Exploring the Use of Monotonic Loading Tests to Evaluate Rutting Performance of Asphalt Mixtures

Many state agencies are considering the use of Balanced Mix Design (BMD) method to design and produce asphalt mixtures for their roadways. The shift to the BMD method is an effort to make roadways more sustainable, longer lasting, and more economical by incorporating performance criteria for asphalt mixtures into mix design and production acceptance. In the BMD method, asphalt mixtures are designed and evaluated through a suite of laboratory tests to ensure adequate performance. Rutting, or permanent deformation, is one of the main distress modes of asphalt pavement. In Virginia, the current rutting test used for mix design acceptance as part of the BMD method is the asphalt pavement analyzer (APA) test, which is both time consuming and costly. Therefore, it is important to have simpler and more practical but still reliable and accurate screening tools to test asphalt mixtures for rutting resistance.

The goal of this study was to investigate more time efficient and cost effective performance-based tests to evaluate the rutting resistance of asphalt mixtures. Based on an extensive review of available literature, three monotonic loading tests that showed promising potential to characterize rutting of asphalt mixtures were evaluated including the indirect tensile (IDT) test at high temperatures, rapid rutting test (RRT or also known as ideal rutting test [IDEAL-RT]), and Marshall stability (MS) test. The experimental program of this study was divided into two phases.

In Phase 1, sixteen asphalt mixtures were tested under the IDT test, IDEAL-RT, MS test, and APA test. These tests were evaluated in terms of variability, sensitivity, and correlations among rutting indices. Phase 1 experimental results were used to assess the ability of the monotonic loading tests to determining rutting resistance of asphalt mixtures and were further used to develop corresponding preliminary threshold criteria. In Phase 2, six asphalt mixtures were further evaluated using Asphalt Mixture Performance Tester (AMPT) tests including the dynamic modulus (DM) test, stress sweep rutting (SSR) test, and the repeated load triaxial test (RLTT). Phase 2 experimental results were compared and correlated to the IDT, IDEAL-RT, and APA test results to further evaluate the ability of monotonic loading tests to characterize rutting resistance of asphalt mixtures.

Additionally, this study looked at the utilization of digital image correlation (DIC) as a technique for examining the failure mechanism of asphalt specimens when tested under monotonic tests typically performed at relatively high temperatures. This was a preliminary study of how DIC can potentially be utilized to select, identify, and/or develop parameters that characterize rutting resistance from these monotonic loading tests.

Based on this study, it was found that monotonic loading tests, specifically the IDT test at high temperature and IDEAL-RT, can be used to evaluate rutting performance of asphalt mixtures. Preliminary rutting performance threshold criteria were developed for the IDT test and IDEAL-RT based on correlations to the APA test. Further evaluation of these two monotonic loading tests and parameters is recommended to fully develop a specification for agencies and contractors to use a monotonic loading test to evaluate rutting performance of asphalt mixtures.