80
70
60
50
40
30
0.3
0.4
0.5
0.6
0.7
Cpp
Figure 26. Correlation between on-road British pendulum number and polishing
by projection coefficient Cpp (Tremblay 1995).
ing and selecting aggregates for use in cold
Summary of test equipment
In summary, the British skid resistance tester is
regions pavements and identifies where addi-
commonly used to characterize the changing fric-
tional research is needed.
tional properties of pavement surface. In the U.S.,
laboratory-accelerated polishing tests are uncom-
Soundness test
mon. The frictional values (BPN) are related to field
Most highway departments rely on the sulfate
measurements of skid number. In other countries,
soundness test to determine the frost resistance of
such as Canada, laboratory accelerated tests on
aggregate. This is one of the earliest laboratory
aggregates are routinely conducted using the
tests used to predict the durability of aggregates;
British wear and polish device or using the pro-
it was mentioned in the 1800s and, after some
jection method (in Quebec).
modifications, it has become today's AASHTO
T 104 (1990) (ASTM C 88 1981). Aggregates are
repeatedly soaked in either sodium sulfate or
EVALUATING FROST RESISTANCE
magnesium sulfate solutions and are then dried
OF PAVEMENT AGGREGATES
in an oven. The test simulates the expansive force
The durability of aggregate depends on the
of water freezing within the aggregate pores by
amount of water that freezes within it and on its
growing sulfate crystals in the pores.
ability to resist and/or accommodate the result-
The appeal of this test is its simplicity and the
ing expansion. Thus, durability is related to pore
speed at which it can be conducted. The major
volume--more precisely to pore size distribu-
problem is that it does not duplicate the freezing
tion--and to the aggregate's modulus. Typically,
process in rock. Crystal growth during oven dry-
only those pores that are narrow enough to absorb
ing creates internal expansion that can dilate the
water and at the same time are large enough that
aggregate, but the internal stresses that develop
water can freeze at normal temperatures are the
due to freezing water are not solely related to
problem.
crystal (ice) growth. A major cause of stress dur-
Numerous tests have been proposed over the
ing freezing is hydraulic pressure produced by
years to study the durability of construction
water expelled from the freeze front due to
materials exposed to freezing and thawing, but
expansion of the ice against the pore walls. More-
pavement performance is still the best measure of
over, it is well known that the temperature at
which water freezes within any porous material
aggregate quality. This section reviews testing
varies with the size of the pore: the smaller the
procedures for those most closely related to test-
17
to contents
Previous Page
