National Academies Press: OpenBook

A Manual for Design of Hot-Mix Asphalt with Commentary (2011)

Chapter: Chapter 3 - Asphalt Binders

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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
×
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Suggested Citation:"Chapter 3 - Asphalt Binders." National Academies of Sciences, Engineering, and Medicine. 2011. A Manual for Design of Hot-Mix Asphalt with Commentary. Washington, DC: The National Academies Press. doi: 10.17226/14524.
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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 −30°C the modulus of this particular asphalt binder was about 37,000 times greater than its modulus at 50°C. 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 C H A P T E R 3 Asphalt Binders

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 16 A Manual for Design of Hot Mix Asphalt with Commentary 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 -40 -20 0 20 40 60 80 Temperature, oC M od ul us , P a Modulus at -30 oC is 37,000 times the modulus at 50 oC Figure 3-1. Change in dynamic shear modulus with temperature for typical asphalt binder (frequency = 10 rad/s).

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 Asphalt Binders 17 (a) (b) Figure 3-2. Traditional test for asphalt binders: (a) penetration test; and (b) viscosity test on asphalt binders.

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. 18 A Manual for Design of Hot Mix Asphalt with Commentary Unaged binder DSR test at high temperature RTFOT aging & mass loss determination DSR test at intermediate temperature PAV aging BBR test at low temperature Direct tension test at low temperature (optional) Figure 3-3. Flow chart for performance grading tests.

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 Asphalt Binders 19 Figure 3-4. RTFOT aging oven.

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 δ (“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. 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. 8-mm plates for intermediate temperature tests 25-mm plates for high temperature tests Figure 3-6. Diagram of DSR test at high and intermediate temperature.

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 δ, in units of kPa. By using both G* and sin δ in the specification, the stiffness and elasticity of the asphalt binder are simul- taneously controlled. Stiff, elastic binders will have a higher G*/sin δ value than soft, fluid binders. Performance-graded binders must have a G*/sin δ 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 δ 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 δ, the specified quantity for the intermediate temperature test is G*  sin δ, since many pavement researchers have found a relationship between G*  sin δ 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 δ 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 Asphalt Binders 21

to determine the low-temperature performance grade for the given binder. The main advantage of the direct tension test and associated analysis compared to BBR grading is that many polymer- modified binders have enhanced fracture properties that will result in a lower grading temperature using the direct tension test compared to that produced by BBR grading. Performance Grading—Specification Table 3-1 lists the various requirements for performance-graded asphalt binders, as described in AASHTO M 320 Table 1. In AASHTO M 320 Table 1, the low-temperature grade of the binder is based on creep stiffness and the m-value of the binder from the BBR. If the binder has a creep stiffness between 300 and 600 MPa, the direct tension failure strain requirement can be used in lieu of the creep stiffness requirement. There is also a Table 2 in AASHTO M 320 which uses the critical low cracking temperature from the direct tension test to determine the low- temperature grade of the binder. AASHTO M 320 Table 1 is the most commonly used specifica- tion for PG binders. 22 A Manual for Design of Hot Mix Asphalt with Commentary Figure 3-7. Sketch of BBR test. (a) (b) Figure 3-8. Photographs of the direct tension test: (a) specimen and mold and (b) test device.

