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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 C H A P T E R 7 Selection of Asphalt Concrete Mix Type
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 92 A Manual for Design of Hot Mix Asphalt with Commentary HMA wearing course HMA intermediate course crushed aggregate base prepared subgrade HMA wearing course HMA intermediate course crushed aggregate subbase prepared subgrade HMA base course HMA wearing course HMA intermediate course prepared subgrade HMA base course HMA wearing course HMA leveling course crushed aggregate subbase prepared subgrade PCC (a) Conventional HMA Pavement (b) Deep-Strength HMA Pavement (c) Full-Depth HMA Pavement (d) Composite Pavement Figure 7-1. Cross-sections for typical asphalt pavements in new construction.
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 Selection of Asphalt Concrete Mix Type 93 full-depth repair HMA overlay HMA leveling course existing HMA pavement subgrade crushed aggregate subbase joint repair HMA overlay HMA leveling course existing PCC pavement subgrade crushed aggregate subbase HMA overlay HMA leveling course Rubblized PCC base subgrade crushed aggregate subbase (a) HMA Overlay on Existing HMA Pavement (c) HMA Overlay on Rubblized PCC Pavement (b) HMA Overlay on Existing PCC Pavement with Joint Repair Figure 7-2. Cross-sections for typical asphalt pavements in rehabilitation.
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: â¢ Adequate wet weather friction for safety â¢ High resistance to load-induced rutting, shoving, and surface cracking â¢ High resistance to thermally induced cracking â¢ Low permeability to minimize surface-water infiltration â¢ High durability to resist disintegration due to the combined effects of aging, traffic loading, and freeze-thaw effects â¢ Appropriate surface texture for noise control, safety, and aesthetics â¢ 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. 94 A Manual for Design of Hot Mix Asphalt with Commentary
The purpose of the intermediate course is to add thickness to the pavement when additional structural capacity is required in new flexible pavements, rehabilitated asphalt pavements, and rubblized PCC pavements. An intermediate course may also be used in overlays of intact PCC pavement to provide additional thickness to delay reflective cracking or to provide an additional layer to improve pavement smoothness. Since intermediate courses are close to the surface of the pavement, they must be resistant to rutting. However, they can be constructed with mixtures having lower binder contents than surface courses because the intermediate course is not directly subjected to traffic loading or the damaging effects caused by water and oxidative hardening of asphalt binder. Binder courses are typically dense-graded mixtures with nominal maximum aggregate sizes of 19 or 25 mm. Base Course The base course consists of one or more lifts of HMA at the bottom of the pavement structure. The base course is the primary load-carrying element in deep-strength flexible pavements, full- depth flexible pavements, and rubblized PCC pavements. Because base courses are deep in the pavement structure, they do not have to be highly rut resistant. Base course mixtures should be relatively easy to compact to ensure that the base course is durable and resistant to bottom-up fatigue cracking. HMA base courses are typically dense-graded mixtures with nominal maximum aggregate sizes ranging from 19 to 37.5 mm. Leveling Course A leveling course is a thin layer of variable thickness used in rehabilitation to correct variations in the longitudinal or transverse profile of the pavement. They are referred to as scratch courses in some areas of the United States. Mixtures used for leveling courses are either 9.5- or 4.75-mm dense-graded mixtures to facilitate placement and compaction in thin layers. Important Factors in Mix Selection Several important factors should be considered when selecting an HMA mixture for a specific application. These include â¢ Traffic loading â¢ Rut resistance â¢ Fatigue resistance â¢ Durability â¢ Environment â¢ Lift thickness â¢ Appearance Traffic Loading Traffic loading, specifically the amount of truck loading, is a major factor affecting the design and performance of HMA pavements. Traffic loading is normally expressed as the number of 18,000 lb (80 kN) equivalent single-axle loads (ESALs) that the pavement is projected to carry over its design life. Traffic loading is a major factor in pavement structural design; it is used to determine the overall thickness of the pavement. The overall thickness of the pavement increases with increasing traffic loading. It is also a factor in the design of dense-graded mixtures and the selection of the high temperature binder grade for all mixtures. Higher traffic levels place greater demands on the HMA mixture used, particularly for surface and wearing courses. Mixtures Selection of Asphalt Concrete Mix Type 95
