Q: I was called in to advise on problems with a parking structure. The owner is unhappy with the finishing work on all four slab levels. He is refusing to accept nearly 400,000 square feet of surfaces with trowel and bull float marks, footprints, and unclosed ragged rough areas where the concrete was not properly consolidated.
Since preliminary proposals for correcting the situation show costs around $6 per square foot, we're looking for alternative ways to fix it. Fortunately the job is in a southern state where freeze-thaw durability is not a concern. The slab concrete meets strength specifications, testing 4000 psi or better. There has been no auto traffic on the slabs yet.
A.: A thin bonded overlay, such as those used for bridge deck repair, is one possible remedy. Probably the thinnest, and among the most costly, would be a polymer concrete made with resin and aggregates, using no portland cement. These overlays are installed from 1/4 to 5/8 inch thick. Latex-modified concrete would probably cost less. It can be installed in a bonded overlay as thin as 1 1/4 inches, although a minimum of 1 1/2 inches is recommended (see Concrete Construction, March 1987, page 261).
Another overlay alternative is the so-called Iowa low-slump concrete, which can be installed 1 3/4 inches thick. This mix, used on more than 1,000 bridge decks in Iowa, contains 823 pounds of cement per cubic yard, and is placed at a 3/4-inch slump. A mix this stiff requires special consolidating equipment, and some states have been trying the same mix with superplasticizers added to ease placing problems (see Concrete Construction, February 1988, page 131).
Since the parking structure is in a mild weather zone, something less than these bridge deck overlays would be adequate. A thin conventional concrete topping 3/4 to 1 inch thick could probably be installed using a good epoxy bonding agent. Ross Martin of Baker Concrete Construction suggested another economical solution--mechanically texturing or grinding the surface, removing about l/2 inch of depth to get a more satisfactory, uniform appearance. But, he cautioned, you must be sure that this doesn't reduce cover of the reinforcing steel below specified limits. Consider also the possibility of placing well-bonded patches in the distressed areas, then installing a traffic-bearing membrane like those described in our June 1990 issue (page 545). These two-layer systems form a tough, impervious coating. Some are flexible enough to bridge small cracks, and they're available in a variety of colors.