Browsing by Author "Y. Richard Kim, Committee Chair"

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Complex Modulus Determination of Asphalt Concrete Using Indirect Tension Test
(2004-11-29) Momen, Mostafa; Murthy N. Guddati, Committee Member; Roy H. Borden, Committee Member; Y. Richard Kim, Committee Chair
The purpose of this research is to present the results from an analytical/experimental study on the dynamic modulus testing of hot mix asphalt (HMA) using the indirect tension (IDT) mode. The analytical solution for dynamic modulus determination in IDT was developed by Kim (14) using the theory of linear viscoelasticity. To verify the analytical solution, temperature and frequency sweep tests were conducted on 24 asphalt mixtures commonly used in North Carolina, using both axial compression and IDT test methods. In doing so, a modified dynamic modulus test protocol is introduced that reduces the required testing time by using more frequencies and fewer temperatures based on the time-temperature superposition principle. A comparison of results from the axial compression and IDT test methods shows that the dynamic modulus mastercurves and shift factors derived from the two methods are in good agreement. It was also found that Poisson's ratio is a weak function of the loading frequency; its effect on the phase angle mastercurve is discussed. After verification of the analytical solution, another study was conducted to evaluate the effect of aggregate size on the variability of test results, where the coefficient of variation (CV) was computed for each aggregate size and the results were compared. It was found that mixes with a larger nominal maximum size of aggregate (NMSA) had a greater CV than those with a smaller NMSA. Digital image Correlation was used to further support the findings and reveal physical explanations for the results obtained from this statistical analysis.
A Comprehensive Study of Crack Growth in Asphalt Concrete Using Fracture Mechanics
(2003-10-29) Seo, Youngguk; Y. Richard Kim, Committee Chair; A.A. Tayebali, Committee Member; F.G. Yuan, Committee Member; M.N. Guddati, Committee Member
This research presents findings from a comprehensive experimental/analytical study of crack growth in asphalt concrete using the theory of fracture mechanics. The primary objective of this study is to provide critical information that is complementary to the use of viscoelastoplastic continuum damage model (Chehab et al., 2002) in simulating crack growth by means of finite element analysis. To simulate opening mode fracture, uniaxial-monotonic and cyclic-tension tests were conducted on prismatic specimens with symmetric double notches. The full post-peak behavior with strain localization is well described by softening function and fracture energy using the cohesive crack model. Digital image correlation method (DIC), a non-contact, full-field, surface displacement/strain measurement technique, was utilized to investigate the characteristics of the fracture process zone (FPZ), a localized damage zone. Irrespective of the notch size and testing conditions, the FPZ was observed to be similar in size and shape for the mixture. In addition, it was found that the strain at the crack tip immediately before crack initiation is a decreasing function of strain rate. It is shown that crack growth rate in asphalt concrete can satisfactorily be predicted using a quasi-elastic approach, based on the linear elastic stress intensity factor criterion. Using a temperature-reduced crack speed concept, the crack growth rate of asphalt concrete was shown to be proportional to temperature. Finally, the time-temperature superposition principle was successfully applied to these crack growth rate laws to develop a single relationship (i.e., a crack growth rate master curve). This analysis was further implemented to investigate the specimen size effect in the crack growth rate prediction model.
Determination of Dynamic Moduli in Uniaxial Compression for North Carolina Hot Mix Asphalt Concrete
(2004-12-01) King, Mark Harley; Murthy Guddati, Committee Member; Y. Richard Kim, Committee Chair; Roy Borden, Committee Member
This thesis presents results from an experimental study on the dynamic modulus testing of hot mix asphalts (HMAs) commonly used in North Carolina in uniaxial compression mode. Forty two mixtures with varying aggregate sources, aggregate gradations, asphalt sources, asphalt grades, and asphalt contents are included in this study. With the dynamic modulus database developed, several issues are investigated in this research. Effects of confining pressure on the dynamic modulus are evaluated by comparing results from uniaxial and triaxial compression tests. A modified dynamic modulus test protocol is developed by reducing the required testing time using more frequencies and fewer temperatures based on the time-temperature superposition principle. Hirsch and Witczak predictive models are evaluated. During this analysis a case study was conducted to determine how much pavement performance changes due to the predictive errors. Finally, effects of different mixture variables on dynamic modulus of asphalt concrete are evaluated.
