Binder Flushing in Low Traffic Volume Superpave Mixes

This paper analyzes early flushing or “bleeding” that occurred in several sections of Superpave pavement designed for low traffic volume roads in Nebraska. The objective of this study was to develop a better understanding of the causes of flushing and to examine the relationship between flushing and the material specifications for low traffic volume Superpave mixes. Records indicate that construction of the flushed sections proceeded within specifications. Moreover, all QA/QC material test results were within acceptable ranges. A review of mix design parameters for all flushed sections was conducted to verify compliance with Superpave specifications. Comparison of specified parameters between flushed and non-flushed sections with identical mix designs were also conducted. Field samples from flushed and non-flushed sections were obtained and analyzed for mix design verification and binder characterization. Additional testing on recovered asphalt binders was conducted to assess binder contribution to flushing. Results indicated that recovered binder properties did not meet Superpave specifications. This research shows the compatibility of asphalt binder dominates the asphalt-aggregate interaction. Flushing in the affected pavement sections resulted from excessive binder content, variations in binder properties due to poor material quality control, and lack of a requirement to test for separation problems during construction