designed for higher traffic loading must have greater resistance to both rutting and fatigue cracking. For dense-graded HMA mixture design, the five traffic levels listed in Table 7-1 have been defined. These traffic levels are also used in the recommendations for mixture type presented later in this chapter. Dense-graded mixtures can be used with all traffic levels. GGHMA and OGFC mixtures are more appropriate for pavements with moderate to high levels of traffic. Rut Resistance The required rut resistance of a mixture depends on the traffic level and the location of the mixture in the pavement structure. Pavements with higher traffic levels require greater rut resistance than pavements with low traffic volumes. Surface and intermediate layers require greater rut resistance than base layers. Rut resistance is a consideration in each of the design procedures presented in this manual. For dense-graded mixtures, aggregate angularity, binder grade, compactive effort, and some volumetric properties vary with traffic level and layer depth to provide adequate rut resistance. GGHMA and OGFC mixtures are designed to ensure stone- on-stone contact to minimize the potential for rutting. Binder grade for these mixtures is also selected considering environment and traffic level. Fatigue Resistance Another important consideration related to traffic loading is the resistance of the HMA mixture to fatigue cracking. As discussed in Chapter 2, two types of fatigue cracks have been identified in asphalt pavements: top-down and bottom-up. Thus, fatigue resistance is an important consid- eration for both surface and base course mixtures. Pavements with higher traffic levels require surface and base courses with greater resistance to fatigue cracking. One of the most important mixture design factors affecting fatigue resistance is the effective binder content of the HMA mixture. Fatigue resistance increases with increasing effective binder content; therefore, to resist top-down cracking, dense-graded mixtures of smaller nominal maximum aggregate size and GGHMA mixtures should be considered for high traffic levels. The dense-graded mixture design procedure presented in Chapter 8 provides the flexibility to increase the design VMA requirements up to 1.0% to produce mixtures with improved fatigue resistance and durability. Increasing the VMA requirement increases the effective binder content of these mixtures over that for normal dense-graded mixtures. The use of dense-graded mixtures with higher effective binder content should be considered for base courses in perpetual pavements. One of the structural 96 A Manual for Design of Hot Mix Asphalt with Commentary Traffic Level, ESAL Description < 300,000 Applications include roadways with very light traffic volumes such as local roads, county roads, and city streets where truck traffic is prohibited or at a very minimal level. Traffic on these roadways would be classified as local in nature, not regional, intrastate, or interstate. Special purpose roadways serving recreational sites or areas may also be included at this level 300,000 to < 3,000,000 Applications include many collector roads or access streets. Medium-trafficked city streets and the majority of county roadways may be included at this level. 3,000,000 to <10,000,000 10,000,000 to < 30,000,000 Applications include many two-lane, multilane, divided, and partially or completely controlled-access roadways. Among these are medium to highly trafficked city streets, many state routes, United States highways, and some rural Interstates. â¥ 30,000,0000 Applications include the vast majority of the U.S. Interstate system, both rural and urban in nature. Special applications such as truck-weigh stations or truck-climbing lanes on two lane roadways may also be included at this level. Table 7-1. Traffic levels for HMA mixture design (AASHTO M 323 and R 35).
