Q.: Recently we completed construction of a hotel parking garage. The lobby level of the garage has a 4-inch topping on a 10-inch post-tensioned slab with post-tensioned beams spaced at 27 feet. The structural slab was not waterproofed before putting on the topping. Because of heavy traffic the topping slab is now cracking and allowing water to pass through and become trapped above the structural slab. Since there is no other escape this water is being forced through small shrinkage cracks in the structural slab, as evidenced by buildup of calcium hydroxide on the bottom of the slab. Is there some way to inject a waterproofing material into these cracks to prevent water penetration?

A.: We have no definite solution but offer the following comment. If there aren't many cracks in the post-tensioned slab and you have good reason to believe that there won't be many more, you might successfully seal them from the bottom by injecting with epoxy resin, as you suggest. You might wish to talk about this with a company with long experience in the field, such as Adhesive Engineering Company, 1411 Industrial Road, San Carlos, California 94070, or Western Waterproofing Company Inc. 1637 North Warson Road, St. Louis, Missouri 63132.

There are also some polyurethane foam grouts that can be injected into moisture-containing cracks to seal them. These are described in the article "An Effective Repair for Leaking Waterstops" published in our June 1984 issue, page 594. One such grout is made by Coatings and Sealers Division, 3M Adhesives, 3M Center, Building 223-IN-071, St. Paul, Minnesota 55144.

If you have many cracks in the post-tensioned slab or have reason to believe that cracks will continue to develop in it, you may want to consider some means of sealing the top surface of the topping to keep water from penetrating it and ultimately the post-tensioned slab as well. There are some elastomeric coatings or membranes that resist wear from traffic and have been used on parking deck slabs. They're applied in thicknesses of about 60 mils. One is made by General Polymers Corporation, 9461 Le Saint Drive, Fairfield, Ohio 45014. Another is made by Dural International Corporation, 95 Brook Avenue, Deer Park, New York 11729. Possibly there are other manufacturers.

If you apply such a coating or membrane over the top it may take awhile before the existing water gets out of the slab. For that reason you may want to devise a way of checking periodically to see that the water in the slab is not corroding the post-tensioning cables.

Two other articles that contain information on sealing parking deck slabs are: "Parking Deck Deterioration the Reasons Why," in our September 1976 issue, page 444, and a short item "High Molecular Weight Methacrylate Used as a Crack Welder," in our October 1984 issue, page 885. The first discusses several approaches to waterproofing a parking deck. The second discusses the squeegee application of liquid methacrylate to the top surface of a bridge deck to seal cracks. High molecular weight methacrylates of this kind are available from Rohm and Haas Company, 727 Norristown Road, Spring House, Pennsylvania 19477.

Silikal North America Inc., 305 Sniffens Lane, Stratford, Connecticut 06497 manufactures a different category of methyl methacrylate used for a similar purpose.