We have installed hundreds of thousands of square feet of suspended floor slab on metal decking using lightweight concrete, and we've found it's next to impossible to get a smooth, level floor. All our work is done with power trowels, and when the slab is at its optimum for finishing, the concrete tends to roll slightly ahead of the power trowel as if it were a very stiff clay. This isn't noticeable during finishing but is certainly apparent in the completed slab, particularly as light passes over the surface at an angle. We have spent a lot of money on grinding and patching to produce surfaces that are suitable for floor finishes. Are there any solutions for this problem?
Eldon Tipping of Structural Services Inc., Dallas, says this problem is related to the entrained air normally added to lightweight concrete to enhance workability and pumpability. The entrained air slows bleeding and can cause the surface to crust over before underlying concrete has set. Crusting is even more likely on elevated slabs because there aren't any natural wind breaks. Because the crusted surface looks ready for troweling, finishers get on it too soon and produce the wavy surface you describe. Tipping offers several remedies. One is use of a spray-on monomolecular film that retards water evaporation from the surface. This product is available from admixture manufacturers. If finishers test a section to see whether it's ready for floating and troweling, and find it hasn't set enough, they should respray the test area. If running water is available, another option is using foggers on the upwind side of the slab pour. Landscape nozzles allow you to dial in the desired fogging rate (from 1 to 5 gallons per minute). These nozzles can be mounted on a series of 3-foot-high vertical pipes connected by water hoses and will help to reduce the evaporation rate. Finally, Tipping suggests trying pan floats instead of float blades to improve floor flatness (see Concrete Construction, May 1995, pp. 439-444). Using pan floats on ride-on, nonoverlapping power trowels can raise F-numbers 15 to 20 points. The pan floats can also be used on walk-behind power trowels, but the pan diameter should be less than 30 inches. Larger-diameter pans make the walk-behind machines too hard to handle because of the high torque.