VAA Virginia Asphalt Fall/Winter 2019

VAASPHALT.ORG 29 A VIEW FROM THE LAB Figure 1. I-FIT Test Results for All Mixtures. I-Bars indicate one standard deviation in results. Figure 2. Cantabro Mass Loss Results for All Mixtures. I-Bars indicate one standard deviation in results. fatigue (AASHTO T 321) and the Texas Overlay Test (TX-248-F-09), involved apply- ing a repeated load. The beam fatigue test incorporated three replicate specimens at three strain levels for a total of nine beams for each mixture type. Specimen failure was defined as the number of cycles at which beam stiffness degraded to 50% of the initial flexural stiffness. With the Texas Overlay test, loading was applied for 1,200 cycles or until a reduction of 93% or more of the maximum load was reached. In each case, more cycles to failure are better. Gener- ally speaking, the Texas overlay results were consistent with beam fatigue results and intuitively consistent with stiffness behavior—when the dynamic modulus and binder stiffness increased, fewer cycles to failure were observed. The third test for cracking resistance was the Illinois Flexibility Index Test (I-FIT), which was conducted per Illinois Test Procedure 405 using an instrumented load frame. The I-FIT is used to produce a Flexi- bility Index (FI)—higher FI’s are associated with better cracking resistance. Virginia has yet to relate FI’s to field performance, but Illinois targets an FI of 8.0 or higher for good cracking resistance. Figure 1 shows the average I-FIT results from all of the mixtures that were sampled during these trials. Consistent with Illinois’ research, the highest FI numbers were observed with virgin mixtures compared to higher RAP mixtures. All of the 45% RAP mixtures showed brittle behavior. With the exception of the lone virgin mixture (0% RAP), the Illi- nois FI criteria would indicate that all these mixtures were cracking susceptible. Durability The final laboratory performance test sought to assess material durability. It did this using the Cantabro Mass Loss Test AASHTO TP 108-14. Relative abrasion loss is considered a durability indicator for dense-graded asphalt. Figure 2 presents the average mass loss results for all the mix- tures. In general, the Cantabro test results showed adequate durability for most of the mixtures. The highest mass losses were experienced by 30% and 40% RAP mixtures. General Observations and Looking Forward Despite mixed results from early laboratory testing, it is important to emphasize that production and laydown went well with these materials, and field performance to date remains good. Except for one 45% RAP mixture, all mixtures in these trials met VDOT’s requirements for volumetrics and gradation. Laydown presented few nota- ble issues, and VDOT’s requirements for in-place density were achieved. Permeabil- ity was acceptable (met requirements) with the presence of natural sand in some mix- tures associated with even better in-place properties. The reader should also note that VDOT has implemented important design revisions since these trial mixtures were designed, produced, and placed. These revisions, which include reduced design compaction and new gradation requirements, would likely have resulted in more balanced results in a similar laboratory evaluation. Moving forward, and speaking of balanced, this generally intuitive interplay of basic con- stituents (recycle vs. virgin) and laboratory test results are reason for some optimism, especially as agencies and industry move together towards performance-based (or balanced) design requirements. Designers (and their technology suppliers) should be encouraged that agencies are developing confidence in laboratory performance tests that will enable innovation that optimizes material use/re-use and maximizes profits. When they can remove or reduce many of the traditional constraints, agencies should expect much improved combinations of pric- ing and performance. For much more on these “very high RAP” trials, please see the full report by Nair and company at vtrc.virginiadot.org . Look for report #VTRC 20-R9. Flexibility Index, Fl 9 8 7 6 5 4 3 2 1 0 SR 3 20% RAP PG 70-22 30% RAP PG 64-22 40% RAP PG 64-22 45% RAP PG 64-22 40% RAP PG 58-28 0% RAP PG 70-22 30% RAP PG 64-22 40% RAP PG 64-22 45% RAP PG 64-22 CR 639 US 60 1.20 1.21 1.11 Brittle Brittle 7.0 2.94 Brittle Brittle Cantabro Mass Loss, % 20% RAP PG 70-22 30% RAP PG 64-22 40% RAP PG 64-22 SR 3 CR 639 US 60 Sherdon Street 45% RAP PG 64-22 40% RAP PG 58-28 30% RAP PG 64-22 40% RAP PG 64-22 0% RAP PG 64-22 30% RAP PG 64-22 40% RAP PG 64-22 45% RAP PG 64-22 12.0 11.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0