Capturing the Durability of High Recycled Binder Ratio Asphalt Mixtures (2024)
Chapter: 6 Summary and Path Forward
CHAPTER 6
Summary and Path Forward
Chapter 6 presents a summary of the key findings from this project and identifies potential areas for future research and implementation activities.
6.1 Summary
This report presents the results of NCHRP Project 09-65, organized into different chapters covering materials, laboratory experiments, and evaluation tools focused on pavement surface mixtures with high RBR, as defined in Chapter 2. Chapter 3 is the assessment of component materials and primary durability issues (surface cracking, moisture susceptibility, and raveling) for the selected mixtures. Further comparisons across all mixtures based on both laboratory and cracking performance modeling efforts are provided in Chapter 4. Chapter 5 presents a cost analysis, comparing each combination of materials that yield durable high RBR mixtures against its corresponding typical RBR mixture.
The extensive data and key outcomes derived from this project are reflected in the draft Standard Practice for the design and evaluation of durable asphalt mixtures, which was sent to the AASHTO COMP for evaluation. Implementation guidelines for field validation of the revised draft Standard Practice were also sent to the AASHTO COMP for evaluation, giving valuable insights to facilitate the development of robust surface mixtures with high RBR content.
There are several factors to consider when aiming to achieve adequate performance and desired durability of high RBR mixtures:
- Component material sources and properties
- Virgin binder
- RAM, including RAM binder and gradation
- Aggregate
- Proportions
- RAM and associated RBR
- RBA
- Total and virgin binder contents
- Additives
- Recycling agent
- Anti-stripping agent
- Warm-mix additive
The development of robust surface mixtures involves selecting material types and sources, balancing material component proportions, and adjusting the mix design to provide sufficient cracking resistance after critical aging while maintaining rutting resistance and without
hindering moisture susceptibility or creating raveling issues. The results of this project highlight that the best high RAM strategy depends on material combinations (component materials, i.e., type and properties, and proportions).
Comparisons with preliminary thresholds in the draft framework (Table 9 and Table 10) informed revision of the draft Standard Practice, which includes a selection of candidate tests to assess resistance to common distresses often observed in asphalt pavements. This framework follows a stepwise approach based on mixture performance tests after appropriate conditioning. Distresses are addressed in the following order: intermediate-temperature surface cracking with aging, rutting, moisture susceptibility, raveling, and, finally, low-temperature cracking for the North/Freeze regions. Similarly, guidelines are provided for screening component materials for their use in high RBR surface mixtures, but final selection is based on material availability and cost, as well as adequate mixture performance. Therefore, the draft Standard Practice was revised to include several updates and modifications, listed as follows:
- Component material selection guidelines with respect to aggregate absorption and refined virgin binder properties
- Additional guidelines on binder blend preparation for stiff RAM materials
- Revised asphalt mixture specimen preparation guidelines
- Mixture performance tests, aging protocols, and thresholds used in this project, organized by climate using the four LTPP climatic zones (Wet-Freeze, Dry-Freeze, Wet–No Freeze, and Dry–No Freeze) and categorized by Freeze or No Freeze
- Stepwise approach to developing robust mixtures that are resistant to surface cracking at intermediate temperatures, balanced with respect to rutting resistance, moisture damage resistant, raveling resistant, and low-temperature-cracking resistant for mixtures in Freeze climates
- Guidelines for high RAM mitigation strategy selection
- Decreased RBA strategy calculation
- Revised RA dose selection method
In addition, this project modeled the cracking performance of pavements with robust surface mixtures under the combined effects of aging, moisture, and traffic following two different approaches. Table 47 provides a detailed summary of the key findings from the project, with check marks indicating the laboratory experiments and their corresponding results that support these observations. These key findings are organized by strategies and their impact on mixture performance, followed by the effects of different component material factors and limitations of the results.
6.2 Path Forward
Based on the results presented in this final report, this section offers suggestions for future research and implementation activities.
6.2.1 Suggested Future Research
Suggested future research based on the outcomes of this project include the following:
- Combined aging and moisture conditioning: Mixtures were characterized after both aging and moisture conditioning in the raveling and pavement performance modeling experiments, but the combined protocols were not included in the comprehensive draft Standard Practice. Further evaluation of these protocols with a wide range of materials combinations may be explored to capture durability more efficiently by considering aging and moisture susceptibility concurrently.
