Q.: I am working on providing material for crane counterweights. What is considered a normal weight loss (in pounds per cubic foot) for concrete, from say its plastic weight to a normal exposure weight, after perhaps a month or two? My guess would be in the area of a pound or two per cubic foot.

A.: The weight loss will be a reflection of how much water is lost over time. That will depend on the cement content and water-cement ratio of the concrete as well as its service environment.

Consider a cubic yard of concrete with 500 pounds of cement and a water-cement ratio of 0.5, which would include 250 pounds of water. Dividing by 27 cubic feet per cubic yard, we find the water content per cubic foot is 9.25 pounds. Half of that water will go toward hydrating the cement, leaving around 4.5 lbs of water as relative humidity. If you assume that the concrete will still have a relative humidity of 75% to 80% several months into its service life, then you only have about a pound of weight loss.

Remember this estimate is based on several assumptions that may or may not be completely accurate, but in general it supports the conclusion that the weight loss per cubic foot of concrete as a hardened material will be pretty small. Of course, if a small weight change is a concern, you should also remember that if the concrete is exposed to rain, any water it absorbs will increase the weight.