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Background 5 Figure 2-1. A compacted HMA laboratory specimen and the aggregate and asphalt used to prepare it. All HMA mixtures contain small amounts of air voids. In the laboratory, HMA mixtures are usually designed to contain about 4% air voids, with a range of about 3 to 5%, depending on the type of mixture being designed and the design procedure being used. Properly constructed HMA pavements will usually contain about 6 to 8% air voids immediately after placement and compaction. After construction, as traffic passes over a pavement, the HMA in the wheel paths will normally gradually compact to air void levels approaching the design value of 3 to 5%. However, if the pavement is not compacted adequately during construction, compaction under traffic will fail to reduce the air void content to the design value and, as a result, the pavement will be permeable to air and water, potentially leading to moisture damage and excessive age hardening. Asphalt Concrete Pavements Asphalt concrete pavements are not simply a thin covering of asphalt concrete over soil-- they are engineered structures composed of several different layers. Because asphalt concrete is much more flexible than portland cement concrete, asphalt concrete pavements are sometimes called flexible pavements. The visible part of an asphalt concrete pavement, the part that directly supports truck and passenger vehicles, is called the surface course or wearing course. It is typically between about 40 and 75 mm thick and consists of crushed aggregate and asphalt binder. Surface course mixtures tend to have a relatively high asphalt content, which helps these mixtures stand up better to traffic and the effects of sunlight, air, and water. Surface course mixtures also are usually made using maximum aggregate sizes less than 19 mm, which helps to provide for a quiet ride. Also, using aggregate sizes larger than 19 mm can make it more difficult to obtain mixtures with sufficient asphalt binder contents to provide adequate durability for surface course mixtures, since the lower aggregate surface area of these aggregates results in a lower demand for asphalt binder. On the other hand, the lower binder content needed for these mixtures can make them more economical than mixtures made using smaller aggregates. Below the surface course of a flexible pavement is the base course. The base course helps provide the overall thickness to the pavement needed to ensure that the pavement can withstand the projected traffic over the life of the project. Base courses may be anywhere from about 100 to 300-mm thick. In general, the higher the anticipated traffic level on a pavement, the thicker
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6 A Manual for Design of Hot Mix Asphalt with Commentary the pavement must be, and the thicker the base course. Thicker pavements will deflect less than thinner ones under traffic loading, which reduces strains within the pavement and makes them more resistant to fatigue cracking. Traditionally, base course mixtures have been designed using larger aggregate sizes than surface course mixtures, with maximum aggregate sizes ranging from about 19 to 37.5 mm. This helps to produce a lean mixture with low asphalt binder content, which helps keep the cost of these mixtures low. Also, using larger aggregate sizes allows base course mixtures to be placed in thicker lifts, which can reduce construction costs. However, many engineers have recently been designing base course mixtures more like surface course mixtures-- with smaller aggregate sizes and higher asphalt binder contents. Using these types of mixtures in base course mixtures can help improve both fatigue resistance and resistance to moisture damage, since increased asphalt binder contents in HMA tends to improve fatigue resistance and will also reduce permeability to water. Sometimes an intermediate course is placed between the surface and base courses of a flexible pavement system. This is sometimes called a binder course. Typically 50 to 100 mm in thickness, it consists of a mixture with intermediate aggregate size and asphalt binder content. The surface, base, and intermediate courses together are referred to as bound material or bound layers, because they are held together with asphalt binder. In a typical asphalt concrete pavement, the bound layers are supported by a granular subbase that in turn lays over the subgrade. Granular subbase is crushed stone or gravel, usually 100 to 300 mm in thickness. The nominal maximum aggregate size varies, but it should always be well compacted prior to placement of the base course. The subgrade is the soil on which the pavement is constructed. If the soil is stable and strong, it may only need compaction prior to placing the granular subbase and remaining pavement layers. However, some soils, including many soils containing clay and clay-like minerals are unstable--that is, they shrink and swell significantly when their moisture content changes, and they can also become very weak when the moisture content becomes too high. Before pavement construction, such a subgrade should be stabilized by blending in lime, portland cement, or other additives, or treating it with asphalt emulsion, and then thoroughly compacting the soil. Sometimes the granular subbase is omitted from a pavement and a relatively thick base course is placed directly on the subgrade soil. Such a pavement structure is called a full-depth asphalt pavement. The advantage of this type of construction is that the overall pavement can be thinner because of the increased strength and stiffness of the supporting pavement. However, it should be remembered that such a base course mixture will be significantly more expensive than granular subbase, since it contains asphalt binder. Figure 2-2 is a cross section of a typical flexible pavement system. Surface course 40-75 mm Intermediate course 50-100 mm Base course 100-300 mm Granular subbase 100-300 mm Subgrade Figure 2-2. Typical asphalt concrete pavement structure. In many cases, the intermediate course is omitted; full-depth asphalt pavements do not include a granular subbase.