Reflective Cracking Study: First-level Report on Laboratory Shear Testing
This report contains a summary of the laboratory repeated load shear tests on mixes used as overlays on the Reflective Cracking Study Test Track at the Richmond Field Station. Evaluation of the results of the laboratory study on shear response of the overlay mixes reported herein included the effects of mix temperatures, air-void content, aging, mixing and compaction conditions, aggregate gradation, and shear stress level. Shear testing was performed to assess expected rutting performance at high temperatures. Mixes with five binders were tested, namely AR4000, asphalt rubber, and three modified binders termed MB4 (meeting the Caltrans MB4 specification ), MB15 (meeting the MB4 specification and containing 15 percent recycled tire rubber, referred to as MAC15), and MAC15TR (Southern California GreenBook specification, containing 15 percent recycled tire rubber, referred to as MAC15). A full factorial considering all the variables required a total of 2,880 tests. This was reduced to 186 tests to accommodate time and fund constraints. Based on the shear test results for the mixes used in the overlay experiment, mix rankings for Cycles to 5 Percent Permanent Shear Strain, Permanent Shear Strain at 5,000 Cycles, and Resilient Shear Modulus (G*) were: • Permanent Shear Strain at 5,000 Cycles (best performance to worst): AR4000-D; MAC15-G; RAC-G; MB4-G; MB15-G. • Cycles to 5 Percent Permanent Shear Strain (best performance to worst): AR4000-D; MAC15-G; RAC-G; MB4-G; MB15-G. • Resilient Shear Modulus (G*) (stiffest to least stiff): AR4000-D; RAC-G; MAC15-G; MB4-G; MB15-G The results show that the rankings of different mixes for expected rutting performance are the same for the two permanent deformation parameters calculated from Repeated Simple Shear Test (RSST) results: Permanent Shear Strain at 5,000 Cycles and Cycles to 5 Percent Permanent Shear Strain. The RSST demonstrated that the densegraded mix with unmodified AR-4000 asphalt performed better than the gap-graded mixes with modified binders. Of the three gap-graded mixes with modified binders, the MAC15 mix exhibited the highest resistance to shear deformation while the MB15 mix exhibited the lowest. Dense-graded mixes with the modified binders (MB4, MB15, MAC15) generally showed an increase in permanent shear deformation resistance and shear stiffness compared to gap-graded mixes with the same binders. Until a range of pavement types and environments are evaluated in the second- level analysis, these results provide only a general indication of the relative performance of the modified binders with respect to rutting performance.