Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

The dynamic properties of clay deposits under seismic or wave loading conditions must be well understood to assure dynamic stability of structures founded on such soil. The dynamic shear modulus and damping appear to be a complex function of many variables, and a wide range of values have been reported in the literature. Consequently, considerable uncertainty exists in choosing the appropriate values of shear modulus and damping for a particular problem. This thesis presents a study of the influence of various factors on the shear modulus and damping of a marine clay using a resonant column/torsional shear device. In particular, the influence of factors such as shear strain amplitude, effective confining stress, stress history, frequency (strain rate), and secondary time-dependent behaviour are examined. The pore pressure response is also studied. The shear modulus was found to degrade for shear strains above 0.005%. The strain dependency was found to be well represented by a single normalized modulus reduction curve regardless of the confining pressure or overconsolidation ratio. Slower strain rates resulted in smaller values of shear modulus. Of the variables studied, the duration of sample confinement was found to be the most imporant factor affecting the material damping. Above 0.005% strain, the damping of the marine clay increased with shearing strain amplitude. No significant effect of confining pressure and stress history on damping was observed at any strain level. As well, the material damping was found to be relatively independent of loading frequency. Both the shear wave velocity and damping obtained in this study were found to be consistent with the in situ values determined using the seismic cone penetration test. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

This thesis presents a finite element (FE) modelling approach investigating the effects of corroded shear reinforcement on the capacity and behaviour of shear critical reinforced concrete (RC) beams. Shear reinforcement was modelled using a “locally smeared” approach, wherein the shear reinforcement is smeared within a series of plane-stress concrete elements at the specific stirrup location. This was done with the objective of incorporating both the reduction in cross-sectional area due to corrosion and the corresponding expansion of corrosion products build up. Corrosion damage was incorporated through equivalent straining induced by the corrosion build up on the affected surrounding concrete where the concrete cover was treated as a thick-wall cylinder subjected to internal pressure. Strains were introduced in the FE model using fictitious smeared horizontal pre-stressing steel, with a compressive pre-straining level related to the degree of corrosion penetration of the reinforcement. The FE modelling approach was first validated against published test data of shear critical RC beams with and without stirrup corrosion. The proposed modelling approach successfully reproduces the load deformation response as well as the failure mode and cracking patterns of the published experimental tests. Upon validation of the FE model, the work was extended to a parametric analysis of important shear design variables, such as the shear span-to-depth ratio, beam width and stirrup spacing The FE analyses were carried out for three increasing levels of corrosion (low, moderate and high) applied to affected stirrups within the critical section of the beams and based on steel mass loss (10%, 30% and 50%, respectively). In general, the results show a reduction in load carrying capacity accompanied by a softening of the load-deformation curves with each increasing level of corrosion. In most of the cases, a reduction in deflection associated to peak loads was also observed for moderate and high levels of corrosion. The impact of the various parameters was studied with respect to strength and deformation, as well as crack angle and mid-height horizontal strain. This was done in an effort to compare FE values to those provided by the CSA A23.3 design equations. The CSA A23.3 shear design equations were compared against FE analysis data in terms of residual shear strength estimation and individual component contributions to shear resistance (i.e., concrete and steel). The comparisons revealed an over conservative estimation for both strength and concrete contributions and an overestimation of the steel contribution. This divergence was attributed to a transition in shear behaviour within the critical section. Based on the progression of the concrete compressive struts with increasing corrosion and predicted crack angle, it was found that stresses in affected sections are redistributed towards adjacent undamaged material. The shear resistance mechanism generally transitioned from typical beam behaviour towards an arching-dominated one. Finally, based on important findings from the literature and the work conducted within this research, important considerations for assessment practice are suggested.

No description available.

As advances in solid rocket technology push rocket motors to more extreme operating speeds and temperatures, it becomes increasingly important to have well-designed material systems capable of surviving these harsh conditions. One common component in these systems is the use of a fiber- and particle-reinforced EPDM insulation layer between the motor casing and the solid fuel to shield the casing from the temperatures of the burning fuel and from the high velocity of gas particles traveling within the motor. This work studies several insulation materials to determine which exhibits the highest shear strength after being charred. Double-notch shear test specimens of three materials, ARI-2718, ARI-2719, and ARI-2750, were charred and tested to measure the failure strength of each charred material. The ARI-2750 showed the highest shear strength when loaded along the material orientation, but the ARI-2719 was strongest when transversely loaded. The strength measurements for ARI-2750 were highly sensitive to loading direction, unlike ARI-2718 and ARI-2719. Extensive scanning electron microscopy to identify correlations between shear strength and microstructure revealed that the amount of fiber orientation and amount of residual matrix material may have significant impacts on charred shear strength in these materials. / Master of Science

