NATIONAL BUREAU OF STANDARDS REPORT

7510

PROGRESS REPORT I STANDARD MEASUREMENTS OF SLIPPERINESS ON

WALKWAY AND ROADWAY SURFACES

by

Thomas H. Boone

and

Charles W, Auld

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March 3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and maintenance of the national standards of measurement and the provision of means and methods for making measurements consistent

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July 1957 to June 1960 (Includes Titles of Papers Published in Outside Journals 1950 to 1959) ($2.25); avail- able from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1004-12-10142 21 May 1962 7510

PROGRESS REPORT I STANDARD MEASUREMENTS OF SLIPPERINESS ON

WALKWAY AND ROADWAY SURFACES

by

Thomas H, Boone

and

Charles W. Auld

Organic Building Materials Section

Building Research Division

Sponsored by

National Bureau of Standards

IMPORTANT NOTICE

NATIONAL BUREAU OF STANC

intended for use within the Govi

to additional evaluation and revie

listing of this Report, either in w

the Office of the Director, Nation however, by the Government age

to reproduce additional copies fo

ess accounting documents Approved for public release by the y published it is subjected

Director of the National Institute of

Standards and Technology (NIST) ^ permission is not needed, on October 9, 2015. ared if that a6

ency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

PROGRESS REPORT X STANDARD MEASUREMENTS OF SLIPPERINESS ON

WALKWAY AND ROADWAY SURFACES

1 . INTRODUCTION

This report covers work done in FY1962 on the development of reference

surfaces to serve as constant standards and for calibrating and comparing

different types of friction-measurement equipment.

The report summarizes certain factors affecting friction, the study of a pendulum-type friction-measurement instrument, and the development and

characteristics of some materials proposed as reference surfaces.

Values of slipperiness measurements on certain walkway and roadway

materials are also given.

2. FACTORS AFFECTING FRICTION OF RUBBER

Friction is not a physical constant of any given material, but a function

of both contacting surfaces, including the properties as well as the

surface geometry of both materials.

The use of rubber in footwear and as tires for vehicles tends to govern

its selection as one of the surfaces used in slipperiness studies.

Tabor [l] has shown that two main causes of rubber friction are shear,

due to adhesion, and hysteresis. Under specific conditions, another

term related to the cohesive properties of rubber may also be involved

in resistance to sliding.

Generally speaking, a specimen of any material sliding on any clean

or contaminated surface develops a resistance force in the plane of

contact. Under specific conditions, this force may be purely viscous

drag. In most practical cases, however, two or more terms are likely

to contribute simultaneously.

The friction attributed to the adhesion, hysteresis, and cohesion terms

are discussed in the descending order of their importance.
2

2.1 Friction Caused by Adhesion

A rubber block placed on a clean surface makes intimate contact with

this surface in small zones, a^, a^, a^, . . . . . .an, distributed irregularly

over the apparent block area, as sfiown" in Figure 1. In these zones, the sum of which is defined as the actual contact area , molecular interaction

forms minute adhesive bonds between rubber and surface. To move the block

in the plane of contact or to slide it with constant velocity requires

force enough to shear these bonds. The shear resistance, F, caused by

adhesion is proportional to the actual contact area; the constant of

proportionality being the shear strength, S„

2.2 Friction Caused by Hysteresis

When rubber slides on a sufficiently lubricated, wavy surface (see Figure

2), direct adhesional contact between rubber and surface cannot develop.

As a consequence, the shear resistance due to adhesion must be zero. Any viscous drag is also negligible if the lubricating film remains thick

enough and the sliding velocity is not excessive. However, a definite

resistance, F, has to be overcome to maintain the sliding of the rubber

block.

On a surface such as that shown in Figure 2, the rubber is subject to

continuous deformation. The process of deformation is restricted to a relatively thin layer of the rubber block, and consists of a compression

phase and an expansion phase. A certain amount of energy is required to

compress the rubber element as it approaches an obstacle. When the block

moves on, the element previously compressed can expand, but owing to

hysteresis, it gives back only part of the stored energy. The difference

between compression energy, Ec, and expansion energy, Ee, is lost to the

rubber and converted into its heat equivalent, AE. To maintain energy

equilibrium, the loss must be compensated by external work done on the

rubber block. Hysteresis is therefore responsible for the resistance

against sliding, F. This resistance, defined as friction caused by

hysteresis is proportional to the hysteresis value of the rubber and the

deformation it undergoes.
FRICTION CAUSED BY ADHESION

L

FIGURE I. SCHEMATIC OF A MICROSCOPIC VIEW OF A SMOOTH, DRY SURFACE.

