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Asphalt Binders 17 (a) (b) Figure 3-2. Traditional test for asphalt binders: (a) penetration test; and (b) viscosity test on asphalt binders. of grading asphalt binders, as described in the remainder of this chapter. Photographs of the penetration test and the capillary viscosity test are shown in Figure 3-2. Performance Grading of Asphalt Binders--Overview Performance grading of asphalt binders was developed during SHRP. The main purpose of this way of classifying and selecting asphalt binders is to make certain that the binder has the correct properties for the given environment. Performance grading was also meant to be based more soundly on basic engineering principles--earlier methods of grading binders often used empirical tests, which were useful but did not provide any information on the fundamental engineering properties of the binder. Performance grading uses various measurements of the binder's flow properties to establish its grade, which is expressed as two numbers, for example "PG 64-22." In this example, the "64" represents the maximum pavement temperature for which this binder can be used at low [moderate?] traffic levels. The second number, "-22," signifies the minimum temperature for which the binder can be used without likelihood of failure. It is essential to understand when using the performance grading system that the numbers in the grade designation represent the most extreme temperatures for which that binder is suited. For example, if a given application requires a PG 58-16 binder, there are many other grades that could meet the requirements: PG 58-22, PG 58-28, PG 64-16, PG 64-22, etc. This is only a simple explanation of the basic features of performance grading; the details of the system are more complicated and are explained below. Performance Grading--Test Methods Most of the tests used in performance grading of asphalt binders involve rheological tests. Rheology is the study of the way materials flow, so a rheological test is one that measures one or more aspects of the way in which a material flows. The dynamic shear rheometer (DSR) and the

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18 A Manual for Design of Hot Mix Asphalt with Commentary bending beam rheometer (BBR) both measure the flow properties of asphalt binders--the DSR at temperatures ranging from about 10 to 82°C, the BBR at temperatures ranging from -20 to 0°C. It may at first be surprising that asphalt binders flow at temperatures below 0°C, where it normally appears to be a brittle, glassy material. But asphalt binder will in fact flow even at very low temperatures, although this flow might take months or even years before it is noticeable to the naked eye. Asphalt binders can be characterized as they are produced at the refinery using these rheological tests. Unfortunately, this is not enough to give technicians and engineers a good idea of how a binder will perform in a pavement, since asphalt binders in a pavement harden during mixing, transport, and placement of the mix, and even after the pavement has been placed. Therefore, laboratory procedures are needed to estimate the amount of such age hardening. Two laboratory age-hardening procedures are used in the performance grading system: the rolling-thin film oven test (RTFOT) and the pressure aging vessel (PAV). The way in which these aging tests are used in combination with the rheological tests is illustrated in Figure 3-3. DSR tests at high temperature are performed on both the unaged binder and on the residue from the RTFOT aging procedure. Residue from the RTFOT procedure is then aged in the PAV, and additional tests are performed on the residue from this procedure. The DSR test at intermediate temperature and the BBR test are always performed. An optional test, the direct tension test, is also sometimes performed on the PAV residue. Unlike the other performance grading tests, the direct tension procedure does not measure flow properties, but instead measures the fracture properties of the asphalt binder at low temperatures. The direct tension test is useful for grading some modified binders with unusually high strength and toughness, since it will improve the low temperature grade. The sections below describe in additional detail each of the grading procedures. The aging procedures are discussed first, followed by the actual binder tests. This is followed by a discussion of the grading procedure and a section covering practical aspects of performance grade selection for HMA mix design. Unaged binder DSR test at high temperature RTFOT aging & mass loss determination PAV aging DSR test at BBR test at intermediate low temperature temperature Direct tension test at low temperature (optional) Figure 3-3. Flow chart for performance grading tests.

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Asphalt Binders 19 RTFOT Aging RTFOT aging is meant to simulate asphalt binder age hardening as it occurs during mixing, transport, and placement. In this procedure, 35 grams of asphalt are carefully weighed into glass bottles, which are placed in a circular rack in a specially designed oven. The rack slowly rotates the bottles while the oven maintains the test temperature of 323°C. During the test, a jet of air is blown into the bottles for a few seconds once every rotation. The test is continued for 75 minutes; the bottles are then removed from the oven, cooled, and weighed. The percent mass loss is calculated from the initial and final weight of the asphalt binder in the bottle. High values of mass loss mean that a significant amount of light oils have volatized during aging and, as a result, the asphalt binder might be prone to excessive age hardening, shrinkage, and cracking. Current performance grading standards require that mass loss during RTFOT aging be no more than 1.0%. After mass loss determination, the bottles are heated, and the asphalt is poured either into a tin for further testing, or into PAV pans for additional aging. Figure 3-4 shows an RTFOT oven. PAV Aging In the PAV aging test, the technician fills 125-mm-diameter stainless steel pans with asphalt that has already been aged in the RTFOT test. Six of these pans are placed in a vertical rack, which is then placed in the pressure vessel, which in turn is placed inside an oven. The pressure vessel is a heavily constructed steel chamber, designed to withstand the high pressure and temperature used in the PAV test. These high temperatures and pressures help accelerate aging of the asphalt binder. At the end of the PAV test, the asphalt binder has aged about as much as would typically occur in a pavement after several years of service. Figure 3-5 shows the various pieces of equipment used in performing the PAV aging procedure. DSR Test at High Temperature The primary purpose of the DSR test at high temperature is to ensure that a properly specified asphalt binder will have the proper engineering properties at high temperature and, when used in an HMA mix, will keep it from rutting and shoving under traffic. The DSR is a torsional test in which a thin specimen of asphalt binder is sheared between two circular plates. It is also Figure 3-4. RTFOT aging oven.

