Transportation Research Record: Journal of the Transportation Research Board

The linear viscoelastic regime is defined in terms of the constitutive relationship between the stress and the strain. The set of equations that define the fundamental linear viscoelastic material properties in the time and frequency domains and their relationship to one another is based on the validity of the linearity principle. A material must obey two simultaneous conditions to be linear viscoelastic: the homogeneity (also called proportionality) condition and the superposition principle. On the basis of these considerations a testing procedure was developed to check linear viscoelastic conditions for tests performed on asphalt binders with the dynamic shear rheometer (DSR), the bending beam rheometer (BBR), and direct tension (DT). The testing procedure for the DSR requires performing strain sweeps and multiwave single-point tests. For the BBR, tests performed using different constant loads are required. In addition, the recovery part of the specification test is recorded. For the DT, tests performed at different strain rates and relaxation tests performed at different strain levels are required. When applied to asphalt binder data, the testing procedure found no departure from viscoelastic conditions for the DSR and BBR test data. However, the DT procedure indicated a departure from linear viscoelastic conditions.

In the current Superpave® binder tests, the rheological properties of asphalt binders are determined by the dynamic shear rheometer. The asphalt binders are tested using a gap size of either 1 or 2 mm. In the asphalt mixture, the thickness of the asphalt film between aggregates can vary between a few microns and a few millimeters. Because of the interaction between the asphalt and the aggregate, rheological behavior in thin films can differ from the behavior determined at large gap sizes. In this study the rheological properties of asphalt binders were measured at different gap sizes that simulate the range in film thickness of a typical mixture. Significant changes in rheological properties were observed as the gap size changed. The results indicate that different modified binders show different trends as the gap size changes. A review of the literature was conducted for an explanation of this behavior. Four mechanisms were offered to explain similar gap effects observed for other materials. Using the data collected in this study, the disentanglement mechanism caused by adhesion to testing plates appears to fit the observations for asphalt binders. The results suggest that the current gap settings in the binder tests might not be sufficient to determine the performance of the binder in the mixture. The findings also suggest that more work is required to understand the effect on the rheological behavior of asphalt binders of physical and chemical interactions with aggregates.

Shear vibrations in conventional and polymer-modified asphalt binders are examined. Using dynamic compliances instead of moduli, viscous deformation effects can be separated from total deformation and the modified dynamic loss compliance and modified loss tangent functions can be defined. These two material functions appear to be more sensitive than viscoelastic moduli to the rheological behavior of asphalt binders and to changes caused by polymer addition. The characteristic temperature of the transition from the viscoelastic to viscous behavior of asphalt binder (T_v) can be identified by using the viscous asymptote J”. The damping of shear vibrations that likely relates to the internal structure of asphalt material can be described by the modified loss tangent. The rheological behavior of the base asphalt 200/300 penetration grade and its blends with different amounts of radial styrene-butadiene-styrene rubber is investigated. Using master curves of dynamic functions and the Williams-Landel-Ferry form of the shift factor, isochrones of the original and modified dynamic material functions are constructed. Characteristic temperatures of the viscous transition (T_v) and the glass transition (T_g) are determined. Damping behaviors of the base and modified asphalts are studied.

Because of several orders of magnitude difference between the stiffness of aggregate and binder and the randomness of the binder domain boundaries, the induced deformation under loading can result in a wide distribution of stresses and strains within each of the components. It is expected that although aggregates undergo small strains, most of the strain will accumulate within the binder. Although studies have covered the micromechanics of hot-mix asphalt (HMA), information about the actual typical distribution of asphalt binder domains in HMA and the resulting distribution of stresses and strains is scarce. In this study, advances in imaging techniques are applied to understand the distribution of binder and air voids in selected HMAs. The objective is to determine the strain distribution within the binder using digitized images analyzed with finite-element procedures. This approach captures the image of the specimen cross section and converts the image into finite-element mesh after image processing. The images are converted to finite-element mesh and the finite-element program ABAQUS provides numerical solutions to relate bulk stresses or strains applied to the asphalt mixture to stresses and strains within the binder domains. The results are presented including a summary of the distribution of directional binder film thickness and maximum strains in the mastic domain. Also included is a discussion of the effect of air voids and mineral fillers.