Evaluation of Rutting Behavior of Density Deficient Asphalt Mixtures
(2003-11-18) El-Haggan, Omar Sherif; Roy H. Borden, Committee Member; Y. Richard Kim, Committee Chair; Murthy N. Guddati, Committee Member
The purpose of this research has been to evaluate the effect of change in density on the rutting performance of the asphalt pavement. This investigation helps in determining the appropriate penalty for density deficient pavements based on the rutting performance. Permanent deformation tests were performed at 30, 40, and 50C on specimens with four different air void contents: 8, 8.75, 9.5, and 11%. More permanent deformation was observed at higher air voids and temperature. Complex modulus tests were also performed at the same four air void contents. Results showed that dynamic modulus decreases with the increase of both temperature and air void content as the asphalt mixture becomes softer at higher temperatures and air voids. Finally, a case study was performed to see the effect of air voids on the rutting behavior of the asphalt pavement. In this case study, the yearly rut depth for a certain pavement structure was predicted for both 8% and 11% air voids. Rut depth was determined to be 0.0074 inches for the 8% air voids pavement and 0.0168 inches for the 11% air voids pavement. This means that the pavement with 3% deficiency in air voids had an amount of rutting which is 2.3 times that of the in-specification pavement.
Experimental Investigation and Constitutive Modeling of Asphalt Concrete Mixtures in Uniaxial Tension
(2006-09-25) Underwood, Benjamin Shane; Murthy N. Guddati, Committee Member; Roy Borden, Committee Member; Y. Richard Kim, Committee Chair
Performance modeling of asphalt concrete pavements is one of the most difficult, but important tasks facing pavement engineers. Experiences at North Carolina State University suggest that this task is best accomplished by utilizing two separate models; one to account for the material behavior and another to account for boundary conditions, such as tire-pavement interaction, temperature gradient along the layer thickness, pavement structural design, etc. The material characterization model should focus on the material irrespective of geometry, i.e., fundamental properties. The structural model should be robust enough to account for the range of conditions experienced by pavements in service. Two peer-reviewed and published papers are presented here which deal with the development of a constitutive material model for asphalt concrete. In the first, the viscoelastoplastic continuum damage model in tension is applied to materials from the Federal Highway Administration's Accelerated Load Facility study on modified mixture performance. It is shown that the material model is capable of describing the behavior of the tested mixtures over a range of conditions from primarily viscoelastic to primarily viscoplastic. Further, the model shows sensitivity to changes in asphalt binder and the ability to predict the behavior of asphalt concrete mixtures containing polymer modified binder. The second paper presents results from an experimental study of anisotropy in asphalt concrete. Anisotropy occurs due to the preferential orientation of aggregate particles in the mixture and is found to have varying levels of significance depending on both the mode of loading and the levels of deformation applied. In the linear viscoelastic range, anisotropy is found to have little effect on the material behavior, whereas under monotonic compressive loading until failure, it is found to contribute significantly. Further, it is found that temperature and rate affect the significance of anisotropy in asphalt concrete. Conclusions and plans for future work are also presented.
Fatigue Performance Prediction of North Carolina Mixtures Using Simplified Viscoelastic Continuum Damage Model
(2009-08-10) Hou, Tian; Y. Richard Kim, Committee Chair; Murthy N. Guddati, Committee Member; Roy H. Borden, Committee Member
Fatigue performance modeling is one the major topics in asphalt concrete modeling work. Currently the only standard fatigue test available for asphalt concrete mixtures is the flexural bending fatigue test, AASHTO T-321. There are several issues associated with flexural fatigue testing, the most important of which are the stress state is not uniform but varies over the depth of the specimen and equipment for fabricating beam specimens is not widely available. Viscoelastic continuum damage (VECD) fatigue testing is a promising alternative to flexural fatigue testing. Different researchers have successfully applied the VECD model to asphalt concrete mixtures using constant crosshead rate direct tension test. However, due to the load level limitation of the new coming Asphalt Mixture Performance Tester (AMPT) testing equipment, there is an immediate need to develop a model that can characterize fatigue performance quickly using cyclic test data. In this study, a simplified viscoelastic continuum damage model developed at NCSU is applied to various North Carolina mixtures, which are used in the NCDOT HWY-2007-7 MEPDG local calibration project. It is shown that the simplified VECD model can predict fatigue tests fairly accurately under various temperature conditions and strain levels. It is also shown that the model can be further utilized to simulate both the strain controlled direct tension fatigue test and the traditional beam fatigue test. In this thesis, simulation results are presented. Conclusions regarding the applicability of the new model are advanced as well as suggestions for further work.