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20 A Manual for Design of Hot Mix Asphalt with Commentary (a) (b) Figure 3-5. PAV aging test: (a) pan; and (b) rack filled with pans. a dynamic test, meaning that the specimen is sheared very quickly in a back and forth cycle of loading. In the high-temperature test, the steel plates are 25 mm in diameter, and the specimen is about 1-mm thick. Figure 3-6 is a sketch of the DSR test, showing both the high temperature setup and the smaller plates used for the intermediate temperature test (described in detail in the following section). The applied strain varies depending on the stiffness of the binder. The test is performed at various temperatures, depending on the grade of the asphalt: 46, 52, 58, 64, 70, 76, and 82°C are the standard temperatures for high-temperature DSR tests. The first number in a performance grade is the standard high temperature for DSR testing for that binder. For example, one of the most common grades of asphalt binder is PG 64-22; the high-temperature DSR test on this binder would be performed at 64°C. The DSR test measures both modulus and phase angle. Modulus is a measure of stiffness-- the higher the value, the stiffer the binder. Because the DSR test is a shear test, the modulus value is called the dynamic shear modulus, abbreviated with the symbol G*. The "G" indicates that the modulus is a shear value, and the "*" indicates that it is a dynamic modulus. In rheological tests like the DSR, the phase angle is a measure of how fluid a material is. The more a material behaves like a fluid, the higher the phase angle. Materials that behave like an elastic solid--that spring back quickly after loading--have a low phase angle. Phase angle is often abbreviated using the Greek letter ("delta"). When a material with a high phase angle is loaded and deforms, and the load is removed, the material will tend to stay in its deformed shape--it will not spring back. 25-mm plates for 8-mm plates for high temperature intermediate tests temperature tests Figure 3-6. Diagram of DSR test at high and intermediate temperature.

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Asphalt Binders 21 Phase angle should not be confused with stiffness or modulus. A very stiff clay might have a higher modulus than a soft rubber, but a higher phase angle. This means it will deform less than the rubber under loading, but will not recover any of this deformation once the load is removed. In the high-temperature DSR test, the quantity specified is G*/sin , in units of kPa. By using both G* and sin in the specification, the stiffness and elasticity of the asphalt binder are simul- taneously controlled. Stiff, elastic binders will have a higher G*/sin value than soft, fluid binders. Performance-graded binders must have a G*/sin value of at least 1.0 kPa at the specified grading temperature in the unaged condition, using a test frequency of 10 rad/s. After RTFOT aging, the minimum value of G*/sin is 2.2 kPa. DSR Test at Intermediate Temperature The DSR test at intermediate temperature uses the same basic principles as the high-temperature test, but there are a few important differences. The DSR test at intermediate temperatures is designed to prevent binders from becoming too stiff at intermediate temperatures, which can contribute to premature fatigue cracking in pavements. This also helps to control the overall flow properties of the asphalt binder. Because the asphalt binder is much stiffer at the lower test temperatures, the plates must be smaller and the specimen thicker, as shown in Figure 3-6. For the intermediate temperature test, 8-mm-diameter plates are used, and the specimen is 2 millimeters thick. Instead of G*/sin , the specified quantity for the intermediate temperature test is G* sin , since many pavement researchers have found a relationship between G* sin and fatigue resist- ance for HMA mixtures. The DSR test at intermediate temperatures is run after RTFOT and PAV conditioning, at temperatures ranging from 4 through 40°C, in 3° increments (4, 7, 10°C, etc). The maximum allowable value for G* sin is 5,000 kPa, at a frequency of 10 rad/s. BBR Test The purpose of the BBR test is to make sure that asphalt binders do not become too stiff and brittle at low temperatures, since this can contribute to transverse cracking in HMA pavements. The BBR test is a flexural stiffness test--a small beam of asphalt is loaded for 1 minute and the deflection is measured. From the applied load and resulting deflection, the creep stiffness of the asphalt binder is calculated. In analyzing the BBR data, another quantity, called the m-value, is also calculated. The m-value is the log-log slope of the creep curve at a given loading time. The BBR specimen is 125 millimeters long, 12.5 millimeters wide, and 6.25 millimeters thick. The test can be run at test temperatures of -36, -30, -24, -18, -12, -6 and 0°C. When performance grading an asphalt binder, the BBR test is run at a temperature 10°C higher than the low grading temperature. A performance 64-22 binder, for example, would be tested using the BBR at -12°C. The maxi- mum allowable stiffness in the BBR test is 300 MPa at 60 seconds, and the minimum m-value is 0.300 at the same loading time. Figure 3-7 is a sketch of the BBR test. Direct Tension Test The direct tension test is unique among the binder specification tests in that it is a fracture test, and not a rheological test. In this procedure, a small specimen of asphalt is slowly pulled apart in tension until it fails. Figure 3-8 illustrates the direct tension test. This test should not be confused with the older ductility test, which is performed at higher temperatures at much higher strains, and is an empirical test that does not provide any useful information on engineering properties. The results of the direct tension test are strain and stress at failure, and the test is performed at low temperatures at very low strains and strain rates. These results can be used to perform an analysis of low-temperature thermal stresses that produces an estimated cracking temperature for the binder, as outlined in AASHTO Provisional Standard PP 42. This temperature is then used