Asphalt Binders 23 PG 46 PG 52 PG 58 Binder Performance Grade: −34 −40 −46 −10 −16 −22 −28 −34 −40 −46 −16 −22 −28 −34 −40 Design high pavement temperature, °C: <46 <52 <58 Design low pavement temperature, °C: ≥34 ≥40 ≥46 ≥10 ≥16 ≥22 ≥28 ≥34 ≥40 ≥46 ≥16 ≥22 ≥28 ≥34 ≥40 Test on Original Binder Flash Point Temperature (T 48), Min., °C 230 Viscosity (T 316) Maximum value of 3 Pa-s at test temperature, °C 135 Dynamic Shear (T 315) G*/sin δ, minimum value 1.00 kPa, at 10 rad/s and Test Temperature, °C 46 52 58 Tests on Residue from Rolling Thin Film Oven (T 240) Mass Loss, Maximum, % 1.00 Dynamic Shear (T315) G*/sin δ, minimum value 2.20 kPa, at 10 rad/s and Test Temperature, °C 46 52 58 Tests on Residue from Pressure Aging Vessel (R 28) PAV Aging Temperature, °C 90 90 100 Dynamic Shear (T 315) G* sin δ, maximum value 5,000 kPa, at 10 rad/s and Test Temperature, °C 10 7 4 25 22 19 16 13 10 7 25 22 19 16 13 Creep Stiffness (T 313) Stiffness, maximum value 300 Mpa m-value, minimum value 0.30, at 60 sec and Test Temperature, °C −24 −30 −36 0 −6 −12 −18 −24 −30 −36 −6 −12 −18 −24 −30 Direct Tension (T 314) Failure strain, minimum value 1.0%, at 1.0 mm/min and Test Temperature, °C −24 −30 −36 0 −6 −12 −18 −24 −30 −36 −6 −12 −18 −24 −30 PG 64 PG 70 Binder Performance Grade: −10 −16 −22 −28 −34 −40 −10 −16 −22 −28 −34 −40 Design high pavement temperature, °C: <64 <70 Design low pavement temperature, °C: ≥10 ≥16 ≥22 ≥28 ≥34 ≥40 ≥10 ≥16 ≥22 ≥28 ≥34 ≥40 Tests on Original Binder Flash Point Temperature (T 48), Min., °C 230 Viscosity (T 316) Maximum value of 3 Pa-s at test temperature, °C 135 Dynamic Shear (T 315) G*/sin δ, minimum value 1.00 kPa, at 10 rad/s and Test Temperature, °C 64 70 Tests on Residue from Rolling Thin Film Oven (T 240) Mass Loss, Maximum, % 1.00 Dynamic Shear (T 315) G*/sin δ, minimum value 2.20 kPa, at 10 rad/s and Test Temperature, °C 64 70 Tests on Residue from Pressure Aging Vessel (R 28) PAV Aging Temperature, °C 100 100 (110) Dynamic Shear (T 315) G* sin δ, maximum value 5,000 kPa, at 10 rad/s and Test Temperature, °C 31 28 25 22 19 16 34 31 28 25 22 19 Creep Stiffness (T 313) Stiffness, maximum value 300 Mpa m-value, minimum value 0.30, at 60 sec and Test Temperature, °C 0 −6 −12 −18 −24 −30 0 −6 −12 −18 −24 −30 Direct Tension (T 314) Failure strain, minimum value 1.0%, at 1.0 mm/min and Test Temperature, °C 0 −6 −12 −18 −24 −30 0 −6 −12 −18 −24 −30 Table 3-1. Specification for performance-graded asphalt binders. (continued on next page)

Critical Temperatures, Specification Values, and Reliability A unique feature of the performance grading system is that it is based not on the values of a given property at a given temperature, but on at what temperature a critical value of that property is achieved. A PG 58-28 binder has a G*/sin δ value of at least 1.0 kPa at 58°C and 10 rad/s in the unaged condition and a maximum flexural creep stiffness of no more than 300 MPa at −18°C at 60 s. The two numbers in the performance grade (PG) refer to extreme high and low pavement temperatures at which the binder is expected to perform adequately. It is important to understand how these extreme pavement temperatures are defined. The high temperature is defined as the yearly, 7-day average maximum pavement temperature, measured 20 millimeters below the pavement surface (referred to as design high pavement temperature). This may seem straight- forward, but because high pavement temperatures are quite variable, the design high pavement temperature will vary from year to year and cannot be defined in a precise, single value. Instead, statistical methods must be used through the concept of reliability. The reliability of a given high pavement temperature refers to the probability that it will not be exceeded in any given year. For example, in Saint Louis, MO, the average value of the design high pavement temperature is 52.9°C. That means that in any given year, there is a 50% chance that the actual high pavement temperature will be lower than this, and a 50% chance that it will be higher. Therefore, the design high pavement temperature at a 50% level of reliability for Saint Louis is 52.9°C. At a 98% level of reliability, the design high pavement temperature is 60.0°C. In other words, in any given year there is a 98% chance that the maximum pavement temperature in Saint Louis will be less than 60°C. The same approach is used in low-temperature performance grading. In this case, the low pavement temperature is defined simply as the minimum pavement temperature at the pavement 24 A Manual for Design of Hot Mix Asphalt with Commentary PG 76 PG 82 Binder Performance Grade: −10 −16 −22 −28 −34 −10 −16 −22 −28 −34 Design high pavement temperature, °C: <76 <82 Design low pavement temperature, °C: ≥10 ≥16 ≥22 ≥28 ≥34 ≥10 ≥16 ≥22 ≥28 ≥34 Tests on Original Binder Flash Point Temperature (T 48), Min., °C 230 Viscosity (T 316) Maximum value of 3 Pa-s at test temperature, °C 135 Dynamic Shear (T 315) G*/sin δ, minimum value 1.00 kPa, at 10 rad/s and Test Temperature, °C 76 82 Tests on Residue from Thin Film Oven (T 240) Mass Loss, Maximum, % 1.00 Dynamic Shear (T 315) G*/sin δ, minimum value 2.20 kPa, at 10 rad/s and Test Temperature, °C 76 82 Tests on Residue from Pressure Aging Vessel (R 28) PAV Aging Temperature, °C 100 (110) 100 (110) Dynamic Shear (T 315) G* sin δ, maximum value 5,000 kPa, at 10 rad/s and Test Temperature, °C 37 34 31 28 25 40 37 34 31 28 Creep Stiffness (T 313) Stiffness, maximum value 300 Mpa m-value, minimum value 0.30, at 60 sec and Test Temperature, °C 0 −6 −12 −18 −24 0 −6 −12 −18 −24 Direct Tension (T 314) Failure strain, minimum value 1.0%, at 1.0 mm/min and Test Temperature, °C 0 −6 −12 −18 −24 0 −6 −12 −18 −24 Table 3-1. (Continued).