Investigation of the Effect of Lime on Performance of Hot Mix Asphalt using Advanced Testing and Modeling Techniques
(2008-08-21) Lee, Sangyum; Kimberly S. Weems, Committee Member; Murthy N. Guddati, Committee Member; Roy H. Borden, Committee Co-Chair; Y. Richard Kim, Committee Chair
The benefits of using hydrated lime as an additive in asphalt concrete are well known. When added to asphalt concrete mixtures hydrated lime shows the beneficial effects of filler, while also improving resistance to moisture damage. This study presents findings from four studies into the impact of hydrated lime, the impact of lime introduction method on the volumetric optimums, and the performance evaluation of unmodified and lime-modified hot mix asphalt (HMA) mixtures at varying asphalt contents using Simple Performance Tests developed from the NCHRP projects 9-19 and 9-29 and the viscoelastic continuum damage (VECD) finite element analysis. The performance characteristics evaluated in this study include fatigue cracking and rutting behavior in both dry and moisture-conditioned states. Test methods adopted in this evaluation are: the dynamic modulus test for stiffness characterization; the triaxial repeated load permanent deformation test for rutting characterization, and the direct tension test for fatigue cracking characterization. From the experimental investigation it is found that the method of lime introduction can have an important effect on the optimum volumetric asphalt content. Regarding dynamic modulus it is found that hydrated lime has a minimal impact on the mixtures in this study. However, the findings from this study support conventional understanding of the effects of asphalt content, lime modification, and moisture conditioning on the fatigue cracking and rutting performance of HMA mixtures. That is, as asphalt content increases, the resistance to fatigue cracking improves and rutting performance worsens. Another accepted fact is that lime modification reduces the susceptibility for moisture damage in terms of both fatigue cracking and rutting. The contribution of this paper, therefore, is to demonstrate advanced test methods and models that can be used in the performance evaluation of various mixtures. With additional validation and calibration, the comprehensive methodology described in this paper may serve as the foundation for a performance-based HMA mix design and performance-related HMA specifications.
Local Calibration of the MEPDG for Flexible Pavement Design
(2007-10-26) Muthadi, Naresh Reddy; Y. Richard Kim, Committee Chair; Murthy N. Guddati, Committee Member; T. Matthew Evans, Committee Member
The 1993 American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement Structures is a mere modification of the empirical methods found in its earlier versions that are based on regression equations relating simple material and traffic inputs. Although the various editions of the AASHTO design guide have served well for several decades, they contain too many limitations to be continued as the nation's primary pavement design procedures. The Mechanistic-Empirical Pavement Design Guide (MEPDG) procedure, on the other hand, provides the tools for evaluating the effect of variations in input data on pavement performance. The design method in the MEPDG is mechanistic because it uses stresses and strains in a pavement system calculated from the pavement response model to predict the performance of the pavement. The empirical nature of the design method stems from the fact that the pavement performance predicted from laboratory-developed performance models is adjusted based on the observed performance from the field to reflect the differences between predicted and actual field performance. The performance models used in the MEPDG are calibrated using limited national databases and, thus, it is necessary to calibrate these models for local highway agencies implementation by taking into account local materials, traffic information, and environmental conditions. Two distress models, permanent deformation and bottom-up fatigue cracking (hereafter referred to as alligator cracking), were employed for this effort. Fifty-three pavement sections were selected for the calibration and validation process: 30 long-term pavement performance (LTPP) pavements, which include 16 new flexible pavement sections and 14 rehabilitated sections, and 23 North Carolina Department of Transportation (NCDOT) sections. All the necessary data were obtained from the LTPP and the NCDOT databases. To provide reasonable values in cases where data were missing, MEPDG defaults, NCDOT typical range of values, and engineering judgment were employed. Finally, an experimental matrix is developed to identify any bias resulting from the use of local materials and conditions. The NCDOT currently relies on a subjective rating or non-numeric rating system of the permanent deformation data, which presented difficulties in the conversion to the MEPDG format. The verification runs for the LTPP sections using the parameters developed during the national calibration effort under the NCHRP (National Cooperative Highway Research Program) 1-37A project showed promising results. Microsoft Excel Solver was used to fit the predicted rut depth values to the measured values by changing the coefficients in the permanent deformation models for hot-mix asphalt (HMA) and unbound materials. This process was employed for each of the permanent deformation models separately. For the alligator cracking model, the only possibility of reducing the standard error and bias is through the transfer function. Again, Microsoft Excel Solver was used to minimize the sum of the squared errors of the measured and predicted cracking by varying the C1 and C2 parameters of the transfer function. It was found that there is no significant difference between the local calibrated standard error and the global standard error for the HMA permanent deformation model as well as the alligator cracking model. Therefore, it was decided to keep both the models for a more robust calibration in the future that would increase the number of sections and include more detailed inputs (mostly Level 1 inputs).
