@article{gulzar_underwood_2024, title={Asphalt binder characterization using waveform-based viscoelastic measures and time-temperature superposition principle under large strains}, volume={438}, ISSN={["1879-0526"]}, url={https://doi.org/10.1016/j.conbuildmat.2024.137177}, DOI={10.1016/j.conbuildmat.2024.137177}, journal={CONSTRUCTION AND BUILDING MATERIALS}, author={Gulzar, Saqib and Underwood, Shane}, year={2024}, month={Aug} }
 @article{fried_boz_preciado_gulzar_underwood_castorena_habbouche_2024, title={Evaluation of Alternative Approaches to Restore the Rheology of Recycled Asphalt Binders}, volume={3}, ISSN={["2169-4052"]}, url={https://doi.org/10.1177/03611981241235179}, DOI={10.1177/03611981241235179}, abstractNote={ This study evaluates the ability of softer virgin binders, recycling agents (RAs), and extenders to restore the rheology of reclaimed asphalt pavement (RAP) binders to that of control binders. Control binders are those binders that would be specified for use with virgin mixtures. Two procedures for RA dosage selection are examined: 1) dose to restore the intended high-temperature grade; and 2) dose to restore the intended low-temperature grade. A broad range of point parameters and rheological parameters that measure the balance of relaxation and stiffness characteristics are examined at two long-term age levels. The results of recycled binder blends are compared with the control virgin binders statistically. Using softer virgin binders generally resulted in blended binder performance-graded properties that were similar to or better than the control. However, these softer binder systems often exhibited other rheological properties that were inferior to the control. The rheological characteristics of the RA blends varied with the RA type and dosage. Blends with dosages that restored the intended high-temperature grade generally yielded the most favorable comparisons to the control binders. RAs derived from triglycerides and fatty acids and from tall oils generally yield more favorable rheological characteristics than those derived from aromatic extracts. In addition, a vacuum gas oil extender achieved similar blend rheological characteristics to RAs but extenders containing re-refined engine oil bottoms generally yielded a poor balance of stiffness and relaxation characteristics. Rheological balance parameters are generally weakly correlated, suggesting they provide unique insight into the rheological behavior of binders. }, journal={TRANSPORTATION RESEARCH RECORD}, author={Fried, Andrew and Boz, Ilker and Preciado, Jaime and Gulzar, Saqib and Underwood, Benjamin Shane and Castorena, Cassie and Habbouche, Jhony}, year={2024}, month={Mar} }
 @article{xue_gulzar_castorena_2024, title={Linear viscoelastic, viscoplastic, and damage characterization of recycled asphalt binders and mixtures containing recycling agents with long-term aging}, volume={6}, ISSN={["1573-2738"]}, url={https://doi.org/10.1007/s11043-024-09721-4}, DOI={10.1007/s11043-024-09721-4}, journal={MECHANICS OF TIME-DEPENDENT MATERIALS}, author={Xue, Lei Gabriel and Gulzar, Saqib and Castorena, Cassie}, year={2024}, month={Jun} }
 @article{preciado_gulzar_castorena_underwood_habbouche_boz_2024, title={Performance Assessment of High Reclaimed Asphalt Pavement Asphalt Mixtures with Recycling Agents}, volume={4}, ISSN={["2169-4052"]}, url={https://doi.org/10.1177/03611981241236483}, DOI={10.1177/03611981241236483}, abstractNote={ The increased use of reclaimed asphalt pavement (RAP) in asphalt mixtures has promoted the use of recycling agents (RAs) to mitigate the effect of oxidized materials on the performance of asphalt mixtures. This laboratory-based study assesses the linear viscoelastic (LVE), cracking resistance, and permanent deformation properties of recycled asphalt mixtures containing RAs. The dosage levels used for this study were determined based on the manufacturer’s recommendation to restore the blended binder system’s low-temperature performance grade (PGL) to −22°C. The study found that blending a performance grade (PG) 64-22 virgin binder with RAs in high RAP mixtures can yield a comparable LVE response to high RAP mixtures designed using a softer binder (PG 58-28). Moreover, the study revealed that the cracking performance of high RAP mixtures could be improved without jeopardizing the rutting potential of the material by using RAs at dosage levels selected to yield a blended binder PGL of −22°C. }, journal={TRANSPORTATION RESEARCH RECORD}, author={Preciado, Jaime and Gulzar, Saqib and Castorena, Cassie and Underwood, Benjamin Shane and Habbouche, Jhony and Boz, Ilker}, year={2024}, month={Apr} }
