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OCR for page 195
Design of Open-Graded Mixtures 195 completely, which reduces the potential for moisture damage and raveling. The use of polymer- modified binders in OGFCs has increased the durability of these mixes. All of these improvements have led to increased OGFC service life. Within the overall category of OGFC, there are two predominant types used in the United States, which can be generically called permeable friction courses (PFCs) and asphalt concrete friction courses (ACFCs). PFCs are an OGFC that are specifically designed to have high air void contents, typically in the range of 18 to 22%, which helps remove water from the pavement surface during a rain event. PFCs have been referred to as porous European mixes in the United States and are effective in improving frictional resistance, reducing splash and spray, improving nighttime visibility, reducing hydroplaning, and reducing pavement noise levels. The term ACFC is applied to OGFC mixes that are not specifically designed for removing water from the pavement surface. Some agencies within the United States use ACFCs as a wearing surface to simply improve frictional resistance and reduce tire/pavement noise levels. These agencies typically include 8 to 9% of rubber modified asphalt binders within ACFCs. Though ACFCs are designed to have relatively high air void contents (10 to 15%), they are not specifically designed to remove large volumes of water from the pavement surface. Of the two OGFC categories, ACFCs are likely more effective at reducing pavement noise levels. However, some agencies have become concerned about the durability of ACFCs, particularly when their air void contents approach 10%. PFCs have become the more common type of OGFC in the United States, and the remainder of this chapter deals specifically with the design of this mixture type. Overview of PFC Mix Design Procedure The design of PFC mixtures is similar to the design of SMA and GGHMA in that PFC should have stone-on-stone contact and low potential for draindown. However, because of the past problems dealing with durability, there is a laboratory test designed specifically to evaluate the potential durability problems of PFC mixtures. NCHRP Report 640 provides much useful information on the mix design, construction, and maintenance of permeable friction courses. The design of PFCs consists of five primary steps (Figure 11-1). The first step is to select suitable materials. Materials needing selection include coarse aggregates, fine aggregates, asphalt binder, and stabilizing additives. Step 2 includes blending three trial gradations using the selected aggregate stockpiles. For each trial gradation, asphalt binder is added and the mixture compacted. The third step in the mix design procedure entails evaluating the three compacted trial blends in order to select the design gradation. Next, the selected design gradation is fixed and the asphalt binder content is varied. The resulting mixtures are evaluated in order to select optimum asphalt binder. Finally, the design gradation at optimum asphalt binder content is evaluated for moisture susceptibility. This manual does not include provisions for performance testing of PFC mixtures, because there is limited experience at present in performing and interpreting rut resistance tests on this type of material. The information given in this chapter is largely taken from Design Construction and Performance of New-Generation Open-Graded Friction Courses, NCAT Report 00-01, by Mallic, Kandhal, Cooley, and Watson. This report is an excellent reference for technicians and engineers involved in the design of OGFC mixtures. Step 1--OGFC Materials Selection The first step in the PFC mix design procedure is to select suitable materials: coarse aggregates, fine aggregates, asphalt binder, and stabilizing additives. Aggregates used in PFC should be cubical, angular, and roughly textured. The stability and strength of PFCs are derived from the stone

OCR for page 195
196 A Manual for Design of Hot Mix Asphalt with Commentary Design of OGFC Mixtures Identify Materials Step 1 Select Aggregates Stabilizer Materials Asphalt Cement Determine VCA of Select Trial coarse aggregate in dry- Gradations rodded condition Step 2 Low % passing Break Medium % passing Break High % passing Break Point Sieve Point Sieve Point Sieve Add Within asphalt cement Specifications and compact Step 3 Analyze data and select optimum gradation Fix gradation and vary asphalt cement content Step 4 Determine optimum asphalt cement content Conduct moisture susceptibility Step 5 No Meet all specifications ? Yes End Figure 11-1. Flow diagram illustrating PFC mix design methodology.

OCR for page 195
Design of Open-Graded Mixtures 197 skeleton and, therefore, the shape and angularity should be such that the aggregates will not slide past each other. Angular, cubical, and textured aggregate particles will lock together providing a stable layer of PFC. Because of the open-graded aggregate structure, the aggregate surface area of PFCs is very low. Like GGHMA and SMA, PFC mixes are required to have a relatively high asphalt binder content. Therefore, the aggregates are coated with a thick film of asphalt binder and the properties of the asphalt binder are important to the performance of PFC. The asphalt binder must be very stiff at high temperatures to resist the abrading action of traffic; however, they should also perform at intermediate and low temperatures. Modified binders are not necessarily required; however, experience indicates better and longer service when modified binders are used. Because of the high asphalt binder content and low aggregate surface area, PFC mixes have a high potential for draindown problems. In order to combat the draindown problems, stabilizing additives are used. The most common stabilizing additive is fiber. Asphalt binder modifiers that stiffen the asphalt binder can also be a considered a stabilizing additive. However, fibers are more effective at reducing draindown potential. The following sections provide requirements for the various materials used in PFC mixes. These requirements are provided for guidance to agencies not having experience with these types of mix- tures. Some agencies have successfully used other test methods and criteria for specifying materials. Coarse Aggregate The success of a PFC pavement depends heavily on the existence of particle-on-particle contact. Therefore, in addition to particle shape, angularity, and texture, the toughness and durability of the coarse aggregates must be such that they will not degrade during production, construction, and service. Table 11-1 presents coarse aggregate requirements for PFC mixtures. Fine Aggregate The role of fine aggregates in a PFC is to assist the coarse aggregate particles in maintaining stability. However, the fine aggregates must also resist the effects of weathering. Therefore, the primary requirements for fine aggregates within a PFC are to ensure a durable and angular material. Requirements for fine aggregates for use in PFCs are provided in Table 11-2. Asphalt Binder Asphalt binders should meet the performance grade requirements of AASHTO M 320-04. Chapter 8 discussed binder selection for dense-graded HMA mixtures in detail; much of this Table 11-1. Coarse aggregate quality requirements for PFC. Spec. Spec. Test Method Minimum Maximum a Los Angeles Abrasion, % Loss AASHTO T 96 - Flat or Elongated, % ASTM D 4791 2 to 1 - 50 Soundness (5 Cycles), % AASHTO T 104 Sodium Sulfate - 15 Magnesium Sulfate - 20 Uncompacted Voids AASHTO T 326 45 - Method A a Aggregates with L.A. Abrasion loss values up to 50 have been successfully used to produce OGFC mixtures. However, when the L.A. Abrasion exceeds approximately 30, excessive breakdown may occur in the laboratory compaction process or during in-place compaction.