In the United States, most floor covering manufacturers require concrete floors to reach a water-vapor emission rate of either 5 or 3 pounds/1,000 square feet/24 hours before workers can place the floor covering. Installers place the floor covering when the specified emission rate has been reached, as confirmed by a calcium chloride cup test. But research conducted by The Aberdeen Group shows that it's difficult to accurately estimate the drying time needed for concrete to reach a specified vapor-emission rate. Some of the findings of the research:
- Water loss increases as the concrete's water/cement ratio increases; there isn't a constant final w/c or percent of initial mixing water that corresponds with the time at which concrete reaches a desired emission rate.
- Re-wetting of a slab has less effect on the emission rate as the concrete matures.
- Covering a floor, even temporarily, significantly affects the water-vapor emission rate.