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OCR for page 98
98 A Manual for Design of Hot Mix Asphalt with Commentary Table 7-2. Recommended lift thicknesses as given in NCHRP Report 531. Mixture Type Minimum Ratio of Lift Maximum Ratio of Lift Thickness to Nominal Thickness to Nominal Maximum Aggregate Size Maximum Aggregate Size Fine, Dense-Graded 3.0 5.0 Coarse, Dense-Graded 4.0 5.0 GGHMA 4.0 5.0 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. Table 7-3. Recommended HMA mixture types. Surface Intermediate Base Leveling Traffic Level, ESAL Mix Type NMAS, Mix Type NMAS, Mix Type NMAS, Mix Type NMAS, mm(a) mm(a) mm(a) mm < 300,000 Dense-graded 4.75, 9.5 Dense-graded 19.0, 25.0 Dense-graded 19.0, 25.0, Dense-graded 4.75, 9.5 37.5 300,000 to < 3,000,000 Dense-graded 4.75, 9.5 Dense-graded 19.0, 25.0 Dense-graded 19.0, 25.0, Dense-graded 4.75, 9.5 37.5 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, Dense-graded 4.75, 9.5 37.5 (b, c) (b) 10,000,000 to < 30,000,000 Dense-graded 9.5, 12.5 Dense-graded 19.0, 25.0 Dense-graded 19.0, 25.0, Dense-graded 4.75, 9.5 GGHMA 9.5, 12.5 37.5 30,000,0000 Dense-graded(b, c) 9.5, 12.5 Dense-graded 19.0, 25.0 Dense-graded(b) 19.0, 25.0, Dense-graded 4.75, 9.5 GGHMA 9.5, 12.5 37.5 a Select nominal maximum aggregate size to meet requirements of Table 7-2 b Consider increasing design VMA by 1.0% c May add OGFC wearing course on pavements with high-speed traffic

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Selection of Asphalt Concrete Mix Type 99 Table 7-4. Definition of fine, dense-graded HMA mixtures (AASHTO M323). Nominal Maximum Primary Control Percent Aggregate Size Sieve Passing 37.5 mm 9.5 mm 47 25.0 mm 4.75 mm 40 19.0 mm 4.75 mm 47 12.5 mm 2.36 mm 39 9.5 mm 2.36 mm 47 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 life--about 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 content--this 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.

OCR for page 98
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. 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.