Two binder technician workshops were held in January 1998, one at the Connecticut Advanced Pavement Laboratory at the University of Connecticut and one at the Northeast Center of Excellence for Paving Technology at Pennsylvania State University. These workshops were followed by a second set in 1999. The overall objective was to improve the repeatability of the test methods used to grade Superpave® asphalt binders. During the workshops, participants demonstrated and discussed the techniques used in their own laboratories. A document, Manual of Practice for Testing Asphalt Binders in Accordance with the Superpave PG Grading System, was developed for use by asphalt binder technicians and as a training aid for a proposed binder technician certification program. The results of the discussions that were held during the workshops and the items that are presented in the manual of practice are summarized. The results were grouped into four main categories: (a) handling, sampling, and sample preparation; (b) temperature measurements; (c) equipment calibration; and (d) testing procedures. The items discussed here and in the manual of practice supplement and clarify the current AASHTO test methods. The test methods in themselves are not sufficient to ensure uniformity in testing practice from one laboratory to the other.

The results of low-temperature fracture testing of a large number of both regular and modified asphalt binders are discussed. Two Strategic Highway Research Program (SHRP) binders (Materials Reference Library Codes AAG-2 and AAN) were evaluated with 5 percent by weight of a variety of commonly used polymer modifiers. Specimens of three different sizes were tested in a three-point bend configuration, both with and without a notch. The original SHRP effort was aware of the need for a rigorous fracture mechanics-type binder test, but because of a lack of time and resources, only the bending beam rheometer and the direct tension test were ultimately developed. Some of the differences between failure strain and fracture toughness measurements are discussed, as well as how these differences may relate to pavement performance. The results of this study demonstrate that there is a large range of notch sensitivities and fracture energies for different polymer-modified binders, suggesting that the ductile-to-brittle transition (as measured with the direct tension test) may not be a totally reliable performance indicator. Fracture energy may be a better choice, since it combines the notched strength with a stiffness to yield a true material property that is independent of sample size and configuration.

Evaluation of a mixture’s moisture sensitivity is currently the final step in the Superpave® volumetric process. This step is accomplished by using AASHTO T-283, which tolerates a range of values in the test variables of sample air voids and degree of saturation. The tensile strength ratios determined for the mixes in this study varied with the air void level and degree of saturation. Although the levels of conditioning were within the specifications for AASHTO T-283, test results both passed and failed the 80 percent criterion, depending on the severity of conditioning. An alternative to measuring indirect tensile strength is a test that evaluates a mixture’s fundamental material properties. A relatively simple test is proposed that measures the cohesion and friction angle for asphalt mixtures. In addition, the Superpave shear tester (SST) was incorporated as a tool in evaluating moisture sensitivity. The proposed axial test determined the cohesion and angle of friction of the mix. The friction angle remained constant for the conditioned and unconditioned samples. Hence, conditioning of the samples had practically no effect on the mixture’s internal friction. The cohesion of the mix decreased when the mix was subjected to conditioning. The reduction in cohesion was greater in the case of the Fountain aggregate, which is known to be highly moisture susceptible. The shear tests to failure performed on the SST confirmed the results of the new apparatus, which provides a simple method for determining a mixture’s cohesion. The loss of cohesion due to conditioning can be used to determine a mixture’s moisture susceptibility. The three antistrip additives used in this study were hydrated lime, a liquid amine, and a liquid phosphate ester.

