Concrete Slab Finishes and the Use of F-number System
Matthew Stuart, P.E., S.E., F.ASCE
Course Outline
The
F-number system is used today to specify and evaluate the levelness and flatness
of both slab-on-grades and elevated slabs. This one hour online course covers
a basic understanding of the need for and the derivation of the F-number system
used to specify and determine slab levelness and flatness.
This
course includes a multiple choice quiz at the end.
Learning Objective
At
the conclusion of this course, the student will:
Course Introduction
Because of the inherent inadequacies of the straightedge specifications historically used in the construction industry a new system to control concrete floor slab flatness and levelness was developed. The new method was made possible through the development of equipment that was able to accurately and systematically quantify the levelness and roughness of slabs. ACI Committee 117 in turn established a system to evaluate the data from the testing equipment. This method is referred to as the Face Floor Profile Numbers or F-number system and is used today to specify and evaluate the levelness and flatness of both slab-on-grade and elevated concrete slab construction.
Course Content
Deficiencies
of the traditional straightedge specifications:
Starting in the 1970’s, ACI targeted the conventional 1/8” in 10’-0” floor finish, or “straightedge spec”, as an area that needed a more uniform and realistic specification and monitoring method. This need was felt because of the ambiguous nature of the traditional straightedge specification. For example 1/8" in 10'-0" could mean either of the two conditions (See Figure 1)
In addition,
the straightedge method had no standard test method; therefore field conditions
were often subject to interpretation. For example does the spec 1/8" in 10'-0"
mean that measurements are taken at points both 360° and 90° from each other?
Are measurements cumulative along lengths greater than 10’-0”? Finally, the
straightedge specs do not address floor flatness (roughness or wash boarding)
or levelness (tilt or slope).
What
are F-numbers?
The F-number
system is a statistical method to evaluate horizontal concrete surfaces for
flatness and levelness. The Face Company, in conjunction with ACI Committee
117, developed the basis for the system through the use of a patented continuous
profileograph machine. For a typical random traffic industrial or other similar
floor slab there are two F-numbers used to evaluate the floor flatness and levelness,
FF and FL, respectively. FF (flatness) is most
affected by the finishing operation. FL (levelness) is most affected
by forming and strike-off. Both numbers are dimensionless and the higher the
number the more accurate the floor construction.
The FF
number relates to the rate of change of the elevation of a concrete floor
over a 12" interval. The rate of change of slope is represented by the
value q. A hump in a slab produces
a negative q value while a trough or depression produces a positive q.
FF is calculated using the following formula:
FF
= 4.57 / ((3 x Sq) + (| q |))
Where:
Sq = the standard deviation of q
| q |
= the absolute value of the mean of q
(See Figure
2)
The FL
number relates to the general floor slope of a concrete slab from an assumed
datum elevation. The difference in elevation of two points which are 10'-0"
apart are measured as the value z. FL
is calculated using the following formula:
FL
= 12.5 / ((3 x Sz) + (| z |))
Where:
Sz = the standard deviation of z
| z |
= the absolute value of the mean of z
(See Figure
3)
For defined
traffic industrial floor slabs, such as very-narrow-aisle warehouses where
forklifts travel the same path, an Fmin numerical system is used.
Fmin is not calculated like FF or FL. Fmin is actually a direct measurement
of the floor surface in the wheel path of the forklift for which the defined
traffic area is designed. An Fmin100
specification is a baseline number corresponding to a flatness tolerance of
1/8" in 10'-0". An Fmin50 specification corresponds to a flatness tolerance
of 1/8" in 5'-0", or 1/4" in 10'-0".
How
are F-numbers specified?
ACI 117
is the industry standard for the use of the F-number system and establishes
the F-numbers of different types of floors as well as the definitions of acceptable
variances. ACI 302 also provides guidelines for the placing and finishing
of concrete floor slab construction for the F-number system.
ASTM E 1155 is the basis of the standard test method for the FF
and FL number system. One
of the most important aspects of establishing F-numbers is to not over specify
beyond that which is really required for a given project. This is because
an increase in required floor quality will almost always result in higher
costs. One of the best methods for determining the
most appropriate criteria to specify for a project is to measure existing
floors that are performing satisfactorily. Specifications should state the
performance criteria only and not specify the method of obtaining the required
tolerances. This is recommended because the quality of work varies from finishing
crew to finishing crew for any given standard technique used in the field.
FF
and FL are typically specified for both overall and minimum
local values. This helps prevent the acceptance of isolated local areas of
low quality construction due to it being averaged in with other areas of the
slab that surpass the minimum requirements. The specified overall values describe the flatness
and levelness values that must be achieved when all measured values on a given
test surface are combined. The minimum
local values describe the flatness and levelness values below which repair
or replacement is required within the effected area. The general rule of thumb
is that the local area minimum must be within 2/3 of the specified overall
value. On elevated slabs the FL
levelness tolerance does not apply because of the deflection of the overall
framing system and should therefore not be specified. Only FF numbers should be specified for elevated slabs.
Lift and
forktruck manufactures publish recommended Fmin floor tolerances
to insure the proper operation of their equipment. The higher the number the
flatter and more level the floor requirement is. Fmin100 is typically
referred to as a "Superflat" floor requirement. Producing an Fmin100
tolerance in any direction on a floor is comparable to producing a floor surface
meeting FF140 and FL100 tolerances.
