60
50
Class I
40
30
20
R = 0.90
10
0
10
20
30
40
50
60
70
0
BPN
Figure 21. Relationship between British pendulum number and Illinois skid
trailer number.
not. Another advantage of this device over the
of the pendulum. Test method ASTM E 303 (1995),
BSRT is that it can be used to determine the fric-
Standard Test Method for Measuring Surface Fric-
tional properties of coarse and open graded
tional Properties Using the British Pendulum Tester,
mixes. As with the BSRT, this test has been stan-
uses the BSRT to measure the frictional properties
dardized by ASTM under E 707-90 (1995).
of the microtexture.
As with the BSRT, the friction value is read
The result from the BSRT is the British pendu-
from the scale and is presented as the variable
lum number (BPN), which relates well to skid
numbers obtained in the field (Fig. 21). (The term
Class I in Figure 21 refers to an Illinois DOT road
100
classification system.) Sometimes, the BPN value
is converted and reported as a polished stone
value (PSV). The PSV value is the BPN value at
the end of 6 hours of testing.
80
The advantages of this device are that it is por-
table, has a low initial cost, and can test in differ-
ent orientations. Its disadvantages include that
60
results from coarse macrotexture are question-
able, it can only simulate low-speed skidding
(780 m/hr), and it requires laborious calibration.
To maintain consistency between different
40
testers, a calibration process is used with a stan-
dard aluminum surface. Good agreement has
been reported between two testers calibrated
with this procedure.
20
NCSU variable-speed friction tester
The variable-speed friction tester (VST) is sim-
ilar to the BSRT except that it has a locked-wheel
0
smooth rubber tire at the bottom of the pendu-
80
0
40
60
20
lum. It can also be used in the laboratory and in
Skid Trailer Number, SN
the field. One advantage of this device is that it
Figure 22. Correlation between variable-speed number
can be used to simulate variable vehicle speed,
and skid trailer number at 40 mph.
something the British skid resistance tester can-
14
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