PERMEABLE FRICTION COURSE MIXTURES ARE DIFFERENT

Permeable friction course (PFC) mixtures are a special type of hot mix asphalt characterized by a high total air voids (AV) content, which confers special properties to this type of mix including high permeability and noise reduction capacity. Although substantial progress was achieved in the last decade in terms of developing a mix design procedure, recent research conducted at Texas A&M University assessed several aspects related to mixture evaluation, including volumetric properties, drainability, durability, and effects of densification to further enhance the design of this type of mixture. This paper summarizes corresponding relevant results and recommendations, which include improvement in the computation of the total AV content based on the application of dimensional analysis (to compute the bulk specific gravity of the compacted mixture (Gmb)) and a calculation procedure to determine the theoretical maximum specific gravity of the mixture (Gmm). In addition, evaluation of field drainability can be conducted in terms of the water flow value (outflow time), and the expected value of permeability (E[k]) was recommended as an estimator to predict permeability. Furthermore, the Cantabro loss test was suggested for evaluating mixture durability. Verification of stone-on-stone contact during mix design, as well as density control during construction should also be specified to obtain a granular skeleton with adequate stability. Alvarez and Epps Martin 3 INTRODUCTION Permeable friction courses (PFC), as defined by the Texas Department of Transportation (TxDOT), or new generation open-graded friction courses (OGFC), are asphalt mixtures designed with high total air voids (AV) content compared to dense-graded hot mix asphalt (HMA). Stone-on-stone contact of the coarse aggregate is also required to ensure adequate mixture stability. Thus, PFC mixtures serve as wearing courses that placed in thin thicknesses improve safety, especially in wet weather conditions due to water traveling through connected AV within the mixture instead of over the surface. In addition, compared to dense-graded HMA, PFC offers cleaner runoff and reduction of pavement noise [1-3]. The particular characteristics of PFC mixtures (e.g., high AV content, open gradation, and high drainability) lead to the necessity of specific approaches to conduct corresponding mix design and evaluation. Current TxDOT PFC mix design relies on volumetric properties to determine the optimum asphalt content (OAC). An evaluation of draindown and moisture susceptibility of the mixture are also conducted on specimens fabricated at the OAC defined by volumetric properties [4]. Recently, the National Center for Asphalt Technology (NCAT) [5-7] also proposed a mix design method that includes an assessment of both functionality and durability. Furthermore, ASTM D 7064-04 [8] contains a mix design procedure based on evaluating volumetric properties to establish the OAC. Although the consolidation of these methodologies have enhanced the design of PFC mixtures, additional research is still required to further improve specific aspects related to mixture characterization and evaluation. This paper summarizes the main findings and recommendations reported in recent research conducted at Texas A&M University on the evaluation of PFC mixtures, including aspects related to: (i) volumetric properties, (ii) functionality-drainability, (iii) durability, and (iv) effects of densification. This research included PFC mixtures fabricated using both Performance Grade (PG) asphalts (PG 76-XX) and Asphalt Rubber (AR) asphalts and corresponding aggregate gradations as currently specified by TxDOT [9], which were evaluated using laboratoryand field-compacted (road cores) specimens. The paper is organized in four main sections related to each mixture aspect evaluated. A summary and recommendations complete the paper. VOLUMETRIC PROPERTIES Total AV content (or corresponding density) is the main parameter used for selecting the OAC of PFC mixtures. The total AV content is determined as: