Most contractors and engineers agree that elevated concrete slabs don't always deflect as intended. Measurements are necessary to identify the areas in which practice departs from theory and to determine what adjustments are needed to improve the quality of the next concrete slab placement. A continuous cycle of placement, measurement, evaluation, and adjustment produces high-quality elevated concrete slabs and satisfied owners.
The F-number system is the approach now recommended by ACI 302 for evaluating floor surfaces. Elevation point measurements are taken on 12-inch centers along a straight line, and the resulting data are used to arrive at a statistical estimate of the floor's flatness and levelness. Flatness numbers of 25 and above can be easily achieved by using a straightedge during finishing. Use screed rails or pipe screeds instead of wet screeds to provide immediate and precise elevation control during strikeoff. Tolerances in ACI 117 provide a 1.5-inch envelope within which floor elevations must fall. Also, for a slab thickness of 12 inches or less, ACI recommends a tolerance of plus three-eigths inch and minus one-quarter inch. However, contractors will find they cannot meet flatness, levelness, and elevation tolerances without the flexibility to adjust concrete thickness.