@article{ashouri_wang_choi_kim_2021, title={Development of healing model and simplified characterization test procedure for asphalt concrete}, volume={271}, ISSN={["1879-0526"]}, DOI={10.1016/j.conbuildmat.2020.121515}, abstractNote={Fatigue cracking is one of the major distresses in asphalt pavement. Numerous and significant efforts have been undertaken to predict the fatigue life of pavements. One of the mechanisms that affects fatigue life is healing, and thus, including healing in fatigue performance prediction models is necessary. For this study, twelve healing tests at three temperatures and four rest periods were conducted to evaluate the healing characteristics of asphalt materials. The percentage of healing (%Hs) used in this study is defined as the ratio of the internal state variable (S) before the rest period to the incremental internal state variable due to the rest period. A %Hs mastercurve was constructed by applying the time–temperature superposition (t-TS) principle to the %Hs versus rest period curves and thereby proved that the t-TS principle works for healing. The %Hs mastercurves at different damage levels were shifted vertically to construct one reference %Hs mastercurve. The amount of vertical shifting is referred to as the vertical healing shift factor. The reference %Hs mastercurve with the vertical healing shift factor and t-TS principle led to the proposed shift healing model as a function of rest period, temperature, and damage level. A test protocol to calibrate the model also is suggested in this work. The protocol requires only three specimens, and thus, testing can be completed within a day. The proposed shift healing model characterized by the suggested protocol test was implemented successfully in twelve healing tests to predict the healing behavior of the test specimens, which indicates the prediction capability of the suggested model and test protocol.}, journal={CONSTRUCTION AND BUILDING MATERIALS}, author={Ashouri, Morteza and Wang, Yizhuang and Choi, Yeong-Tae and Kim, Youngsoo}, year={2021}, month={Feb} }
 @article{sabouri_choi_wang_hwang_baek_kim_2016, title={Effect of Rejuvenator on Performance Properties of WMA Mixtures with High RAP Content}, volume={11}, ISBN={["978-94-017-7341-6"]}, ISSN={["2211-0852"]}, DOI={10.1007/978-94-017-7342-3_38}, abstractNote={The production of warm mix asphalt (WMA) mixtures with high percentages of reclaimed asphalt pavement (RAP) is gaining attention as a way to save costs and efficiently utilize existing resources. However, WMA must perform at least as well as hot mix asphalt (HMA) before it can be used as a replacement for HMA. In this study, the performance of a WMA mixture with a high percentage of RAP (40 % RAP) and a WMA additive (1.5 % of binder weight) that works as a rejuvenator was evaluated and compared with the performance of a HMA mixture with the same amount of RAP in order to evaluate the effects of the WMA rejuvenator. These mixtures were evaluated in terms of fatigue cracking using the simplified viscoelastic continuum damage (S-VECD) model and in terms of rutting using the triaxial stress sweep (TSS) test. In addition, layered viscoelastic pavement analysis for critical distresses (LVECD) was used to predict the fatigue resistance of these mixtures for future use. The WMA rejuvenator was found to improve the mixing and compaction ability of the WMA mixture. Also, compared to the HMA mixture, the WMA mixture showed better fatigue resistance, but the rejuvenator found to have an adverse effect on the rutting resistance of the mixture.}, journal={8TH RILEM INTERNATIONAL SYMPOSIUM ON TESTING AND CHARACTERIZATION OF SUSTAINABLE AND INNOVATIVE BITUMINOUS MATERIALS}, author={Sabouri, Mohammadreza and Choi, Yeong-Tae and Wang, Yizhuang and Hwang, Sungdo and Baek, Cheolmin and Kim, Richard Y.}, year={2016}, pages={473–484} }
 @article{choi_kim_2014, title={Implementation and verification of a mechanistic permanent deformation model (shift model) to predict rut depths of asphalt pavement}, volume={15}, ISSN={["2164-7402"]}, DOI={10.1080/14680629.2014.927085}, abstractNote={The shift model is implemented in the layered viscoelastic asphalt pavement analysis for critical distresses (LVECD) program to predict the rut depth of asphalt pavements. The rut depth measurements taken at the National Center for Asphalt Technology (NCAT) test track and the Federal Highway Administration Accelerated Facility (FHWA ALF) test sections are evaluated using the model. The model can successfully evaluate rut depth, which proves the capability of the model implemented in the LVECD program. The slight over-prediction of the NCAT sections can be explained by ageing in the field that increases the pavement's resistance to rutting. The simulation results support the hypothesis that triaxial stress sweep tests with confinement can represent the permanent deformation behaviour of asphalt concrete in the field. In this regard, excessive shear flow may be the reason for the under-prediction of the FHWA ALF mixtures. For better predictions, a correction factor (i.e. a transfer function) is suggested, which is quantified via the ratio of shear stress to shear resistance. After applying individual transfer functions, the permanent deformation model in the LVECD can evaluate the growth of the rut depth. Therefore, even though the shift model is a uniaxial model, the model can predict the rut depth of asphalt concrete by employing the transfer function.}, journal={ROAD MATERIALS AND PAVEMENT DESIGN}, author={Choi, Yeong-Tae and Kim, Y. Richard}, year={2014}, pages={195–218} }
