Concrete Slab Flatness

Slab flatness

The degree of a slab’s flatness is expressed by two numbers, the FF and the FL.  Together, these are the F-numbers.  They are the Face floor profile numbers developed by Allen Face from 1978 to 1987.  They describe a concrete slab’s flatness (FF) and levelness (FL).  Flatness is how close to geometric planarity a slab approaches.  The profile of a slab consists of concrete waves.  These waves have amplitudes and wavelengths.  FF is a statistical measure of the roughness of a slab’s surface caused by the presence of waves.

Levelness is the amount of slope (or pitch or tilt) in a slab.  This is the degree to which the surface of the slab approaches true horizontal perfection. 

Flatness numbers run on a scale from 10 to 150 and most slabs fall into the range of 15 to 100.  The higher the FF number, the flatter the slab.

The waves are omnidirectional and are not as if a concrete slab was the ocean and the waves are rolling in toward the shore in a single profile.  The slab’s waves are in all directions and so there is an almost infinite number of wave profiles in a slab.  The FF number is a statistical measure of this omnidirectional unevenness.

Needs for different amounts of flatness

Depending on the building type, different minimum FF and FL values are needed.  Some need very flat slabs.  Some, not so much.  Here are some examples.

Offices with fixed partitions               FF 20 / FL 17
Offices with moveable partitions        FF 30 / FL 25
Stores                                            FF 20 / FL 17
School classrooms                          FF 20 / FL 17
Gymnasiums                                   FF 40 / FL 30
TV studios                                       FF 100 / FL 50
Narrow aisle warehouses                   FF 100 / FL 50

FF values use these terms as convention:

Conventional   FF 25
Good              FF 38
Flat                FF 50  
Very flat          FF 75
Superflat         FF 100
Ultraflat           FF 150

The average industrial floor is FF 22 and FL 18.  This is sufficient for many uses.  A general purpose warehouse needs from FF 50 / FL 30 to FF60 / FL 40.  From 1960 to this day, most slabs fall in between FF 15 and FF35.  A slab with FF 13 / FL 10 is the lowest acceptable for any slab for any use.  A slab whose F-numbers are less than these is just not very good.

Another example of the need for different degrees of flatness is that a surface that is to be tiled needs to be more flat than a surface that is to be carpeted.

Floor traffic

The advent of warehouses using high stacking narrow aisle warehouse trucks in 1975 caused a need for slabs even flatter than “very flat”.  These are also called Very Narrow Aisle (VNA) lift trucks.  There are two types of floor traffic on concrete slabs:  Random traffic and Defined traffic.  A defined traffic floor has vehicle traffic that moves along the same tracks on the slab all of the time.  There is never a differing or random movement of any vehicle on the slab ever.  So, only those specialized buildings such as warehouses using VNA lift trucks in a narrow aisle storage environment would have defined traffic.  A typical industrial building has random traffic. Random traffic floors use the normal FF / FL F-numbers.  Defined traffic floors use an Fmin number.  It differs from the normal FF / FL F-numbers in that it is measured with a different instrument and is a continuous measurement.  A defined traffic floor cannot be described with the random traffic floor F-numbers.  They are measured differently. 

A random traffic floor can be superflat by having an FF 100.  A superflat defined traffic floor is defined by Fmin = 100.  However, an Fmin 100 surface is comparable to an FF 140 one.  This is kind of like figuring out a man’s IQ from his Wonderlic test score.  IQ = (2 x Wonderlic score) + 60 gives you a fair approximation.

Though superflat and ultraflat floors can be constructed as random traffic slabs, this is a very, very unusual need.  Something very special needs to be going on to have a superflat random traffic concrete slab.  A superflat slab costs an additional $1 to $1.50 per sqft more than a less flat slab.  This is due to the extra labor costs needed to produce such a slab.

Measuring and calculating flatness

F-numbers are dimensionless.  They are linear in value.  That means that an FF 18 slab is exactly half as flat as an FF 36 slab.  An FF 99 slab is three times as flat as an FF 33 one.  The F-number is inversely proportional to the standard deviation of the curvature of the slab’s waves for FF and of the slope for FL.

Flatness formula
FF = 4.57 / (( 3 x std deviation of a ) + ( | a | ))
            where a is the change in height over a one foot measurement interval and | a |
            is the absolute value of the mean of a

Levelness formula
FL = 12.5 / (( 3 x std deviation of b ) + ( | b | ))
            where b is the slope of the slab and  | b | is the absolute value of the
            mean of b

There are two kinds of F-number specifications, SOV’s and MLV’s.  Specified Overall Values and Minimum Local Values.  The SOV is the combined FF and FL values on the entire tested surface.  The MLV’s are within smaller areas of the slab’s surface.  Each individual MLV must be at least 67% of the SOV.  If an MLV is not, it has to be fixed so that it reaches the 67% value.

F-numbers are statistics of the amount of waviness in the slab.  They are an analysis of a collection of many measurements made in a defined manner at specified intervals over the surface of the slab.  ASTM tells you how to make and collect these measurements to arrive at the slab’s F-numbers.  You make 34 measurements per 1,000 sqft of slab surface area.

Slab on grade and elevated slabs

Since elevated slabs possess deflection due to the overall framing system and not just of the concrete slab in theoretical isolation, you cannot have meaningful FL numbers for them.  Elevated slabs can only have FF numbers.  But, under special circumstances, they can be determined and considered.  This would be when the slab remains supported in its as-cast position and there is no camber present.

Elevated slabs should never have an FF below 20.  SOG’s whose use doesn’t need to be very flat can go on down to FF 13 (the minimum).  An SOG with an FF 12 or less is not good.  It is a pretty wavy slab.

What causes the two F-numbers?

Flatness, FF, shows you how well the finisher worked the surface.  Levelness, FL, tells you how skillfully the contractor set the side forms and struck off the concrete.  The FL number has nothing to do with the finisher’s workmanship or knowledge in producing an FF number.

A flatness timeline

A history of events in modern flatness

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