 @article{gulzar_xue_preciado_fried_castorena_underwood_habbouche_boz_2024, title={Phenomenological modeling of aging and rejuvenation on asphalt binder fatigue characteristics}, volume={425}, ISSN={["1879-0526"]}, DOI={10.1016/j.conbuildmat.2024.135929}, abstractNote={Recycling agents (RAs) have been explored as additives to enhance the properties of recycled asphalt binders. Despite extensive research on RAs, a critical knowledge gap exists regarding their impact on fatigue cracking of asphalt binders, especially considering the aging processes. This study aims to comprehensively investigate the effects of recycling agents on the fatigue properties of asphalt binders, with a specific focus on the influence of aging and rejuvenation. The current Linear Amplitude Sweep (LAS) test analysis procedure is utilized to assess the fatigue properties, and the efficacy of existing LAS test-based indices is evaluated. Furthermore, a novel index based on the simplified viscoelastic continuum damage (S-VECD) theory framework is proposed to monitor changes in fatigue properties with aging and rejuvenation. A total of 26 recycled binder blends are studied and the results demonstrate that the proposed index consistently tracks the effects of aging and rejuvenation in recycled binder blends containing recycling agents. Additionally, cross-scale assessments are conducted by correlating the proposed index with mixture performance, specifically the Indirect Tensile-Cracking Test parameter (CTindex). The findings indicate that the proposed index exhibits a correlation with the CTindex after 1-day Long Term Aging testing and effectively captures the aging sensitivity across different scales.}, journal={CONSTRUCTION AND BUILDING MATERIALS}, author={Gulzar, Saqib and Xue, Lei and Preciado, Jaime and Fried, Andrew and Castorena, Cassie and Underwood, Benjamin Shane and Habbouche, Jhony and Boz, Ilker}, year={2024}, month={Apr} }
 @article{gulzar_castorena_underwood_2023, title={An investigation into the nonlinear rheological behavior of modified asphalt binders using large amplitude oscillatory shear rheology}, volume={24}, ISSN={["1477-268X"]}, url={https://doi.org/10.1080/10298436.2023.2211211}, DOI={10.1080/10298436.2023.2211211}, abstractNote={ABSTRACT Asphalt binders have been studied extensively with respect to their linear viscoelastic properties. From these properties, performance metrics have been derived and used for specification purposes. However, these materials are used in asphalt concrete pavements where they may experience time-dependent loads in the nonlinear regime. In the past, the vast majority of asphalt binders exhibited strong correlations between their linear and nonlinear properties and so despite this mismatch in characterization and the actual use phase domains, a system based on linear properties could be reliably deployed. More recently though, novel binders and additives have been introduced with differing relationships between linear and nonlinear behaviors. As such there is a need to characterize the asphalt binder response under large strains and use specifications that more explicitly account for the binder nonlinearity. Here, an attempt has been made to characterize the nonlinear rheological properties of asphalt binder using large amplitude oscillatory shear. The response of a single terminal blend, crumb rubber modified binder under large strains is analyzed using Lissajous-Bowditch plots while the contribution of higher harmonics is evaluated using Fourier-transform rheology. Finally, the elastic and viscous contributions are obtained using stress decomposition to an orthogonal set of Chebyshev polynomials. It is found that the relative nonlinearity increases with increasing strain and frequency under the tested conditions. The asphalt binder evaluated in this study predominantly exhibits shear thinning and either strain stiffening or softening behavior depending upon the test conditions. The applicability of time-temperature superposition in the nonlinear regime for asphalt binders is also evaluated.}, number={1}, journal={INTERNATIONAL JOURNAL OF PAVEMENT ENGINEERING}, author={Gulzar, Saqib and Castorena, Cassie and Underwood, Shane}, year={2023}, month={Dec} }
