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Revision of the Superpave High Temperature Binder Specification: The Multiple Stress Creep Recovery Test (With Discussion)

The inadequacy of the Superpave high temperature specification parameter, G*/sin δ, to correctly grade the superior field performance of modified asphalt binders has been demonstrated by several researchers. A new parameter that is blind to modification type and is performance based is now needed. As a replacement for the existing high temperature binder test (G*/sin δ), the FHWA has developed an easy to use Multiple Stress Creep and Recovery Test (MSCR) that measures fundamental characteristics of asphalt binders. In this study, several binder parameters proposed to replace the existing Superpave rutting parameter were validated using hotmix testing. Several different binder tests were evaluated to determine which would provide a replacement for the Superpave high temperature binder criteria. The new test and criteria will have to be performance related and blind to modification. The results from these binder tests were compared against hot-mix rutting results from the Asphalt Pavement Analyzer, the Hamburg Wheel Tracking, the ALF test sections and actual roadway sites. The results from the mixture rut testing showed that different rut testers will provide completely different ranking of binders. This difference is related to the stress level applied by the different testers. This hot-mix testing indicates that the different binders, specifically the polymer modified binders, have different stress dependencies. The binder criteria currently used to specify the high temperature properties are specifically intended to be run in the linear viscoelastic range and therefore can not determine the stress dependency on binder materials. The multi step creep and recovery test can be run at multiple stress levels and can characterize the stress dependency of polymer modified binders. The MSCR test was developed as a result of these findings and other results from various internal studies conducted by FHWA. A separate sub-study was also conducted in this research to understand the effect of stress and strain on the microstructure of polymer modified binders. It was found that in MSCR data there is a clear relationship between %recovery and %strain in the creep portion of the test. In some cases, at least, this is the dominant relationship. Very high strain causes yield behavior in polymer modified asphalt binders (PMA). After high strain, PMAs still exhibit recovery but the rate of recovery is reduced. At high strain, binder morphology, tensile and shear properties change. A test procedure was developed to run creep and recovery testing on one sample at multiple stress levels (MSCR). This test procedure makes it easy to evaluate how the binder response will change under different stress conditions. A property called non-recoverable compliance Jnr was developed based on the non-recovered strain at the end of the recovery portion of the test divided by the initial stress applied during the creep portion of the test. The Jnr value normalizes the strain response of the binder to stress which clearly shows the differences between different polymer-modified binders.

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  • English

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  • Accession Number: 01080569
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Nov 14 2007 11:59AM