This was presented at the Utah Department of Transportation
(UDOT) Engineer's Conference held November 20-22, 1997 at Snowbird,
UT. The conference was attended by representatives from UDOT,
Industry, and Academia.
Utah has been one of the lead states in adopting the PG binder grading system proposed by SHRP. Experiences here and elsewhere have been generally positive. Some complications have arisen as validation work continues. Most laboratory and field studies reflect very positive correlations between the rutting parameter, G*/sin d, measured by the Dynamic Shear Rheometer (DSR) and rutting for any given mix design. Long Term Pavement Performance (LTPP) studies are continuing to perfect the air-to-pavement temperature algorithms at high summertime temperatures. Results to date do not suggest that any serious problems exist in the current system. One important issue not yet resolved is the question of "grade bumping" to stiffer binders based on traffic speed and load.
Low temperature cracking also appears to be closely correlated with the "S" and "m" values from the Bending Beam Rheometer (BBR) when conventional straight-run bitumens are used in the mixture. However, the original SHRP assumption that the air and pavement temperatures are equal on the coldest winter night have been found to be inaccurate. Pavements in the LTPP pavement studies are currently being monitored to develop a corrected equation. Significant changes can be expected once these equations become available. The problem is also complicated when asphalt modifiers are used. The Direct Tension (DT) test may be a better predictor of low temperature cracking damage. Utah is researching this problem and has included the Direct Tension test for 1997 specifications.
The fatigue parameter, G*sin d, has not been validated, and continues to be questioned by several asphalt mixture researchers. The binder Expert Task Group (ETG) has taken these questions under consideration and is currently reviewing this parameter. The parameter is important since badly fatigued pavements must be removed and replaced. CALTRANS data shows that highly oxidized asphalts may perform very well in very low strain environments, but may crack severely in fatigue when pavement strains are large. Elastomeric polymers, on the other hand, would appear to offer the greatest economic benefit in higher strain environments along with benefit in low strain environments. This complicates the grade selection issue because both modification techniques (oxidizing and polymer modification) can be used to extend the temperature range to meet current PG specifications. Both modification methods could be used to make the same PG 76-22, but their relative performance in fatigue will vary greatly depending upon the strains developed during loading of the pavement structure. The complexity of the issue has led some states to add a polymer specific identifier test to some grades to insure polymer modification. For example, Georgia is using a DSR maximum phase angle requirement; Kentucky is specifying a elastic recovery test; and Ohio is specifying 5 percent SBS or 3 percent SBR modifier.
The PG specification appears to be working well for straight run bitumens, as long as binder grades are carefully selected to meet the needed mechanical properties of the mixture. This includes consideration of climate, traffic, aggregate quality, pavement structure, moisture damage, etc. Some agencies are now experimenting with Superpave mixture tests or various proof tests to verify the expected mixture performance. Examples include Colorado DOT's Eurolab, Georgia DOT's Asphalt Pavement Analyzer, and Utah's newly purchased Hamburg Wheel Tracking Device.
For modified asphalts, the outlook is more complex. NCHRP Project
9-10, recently awarded to the Asphalt Institute and the National
Center for Asphalt Technology, is specifically charged with evaluating
numerous asphalt modifiers to determine whether the PG specification
appropriately ranks their performance in mixtures.
Return to Articles about Superpave
Return to Superpave Home Page