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OCR for page 24
24 A Manual for Design of Hot Mix Asphalt with Commentary
Table 3-1. (Continued).
PG 76 PG 82
Binder Performance Grade:
-10 -16 -22 -28 -34 -10 -16 -22 -28 -34
Design high pavement temperature, °C: <76 <82
Design low pavement temperature, °C: 10 16 22 28 34 10 16 22 28 34
Tests on Original Binder
Flash Point Temperature (T 48), Min., °C 230
Viscosity (T 316)
Maximum value of 3 Pa-s at 135
test temperature, °C
Dynamic Shear (T 315)
G*/sin , minimum value 1.00 kPa, at 76 82
10 rad/s and Test Temperature, °C
Tests on Residue from Thin Film Oven (T 240)
Mass Loss, Maximum, % 1.00
Dynamic Shear (T 315)
G*/sin , minimum value 2.20 kPa, at 76 82
10 rad/s and Test Temperature, °C
Tests on Residue from Pressure Aging Vessel (R 28)
PAV Aging Temperature, °C 100 (110) 100 (110)
Dynamic Shear (T 315)
G* sin , maximum value 5,000 kPa, at 37 34 31 28 25 40 37 34 31 28
10 rad/s and Test Temperature, °C
Creep Stiffness (T 313)
Stiffness, maximum value 300 Mpa
0 -6 -12 -18 -24 0 -6 -12 -18 -24
m-value, minimum value 0.30, at
60 sec and Test Temperature,°C
Direct Tension (T 314)
Failure strain, minimum value 1.0%, at 0 -6 -12 -18 -24 0 -6 -12 -18 -24
1.0 mm/min and Test Temperature, °C
Critical Temperatures, Specification Values,
and Reliability
A unique feature of the performance grading system is that it is based not on the values of a
given property at a given temperature, but on at what temperature a critical value of that property
is achieved. A PG 58-28 binder has a G*/sin value of at least 1.0 kPa at 58°C and 10 rad/s in the
unaged condition and a maximum flexural creep stiffness of no more than 300 MPa at -18°C at
60 s. The two numbers in the performance grade (PG) refer to extreme high and low pavement
temperatures at which the binder is expected to perform adequately. It is important to understand
how these extreme pavement temperatures are defined. The high temperature is defined as
the yearly, 7-day average maximum pavement temperature, measured 20 millimeters below the
pavement surface (referred to as design high pavement temperature). This may seem straight-
forward, but because high pavement temperatures are quite variable, the design high pavement
temperature will vary from year to year and cannot be defined in a precise, single value. Instead,
statistical methods must be used through the concept of reliability. The reliability of a given high
pavement temperature refers to the probability that it will not be exceeded in any given year.
For example, in Saint Louis, MO, the average value of the design high pavement temperature
is 52.9°C. That means that in any given year, there is a 50% chance that the actual high pavement
temperature will be lower than this, and a 50% chance that it will be higher. Therefore, the design
high pavement temperature at a 50% level of reliability for Saint Louis is 52.9°C. At a 98% level of
reliability, the design high pavement temperature is 60.0°C. In other words, in any given year there
is a 98% chance that the maximum pavement temperature in Saint Louis will be less than 60°C.
The same approach is used in low-temperature performance grading. In this case, the low
pavement temperature is defined simply as the minimum pavement temperature at the pavement
OCR for page 25
Asphalt Binders 25
64 -4
design low pavement
Design High Pvmt. Temp., C
Design Low Pvmt. Temp., C
temperature
-7.1oC
58 design high pavement -10
temperature
54.7oC
65%
52 -16
40 50 60 70 80 90 100
Reliability, %
Figure 3-9. Example of PG binder grade reliability
for Atlanta, GA.
surface experienced at a given location in a given year. For Salt Lake City, UT, the average low
pavement temperature is -13.6°C. Thus, the design low pavement temperature at 50% reliability
is -13.6°C. At a 98% reliability level, the design low pavement temperature at Salt Lake City is
-21.3°C. It should be emphasized that the design low pavement temperature is not the same as
the minimum air temperature. Typically, the design low pavement temperature is significantly
higher than the minimum air temperature for a given location. In Salt Lake City, for example,
the average minimum air temperature is -19.6°C, 6 degrees colder than the average design low
pavement temperature.
Figure 3-9 is a plot of performance grade reliability for design high and low pavement
temperatures for Atlanta, GA. In the example illustrated in this plot, at a 65% reliability level, the
design high pavement temperature is 54.7°C, and the design low pavement temperature is 7.1°C.
Calculation of design high and low pavement temperatures at different reliability levels involves
compilation of a wide range of weather data and analysis of this data to produce both average
values and standard deviations for design high and low pavement temperatures for thousands of
sites throughout the United States and Canada. Fortunately, the software package LTPPBind has
been developed to perform these calculations for pavement engineers and technicians. The values
in the examples given above were taken from LTPPBind, Version 2.1. LTPPBind also can generate
various useful plots, including reliability plots like that shown in Figure 3-9. At the time this
manual was being compiled, a new version of LTPPBind--Version 3.0--was in beta release.
This newer version of LTPPBind differs substantially from Version 2.1. The most important
of these differences is that in Version 3.0, critical high temperatures are based not just on pavement
temperatures calculated from historical weather data, but from damage analyses performed using
a newly developed rutting model. Version 3.0, once in full release, should provide better estimates
for design high pavement temperatures in hot, dry climates--situations where earlier versions of
LTPPBind appeared to under-predict high pavement temperatures. The LTPPBind program can
be downloaded from the LTPPBind website maintained by the FHWA.
An important question is what level of reliability should be used when selecting binders.
Engineers and technicians should keep in mind that if a PG binder is selected at a 50% reliability
level, there is a 50-50 chance in any year that the high and/or low pavement temperature will exceed
those for which the binder has been developed. That is, a pavement made using a binder selected
at a 50% reliability level is likely to exhibit rutting and or low-temperature cracking within a few
years. Therefore, high reliability levels should be used when selecting binders. For lightly traveled
rural and residential roads, reliability levels of at least 90% should be used. For interstate highways
