Hydraulic Conductivity (Permeability) of Laboratory-Compacted Asphalt Mixtures

A flexible-wall permeameter was used to study the hydraulic conductivity (also referred to as “permeability”) of asphalt mixtures commonly used in Wisconsin. Effects of saturation, hydraulic gradient, and sidewall leakage were studied to determine an appropriate testing procedure. The test procedure was then used to study how mixture design variables affect hydraulic conductivity. Four aggregate gradations were tested to develop relationships between hydraulic conductivity and volumetric properties. Two mixtures were also compacted to different heights (ranging from 37.5 mm to 110.0 mm) to evaluate how lift thickness affects hydraulic conductivity. Results of the tests indicate that back-pressure saturation can be used to ensure that specimens are saturated and that the saturated hydraulic conductivity is measured. Sidewall leakage was eliminated by using bentonite clay as a sealing agent. The hydraulic gradient was selected on the basis of a parametric experiment showing how the gradient affects hydraulic conductivity. Tests on the various mixtures showed that a power-law relationship exists between air voids content and hydraulic conductivity, but the hydraulic conductivity also depends on the gradation. Lift thickness also appears to be important, with lower hydraulic conductivity obtained from greater lift thickness at a given air void content. The results show that hydraulic conductivity cannot be controlled only by limiting air voids and that a hydraulic conductivity test is necessary for a mixture design that includes hydraulic conductivity as a criterion.