L, NORMAL LOAD; F, PULL FORCE REQUIRED TO OVERCOME ADHESIVE

SHEAR; S, SHEAR STRENGTH; AND a., a

2 , a3 .....a^ LOCAL

CONTACT AREAS.

FRICTION CAUSED BY HYSTERESIS

L

FIGURE 2 . SCHEMATIC OF A WAVY, LUBRICATED SURFACE. L, NORMAL LOAD;

F ^ PULL REQUIRED TO COMPENSATE FOR HYSTERESIS LOSSES;

Ec, COMPRESSION ENERGY; Ee, EXPANSION ENERGY; AND AE,

ENERGY LOSS (CONVERTED INTO HEAT).
3

2.3 Friction Caused by Cohesion

A knife-edge of finite length by negligible thickness and depth slicing

through rubber has to overcome molecular cohesion. Because of the

finite dimensions of the knife-edge or surface irregularities, this term

is always accompanied by adhesion and hysteresis, and it seems impossible

to separate the two latter terms from the first. It is likely that

cohesion, as such, has very little effect on friction, but contributes

indirectly by augmenting adhesion and hysteresis resistance.

In the general case, friction, F, of the rubber is the sum of the above

three causes:

F = F (adhesion) + F (hysteresis) + F (cohesion)

and the rubber properties influencing these three causes are: 1) composition, 2) physical properties, and 3) geometric properties.

3. PENDULUM METHOD OF MEASURING FRICTION

The pendulum method, based on conservation of energy, is a handy way to

measure the relative slipperiness of surfaces, either in the laboratory or in the field. The principle was first reported by Sigler in 1943 on a large, table-mounted instrument [2], and again by Sigler, Geib, and Boone,

in 1948 on a smaller, portable instrument [3]« The difference between the

initial and final heights of the pendulum head is proportional to the

average frictional force encountered by a rubber block (slider) when

sliding over the surface. The value measured is relative, because the

friction does not reach steady-state conditions. It is therefore referred

to as "antislip coefficient" or "relative skid resistance".

This method approximates the conditions encountered by a tread element

of a tire running under slip, or when the rear edge of a shoe heel

contacts a surface, and in fact is believed to produce values more

nearly representing the critical coefficient of friction than sliding

coefficient. The sliders can be changed readily, but the design of the

pendulum head requires considerable attention.

The Road Research Laboratory of England has developed a pendulum- type

machine known as the British Portable Skid-Resistance Tester (Figure 3.).

This tester is identical in principle with the NBS slipperiness tester.

Considerable attention was given to construction details to insure that

the apparatus would be quick and easy to use, and would maintain accuracy

over long periods. The slider, which is spring loaded, is a three-inch

by one-inch block of rubber that comes in contact with the suface along
Sllflfi

FIGURE

3.

BRITISH

PORTABLE

SKID-RESISTANCE

TESTER

DEVELOPED

BY

THE

ROAD

RESEARCH

LABORATORY

OF

ENGLAND
one edge of the three-inch length. The machine can be adjusted vertically,

so that the length of surface the slider traverses can be controlled.

After the proper adjustments are made, the pendulum and & pointer (the

pointer acts from the same axis as the pendulum) are cocked in a righthand horizontal position. Upon release, the pendulum carries the pointer

through an arc and falls away, leaving the pointer at the furthest point

of the arc traversed by the pendulum. At this point, a. measurement is

recorded from a direct scale. The scale is numbered from 0 to 150, and

the quantity measured is termed '“skid resistance'® by the British.

The British tester, like other pendulum testers, is built around the

principle of the conservation of energy. This principle states that the

energy of a system at given State 1 (E^) minus the energy at State 2 (E2 ) must equal energy lost (Er) when the system is brought from State 1 to

State 2. See Figure 4.

