Browsing by Author "Murthy N. Guddati, Committee Member"

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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.
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.
Fuzzy and Neural Network Models for Analyses of Piles
(2007-11-19) Jeon, JongKoo; M.S.Rahman, Committee Chair; Roy H. Borden, Committee Member; Mohammed A. Gabr, Committee Member; Murthy N. Guddati, Committee Member; Deepak Sirdeshmukh, Committee Member
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).
Long-Term Performance Assessment of Asphalt Concrete Pavements Using the Third Scale Model Mobile Loading Simulator and Fiber Reinforced Asphalt Concrete
(2004-03-14) Lee, Sugjoon; Roy H. Borden, Committee Co-Chair; Hechmi Hamouda, Committee Member; Jon P. Rust, Committee Co-Chair; Y. Richard Kim, Committee Co-Chair; Murthy N. Guddati, Committee Member
Long-term pavement performance such as fatigue and rutting is investigated using the third scale Model Mobile Loading Simulator (MMLS3). Prediction algorithms are proposed that can account for the loading rate of MMLS3 and temperature variation along the depth of pavement. In a separate study, influence of fibers on the fatigue cracking resistance is studied. In this research, laboratory asphalt pavement construction technique, sensor instrumentation, and test conditions are evaluated to establish effective test protocols for fatigue cracking and rutting evaluation using the MMLS3. The investigated results present that: (1) the MMLS3 with wheel wandering system can induce the realistic fatigue (alligator pattern) cracks; (2) using wavelet correlation method (WCM), fatigue damage growth and microdamage healing are observed; (3) the algorithm for the fatigue life prediction of laboratory pavement is established using the indirect tension testing program and linear cumulative damage theory; (4) the MMLS3 performs a rapid assessment of the rutting potential under controlled conditions; (5) the predictive algorithm predicts rutting performance of asphalt pavements loaded by the MMLS3 using the repetitive cyclic triaxial compression testing program. It was found that fiber inclusion can improve the mechanical properties of asphalt concrete. Single nylon fiber pullout test was used to investigate debonding and pulling behavior. As for indirect tension strength tests, asphalt concrete containing nylon fibers showed the potential of improving fatigue cracking resistance by an increase of the fracture energy.
A Numerical Investigation of the Effects of Loading Conditions on Soil Response
(2009-03-27) Zhao, Xueliang; T. Matthew Evans, Committee Chair; Roy H. Borden, Committee Co-Chair; Mohammed A. Gabr, Committee Member; M. Shamimur Rahman, Committee Member; Murthy N. Guddati, Committee Member; C. C. David Tung, Committee Member
All three principal stresses play a part in the stress-strain-strength response and volumetric behavior of solids and granular materials. In geotechnical engineering, conventional triaxial compression (CTC), plane strain (PS), and direct shear (DS) are the three most commonly used laboratory tests to simulate the field conditions. It is natural to assume that specimens subjected to different loading conditions will show different responses and behaviors. In reality, many soil problems involving shear strength approximate to PS loading conditions in the field (e.g., earth dam, embankment, and retaining wall). However, CTC or DS test is typically used to measure the stress-strain-strength parameters for design because of their simplicity and versatility compared with the complexity and difficulty of the PS test, even though they might not closely mimic the field condition. The current research focuses on the numerical analysis of effects of different loading conditions (e.g., CTC, PS, and DS) on the macro- and micro-behaviors of granular materials using discrete element method (DEM). Analytical, statistical, and stereological approaches are employed. It is the first work to compare the results under the three most common loading conditions (PS, CTC, and DS) in DEM modeling. Models of the CTC, PS, and DS tests are developed. A new method to simulate the membrane behavior is proposed. Parametric analyses to qualitatively assess the effects of the specific parameter on the macroscale response of the specimen are performed. Macroscale responses of sets of simulations of assemblies under PS, CTC, and DS loading conditions are studied. Small-strain responses, shear strengths, and volumetric behaviors of the assemblies under different loading conditions are investigated. Microscale analyses on the assembly behaviors (e.g., void ratio and coordination number) and particle behaviors (e.g., particle rotation and displacement) are conducted. Particle orientation and contact properties (e.g., contact normal and contact force) are investigated using statistical analysis method. An algorithm to generate numerical slicing images which is to simulate the way in laboratory experiments is proposed. The local void ratio distribution analysis and particle orientation distribution analysis are performed using stereological method. Integrating macro-, micro-, and stereological methods, some issues such as strain localization, critical state, and principal stress direction rotation of DS test are investigated.