Investigation of Over-Permeable Superpave Surface Mixes in North Carolina

Abstract

Superpave was introduced in 1994, as a result of the Strategic Highway Research Program (SHRP) and was aimed at addressing the performance of asphalt concrete pavements. This SHRP study recommended new specifications for asphalt mixture designs which included performance graded binders, performance related aggregate properties, and new principles for volumetric mix designs. With these new mix design principles, it was evident that Superpave mix designs would contain more coarse aggregates and less natural sands. With the introduction of Superpave, many in the Asphalt industry feared that the recommended coarse structured mixes would be very difficult to compact and highly permeable compared to the Marshall mixes they were replacing. In North Carolina, we have seen instances where this theory was proven true and have witnessed coarse Superpave mixes that have been permeable. In general, the cause of this permeability may be due to increased voids caused by improper compaction that results from the coarse nature of the designs, which in turn creates interconnected voids that will transmit water through the pavement. A remedy to the permeability problem may be a new test that can determine in the laboratory if certain mixes are susceptible to permeability. This test is discussed in this study.

The data represented in this study shows the causes of permeability in a small group of pavements that were sampled and should not be misinterpreted to imply that this small number of distressed pavements is indicative of all Superpave mixes. The selected mixes are a cross section of surface mixes used across the State of North Carolina.

The permeability test method discussed in this study is a very simple procedure and the data presented show that this test can be used for predicting field permeability of Superpave surface mixtures. The coarse graded mixes performed poorly in the lab permeability tests and the fine graded mixes proved to be very impermeable. This study also evaluated mixes using laboratory performance tests. The results show that the coarse graded mixes and fine graded surface mixes perform equally well under laboratory performance testing at a given void content that matches the in-situ properties.

Using these lab tests and the results of this study, a clearer understanding of the effects of water in asphalt pavements and how to prevent this water infiltration has been obtained and new specification changes have been recommended to reduce any future permeability problem with Superpave surface mixes.