Section 5: Performance Graded Binders (PG Binders)Anchor: #i1013829
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 (PG) binders in contrast to the older system of viscosity graded (AC) binders, which are typically used for surface treatments and aggregate precoating (see Item 300). Generally speaking, any paving mix produced hot through a hot-mix plant should be specified with a PG binder.
In the PG binder system, engineering properties believed to be related to the expected performance are measured at temperatures corresponding to the climatic and traffic conditions (maximum 7-day pavement temperature, minimum pavement temperature, loading duration based on truck speed, and traffic volume) of the pavement location. This allows selection of a binder grade that is specifically suited to the particular highway application.
PG binder grade designations are based on climate parameters, as explained below:
Figure 3-2. Performance Grade Binder.Anchor: #i1013858
5.2 Selecting a PG Binder
A “base” PG binder can be established for any region within the United States with sufficient climate data. Initial selection can be made using TxDOT’s computer program, PG-SPG Grade Selection, or the Federal Highway Administration’s LTPPBind by specifying the geographic location and the desired level of confidence (usually 95% or 98%). TxDOT spreadsheet uses the older version of the LTPP temperature model that was in place when TxDOT implemented PG. The current LTTPbind model is a little different, and may give different grade recommendations. (Usually if there is a difference it’s one grade higher on the top end.). TxDOT has also generated maps ( 95% confidence level, 98% confidence level) showing the climate based grade for each Texas county.
For Texas, at the 95% confidence level, a PG 64-22 base binder can be used in most locations. It is the most commonly used binder by the department and is frequently used for subsurface mixes and low volume pavements. A few areas of the state with desert conditions may require PG 70 rather than 64. Likewise, some of the northern parts of the state use PG XX-28, rather than -22, due to the colder conditions.
“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 stone-matrix asphalt (SMA) and permeable friction course (PFC) require a minimum PG 76-XX binder) to insure reliable performance. Stiffer PG 70 or PG 76 binders are also commonly used for surface mixes in high traffic areas.
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. Also, the term “grade dumping” (decreasing the high binder grade and, in some cases, the low binder grade) has gained usage with the advent of using higher percentages of RAP (reclaimed asphalt pavement) and RAS (recycled asphalt shingles) in new mixtures, to mitigate the effects of binder stiffening caused by recycled binder substitution.
Where the potential for increased cracking problems exist, such as thin HMA surfaces on resilient bases, use caution when bumping the binder high temperature rating higher to address traffic issues. These conditions would typically require a stiffer binder, but 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 of 92°C or more between the low and high temperature portions of the grade. 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, use caution with grade bumping under these circumstances. Strain levels are typically maximized by traffic loading at the bottom of a 2- to 4-in. thick hot-mix asphalt (HMA) layer. Use of a stiff PG binder (such as PG 70 or higher) with recycled materials in these structures will severely limit the fatigue life of the mix. Chemically treating the lower pavement structure is an alternative if thin ACP surfaces are desired.