ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 343

parameters at the specified test temperatures. One important difference be-
tween the Performance Grade specifications and the traditional specifica-
tions is in the way the specifications work. As shown in Table 9.3, the
physical properties (criteria) remain constant for all grades, but the temper-
atures at which these properties must be achieved vary, depending on the
climate at which the binder is expected to be used. The temperature ranges
shown in Table 9.3 encompass all pavement temperature regimes that exist in
the United States and Canada.
The binder is selected to satisfy the maximum and minimum design pave-
ment temperature requirements. The average seven-day maximum pavement
temperature is used to determine the design maximum, whereas the design
minimum pavement temperature is the lowest pavement temperature. Since
the maximum and minimum pavement temperatures vary from one year to
another, a reliability level is considered. As used in the Performance Grade,
reliability is the percent probability in a single year that the actual pavement
temperature will not exceed the design high pavement temperature or be
lower than the design low pavement temperature.
It is assumed that the design high and design low pavement temperatures
throughout the years follow normal distributions as illustrated in Figure 9.22(a).
In this example, the average seven-day maximum pavement temperature is
56°C and the standard deviation is 2°C. Similarly, the average one-day mini-
mum pavement temperature is and the standard deviation is 4°C. Since
the area under the normal distribution curve represents the probability as il-
lustrated in Figure 1.19, the range of temperature that satisfies the assumed
probability can be calculated. For example, the range between and 56°C
results in a 50% reliability for both high and low temperatures. By subtracting
two standard deviations from the minimum pavement temperature and
adding two standard deviations to the maximum pavement temperature, the
range between and 60°C results in 98% reliability. In selecting the
appropriate grade, the designer should select the standard PG grade that most
closely satisfies the required reliability level. This “rounding” typically results
in a higher reliability level than is intended, as shown in Figure 9.22(b). Note
that the reliability levels at the high- and low-temperature grades do not need to
be the same, depending on the specific pavement conditions.

- 23°C

- 23°C

- 31°C

Sample Problem 9.1

What standard PG asphalt binder grade should be selected under the following
conditions:

The seven-day maximum pavement temperature has a mean of 57°C and a standard
deviation of 2°C.
The minimum pavement temperature has a mean of

- 6 ° C and a standard deviation

of 3°C.
Reliability is 98%.