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10 A Manual for Design of Hot Mix Asphalt with Commentary Raveling Raveling occurs when tires dislodge aggregate particles from the surface of an HMA pavement. Many of the same factors that contribute to poor fatigue resistance will also contribute to raveling, including low asphalt binder contents and poor field compaction. Because the pavement surface is exposed to water from rain and snow, poor moisture resistance can also accelerate raveling in HMA pavements. Asphalt Concrete Mixtures Asphalt concrete mixtures can be classified in many different ways. Perhaps the most general type of classification is by whether or not the mix must be heated prior to transport, placement, and compaction. HMA concrete, or simply HMA, must be thoroughly heated during mixing, transport, placement, and compaction. The asphalt binder used in HMA is quite stiff at room temperatures, so that once this type of asphalt concrete cools it becomes stiff and strong enough to support heavy traffic. Cold mix asphalt, on the other hand, is normally handled, placed, and compacted without heating. This material can be handled cold because it uses liquid asphalts in the form of emulsions and cutbacks that are fluid at room temperature. Asphalt emulsions are mixtures of asphalt, water, and special chemical additives called surfactants that allow the other two materials to be blended into a stable liquid. When blended with aggregate, the emulsion "breaks," meaning the asphalt separates from the water and thoroughly coats the aggregate. Cutback asphalts are blends of asphalt binder and petroleum solvents. Once placed, cold mix made with cutback asphalts gradually cure as the solvent evaporates from the asphalt concrete. Many engineers now avoid the use of cutback asphalts because of environmental concerns. Cold mix is economical because it does not require large amounts of energy to heat the mix during production and placement. However, it is difficult to compact thoroughly and in general is not as durable as HMA. Cold mix is sometimes used for base course construction and is also commonly used for patching and repairing pavement. A new, third type of mix--called warm-mix asphalt (WMA)--has recently become increasingly popular. In this type of mixture, various different methods are used to significantly reduce mix production temperature by 30 to over 100F. These methods include (1) using chemical additives to lower the high-temperature viscosity of the asphalt binder; (2) techniques involving the addition of water to the binder, causing it to foam; and (3) two-stage processes involving the addition of hard and soft binders at different points during mix production. WMA has several benefits, including lower cost (since significantly less fuel is needed to heat the mix), lower emissions and so improved environmental impact, and potentially improved performance because of decreased age hardening. There is some concern that WMA might in some cases be more susceptible to moisture damage, but this has yet to be clearly demonstrated. This manual deals exclusively with HMA of which there are three different major types-- dense-graded mixtures, gap-graded mixtures, and open-graded mixtures. Dense-graded mixtures are the most common HMA mix type. The term dense-graded refers to the dense aggregate gradation used in these types of mixtures, which means that there is relatively little space between the aggregate particles in such mixtures. Historically, dense-graded mixtures were popular because they required relatively low asphalt binder contents, which kept their cost down. However, experience has shown that HMA with binder contents that are too low can be difficult to place and compact and may be prone to surface cracking and other durability problems. Therefore, many "dense-graded" HMA mixtures do not use a true maximum density gradation, but use somewhat "open" gradations that deviate slightly from maximum density; such mixtures have more space between the aggregate particles and can be designed to contain more asphalt binder. Mixtures that are somewhat coarser than the maximum density gradation are called coarse-

OCR for page 10
Background 11 graded mixtures, while mixtures somewhat finer than the maximum density gradation are called fine-graded mixtures. This terminology can be somewhat confusing, since both coarse- and fine-graded mixtures should be considered variations of dense-graded HMA. A more appropriate terminology is to refer to the three types of dense-graded HMA as dense/dense-graded, dense/ coarse-graded, and dense/fine-graded mixtures. When engineers and technicians first began developing mix designs using the Superpave system in the 1990s, there was a clear trend toward dense/coarse-graded mixtures, in order to increase the rut resistance of HMA pavements. However, in the past few years, many agencies have shifted back toward finer mixtures (dense/dense or dense/fine), to help improve the durability of surface course mixtures. Also, recent research has suggested that dense/fine HMA mixtures can, in most cases, be designed to have just as much rut resistance as dense/coarse mixtures. The procedure for designing dense-graded HMA mixtures given in this manual (in Chapter 8) suggests that a range of gradations be evaluated during the mix design process and that the gradation most effective in meeting the given mixture specifications should be selected. The suggested volumetric requirements do include a slight increase in the allowable range for dust-to-binder ratio and an optional table for high-durability mixtures that includes an even higher dust-to-binder ratio and an increase in minimum VMA; both of these changes will probably reduce the number of dense/coarse-graded HMA mixtures being designed under this system. During the past 20 years, stone-matrix asphalt (SMA) has become increasingly common in the United States and Europe. SMA is a special type of HMA designed specifically to hold up under very heavy traffic. SMA is composed of high-quality coarse aggregate, combined with a large amount of mastic composed of a high-performance asphalt binder, mineral filler, and a small amount of fibers. The aggregate used in SMA contains a large amount of coarse aggregate and a large amount of very fine material (called mineral filler), but not much sand-sized material. For this reason, such aggregates are called gap-graded, and SMA and similar HMA types are referred to as gap-graded mixtures, or gap-graded HMA (GGHMA)--the term used in this manual. A well-developed coarse aggregate structure in combination with a relatively large volume of high performance binder helps ensure that a properly designed SMA mixture will exhibit excellent per- formance. SMA is usually only used on very heavily trafficked roadways, where its excellent per- formance makes it cost-effective despite the high initial investment required to construct SMA pavements. The design of GGHMA mixtures is discussed in detail in Chapter 10 of this manual. Figure 2-6 shows an SMA surface course on a dense-graded HMA base. Figure 2-6. SMA surface course on dense-graded HMA base.