 @article{choi_kim_2013, title={A Mechanistic Permanent Deformation Model for Asphalt Concrete in Compression}, volume={82}, ISSN={["0270-2932"]}, DOI={10.1080/14680629.2013.812847}, abstractNote={Permanent deformation modelling research at North Carolina State University has produced the so-called incremental model that fits the primary and secondary regions in permanent strain growth. Triaxial repeated load permanent deformation tests are conducted on Federal Highway Administration-Accelerated Loading Facility and NY9.5B mixtures to evaluate the effects of temperature, stress, and load time on permanent deformation and, therefore, to determine the form of the incremental model to account for these effects. The test results suggest that the slope in the log(ϵvp)−log(N) plot is constant regardless of these three major factors. This observation provides the basis for two modelling approaches: the functionalised model and the shift model. The functionalised model is formulated by expressing the coefficients of the incremental model in terms of the reduced load time and deviatoric stress. The shift model, based on the time-temperature–stress superposition principle, utilises the strain mastercurve and reduced load time and deviatoric stress shift functions. A composite loading test that is composed of varying load times and deviatoric stresses is proposed as the model calibration test. It is found that the permanent strain growth under the complex loading histories predicted by the calibrated models is in good agreement with the measured permanent strain growth.}, journal={ASPHALT PAVING TECHNOLOGY 2013, VOL 82}, author={Choi, Yeong-Tae and Kim, Y. Richard}, year={2013}, pages={617–649} }
 @inproceedings{choi_kim_2013, title={A mechanistic permanent deformation model for asphalt concrete in compression}, volume={82}, booktitle={Asphalt paving technology 2013, vol 82}, author={Choi, Y. T. and Kim, Y. R.}, year={2013}, pages={617–649} }
 @article{choi_kim_2013, title={Development of Calibration Testing Protocol for Permanent Deformation Model of Asphalt Concrete}, ISSN={["2169-4052"]}, DOI={10.3141/2373-04}, abstractNote={Recent permanent deformation modeling research at North Carolina State University has resulted in the shift model, which is capable of expressing the permanent strain growth of asphalt concrete as a function of deviatoric stress, load time, and temperature on the basis of the time–temperature superposition and time–stress superposition principles. This paper presents an efficient calibration test protocol for the shift model as well as verification of the model. The proposed test protocol is comprised of triaxial stress sweep (TSS) tests and a reference test. The TSS test is suggested to reduce the number of tests required by applying three deviatoric stresses within one test. Each TSS test was performed at three temperatures: high (TH), intermediate (TI), and low (TL). The reference test was a triaxial repeated load permanent deformation test conducted at TH only. The shift model was calibrated for the polymer-modified dense-graded NY9.5B mix, and the calibrated model was applied successfully to predict strain growth for the composite tests at the three study temperatures and for random load tests at TH. The calibration testing procedure was optimized for the asphalt mixture performance tester. The TSS tests take approximately 2.9 h at TH and 1.5 h at TI and TL. Thus, about a day was required to complete one set of calibration tests under the proposed test protocol. Within 2 to 3 days of testing, depending on the number of replicates, the calibrated shift model is capable of predicting permanent strain growth for different temperatures, load times, and deviatoric stresses.}, number={2373}, journal={TRANSPORTATION RESEARCH RECORD}, author={Choi, Yeong-Tae and Kim, Y. Richard}, year={2013}, pages={34–42} }
 @article{choi_subramanian_guddati_kim_2012, title={Incremental Model for Prediction of Permanent Deformation of Asphalt Concrete in Compression}, ISSN={["2169-4052"]}, DOI={10.3141/2296-03}, abstractNote={Permanent deformation (rutting) is one of the major distresses in asphalt pavement. To predict permanent deformation of asphalt concrete, repeated creep and recovery (or flow number) tests are typically used in the laboratory. However, models for the prediction of permanent deformation that incorporate flow number testing cannot represent the primary region because they concentrate on the secondary region. A new simple permanent deformation model called the incremental model is proposed. The proposed model is derived from the rate model, which is a rigorous mechanical model based on viscoplasticity. Four parameters of the new model provide an understanding of the permanent deformation. Parameter A is related to the initial permanent strain level, and Parameter C provides information about where the secondary region starts. That is, Parameters A and C govern the primary region, where α (alpha) is the slope of the secondary region, and B represents the permanent deformation level of the secondary region. Two mixtures are selected to investigate the deformation characteristics, and repeated creep and recovery tests are performed in compression. The incremental model is verified by applying it to various loading conditions for two mixtures. Furthermore, it is found that α is the material constant and the time-temperature superposition principle is applicable to each parameter. All parameters, except a, depend on both deviatoric stress and reduced load time, which is the product of load time and temperature. The incremental model describes ways to apply the time-temperature supposition principle to permanent deformation.}, number={2296}, journal={TRANSPORTATION RESEARCH RECORD}, author={Choi, Yeong-Tae and Subramanian, Vijay and Guddati, Murthy N. and Kim, Y. Richard}, year={2012}, pages={24–35} }