design considerations for a perpetual pavement is that bottom-up fatigue cracking never occurs in the pavement. Durability Durability is the resistance of an HMA mixture to disintegration due to exposure to the combined effects of weathering and traffic. HMA surface and wearing courses have the most severe exposure, because they are subjected directly to damage by both traffic loading and the environment. The exposure for intermediate and base courses is less, except during staged construction when the intermediate or base layer may temporarily carry traffic for extended time periods. Mixtures subjected to more severe exposure conditions must have greater durability. NCHRP Report 567 summarizes the relationships among HMA composition and performance; for the most durable mixesâones with good fatigue resistance and low permeability to air and waterâhigh binder contents are needed, along with a reasonable amount of fine material in the aggregate. Perhaps most importantly, the mix should be well compacted during construction. In general, both the binder content and the amount of fines in the aggregate blend will increase with decreasing aggregate nominal maximum aggregate size (NMAS). This is one of the reasons that smaller NMAS mixtures are used in surface courses. The effective binder content of GGHMA mixtures is very high due to the gap-graded structure of these mixtures. OGFC mixtures typically incorporate modified binders and fibers to increase the binder content of these mixtures and improve their durability. Environment Environment is a direct consideration in each of the design procedures presented in this manual. The environment in which the pavement will be constructed determines the performance grade of binder that will be used for all mixture types. When considering an OGFC as a wearing course in freezing climates, it is important to recognize that these surfaces may require somewhat different winter maintenance practices. The open structure of OGFCs causes these mixtures to freeze more quickly than dense-graded and GGHMA mixtures, resulting in the need for earlier and more frequent application of deicing chemicals. Additionally, sand should not be used with the deicing chemicals because the sand will plug the pores of the OGFC, decreasing their effectiveness. Lift Thickness Proper compaction of HMA is critical to its long-term performance. Unfortunately, many design engineers consider compaction to be a detail to be worked out by the paving contractor at the time of construction. Adequate compaction may not be possible if lift thickness is not properly considered during pavement design and mixture selection. NCHRP Project 9-27 included field studies to evaluate the effect of lift thickness on the density and permeability of HMA layers. One of the recommendations of this study, as given in NCHRP Report 531, is that the ratio of the lift thickness to nominal maximum aggregate size be 3.0 to 5.0 for fine, dense-graded mixtures and 4.0 to 5.0 for coarse, dense-graded mixtures and GGHMA. OGFCs are typically constructed 19 to 25 mm (3â4 to 1 in) thick. Table 7-2 summarizes the recommendations given in NCHRP Report 531 considering HMA lift thickness. Appearance In some cases, the appearance of the surface is an important consideration. Mixtures with larger aggregate sizes have coarser surface textures, which may not be appropriate for some applications like city streets. Selection of Asphalt Concrete Mix Type 97
Recommended Mix Types This manual presents detailed design procedures for three types of HMA mixtures: dense- graded, GGHMA, and OGFC. Table 7-3 presents recommended mixture types based on traffic level and layer. Dense-Graded Dense-graded HMA mixtures are the most commonly used mixtures in the United States. They can be used in any layer of the pavement structure for any traffic level. Traffic level is a direct consideration in the design of dense-graded mixtures. Aggregate angularity, clay content, binder grade, compactive effort, and some volumetric properties vary with traffic level in the dense-graded mixture design procedure. Dense-graded mixtures also provide the mixture designer with the greatest flexibility to tailor the mixture for the specific application. The dense-graded mixture design procedure presented in Chapter 8 provides the flexibility to increase the