An additional
ASTM specification, E 1486 can be used to determine the deviation from a 10-foot
straightedge for both random and defined traffic floors in order to determine
the "Waviness Index". In addition this specification provides defined
wheel track equations for specifying elevation differences and change in elevation
transversely between left and right wheel tracks and longitudinally from front
to rear axle and can therefore be used as a standard for evaluating Fmin
floor tolerances.
The following
table can be used as a guideline for specifying F-numbers for various floor
profile categories. In addition, although there is no direct equivalence
between F-numbers and traditional straightedge specifications the second table
provides an approximate correlation between straightedge measurements and
FF numbers.
(See Table
1)
Recommended
F-Numbers for Various Floor Profile Categories
|
|||||
|
Floor Profile Category |
Random Traffic Floors |
Defined
Traffic Floors |
|||
|
Specified Overall Value |
Minimum Local Value |
||||
|
FF |
FL |
FF |
FL |
Fmin |
|
|
Conventional (using bullfloat) |
19 |
13 |
13 |
10 |
19 |
|
Conventional (Using highway straightedge) |
25 |
17 |
13 |
10 |
25 |
|
Good |
38 |
25 |
19 |
13 |
38 |
|
Flat |
50 |
33 |
25 |
17 |
50 |
|
Very Flat |
75 |
50 |
38 |
25 |
75 |
|
Superflat |
100 |
66 |
50 |
33 |
100 |
|
Ultraflat |
150 |
100 |
75 |
50 |
150 |
Approximate Equivalence of FF Numbers and Straightedge
Measurements
|
|
|
F-Number |
Gap Under 10’-0” Straightedge |
|
12 |
1/2” |
|
20 |
5/16” |
|
25 |
1/4” |
|
32 |
3/16” |
|
50 |
1/8” |
How
are F-numbers measured ?
Floor
flatness and levelness tests should be conducted anywhere from within 16
to 72 hours after completion of the final troweling operation, depending
on the project type and requirements. As it is typically not practical to
obtain acceptable local values across construction joints it is recommended
that these areas be omitted from consideration for comparison to the specified
minimum local value. It should also be noted that it is entirely possible
that the floor can be within specified tolerances during the initial testing
process but become out of spec prior to actual use because of slab cracking,
curling or sub-grade settlement.
ASTM E1155
is the governing specification for the testing procedures used to determine
floor flatness and levelness for random traffic floors. This standard allows
for numerous techniques for gathering the required date including levels,
inclinometers, profilometers and other means. One of the most often used
pieces of equipment for these measurements is the Dipstick floor profiler,
an apparatus that can field measure both floor flatness and levelness.
This equipment can be programmed to measure floor slabs on slopes
also.
For
defined traffic floors such as rack aisleways where the wheel traffic occurs
in the same location for all operations, instead of random sampling, traffic
paths are directly measured using a continuous recording floor profilometer
configured to run in exactly the same specified wheel tracks as the forktrucks.
The results of the measurements taken do not produce F-numbers, but
instead simply confirm compliance with the minimum requirements specified. Changes in elevation and rate of change are confirmed in both the
longitudinal and transverse direction of the wheel travel. The record of
measurements indicates the exact location of any areas that are not in compliance.
Concrete
Slab Finishing Specification Example:
The following
is an example of a performance-based specification for concrete finishing
as it relates to the F-number system. Text highlighted and located within
brackets indicates alternate or optional items depending on the nature of
the project:
Concrete
Finish: Concrete finish floors shall have a [hard steel troweled finish; light broom finish] unless
indicated otherwise on the drawings. Place, strike off, consolidate, level
and float to the proper elevation. Troweling
shall begin after surface has received a float finish. The slab drying must
proceed naturally and must not be hastened by the dusting on of dry cement
or sand. Lightly tool all edges at construction joints and exercise care
that slab edges are not depressed along bulkheads during finishing operations,
particularly hand troweling. Exterior slabs, sidewalks, pads and ramps shall
have a light broom finish unless indicated otherwise on the drawings.
Provide standard trowel finish at all sub-slabs.
Random
traffic floors shall conform to the following surface profile tolerances:
Floor
Flatness Number: FF
Specified
Overall Value = [35]
Minimum
Local Value = [24]
Floor
Levelness Number: FL
Specified
Overall Value = [25]
Minimum
Local Value = [18]
Defined
traffic floors shall conform to the following surface profile tolerances:
If a portion
of a floor does not meet the specified F-number the following remedies are
recommended:
Course Summary
The F-number
system established by ACI 117 should be used in lieu of the old straightedge
specification for the construction of concrete floor slabs. Two distinct
and separate F-numbers are used for specifying floor flatness (roughness)
and levelness (tilt or slope) for slabs subject to either random or defined
traffic patterns. ASTM E 1155 is
the industry standard for testing of random traffic floors. The ASTM E 1486
test procedure can be used to determine the Waviness Index for either random
or defined traffic floor slabs. F-numbers are measured in the field typically
using either a Dipstick (for random traffic floors) or a profilometers (for
defined traffic floors). Project specifications should provide for a remedy
for out-of-tolerance floor slabs.
Additional Resource
Effective
F-Number Specification by Allen Face, FACI, President, Allen Face &
Associates
Related Links
For additional technical
information related to this subject, please visit the following websites or
web pages:
Allen Face & Company,
L.C.
American Concrete Institute
Acceptable Tolerances
for Flatness in Concrete Floors
Concrete Slabs and the Use of the F-Number System
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.