 @article{preciado_gulzar_fried_castorena_underwood_habbouche_boz_2023, title={Benchmarking Recycled Binder Blends Using Statistical Analysis: A Case Study of Virginia and North Carolina}, volume={2678}, ISSN={["2169-4052"]}, url={https://doi.org/10.1177/03611981231178812}, DOI={10.1177/03611981231178812}, abstractNote={ Recycling agents (RAs) are added to asphalt binder blends containing recycled materials to mitigate the negative effects of adding oxidized material to asphalt mixtures. Several methods exist to select a particular RA dosage to restore a specific Superpave performance parameter. However, the rheological similarities between the RA-modified systems and typical virgin binders across the complete range of temperatures remain to be ascertained. In this study, two RA dosage selection procedures, one aiming to restore high-temperature performance grade and the other aiming to restore low-temperature performance grade were statistically compared. Quality assurance datasets from Virginia and North Carolina were used to benchmark the typical virgin binder properties, and 17 recycled binder blends using constituent materials from the two states were studied with respect to Superpave binder properties. Both univariate and bivariate statistical analyses were applied to compare and contrast the rheological properties of these blends with the benchmarked virgin binder properties for the two states. Then, a statistical similarity analysis was done using the Mahalanobis distance to evaluate the rheological properties of these blends with those of the reference binders. The study found that both dosage selection procedures were successful in achieving rheological similarity at target-specific temperatures and for specific binder properties but might not necessarily yield similar properties at other temperatures. Finally, an example application is presented using statistical analysis to select an RA dosage that yields a rheological fingerprint more similar to that of virgin binder systems. }, number={3}, journal={TRANSPORTATION RESEARCH RECORD}, author={Preciado, Jaime and Gulzar, Saqib and Fried, Andrew and Castorena, Cassie and Underwood, Benjamin Shane and Habbouche, Jhony and Boz, Ilker}, year={2023}, month={Jul} }
 @misc{gulzar_fried_preciado_castorena_underwood_habbouche_boz_2023, title={Towards sustainable roads: A State-of-the-art review on the use of recycling agents in recycled asphalt mixtures}, volume={406}, ISSN={["1879-1786"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85152237477&partnerID=MN8TOARS}, DOI={10.1016/j.jclepro.2023.136994}, abstractNote={The use of asphalt mixtures containing previously paved asphalt mixture from milled pavements, so-called reclaimed asphalt pavements or RAP, has been around for more than half a century. Historically, relatively low RAP contents were used, around 15% of the total mix. Recently, the use of high RAP contents has increased and now it is common to design mixtures with 30% or higher RAP contents, especially in the US. At these higher recycled contents, the characteristics of the recycled material begin to significantly affect the behaviors of the asphalt mixtures and alter the pavement performance. The use of recycled materials in asphalt mixtures at these contents may result in diminishing returns in terms of prospective performance at or beyond a certain incorporation dosage level. To overcome this barrier, recycling agents (RA) have been proposed to extend the break-even point and/or provide savings, both in terms of cost-effectiveness as well as environmental impacts. This paper is an examination of the use of RAs in recycled asphalt mixtures. There have been many reviews regarding RAs in the recent past, notably the review compiled as part of the United States National Cooperative Highway Research Program (NCHRP) Project 09–58. However, this field is evolving very quickly and there have been notable advances that require a renewed examination of the state of the art with respect to RA usage. In these intervening years, some specific advances have been made to propose novel methods for understanding the chemical nature of RAs, how RAP interacts with the virgin binder and RA, and how acceptable performance of recycled asphalt binder blends and mixtures can be achieved through dosage selection methods. However, the scientific knowledge on the selection and use of RAs in asphalt pavements is limited, and there is a need to develop robust methodologies that establish threshold criteria and performance metrics to facilitate their use on a regular basis. While there do exist some general trends with respect to RA effectiveness, the specific benefits and limitations found in individual studies vary greatly. Thus, given the current state of knowledge, RAs should be evaluated on a material-by-material basis using asphalt mixtures designed and delivered using local practices.}, journal={JOURNAL OF CLEANER PRODUCTION}, author={Gulzar, Saqib and Fried, Andrew and Preciado, Jaime and Castorena, Cassie and Underwood, Shane and Habbouche, Jhony and Boz, Ilker}, year={2023}, month={Jun} }