Strain localization, the phenomenon of large shear deformation within thin zones of intensive shearing, commonly occurs both in-situ and in the laboratory tests on soils specimens. The intriguing mechanism of strain localization and how it will affect the general behavior of soil specimens have been investigated by many researchers. Some of the efforts have focused on finding the links between material properties (void space, fabric tensor) and mechanical behavior (stress, strain, volumetric strain). In the last ten years, several extensive studies have been conducted at Georgia Tech to investigate the mechanism of strain localization and link the microstructural properties with the engineering behavior of Ottawa sands. These studies have included 2-D and 3-D characterization of soil microstructures under either triaxial or biaxial shearing conditions. To extend and complement these previous studies, the current study focuses particularly on 3-D reconstruction, analysis and modeling of specimens of Ottawa sand subject to triaxial or biaxial loading. The 3-D microstructure of biaxial specimens was reconstructed using an optical microscopy based montage and serial sectioning technique. Based on the reconstructed 3-D digital volumes, a series of 2-D and 3-D characterizations and analyses, including local void ratio distributions, extent of shear bands, influence of soil fabrics and packing signature effects, were conducted. In addition to the image analysis based reconstruction and characterization, the 3-D discrete element method (DEM) code, PFC3D, was used to explore both biaxial and triaxial shear related soil behaviors at the global and particulate scale. Void ratio distributions, coordination numbers, particle rotations and displacements, contact normal distributions and normal contact forces as well as global stress and strain responses were investigated and analyzed to help understand the mechanism of strain localization. The microstructures of the numerical specimens were also characterized in the same way as the physical specimens and similar strain localization patterns were identified. Combined with the previous related studies, the current study provides new insights into the strain localization phenomenon of Ottawa sands subject to triaxial and biaxial loading. In addition, the reconstructed digital specimens were subject to a series of dissection studies which revealed exciting new insights into "microstructure signatures" which exist at both meso and micro scales within the real and simulated specimens.

Most models available today for predicting the forces encountered by tillage tools apply to slow moving tools and do not take into account speed effects. However, most tillage operations are performed at speeds in the range of 2-8 km/h, and experimental studies show that tool forces increase significantly with tool speed. This effort of developing a model for predicting the forces on narrow tools under dynamic conditions was carried out in three steps. First, a series of laboratory tests was conducted to determine the effect of shear rate on soil shear strength and soil-metal friction parameters. Second, a model was developed to include dynamic effects. Third, the model was verified experimentally under laboratory conditions. Direct shear tests using a conventional shear box were conducted on an artificial soil at shear rates between 0. 5 and 12 7 cm/min. Experimental results showed that for the soil tested, the angle of internal friction, soil-metal friction angle, cohesion, and adhesion are independent of shear rate. A soil-tillage tool interaction model developed for quasi-static soil failure was modified to include shear rate effects and accelerational force effects. Experimental verification tests for the model were conducted under controlled conditions using an indoor soil bin facility. Tests were conducted with flat tines at speeds from 5. 4 to 120 cm/s. The overall trend was for the model to underpredict the observed total tool force by 16 %. However, the model demonstrated that terms including accelerational force effects can account for a large portion of the increase in tool force observed to occur with an increase in tool speed. / Master of Science

