This study evaluated a limited number, but well-controlled group of asphalt binders of the same PG grade made with a wide range of REOB contents; 0%, 2.5%, 6% and 15%. The most practical indicator of the possible presence of a considerable quantity of REOB with measureable changes to the rheological and performance characteristics was the difference between the BBR m-value temperature grade and the BBR S stiffness temperature grade, DTcritical. When a binder exhibited a large DTcritical it was associated with larger differences in performance losses depending on the binder tests and to a lesser extent the mixture test. The DTcritical performance disruption was made worse by oxidative aging by means of double PAV conditioning as well as holding the binder at extended low temperatures before testing.
The impact of REOB on moisture damage resistance showed higher moisture sensitivity with increasing REOB content, but it did not interfere with liquid anti-strip additives. Mixture cracking tests results were mixed. Low temperature relaxation, strength and fracture measured with TSRST showed the fracture strength had slight increases or decreases with 2.5% and 6% REOB and could be interpreted as unaffected. Aging also improved the average strength of these mixes. However, the strength of the highest 15% REOB mix was measurably decreased and made worse by aging. The impact of REOB on intermediate temperature fatigue cracking performance depended on the aging condition and whether stress-control or strain-control performance was considered.
To minimize risks a best practice needs to be developed which includes a maximum use level, taking into account both the variability of REOB and the effects on asphalt binders from different sources. Asphalt binder specifications might be refined by placing lower limits to BBR stiffness or a maximum allowable DTcritical. More focus should be placed on intermediate REOB levels such as near 10% to the 6% used in this study to better identify performance pitfalls and performance benefits from REOB and the corresponding DTcritical.
Re-refined engine oil bottoms (REOB) are one of several products obtained in the refining of recovered engine oil and have been used in the asphalt industry since the 1980’s. Generally, REOB is used to help soften the base asphalt binder and is commonly used from 3 to 10% by weight in order to achieve desired low temperature asphalt binder properties. Recently, poor cracking performance in a number of Canadian and northern United States pavement sections have been blamed on the use of REOB to modify the asphalt binder. This has prompted state agencies in the northeast United States to ban its use, without necessarily understanding how REOB affects asphalt binder and mixture performance.
A research effort was conducted to evaluate the laboratory performance of asphalt binders and mixtures modified with REOB. Two different sources of REOB were blended with different base asphalt grades at varying dosage rates in the study to achieve “softer” asphalt binders—similar to the current practice of REOB modification in the asphalt industry. Performance grading, master stiffness curves, double edged notch tension test, and Black Space analysis were conducted on the asphalt binders at different levels of laboratory aging. Additionally, the asphalt binders were used to produce asphalt mixtures for stiffness, permanent deformation, fatigue cracking and low temperature cracking performance. The research study showed that while being able to achieve softer asphalt binder grades in accordance with AASHTO R 29, the addition of REOB accelerates the age hardening effects in the asphalt binder, with higher levels of age hardening occurring at higher REOB dosage rates. The study also indicated that while the stiffness properties at low temperatures are not impacted by the REOB, the relaxation properties, as measured using m-slope of the Bending Beam Rheometer (BBR), are highly affected. Both the Black Space analysis, using the Glover-Rowe approach, and the DENT test show promise at identifying the age hardening effects and correlated well to mixture fatigue cracking in the Overlay Tester. Differences were found between the various asphalt mixture fatigue cracking tests in their respective ranking of fatigue cracking performance. Asphalt mixture permanent deformation and dynamic modulus of REOB modified asphalt mixtures showed similar performance to the neat asphalt binders when compared to the REOB modified asphalt binders of the same PG grade.
Dynamic modulus has several useful functions in flexible pavements, including stress/strain characterization, rutting and cracking characterization, an input into several analytical and numerical models, and a primary input into PavementME Design. While the traditional dynamic modulus test is run in the uniaxial configuration, this is not possible for field cores. Therefore, the Indirect Tension dynamic modulus (IDT E* ) and torsion bar shear modulus (torsion bar G* ) have been developed. However, there has been limited research looking at analyzing the data from field cores for these two geometries, comparing modulus data from the two geometries, examining in-service aging of dynamic modulus, and quantifying pavement conditions using dynamic modulus. This research examines ten field sections in Arkansas, comprised of four “good” performing sections, two “medium” performing sections, and four “poor” performing sections in an attempt to address these four questions. First, this research found that using AASHTO T 342 and AASHTO R 62 can lead to irrational coefficients but provide rational results. Second, while the IDT E* and torsion bar G* values were similar at high modulus values, the IDT E* values began to increase as the modulus decreased compared to the torsion bar G* values, increasing to over a decade of difference. Third, a noticeable difference was observed between the modulus values of the bottom surface layer and top surface layer, with the bottom surface layer showing higher modulus values in all cases. While the upper surface layer showed higher oxidation, other weathering effects such as moisture and traffic appear to have overwhelmed the oxidation effect and pavement deterioration has reduced the integrity of the mix. Finally, both the IDT E* and torsion bar G* were not able to quantify a noticeable difference between poor and medium performing sections, and medium and good performing sections, but were able to quantify a difference between the poor and good behaving sections. Overall, the IDT E* and torsion bar G* tests were able to produce consistent master curves, correlate to each other, identify differences between surface course lifts, and quantify differences in field performance.
