Nathan Morian

Oxidative Aging of Asphalt Binders in Hot-Mix Asphalt Mixtures

This study evaluated the effect of different aggregate sources along with their Corresponding change in mixture characteristics to determine the influence of both on binder oxidation rates and changes in mixture stiffness when compacted mixtures were exposed to laboratory aging conditions. The two aggregate sources, Colorado and Nevada, had different gradations and different water absorption rates, which led to differences in the calculated asphalt binder apparent film thicknesses (AFT) for each mixture. Two asphalt binders, an unmodified PG 64-22 and a styrene–butadiene–styrene–modified PG 64-28, were used. The overall findings of the study indicated that both the aggregate and mixture characteristics influenced the oxidation rates of the binder, with the two binders oxidizing by similar amounts when aged in mixtures with the same characteristics (AFT and mixture air voids). The oxidation changes in the binder had differing effects on the stiffness of the mixture as a function of age. Not only were the aggregate and mixture characteristics important to the mixture stiffness and aging relationship, but the binder characteristics themselves, in particular polymer modification, influenced the aging and stiffness relationships of mixtures with age.

Low Temperature Characterization of Asphalt Mixtures by Measuring Visco-Elastic Properties under Thermal Loading

Low temperature thermal cracking is a common type of failure in asphalt pavement that occurs particularly in cold regions or locations with significant daily temperature fluctuations. The resistance of asphalt mixtures to low temperature cracking is generally influenced by the thermal contraction/expansion, visco-elastic, and fracture properties of the asphalt mixture. The accurate characterization of these properties is essential to the meaningful modeling of thermal cracking in asphalt pavements and, thus, the design of thermal cracking resistant mixtures. This paper describes a fundamental approach to determine the visco-elastic properties of asphalt mixtures from direct measurements of thermal stress and strain using a uniaxial test device recently developed at the University of Nevada, Reno. The relaxation modulus was computed in the temperature domain using linear visco-elastic constitutive equation, known as Boltzmanns superposition principle, from the measured, thermally-induced stress and strain during the test. Five distinct stages were identified from the relaxation modulus change with temperature: viscous softening, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages. The proposed approach was used to assess four hot mixed asphalt mixtures made from three aggregate sources with different mineralogy (Quartzite, Limestone and Rhyolite) and two different binder grades (PG64-22 and PG64-28) from different sources. It was found that the evolutions of thermal stress and strain are significantly influenced by the grade of asphalt binder and aggregate source. The relaxation modulus and the derived thermal visco-elastic properties were highly affected by the grade of asphalt binder.

Approach for Quantifying the Effect of Binder Oxidative Aging on the Viscoelastic Properties of Asphalt Mixtures

Because of the noted influence of oxidative aging on mixture properties and pavement performance, it is becoming imperative to have a more complete understanding of the influence of asphalt binder aging on the viscoelastic behavior of asphalt mixtures. This study proposes a new approach to correlating the oxidative aging of asphalt binder for carbonyl functional groups with the viscoelastic behavior of asphalt mixtures for a continuous relaxation spectrum. The asphalt mixture complex modulus and the carbonyl area for the recovered asphalt binder were measured for mixtures subjected to varying durations of long-term aging in the laboratory. The continuous relaxation spectrum was obtained analytically from the 2S2P1D model of complex modulus of asphalt mixture through the inverse Fourier–Laplace transform approach. A consistent horizontal shift in the continuous relaxation spectrum was observed for all mixtures with the increase in aging duration. However, the shape and the amount of shifting of the spectra were mixture dependent. In particular, mixtures with higher asphalt binder absorption exhibited the greatest shift in the continuous spectra for both unmodified and polymer-modified asphalt mixtures. Good correlations were observed between the carbonyl in asphalt binder and the continuous relaxation spectrum parameters of the asphalt mixture. Such relationships should permit the incorporation of long-term oxidative aging directly into the constitutive equation used in pavement response analyses.