- Rheological evaluation of binders at intermediate temperatures: A unified methodology is needed to select critical test parameters and conditions (temperature, amplitude, and frequency of
Table 47. NCHRP 09-65 key findings.
| Key Findings toward Developing Balanced and Durable High RBR Mixtures | Component | Design | Mixture | Modeling | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| High RAM Strategies Decreased RBA Alternate binder (softer grade, polymer-modified, different source) Recycling agent Adjusted aggregate gradation Combined strategies | Moisture Strategies Liquid anti-stripping agent Lime Other additives Different aggregate type | Binder Aggregate RAM | Binder Content | CTIndex | N12.5 | SIP, SN | TSR | %AL, %AL Ratio | %Cracks (CZM) | Sapp (FlexMAT) | %Damage (FlexPAVE) |
| A stepwise approach can be used to develop robust high RBR mixtures assessed by laboratory mixture performance tests and long-term pavement cracking modeling. | – | – | √ | √ | √ | √ | √ | √ | √ | √ | |
| The use of high RAM strategies to enhance cracking resistance with critical aging and preliminary thresholds did not result in rutting, moisture susceptibility, or raveling issues. The only strategy considered to increase binder content was decreased RBA. | – | – | √ | √ | √ | √ | √ | – | – | – | |
| Selection of the most effective high RAM strategy depends on the material combination (component materials and proportions). For practicality and cost, consider decreasing RBA, incorporating an RA, or employing a combination of strategies for extremely stiff RAP (or high RAM or RAS contents). However, high doses of recycling agents may not be suitable because of their cost or impact on moisture susceptibility (by TSR). | – | – | √ | √ | √ | √ | √ | – | – | – | |
| The most effective high RAM strategies for improving CTIndex involved increasing l75/m75 to improve mixture flexibility and ductility. However, an alternate virgin binder source with higher ΔTc was not effective. | Binder: PG & ∆Tc | – | √ | – | – | – | – | – | – | – | |
| Binder quantity and quality parameters influence mixture durability. The ratio of total binder content to binder blend PGH (Pb/blend PGH) ≥ 7.5 provided acceptable moisture resistance by TSR, and the ratio of RAP binder content to RAP PGH correlated with RAP DoA. | Binder: PG & ∆Tc RAM: PGH, DoA, Binder Content | √ | – | – | – | √ | – | – | – | – | |
| The G-R10Hz and G-R10rad/s parameters with 40-h PAV aging exhibited the best correlations to the CTIndex, and NCHRP 09-59 proposed 10 rad/s loading frequency and temperature based on PGL for this parameter. | Binder: G-R | – | √ | – | – | – | – | – | – | – | |
| RAM properties that facilitate high RAM contents include lower G-R and lower PGH to increase DoA and DWT. ASAs were effective moisture strategies at typical and high RBR values, especially with high DoA. | RAM: PGH, G-R, DoA, DWT | – | √ | – | – | √ | – | – | – | – | |
| Aggregate quantity and (water) AC also influence mixture moisture susceptibility. Mixtures with the maximum product of the aggregate bin percentage and its (water) AC < _30, while also showing Pb/blend PGH ≥ 7.5, provided acceptable moisture resistance by TSR. | Aggregate: AC | – | – | – | – | √ | – | – | – | – | |
| Combined aging and moisture conditioning and characterization of PMA and mixtures need further evaluation. | Binder: ∆Tc, G-R | – | √ | – | – | – | √ | √ | √ | √ | |
- loading) for the evaluation of binders with respect to their contribution to cracking resistance at intermediate temperatures.
- Characterization of polymer-modified binders and mixtures: Current evaluation tools for PMA and corresponding asphalt mixtures are inadequate with respect to capturing the enhanced performance documented in the field and shown in pavement performance modeling.
6.2.2 Suggested Implementation Activities
The following activities are suggested to implement the results of this project:
- Construct field pilot projects: This research effort did not include construction of field projects. Therefore, development and construction of field pilot projects using the provided guidelines would demonstrate the application of the stepwise evaluation approach in the draft Standard Practice. The use of additional material combinations in different climates across the United States would also further verify the framework and preliminary thresholds.
- Monitor performance of existing field projects: Revisiting the seven existing field projects, each with multiple field sections constructed in conjunction with NCHRP Project 09-58 and NCHRP Project 20-44(24), would be invaluable in refining the guidelines and performance thresholds provided in the revised draft Standard Practice (Epps Martin et al. 2019; Leavitt, Epps Martin, and Arámbula Mercado 2023). Additional field distress monitoring would help assess the performance of high RBR mixtures with and without high RAM strategies and associated control mixtures with typical RBR over time, based on corresponding specifications in six SHAs.
- Review existing thresholds: The tools, models, and guidelines developed in this project can be used to review SHA specifications and propose revisions to successfully use RAM, including establishing thresholds for mixture performance tests based on benchmarking, existing field projects, and new field pilot projects. These efforts may be coordinated through one or more national working groups or other research and implementation projects.