surface experienced at a given location in a given year. For Salt Lake City, UT, the average low pavement temperature is −13.6°C. Thus, the design low pavement temperature at 50% reliability is −13.6°C. At a 98% reliability level, the design low pavement temperature at Salt Lake City is −21.3°C. It should be emphasized that the design low pavement temperature is not the same as the minimum air temperature. Typically, the design low pavement temperature is significantly higher than the minimum air temperature for a given location. In Salt Lake City, for example, the average minimum air temperature is −19.6°C, 6 degrees colder than the average design low pavement temperature. Figure 3-9 is a plot of performance grade reliability for design high and low pavement temperatures for Atlanta, GA. In the example illustrated in this plot, at a 65% reliability level, the design high pavement temperature is 54.7°C, and the design low pavement temperature is 7.1°C. Calculation of design high and low pavement temperatures at different reliability levels involves compilation of a wide range of weather data and analysis of this data to produce both average values and standard deviations for design high and low pavement temperatures for thousands of sites throughout the United States and Canada. Fortunately, the software package LTPPBind has been developed to perform these calculations for pavement engineers and technicians. The values in the examples given above were taken from LTPPBind, Version 2.1. LTPPBind also can generate various useful plots, including reliability plots like that shown in Figure 3-9. At the time this manual was being compiled, a new version of LTPPBind—Version 3.0—was in beta release. This newer version of LTPPBind differs substantially from Version 2.1. The most important of these differences is that in Version 3.0, critical high temperatures are based not just on pavement temperatures calculated from historical weather data, but from damage analyses performed using a newly developed rutting model. Version 3.0, once in full release, should provide better estimates for design high pavement temperatures in hot, dry climates—situations where earlier versions of LTPPBind appeared to under-predict high pavement temperatures. The LTPPBind program can be downloaded from the LTPPBind website maintained by the FHWA. An important question is what level of reliability should be used when selecting binders. Engineers and technicians should keep in mind that if a PG binder is selected at a 50% reliability level, there is a 50-50 chance in any year that the high and/or low pavement temperature will exceed those for which the binder has been developed. That is, a pavement made using a binder selected at a 50% reliability level is likely to exhibit rutting and or low-temperature cracking within a few years. Therefore, high reliability levels should be used when selecting binders. For lightly traveled rural and residential roads, reliability levels of at least 90% should be used. For interstate highways Asphalt Binders 25 52 58 64 40 50 60 70 80 90 100 Reliability, % D es ig n Hi gh P vm t. Te m p. , C -16 -10 -4 D es ig n Lo w P vm t. Te m p. , C design low pavement temperature design high pavement temperature -7.1oC 54.7oC 65% Figure 3-9. Example of PG binder grade reliability for Atlanta, GA.

