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Stone-on-stone contact of the coarse-aggregate fraction is one of the main characteristics of permeable friction course (PFC) asphalt mixtures that is required to provide adequate resistance to both raveling and permanent deformation. Currently, stone-on-stone contact is determined by comparing the air voids content in the coarse aggregate (VCA), assessed in both the dry-rodded condition (VCADRC) and the compacted PFC mixture (VCAmix). The underlying assumption is that the coarse aggregate of a compacted PFC mixture with VCAmix equal to VCADRC would develop a stone-on-stone contact condition equivalent to that existing in the dry-rodded aggregate. This study focused on proposing enhancements for the quantitative determination of stone-on-stone contact of PFC mixtures. The assessment supported on both laboratory testing and application of the discrete element method and image analysis techniques, led to recommendation of a criterion to determine the breaking-sieve size. In addition, verification of stone-on-stone contact using a maximum VCA ratio of 0.9 was recommended to ensure the design and construction of PFC mixtures with fully developed stone-on-stone contact. [ABSTRACT FROM AUTHOR]

Drainability is one of the main characteristics of permeable friction course (PFC) mixtures and is the primary reason for using these mixtures as the surface course in asphalt pavements in the United States. Current approaches suggested for PFC mix design to evaluate drainability (using gyratory-compacted specimens) include: (1) achieving a target total air void (AV) content as an indirect indication of permeability and (2) direct measurement of permeability in the laboratory. The assessment conducted in this study suggested that these approaches are not effective in ensuring adequate drainability in field-compacted mixtures. Thus, different alternatives were evaluated to improve this assessment. Corresponding analysis suggested that: (1) the water-accessible AV content can be used as a surrogate of the total AV content to indirectly assess permeability and (2) the water flow value (outflow time) can be applied to evaluate the field drainability of PFC mixtures. The expected value of permeability, determined using a modified version of the Kozeny-Carman equation, was recommended to analytically predict permeability for mix design and evaluation purposes. [ABSTRACT FROM AUTHOR]

Abstract: Durability and functionality (i.e., noise reduction effectiveness and drainability) of permeable friction course (PFC) mixtures depend on the characteristics of the air voids (AV) contained in the mixture. This study analyzes the internal structure of PFC mixtures, assessed in terms of AV characteristics; determined using X-ray Computed Tomography and image analysis techniques. Corresponding results showed: (i) heterogeneous distributions of AV in the horizontal direction of both field-compacted mixtures (road cores) and specimens compacted using the Superpave Gyratory Compactor (SGC) and (ii) limitations to compare their vertical AV distributions. Recommendations to reduce the horizontal heterogeneity included using road cores with a minimum 152.4mm diameter and coring SGC specimens to 101.6mm in diameter. Implementation of field-compaction control and future analysis of mixtures prepared accordingly was recommended to determine the pattern of vertical AV distribution that should be reproduced in SGC specimens and corresponding modifications required for fabrication of these specimens. [Copyright &y& Elsevier]

Durability of porous or permeable friction courses (PFC) is an important aspect to address when designing this type of hot mix asphalt. At present, several agencies perform the mix design of PFC primarily by determining volumetric mixture properties. This approach ensures adequate mixture functionality, but it does not guarantee mixture durability. This paper evaluates the Cantabro loss test, the Hamburg Wheel-Tracking test (HWTT) and the Overlay test (OT) to determine the one most appropriate for mix design and laboratory performance evaluations. The Cantabro loss test, performed in both dry and wet conditions, is recommended for PFC mix design to corroborate the suitability of the optimum asphalt content defined based on volumetric determinations. The HWTT and the OT are not recommended, since the variability of the test results indicated that these tests may not be suitable for PFC mixtures. [ABSTRACT FROM AUTHOR]

Current hot mix asphalt (HMA) mix design procedures used to determine the optimum asphalt content for permeable or porous friction course (PFC) mixtures are based primarily on total air void (AV) content. Durability and functionality of PFC mixtures are also related to the total AV content. However, the connected AV content (defined as the proportion of AV that form connected pathways for air and water transport through PFC mixtures) may provide more insight into the mixture structure in terms of the AV content directly associated with functionality and durability properties and constitute an alternative parameter to conduct PFC mix design and evaluation. This study evaluated two laboratory methodologies (vacuum and dimensional analysis) for determining water-accessible AV content and two types of analysis to compute interconnected AV content based on X-ray Computed Tomography (X-ray CT) and image analysis techniques. Although both the interconnected AV content and water-accessible AV content constitute determinations of connected AV content, different nomenclature was used to differentiate the origin of the calculation. Dimensional analysis with application of vacuum and X-ray CT and image analysis with inclusion of surface AV are recommended for determining water-accessible AV content and interconnected AV content, respectively. Future work should focus on investigating the use of connected AV content as an alternative parameter to integrate in mix design and laboratory and computational evaluation of PFC mixtures. [ABSTRACT FROM AUTHOR]

