Acid etch on lightweight deck

Concrete Construction, August, 2002

Q. I am planning to pour a 3-inch-thick concrete slab over a metal deck supported by bar joists. This slab will contain radiant heating tubes and will be the finished floor surface, which will then be used as a platform for the wood-framed house above. Three inches of regular concrete will be very heavy. Is there a composition of concrete that will be lighter and still maintain its durability as a finished floor surface? Note that we are planning an acid-etch stain as the finished surface of the concrete slab.

A. For a lightweight concrete floor, you could use a lightweight (actually low-density) aggregate concrete or cellular concrete. Lightweight concrete, made using a low-density aggregate, can vary in weight from as little as 20 to about 120 pounds/cubic foot. The very-low-density concretes (below 50 pounds/cubic foot) typically have low compressive strength (less than 1000 psi), so those mixes probably would not be appropriate for your application.

Cellular concrete contains stable air or gas cells created either by mixing the concrete with a foam or by a chemical reaction that generates gas. It ranges in density from 50 to 120 pounds/cubic foot. The compressive strength is directly related to the density (lower density equals lower strength) and ranges from as low as 250 psi up to more than 3000 psi.

We don't see why you couldn't do an acid-etch stain on these concretes. The binder is still portland cement; only the aggregate is different (and the bubbles in cellular concrete), and the acid-etch stain reacts with the calcium hydroxide produced by the cement paste in. the mix. We have heard of people doing this and are unaware of any problems.

You can research these materials by checking ACI 523.3 "Cellular Concrete," ACI 213 "Lightweight Aggregate Concrete," or ACI 304.5 "Batching, Mixing, and Job Control of Lightweight Concrete." You could also check with the Expanded Shale, Clay, and Slate Institute, which has lots of information on lightweight (801-272-7070, www.escsi.org).

One last note, be sure the bar joists below are sized to support this floor with little deflection, or you will get a lot of cracking.