Pilot Implementation of the Overlay Tester and Double-Blade Saw (5-4467-01-1)

Past experience and recent field performance data has shown that several of TxDOT's widely used asphalt mixtures are rut resistant but increasingly prone to cracking. Under Research Project 0-4467 the Overlay Tester (OT) was successfully demonstrated as a simple performance-related test to identify the crack resistance of asphalt mixtures. This implementation project focused on developing specifications for the OT, purchasing three units, and conducting shakedown tests on the new units. Within lab testing at the Texas Transportation Institute produced very consistent results. However, variability problems were encountered in the initial Round-Robin tests between labs. An investigation was conducted to identify the source of the differences. Several factors were identified and these are described in this report. Recommendations were made to modify the test protocol and it is highly recommended that a second Round-Robin test be conducted to evaluate the repeatability and reproducibility of the overlay tester. Additionally, specifications for double-bladed saws were developed and three saws were purchased and delivered to Districts labs. The samples cut with these saws meet the required operational tolerances. However, it was recommended that calibration may be required on mixtures made with extremely hard rock, such as river gravels. The results obtained with both the new overlay testers and double bladed saws are presented in this report. The specifications for each device are given in the Appendices.