Compaction of permeable or porous friction course (PFC) mixtures is generally considered a process without major issues, and field density requirements (or corresponding total air voids [AV] content) are not currently specified for this type of hot mix asphalt. However, proper densification is one of the most important aspects to control during construction to prevent raveling, the distress most frequently reported as the cause of failure in these mixtures. This paper presents an evaluation of the effect of densification on PFC mixtures. This evaluation included both the study of the internal structure of compacted mixtures and a comparison of performance based on macroscopic response. Results from this study showed that differences encountered in the internal structure of road cores and specimens compacted using the Superpave Gyratory Compactor limit the use of these laboratory compacted specimens in durability and functionality evaluations of PFC mixtures. In addition, changes in densification, after reaching stone-on-stone contact, modified the mixture properties and performance. The magnitude of these modifications provided evidence of the ease of verifying not only stone-on-stone contact during mix design, but also of the importance of controlling the density during construction to ensure an equilibrium density that guarantees the balance between mixture durability and mixture functionality. [ABSTRACT FROM AUTHOR]

Current hot mix asphalt (HMA) mix design procedures used to determine the optimum asphalt content (OAC) for permeable or porous friction course (PFC) mixtures are based on volumetric properties, primarily total air void (AV) content. This calculated volumetric parameter depends on the bulk specific gravity (Gmb) and the theoretical maximum specific gravity (Gmm) of the mixture, which are generally difficult to measure in a laboratory due to the high asphalt contents, high total AV contents, and the use of modified asphalts for PFC mixtures. This study evaluated two methodologies for determining Gmb (vacuum and dimensional analysis) and two methodologies for determining Gmm (measured and calculated) for use in calculations of total AV content. For the mixtures assessed in this study, originally designed with a total AV content of 20 %, the alternative methodologies studied led to total AV content values outside the design range (18 to 22 %), which implies the necessity of gradation modifications or changes in the fiber content to meet AV requirements and define an OAC. Dimensional analysis and a calculation procedure, based on values of Gmm measured in the laboratory at low asphalt contents, are recommended for determining Gmb and Gmm values, respectively. In addition, dimensional analysis is preliminarily recommended to compute the water-accessible AV content of PFC mixtures based on the assessment of two methods (vacuum and a methodology proposed for dimensional analysis) to compute this parameter. Water-accessible AV content is considered as an alternative parameter for mix design and evaluation. [ABSTRACT FROM AUTHOR]

Abstract: Permeable friction course mixtures (PFC) are special hot mix asphalt (HMA) mixtures that are designed to improve motorist safety and reduce traffic noise (i.e., tire–pavement noise). In terms of pavement structural design and construction, PFC mixtures are typically used as the surfacing course layer where in addition to improving the skid resistance, also serve as the surface drainage layer. This paper compares PFC mixtures designed and fabricated using performance grade (PG) and asphalt rubber (AR) asphalt-binders (or asphalts). The experimental design included assessment of total air voids (AV) content, durability, drainability, stone-on-stone contact, and internal structure of the PFC mixtures fabricated using both AR and PG asphalts; denoted herein as AR–PFC and PG–PFC mixtures, respectively. The analysis conducted provided evidence of differences between AR–PFC and PG–PFC mixtures that suggest the need for differentiation and refinement of the existing specifications to consider these mixtures as independent materials. Future research should, therefore, focus on defining particular specifications for mix design and control for AR–PFC and PG-PFC mixtures. [Copyright &y& Elsevier]

Permeable Friction Courses (PFC) are asphalt mixtures characterized by high air void contents that are used as surface layers in conventional pavements, principally to achieve safer roads and control noise pollution. This paper summarizes existing information related to the performance of PFC mixtures in the United States (US), including properties of the mixture that determine the durability and functionality of the pavement and mix design methodologies that are currently being used in the US. The objective of this work is not only to analyze this information but also to identify current and future challenges to enhance the performance of PFC mixtures and promote its responsible use. [ABSTRACT FROM AUTHOR]