Q.: In concrete buildings I am designing it is important that exposed concrete decks over interior spaces be free of leaks. I need a tested and proven admixture or sealer to provide a permanently waterproof deck. The deck will be exposed concrete, subject to traffic and to the Florida sun.

A.: There is considerable difference of opinion about whether admixtures can effectively reduce the permeability of high quality concrete. Certainly the watertightness of any concrete, with or without an admixture, is lost if the concrete should crack. For this reason a sealant is usually chosen where leakage cannot be tolerated.

Methods of sealing concrete against water are discussed in the articles "Waterproofing Materials for the Earth-Sheltered Contractor" and "Construction Recommendations for Moisture Control," both of which appeared in the September 1980 issue. These supply information that should be useful for your purpose.

You might want to consider post-tensioning, which should guarantee a crack-free roof if done properly. (See the article "Post-Tensioning: It Makes Sense for Earth-Sheltered Homes," February issue, page 105.)