The Superpave mix design system was originally intended to include performance testing in addition to volumetric design for moderate and high traffic level pavements. However, the proposed tests for rutting and cracking resistance were not practical for routine use and were never implemented. During the early days of Superpave implementation, rutting resistance was the primary focus, prompting changes such as increased compactive effort, stiffer binders, and more angular aggregates. Many states also added testing requirements for the Asphalt Pavement Analyzer (APA) or the Hamburg Wheel-Tracking Test (HWTT). Today, most states indicate that rutting has been virtually eliminated and that various forms of cracking are now the controlling factors in asphalt pavement service life. Construction quality issues and failure to address underlying pavement distress can contribute to increased cracking. However, issues related to laboratory compactive efforts, aggregate specific gravity measurements, and the increased use of recycled and innovative materials, which cannot be fully assessed with volumetric mix designs, are also believed to contribute to cracking. Many highway agencies and contractors are contemplating a new era of mix design and quality assurance using mixture performance tests to improve asphalt pavement performance.
Balanced mix design (BMD) is an alternative method of designing asphalt mixes using performance tests on appropriately conditioned specimens to address multiple modes of distress while considering mix aging, traffic, climate, and location within the pavement structure. With BMD, mixes are designed to achieve a balance between rutting and cracking resistance using practical mixture performance tests. The BMD concept was initially developed at the Texas A&M Transportation Institute (TTI) using the HWTT and the Overlay Test (OT) to evaluate rutting and cracking resistance, respectively. Performance test results were used in addition to traditional volumetric criteria to determine the design asphalt binder content and grade that provided satisfactory resistance to both rutting and load-associated cracking.
NCAT recently completed NCHRP Project 20-07/Task 406, with the objective of developing a framework to address alternate approaches for BMD. Existing knowledge gaps were also identified, allowing research problem statements (RPSs) to be developed for future research that will help facilitate BMD implementation.
As part of the NCHRP project, NCAT conducted a survey of state DOTs regarding the current use of performance testing and BMD practices. Of the 47 states that responded, six already use BMD. Iowa, Illinois, Louisiana, New Jersey, Texas, and California all use some form of BMD, but the performance tests, criteria, and adherence to Superpave volumetric criteria differ among these DOTs. Many other DOTs require a single performance test in their mix design specifications but are not using a “true” BMD approach, which the FHWA Task Force on BMD defined as addressing multiple modes of distress. A majority of states showed interest in constructing BMD trial projects to compare performance with mixes designed using volumetric criteria alone.
Based on the survey results and a literature review, NCAT developed a framework for BMD in the form of a draft AASHTO standard practice and standard specification. Agencies may select the performance tests of their choice for rutting resistance, cracking resistance, and moisture susceptibility. Existing criteria used by different state DOTs are given in the draft standard specification. The draft standard practice includes four BMD approaches. Approach A, Volumetric Design with Performance Verification, is more restrictive than AASHTO R 35 in that the completed Superpave mix design must meet the additional performance test requirements. Approach B, Volumetric Design with Performance Optimization, allows adjustment of the optimum AC by ±0.5% to meet performance test criteria. Approach C, Performance-Modified Volumetric Design, uses AASHTO R 35 through the evaluation of trial blends, at which point performance testing is conducted to determine optimum AC. In Approach C, the agency may relax some of the volumetric criteria in AASHTO M 323 provided that performance test criteria are met. Approach D, Performance Design, relies entirely on performance test results to select all mix component proportions. This approach is the least restrictive, allowing the highest level of innovation.
The centerpiece of BMD is performance testing to evaluate mix resistance to rutting and cracking. Cracking tests can be further categorized by the mechanism involved in crack initiation and propagation: thermal cracking, reflective cracking, bottom-up fatigue cracking, and top-down fatigue cracking. Most states currently require the testing of mix resistance to moisture damage, another common distress of asphalt pavements. Perhaps the biggest question for implementing BMD is: what are the “best” performance tests for each type of distress? Numerous tests have been developed over the past few decades. Some are better suited for routine use in mix design and quality assurance, while others are primarily focused on characterizing the fundamental properties of a mix to predict pavement response. For a performance test to be included in BMD procedures, criteria must first be established based on good correlations between laboratory and field performance. Other practical considerations include testing time, complexity of data analysis, test result variability, cost and availability of equipment, and sensitivity to mix parameters. Another primary concern is establishing appropriate mix conditioning and aging protocols for performance tests.
Many DOTs and contractors recognize the benefits of BMD and are working toward implementation. However, gaps still exist in the knowledge needed to reach full implementation. NCAT researchers identified nine important steps necessary for moving a test method from concept to implementation and conducted an extensive literature review to determine which steps have been completed for candidate test methods in each distress category. This process allowed the research team to identify existing knowledge gaps and test development steps needed to advance the most promising test methods into mainstream practice. Each distress category is summarized below.
