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CHAPTER 3 Asphalt Binders Asphalt binders, sometimes referred to as asphalt cement binders or simply asphalt cement, are an essential component of asphalt concrete--they are the cement that holds the aggregate together. Asphalt binders are a co-product of refining crude petroleum to produce gasoline, diesel fuel, lubricating oils, and many other petroleum products. Asphalt binder is produced from the thick, heavy residue that remains after fuels and lubricants are removed from crude oil. This heavy residue can be further processed in various ways, such as steam reduction and oxidation, until it meets the desired set of specifications for asphalt binders. For demanding, high-performance applications, small amounts of polymers are sometimes blended into the asphalt binder, pro- ducing a polymer-modified binder. Asphalt binders have been mixed with crushed aggregate to form paving materials for over 100 years. They are a very useful and valuable material for constructing flexible pavement world- wide. However, asphalt binders have very unusual engineering properties that must be carefully controlled in order to ensure good performance. One of the most important characteristics of asphalt binders that must be addressed in test methods and specifications is that their precise properties almost always depend on their temperature. Asphalt binders tend to be very stiff and brittle at low temperatures, thick fluids at high temperatures, and leathery/rubbery semi-solids at intermediate temperatures. Such extreme changes in properties can cause performance problems in pavements. At high temperatures, a pavement with a binder that is too soft will be prone to rutting and shoving. On the other hand, a pavement that contains a binder that is too stiff at low temperatures will be prone to low-temperature cracking. Figure 3-1 illustrates the extreme change in modulus that occurs in asphalt binders over the range of temperatures likely to occur in pavements; at -30C the modulus of this particular asphalt binder was about 37,000 times greater than its modulus at 50C. Specifications for asphalt binders must control properties at high, low, and intermediate temperatures. Furthermore, test methods used to specify asphalt binders usually must be conducted with very careful temperature control; otherwise, the results will not be reliable. Asphalt binders are also very sensitive to the time or rate of loading. When tested at a fast loading rate, an asphalt binder will be much stiffer than when tested at a slow loading rate. Therefore, time or rate of loading must also be specified and carefully controlled when testing asphalt binders. Another characteristic of asphalt binders that complicates specification and testing of these materials is that, for various reasons, such binders tend to harden with time. For example, when asphalt binders are heated to high temperatures, as happens when mixing and transporting HMA, some of the lighter volatile oil fractions of the asphalt vaporize, which can harden the remaining asphalt binder. At the same time, some of the chemical compounds making up asphalt binders can oxidize, which can also result in an increase in stiffness. Some oxidation occurs during mixing, transport, and placement of the HMA. However, slow, long-term oxidation will continue to occur in the asphalt binder in a pavement for many years, resulting in a slow but sometimes very 15

OCR for page 15
16 A Manual for Design of Hot Mix Asphalt with Commentary 1.E+09 Modulus at -30 oC is 37,000 1.E+08 times the modulus Modulus, Pa 1.E+07 at 50 oC 1.E+06 1.E+05 1.E+04 -40 -20 0 20 40 60 80 Temperature, oC Figure 3-1. Change in dynamic shear modulus with temperature for typical asphalt binder (frequency = 10 rad/s). significant increase in stiffness. Sometimes asphalt binder age hardening can be so severe that it can lead to serious premature surface cracking of the pavement surface. Several other types of hardening occur in asphalt binders without any loss of volatiles or oxidation; these include steric hardening and physical hardening. These phenomena are not yet well understood, but appear to be caused by a slow rearrangement of the molecules in the asphalt binder over time, resulting in a gradual increase in stiffness. Unlike other types of hardening, steric hardening and physical hardening are reversible--if the asphalt is heated until fluid and then cooled, all or most of the hardening will be removed. This is one of the reasons it is important to thoroughly heat and stir asphalt samples prior to performing any laboratory tests. Asphalt binders are complex materials that are difficult to specify and test. Pavement engineers and technicians have struggled for over 100 years to develop simple tests and effective specifica- tions for asphalt binders. One of the earliest tests for asphalt binders was the penetration test, in which a small lightly weighted needle was allowed to penetrate the asphalt for a set period of time (typically 5 or 60 seconds). The distance the needle penetrated into the asphalt was measured and was used as an indication of its stiffness. Other such empirical tests were the ring and ball softening point temperature, and the ductility test. These tests were useful (many are still used in specifications in Europe and other parts of the world), but had shortcomings. They did not measure any fundamental property of the asphalt binder, like modulus or strength. The results were also sometimes highly variable and were not always in close agreement from laboratory to laboratory. In the 1960s, specifications based on viscosity measurements began to be adopted by many highway agencies. Viscosity tests are superior to the earlier empirical tests--they provide information on a fundamental characteristic of the asphalt binder and provide reasonably repeatable results among laboratories. However, there are drawbacks to viscosity testing. First, it is best used at high temperatures, where the behavior of the asphalt binder approaches that of an ideal fluid. At low and intermediate temperatures, viscosity tests become difficult to perform and even more difficult to interpret. Second, viscosity tests only provide a limited amount of infor- mation on the flow properties of a material. Two different asphalt binders can have identical vis- cosity values at a given temperature but might behave very differently because of differences in the degree of elasticity exhibited in their behavior. When loaded, the asphalt binders might deform the same amount, but when the load is removed, one might spring back, or recover, to nearly its initial shape. The other might hardly recover at all, staying in its deformed shape. The asphalt binder that showed more recovery--that behaved in a more elastic fashion--would tend to provide better rut resistance in paving applications compared to the other binder with poor recovery. However, viscosity tests provide no information about recovery or about the degree of elasticity exhibited by a material under loading. The shortcomings in both older empirical tests and in the newer viscosity tests eventually led to the development of a more effective system