ACI 302.1R states the minimum local F-number values are set at 67 percent of the specified overall values; and thus the minimum local value for flatness is FF 13 and for levelness is FL 10. However,
ACI 302.1R also states levelness does not apply to sloped surfaces and, since most exterior concrete surfaces are sloped, the levelness requirement would not apply.
Flatness specification clauses
Flatness specification requirements from MasterSpec and ACI specifications and guides are summarized as follows.
MasterFormat Section 03 30 00–Cast-in-place Concrete
This requires a straightedge or F-number tolerance for trowel finish but no tolerances for scratch, float, or broom finish.
MasterFormat Section 32 13 13–Concrete Paving: Surface
This requires a gap below 3 m (10 ft) long, unleveled straightedge not to exceed 13 mm (1⁄2 in.).
ACI 117-10, Standard Specifications for Tolerances for Concrete Materials and Construction, Section 12, Exterior Pavements and Sidewalks
Surface tolerances specified only for ramps, sidewalks, and intersections. The commentary indicates conventional floor surface tolerances are appropriately applied to areas such as mechanical rooms, non-public areas, or surfaces under raised computer flooring or thick-set tile. Unlike ACI 302.1R, it does not mention parking garage structures.
ACI 301-10, Specifications for Structural Concrete
This requires flatness surface tolerances for scratch, float, and trowel finish, but not for broom finish.
ACI 302.1R-04, Guide to Concrete Floor and Slab Construction
This provides recommendations for trowel finish for different applications, but no tolerance recommendations for scratch, float, or broom finish.
ACI 330.1-03, Specification for Unreinforced Concrete Parking Lots
In any direction, the gap below a 3-m unleveled straightedge resting on high spots shall not exceed 13 mm.
ACI 362.1R-12, Guide for the Design and Construction of Durable Parking Structures
This does not recommend any surface tolerances.
Straightedge vs. F-number
The Commentary for ACI 117-10 indicates an FF of 20 defined as a “conventional” floor classification is approximately equal to a 6 to 15-mm (1⁄4 to 5⁄8-in.) gap under a 3-m (10-ft) straightedge. Conversely, the Commentary states a 13-mm (1⁄2-in.) gap under a 3-m straightedge would be equal to a floor flatness ranging from FF 17 to 28. The Commentary for ACI 117-90 had previously stated a “rough correlation” for a 13-mm gap under a 3-m unleveled straightedge would be an FF of 12.
F-numbers are used extensively for specifying flatness and levelness of interior floor slabs. It appears because of this, and limitations of the straightedge method, some specifiers use F-number tolerances for exterior concrete slabs. Currently, there are no industry recommendations for F-numbers on exterior broomed concrete surfaces. These authors have seen a specification requiring an overall FF/FL of 35/25 for a project. It should be noted that the levelness, FL, specification would be inappropriate if the surface was sloped. However, what is a reasonable FF requirement for a sloped and broomed surface?
Measured flatness on a bullfloat surface for buildings
The lowest FF for a broomed surface would be that resulting from a bullfloat and broom finishing technique. Released in 1989, “Measuring the Quality of Floor Finishes” by D.E. Stephan reported floor flatness measurements from over 4000 readings taken on 11 buildings representing bullfloated concrete surfaces. The average floor flatness for all the buildings was an FF of 20 with individual building measurements ranging from 15 to 25.
Brooming the bullfloated surface imparts a surface texture that reduces the floor flatness below that for just a bullfloated surface. Thus, Stephan’s data sets an upper limit on floor flatness that can be achieved with a bullfloat and broom surface.
Measured flatness on a broomed surface
for parking structures
Floor flatness measurements for broomed slab-on-ground and suspended slabs for two parking structures in Miami, Florida, and Quincy, Washington, are shown below:
- Miami: overall FF = 24; ranges from 21 to 28 (specified 13-mm [1⁄2-in.] gap under a 3-m straightedge);
- Quincy: overall FF = 22; ranges from 19 to 26 (specified overall FF = 25).