In equation form:

Er = E

1 - E

2 (1)

In the present case, Er is the energy lost by the pendulum as the slider

is dragged over the surface under test and is given by

Er = d • F (2a)

where d is the sliding length and F is the resistance experienced by the

slider over the sliding length d.

Since a coefficient of sliding friction can be defined by

p = F/P (3)

where P is the average pressure on vertical load with which the slider is

pressed against the surface under test, equation 2a may be written as

Er = d|iP (2b)

The energy at State 1 is the potential energy of the pendulum prior to

release and is given by product (ignoring the force of gravity which is

constant for a given location)

E

1 « ho . W (4)

where ho is the height of the center of gravity (C.G) of the pendulum with
CONCEPT

OF

ENERGY

CONSERVATION
5

respect to an arbitrary reference plane (most conveniently the test surface)

and W is the weight of the pendulum head which can be assumed to be concentrated in the C.G. The energy at State 2 is the potential energy of the

pendulum after the slider has been dragged over the surface and is given by

E2 = h • W (5)

analogous to equation (4) . Here h is the height of the C.G. with respect

to the reference plane at the end of the pendulum swing; W having the same meaning as before.

When equations (2b), (4), and (5) are substituted into equation (1), one obtains

W(ho “ h)

dP

4. SELECTION OF RUBBER SLIDERS

( 6 )

A large number of slipperiness tests were conducted with the British pendulum

tester on concrete surfaces and special laboratory constructed surfaces,

using controlled natural rubber sliders provided by the British. Road Research

Laboratory and synthetic rubber sliders made of the formulation used by ASTM

Committee E - 1 7 for test tires. J/

1 /

Oil Extended SBR Tread

SBR 1712 137.50

Zinc Oxide 3.0

Stearic Acid 2.0

Sunproof wax 2.0

Sundex 53 4.0

ISAF 69.0

Sulfur 2.0

Santo Cur e 1.1

Flexamine 1.5

Sheet Cure 60 Min. at

Tire Tread - Hardness (Durome ter

300% Modulus 1400 psi

57 ±2 Taken 24 Hours after Cure.

The specifications for this tire do not appear in an ASTM Standard at the

present time. The tire was developed as a research tool in connection

with the evaluation of road surfaces by Committee E-17 on Skid Resistance,

whose objective is to develop standard methods of testing road surfaces for

their non-skid properties and possibly to develop standard surfaces for use

in this research. The tire was actually developed by the General Tire and

Rubber Co., Akron, Ohio.
- 6 -

The results of these tests are shown in Figure 5. The plotted points represent the average of 15 readings taken at 3 different places on the concrete

surfaces and an average of 10 readings taken at 2 different places on the

laboratory constructed surfaces. It may be noted that most of the points

fall below the line of equality indicating a difference in the two rubbers.

In the area of a 40 to 50 scale reading, the values for synthetic rubber

are approximately 207, greater than those observed with the natural rubber.

Results of Durometer Hardness tests reveal that there is a small but

measurable difference (2 units) in the hardness of these two materials,

with the synthetic rubber being the harder of the two, in tests made at

temperatures of 30°F to 110°F. A 207, lower hysteresis loss of the natural

as compared with the synthetic rubber was also noted.

Additional tests were made with seven British pendulum testers on five

surfaces to compare the two rubber materials and to determine possible

machine variables. The controls instituted in these tests were:

(1) Preconditioning the rubber sliders before testing by making 10 passes

over silicon carbide surfaces with particle size grades of 50, 60,

and 80.

(2) Setting the distance of contact of the rubber with the test surface

at 5 ±1/16 inches.

(3) Making all tests under wet conditions, i.e. uniformly flooding the

test surface between each test with water at 73 ±3°F.

(4) Testing within the same day at one location on the same surfaces.

The plotted points of the average of seven tests with each machine on the

five surfaces are given in Figure 6. On most surfaces, the results give

a high reading (more s lip-resistance) with the synthetic rubber. The

greatest difference between the values for the" natural and synthetic

rubber is shown on the epoxy surface, which has a pyramid surface design.

This is believed to be caused by the difference in hysteresis of the

two rubbers.

