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CHAPTER 7 Selection of Asphalt Concrete Mix Type The selection of an appropriate HMA mixture for a specific paving application is important in designing new pavements and for rehabilitation strategies for existing pavements. The type of mixture selected for the various layers of a pavement has a major effect on the cost, constructability, and long-term performance of the pavement. Mixtures with lower binder contents and lower quality aggregates are less expensive. To facilitate placement and compaction, thinner layers should be made with smaller nominal maximum aggregate size mixtures, while thick base layers should be made with larger nominal maximum aggregate sizes. Mixtures at the surface of a pavement should have relatively high binder content to make them more resistant to the damaging effects of traffic and the environment. Lower binder contents can be used in mix- tures for intermediate and base courses because they are protected by the layers above them. Careful consideration of mix type is an important factor when staged construction is used, because the base or intermediate courses must serve temporarily as the surface during the first stages of construction. This chapter provides recommendations for mixture type selection considering traffic, environ- ment, constructability, and economics. It discusses the appropriate use of the three HMA mix types that can be designed using the procedures presented in this manual: dense-graded, gap-graded (GGHMA), and open-graded friction course (OGFC). Although the types of mixtures to be used in a project are usually selected during the design phase, it is important that mixture designers understand the rationale behind the selection of mixtures for specific applications. In some cases, the engineer responsible for a mix design may be asked to suggest a mix type for a given application. The recommendations presented in this chapter largely follow those contained the National Asphalt Pavement Association (NAPA) Publication IS 128, HMA Pavement Mix Type Selection Guide. The interested reader should refer to this publication for additional information concerning mixture type selection. Pavement Structure and Construction As discussed in Chapter 2, asphalt concrete pavements are engineered structures consisting of multiple layers or courses of hot-mix asphalt (HMA) and other materials. The structural HMA layers are usually referred to as surface, intermediate, and base courses depending on their location in the pavement structure. The intermediate course is sometimes called the binder course. Some pavements with higher traffic volumes may also include a wearing course composed of OGFC placed over the surface course. Each HMA layer in a pavement is composed of different materials and is placed in one or more lifts using separate paving operations. Each layer has a specific function that affects the type of mixture that should be specified and used. Figures 7-1 and 7-2 show typical cross sections for asphalt pavements commonly encountered in new construction and rehabilitation. 91

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92 A Manual for Design of Hot Mix Asphalt with Commentary HMA wearing course HMA wearing course HMA intermediate course HMA intermediate course crushed HMA base aggregate course base crushed aggregate prepared subgrade subbase prepared subgrade (a) Conventional HMA Pavement (b) Deep-Strength HMA Pavement HMA wearing course HMA wearing course HMA leveling course HMA intermediate course PCC HMA base crushed course aggregate subbase prepared subgrade prepared subgrade (c) Full-Depth HMA Pavement (d) Composite Pavement Figure 7-1. Cross-sections for typical asphalt pavements in new construction. As shown in Figure 7-1, there are four types of new pavements depending on the type of base and the overall thickness of the HMA layers. Conventional flexible pavements, shown in Figure 7-1a, consist of relatively thin layers of HMA constructed over an unbound aggregate base. In this type of pavement, the unbound aggregate base is thick and is the major load-carrying element in the pavement. Conventional flexible pavements are primarily used on roadways with low traffic volumes. Flexible pavements that carry moderate to high traffic volumes are either deep-strength or full-depth. Deep-strength HMA pavements, shown in Figure 7-1b, have a relatively thick HMA base constructed on an unbound aggregate subbase, while in full-depth HMA pavements, shown in Figure 7-1c, all layers above the prepared subgrade are constructed with HMA. The HMA base is the primary load-carrying element in both of these pavement types. The unbound aggregate subbase in deep-strength HMA pavements provides a working platform for paving, and in some areas, additional thickness for frost protection. Composite pavements, shown in Figure 7-1d, consist of an HMA surface constructed on Portland cement concrete (PCC). The PCC is the primary load-carrying element in composite pavements. Composite pavements are constructed by design in some urban areas or during lane widening on PCC rehabilitation projects that include an HMA overlay where it is desired to maintain the same pavement cross section in the new lanes and the existing lanes. Perpetual pavement, a relatively new concept, is intended to provide a pavement with a very long-lasting underlying structure combined with a durable wearing course. Ideally, the pavement structure should last 50 years or more without replacement, while the surface course might need replacement every 20 years. Selection of mixtures for perpetual pavements is discussed at the end of this chapter. Pavement rehabilitation with HMA can result in two types of pavements as shown in Figure 7-2. Rehabilitation of existing asphalt pavements, shown in Figure 7-2a, is almost always accomplished using an HMA overlay. Prior to constructing the overlay, areas of the pavement that exhibit alligator or fatigue cracking must be repaired to full depth because the base of the existing pavement remains the primary load-carrying element in the flexible pavement after construction

