3. Asphalt Materials and Mix Design

3.2.1 Penetration Grading Systems

The penetration of an asphalt binder (AASHTO T 49, ASTM D946) is the penetration of a weighted needle into a binder sample measured in units of decimillimeters (0.1 mm) at 25 °C (77 °F). Asphalt mixtures with stiffer binders (i.e., with a lower penetration) will be stiffer at a given temperature than mixes with softer binders (i.e., with a higher penetration). For example, at a given temperature, a mix containing binder classified as 60–70 penetration grade will be stiffer and may require more compactive effort by the rollers to achieve the desired density than will a mix containing a 120–150 penetration grade asphalt binder. This empirical test is an indicator of stiffness at one temperature, but any relationship with overall mix performance can be significantly variable.

3.2.2 Viscosity Grading Systems

Grading of asphalt binders by viscosity (resistance to flow) is defined by a viscosity measurement at 60 °C (140 °F) on the material in its original (as received from the refinery) condition (termed AC, for asphalt cement) or on a binder considered to be comparable to the material after it has passed through the high temperatures of the plant production process (termed AR, for aged residue). In the AC grading system, a mix containing an AC-20 will be stiffer than a mix containing an AC-10. Similarly, in the AR grading system, a mix containing an AR-4000 will be stiffer than one containing an AR-2000 at the same temperature. The ASTM viscosity standard is D3381, while the AASHTO viscosity standard is M 226.

3.2.3 Superpave Performance-Graded System

Grading systems based on penetration and viscosity have worked reasonably well for many years and are still used in many countries because of the simplicity and portability of the testing equipment. However, these simpler grading systems can categorize binders within the same grade even though they may exhibit very different temperature and performance characteristics in the environment in which they are used.

The PG system was developed to provide an improved set of asphalt binder specifications (AASHTO M 320, ASTM D6373). This system endeavors to measure physical properties that can be related directly to field performance by engineering principles. The PG tests are performed at loading times, temperatures, and aging conditions that more realistically represent those encountered by in-service pavements. The PG specifications help in selecting a binder grade that will limit the contribution of the binder to low-temperature cracking, permanent deformation (rutting), and fatigue cracking of the asphalt pavement within the range of climate and traffic loading found at the project site.

An important difference between the PG specifications and those based on penetration or viscosity is the overall format of the requirements. For the PG binders, the performance criteria remain constant; however, the temperatures at which those properties must be achieved vary depending on the climate in which the binder is expected to serve. The binder is graded in 6 °C increments of pavement temperature. An example of a binder designation in this system is PG 64-22. For this example, the binder is selected to resist environmental conditions in which the average 7-day maximum pavement temperature is at least 64°C (147 °F) but lower than 70 °C (158 °F). On the low-temperature side, the binder is selected to perform in pavement temperatures from −22 °C (−8 °F) down to just above the next grade at −28 °C (−18 °F).

The PG specifications help in selecting a binder grade that will limit the contribution of the binder to low-temperature cracking, permanent deformation (rutting), and fatigue cracking of the asphalt pavement within the range of climate and traffic loading found at the project site.

While this concept worked well for conventional speed, moderate traffic-volume pavements and airfields, research indicated that it needed some refinement for pavements that had slow-speed loading and heavier loading. Rather than change criteria and/or test conditions to reflect a change in loading time and traffic volume, the architects of the PG system elected to simply adjust for traffic speed and volume by “grade bumping,” using stiffer grades than indicated by the climate alone. This was a simple way to ensure adequate support in high-volume and/or slow-loading conditions. Requirements for grade bumping can be found in the current AASHTO M 323 for roadways, and for airfields, the requirements can be found in the DoD UFGS 32 12 15.13 and FAA P-401/P-403 specifications.

