Modified Asphalt Binders in Mixtures - Topical Report: Permanent Deformation Using A Mixture With Diabase Aggregate

11. Conclusions

a. Conclusions Provided by the 11 Mixtures

The G*/sind's of the asphalt binders at 50°C and 0.125 rad/s had a high correlation to mixture rutting resistance as measured by the cumulative permanent shear strains from RSCH. The r² was 0.89. (See figure 15.) The number of data points was insufficient for determining if there was a relationship between the continuous high-temperature PG and cumulative permanent shear strain.

The G*/sind's of the asphalt binders at 70°C and 0.9 rad/s had a weak correlation to mixture rutting resistance as measured by the French PRT at 70°C. The r² was 0.70. (See figure 20.) Without EVA, the r² would be 0.88. The continuous high-temperature PG's provided a fair correlation. The r² was 0.80. (See figure 23.)

Grafting and the geometry of the EVA- and SBS-modified asphalt binders had no effect on their rutting resistances at a 5-percent level of significance.

The main objective of this study was to determine which asphalt binders provide high-temperature properties that do not agree with mixture rutting resistance. In general, the number of obvious discrepancies was low. The G*/sind for EVA at 70°C and 0.9 rad/s was found to be low, based on the French PRT.

b. Conclusions Provided by All Mixtures

The G*/sind's of the asphalt binders at 50°C and 0.125 rad/s had a high correlation to mixture rutting resistance as measured by the cumulative permanent shear strains from RSCH. The r² was 0.93. (See figure 16.) The continuous high-temperature PG's provided a fair correlation. The r² was 0.76. (See figure 14.)

The G*/sind's of the asphalt binders at 70°C and 0.9 rad/s had a high correlation to mixture rutting resistance as measured by the French PRT at 70°C. The r² was 0.88. (See figure 22.) Without EVA, the r² would be 0.93. The continuous high-temperature PG's provided a fair correlation. The r² was 0.80. (See figure 24.)

The best correlations between the mixture tests were: (1) RSCH vs. French PRT, r² = 0.76; (2) RSCH vs. FSCH, r² = 0.73; and (3) French PRT vs. Hamburg WTD, r² = 0.69. These relationships should not be used to predict one property from the other. The r²'s are too low for prediction purposes.

The creep slopes from the Hamburg WTD had very low repeatability.

A change in high-temperature PG from 70 to 76 significantly increased rutting resistance based on both RSCH and the French PRT. The reduction in cumulative permanent shear strain from RSCH at 50°C was 37 percent. The reduction in rut depth from the French PRT at 70°C was 21 percent. Based on these reductions, it could be concluded that there can be differences in rutting performance for asphalt binders within a grade, but this conclusion has to be balanced against the increase in the number of grades if the increment between grades was to be reduced.

12. Recommendations

The correlations between mixture G*/sind and binder G*/sind were fair to good. The r² for the 16 materials was 0.79 using 10.0 Hz and 10.0 rad/s, and 0.85 using 2.0 Hz and 2.0 rad/s. There is no fundamental reason for choosing these pairs of frequencies, and they do not relate mathematically to each other. Therefore, the data could be correlated using a matrix of several asphalt mixture frequencies vs. several asphalt binder frequencies.

The asphalt binders should be tested using other aggregate types or gradations, and, if possible, the test temperature for the SST should be increased so that it is closer to the PG's of the asphalt binders.

Determine whether the elimination of the hydrated lime from the mixture caused the change in ranking for the Novophalt and Styrelf mixtures and the changes in the moduli shown in table 1.

13. References

1. K. D. Stuart, W. S. Mogawer, and P. Romero, Validation of Asphalt Binder and Mixture Tests That Measure Rutting Susceptibility Using the Accelerated Loading Facility, Publication No. FHWA-RD-99-204, Federal Highway Administration, McLean, VA, December 1999, 348 pp.
   
   
2. AASHTO Provisional Standards, American Association of State Highway and Transportation Officials, Washington, D.C., April 2000 Edition.
   
   
3. NCHRP Project 90-07, "Understanding the Performance of Modified Asphalt Binders in Mixtures," Work Plan, Study in Progress, National Cooperative Highway Research Program (NCHRP), Transportation Research Board, National Research Council, Washington, D.C., 2001.
   
   
4. AASHTO TP5, "Method for Determining the Rheological Properties of Asphalt Binder Using a Dynamic Shear Rheometer," AASHTO Provisional Standards, American Association of State Highway and Transportation Officials, Washington, D.C., April 2000 Edition.
   
   
5. T. Aschenbrener, "Evaluation of the Hamburg Wheel-Tracking Device to Predict Moisture Damage in Hot-Mix Asphalt," Transportation Research Record 1492, Transportation Research Board, Washington, D.C., 1995, pp. 193-201.
   
   
6. T. Aschenbrener, R. Terrel, and R. Zamora, Comparison of the Hamburg Wheel-Tracking Device and the Environmental Conditioning System to Pavements of Known Stripping Performance, Publication No. CDOT-DTD-R-94-1, Colorado Department of Transportation, Denver, CO, January 1994.
   
   
7. M. Hines, "The Hamburg Wheel-Tracking Device," Proceedings of the Twenty-Eighth Paving and Transportation Conference, Civil Engineering Department, The University of New Mexico, Albuquerque, NM, 1991.
   
   
8. K.D. Stuart, J.S. Youtcheff, and W.S. Mogawer, "Understanding the Performances of Modified Asphalt Binders in Mixtures: Evaluation of Moisture Sensitivity," Publication No. FHWA-RD-02-029, Federal Highway Administration, McLean, VA, December 2001, 17 pp.

Table 15. G*/sind's of the binders vs. the creep slopes from
the Hamburg WTD with the materials listed from highest
to lowest slope (highest to lowest resistance to rutting).

Table 16. Replicate data for the Hamburg WTD.

¹CV = Coefficient of Variation, percent = (standard deviation ÷ average)*100.

Table 17. Rankings by test type with the material
having the most resistance to rutting listed at the top.

Table 18. Numerical rankings by test type where No. 1 has the most resistance
to rutting according to the test and No. 11 has the least resistance to rutting.

Table 19. Coefficients of determination, r², using the data from the 11 mixtures.

Table 20. Coefficients of determination, r², using the data from all mixtures.

Figure 27. RSCH cumulative permanent shear strain at 50°C
vs. French PRT rut depth at 70°C for the 11 mixtures.

Figure 28. Log RSCH cumulative permanent shear strain at 50°C vs.
log French PRT rut depth at 70°C for all mixtures.

Figure 29. Log RSCH cumulative permanent shear strain
vs. log FSCH G*/sind for all mixtures.

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