There are six candidate tests for this distress mode, including the Low-Temperature Semi-Circular Bend Test (SCB), the Disk-Shaped Compact Tension Test (DCT), and the Illinois Flexibility Index Test (I-FIT). A comprehensive sensitivity study is needed to evaluate the top three candidate tests. DCT has more critical steps completed than the others and seems to be the most preferred test based on survey responses and existing literature. If two major ongoing studies have positive results, the research needs for implementation of the DCT should be complete; the only additional need is training for other states that plan to implement it. Lower-priority needs are conducting round-robin testing and training for the low-temperature SCB test.
Five candidate tests are included for this distress mode. Based on survey results and the literature review, OT and DCT appear to have the most potential for evaluating resistance to reflective cracking and have completed more critical steps required for implementation than other candidate tests. If several ongoing studies provide positive outcomes, only robust field validation and training would be needed for implementation of these two tests. The SCB-Jc and Flexural Bending Beam Fatigue Test (BBF) are not recommended for implementation because multiple critical steps are incomplete; furthermore, the BBF test is considered impractical for routine use.
Bottom-Up Fatigue Cracking
There are six candidate tests for this distress mode, including the BBF, SCB-Jc, OT, and I-FIT. OT and I-FIT appear to be the top candidates for BMD implementation. For both tests, a robust validation experiment to set test criteria for bottom-up fatigue cracking is the largest knowledge gap. Although research is ongoing to validate these tests for other modes of cracking, no validation experiments are planned for bottom-up fatigue cracking. This is a lower-priority research need for several reasons. First, most asphalt tonnage in the U.S. is produced for rehabilitation of existing pavements rather than new construction, where bottom-up fatigue cracking would be a design consideration. Second, the use of a valid perpetual pavement design strategy could eliminate bottom-up fatigue cracking as a mode of pavement failure for new asphalt pavements.
Five test methods are candidates for this mode of distress. I-FIT appears to have the most potential for BMD implementation since all nine critical steps have been completed or are ongoing. Another promising candidate is the Indirect Tensile Asphalt Cracking Test (IDEAL-CT), a relatively new test developed at TTI. Several critical steps still need to be addressed, but the IDEAL-CT’s primary advantage is that specimens do not require cutting, notching, or gluing, making the test method much faster than any other cracking test.
There are five candidate test methods for rutting resistance, including the HWTT and APA, which are already used by numerous state DOTs. All critical steps have been completed for the HWTT, and the only gap for the APA is a round robin study. Most steps have also been completed for the Flow Number Test (FN), but no states have implemented it for routine use as a rutting performance test.
The HWTT and Tensile Strength Ratio (TSR) are most commonly used to evaluate resistance to moisture damage. However, there is serious concern that test results may not be reliable indicators of field performance. Thus, a robust validation experiment is needed to evaluate both test methods for reliability.
Based on the survey results, the top five distresses that agencies wish to address with performance testing are fatigue cracking, rutting, thermal cracking, moisture damage, and reflective cracking. This ranking was used as a guide, along with the literature review and analysis of knowledge gaps, to establish priorities for research needed to facilitate BMD implementation. Nine research problem statements (RPSs) were developed to aid continued advancement of BMD, as listed below. Detailed descriptions of each project, as well as cost estimates and suggested schedules of completion, are found in Appendix B of the report for NCHRP Project 20-07/Task 406.
- Laboratory aging protocols for cracking tests
- Validation of reflective cracking tests
- Further validation of top-down cracking tests
- Validation of moisture susceptibility tests
- Refinement of AASHTO T 324 (HWTT)
- Establishing precision estimates for AASHTO T 340 (APA)
- Validation of bottom-up fatigue cracking tests
- Sensitivity of thermal cracking tests to mix design variables
- Establishing precision estimates for AASHTO TP 105 (Low-Temperature SCB)
The ongoing NCAT/MnROAD Cracking Group Experiment will contribute to the validation of top-down and thermal cracking tests by correlating laboratory results with field measurements. Findings will help agencies select the most suitable cracking tests for BMD as well as provide preliminary test criteria. Additional BMD-focused research is ongoing at the NCAT Test Track, with the placement of four test sections sponsored by Oklahoma and Texas in the 2018 research cycle.
NCAT offers a Balanced Mix Design Course, which is a 2 1/2-day workshop that provides hands-on training with all laboratory performance tests used in BMD. Attendees also gain a better understanding of the BMD process. Upcoming course dates are February 5-7, 2019, and registration information is available at http://ncat.us/education/training.
Fuente de la noticia: http://eng.auburn.edu/research/centers/ncat/newsroom/2018-fall/framework.html