The main focus of the paper is to present the concept of a newly developed Uniaxial Shear Tester (UST) and to investigate the correlation between results from the UST and the Superpave Shear Tester (SST), a tool broadly recognized for asphalt mix design and rutting susceptibility evaluation. In this study, the UST testing principles, finite element analysis of stresses, and comparison of measured data are presented. The correlation was assessed on the basis of two tests, the repeated shear test and the small amplitude oscillation test also referred as the shear frequency sweep test. It was shown that the material characteristics determined from UST and SST are highly correlated. The dependencies are discussed in the sense of linear correlation and correlation coefficients. Test variability is discussed in the paper.
Cracking has become a primary mode of distress in recent years that frequently drives the need for rehabilitation of asphalt pavements. Meanwhile, asphalt mix designs are becoming more and more complex with the increasing uses of recycled materials, recycling agents, binder additives/modifiers, and multiple warm mix asphalt technologies. Thus, there is an urgent need to identify reliable cracking tests that can be used for routine mix design to eliminate brittle mixes. This paper includes a critical review of cracking mechanisms and laboratory tests. A total of twelve cracking tests were discussed at a cracking test workshop held as part of the National Cooperative Highway Research Program (NCHRP) Project 9-57. Seven cracking tests were selected for further laboratory evaluation and field validation. Four of the simpler cracking tests from the seven were evaluated in this paper, these being Texas Overlay Test (OT), Disk-shaped Compact Tension (DCT) test, Semi-Circular Bend test from the Louisiana Transportation Research Center (SCB-LTRC), and SCB test at room temperature from Illinois (SCB-IL). A laboratory sensitivity study was performed, and the results showed that all four cracking tests were generally sensitive to asphalt mix components. However, there were some concerns with the DCT, SCB-LTRC, and SCB-IL. Both the DCT and SCB-IL were found to be not sensitive to asphalt binder content; and both the DCT and SCB-LTRC showed an unexpected increase in cracking resistance when adding RAS to the mix. Additionally, two sets of field test sections were used for preliminary validation of these four cracking tests. It was found that the OT, DCT, and SCB-IL provided rankings which matched the measured field performance for the two sections on US62, Texas; and the OT and SCB-LTRC were valid for six APT test sections. Further validation with different mixes, traffic and climate is needed.
Aging has long been recognized as a major distress mechanism for asphalt concrete and, by extension, asphalt pavements. Aging causes the material to stiffen and embrittle, which leads to a high potential for cracking. Although a significant amount of effort has been placed on understanding the aging process of asphalt binder, less effort has been put forth to develop laboratory aging procedures for producing aged mixture specimens for performance testing. An optimal laboratory conditioning procedure to simulate long-term aging for performance testing and prediction is required in order to integrate the effects of long-term aging in pavement prediction models and other mechanistic design and analysis methods. In this study, oven aging and pressure aging vessel aging are applied to both loose mix and compacted specimens in order to evaluate and select an aging method to simulate long-term aging for performance testing and prediction. The selected method must be able to maintain specimen integrity in order to be used for performance testing and prediction. Efficiency, practicality, and versatility also are considered in evaluating the aging methods. The results demonstrate that loose mix aging in an oven is the most promising aging method to produce mixture specimens for performance testing in terms of efficiency, specimen integrity, versatility, and cost.
