Evaluating the Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios (2020)

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145 Chapter 6 presents a summary of the key findings from this study and suggests further research and implementation activities. 6.1 Summary The results from this study are presented in this report and organized with key results from Phase 1 and Phase 2 highlighted in Chapter 1 and Chapter 2, respectively. Field performance of the test sections in the five field projects is provided in Chapter 3 and compared to corre- sponding laboratory performance at low recycling-agent doses, and expanded laboratory per- formance results at higher recycling agent-doses are shown in Chapter 4. Chapter 5 summarizes the evaluation tools developed as a result of the extensive data generated and analyses conducted in all three phases of this study and included in a draft AASHTO standard practice provided in Appendix I. The comprehensive results presented in this report and documented in multiple publications and presentations indicated that the following factors are not distinct but instead contribute concurrently to determine the performance of mixtures with high RBRs and recycling agents initially and with aging: • Base binder PG and quality (ΔTc); • Binder modification by polymers or WMA or other additives; • Proportions of recycled materials (RAPBR and RASBR); • Recycling-agent type; • Recycling-agent dose; and • Recycled binder availability, which is a function of its aging state and production temperature. These overlapping factors and their interplay highlighted the need to evaluate mixture per- formance and necessitated the development of the following tools that are included in the draft AASHTO standard practice (Appendix I) to facilitate the evaluation of the effectiveness of recy- cling agents in high RBR binder blends and corresponding mixtures initially and with aging: • Component materials selection guidelines based on PGH and ΔTc and limiting RAS content and total RBR. • Recycling-agent dose selection method and materials proportioning based on PGH. • Binder blend rheological evaluation tools with thresholds for PGH and the G-R parameter or Tδ = 45° from DSR testing and ΔTc from BBR testing. • Mixture performance evaluation tools with thresholds for N12.5 from HWTT or APA testing, G-Rm parameter from |E*| testing, FI from I-FIT testing, Sm and m-valuem from BBRm testing, and CRIEnv from UTSST testing. • Recycled binder availability factor for RAP that correlates with PGH. C H A P T E R 6 Summary and Path Forward

146 Evaluating the Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios The evaluation tools for binder blends and mixtures include aging protocols, recycling-agent blending methods, and laboratory tests and corresponding thresholds for adequate perfor- mance. Additional investigations of chemical compatibility of recycling agents with base and recycled binders and representative binder blending were also completed in Phase 2, and labora- tory aging and climate effects were explored for both binder blends and mixtures. Table 41 provides a summary of the key findings from this study that were highlighted at the end of each chapter, and cross-references all of the laboratory and field experiments that justify these findings (Epps Martin et al. 2015, 2017). 6.2 Path Forward Based on results presented in this final report, challenges remain for evaluating recycling- agent effectiveness initially and with aging for mixtures with high RBRs and recycling agents. This section provides suggested areas of future research and implementation activities. 6.2.1 Suggested Future Research Ideas for suggested future research generated during this study include the following: • Moisture Susceptibility: In the process of determining the selected recycling-agent dose to match continuous PGH for the target climate and verifying the rutting resistance of rejuve- nated mixtures, limited HWTT testing was performed. Some mixtures failed HWTT criteria; however, additional dry HWTT testing conducted in the APA Junior indicated these same mix- tures had adequate rutting resistance. These results suggest that adequate rutting resistance can be achieved, but moisture susceptibility may be an issue when recycling agents are used. Further research outside the scope of this study is needed. • Long-Term Aging: Based on the data from NCHRP 09–52, a more significant laboratory LTOA protocol compared to the 5 days at 85°C (185°F) is needed to simulate approximately 7 to 10 years of field aging, when asphalt pavements are most vulnerable to cracking. Recently, several studies including NCHRP Project 09–54 “Long-Term Aging of Asphalt Mixtures for Performance Testing and Prediction” (Kim et al. 2017) have evaluated additional laboratory LTOA protocols, and the findings from these studies are summarized as follows: – Reinke (2015): LTOA protocol of 12 h to 24 h of loose mix aging at 135°C (275°F) was representative of approximately 8 years in Minnesota. – Blankenship and Zeinali (2016): LTOA protocol of 24 h of loose mix aging at 135°C (275°F) was equivalent to 5 to 7 years of field aging. – Elwardany et al. (2016): Oven aging of loose mix was more promising than aging com- pacted specimens; LTOA protocol of 13 to 21 days of loose mix aging at 95°C (203°F) was equivalent to approximately 8 years in Virginia. – Hanz et al. (2016): LTOA protocol of 12 h of loose mix aging at 135°C (275°F) was equivalent to that of 5 days at 85°C (185°F) for compacted specimens. – Kim et al. (2017): Loose mix aging at 95°C for predefined durations (9–21 days) based on climatic data can approximately represent 4 to 16 years of service. Therefore, in future research, LTOA protocols of loose mix aging at 95°C (203°F) or 135°C (275°F) prior to compaction need to be explored. • Rheological Evaluation of Modified Binders: – Historical data from evaluation of unmodified base binders indicate that fracture proper- ties are related to stiffness and linear viscoelastic (LVE) characteristics (Heukelom 1966) and that the embrittlement of binders with aging correlates to the observed reduction in phase angle (Ruan et al. 2003). Furthermore, other frequency- and temperature-dependent

