Field Verification of Proposed Changes to the AASHTO R 30 Procedures for Laboratory Conditioning of Asphalt Mixtures (2019)

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1 S U M M A R Y Field Verification of Proposed Changes to the AASHTO R 30 Procedures for Laboratory Conditioning of Asphalt Mixtures NCHRP Project 09-52, “Short-Term Laboratory Conditioning of Asphalt Mixtures,” had two goals: (a) to develop a laboratory short-term aging protocol to simulate the aging and asphalt absorption of an asphalt mixture during production and transportation based on factors thought to affect aging, and (b) to develop a laboratory longer-term aging protocol to simulate the aging of the asphalt mixtures after construction. The project was divided into two phases. Phase I evaluated short-term aging protocols for hot mix asphalt (HMA) and warm mix asphalt (WMA) to simulate plant aging, and Phase II evaluated long-term aging protocols to quantify field aging after construction. The factors included in the selection of test sites were WMA technologies, plant mixing temperatures, aggregate absorption of asphalt binder, mixing plant type, climate, reclaimed asphalt pavement (RAP) and/or recycled asphalt shingles (RAS) content, and asphalt source. Nine field sites in eight states were constructed to provide the factors and materials necessary to complete this research. Each field site had one or more of the factors listed pre- viously as its primary study component. During the construction of the field sites, mineral aggregates and binders were sampled to fabricate laboratory-mixed, laboratory-compacted (LMLC) specimens to replicate conditions at mixture design. Mixtures produced by the asphalt plant were sampled and compacted on or near the job site to provide plant-mixed, plant-compacted (PMPC) specimens, and roadway field cores were obtained immediately after construction and, if possible, at intervals up to 2 years after construction. Findings from NCHRP Project 09-52 were published in NCHRP Report 815: Short-Term Laboratory Conditioning of Asphalt Mixtures (Newcomb et al. 2015). The laboratory short-term oven aging (STOA) protocol used in NCHRP Project 09-52 was based on the procedure developed for NCHRP Project 09-49, “Performance of WMA Technologies: Stage I—Moisture Susceptibility” [published in NCHRP Report 763: Evaluation of the Moisture Susceptibility of WMA Technologies (Epps Martin et al. 2014)]. This procedure was to age the loose mix sample at 275°F (135°C) for 2 hours for HMA and at 240°F (116°C) for 2 hours for WMA. The STOA matched PMPC very closely. For the laboratory long-term oven aging (LTOA) protocol, two procedures were evaluated. The LTOA procedures were to apply the STOA conditioning on loose mix and then subject the compacted samples to either (a) 185°F (85°C) for 5 days, or (b) 140°F (60°C) for 2 weeks. Of the factors listed, all except mixing plant type and production temperature were found to affect the short-term aging of the mixtures. The LTOA procedures resulted in plant stiffness values for LMLC mixtures that mimicked 9,100 cumulative degree-days (CDDs) for the material aged at 140°F (60°C) for 2 weeks and 16,000 CDDs for material

2 aged at 185°F (85°C) for 5 days. These CDD values reflected 7- and 11-months’ time, respectively, in warmer climates and 12 and 22 months, respectively, of field aging in colder climates. After NCHRP Project 09-52 was completed, the panel determined that a project extension was needed to explore longer ranges of aging to obtain more data on longer-range models and the longer-term effects of the factors listed previously. With the help of the state departments of transportation (DOTs) and contractors involved in the first effort, NCHRP Project 09-52A accomplished this. It provided data that showed the plateauing of stiffness with time and allowed for the development of better long-range models for aging of asphalt mixtures. Separating the data between cold and warm climates provided better fitting models for long-term aging and showed that the rate of aging was essentially the same between the two, although the magnitude of aging was much higher for the warm climate model than for the cold climate model. The LTOA procedures used in NCHRP Project 09-52 simulated a short period of time for field aging. For NCHRP Project 09-52A, leftover loose plant mix from South Dakota was used to investigate whether a more aggressive LTOA method would better simulate a longer period of field aging. The LTOA on the South Dakota mixture entailed aging the loose mixture at 185°F (85°C) for 5 days. Because a greater amount of the surface area of the mixture was exposed to air, more oxidation took place, resulting in a stiffness that would have taken place at 134,000 CDDs. Then, compacted specimens were aged at 185°F (85°C) for 5 days. Results show that the more aggressive method using loose mix resulted in stiffnesses simulating 7 to 10 years of field aging for warmer climates and 12 to 14 years for cooler climates. The effects of the factors used in the construction of the NCHRP Project 09-52 test sections were evaluated to see if their impacts had changed with time. All factors that were shown to have an impact on aging during NCHRP Project 09-52 continued to show impacts in NCHRP Project 09-52A. The effect of aggregate absorption with time for the Florida and Iowa test sites showed that the initial differences that occurred between the high- and low-absorptive aggregates diminished with time. The differences in asphalt source were maintained between the two binders in the Texas II field site, although there was not a substantial difference in the initial AASHTO M320 grading of the binders. Mixtures con- taining RAP at the Texas I and New Mexico field sites continued to show higher stiffness values than virgin mixtures. The impact of WMA technologies decreased with time for most of the field sites so that the stiffness values converged with time. The Florida site was the only exception, in which the difference in the WMA stiffness remained substantially lower than in the HMA stiffness. Production temperature continued to show no impact on aging. Finally, samples left over from NCHRP Project 09-52 and extra samples from NCHRP Project 09-52A were tested to show how aging impacts the cracking resistance of asphalt mixtures. Using the Indirect Tensile Asphalt Cracking Test (IDEAL-CT), samples were tested over a range of 5,000 to 90,000 CDDs. Although this was just an exploratory effort, there was a clear trend showing a decreasing cracking resistance with aging. Suggestions for future studies include the following: • Refine the global aging models’ relationship to climate. • Find a better means to characterize asphalt binder aging to differentiate asphalt sources. • Conduct a study on the impacts of aging on the fracture resistance of asphalt mixtures.