Optimizing the Design of Permeable Friction Course Mixtures
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Permeable friction course (PFC) mixtures, or new-generation open-graded friction course mixtures, are one of the safest and quietest available alternatives for surface paving. Advantages to the use of PFC mixtures are improvements in safety, reduction in environmental impact (e.g., cleaner runoff), and economy. These advantages are closely related to their high total air void (AV) content and aggregate gradation, which confers high permeability and noise reduction properties to PFC mixtures. Research was completed at Texas A&M University to improve PFC mix design. Findings included improved determination of the total AV content through the use of a calculation procedure to obtain the mixture theoretical maximum specific gravity, Gmm, and dimensional analysis to determine the mixture bulk specific gravity, Gmb. Water-accessible AV content was proposed as a surrogate of the total AV content for mix design and evaluation. In addition, the water flow value (outflow time) and the expected value of permeability (E[k]) were recommended, respectively, to assess field drainability and to estimate the permeability of both laboratory- and field-compacted (i.e., road cores) mixtures. The Cantabro loss test was suggested for assessing mixture durability. Furthermore, an improved criterion for verification of stone-on-stone contact for mix design and a field-density control during construction were recommended to guarantee adequate stability and durability. Recommendations to improve fabrication of Superpave gyratory compactor specimens and sampling of field-compacted mixtures (by using road cores) were included. This set of recommendations was integrated in an improved mix design method for PFC mixtures.
