IMPROVING MIX DESIGN AND CONSTRUCTION OF PERMEABLE FRICTION COURSE MIXTURES

Abstract

Permeable friction course (PFC), or new generation open-graded friction course (OGFC) mixtures, are hot mix asphalt (HMA) characterized by high total air voids (AV) content (minimum 18 %) as compared to the most commonly used dense-graded HMA. The high AV content confers to PFC mixtures both high permeability and noise reduction effectiveness. These characteristics and the high values of surface friction exhibited by PFC mixtures, as compared to dense-graded HMA, lead to improvements in safety and the environment, which make PFC one of the safest, cleanest, and quietest alternatives currently available for surface paving. The main objective of this study was improving the current PFC mix design method and construction practices in terms of compaction control. Corresponding results were integrated in an improved mix design method that is based on the guidelines of the current mix design method used by the Texas Department of Transportation. The improved mix design included modified computation of the inputs required to determine mixture density (or corresponding total AV content). These changes led to a proposed modification of the density specification for mix design from 78-82 % to 76-80 %. In addition, the water-accessible AV content was proposed as a surrogate of the total AV content for mix design and evaluation. The improved mix design method also includes verification of drainability, durability, and stone-on-stone contact. Computation of the expected value of permeability (E[k]) and measurement of the water flow value were recommended, respectively, for verification of drainability in the laboratory (using specimens compacted in the Superpave Gyratory Compactor (SGC)) and in the field. The Cantabro loss test conducted in both dry- and wet-conditions was suggested for assessing mixture durability. Improved criteria were proposed for verification of stone-on-stone contact based on the evaluation of the AV content in the coarse aggregate fraction of the mixture. In addition, comparison of the internal structure of field-and laboratory-compacted mixtures supported recommendation of a field-compaction control. Recommendations to reduce the horizontal heterogeneity of AV encountered in PFC specimens included using road cores with a minimum 152.4 mm diameter and coring SGC specimens from 152.4 to 101.6 mm in diameter