Section 4: Performance Graded Binders (PG Binders)Anchor: #i1010677
The Strategic Highway Research Program (SHRP) produced a system of materials selection, testing, and mixture design named Superpave, for Superior Performing Asphalt Pavements. The Superpave binder specifications are performance-based, therefore, these binders are known as performance-graded binders in contrast to the older system of viscosity AC-graded binders. Generally speaking, any mix used hot through a hot mix plant should be specified with a PG binder.
AC (-5, -10) binders are typically used in surface treatment operations and aggregate pre-coating (see Item 300). Engineering properties believed to be related to the expected performance (maximum 7-day pavement temperature, minimum pavement temperature, loading duration based on truck speed, and traffic volume) are featured in the PG binder specifications to allow proper selection for a particular highway application.
Performance graded binder specifications carry a short-hand designation as explained below:
Figure 3-2. Performance Grade Binder.Anchor: #i1010703
Selecting a PG Binder
A “base” PG binder can be established for any region within the United States with sufficient climate data. For Texas, at the 95% confidence level, a PG 64-22 base binder can be used in most locations. Initial selection can be made using the computer program ( PG Binder Grade Selection), or by using a set of maps ( 95% confidence level, 98% confidence level).
“Bumping” or increasing the binder high temperature rating by one or even two grades is predicated on building in stiffness to handle slow moving or standing traffic or very high traffic volumes. Stiffer binders may be required for certain mix types (e.g., both SMA and PFC both require a minimum PG 76-XX binder) to insure reliable performance.
Another practice not suggested by the SHRP research, but equally valid, is bumping the low temperature rating downward. This practice could be used to address cracking problems in a specific region by expanding the temperature range of the grade without stiffening the binder.
Where the potential for increased cracking problems exists, such as thin ACP surfaces on resilient bases, use caution when bumping the binder high temperature rating higher to address traffic issues. These conditions would require a stiffer binder and may cause or worsen cracking.
In addition to the PG requirement, the TxDOT specification contained in Item 300 also requires an elastic recovery test ( ASTM 6084, Standard Test Method for Elastic Recovery of Bituminous Materials by Ductilometer) for all binders with a spread between the low and high temperature portions of the grade of 92°C or more. The effect of this requirement is to ensure that a polymer modifier is used in producing these binders. Higher temperature spread binders are generally more costly.
Binder selection also depends on the pavement cross section. Thin-surfaced flexible pavements are designed to deflect and rebound once a load has passed. Because sensitivity to strain levels increases with stiffness, stiff PG binders are not designed to be used under these circumstances. Strain levels are typically maximized by traffic loading at the bottom of a 2- to 3-inch thick hot mix asphalt (HMA) layer. Use of a stiff PG binder in these structures will severely limit the fatigue life of a stiff mix.
Additional guidelines in selecting a PG binder can be found in the document, “ Superpave Binder Materials Selection Procedures.”