A new class of reactive polymer modifiers designed to improve binder-aggregate adhesion in asphalt concrete was evaluated. Using a controlled free radical polymerization process, polyisoprene was prepared with short blocks of reactive amino- or silane-functional monomer at one end of the polymer chain. The reactive polymers so synthesized were tested with a modified version of the Tunnicliff-Root method (ASTM D4867) for measuring retained tensile strengths after water immersion moisture conditioning at 60°C for 24 h. It was found that the retained tensile strength of the unmodified samples was 53 percent, and the retained tensile strength of the regular polyisoprene-modified control samples was between 57 and 69 percent, depending on the polymer molecular weight and content. The best retained tensile strengths of 86 and 90 percent, respectively, were obtained with samples modified with 3 and 5 percent by weight of the higher-molecular-weight silane-functional polyisoprene. The method for obtaining the desired effect is flexible; lower-cost monomers, such as butadiene, and more common polymerization methods, such as emulsion or anionic polymerization techniques, may be used equally well to produce polymers with similar or better performance characteristics. An added benefit is that the tested polymers imparted significant improvements in low-temperature performance measured with the thermal stress restrained cooling test.

A new specification procedure was proposed recently to determine the low-temperature performance grade of asphalt binders. This new procedure uses bending beam rheometer (BBR) and direct tension test (DTT) data at two temperatures to determine the low-temperature grade of an asphalt binder. A study was conducted to validate this procedure by using asphalt binder retained samples from the widely published test road in Pennsylvania. This road was constructed in 1976 in Elk County, and performance—including low-temperature transverse cracking index—was monitored over 6 years. In this study, the retained binder samples from test sections T-1 to T-6 were tested with the BBR and the new Superpave® DTT. The data from these two tests were analyzed to obtain critical cracking temperatures and low-temperature performance grades. Results indicate that the new procedure correctly predicts the temperature at which cracking was observed in the field.

Results from the low-temperature performance evaluation of a new class of reactive polymer modifiers designed to improve binder-aggregate adhesion in asphalt concrete are discussed. A living free radical polymerization process was used to prepare polyisoprene (PI) with short blocks of reactive silane-functional monomer at one end of the polymer chain. Performance was evaluated with the thermal stress restrained specimen test at a cooling rate of-10°C/h. The results obtained were compared with those for unmodified and styrene-butadiene (SB)-modified mixes. When added at 3 and 6 percent by weight of the binder, SB reduced the fracture temperature of the asphalt concrete by 6°C and 9°C, respectively. Similar results were obtained with the reactive polymers. However, although all the SB-modified samples failed in a catastrophic mode, none of the samples containing the silane-functional PI did. It is hypothesized that these polymers toughened the asphalt to an extent that cracks were only able to grow in a stable mode as opposed to an unstable mode or that localized yielding occurred. The method for obtaining the desired toughening effect is flexible; lower-cost monomers, such as butadiene, and more common polymerization methods, such as emulsion or anionic polymerization techniques, could be used equally well to produce polymers with similar or better performance characteristics. An added benefit is that the tested polymers were able to improve the stripping resistance of the mix significantly.

The effects of finely dispersed fillers on the fatigue performance of asphalt binders and asphalt concrete mixes at relatively low temperatures are examined. A series of model binder systems containing glass spheres with narrow particle size distributions were used to study the effect of filler particle size on the fatigue performance of the asphalt mastic. Two mastic systems containing ground limestone fillers, which possessed significantly different gradations, also were tested. Fatigue performance was evaluated by applying a constant torsional strain to each specimen in a dynamic rheometer at 10°C and 40 Hz. Testing at various strain levels allowed the relationship between fatigue life and strain to be determined for the different systems. The results indicate that as the particle size of the filler decreases, the fatigue life of the asphalt mastic increases. This observation is a direct result of the mode of fatigue failure in the asphalt mastics and is in agreement with Evans’s theory on crack pinning for failure in filled brittle solids. Constant stress asphalt concrete fatigue tests on both dense- and gap-graded systems prepared with the two different ground limestone fillers show that the particle size does not significantly affect the fatigue life of the mixes. These results also confirm that crack pinning is the major mechanism responsible for improved fatigue performance.