Q.: I am designing heated slabs for a do-it-yourself car wash. These will be located in single-car compartments that will be open to the weather on one side. The 5-inch-thick slabs will contain plastic pipe to maintain heat in the slabs at all times during cold weather. The concrete will undoubtedly pick up deicer salts brought in by automobiles and in case of a power failure or other unexpected circumstance the concrete could freeze, so I would like to use air-entrained concrete. Is it all right to use air-entrained concrete in a slab that is to be heated?
Would it help to apply linseed oil or other sealer to minimize the amount of moisture and deicers absorbed? Would it be wise to omit the vapor barrier at the bottom of the slab? Is a 3500-psi concrete adequate? If we use a broom finish for slip-resistance will this detract from resistance to salt scaling?
A.: To answer your questions in sequence: Yes it is all right to use air-entrained concrete in a slab that will be heated, and this will improve the resistance to salt scaling during any shutdowns. If the slab is made with 3/4-inch or 1-inch maximum size aggregate it ought to have 6 percent entrained air. (If the aggregate happens to be only 1/2 inch maximum size it should have 7 percent air, and if 3/8 inch, 71/2 percent air.) A linseed oil treatment would be a good safeguard if the concrete can be adequately cured and then dried for about a month before the sealer is applied (see ACI 302.1R-80, Chapter 9 for method of applying).
It would probably be advantageous to omit the vapor barrier below the slab to allow water to evaporate from the lower surface. A 4000-psi air-entrained mix has been found to have better durability against deicers than a 3500-psi air-entrained mix when used in exposed flatwork. A broom finish is desirable for slip resistance. Such a finish has not been found to make the slab less durable. If possible you should make the broom-channel rivulets run toward the drain.