Browsing by Author "Dr. M. Shamimur Rahman, Committee Member"

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Efficient Absorbing Boundary Conditions for Modeling Wave Propagation in Unbounded Domains.
(2005-05-02) Zahid, Md. Anwar; Dr. Murthy N Guddati, Committee Chair; Dr. C. C. David Tung, Committee Member; Dr. M. Shamimur Rahman, Committee Member; Dr. Mansoor Haider , Committee Member
Many engineering problems (e.g. soil-structure interaction, medical imaging and nondestructive evaluation) encounter the phenomena of wave propagation. Among these problems some involve domains of infinite extent. Standard numerical methods such as finite element and finite difference methods cannot handle the unbounded domain as they are designed for the analysis of bounded domains. In order to solve an unbounded-domain problem, the domain is truncated around a region of interest, and absorbing boundary conditions (ABCs) are applied on the truncation boundary. These ABCs are expected to absorb outgoing waves and mimic the effect of the truncated exterior. Continued-fraction absorbing boundary conditions (CFABCs) are a class of highly efficient ABCs for modeling acoustic wave absorption into unbounded domains. The current versions of CFABCs are applicable only to non-dispersive scalar wave equation and are not effective for dispersive or elastic wave propagation problems. This dissertation contains extensions of CFABCs to dispersive and elastic wave propagation problems. The main difficulty in the case of dispersive wave propagation is that evanescent waves have significant presence and are not treated accurately by original CFABCs. In the first part of the dissertation, CFABCs are modified to effectively absorb propagating as well as evanescent waves. This is achieved with the help of special padding elements that absorb the evanescent waves and standard CFABC elements that are effective in absorbing propagating waves. Called the "padded CFABC", this combination is shown to be a highly efficient and accurate ABC for dispersive wave equations. Numerical results are presented to illustrate the effectiveness of these ABCs. The second part of the dissertation involves the extension of CFABCs to elastic wave propagation problems. Elastic wave propagation is inherently complex because of the strong coupling of pressure and shear waves that propagate at different speeds. It turns out that straightforward extension of acoustic CFABC tends to be unstable for elastic wave propagation problems. Modifying the CFABC by altering the parameters to complex numbers appears to rectify the stability problem. This stabilized CFABC, named "complex CFABC", is not as efficient as the original CFABC, but is superior to existing ABCs for elastic media. The complex CFABC necessitates modification of the implementation including careful operator splitting to achieve efficient explicit computational procedure. These modifications result in an effective and stable complex CFABC for elastic wave propagation, which is illustrated with the help of numerical examples. The ABCs developed in this dissertation are expected to aid in increasing the simulation efficiency for various unbounded domain problems, thus have impact on various fields including earthquake engineering, seismology, soil-structure interaction and shallow water problems.
Evaluation of Indirect Tensile Strength as Design Criteria for Superpave Mixtures
(2008-10-14) Krishnankuttynair, Harikrishnan; Dr.N.Paul Khosla, Committee Chair; Dr. Michael Leming, Committee Member; Dr. M. Shamimur Rahman, Committee Member; Dr. David Dickey, Committee Member
Many factors contribute to the degradation of asphalt pavements. When high quality materials are used, distresses are typically due to traffic loading, resulting in rutting or fatigue cracking. The presence of water (or moisture) often results in premature failure of asphalt pavements in the form of isolated distress caused by debonding of the asphalt film from the aggregate surface or early rutting/fatigue cracking due to reduced mix strength. Moisture sensitivity has long been recognized as an important mix design consideration. The tensile strength is primarily a function of the binder properties. The amount of asphalt binder in a mixture and its stiffness influence the tensile strength. Tensile strength also depends on the absorption capacity of the aggregates used. At given asphalt content, the film thickness of asphalt on the surface of aggregates and particle-to-particle contact influences the adhesion or tensile strength of a mixture. Various studies have repeatedly proved that the tensile strength increases with decreasing air voids. The tensile strength of a mixture is also strongly influenced by the consistency of the asphalt cement, which can influence rutting. Thus, tensile strength plays an important role as a design and evaluation tool for Superpave mixtures Moisture damage of asphalt pavements is a serious problem. The presence of moisture tends to reduce the stiffness of the asphalt mix as well as create the opportunity for stripping of the asphalt from the aggregate. This, in combination with repeated wheel loadings, can accelerate pavement deterioration. Strength loss is now evaluated by comparing indirect tensile strengths of an unconditioned control group to those of the conditioned samples. If the average retained strength of the conditioned group is less than eighty-five percent of the control group strength, the mix is determined to be moisture susceptible. This research study shows that reliance on the Tensile Strength Ratio (TSR) values only may be misleading in many cases. The individual values of tensile strength of conditioned and unconditioned specimens in conjunction with TSR values should be employed in assessing the effect of water damage on the performance of pavements. This study found that a minimum tensile strength should be established for a given ESAL range. The fatigue life of the mixtures decrease exponentially with decreasing tensile strength. This trend is justified by the loss in stiffness and thereby initiating cracks and stripping. There exists a minimum tensile strength for a given ESALs level that can be used as a surrogate criterion for fatigue life estimation. This research study also shows that the mixtures with lower tensile strength have higher rut depths. Rut depths of mixtures were shown to increase with decreasing tensile strength, which can be attributed to the fact that the aggregate structure is affected due to moisture damage and subsequent loss in tensile strengths of the mixtures. This study suggests that tensile strength can be used as a design tool in the Superpave mix design stage and a modified mix design procedure is proposed based on individual tensile strength.
