Section 4: Performance Graded Binders (PG Binders)
Anchor: #i1010677General
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, hence 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 still 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: #i1010703Selecting 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 (PGEXCEL3.EXE), 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 are also required/desired for certain mix types (SMA and PFC both require a minimum PG 76-XX binder) to insure reliable performance.
Another practice that was not suggested by the SHRP research, but which is equally valid, is bumping the low temperature rating downward. This could be done to address cracking problems in a specific region, by expanding the temperature range of the grade without stiffening the binder. Bumping the binder high temperature rating higher to address traffic issues where there tend to be cracking problems or resilient bases may cause or worsen cracking, since that would ultimately call for a stiffer binder.
In addition to the PG requirement, the TxDOT specification contained in Item 300 also requires an elastic recovery test ("Tex-539-C, Measurement of Elastic Recovery of Tensile Deformation Using a 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 generally are more costly.
Binder selection will also depend upon the pavement cross section. Thin-surfaced flexible pavements are designed to deflect and rebound once a load has passed. Stiff PG binders are not designed to be used under these circumstances since sensitivity to strain levels increases with stiffness; strain levels typically developed at the bottom of a 2- to 3-in. thick hot mix asphalt (HMA) layer 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.”