3.2.4 Multiple Stress Creep Recovery Grading System

The MSCR specification (AASHTO M 332, ASTM D7405) endeavors to solve issues with the Superpave PG system, improving the way the high-temperature behavior of polymer-modified asphalts is addressed and removing the need to grade bump. It uses the creep and recovery test concept to evaluate the asphalt binder’s potential for permanent deformation. Using the dynamic shear rheometer, a 1-s creep load is applied to the rolling thin-film oven-aged asphalt binder sample. After the 1-s load is removed, the sample is allowed to recover for 9 s. The test begins with the application of a low stress (0.1 kPa) for 10 creep/recovery cycles, and then the stress is increased to 3.2 kPa and repeated for an additional 10 cycles.

In the MSCR test, two separate parameters can be determined during each loading cycle: non-recoverable creep compliance (J_nr) and percentage of recovery (MSCR Recovery). Figure 4 shows a typical result from the MSCR test. The test specimens are creep loaded at 0.1 kPa and 3.2 kPa. After each 1-s creep load, the binder is allowed to recover for 9  s. J_nr is a measure of the residual strain left in the specimen after repeated creep loading and recovery, relative to the amount of stress applied. This parameter has been shown to be better correlated with rutting potential than the G*/sin δ parameter used in the Superpave system.

Source: U.S. Department of Transportation
Figure 4. MSCR Stress and Strain Responses

Figure 5 shows the acceptance criteria of AASHTO R 92 for MSCR Recovery. After the average amount of recovery is calculated, the results are used in combination with Jnr to indicate whether a binder has a significant elastic component.

Source: AASHTO (AASHTO R 92, Figure 1)
Figure 5. MSCR Recovery Acceptance Curve

Unlike the AASHTO M320 system, the test temperature used for the MSCR test is selected based on actual high pavement temperatures with no grade bumping. For example, if a binder grade would need to perform in an environment with average high pavement temperatures of 64 °C and low pavement temperatures reaching −22 °C, the MSCR test would be performed on the binder at a high temperature of 64 °C regardless of the traffic speed and loading. Higher loading is accounted for by increasing the stiffness (reducing the compliance) required for the asphalt binder at the grade temperature.

Unlike the AASHTO M320 system, the test temperature used for the MSCR test is selected based on actual high pavement temperatures with no grade bumping.

The designations shown in Table 1 are based on information found in AASHTO M 332.

Table 1. MSCR Designations

Source: AASHTO
km/h = kilometers per hour Source: AASHTO
Note: Grade bumping is accomplished by using “H,” “V,” or “E” designations and not by increasing the PG hightemperature grade as recommended in AASHTO M 320.

For standard traffic loading, J_nr (determined at 3.2 kPa shear stress) is required to have a maximum value of 4.5 kPa⁻¹. Continuing the example, the subsequent grade would then be a PG 64S-22. As traffic increases to heavy and very heavy loading, the J_nr of the asphalt binder needs to be lower—allowing maximum values of 2.0 and 1.0 kPa⁻¹, resulting in binder grades of PG 64H-22 and PG 64V-22. For extremely heavy traffic loading, the J_nr of the asphalt binder could only have a maximum non-recoverable creep compliance of 0.5 kPa⁻¹, resulting in a PG 64E-22.

Although the MSCR grading system is specified by many State DOTs, at the time of this writing it is not specified for airfields, neither in the DoD UFGS 32 12 15.13 nor the FAA P-401/P-403 specifications.

3.2.5 Temperature-Viscosity Characteristics

The viscosity of an asphalt binder changes with temperature, with lower temperatures resulting in higher viscosity (higher “stiffness”). This temperature-viscosity relationship impacts several aspects of asphalt testing, production, placement, and compaction.

During lab testing of both asphalt binder and asphalt mixtures, the testing temperature is always specified and must be tightly controlled. Without this control, test results lose their meaning. For example, the test temperature for the Hamburg Wheel-Tracking Test might be 50±1°C for a particular agency. If the actual test temperature is lower, then the binder will be stiffer and the results will exhibit artificially low rutting. If the temperature is allowed to wander higher than the test temperature, the rutting results would falsely indicate that the mix is more rut-susceptible than it actually is.