Micromechanics-based Multiscale Lattice Modeling of Fatigue Cracking in Hot Mixed Asphalt
(2003-10-08) Feng, Zhen; John W. Baugh, Committee Member; Robert E. White, Committee Member; Murthy N. Guddati, Committee Co-Chair; Y. Richard Kim, Committee Chair
This dissertation presents a novel micromechanics-based lattice procedure designed to characterize the cracking performance of hot mix asphalt (HMA) from its constituent material properties. The approach essentially consists of a series of direct-lattice models integrated with multiscale technique. A typical direct-lattice modeling starts with a preprocessor that discretizes the microstructure of a specimen, which is either physically or virtually fabricated, into a random truss lattice. The mixture performance can be predicted by analyzing such lattice network using a general-purpose finite element program FEP++. The cracking process in HMA is simulated by successive removal of failed links representing microcracks. In this study, a surface energy based failure criterion is developed to trigger the creation and subsequent extension of microcracks. Due to the disparate length scales associated with microcracking and specimen size, the direct-lattice modeling described above would demand unrealistic computational cost for modeling even a laboratory scale HMA specimens. In order to make the procedure more practical, the multiscale approach is implemented. Essentially, multi-scale model considers the effect of different-sized aggregates at different length scales. Such an approach reduces the computational cost significantly, while capturing the mechanical phenomenon at various length scales. Finally, the effectiveness of the proposed multiscale lattice procedure is illustrated by modeling an actual indirect tensile (IDT) test on a thin cylindrical HMA specimen and making quantitative comparisons with experimental measurements, including full-strain fields measured by the digital image correlations (DIC) technique.
Nonlinear Finite Element Analysis of Pavements and Its Application to Performance Evaluation
(2003-07-24) Mun, Sungho; Mansoor Haider, Committee Member; Tasnim Hassan, Committee Member; Murthy N. Guddati, Committee Co-Chair; Y. Richard Kim, Committee Chair
This research documents the findings from the study of failure mechanisms of fatigue cracking in asphalt pavements using the finite element program that employs the viscoelastic continuum damage model for asphalt layer and a nonlinear elastic model for unbound layers. Both bottom-up and top-down cracks are investigated by taking several important variables into account, such as asphalt layer thickness, layer stiffnesses, pressure distribution under loading, and load levels applied on the pavement surface. The crack initiations in different pavement structures under different loading conditions are studied by monitoring a damage contour. The developed finite element code, called VECD-FEP++, employs the viscoelastic continuum damage model as the constitutive model of asphalt concrete and the universal model (or so-called Uzan-Witczak resilient modulus model) for unbound materials. The finite element analysis of various pavement-load combinations showed significantly different failure mechanisms. Details on the VECD-FEP++ and the findings are given in the following chapters.
Quantifying the Benefits of Improved Rolling of Chip Seals.
(2009-04-23) Lee, Jaejun; Roy H. Borden, Committee Member; Richard L. Lemaster, Committee Member; Y. Richard Kim, Committee Chair; Akhtarhusein A. Tayebali, Committee Member
This dissertation presents an improvement in the rolling protocol for chip seals based on an evaluation of aggregate retention performance and aggregate embedment depth. The flip-over test (FOT), Vialit test, modified sand circle test, digital image processing technique, and the third-scale Model Mobile Loading Simulator (MMLS3) are employed to evaluate the effects of the various rolling parameters and to measure chip seal performance. The samples used to evaluate the chip seal rolling protocol were obtained directly from field construction. In order to determine the optimal rolling protocol, the effects of roller type, number of coverages, coverage distribution on the sublayers of a multiple chip seal (i.e., the split seal and triple seal), and rolling pattern are evaluated using the results of aggregate retention performance tests, the modified sand circle method, and the digital image process. It is found that two types of roller, the pneumatic tire roller and the combination roller, are recommended as the optimal rollers for the chip seal. In addition, it is found that the optimal number of coverages for the chip seal, as determined from measurements of the aggregate, is three coverages. Moreover, the performance of the triple seal without coverage at the bottom layer does not affect the aggregate retention performance, although the split seal does require coverage at the bottom layer. Finally, it is found that the rolling pattern is strongly related to a delayed rolling time between the aggregate spreader and the initial rolling time. Therefore, it was recommended that two pneumatic tire rollers applied initial one coverage with optimal delayed rolling time (between 2 min. and 4 min.) to entire lane width, and then the combination roller applied two additional coverages. Further, it is confirmed that the delayed rolling time is related to the aggregate moisture condition and the ambient temperature.