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-number specification
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).
It seems like a lot more companies are using that ready-mix concrete. They are able to make up the mix twice as fast. It is yet to be determined if it is as good of quality. We’ll see once a few driveways get built.
It is interesting that a slip-resistant surface is actually required for this type of thing. This makes sense especially in areas where snowy and rainy weather is really common. I know that where I live, parking lots would be treacherous if this type of thing wasn’t required. I appreciate your helpful information on how and why they do this type of thing to exterior concrete.
There is a lot you can do with concrete. I remember laying concrete once and to see all of the work that goes into it is cool. If you want the job done right you have to make sure you lay it correctly and finish it with the broom finish to kept people from slipping.
There are so many techniques and ways of putting in concrete. For something that seems so simple, it gets really complex. It seems like more and more methods are created all the time. Being someone who installs concrete for work, I have to stay on top of all the methods and figure out which one to use.
Thanks for all of the insight about broomed exterior concrete surfaces. You talk about how this method is used to achieve the size effect because with larger surfaces, it becomes difficult to give the concrete a unified and consistent appearance, so the broom finish accomplishes this task. I can see how this is a fairly easy way to make sure that the surface ends up looking polished, smooth, and unified. Thank you again for the insight!
Thanks for your article about concrete construction an safety. I didn’t know that exterior concrete surfaces must have some form of slip-resistant surface. I grew up in the Northwest, and it rained for most of the year. This makes perfect sense, though. You can get highly polished surfaces with concrete. With a slip resistant surface it should help to minimize falls.
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Thanks a lot for sharing info about concrete surfaces in such detail. I found the size effect aspect especially interesting and relevant, being in the commercial and industrial flooring industry myself.
1-how answered about these comments:
-Broom finish is not straight, not consistent, and not uniform at many panels.
-The corrugated depth of broom finish is not uniform, Texture finish appeared rough &light at many places, discoloration on concrete finish observed and marks appeared on the texture finish.
Great share. I appreciate your post as it is a good tip for homeowners. It is quite useful to know that asphalt driveway needs regular seal coating. My parents living in new Bedford ma, they are going to update with a new asphalt coating because their old concrete driveway is cracked and falling apart. They need to find a good contractor for getting the job done with a low-cost budget on stamped concrete cape cod, so it’s a simple thing for them if you can recommend one. Thanks, Doris Brown