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OCR for page 95
Selection of Asphalt Concrete Mix Type 95 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 Traffic loading Rut resistance Fatigue resistance Durability Environment Lift thickness 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

OCR for page 95
96 A Manual for Design of Hot Mix Asphalt with Commentary Table 7-1. Traffic levels for HMA mixture design (AASHTO M 323 and R 35). Traffic Level, ESAL Description Applications include roadways with very light traffic volumes such as local roads, county roads, and city streets where truck < 300,000 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 Applications include many collector roads or access streets. 300,000 to < 3,000,000 Medium-trafficked city streets and the majority of county roadways may be included at this level. 3,000,000 to <10,000,000 Applications include many two-lane, multilane, divided, and partially or completely controlled-access roadways. Among these 10,000,000 to < 30,000,000 are medium to highly trafficked city streets, many state routes, United States highways, and some rural Interstates. Applications include the vast majority of the U.S. Interstate system, both rural and urban in nature. Special applications such as 30,000,0000 truck-weigh stations or truck-climbing lanes on two lane roadways may also be included at this level. 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

OCR for page 95
Selection of Asphalt Concrete Mix Type 97 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 mixes--ones with good fatigue resistance and low permeability to air and water--high 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 (3/4 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.