Table 1 . lists the instruments, calibration dates, and results of the

above control tests. The following conclusions may be drawn from this

table

(1) With the exception of the steel surface, the synthetic rubber

sliders gave higher readings than the natural rubber sliders (as

shown in Figure 6). On the steel surface, where the hysteresis and

cohesive forces of the rubber were of less significance, the softer

natural rubber with its greater surface contact gave higher results.
SLIP

RESISTANCE

(Natural

Rubber)

70

60

50

40

30

20

.

Key

•-Concrete Surfaces

A~Laboratory Constructed Surfaces

7

/

/

4 /®Sl

/ • « ' 0a

/

'

\ •

W i

•

•

it®

\ \

\

V

9

*«• •• A.

/

/A A

/

a/

10

V

20 30 40 50 60 70

SLIP RESISTANCE (Synthetic Rubber)

FIGURE 5, CORRELATION BETWEEN NATURAL AND SYNTHETIC RUBBER AS

DETERMINED BY ONE BRITISH PENDULUM TESTER.
70

60

50

40

30

20

Key

A-Epoxy surface

-Polished stainless

Steel Surface

•-Concrete surface 1 O-Concrete surface 2

©-Concrete surface 3

ST \\

4

/

/

/

/

a a

A A

/

\\\

’ •• A

3

>0

/

/ /

/ ®

0

o

^

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zpp

0,0

rf*

V \

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20 30 40 50 60 70

SUP RESISTANCE (Synthetic Rubber)

FIGURE 6. CORRELATION BETWEEN NATURAL AND SYNTHETIC RUBBER AS

DETERMINED BY SEVEN BRITISH PENDULUM TESTERS.
TABLE 1.

TABULATED

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(2) Instrument No. 6026 gave consistently lower readings.

(3) No changes in the test surfaces were noted during the test.

5. RESULTS OF SLIPPERINESS TESTS ON FLOORING MATERIALS

To determine the suitability of the British pendulum tester for the evaluation of flooring materials, selected surfaces from different manufacturers

were tested with the synthetic rubber slider (see Table 2). The samples

of the walkway surfaces were tested both before and after four years

exposure to pedestrian traffic. They were applied to one-inch plywood

with adhesive. No floor surface treatment or finish was applied to the

samples during exposure to traffic. Some samples, however, had a factoryapplied finish.

6. REFERENCE SURFACES

Since the scope of this project is the preparation of reference surfaces,

it seems evident that a reliable method of measuring the slip-resistance

of these surfaces must be obtained and that the reference surfaces so developed must react toward the measuring device in the same manner as actual walkway and roadway surfaces.

Some of the variables which affect the results by the British Pendulum

Tester have been discussed. All these testers give useful and valuable

information, but they would be even more useful if correlated with some standard reference surface. When the instrument variables such as rubber

slider, spring pressure, and method of operation are controlled, a more exacting study may be made on possible materials for reference surfaces.

Some materials have been investigated to a limited extent. These include

epoxy surfaces with pyramid surface designs and polished stainless steel

surfaces (see Table 1). Other surfaces such as different grades of silicon

carbide cloth, ground glass, and canvas have also been tried.

The following desirable characteristics for reference surfaces are suggested

1. Ease of reproduction.

2. Ease of shaping or obtaining geometric designs.

3. General magnitude of resistance (i.e. ability to withstand test

conditions)

4. Surfaces within and including the range commonly experienced in

walkway and roadways.

5. Suitable for laboratory use and handling.

—'6. Materials of long-term stability and durability. —^Expendable materials with uniform, reproducible surfaces meeting the

other listed characteristics are also suggested.
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7. Relation with water

a. wettability

b. inert to water

c. texture with respect to water.

7. REFERENCES

[1] Tabor, D., "Friction Between Tire and Road", Engineering 186s

838-840 (1958).

[2] Sigler, P. A., "Relative Slipperiness of Floor and Deck Surfaces",

Building Materials and Structures Report . National Bureau of

Standards, (July 1943), BMS-100.

[3] Sigler, P. A., Geib, M. N e , and Boone, T. H., "Measurement of the

Slipperiness of Walkway Surfaces, Journal of Research . National

Bureau of S

t andards, Vol. 40, p. 339 (May 1948), RP1879.

USCOMM-NBS-DC

U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries out specialized research, development, and engineering in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside of the

front cover. WASHINGTON, D. C.

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