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3 Background Porous Friction Course (PFC) has been used in Europe and the United States for many decades. PFC is also known as Porous European Mix (PEM), Open-Graded Friction Course (OGFC), and Porous Asphalt (PA). PFCs are primarily used to improve safety by increasing the frictional prop- erties of the pavement surface along with allowing surface water to drain through the pavement. Despite the benefits, the use of PFC has diminished over the years due to durability and service- life issues. The durability problems are generally evidenced by raveling, and once the distress begins, it progresses rapidly. A survey conducted by the National Center for Asphalt Technol- ogy (NCAT) showed that in 1998, 22 states had discontinued use of PFC (Kandhal and Mallick, 1998). A 2014 survey conducted by NCAT as part of NCHRP Project 01-55 showed that only about half of 41 responding agencies (40 states and Puerto Rico) were using PFC mixes. Figure 1 depicts the results of the 2015 survey in regard to PFC usage. The survey revealed that agencies that did not use PFC felt that their designs were not adequate to maintain the expected perfor- mance life of PFC mixes. The primary distresses that reportedly caused premature failure were identified as raveling and top-down cracking. Examples of raveling and cracking in a PFC mix can be seen in Figure 2. There are numerous factors that can lead to these types of distress. For example, the nomi- nal maximum aggregate size (NMAS) and lift thickness play a part in raveling. The amount of asphalt binder and air voids in the mix can influence a mixâs susceptibility to raveling and crack- ing. The use of modified asphalts with specified minimum asphalt contents can help to prevent raveling and cracking, but there is still a need to address how to determine an optimum binder content. Currently, a glass pie plate draindown method is primarily used for determining opti- mum asphalt contents. Based on this test, binder content is more often determined in relation to draindown than to performance criteria that address the visible distresses. The mix susceptibility to top-down cracking may be attributed to higher air voids and binder draindown. There is no test currently being used in PFC mix design to evaluate the cracking potential. This study will address durability and cracking issues during mix design and will also evaluate an array of per- formance tests to help determine what testing should be included in PFC mix design procedures. Objective and Scope The objective of this research was to develop a PFC mix design method that includes performance-based test procedures that address the types of distresses commonly seen and are viable procedures whether virgin aggregates, reclaimed asphalt pavement (RAP), or recycled asphalt shingles (RAS) are used in the mix design. Several laboratory tests were evaluated, and C H A P T E R 1 Introduction
4 Performance-Based Mix Design of Porous Friction Courses those that are most discriminating of successful performance were selected for the design proce- dure. This research included determining the optimum asphalt content and evaluating the effect of dust and fiber content of the mixes along with using lab performance tests to help determine performance threshold criteria. Typical asphalt contents range from 5.5%â7.0% and dust con- tent ranges from 1%â6% (Cooley et al., 2009). A sample matrix with varying asphalt and dust contents was tested for durability using an array of performance tests. Figure 1. 2014 Survey of PFC usage. Figure 2. Primary distresses observed in PFC mixes (NCAT, 2014). Top-Down Cracking Raveling