Impact of lime application method on ravelling and moisture sensitivity in HMA mixtures

This study is sponsored by the Regional Transportation Commission of Washoe County to assess the impact of lime treatment on the moisture damage of hot-mixture asphalt (HMA) mixtures near Reno, Nevada. This paper presents the laboratory evaluation of lime application in HMA mixtures, mechanistic analyses of those results and an evaluation of plant-produced HMA mixtures to evaluate the variability of moisture susceptibility. The laboratory evaluation based upon resilient modulus testing detected moisture sensitivity benefits with the use of lime over multiple freeze–thaw cycles, but could not distinguish between the marinated and non-marinated laboratory mixtures. The mechanistic analysis based upon laboratory mixtures indicated similar results in terms of the presence of lime and showed slight signs of improvement with the marination process. The field variability based on tensile strength ratios (TSR) indicated that the marination process does reduce the variability of the benefits of lime treatment on moisture sensitivity.

Significance of Mixture Parameters on Binder Aging in Hot-Mix Asphalt Mixtures

Traditionally, aging studies of asphalt materials have been conducted primarily on asphalt binders only and have omitted any potential influence of the aggregate. These results are commonly extrapolated to the mixture behavior without a comprehensive understanding of how the mixture characteristics may potentially influence the aging of the binder within the mixture. This study was initiated by FHWA to evaluate the effects of mixture parameters—namely, different aggregate and asphalt binder sources, air voids, and binder contents—on the aging characteristics of asphalt binders aged in mixtures. Those same binders were also aged in pressure aging vessel (PAV) pans in laboratory ovens to determine the aging characteristics of the asphalt binders, as is the current state of practice. These characteristics were determined by the binder kinetics, with respect to the carbonyl area measured from the Fourier transform infrared spectroscopy spectra as a function of aging time and temperature. The hardening susceptibility (HS) of the asphalt binders, as defined by the relationship between the low shear viscosity and the carbonyl area, was also determined. The HS of the asphalt binders aged in the mixtures was compared with the HS of the binders aged in the PAV pans and was found to be statistically dissimilar for some mixtures and statistically similar for others. The analysis revealed that the effective binder content of the mixtures provided the strongest indicator of whether the evaluated mixture parameters would have a significant influence on the aging characteristics of the asphalt binder in the mix.

Effect of select warm-mix additives on thermo-viscoelastic properties of asphalt mixtures

In this paper, the thermo-viscoelastic properties of hot and warm asphalt mixtures from South Dakota, USA, were determined from direct measurements of thermally induced stress and strain. The approach to determine the relaxation modulus, viscous flow, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages was described and used to assess the influence of select warm-mix additives on low-temperature properties. The evaluated warm-mix additives as well as the aggregate source were found to have inconsistent effects on the thermo-viscoelastic properties of the mixtures. It was observed that mixtures with similar thermal stress build-up can have significantly different relaxation modulus values depending on the coefficient of thermal contraction of the respective mixture. In summary, the proposed methodology was able to discern the slight differences caused by the warm-mix asphalt treatments on the same binder and two aggregates.

Rheological Indexes: Phenomenological Aspects of Asphalt Binder Aging Evaluations

An investigation was conducted into several commonly used rheological indexes and their influence on oxidation studies. The evaluation considered the low shear viscosity; the dynamic shear rheometer function, or Glover–Rowe parameter; the dynamic shear crossover modulus (G*c) value; and parameters from the Williams–Landel–Ferry and Kaelble shift functions as the rheological indexes in a comparative oxidation characterization between two example asphalt binders. In general, each respective methodology was found to correlate with increasing levels of oxidation but resulted in differing outcomes with respect to oxidation, which depended on the evaluation methodology used. Although each method presented its own merit and respective limitations, a single preferential method was not readily identified. However, the critical need for reliable and sound rheological measures and data processing procedures was clearly demonstrated. This evaluation supported the need for a clear and logical evaluation of the rheological parameters used to characterize asphalt binders. It has been documented that certain rheological indexes have strong correlations to specific distress types or mechanical behaviors and thus may prove beneficial to a particular analysis. However, evaluations must also acknowledge the limitations of selected techniques. When possible, complete characterization through means of the development of full master curves should be beneficial. With sound rheological data and comprehensive measures, several indexes can be determined and provide a more comprehensive view of the investigated material responses.