Aging of asphalt mixtures occurs during production and construction and continues throughout the service life of the pavement. Although this topic has been studied extensively, recent changes in asphalt mixture components, production parameters, and plant design have raised a need for a comprehensive evaluation that considers the impacts of climate, aggregate type, recycled materials, WMA technology, plant type, and production temperature. In this study, field cores were acquired from seven field projects at construction and several months afterwards, and raw materials were also collected for fabricating laboratory specimens that were long-term oven aged (LTOA) in accordance with selected protocols. The resilient modulus and Hamburg wheel tracking tests were conducted on both specimen types to evaluate the evolution of mixture stiffness and rutting resistance with aging. The concepts of cumulative degree-days and mixture property ratio were proposed to quantify field aging and its effect on mixture properties. Test results indicated that the LTOA protocols of two weeks at 140°F (60°C) and five days at 185°F (85°C) produced mixtures with equivalent in-service field aging of 7–12 months and 12–23 months, respectively, depending on climate. Finally, among the factors investigated in the study, WMA technology, recycled materials, and aggregate absorption exhibited a significant effect on the long-term aging characteristics of asphalt mixtures, while production temperature and plant type had no effect.
Bus Rapid Transit (BRT) has generated great interest among small and large cities across the United States (e.g., Detroit, MI, Grand Rapids, MI, and Aspen, CO) as a means of improving mobility and accessibility, and optimizing the use of street space, at a relatively modest cost per mile ($10-$27 million). The main advantage of BRT is its ability to operate on all types of road infrastructures: mixed-flow arterials, mixed-flow freeways, dedicated arterial lanes, at-grade or fully grade-separated transitways, managed lanes, and tunnels. The purpose of this study is to identify BRT-advantaged age-groups and income level groups by examining various BRT cities. A group or sector is said to be “BRT-advantaged” when its population grows at a higher rate within a BRT shed than within the larger metropolitan region during the same time period. Shift-share analysis was conducted to identify various BRT-advantaged attributes. Shiftshare analysis is used to decompose changes in an attributes (such as age-group and income level) in local areas. For example, the analysis identifies age groups that have comparative advantage in local areas. The technique provides a picture of how well a region’s income level group and age groups are growing at a given moment in time. As a part of this effort, age-group and income level data of five BRT cities were collected before and after the implementation of BRT at region and BRT-shed level. BRT-advantaged attributes by each city, as well as combined were identified. With the precedent of specific populations thriving in a BRT shed, communities and their planners can target the appropriate age and income level groups in their marketing efforts. The author discussed the causes behind the influence of BRT on the various population groups.
This paper describes the outcome of an exploratory step in the lead author’s broader research agenda funded through a Discovery Grant by the Natural Sciences and Engineering Research Council of Canada (NSERC) called “Developing planning and forecasting tools for age-friendly rural and community transportation alternatives: a focus on volunteer driver programs to facilitate older person mobility and safety”. It incorporates the results of an undergraduate special study that involved working with seven different regional volunteer driver programs in New Brunswick to catalogue the type and extent of trip and contextual information that they collect. The goal was to identify opportunities for consistent reporting practices that could enhance day-to-operations, as well as contribute to long term and strategic transportation planning incorporating these programs.
A bicycle-sharing system (BSS) is intended to provide increased convenience to individuals because they can use the service without the costs and responsibilities associated with owning a bicycle for short trips within the service area of the system. These systems are recognized to have traffic and health benefits such as flexible mobility, physical activity, and support for multimodal transport connections (Shaheen et al., 2010). Given the recent rapid growth of bicycle-sharing systems as a viable and sustainable mode of transportation for short trips, there is substantial interest in identifying contributing factors that encourage individuals to use these systems. This paper looks at BSS behavior at a trip level to analyze bicyclists’ destination preferences using a random utility maximization approach. Understanding the individuals’ decision processes in adoption and usage of bicycle-sharing systems will enable bicycle-sharing system operators/analysts to enhance their service offerings. Specifically, we study the decision process involved in identifying destination locations after picking up the bicycle at a BSS station. There have been several location choice studies in traditional travel demand literature that adopt a random utility maximization approach for understanding destination/location preferences (Chakour and Eluru 2014 Waddell et al. 2007; Sivakumar and Bhat, 2007). In this paper, we adapt this approach to the bicycle-sharing system data.
The aim of this paper is to investigate the impact of stated adaptation and opinion responses on individuals’ mode choice before and after travel demand management (TDM) strategy implementation. This investigation makes use of econometric modelling approaches, including the scaled multinomial logit model to capture these trends. This approach allows for the interpretation of differences in the decision making process based on the parameterization of the scale. The key outcomes of this analysis provide new insights into the effectiveness of TDM policies drawn from the perceptions and attitudes of travelers. These insights will permit policy/decision makers to justify the allocation of resources to different policies based on small scale, easy to implement stated adaption surveys.
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