7 days if made with Type I cement, or 3 days with Type III cement, if kept above 50 degrees F (10 degrees C), unless otherwise prescribed in the project specifications." Natural curing would seem to be practical on your project, so long as the moist weather continues, provided the engineer who wrote the job specifications agrees. Otherwise, you may want to use some kind of curing enclosure that does not bring the curing membrane into contact with the concrete surface, which could also cause discoloration.