The Limitations of F-number Specifications

by Bruce Suprenant, P.E. and Ron Kozikowski
Created: January 16, 2013

The penetrations required in this depressed area in an elevated floor slab make it difficult to use 10-foot straightedges and riding power trowels. Specifiers should consider these multiple...
The penetrations required in this depressed area in an elevated floor slab make it difficult to use 10-foot straightedges and riding power trowels. Specifiers should consider these multiple obstructions to finishing operations when determining F-number values. In this case, the specifier required F-numbers in the small depressed floated area that was to receive ceramic tile for bathrooms.
A bullfloat and broom finish for exterior pavement at Baker Concrete Construction headquarters in Monroe, Ohio. Measured floor flatness values ranged from 15 to 20.
A bullfloat and broom finish for exterior pavement at Baker Concrete Construction headquarters in Monroe, Ohio. Measured floor flatness values ranged from 15 to 20.

Construction joints

F-numbers are measured in accordance with the restrictions set forth in ASTM E 1155. One important restriction is that when testing a floor, no test section boundary shall cross any construction joint. As ASTM E 1155 states in Note 4, shown below, traffic usually crosses many construction joints.

NOTE 4—Since construction joints are a discontinuity in the floor surface, measuring across them would introduce statistical anomalies into this test method. Construction joints are therefore excluded from the generation of F-Number statistics. However, since traffic will nevertheless pass across many of the construction joints, a separate measurement and analysis of the joints may be required in order to provide a quantitative measure of the roughness of the joints themselves. Some joints may never see traffic, for example, those along a wall. The particular joints required to be analyzed may be specified in contract specifications, along with a maximum allowable value for qi.

However, in addition to forklift traffic, floor coverings must also cross construction joints. When using sheet vinyl tile over construction joints, a line above the tile that mirrors the joint below is sometimes noticeable. In an attempt to remedy this issue, specifiers are requiring sample measurement lines to be run across the construction joint.

However, section 7.7 of ASTM E 1155 has a provision for separately measuring across construction joints.

7.7 Construction Joints—Where construction joints are required to be measured, periodic measurements of the 24-in. curvature qi shall be taken, transverse to and centered on the construction joint. At least one qi measurement shall be taken on each straight section of joint, with a maximum interval between measurement locations not to exceed 10 ft. These measurement locations shall be recorded.

As Note 4 indicates, construction joints are excluded from the sample measurement lines when obtaining F-numbers for random floors. Measuring across construction joints lowers the F-number values for the entire floor.

Specifiers have decided to ignore ASTM E 1155 and use the Project Specifications to void it by requiring that sample measurement lines cross construction joints when obtaining F-numbers on random traffic floors. This in effect lowers the F-numbers for the floor and subjects the contractor to remediation or payment penalty requirements for the entire floor when there may only be an issue at the construction joint.

Consider for example a floor that measures an FF 68 and an FL 30. What happens if the sample measurement line runs across a 18-inch elevation change at the construction joint? Based on ASTM E 1155 calculations, the FF and FL reduce to 41 and 25, respectively. And if the construction joint elevation changes by 14 inch, the FF is 22 and FL 21.

Specifers should follow ASTM recommendations and separate the measurement of construction joints from the random traffic floors. However, if the sample measurement line crosses the construction joint, the specification limits for F-numbers should be reduced.

Ron Kozikowski, PE, a construction and materials engineer with North Starr Concrete Consulting, P.C., can be reached at Ron@NorthStarrConcrete.com. Bruce A. Suprenant, PE, PhD, FACI, concrete consultant, can be reached at bsuprenant@bsuprenant.com.