Key Findings Binder Blend Results Mortar Results Mixture Results PG & Δ T c G -R Pa ra m et er T δ = 45 ° SA R -A D , T g, T g E nd C A g A gi ng P re di ct io n PG & Δ T c C I B in de r C on te nt P b M R |E *| & G -R m FI S m , m -v al ue m N 12 .5 C R I E nv D R , G R vs N f Recycling-agent effectiveness must be characterized in high RBR binder blends or mixtures initially and with long-term aging to capture initial compatibility and rheological response to oxidation. — — — — — — — Recycling-agent dose to match continuous PGH for target climate is required for high RBR binder blends and mixtures to maintain durability with long-term aging, with lower dose to restore PGL only sufficient with short-term aging. Recycling agent doses used in the field projects in this study were insufficient with aging. — — — — — — — Recycling agents are more effective in rejuvenating less-aged recycled materials (RAP more than RAS and MWAS more than TOAS) in balanced, limited proportions (< 0.5 RAPBR + RASBR and < 0.15 RASBR). RAS contents should be limited because at typical production temperatures, RAS likely acts as a filler with none of the stiff, brittle recycled binder available for blending. — — — — — — — — — Rejuvenation mechanisms differ by recycling-agent type. — — — — — — — — — — — Chemical analysis of high RBR binder blends with recycling agents is challenging, and additional evaluation tools are needed. — — — — — — — — — — — — — — Use of high-quality base binders (ΔTc > −3.5) improves performance of high RBR binder blends and mixtures with recycling agents. — — — — — — — Recycled binder in RAP and RAS is not 100% available in mixtures, with binder availability dependent on age and climate and proposed maximum limits on PGH of 100°C and 150°C, respectively. — — — — — — — — — — — — — — — Adequate performance for high RBR binder blends with recycling agents can be controlled with proposed thresholds for PGH, G-R parameter, and ΔTc. Crossover temperature (Tδ = 45°) can be used as an alternative approach to the G-R parameter. — — — — — — — — — — — — — Mortar procedures provide realistic assessment of binder blending and narrow the PG UTI as compared to that of a corresponding binder blend. — — — — — — — — — — — — — — Table 41. NCHRP 09–58 key findings. (continued on next page)

Key Findings Binder Blend Results Mortar Results Mixture Results PG & Δ T c G -R Pa ra m et er T δ =4 5 SA R -A D , T g, T g En d C A g A gi ng P re di ct io n PG & Δ T c C I B in de r C on te nt P b M R |E *| & G -R m FI S m , m -v al ue m N 12 .5 C R I E nv D R , G R vs N f Adequate performance for high RBR mixtures with recycling agents can be controlled with proposed thresholds for N12.5, G-Rm, FI, Sm and m-valuem, and CRIEnv. Field performance can be used to establish or verify thresholds for adequate mixture cracking performance or performance of recycled asphalt mixtures with high RBR, and recycling agents can be compared to that of DOT control mixtures. — — — — — — — — — — — Some high RBR mixtures with recycling agent may be moisture susceptible. — — — — — — — — — — — — — — — A binder oxidative aging model can be used to evaluate different binders in different climates and explore the tie between field and laboratory aging. — — — — — — — — — — — — — — — Based on CDD, laboratory STOA of 2 h at 135°C (275°F) plus LTOA of 5 days at 85°C (185°F) was equivalent to approximately 8 or 12 months in service in warmer climates and 20 or 24 months in service in colder climates for mixture cracking resistance and stiffness, respectively. — — — — — — — — — — — — — — Standard laboratory fabrication protocols with STOA produce specimens that represent cores for high RBR mixtures with and without recycling agent. — — — — — — — — — — — — — — 100% addition with a mandatory requirement to ensure adequate mixture rutting resistance is recommended to add recycling agents at doses > 5.0% in mixtures with RAS. — — — — — — — — — — — — — — Modifications are needed for testing high RBR mixtures after long-term aging. — — — — — — — — — — — — — Reheating to produce RPMLC specimens is especially detrimental to high RBR mixtures with recycling agents. — — — — — — — — — — NOTE: — = not applicable. Table 41. (Continued).

