Common problems in F-number measurement arise on many jobs. Following ASTM E 1155 "Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers," is essential.
During the summer of 1999, a concrete contractor placed an exterior parking lot for a commercial building in the Midwest. The concrete was air entrained and received a broomed finish, followed by 3 days of moist curing with plastic-coated burlap. Because some of the concrete set at a different rate than the surrounding concrete, the appearance of the broomed finish wasn't uniform and "soft surface conditions" were reportedly exhibited. The parking lot owner questioned the quality of the concrete, and had cores sent to a test lab. The petrographic report stated that the coarse aggregates were generally sound but the entrained-air content in the top 1/4 inch of the two samples was inadequate to resist freeze/thaw damage. The petrographer further concluded that freeze/thaw damage was expected to be a problem for the concrete slab.
In structures for which rebar corrosion isn't an issue, calcium chloride is the most economical and effective accelerating admixture. It's widely used in a liquid solution in unreinforced pavements, residential foundation walls, and floors placed in cold weather. It helps concrete set and gain strength faster, allowing finishers to complete their work sooner and decreasing the likelihood of damage caused by early freezing.
Many building owners and contractors have reported experiencing project delays while waiting for slabs made with lightweight concrete to dry to the moisture-vapor-emission rate typically required by floor-covering manufacturers (3 pounds per 1000 square feet in 24 hours). Though we couldn't find any data regarding the drying time of lightweight concrete, field experience tells us that lightweight concrete takes longer to dry than normal-weight concrete. To help fill this information gap, Concrete Construction devised a testing program to find out how long it takes lightweight concrete to dry.