 @article{fried_xue_gulzar_preciado_underwood_castorena_2022, title={Evaluation of Recycling Agent and Extender Dosage Selection Procedures to Restore the High-Temperature Climatic Performance Grade}, volume={2676}, ISSN={0361-1981 2169-4052}, url={http://dx.doi.org/10.1177/03611981221074361}, DOI={10.1177/03611981221074361}, abstractNote={ Dosage selection is important for effective use of recycling agents and extenders in asphalt pavements. A standardized protocol for extender and recycling agent dosage selection does not presently exist. NCHRP Project 09-58 established a recycling agent dosage selection procedure that aims to restore the desired high-temperature grade of a blend of recycled binder, virgin binder, and additive. Their simplified procedure relies on the existence of class-specific relationships between the blended system high-temperature grade and additive content. Further, the dosage selection procedure assumes complete recycled binder availability. There is general consensus that complete recycled binder availability is not achieved in practice, which may yield asphalt mixtures with poor rutting performance. This study seeks to evaluate the universality of the slopes proposed in NCHRP Project 09-58 to both extenders and recycling agents and evaluate the rutting resistance of asphalt mixtures prepared using additive dosages selected to restore the high-temperature grade of the blended binder system. This study encompasses one reclaimed asphalt pavement (RAP) and one recycled asphalt shingles (RAS) mixture. Two extender and two recycling agent products were evaluated. The results indicate that different additive types yield different slopes of the blended system high-temperature grade versus additive content. The results also show that uncertainty should be accounted for when establishing a dosage selection procedure to ensure that the blended binder grade does not fall below the desired value. The rutting resistance of mixtures prepared using additive dosages intended to restore the high-temperature grade all passed recommended Hamburg wheel-track criteria. }, number={6}, journal={Transportation Research Record: Journal of the Transportation Research Board}, publisher={SAGE Publications}, author={Fried, Andrew and Xue, Lei Gabriel and Gulzar, Saqib and Preciado, Jaime and Underwood, B. Shane and Castorena, Cassie}, year={2022}, month={Feb}, pages={302–314} }
 @article{matini_gulzar_underwood_castorena_2022, title={Evaluation of Structural Performance of Pavements under Extreme Events: Flooding and Heatwave Case Studies}, volume={3}, ISSN={["2169-4052"]}, url={https://doi.org/10.1177/03611981221077984}, DOI={10.1177/03611981221077984}, abstractNote={ The recent increase in the frequency of extreme weather events has raised awareness and interest in the need for transportation infrastructure resilience. In this paper, the issue of pavement resilience is discussed with the goal of refining the idea for its use in pavement design. It is argued that one critical knowledge gap to advancing the state of the art in this area is distinguishing between functional and structural resiliency. The arguments here are framed using floods and heatwaves to demonstrate the importance of structural resilience. Under extreme event disruptions, structural damage is inevitable. The case study simulations in this paper suggest, depending on the pavement structure, intensity, and frequency of flooding events over the analysis period, that pavement rutting performance can decrease by 15.5% in the case of a structure with sand subgrade and 18.8% in the case of a structure with clay subgrade. In the case of heatwaves, the increase in rutting was found to be 2.9% in a structure with sand subgrade. To move toward more resilient pavement infrastructure, it is important to continuously monitor pavements after extreme events, develop methodologies to predict their performance, incorporate the findings in the current pavement management systems, and adapt design and management strategies accordingly. Future design and management of pavement systems should consider both structural and functional resiliency. This study shows that pavement performance simulations predict a long-term decrease in structural performance as a result of extreme events. }, number={7}, journal={TRANSPORTATION RESEARCH RECORD}, publisher={SAGE Publications}, author={Matini, Narges and Gulzar, Saqib and Underwood, Shane and Castorena, Cassie}, year={2022}, month={Mar} }
