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

A short coming of the old methods of grading asphalt is
that under the penetration and viscosity grading
systems, two asphalts may have the same properties
at the tested temperature and be much different at
another temperature. This variation is illustrated in
Figure 1-2.
The intent behind the new Superpave system asphalt
binder specification was to enhance an asphalt
pavement’s ability to prevent low temperature thermal
cracking at the cold temperatures and to prevent
permanent deformation or rutting at the high
temperatures.
Performance Graded (PG) binders have two numbers in their designation such as PG 58-34. Both numbers
describe the pavement temperatures in Celsius degrees at which the pavement must perform. The first
number (58 [136E F] in the example) is the high temperature standard grade for the pavement, and the
second number (Minus 34 [29E F] in the example) is the low temperature standard grade. This pavement
service temperature range (58E C to minus 34E C) is the range the pavement is expected to perform. The
high pavement temperature is measured 20 mm below the pavement surface while the low temperature is
that of the pavement surface.
A comparison of viscosity graded binders to approximate PG graded binders is shown in Table 1-3. For
severe temperature variation it is necessary to modify the PG graded binders. Typical modifiers are
polymers and latex. For example, a soft base AC binder is used for the cold properties of the binders and a
modifier is added to control the hot properties of the binder's performance. A general rule of thumb is that if
the sum of the two numbers in the designation of the binder is greater than 90 (know as the rule of 90's), the
asphalt binder in more likely to be a modified binder. For example, PG 58-22, the sum of the two numbers is
80, therefore, PG 58-22 is a non-modified asphalt binder. A PG 76-28, the sum of the two numbers is 104,
therefore, PG 76-28 is a modified asphalt binder. The cost of the asphalt binder increases as the extent of
the modification increases.
Table 1-3
Comparison of Viscosity Graded Binder to Approximate PG Graded Binders 1
Non - Modified Binders Modified Binders
Viscosity Graded PG Graded Viscosity Graded PG Graded
AC- 5 PG 58-28 PG 58-34
AC – 10 PG 58-22 AC – 20 R PG 64-28
AC – 20 PG 64-22 PG 70-28
AC – 20 P PG 76-28
1 Depending on binder supplier –
Chart is used as an approximate comparison of viscosity grades to PG grades
Most geographic regions in Colorado utilize two primary PG grades of asphalt binder. Typically, one nonmodified
grade and one modified grade. For example, CDOT Region 4 typically uses a PG 58-22 and a PG
64-28. Table 1-4 lists the PG graded binders that are utilized in each CDOT region.