Evaluation of the Effects of Compaction Methods on the Predicted Performance of Superpave Mixtures
(2004-11-14) Sadasivam, Suriyanarayanan; Dr. M. Shamimur Rahman, Committee Member; Dr. Pierre A. Gremaud, Committee Member; Dr. N. Paul Khosla, Committee Chair; Dr. Michael L. Leming, Committee Member
Several compaction methods are used to fabricate specimens for performance testing in the laboratory. As the physical properties of the specimens depend on the method of compaction used for fabrication, the compaction methods adopted in the laboratory should simulate the properties of the pavement in the field. The effects of different compaction methods on the performance of mixtures were investigated in this study. Laboratory compaction methods such as Superpave Gyratory Compaction (SGC) and Rolling Wheel Compaction (RWC) were compared with field compaction. Four field mixtures were selected. Various performance evaluation tests were conducted on the field cores and specimens fabricated using SGC and RWC. The mixtures were evaluated using the Shear tester, the Asphalt Pavement Analyzer (APA) and the NCSU Wheel Tracking Device. The results indicate that the laboratory compacted mixtures tend to be superior in their performance than the field cores. The laboratory compacted mixtures have higher stiffness values and lower shear strain values than the field cores. The RWC seems to simulate field compaction better than the SGC. A good correlation was found between the results of the Repeated Shear tests at Constant Height tests, the APA tests and the NCSU WTD rut tests. The mixtures which failed to satisfy the RSCH test criteria had rut depths greater than 0.5 inch, as measured by the APA and NCSU WTD. The mixtures that passed the RSCH tests had rut depths less than 0.5 inch. The APA test and the NCSU WTD test can be used as a simulator to examine the rutting susceptibility of a mixture. The compactability of the mixtures is studied using the SGC, IPC Servopac Compactor and GLPA. The compaction parameters that relate with the rutting behavior of mixtures were compared with the RSCH shear strains at different air void contents. The correlations showed different trends with two sets of parameters with one set measuring the compactability of the mixtures and another set measuring the shear behavior of the mixtures. The compaction parameters measured based on the theory of rate of densification could not satisfactorily predict the rutting behavior of the mixtures.
Influence of Water Infiltration on the Deformation of Mechanically Stabilized Earth (MSE) Walls
(2008-12-07) Kim, Wan Soo; Dr. Mohammed A. Gabr, Committee Member; Dr. M. Shamimur Rahman, Committee Member; Dr. R. Wayne Skaggs, Committee Member; Dr. Roy H. Borden, Committee Chair
The use of locally available soils, often with significant fines contents, clearly has great economic impact on the cost of mechanically stabilized earth (MSE) wall construction. However, numerous problems associated with large deformations and some failures after surface water infiltration into these ?marginal soils? have been documented in the literature (e.g. Soong and Koerner, 1999). The documented problems can be better understood using the framework of unsaturated soil mechanics because compacted soils are typically found in an unsaturated state. This thesis presents the implementation of infiltration, seepage, volume change, and shear strength modeling of unsaturated conditions appropriate for compacted soils into the FLAC (Fast Lagrangian Analysis of Continua) code to study the behavior of an MSE wall constructed with marginal soils. The effect of matric suction (as well as net normal stress) on the soil properties such as elastic modulus and shear strength was considered in the developed FLAC model. The stress dependent elastic modulus was determined from the results of the one-dimensional oedometer test. The elastic moduli of specimens prepared under various compaction conditions were investigated for soaked specimens as well as specimens tested at as-compacted moisture contents while considering both drained and undrained loading conditions. A procedure to estimate the undrained modulus from the constitutive relation for drained loading and the predicted pore pressure due to loading was suggested. The prediction of pore pressure was computed by modifying Hilf?s equation. Comprehensive studies on the prediction on unsaturated shear strength were performed using the three most commonly used empirical procedures (Vanapalli et al.?s 1st and 2nd approaches and the Khalili and Khabbaz method). The influence of the shape of Soil Water Characteristic Curve (SWCC) on the predicted shear strength was investigated through a parametric study. Moreover, shear strength data published in the literature for fifteen soils were examined using these procedures. Comparisons between measured and predicted values of unsaturated shear strength were presented for different soil types. The effect of stress state on the prediction of shear strength was also discussed. A series of numerical simulations including transient seepage analyses and stress deformation analyses were performed in order to predict the behavior of a MSE wall subjected to surface-water infiltration. In this research, two mechanisms to cause the deformation due to wetting were considered in simulations; (1) the deformation induced by shear strength decreases and (2) the volumetric deformation (swell or collapse) due to wetting. The effect of a low-quality compaction zone behind the wall face and the resulting higher permeability on the wall behavior was investigated. As result of the simulations, the wall deformations (face deflections and settlements) and reinforcement tensions (average and maximum tensions) were presented during construction and after periods of infiltration.