Abstract
Asphalt mixes are designed to provide adequate resistance to various distresses including cracking, rutting, and moisture damage. Recently, more efforts are directed towards including performance assessment tests during the design and production of asphalt mixes. Performance-Engineered Mix Design (PEMD) or Balanced Mix Design (BMD) is a new and innovative design approach that incorporates performance assessment tests to optimize the design of asphalt mixes to provide adequate performance. Although transportation agencies are motivated to implement the PEMD approach, several research knowledge gaps and concerns need to be addressed before PEMD successful implementation. This research study aims to advance, develop, and implement performance-engineered design approach and specifications to extend the service life of asphalt pavements.
The first phase of this research developed and evaluated a new and innovative monotonic cracking performance indicator called Weibull Cracking Resistance Index (WeibullCRI). The proposed indicator describes the entire load-displacement curve, which overcomes the limitations of the existing performance indicators. First, WeibullCRI was examined using an extensive laboratory evaluation of 16 different asphalt mixes. The results indicated that WeibullCRI was sensitive to variation in binder content and binder PG and the results were in good agreement with the expected cracking resistance based on the composition of the studied mixes. In addition, WeibullCRI had low variability in test results and higher number of various statistical groups. Next, the applicability of WeibullCRI as a unified approach to analyze the results of various monotonic assessment tests was investigated using data generated by other researchers and reported in the literature. The results indicated that WeibullCRI is able to interpret the testing results of various monotonic performance assessment tests (i.e., IDT- intermediate temperature, Semi-Circle Bending [SCB]- intermediate temperature, SCB-low temperature, Disk-Shaped Compact Tension [DCT], and Simple Punching Shear Test [SPST]) and various displacement measurement methods (i.e., actuator vertical displacement and Crack Mouth Opening Displacement [CMOD]). WeibullCRI was also sensitive to variation in test conditions (i.e., specimen notch depth, thickness, and air void content) and mix composition proportions (i.e., binder content, binder grade, aggregate type, NMAS, aging, rejuvenator dosages, and Recycled Asphalt Pavement [RAP] materials).
The second phase of this study reviewed and evaluated the current monotonic cracking performance assessment tests and indicators including the developed WeibullCRI used to assess asphalt mix resistance to cracking. In this phase, the testing requirements of various test standards, key publications, concepts, calculation methods, physical meaning, and advantages and disadvantages of various performance indicators were reviewed. Then, the study investigated the validity of the most promising testing standards and indicators. Three testing standards and 12 performance indicators were considered. Several aspects were examined including 1) investigate the fundamental meaning of the variation in the load-displacement curve in terms of the change in mix resistance to cracking, 2) sensitivity of performance indicators to mix compositions, 3) variability in test results, 4) number of various statistical groups, 5) correlation between various performance indicators, 6) direct correlation between laboratory results of monotonic performance tests and indicators with the observed field cracking, and 7) ability to develop PEMD specifications. A comprehensive laboratory investigation was conducted using 33 different asphalt mixes included six Laboratory Mixed-Laboratory Compacted (LMLC) and 10 Plant Mixed-Laboratory Compacted (PMLC) asphalt mixes, and 17 field projects with known cracking performance. The results showed that WeibullCRI calculated from the IDT test to have the lowest test variability, maximum number of Tukey’s honestly significant difference (HSD) groups, and have excellent correlation with cyclic cracking resistance assessment indicators as compared to the other monotonic performance indicators. In addition, the results demonstrated that there was no direct correlation between all monotonic performance indicators and the observed field cracking performance, therefore an alternative approach was proposed, evaluated, and validated to develop performance thresholds for the selected performance indicators. Three pass/fail cracking performance thresholds were proposed for WeibullCRI to distinguish between asphalt mixes with good, fair, and poor cracking resistance using the proposed approach.
The third phase of this study focused on the development and evaluation of a new cyclic cracking assessment test called Multi-Stage Semi-circle bending Dynamic (MSSD). The test offers advantages over the available monotonic and dynamic cracking assessment tests and addresses major concerns to implement the PEMD (i.e., performance test validity, complex specimen preparation, and testing time). The developed MSSD test simulates the repeated loading (cyclic) in a reasonable testing time (less than 9 hours per test regardless of mix type), has a fixed loading sequence that works for mixes with different characteristics (e.g., mix composition, percent air void content, thickness, etc.), and utilizes testing equipment and specimen geometry similar to that used in monotonic tests. The laboratory evaluation results showed that the proposed test and its derived performance indicators were sensitive to mix composition and had lower variability compared to other dynamic tests. In addition, the MSSD performance indicators correlated well with the observed cracking performance in the field and were able to distinguish between projects with good and poor resistance to cracking. Based on the evaluation results, three pass/fail cracking performance thresholds were proposed to distinguish between asphalt mixes with good, fair, and poor resistance to cracking.
The fourth phase of this research examined the most promising tests and performance indicators to evaluate the resistance of asphalt mixtures to rutting. Two tests (i.e., Hamburg Wheel Tracking test [HWTT], and Asphalt Pavement Analyzer [APA] rut test) and three rutting performance indicators (i.e., HWTT rut depth after 15,000 passes [HWTT15000], HWTT rut depth at 20,000 passes [HWTT20000], and APA rut depth after 8,000 cycles [APA8000]) were considered. An intensive laboratory investigation was conducted that included six LMLC, 10 PMLC, and field cores extracted from 17 field projects. The research findings showed that both HWTT and APA rut test provided similar rutting assessment for the evaluated mixes. The study recommended using the HWTT over the APA rut test since HWTT can be also used to assess the resistance of asphalt mixtures to moisture damage to moisture damage. Also, the study recommended using HWTT15000 over HWTT20000 as a performance indicator since it requires less testing time.
The final phase of this research provided recommendations of the best testing standards, performance indicators, and performance specifications to assess asphalt mix resistance to cracking and rutting. In addition, it provided guidelines to demonstrate the use of the proposed tools during the design and/or production of asphalt mixes. It also proposed standards testing procedures for the newly developed WeibullCRI performance indicator and MSSD test.