Evolution of Thermoviscoelastic Properties of Asphalt Mixtures with Oxidative Aging

The uniaxial thermal stress and strain test (UTSST) provides a fundamental approach to characterize the thermoviscoelastic properties of asphalt mixtures and permits the pragmatic evaluation of changes in the stiffness and overall behavior of mixtures as a function of oxidative aging. The UTSST modulus was computed in the temperature domain with a linear viscoelastic constitutive equation from the measured thermally induced stress and strain. Five distinct stages, here named thermoviscoelastic properties, are identified from the modulus as a function of temperature: viscous softening, viscous-glassy transition, glassy hardening, crack initiation, and fracture stages. Through consideration of the thermoviscoelastic properties, marked differences in the aging process were noted in the evaluation of two binders and two aggregate sources over a range of air void levels. Typically, decreases in the viscous response of the mixtures as well as corresponding increases in both the stiffness and brittle behavior are presented as a function of aging. The evaluated behavior of the mixtures also provides a clearer understanding of the significant influence the air void level, or mixture density, has on the binder oxidation and overall mixture performance. The evaluation method provides definitive measures to monitor the progression of multiple aspects of the response of asphalt mixtures to thermally induced loading.

Recommendations for the characterization of RAP aggregate properties using traditional testing and mixture volumetrics

A Federal Highway Administration (FHWA) funded study was conducted to investigate the influence of extraction methods on aggregate properties. The properties of the virgin aggregates were compared with those of aggregates extracted from laboratory-produced recycled asphalt pavement (RAP) from four different aggregate sources. The extracted and actual asphalt binder contents were also compared. The study investigated the influence of the extraction method on tendencies to under- or over-estimated certain mix design properties. The test results were also examined to determine the impact of the RAP aggregate properties on the voids in mineral aggregate (VMA) over different RAP percentages. Recommendations were made for the most appropriate method to estimate the RAP aggregate specific gravities based on acceptable levels of error in VMA for mixtures with varying levels of RAP.

Impact of Recycled Materials and Recycling Agents on Asphalt Binder Oxidative Aging Predictions

The overall objective of this study was to evaluate the influence of selected recycling agents (RAs) and recycled materials on the development of cracking potential with respect to oxidative aging. Given the complex nature of varying base asphalt binders, recycled materials, whether recycled asphalt pavement (RAP), reclaimed asphalt shingles (RAS), or both, and the complexity of their combined interactions with recycling agents, standard evaluation protocols for binder grading and evaluation may be insufficient. The binder blend aging predictions or oxidation modeling evaluation was introduced as a means to evaluate the combined influence of both binder oxidation kinetics and resulting rheological changes on the measured cracking potential of the various binder blends—that is, Glover–Rowe (G-R) parameter—driven by temperature estimation modeling over simulated in-service durations at example geographic locations. This evaluation has demonstrated the importance of adequate characterization of the specific materials being used in conjunction with selection of the correct dose of the appropriate recycling agent to ensure sufficient resistance to cracking and embrittlement of proposed material combinations. The combined influence of all the interested components did not always add up to the sum of the individual parts, nor are the measured interactions consistent with increased levels of oxidation. Therefore, the prevailing conclusion of the study as a whole indicated that material-specific evaluations are needed to identify the complex interactions taking place within the material combinations of interest, but also multiple levels of aging at appropriate intervals may be necessary for comprehensive characterization.