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CHAPTER 2 Background To thoroughly understand the mix design procedures and test methods presented herein, a basic knowledge of construction materials and paving technology is needed. This information is summarized below. Engineers and technicians with a broad range of experience in materials testing and the design of HMA mixtures and flexible pavement structures need not read this chapter in detail. Individuals who are relatively new to asphalt pavement technology will find the information on materials, asphalt pavements, asphalt concrete mixtures, and mix design methods helpful when reading the later chapters of this manual. Materials Used in Making Asphalt Concrete Asphalt concrete is composed primarily of aggregate and asphalt binder. Aggregate typically makes up about 95% of an HMA mixture by weight, whereas asphalt binder makes up the remaining 5%. By volume, a typical HMA mixture is about 85% aggregate, 10% asphalt binder, and 5% air voids. Small amounts of additives and admixtures are added to many HMA mixtures to enhance their performance or workability. These additives include fibers, crumb rubber, and anti-strip additives. Figure 2-1 shows a typical HMA laboratory specimen and the materials used to produce it. Asphalt binder holds the aggregate in HMA together--without asphalt binder, HMA would simply be crushed stone or gravel. Asphalt binder is the thick, heavy residue remaining after kerosene, gasoline, diesel oil, and other fuels and lubricants are refined from crude oil. Asphalt binder consists mostly of carbon and hydrogen, with small amounts of oxygen, sulfur, and several metals. The physical properties of asphalt binder vary tremendously with temperature. At high temperatures, asphalt binder is a fluid with a consistency similar to that of motor oil. At room temperature most asphalt binders will have the consistency of putty or soft rubber. At subzero temperatures, asphalt binder can become very brittle--asphalt samples stored in a freezer will shatter like glass if dropped on a hard surface. Many asphalt binders contain small percentages of polymer to improve their physical properties; these materials are called polymer- modified binders. Much of the current asphalt binder specification used in the United States was designed to control changes in consistency with temperature. This specification and the associated test methods are discussed in more detail in Chapter 3. Because HMA mixtures are mostly aggregate, aggregates used in HMA must be of good quality to ensure the resulting pavement will perform as expected. Aggregates used in HMA mixtures may be either crushed stone or crushed gravel. In either case, the material must be thoroughly crushed, and the resulting particles should be cubical rather than flat or elongated. Aggregates should be free of dust, dirt, clay, and other deleterious materials. Because aggregate particles carry most of the load in HMA pavements, aggregates should be tough and abrasion resistant. Properties of aggregates and the tests that technicians use to evaluate them are discussed in detail in Chapter 4 of this manual. 4