 @article{gulzar_goenaga_underwood_2022, title={Novel Index for Vulnerability Assessment of Flexible Pavement Infrastructure to Temperature Rise: Case Study of Developing Countries}, volume={28}, ISSN={["1943-555X"]}, url={https://doi.org/10.1061/(ASCE)IS.1943-555X.0000669}, DOI={10.1061/(ASCE)IS.1943-555X.0000669}, abstractNote={Road infrastructure is engineered to perform through constant exposure to climatological stressors. Due to climate change in particular, expected rises in the average ambient and extreme temperatures, pavement performance might deviate from what was originally predicted at the design stage. This poses serious risks to the long-term performance of this infrastructure because the capacity of a transportation network to provide an acceptable level of service over the period that it was intended for is also seriously affected. However, identifying where potential risks are greatest and where more involved planning and prioritizing is needed is hampered by the lack of vulnerability assessment methods that appropriately integrate failure pathways and engineering decisions. In this study, a new vulnerability index is proposed—extent of pavement grade reliability loss (EPGRL), which has the capacity to capture both flexible pavement performance and transportation serviceability. The index is based on two assumptions. First, flexible pavement performance is directly related to the inherent rheological properties of the asphalt binder, which can be estimated from the performance binder grade requirements for a locale. Second, the level of service of a road network can be represented by its connectivity, expressed here by a network centrality measure, the betweenness score. Two developing countries, Colombia and India, were selected to analyze the capabilities of the EPGRL. The results obtained indicate the EPGRL captures the available performance capability of pavement infrastructure together with the importance of transportation network elements. Hence, it can be used as a tool to evaluate and quantify the vulnerability of transportation infrastructure to future climate change.}, number={1}, journal={JOURNAL OF INFRASTRUCTURE SYSTEMS}, author={Gulzar, Saqib and Goenaga, Boris and Underwood, B. Shane}, year={2022}, month={Mar} }
 @article{gulzar_richard kim_castorena_shane underwood_2021, title={COVID-19, Uncertainty, and the Need for Resilience-Based Thinking in Pavement Engineering}, volume={147}, ISSN={["2573-5438"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85094102919&partnerID=MN8TOARS}, DOI={10.1061/JPEODX.0000235}, abstractNote={Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal © 2020 American Society of Civil Engineers}, number={1}, journal={JOURNAL OF TRANSPORTATION ENGINEERING PART B-PAVEMENTS}, author={Gulzar, Saqib and Richard Kim, Y. and Castorena, Cassie and Shane Underwood, B.}, year={2021}, month={Mar} }
 @article{daryaee_habibpour_gulzar_underwood_2021, title={Combined effect of waste polymer and rejuvenator on performance properties of reclaimed asphalt binder}, volume={268}, ISSN={["1879-0526"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85095820264&partnerID=MN8TOARS}, DOI={10.1016/j.conbuildmat.2020.121059}, abstractNote={In hot asphalt recycling, use of reclaimed asphalt binder has resulted in stiffer binders. To counter this phenomenon, rejuvenators are utilized. Further, research has shown that polymers can improve the performance properties of the aged bitumen. The study of the combined effect of both (rejuvenator and polymer) on the rheological properties of reclaimed asphalt binder can provide useful insights about the performance of the binder in hot asphalt recycling projects. In the current study, the combined effect of a rejuvenator and polymer derived from a waste source on the mechanical properties of bitumen containing varying percentages of reclaimed asphalt binder (RAB) is evaluated for the first time. The fatigue behavior was evaluated using linear amplitude sweep (LAS), and time sweep (TS) tests while rutting resistance was assessed using multiple stress creep recovery (MSCR) test. Moreover, bending beam rheometer (BBR) test is also performed to compare the sample’s low-temperature cracking. Results indicate that the synergistic use of waste polymer along with rejuvenator substantially improves the high, intermediate, and low-temperature properties of the blends with 50% RAB content compared to that of the control bitumen. Therefore, the use of waste PBR in hot asphalt recycling (HIR) projects, in addition to improving the performance of recycled bitumen can also be introduced as an environmental-friendly option.}, journal={CONSTRUCTION AND BUILDING MATERIALS}, author={Daryaee, Daryoosh and Habibpour, Mahdi and Gulzar, Saqib and Underwood, Benjamin Shane}, year={2021}, month={Jan} }
