Addressing Safety Through Evaluation and Optimization of Permeable Friction Course Mixtures
Allex E. Alvarez, Amy Epps Martin and Cindy K. Estakhri, Texas A&M University, January 2010, 91 pp. (476660-00013-1)
Permeable friction course (PFC) mixtures are a special type of hot mix asphalt characterized by a high total air voids content to guarantee proper functionality and stone-on-stone contact of the coarse aggregate fraction to ensure adequate mixture stability. Thus, PFC mixtures constructed in a thin layer at the surface of a pavement structure produce several benefits in terms of economy, safety, and the environment. This project focused on the analysis of functionality (i.e., drainability), stone-on-stone contact, and mixture internal structure to propose improvements in these three aspects for PFC mix design and evaluation. The analysis was based on both a macroscopic assessment of mixture properties and a study of internal structure using X-ray Computed Tomography (X-ray CT) and image analysis techniques.
The assessment of drainability led to a recommendation to use the expected value of permeability (based on a modified Kozeny-Carman equation) for analytical prediction of permeability. The water-accessible AV content was also proposed as a surrogate of the total AV content for indirect assessment of permeability. In addition, field drainability of PFC mixtures can be evaluated in terms of the water flow value (outflow time). Proposed enhancements for the quantitative determination of stone-on-stone contact were established using the Discrete Element Method and image analysis techniques. This analysis allowed recommendation of: (i) a criterion to determine the breaking-sieve size (sieve size differentiating the coarse and fine aggregate fractions) and (ii) verification of stone-on-stone contact using a maximum voids in coarse aggregate (VCA) ratio of 0.9 over the current criterion (VCA=1.0). The analysis of mixture internal structure led to recommend reduction of the horizontal heterogeneity of total AV content by using road cores with a minimum 152.4 mm diameter and coring SGC specimens from 152.4 to 101.6 mm in diameter. In addition, limitations in comparing the vertical distribution of AV of field- and laboratory-compacted mixtures supported recommendation of field-compaction control and future analysis of mixtures produced accordingly.
Keywords: Permeable Friction Course, Open-Graded Friction Course, Permeability, Stone-on-Stone Contact, Mixture Internal Structure, X-ray Computed Tomography, Image Analysis, Total Air Voids Content
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