Q.: We are the architects on a concrete house that is to have an exposed-aggregate finish produced by sandblasting. A 12-foot-high continuous wall, which comes out of a hill, is more than 200 feet long and is being placed in sections, using 2500-psi concrete. Now, when an infill section is about to be placed, the owner is panicking. He is worried that the strength is not sufficient for so high a wall and wants to switch to 3000-psi concrete. Our design shows the higher strength is not needed and the mix has been placing well up to now. We are concerned that additional cement would cause an objectionable color difference. This wall is the most obvious part of the house.

A.: You would be inviting trouble with appearance if you were to change the mix at this point. If your wall is already satisfactory in all respects it would be best to stick with it.