Several test methods are in practice to assess the rutting potential of a mixture. The commonly used procedures are Diametral tests, Uniaxial test, Triaxial tests, Shear tests, Empirical tests, and Simulative tests. Of all these test methods, simulative test methods are relatively easier to use and ready for immediate adoption. The APA test is the most widely used simulative test. It is imperative that the rut depth criteria for the APA test should be developed for its employment. In a recent study conducted at NCSU, it was concluded that the APA could clearly detect poorly performing mixtures. With the limited availability of data, a reasonable correlation was observed between the APA tests and Repeated Shear at Constant Height (RSCH) tests.
A comprehensive research study was conducted on all surface mixtures of NCDOT. The rut depths measured from the APA test were compared with the corresponding shear strains of the RSCH test. From HWY-2005-13 project, regression models were developed to characterize the rutting behavior of each individual mix by considering effects of test temperature, aggregate types and traffic volumes. Validation of these models was done using the field cores obtained from NCDOT and further modified models were developed. In addition, the shear and APA tests were fine-tuned by addressing issues related to air voids, test temperatures and aggregate sources. Statistical analysis was performed on the test results as measured by the APA and the RSCH tests. This was used to characterize rut resistance of mixtures used in this study. The correlations estimated using the data obtained from the APA and Shear tests were used to develop rut depth criteria for the APA test. The developed rut depth criteria for the APA test could be adopted for immediate use in practice.