Corrosion is a concern in old prestressed concrete bridges, especially bridges built in marine environments. Corrosion induces cracks in the concrete superstructure which accelerates the deterioration rate and can result in a complete loss of the concrete cover and exposure of the reinforcing and prestressing steel. This causes degradation in the load-carrying capacity of the bridge girders. Consequently, decisions need to be made on whether to replace, retrofit, or load post these bridges. Extensive research has focused on the flexural strength of corroded prestressed concrete girders. This research studies the shear strength of corroded prestressed concrete girders which can, then, be expanded further to evaluate the possible retrofitting techniques for restoring, or enhancing, their shear strengths. Two old prestressed concrete girders built in the 1960's and 1970's were delivered to the Murray Structural Engineering Laboratory at Virginia Tech from two decommissioned bridges in Virginia. The two girders showed signs of deterioration due to corrosion. Non-destructive testing was performed to evaluate their in-situ conditions. For both girders, each end was tested in the lab in three-point loading condition to make full use of the girders. Shear capacities of the girders were predicted using four methods in the current AASHTO LRFD and the ACI codes. In addition, analysis using Response2000 and strut-and-tie modelling were also carried out. Evaluation of these methods and comparisons with the experimental results were performed to reach to conclusions and recommendations for future work. Corrosion in strands seemed to not have as much influence on the shear capacity as on the flexural capacity. Destructive shear tests indicated that the actual shear capacities of the girders investigated in this research exceeded nominal capacities predicted by the current codes. However, the flexural capacities were reduced. Possible reasons for the girders' behaviors are discussed. / Master of Science / Many bridges in the United States were built using longitudinal members, called girders, made of prestressed concrete. In prestressed concrete, because concrete cannot resist high tensile forces, tensioned steel cables, called strands, are used to produce compression on the concrete member to improve its behavior when it is in service. Corrosion is a concern in old prestressed concrete bridges, especially bridges built in marine environments. Corrosion induces cracks in the concrete superstructure which accelerates the deterioration rate and can result in a partial loss of the concrete body and exposure of the embedded steel. This causes degradation in the load-carrying capacity of the bridge girders which raises a danger to vehicles, passengers, and pedestrians. Consequently, decisions need to be made by authorities on whether to replace, repair, or load post these bridges. Two main types of loads exist in bridge girders, namely shear forces and bending moments. Extensive research has focused on the ability of corroded prestressed concrete girders to resist stresses produced by moment, or flexure. However, bridge girders must also resist shear forces. This research studies the shear strength of corroded prestressed concrete girders which can, then, be expanded further to evaluate the possible retrofitting techniques for restoring, or enhancing, their shear strengths. Two old prestressed concrete girders built in the 1960's and 1970's were delivered to the Murray Structural Engineering Laboratory at Virginia Tech from two decommissioned bridges in Virginia. The two girders showed signs of deterioration due to corrosion. These signs include concrete losses, cracks, areas of unsound concrete, and exposed strands. Non-destructive testing was performed on the girders to evaluate the severity of their in-situ conditions. Then, two destructive full-scale tests were performed on each girder in the lab to estimate their actual shear strengths. Shear strengths of the girders were also predicted using four methods present in the current American Association of State Highway and Transportation Officials, AASHTO, and the American Concrete Institute, ACI, codes. In addition, analyses using other advanced tools were also carried out. Evaluation of these methods and comparisons with the experimental results were performed to reach to conclusions and recommendations for future work. Corrosion in strands seemed to not have as much influence on the shear strength as on the flexural strength. Destructive shear tests indicated that the actual shear strengths of the girders investigated in this research exceeded nominal strengths predicted by the current codes, the AASHTO and the ACI. However, the flexural strengths were reduced. Possible reasons for the girders' behaviors are discussed.

ABSTRACT SHEAR STRENGTH BEHAVIOUR OF SAND - CLAY MIXTURES &Ouml / LMEZ, Mehmet Salih M.S., Department of Civil Engineering Supervisor: Prof. Dr. Mehmet Ufuk ERGUN May 2008, 106 pages A clean sand having about 5 % fines has been mixed with 5 to 40 % commercial kaolin to form different sand-clay soil mixtures. The purpose of making this study is to observe the effects of fraction of fine materials in the soil mixture on the behavior of shear strength. Three series of experiments have been performed throughout the study. Undrained triaxial compression tests (series 1) are performed on specimens taken out from homogeneously mixed soil mixtures at specified kaolin contents consolidated in a box without keeping the mixture under water. In series 2 experiments specimens are taken from a box where soil mixtures are consolidated under water and undrained triaxial compression tests are performed on the samples. Drained direct shear tests are performed on samples prepared without performing initial consolidation in large boxes but directly prepared in the direct shear boxes and consolidated prior to shear (series 3). It has been found that about 20 % kaolin - 80 % sand mixture seems to be a threshold composition and changes in both undrained and drained shear stress-strength behaviour occur afterwards with increasing fine material content.

Particulate materials are ubiquitous in the natural environment and have served throughout human history as one of the basic materials for developing civilizations. In terms of human activity, the handling of particulate materials consumes approximately 10% of all the energy produced on earth. Advances in the study and understanding of particulate materials can thus be expected to have a major impact on society. Geotechnical engineers have a long history of studying particulate materials since the fundamental building blocks of the profession include sands, silts, clays, gravels and ores, all of which are in one form or another particulates. The interface between particulates and other engineered materials is very important in determining the overall behavior of many geotechnical systems. Laboratory experimental studies into interface shear behavior has until now, been largely confined to systems involving uniformly graded sands comprised of a single particle size. This study addresses these potential shortcomings by investigating the behavior of binary particle mixtures in contact with surfaces. The binary nature of the mixtures gives rise to a changing fabric state which in turn can affect the shear strength of the mixture. Accordingly, packing limit states and the shear strength of binary mixtures were investigated across a range of mixtures, varying in particle size ratio and the proportion of fine particles to provide a reference. Binary mixtures in contact with smooth surfaces were investigated from both a global shear response and a contact mechanics perspective. A model was developed that allowed for the prediction of an interface friction coefficient based on fundamental material properties, particle and mixture parameters. Surface roughness changes as a result of shearing were also examined. The interface shear behavior with rough interfaces was examined in the context of the relative roughness between particles and surface features. The interpretation of traditional measures of relative roughness suffer from the need for a definitive average particle size, which is ambiguous in the case of non-uniform mixtures. Measures of an applicable average particle size for binary mixtures were evaluated.