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May/June 2009

On the Level

How Flat Is Flat?

How Face Numbers Caused a Revolution in Floor Construction

In the 1980s, dramatic changes swept the concrete floor industry. The changes affected many kinds of floors but were felt most powerfully in the field of industrial slab construction — the laying of ground-supported concrete floors in warehouses and factories. Slab pours doubled and tripled in size. Worker productivity went up. And floors became much flatter and more level. These changes were not independent but connected to each other and to new technology.

At the center of this revolution lay an ASTM standard, E1155, Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers.

Flatness and a Straightedge

Industrial concrete floors have been around for more than a century. Throughout that time, building users have preferred flat floors to bumpy ones, and builders have tried, in varying degrees, to satisfy that preference. Before the 1980s, however, there was little science to their efforts, and the great majority of floors were never tested for flatness. On those rare occasions when testing did take place, it was usually crude and haphazard. Anyone trying to understand floor flatness, whether to solve a problem on an existing floor or to lay a new floor to a high standard, had almost no information to go on.

For most of the 20th century, the method used to define floor flatness was a 10-ft. straightedge.1 But how straight did the straightedge need to be? No one knew. Did you level the straightedge or just lay it on the floor? Some contract documents made the answer clear, but many did not. How many tests did you make? This question was important, but the answer often depended on what the tester wished to prove. A builder who wanted a floor to pass muster might take three or four readings in an area that looked good and then call it a day. An owner who wanted to reject a floor might take a hundred readings and complain if one or two exceeded the tolerance.

With all those uncertainties, it’s a wonder the straightedge tolerance lasted as long as it did without replacement or standardization. Its long survival probably owes less to its merits than to the fact that, for the first three quarters of the 20th century, floor flatness just didn’t matter that much. People might have wished for flatter floors, but the average floor, without any testing, was usually flat enough for most ordinary purposes.

Changing Trucks, Changing Floors

In the 1970s, high-stacking, very narrow-aisle warehouse trucks came into use. Running in aisles about 2 m wide and lifting pallets to 10 m or more, these vehicles needed a flat and level floor, and ordinary industrial slabs proved inadequate for the new vehicles. Some warehouse owners found themselves stuck with costly equipment that could not be used or had to be operated at a greatly reduced capacity because their floors didn’t make the grade.

When these problems appeared, people asked questions. How flat did a floor need to be for the new warehouse machines? Assuming you answered that first question, then how did you go about making a floor so flat? And how flat was the typical floor, anyway?

The first response to those questions was to drag out, both figuratively and literally, the 10-ft. straightedge. Fearing that the typical tolerance, 1/8 in. in 10 ft., was too loose, some designers tightened it to 1/16 in. But when people actually applied straightedges to concrete, they found that almost no floors fully met the 1/8-in. tolerance, much less 1/16 in. Many floors didn’t even meet a tolerance of 1/4 in. in 10 ft. Everywhere you looked, floors were out of tolerance, and yet some of those floors were providing good service.

In 1983 the American Concrete Institute took the step of issuing a public warning. Readers of Concrete International were told that “indiscriminate use of this tolerance [1/8 in. in 10 ft.] when not needed may raise the potential of unnecessary contractual and legal disputes.”2 But the ACI warning only identified the problem; it did not propose a solution. More than six years would pass before ACI officially adopted a replacement for the 10-ft. straightedge.

It slowly became clear that the straightedge method had three serious flaws. Two of those flaws — the lack of a standard test and the unrealistically strict tolerances — could have been remedied with a bit of effort in an ACI or ASTM International committee. A third flaw was more fundamental, however. If we think of the floor profile as forming a wave, the 10-ft. straightedge revealed the wave’s height but not its length (for wavelengths under 10 ft. (3 m)) (see Figure 1). This proved a drastic limitation, because floor bumps are often spaced well under 10 ft. (3 m) apart, and those short-wavelength features have a big effect on both a floor’s appearance and its suitability for wheeled traffic.

Figure 1 — Variation of Flatness Number with Wavelength

A New Floor Flatness Test

A solution came from Allen Face, then part-owner of a small consulting and testing firm that specialized in floors for high-stacking warehouse trucks. Recognizing that the 10-ft. straightedge was unlikely to meet the needs of modern industrial vehicles, Face developed a measuring method over the late 1970s to the mid-1980s. By 1987 it had largely assumed its present form, as presented to the public in The Construction Specifier magazine.3

At the heart of the new method lay a pair of numbers called Face floor profile numbers, or F-numbers for short. The flatness number, FF, measures flatness or planarity — the degree to which a surface approaches a geometric plane. The plane is not necessarily horizontal. The levelness number, FL, measures levelness — the degree to which a surface approaches the horizontal. FF is based on readings of vertical curvature measured over a span of 600 mm. FL is based on readings of slope (difference in elevation) measured over a span of 3 m. Both F-numbers can be, and in practice always are, derived from a single test, which involves taking elevation or slope readings on 300-mm centers along survey lines spaced a few meters apart.

Readings are analyzed statistically to determine the F-numbers with both the mean and the standard deviation entering into the calculations. But the mean is almost always close to zero so that in the end the F-number is inversely proportional to the standard deviation of the property being controlled — 600-mm curvature in the case of FF, or 3-m slope in the case of FL. A higher value for FF or FL denotes a flatter or more level floor. The scale was set up so that most floors would fall between 10 and 100 for both FF and FL.

