Guideline for the Design and Use of Asphalt Pavements for ...

Using theoretical analyses of actual conditions performed with models for net heat flow and energy balance, and
assuming typical values for solar absorption (0.90), radiation transmission through air (0.81), atmospheric radiation
(0.70), and wind speed (4.5 m/sec), this equation was developed for the high pavement design temperature:
T20mm = (Tair - 0.00618 Lat 2 + 0.2289 Lat + 42.2)(0.9545) - 17.78
Where T20mm = high pavement design temperature at a depth of 20 mm
Tair = seven-day average high air temperature
Lat = the geographical latitude of the project in degrees.
The low pavement design temperature at the pavement surface is calculated as a function of the low air
temperature using the equation:
Tsurf = 0.859 Tair + 1.7
Where Tair
= 1-day minimum air temperature
The Superpave system allows the designers to use reliability measurements to assign a degree of design risk to the
high and low pavement temperatures used in selecting the binder grade. As defined in Superpave, reliability is the
percent probability in a single year that the actual temperature (one-day low or seven-day average high) will not
exceed the design temperatures. A higher reliability means lower risk. For example, consider summer air
Air Temperature Selection
temperatures in Cleveland, Ohio, which has a mean seven-day
maximum of 32EC and a standard deviation of 2EC. In an average
year, there is a 50 percent chance that the seven-day maximum air
temperature will exceed
32EC. However, assuming a normal statistical frequency distribution, there is only a two percent chance that the
seven-day maximum will exceed 36EC (mean plus two standard deviations); therefore, as shown in Figure 5.1; a
design air temperature of 36EC will provide 98 percent reliability.
Continuing the example, assume that an asphalt mixture is to be designed for Cleveland. Figure 5.2 graphically
represents the statistical variation of the two design air temperatures. In a normal summer, the average seven-day
maximum air temperature is 32EC and in a "very hot" summer, this average may reach 36EC. Using a similar
approach for winter conditions, Cleveland has an average minimum air temperature of -21EC with a standard
deviation of 4EC. Consequently, in an average winter, the coldest temperature is -21EC. For a "very cold" winter,
the air temperature may reach -29EC. The standard deviations show there is more variation in the one- day low
temperatures than the seven-day average high temperatures.