During production, the binder viscosity must be low enough that the binder can be pumped and handled in the facility. Both PG and MSCR specifications require that the binder viscosity be lower than 3 Pa·s at 135 °C.

During mix placement, viscosity has a large influence on the workability of the mixture. Hand work—when an asphalt crew laborer is placing the mixture with a shovel or an asphalt rake (lute)—becomes increasingly difficult as the viscosity of the binder increases. The mix flows through the paver more easily when the binder viscosity is lower.

The viscosity of an asphalt binder changes with temperature, with lower temperatures resulting in higher viscosity (higher “stiffness”). This temperature-viscosity relationship impacts several aspects of asphalt testing, production, placement, and compaction.

During compaction, the total time available to achieve proper mat density is influenced by the viscosity of the binder. As the binder becomes stiffer or more viscous as the mixture cools, a greater compactive effort is required to achieve a given density. The binder viscosity is not only affected by the inherent binder properties, but also the thickness of the compacted layer and environmental conditions during construction. Thinner layers will cool more quickly, significantly affecting the allowable time for compaction. Cooler, windier weather can drastically shorten the time available to achieve proper compaction or even make it impossible to achieve.

PaveCool and MultiCool are public domain applications that estimate the time available for compaction operations based on a variety of user-input variables. Figure 6 and Figure 7 demonstrate the effect of decreasing lift thickness on time for compaction, with all other variables held constant. Figure 6 estimates 44 min available for compaction for a 3-inch (76-mm) mat given several constants, including 50 °F (10 °C) air and existing surface temperatures and a fine/dense-graded mix with a PG 64-22 binder. Figure 7 shows that when using the same variables for a 1-inch (25-mm) mat, the time available for compaction drops to 7 min.

Source: Minnesota DOT
Figure 6. Available Compaction Time Example for a 3-inch Mat

Source: Minnesota DOT
Figure 7. Available Compaction Time Example for a 1-inch Mat

The rate of change in viscosity with change in the temperature of a binder is referred to as the binder’s temperature susceptibility. A material that is highly temperature-susceptible is one that exhibits a large change in viscosity for a small change in temperature. Multiple binders that have the same penetration at 25°C (77 °F) may not necessarily have the same viscosity at 135 °C (275 °F) since their temperature susceptibility characteristics may vary. In the lab, reporting this temperature-versus-viscosity relationship is required for some mix design procedures since lab mixing and compaction temperatures are typically based on a prescribed viscosity level.

The rate of change in viscosity with change in the temperature of a binder is referred to as the binder’s temperature susceptibility.

3.2.6 Polymer Modification

Polymer-modified asphalts (PMAs) work to flatten the temperature-viscosity relationship to make the binders less susceptible to changes in temperature. One way in which polymers are classified is based on their physical properties. Depending on their behavior when stretched with sufficient force, polymers are classified as plastomers (plastics) or elastomers (elastics). When stretched, plastomers will yield and remain in their stretched position when the load is released. Elastomers will yield under load (stretch) but will return to their original shape when the load is released. Most polyolefins behave as plastomers, while styrene-butadiene copolymers behave as elastomers.

When blended into asphalt, polymers tend to behave in two different ways. If the polymer forms discrete particles in the asphalt binder, then it functions primarily as a thickener or filler. This increases the viscosity of the asphalt binder while having no significant effect on low-temperature properties. If the polymer forms a continuous network in the asphalt binder, it functions as a homogeneous blend. This blend may impart some of the physical characteristics of the polymer to the binder, which may affect both the high- and low-temperature properties of the asphalt binder.

The task of the designer is to determine whether the extra cost of using PMA mixtures is worth the anticipated extra performance. For example, using a PMA mixture on a walking trail in a park may be an unnecessary extra expenditure. The use of a PMA mixture on a high-volume taxiway or roadway may easily be a better investment.