ASTM Committee E06 on Performance of Buildings developed and standardized the test method for FF and FL. On a parallel track, ACI Committee 117 was working on ways to use F-numbers in concrete specifications, and ACI Committee 302 was looking at the connection between F-numbers and floor slab construction methods.

ASTM’s F-number standard came out in 1987 in two documents: ASTM E1155 for those using inch-pound units and ASTM E1155M for those working in metric units. Then, in 1990, ACI published ACI 117, Standard Specification for Concrete Tolerances, to guide floor designers on how to use F-numbers. The standards needed to replace the old straightedge tolerances were now in place: ACI 117 for design and specification and ASTM E1155 for field testing and analysis.

F-numbers caught on quickly after that. In just a few years ASTM E1155 became the normal method for measuring floor profiles in the United States and Canada. It also found wide usage in Mexico, Brazil and Australia. The United Kingdom also proved receptive to the F-number concept, though not to ASTM E1155 itself, and developed its own method.

The Industrial Floor Tripod

About the time ASTM E1155 went into effect, F-numbers became one leg of a tripod that transformed the industrial floor industry in North America. The tripod’s other legs were the laser screed and the ride-on power trowel. The laser screed, invented in 1985, allowed a single operator to level about 24 m2 of floor area in a single pass, relying on a laser level to establish the floor finish elevation. The ride-on power trowel mounted two (or, less often, three) rotary trowels on a single machine.

Productivity per worker rose dramatically with the introduction of these two pieces of equipment. Seated atop a laser screed, one skilled operator could replace several workers bent over hand straightedges. As the laser screed was helping workers place and level concrete more efficiently, the ride-on power trowel was making them more productive in floating and trowelling the concrete to a smooth finish. One operator on a big ride-on could do the work of several finishers using walk-behind power trowels. According to one claim, “four to five riders can do the work of 25 to 30 walk-behind trowels — or 125 to 150 finishers working on their hands and knees.”4

The standard and the new equipment also enabled bigger slab pours, higher productivity and better floor profiles. Before the change, concrete contractors considered 500 to 1,000 m2 a reasonable day’s work. After the revolution, pour sizes went up to 2,000 m2 and then 3,000 m2. Today, daily slab pours of 3,500 m2 are routine on big warehouse jobs, and pours twice that size no longer make headlines.

Before the revolution, the typical ground-supported floor had flatness (FF) numbers in the twenties. Levelness (FL) numbers typically ranged from the mid-teens to the mid-twenties. After the revolution, it became common to specify FF35 and FL25 for ordinary warehouse and factory floors. By using the new technology tripod, floor contractors found that they could achieve those higher F-numbers at little or no extra cost, even as the amount of concrete placed per day went up. Some designers now specify FF50 and FL30, or even FF60 and FL40, for general purpose warehouses. That would have been unheard of, if not impossible, in the 1980s. And contractors were not working harder; they had just learned how to do their jobs better — thanks to the feedback provided by the ASTM E1155 test. In recent years F-numbers have been edging even higher.

Without ASTM E1155, the laser screed, the slab placing tool used in most of the industrialized world, would not have distinguished itself from other previous ways of placing big slabs such as with a long vibrating screed or even by hand. But those methods produced floors markedly less flat and level than floors placed the traditional way, in narrow strips. What set the laser screed apart was that it allowed big pours; it could put a high degree of flatness and levelness on them — confirmed by ASTM E1155.

And without the laser screed, ride-on power trowels might have remained scarce. Just as big pours existed before the laser screed, ride-on power trowels predate both the laser screed and the introduction of F-numbers. But it took the laser screed, and the big pours that came with it, to justify the widespread use of riding machines.

Effects of the Floor Revolution

ASTM E1155 transformed floor profile testing and advanced a segment of industry. Before the standard was adopted, it’s doubtful anyone in the world made a living by measuring the flatness and levelness of ordinary industrial floors. Today, at least five businesses in North America — four in the U.S. and one in Canada — generate most or a substantial part of their income by measuring floors to ASTM E1155. Dozens of testing laboratories and engineering firms own the instruments for measuring F-numbers and offer ASTM E1155 testing along with their other services. Before ASTM E1155, one company manufactured an instrument designed to measure F-numbers. Now five companies do so: three in the U.S. and two in the United Kingdom. Worldwide, several thousand instruments have been sold.

E1155’s effects have reached far beyond the testing business. When Concrete Construction magazine, in its 50th year, looked back on big stories it had covered in the field of concrete floors, the first story described was the introduction of F-numbers.5

People may disagree on which leg of the tripod — ASTM E1155 F-numbers, laser screed, or ride-on power trowel — contributed most to the revolution in floor flatness. But ASTM E1155 had to come first because the success of the other two depended on it. Without ASTM E1155, there could have been no revolution in the construction of industrial floors.

1. Although the metric equivalent for the tool’s length is 3 m, in the heyday of the straightedge tolerance no one in the U.S. or Canada would have described its length in any units besides feet. Present-day ACI documents, in contrast, explicitly allow either a 10-ft. or a 3-m straightedge, and ASTM E1155 is available in both inch-pound and metric versions.
2. Concrete International, Design & Construction, December 1983.
3. Allen Face, “Floors that Pass the Test,” The Construction Specifier, April 1987.
4. “The Equipment Revolution,” Concrete Construction, January 2000.
5. “Flat Floors, Mega Pours: 1980s Mark the Turning Point in Slab Construction,” Concrete Construction, February 2006.


George Garber is a partner in Face Consultants, Lexington, Ky., and an ASTM International member since 1989 who serves on Committee E06 on Performance of Buildings.