 @article{habbouche_boz_underwood_castorena_gulzar_fried_preciado_2021, title={Review From Multiple Perspectives for the State of the Practice on the Use of Recycled Asphalt Materials and Recycling Agents in Asphalt Concrete Surface Mixtures}, volume={2676}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85128770728&partnerID=MN8TOARS}, DOI={10.1177/03611981211061130}, abstractNote={ The objective of this paper is to provide information from multiple perspectives on the current state of the practice with regard to using recycled materials and recycling agents (RAs) in asphalt concrete mixtures. This information was collected through a survey of U.S. transportation agencies and RA suppliers combined with a search of RA-related specifications and pilot projects previously constructed. Moreover, a case study describing the Virginia Department of Transportation’s experience with RAs provides a tangible example of how at least one agency is approaching the potential implementation of these technologies. This practice review was achieved by documenting the experience, lessons learned, and best practices of multiple asphalt experienced contractors and asphalt binder suppliers in the Virginia area. This paper follows a similar survey conducted in 2014 as part of NCHRP 09-58 and provides a second look at the use of RAs across North America. Not all state departments of transportation have experience with using RAs. Factors preventing the use of RAs included specification limitations, lack of expertise in processing recycled materials, supporting data, and negative prior experiences. Developing a performance-based testing framework is mandatory for the successful use of RAs. In general, good and frequent communication with the RA supplier is critical and necessary during the planning stages, the production of mixtures, and the continuous quality control by the supplier to resolve issues when they arise. Finally, a strong quality control and quality assurance-testing program should be implemented to ensure that materials meet the properties needed to produce a good-performing mixture. }, number={4}, journal={TRANSPORTATION RESEARCH RECORD}, publisher={SAGE Publications}, author={Habbouche, Jhony and Boz, Ilker and Underwood, Benjamin Shane and Castorena, Cassie and Gulzar, Saqib and Fried, Andrew and Preciado, Jaime}, year={2021}, month={Dec} }
 @inproceedings{gulzar_underwood_2020, title={Fourier transform rheology of asphalt binders}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85117414974&partnerID=MN8TOARS}, booktitle={Advances in Materials and Pavement Performance Prediction II - Contributions to the 2nd International Conference on Advances in Materials and Pavement Performance Prediction, AM3P 2020}, author={Gulzar, S. and Underwood, S.}, year={2020}, pages={427–431} }
 @inproceedings{gulzar_underwood_2020, title={Large amplitude oscillatory shear of modified asphalt binder}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85117372082&partnerID=MN8TOARS}, booktitle={Advances in Materials and Pavement Performance Prediction II - Contributions to the 2nd International Conference on Advances in Materials and Pavement Performance Prediction, AM3P 2020}, author={Gulzar, S. and Underwood, S.}, year={2020}, pages={432–435} }
 @article{gulzar_underwood_2020, title={Nonlinear Viscoelastic Response of Crumb Rubber Modified Asphalt Binder Under Large Strains}, volume={2674}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85085990773&partnerID=MN8TOARS}, DOI={10.1177/0361198120907097}, abstractNote={ Agencies have been increasing their use of polymer modified asphalt binders in recent years to address performance issues and lengthen the useful life of their pavements. When deployed these materials likely experience strain levels exceeding their linear viscoelastic (LVE) limits. The same situation exists in non-polymer modified asphalt binders as well, but the effect may be more pronounced in polymer modified systems because of their bi-phasic nature. In this study, terminally blended crumb rubber (CR-TB) modified asphalt is studied to understand and quantify the nonlinear viscoelastic response under large strains. The CR-TB binders are extensively used in pavements subjected to high vehicular loads and extreme climatic conditions; thereby, their response under large strains becomes more critical. The current standard