Feasibility Evaluation and Guidance Development for Implementing Practical Aging Protocols for Balanced Mix Design (BMD) Verification and Acceptance

There is a consensus among state highway agencies (SHAs) and the asphalt pavement industry that mixture aging should be considered when evaluating the cracking resistance of asphalt mixtures in a BMD framework. In general, aging yields asphalt mixtures with improved rutting resistance but reduced cracking resistance and durability. Field aging of a specific mixture is influenced by production conditions, climate, and sensitivity to oxidation and subsequent rheological response that can vary greatly depending on many mix design factors, which include but are not limited to: selection and proportions of virgin component materials of different sources and types; the use of reclaimed asphalt pavement (RAP), recycled asphalt shingles (RAS), or other recycled materials; and the incorporation of asphalt additives (e.g., warm mix asphalt additives, anti-stripping agents, recycling agents, etc.). Over the years, different long-term mixture aging protocols have been developed for mixture performance testing and prediction. Although some of the protocols have shown promising results in a research environment, they are not practical for use in BMD verification and acceptance (i.e., production quality assurance) where a quick turnaround on the test results for plant-produced mixtures is desired. Because of this limitation, many SHAs elect to either not require BMD performance tests during production or only conduct the tests on specimens that are reheated or short-term aged. Although using the short-term aged test results sometimes discriminates the cracking resistance of asphalt mixtures, this practice may not be sufficient to ensure that the mixtures will have adequate long-term cracking resistance in the field, especially for those containing additives that could affect the aging sensitivity of asphalt mixtures.

To address this limitation, the asphalt research community has suggested a potential approach of using the short-term aged cracking test results along with an aging correction factor (ACF) to estimate the long-term results for mix design verification and acceptance. For this approach, an ACF must be established as part of the mix design development process, or possibly during trial mix production (i.e., test strip), by conducting the selected cracking test(s) at multiple aging conditions. Although this approach appears promising conceptually, its feasibility, robustness, and practicality for use in field practice is yet to be determined. There is limited information available on how an ACF can be developed from cracking test results at different mixture aging temperatures and durations, and how reliable the ACF is in terms of its ability to predict the long-term aged cracking test results. Furthermore, it remains unknown how the ACF varies among asphalt mixtures with a wide range of mix design factors and production and climatic conditions, and whether a relationship between binder aging and mixture aging exists that can possibly be used to simplify the ACF development process.

The overall objective of the proposed research is to evaluate the feasibility and develop guidelines for implementing practical asphalt mixture aging protocols for BMD verification and acceptance. Specifically, this research seeks to: 1) determine the feasibility, robustness, and practicality of using the aging correction methodology to predict the long-term aged cracking test results of plant-produced mixtures (using either hot-compacted or reheated plant mixed-laboratory compacted specimens) for mix design verification and production acceptance; 2) evaluate the sensitivity of the developed methodology to different asphalt mixture components and proportions; 3) provide guidance in the form of a framework for developing a generic or mixture-specific aging correction methodology based on BMD cracking test results at multiple aging conditions during mix design; and 4) suggest a work plan that can be used by SHAs to verify the developed guidelines using BMD shadow or pilot projects.

The results of this research will further enhance the benefits of BMD, yielding improvement in the long-term cracking resistance of asphalt mixtures and reducing SHA’s risk of approving field mixtures with unacceptable cracking performance. Furthermore, this research will help SHAs make more informed decisions with the evaluation and approval of asphalt additives by taking into consideration their potential impacts on aging sensitivity and long-term cracking resistance of asphalt mixtures. Designing and producing asphalt mixtures with adequate long-term cracking resistance in a BMD framework will contribute to extending the longevity and reducing the life-cycle cost of asphalt pavements, while allowing contractors to be innovative with component material selection, mix design, and production practices to ensure performance.

Among the existing long-term aging protocols for mixture performance testing and prediction, the most used ones include compacted specimen aging for 5 days at 85°C (per AASHTO R 30), loose mix aging for 3 to 5 days at 95°C (per recommendations of NCHRP 09-54), and loose mix aging for 6 to 8 hours at 135°C (per recommendations of research studies at NCAT, UW-Madison, and other research organizations). Considerable research has evaluated the effectiveness of these protocols in discriminating the cracking resistance of asphalt mixtures and obtained reasonably promising results in most cases. However, because of their long durations, these aging protocols may not be practical for use in mix design verification and production of asphalt mixtures where a quick turnaround on the test results is desired for quality control and acceptance purposes. Another limitation associated with some of these aging protocols is that they are difficult to accommodate within the laboratory work schedule of SHAs and asphalt contractors. One potential approach to overcome this limitation is to predict the long-term aged cracking test results based on the short-term aged results and a mixture-specific ACF developed during mix design. However, a preliminary literature search only identified a few studies containing BMD cracking test results at multiple long-term mixture aging protocols that can be used to determine the feasibility and robustness of the ACF concept. Furthermore, there is no information or guidance available on how to implement this concept into BMD verification and acceptance in the field.

The results of this research will be used by SHAs and asphalt contractors to produce BMD mixtures with adequate cracking resistance and durability in the field, which has the potential to extend the service life of asphalt pavements while reducing the maintenance and rehabilitation costs from the life-cycle cost perspective. Implementation activities may include: ·

• Development of guidelines (in the form of a framework) to help SHAs implement practical aging protocols for BMD verification and acceptance to ensure satisfactory long-term cracking performance.* If applicable, suggested modifications to AASHTO R 30 on implementing practical aging protocols for field verification and acceptance of asphalt mixtures in a BMD environment.* A national workshop or webinar with invited participants to discuss strategic implementation of the developed guidelines into existing mix design verification and acceptance practices.* Partnership with SHAs that have implemented BMD or are in the process of BMD implementation to conduct field verification of the developed guidelines using shadow or pilot projects in different climatic regions.