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Selection of Asphalt Concrete Mix Type 93 HMA overlay HMA leveling course full-depth existing HMA repair pavement crushed aggregate subbase subgrade (a) HMA Overlay on Existing HMA Pavement HMA overlay HMA leveling course joint existing PCC repair pavement crushed aggregate subbase subgrade (b) HMA Overlay on Existing PCC Pavement with Joint Repair HMA overlay HMA leveling course Rubblized PCC base crushed aggregate subbase subgrade (c) HMA Overlay on Rubblized PCC Pavement Figure 7-2. Cross-sections for typical asphalt pavements in rehabilitation. of the overlay. If the existing surface course is in reasonably good condition, there is adequate vertical clearance, and safety hardware can accommodate an increase in pavement elevation, the overlay may be placed directly on the existing surface course. If the existing pavement includes an OGFC; the surface course is rutted, cracked or highly weathered; or it is important to maintain the existing elevation of the pavement, then the existing pavement is milled to an appropriate depth prior to placement of the overlay. A thin leveling or scratch course of variable thickness may be placed on the existing or milled pavement to improve smoothness prior to placing the HMA overlay. If strengthening is required due to an anticipated change in traffic volume, an intermediate course may also be added. Rehabilitation of existing PCC pavements with HMA involves placing one or more layers of HMA over the PCC. The HMA may be placed directly on the existing PCC, shown in Figure 7-2b, after repair of cracked PCC slabs and joints that exhibit poor load transfer. When the HMA is placed directly on the intact PCC, the PCC is the primary load-carrying element of the rehabilitated pavement. The HMA overlay is often saw cut at the location of the PCC joints to control reflective cracking in the HMA. The saw cuts are sealed at the time of construction. Alternatively, as shown in Figure 7-2c, the PCC slab may be broken or rubblized to control reflective cracking. In this case much thicker HMA layers are placed over

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94 A Manual for Design of Hot Mix Asphalt with Commentary the broken or rubblized PCC. The new HMA base serves as the primary load-carrying element in the rehabilitated pavement. A thin leveling course of variable thickness may be placed on the broken or rubblized PCC to improve smoothness prior to placing the HMA layers. The sections that follow describe in greater detail the function and characteristics of each of the HMA layers shown in Figure 7-1 and 7-2. These characteristics are important factors in the selection of appropriate mixture types for each layer. Surface Course The surface course is the uppermost structural layer in an asphalt pavement. In most cases it is the top layer of the pavement and also serves as the wearing course. Since it is directly exposed to traffic and environmental forces, it must be produced with the highest quality materials. The surface course provides the following characteristics for an asphalt pavement: Adequate wet weather friction for safety High resistance to load-induced rutting, shoving, and surface cracking High resistance to thermally induced cracking Low permeability to minimize surface-water infiltration High durability to resist disintegration due to the combined effects of aging, traffic loading, and freeze-thaw effects Appropriate surface texture for noise control, safety, and aesthetics Smoothness. Because the surface course is made with the highest quality materials, economics dictate that it be the thinnest pavement layer, typically 25 to 75 mm (1.0 to 3.0 in) thick. Surface course mixtures are typically only one lift thick and made with nominal maximum aggregate sizes of 12.5 mm or less. Smaller nominal maximum aggregate size mixtures can be placed in thinner layers, have higher binder contents, and, when compacted to the same in-place air void content, have lower permeability than larger nominal maximum aggregate size mixtures. Surface courses contain highly angular aggregates and an appropriate performance-graded binder to resist traffic and environmental forces. If the surface course is also the top layer in the pavement, then the aggregates must be resistant to polishing under traffic loading to provide appropriate skid resistance over the service life of the pavement. Dense-graded and GGHMA mixtures are commonly used as surface courses. OGFC Wearing Course Some moderate- to high-traffic pavements may include an OGFC as a wearing course on top of the surface course to improve skid resistance, reduce splash and spray, and reduce noise. These characteristics of OGFCs are the result of the open pore structure of these mixtures. OGFCs are made with durable crushed aggregates and often include modified binders and fibers to increase the binder content and improve durability. Because OGFCs are very permeable, the surface course directly beneath them must be impermeable to minimize infiltration of water into the pavement structure. To avoid trapping water in the pavement structure, OGFCs should be daylighted at the shoulders and milled from the pavement before placing future overlays. Intermediate Course The intermediate or binder course consists of one or more lifts of HMA between the surface and base courses. Not all pavements have an intermediate course; the need for an intermediate course depends on the overall thickness of the HMA and the thickness of the base and surface courses.