Q.: The exposed smooth walls of a commercial office building are not uniform in color. The plywood forms used were new when the job began. We noticed that the forms absorbed a considerable amount of form release agent the first time it was applied to them. As time went on, less form release agent seemed to be needed. The walls also seemed to be a little bit more uniform in appearance towards the end of the job. What could have caused the nonuniformity of color?
A.: Your description suggests that you used too highly absorptive plywood. Nonabsorptive plywood should have been used. Absorptive plywood removes moisture from the concrete mix at the surface next to the form. Some calcium hydroxide from hydration of the cement dissolves in the mix water and this also becomes absorbed. Loss of this light-colored portion of the mix causes the concrete to become darker. The solution to this problem is to use a less absorptive plywood.