and other major construction projects, reliability levels of at least 95% should be used when selecting performance-graded binders. Practical Selection of PG Binder Grades for HMA Mix Design Although the LTPPBind computer program is very useful, in practice most highway agencies have, through experience, developed their own systems for selecting binder performance grades depending on traffic level and location. This has been done in part because refineries are able to produce only a limited number of binder grades, so engineers must determine two or three per- formance grades that can be used to meet most or all of the paving needs in a given region. This is sometimes referred to as a “binder slate” for a given state or region. For example, a common binder slate in the Mid-Atlantic states involves only three performance grades: 58-28, 64-22, or 76-22. Other binders might be occasionally used in this region, but typically only for small demonstration projects. Engineers selecting performance-graded binders for paving applications should refer to the appropriate specifications for their state or, if there are none, to those in neighboring states with similar climates and conditions. Binder producers may also be useful in providing information concerning what binder performance grades are available locally and which might be most appropriate for a given application. Engineers and technicians using the LTPPBind program without referring to the binders used by the local highway agency may find that the binder they have specified for a given application is not locally available. In selecting performance-graded binders from an available slate, it must be remembered that a given performance grade will meet the requirements of many less extreme situations. For example, in many areas of the Mid-Atlantic, LTPPBind (version 3.1) indicates that a PG 58-22 binder should be used for light traffic. However, this binder may not be available in some Mid-Atlantic states. If the PG 58-22 binder cannot be found (or found at a reasonable price), a PG 64-22 binder would be selected and would perform perfectly well, since its extreme high and low temperature ratings meet or exceed those for these applications. Care should however be used in selecting binders that are much stiffer than required for a given application. Recently, many highway agencies have noticed an increase in surface cracking in HMA pavements. Although such top-down cracking is not yet fully understood, using unnecessarily stiff binders may contribute to the problem. Additional details concerning the selection of asphalt binders for HMA mixtures are given in Chapter 8 of this manual. Bibliography AASHTO Standards M 320, Performance-Graded Asphalt Binder M 323, Superpave Volumetric Mix Design PP 42, Determination of Low-Temperature Performance Grade (PG) of Asphalt Binders R 28, Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel (PAV) R 29, Grading or Verifying the Performance Grade of an Asphalt Binder R 35, Superpave Volumetric Design for Hot-Mix Asphalt (HMA) T 48, Flash and Fire Points by Cleveland Open Cup T 49, Penetration of Bituminous Materials T 51, Ductility of Bituminous Materials T 53, Softening Point of Bitumen (Ring-and-Ball Apparatus) T 202, Viscosity of Asphalts by Vacuum Capillary Viscometer T 240, Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test) T 313, Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR) 26 A Manual for Design of Hot Mix Asphalt with Commentary

T 314, Determining the Fracture Properties of Asphalt Binder in Direct Tension (DT) T 315, Determining the Rheological Properties of Asphalt Binder Using a Dynamic Shear Rheometer (DSR) T 316, Viscosity Determination of Asphalt Binder Using Rotational Viscometer Other Publications The Asphalt Institute, Asphalt Binder Test Manual (MS-25). The Asphalt Institute (2007) The Asphalt Handbook (MS-4A), 7th Ed., 832 pp. The Asphalt Institute (2003) Superpave Performance Graded Asphalt Binder Specifications and Testing, 3rd Ed., 72 pp. Asphalt Binders 27

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 673: A Manual for Design of Hot-Mix Asphalt with Commentary incorporates the many advances in materials characterization and hot-mix asphalt (HMA) mix design technology developed since the conclusion of the Strategic Highway Research Program (SHRP).

The final report on the project that developed NCHRP Report 673 and Appendixes C through F to NCHRP Report 673 were published as NCHRP Web-Only Document 159. The titles of the appendixes are as follows:

• Appendix C: Course Manual

• Appendix D: Draft Specification for Volumetric Mix Design of Dense-Graded HMA

• Appendix E: Draft Practice for Volumetric Mix Design of Dense-Graded HMA

• Appendix F: Tutorial

The companion Excel spreadsheet HMA tool and the training course materials described in NCHRP Report 673 are available for download from the Internet.

In January 2012, TRB released NCHRP Report 714: Special Mixture Design Considerations and Methods for Warm Mix Asphalt: A Supplement to NCHRP Report 673: A Manual for Design of Hot Mix Asphalt with Commentary. The report presents special mixture design considerations and methods used with warm mix asphalt.

In January 2012, TRB released an errata to NCHRP Report 673: Page 41, Table 4-7, and page 123, Table 8-10, respectively, should be replaced with a new table.

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