design VMA requirements up to 1.0% to produce mixtures with improved fatigue resistance and durability. Increasing the VMA require- ment increases the effective binder content of these mixtures over that for normal dense-graded mixtures. The use of higher effective binder content dense-graded mixtures should be considered for surface and base layers when the traffic level exceeds 10,000,000 ESALs. Dense-graded mixtures can also be designed as fine or coarse mixtures. Fine mixtures generally have a gradation that plots above the maximum density line while coarse mixtures plot below the maximum density line. The definition of fine and coarse mixtures used in AASHTO M 323 is summarized in Table 7-4. For each nominal maximum aggregate size, a primary control sieve has been identified. If the percent passing the primary control sieve is equal to or greater than the specified value in Table 7-4, the mixture classifies as a fine mixture; otherwise it classifies as a coarse mixture. Fine mixtures have smoother surface texture, lower permeability for the same in-place density, and can be placed in thinner lifts than coarse mixtures. 98 A Manual for Design of Hot Mix Asphalt with Commentary Mixture Type Minimum Ratio of Lift Thickness to Nominal Maximum Aggregate Size Maximum Ratio of Lift Thickness to Nominal Maximum Aggregate Size Fine, Dense-Graded 3.0 5.0 Coarse, Dense-Graded 4.0 5.0 GGHMA 4.0 5.0 Table 7-2. Recommended lift thicknesses as given in NCHRP Report 531. Surface Intermediate Base Leveling Traffic Level, ESAL Mix Type NMAS, mm(a) Mix Type NMAS, mm(a) Mix Type NMAS, mm(a) Mix Type NMAS, mm < 300,000 Dense-graded 4.75, 9.5 Dense-graded 19.0, 25.0 Dense-graded 19.0, 25.0, 37.5 Dense-graded 4.75, 9.5 300,000 to < 3,000,000 Dense-graded 4.75, 9.5 Dense-graded 19.0, 25.0 Dense-graded 19.0, 25.0, 37.5 Dense-graded 4.75, 9.5 3,000,000 to <10,000,000 Dense-graded 9.5, 12.5 Dense-graded 19.0, 25.0 Dense-graded 19.0, 25.0, 37.5 Dense-graded 4.75, 9.5 10,000,000 to < 30,000,000 Dense-graded(b, c) GGHMA 9.5, 12.5 9.5, 12.5 Dense-graded 19.0, 25.0 Dense-graded(b) 19.0, 25.0, 37.5 Dense-graded 4.75, 9.5 â¥ 30,000,0000 Dense-graded(b, c) GGHMA 9.5, 12.5 9.5, 12.5 Dense-graded 19.0, 25.0 Dense-graded(b) 19.0, 25.0, 37.5 Dense-graded 4.75, 9.5 aSelect nominal maximum aggregate size to meet requirements of Table 7-2 bConsider increasing design VMA by 1.0% cMay add OGFC wearing course on pavements with high-speed traffic Table 7-3. Recommended HMA mixture types.
GGHMA GGHMA is a gap-graded, densely compacted HMA designed to maximize rut resistance and durability. The principal design consideration in GGHMA is to maximize the contact between particles in the coarse aggregate fraction of the mixture. This fraction provides stability and shear strength to the mixture. The coarse aggregate fraction is then essentially glued together by a binder-rich mastic consisting of a properly selected asphalt binder, mineral filler, and fibers. The fibers are included to stabilize the mixture during handling and placement. The advantages of GGHMA mixtures over dense-graded mixtures include (1) increased resistance to permanent deformation, cracking, and aging and (2) improved durability, wear resistance, low-temperature performance, and surface texture. GGHMA mixtures generally cost more than dense-graded mixtures due to their higher binder content, high filler content, stringent aggre- gate requirements, and the use of polymer-modified binders and fibers. GGHMA should be considered for surface courses when the traffic level exceeds 10,000,000 ESALs. The design of GGHMA mixtures is discussed in Chapter 10. Open-Graded Friction Course (OGFC) OGFC is a gap-graded mixture with a high air void content. The high air void content and open structure of the mixture provides macrotexture and high permeability to drain water from the tire-pavement interface. This minimizes the potential for hydroplaning, improves wet weather skid resistance, and reduces splash and spray. Other benefits of OGFC include reduced noise levels, improved wet weather visibility of pavement markings, and reduced glare. OGFCs are made with durable, polish-resistant aggregates and usually contain modified binders and fibers to increase the binder content and improve their durability. OGFCs generally cost more than dense-graded mixtures. An OGFC may be