Q.: Is there any current practice or potential for reducing required concrete cover when silica-fume concrete is used? With its greatly reduced permeability, shouldn't you be able to get by with less clear cover? This would be especially helpful for reducing weight in precast products.

A.: Consultant Terry Holland says there have been no reductions in cover requirements for structural members containing silica-fume concrete and none is expected. Silica-fume concrete hasn't been in service long enough to permit evaluation of its overall impact on long-term concrete performance. Only after a great deal of longterm data is available should any such reduction be considered. Until then, use the belt-and-suspenders approach: Add silica fume to cut permeability and resist chloride intrusion, but specify adequate cover as well.