characterization techniques are based on LVE response using small amplitude oscillatory shear rheology only and do not consider the behavior of binders under large strains. In this study, large amplitude oscillatory shear (LAOS) rheology is used as a framework to more thoroughly investigate the complete response of the CR-TB modified asphalt binder under large strains at 30°C, 40°C, 50°C, and 60°C and at the frequencies of 0.5, 1, and 5 Hz. The LAOS response is analyzed using Fourier-transform rheology and the orthogonal stress decomposition method involving Chebyshev polynomial representation. It is found that nonlinearity manifests greatly in this study material as strain levels increase and frequencies decrease. The relative nonlinearity increases with increasing strain amplitude and is more significant towards lower end of the tested temperature range. The CR-TB binder shows strain-stiffening/softening and shear-thinning/thickening behavior depending upon a specific temperature, strain level, and frequency. }, number={3}, journal={TRANSPORTATION RESEARCH RECORD}, author={Gulzar, Saqib and Underwood, B. Shane}, year={2020}, month={Mar}, pages={139–149} }
 @inproceedings{gulzar_underwood_2020, title={Stress decomposition of nonlinear response of modified asphalt binder under large strains}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85117403644&partnerID=MN8TOARS}, booktitle={Advances in Materials and Pavement Performance Prediction II - Contributions to the 2nd International Conference on Advances in Materials and Pavement Performance Prediction, AM3P 2020}, author={Gulzar, S. and Underwood, S.}, year={2020}, pages={436–440} }
 @article{ding_wang_gulzar_kim_underwood_2020, title={Uncertainty Quantification of Simplified Viscoelastic Continuum Damage Fatigue Model using the Bayesian Inference-Based Markov Chain Monte Carlo Method}, volume={2674}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083645522&partnerID=MN8TOARS}, DOI={10.1177/0361198120910149}, abstractNote={ The simplified viscoelastic continuum damage model (S-VECD) has been widely accepted as a computationally efficient and a rigorous mechanistic model to predict the fatigue resistance of asphalt concrete. It operates in a deterministic framework, but in actual practice, there are multiple sources of uncertainty such as specimen preparation errors and measurement errors which need to be probabilistically characterized. In this study, a Bayesian inference-based Markov Chain Monte Carlo method is used to quantify the uncertainty in the S-VECD model. The dynamic modulus and cyclic fatigue test data from 32 specimens are used for parameter estimation and predictive envelope calculation of the dynamic modulus, damage characterization and failure criterion model. These parameter distributions are then propagated to quantify the uncertainty in fatigue prediction. The predictive envelope for each model is further used to analyze the decrease in variance with the increase in the number of replicates. Finally, the proposed methodology is implemented to compare three asphalt concrete mixtures from standard testing. The major findings of this study are: (1) the parameters in the dynamic modulus and damage characterization model have relatively strong correlation which indicates the necessity of Bayesian techniques; (2) the uncertainty of the damage characteristic curve for a single specimen propagated from parameter uncertainties of the dynamic modulus model is negligible compared to the difference in the replicates; (3) four replicates of the cyclic fatigue test are recommended considering the balance between the uncertainty of fatigue prediction and the testing efficiency; and (4) more replicates are needed to confidently detect the difference between different mixtures if their fatigue performance is close. }, number={4}, journal={TRANSPORTATION RESEARCH RECORD}, author={Ding, Jing and Wang, Yizhuang David and Gulzar, Saqib and Kim, Youngsoo Richard and Underwood, B. Shane}, year={2020}, month={Apr}, pages={247–260} }
 @inbook{gulzar_underwood_2019, title={Use of polymer nanocomposites in asphalt binder modification}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85086007280&partnerID=MN8TOARS}, DOI={10.1002/9781119605843.ch14}, abstractNote={This chapter contains sections titled: Introduction Background Polymer Nanocomposites Rheological Impacts Suggested Evaluation Method for PNC Modified Asphalt Binders Summary}, booktitle={Advanced Functional Textiles and Polymers: Fabrication, Processing and Applications}, author={Gulzar, S. and Underwood, S.}, year={2019}, pages={405–432} }