Summary and Path Forward 149 properties of viscoelastic materials (such as fracture properties) can be shifted by the LVE time–temperature superposition principles provided that the same molecular motions govern both LVE and fracture properties (Tabatabaee et al. 2013; Roland 2011). A rela- tionship between LVE and fracture properties is commonly accepted for neat asphalt binders and mixtures (not polymer-modified); nevertheless, characterization of polymer- modified binders in the LVE range is arguably effective in capturing the benefit of polymer modification in improving binder cracking resistance. The current PGI specifications, G-R parameter, and Tδ = 45° commonly rank polymer-modified materials in the same range or with poorer performance compared to unmodified binders with respect to cracking, while field experience supports the benefit of polymer modification (Von Quintus et al. 2007). – Considering the Tδ = 45° approach developed in this study, at a given frequency, the polymer- modified materials may exhibit the transition from solid- to fluid-like behavior at a higher temperature compared to unmodified binders. A higher Tδ = 45° implies that there is a wider temperature range in which the material is predominantly storing (G’) stress instead of dissipating (G”) stress due to viscous flow. However, if the failure strength of the polymer- modified material is significantly higher than the accumulated thermal and/or load induced stresses, the material may exhibit satisfactory performance with respect to low- or intermediate-temperature cracking. It is important to highlight that in the parallel aging process of the asphalt binder and the polymer modifier, the first can increase while the second can reduce Tδ = 45°, while both mechanisms are expected to contribute to an overall loss in fracture/fatigue resistance. Therefore, an overall increase in Tδ = 45° with aging is more than likely an indicator of reduced fracture/fatigue resistance in a polymer- modified binder. – Further research is needed for improved characterization/ranking of the cracking resis- tance of polymer-modified binders. DSR-based experimental methods such as the linear amplitude sweep (LAS) test, the multiple stress creep and recovery (MSCR) test, or the bitumen yield energy test (BYET) are available to characterize polymer-modified binders with respect to rutting and cracking resistance. The BYET approach was developed as a DSR surrogate for the ASTM D113 ductility test as a performance index for specification of polymer-modified binders to measure fracture properties (Tabatabaee et al. 2013). – For fatigue evaluation of binders, test temperature selection becomes critical since differ- ent temperatures may result in different rankings. To overcome this issue, test tempera- tures could be normalized to Tδ = 45°, similar to the normalization to glass transition tem- perature previously presented for evaluating low-temperature time-dependent fracture mechanics properties of asphalt binders (Gauthier and Anderson 2006). – Climate-based adjustment of Tδ = 45° thresholds should also be considered in future research to possibly determine an intermediate-temperature PG grade analogous to PGH and PGL. • Chemical Assessment of Recycling Agents: Differences other than carbonyl need to be explored in FT-IR spectra. Although CAg is typically tied to increases in binder stiffness, results in this study suggest that the oxygen uptake versus binder embrittlement in terms of G-R/CAg HS may change significantly when a recycling agent is added to a recycled binder, resulting in additional benefits from recycling agents beyond the initial impact on rheology. • Climate Effects: More research is needed, with additional field projects, to develop more refined mixture cracking resistance thresholds at both intermediate and low temperatures for different climates across the United States. • Specimen Fabrication: Additional validation of the recommended specimen fabrication aging protocols and guidelines for recycling-agent blending by addition, replacement, or a combination of both methods should be completed for mixtures with various optimum binder contents since the amount of total binder in the mixture and other factors, such as binder availability/contribution of the recycled materials and RBR, will likely have an effect on coatability.

150 Evaluating the Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios 6.2.2 Suggested Implementation Activities Ideas for suggested implementation activities generated to apply the results of this study include the following: • Field Demonstration Project: Unfortunately, the majority of the field projects in this study used significantly lower recycling-agent doses than those selected to match continuous PGH for the target climate. Thus, a field demonstration project with a recycling-agent dose selected by the method developed in this study is needed for validation of the evaluation tools also developed in this study. This demonstration project requires a minimum of the following two test sections: one with recycled materials at the maximum proportion allowed by current specifications and one with recycled materials at a higher RBR than that allowed by current specifications and a recycling agent at the dose selected by the method developed in this study. • Review of State Specifications: The tools developed in this study can be utilized to review state specifications limiting recycled materials (RAP and RAS) and propose revisions as necessary based on characterization of commonly used base binders and recycled materials sources and application of these tools. • Field Performance Monitoring: Additional data gathered through continued field perfor- mance monitoring of the field projects in this study could be invaluable in adjusting proposed mixture performance thresholds.