considered as a wearing course on high-speed pave- ment sections when traffic levels exceed 10,000,000 ESALs. High-speed traffic is an important consideration because it helps keep the pores from clogging with debris. The design of OGFC mixtures is discussed in Chapter 11. Materials Selection for Perpetual Pavements As discussed in the introduction to this chapter, perpetual pavements are intended to provide an exceptionally long service lifeâabout 20 years for the surface course and 50 years or more for the underlying pavement layers. Figure 7-3 illustrates the typical structure of a perpetual pavement. The base material should be flexible and fatigue resistant, meaning it should be designed as either a 9.5-mm or 12.5-mm NMAS mixture. Improved fatigue resistance will usually be obtained through the use of fine aggregate gradations and increased asphalt binder contentâthis means increasing the target VMA by 0.5 to 1.0% over typical design values for the given aggregate size. The high temperature asphalt binder grade for the base material should be high enough to prevent any rutting, but no higher. Otherwise the fatigue resistance of the material might be compromised. The low temperature binder grade should, in general, be one grade higher than that required at the surface. Selection of Asphalt Concrete Mix Type 99 Nominal Maximum Aggregate Size Primary Control Sieve Percent Passing 37.5 mm 9.5 mm â¥ 47 25.0 mm 4.75 mm â¥ 40 19.0 mm 4.75 mm â¥ 47 12.5 mm 2.36 mm â¥ 39 9.5 mm 2.36 mm â¥ 47 Table 7-4. Definition of fine, dense-graded HMA mixtures (AASHTO M323).
The intermediate layer should be a strong, rut-resistant mixture. Although in the past it was believed that relatively coarse-graded mixtures with large NMAS provide optimum rut resistance, more recent research has suggested that equal or even better rut resistance can be obtained using fine-graded mixtures with 9.5- or 12.5-mm NMAS aggregate gradations. Selection of the mix type should be based on obtaining the best rut resistance at a minimum cost. This can probably be best achieved in most cases with a standard, dense-graded HMA mixture. The high temperature binder grade for this layer should be the same as that required for the surface mixture. To ensure that the intermediate layer has a high modulus, the low temperature binder grade should be one grade higher than that used for the surface mixture. Selection of mix type for the surface coarse mixture will depend on the traffic level. For very heavy traffic levels, GGHMA mixtures will provide the best performance and greatest assurance of a long pavement life. At intermediate to high traffic levels, carefully designed dense-graded HMA mixtures should perform well. Normal procedures for binder grade selection should be followed in designing the HMA for the surface course of a perpetual pavement. Engineers and technicians performing mix designs for perpetual pavements should keep in mind that this is a relatively new technology that is likely to undergo changes in the near future. The Asphalt Alliance currently maintains a very useful website providing up-to-date information on perpetual pavements. Additional information on perpetual pavements can also be found in TRB Circular 50: Perpetual Bituminous Pavements Bibliography AASHTO Standards M 323, Superpave Volumetric Mix Design R 35, Superpave Volumetric Design for Hot-Mix Asphalt (HMA) Other Publications Brown, E. R., et al. (2004) NCHRP Report 531: Relationship of Air Voids, Lift Thickness, and Permeability in Hot-Mix Asphalt Pavements, TRB, National Research Council, Washington, DC, 48 pp. Christensen, D. W., and R. F. Bonaquist (2006) NCHRP Report 567: Volumetric Requirements for Superpave Mix Design, TRB, National Research Council, Washington, DC, 57 pp. NAPA, Informational Series 128 (2001) HMA Pavement Mix Type Selection Guide, NAPA, Lanham, MD. TRB Committee on General Issues in Asphalt Technology (A2D05) (2001) TRB Circular 503: Perpetual Bituminous Pavements, TRB, National Research Council, Washington, DC, December, 116 pp. 100 A Manual for Design of Hot Mix Asphalt with Commentary 37 to 50 mm of high-quality HMA or GGHMA 100 to 175 mm of high modulus, rut-resistant HMA 75 to 100 mm of flexible, fatigue- resistant HMA Crushed aggregate subbase or prepared subgrade Figure 7-3. Typical structure for perpetual pavement.