Comparison of Falling-Head and Constant-Head Techniques: Estimating Field Permeability of Hot-Mix Asphalt Pavements

Permeability of hot-mix asphalt continues to generate considerable interest in the asphalt materials, design, and construction community. Ongoing studies of permeability report advances in permeability testing techniques for both laboratory-based and field-based testing methods. The University of Arkansas (UAF) has developed a constant-head field permeability test device for use on in-place asphalt pavements. One constructed test section and 16 in-service pavements, including both new construction and overlays of existing pavements, were tested with the UAF device and a falling-head field permeameter originally developed by the National Center for Asphalt Technology. At each field test site, cores were taken from the pavement and tested in the laboratory by the falling-head, constant-tail method described in ASTM PS-129. At a minimum, two field permeability measurements were taken at each site with each permeability device; at least three cores were taken from each test site for testing in the laboratory. The total data set for this study includes more than 40 direct comparisons of each field method and the laboratory results. The data indicate that falling-head and constant-head field methods yield significantly different permeability results but display similar test variability (based on replicate tests). Further, field permeability measured by the falling-head method was significantly different from corresponding laboratory results, while constant-head field results were not significantly different from laboratory results. Relative differences between field devices in the initial applied hydraulic head and the test area footprint may contribute to observed differences.