The strength and behavior of composite slabs are governed by the shear interaction between the concrete and the steel deck. The interaction property depends on several factors and it is not possible to express the relationship from a purely analytical basis. As such, analysis and design methods available today use the interaction property derived from full scale performance tests. In numerical modeling, the interaction property is obtained from a variety of elemental push off tests which, for the most part, do not represent actual slab bending. This research comprises experimental, analytical and numerical investigations of composite slabs. The central objective of the experimental work is to develop a new small scale test method for evaluating the performance and behavior of composite slabs and also for determining the shear interaction property for use in numerical analysis. The characteristics of the new test specimen are simple, easy and economical to conduct, as well as comparable in performance and behavior with the more common full slab test. The analytical study was conducted to determine whether data from small scale tests can be used in the present analytical methods to predict the strength of the actual slabs, to use the same test data for input in the numerical analysis, and to improve the present Partial Shear Connection (PSC) design procedure. A model that relates the shear bond stress to slab slenderness, which can be used to estimate the shear interaction property for slabs with any slenderness, was developed. Finally, a finite element study was conducted to develop a simple modeling method that is suitable for analyzing composite slabs with variable slenderness. Parametric analyses to determine the effect of slenderness on the performance and behavior of composite slabs, and on the accuracy of the present design methods were also conducted. The results of this investigation demonstrate that the small scale test is feasible as a replacement for the full scale test. Data from the small scale test can be used not only in the analytical methods but also in the numerical analysis, thus eliminating the need for separate push off type tests. / Ph. D.

Additive Friction Stir Deposition (AFSD) is a solid state, bulk, metal additive manufacturing technology that seeks to replace certain castings and forgings wherever it is economically feasible among other applications. Critical to its deployment is an in depth understanding of how the solid state deposition process effects engineering alloys used in relevant applications. In this work, an aerospace aluminum alloy 7075 is evaluated both in the as deposited and heat treated condition via age hardening studies and tensile testing. It is found that an embrittlement phenomena occurs that is sensitive to processing parameters and quench rate during heat treatment. Through the use of SEM, TEM, and APT the embrittlement phenomena has been linked to excessive grain boundary precipitation caused by a combination of shear induced mixing and shear induced segregation which allow for the formation of phases at grain boundaries that are slow to dissolve, leaving the grain boundary in a non-equilibrium solute rich state. Critical to this process is the role of dispersoid particles, which are modified by shear processes which provide high energy spots for thermally stable precipitate nucleation. Removal of these dispersoid particles by an alloy modification had been shown to eliminate the embrittlement effect after depositing in a condition where embrittlement is expected for the unmodified 7075. Further work demonstrates the different relationships between processing conditions and the degree of embrittlement for three different tool types. Beyond the implications of the particular alloy studied, this work highlights the fundamental concepts involved when a manufacturing process operates at high strain rates and total strains which can be used for the design of alloys meant for AFSD. / Doctor of Philosophy / Additive Friction Stir Deposition (AFSD) is a new 3D printing process for metals where deformation is used to deposit material in an additive fashion. This work involves understanding and solving an embrittlement issue that occurs during heat treating after deposition for a particular aluminum alloy (7075). In this work, the origins of the embrittlement phenomena are uncovered which have to do with the degree and severity of deformation. Several solutions including alloy development and process control are successfully demonstrated.

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

Reinforced masonry (RM) shear walls are a common lateral load-resisting system for building structures. The seismic design guidelines for such systems are based on relatively limited experimental data. Given the restrictions imposed by the capabilities of available experimental equipment, analytical modeling is the only means to conduct systematic parametric studies for prototype RM wall systems and quantify the seismic safety offered by current design standards. A number of modeling approaches, with varying levels of complexity, have been used for the analysis of reinforced concrete (RC) and masonry wall structures. Among the various methods, the truss analogy is deemed attractive for its conceptual simplicity and excellent accuracy, as indicated by recent studies focusing on RC walls. This thesis uses an existing modeling method, based on nonlinear truss models, to simulate the behavior of fully grouted reinforced masonry shear walls. The modeling method, which was originally created and used for RC walls, is enhanced to capture the effect of localized sliding along the base of a wall, which may be the dominant mode of damage for several types of RM walls. The truss modeling approach is validated with the results of quasi-static cyclic tests on single-story isolated walls and dynamic tests on a multi-story, three-dimensional wall system. For the latter, the truss model is found to give similar results to those obtained using a much more refined, three-dimensional finite element model, while requiring a significantly smaller amount of time for the analysis. Finally, truss models are used for the nonlinear static analysis of prototype low-rise walls, which had been analyzed with nonlinear beam models during a previous research project. The comparison of the results obtained with the two modeling methods indicates that the previously employed beam models may significantly overestimate the ductility capacity of RM squat walls, due to their inability to accurately capture the shear-flexure interaction and the effect of shear damage on the strength of a wall. / 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.

Precast girders and cast-in-place decks are a typical type of concrete bridge construction. A key part of this type of construction is developing composite action between the girder and deck. In order to develop composite action, adequate horizontal shear resistance must be provided at the interface. As lightweight concrete is increasingly being used in bridge designs, it is important to understand the horizontal shear behavior of lightweight concrete. The current AASHTO LRFD Specification provides design equations for horizontal shear strength of both lightweight and normal weight concrete. Thirty-six push-off tests were performed to determine if the current code equations accurately predict the horizontal shear strength of precast girders and cast-in-place decks for both normal weight and lightweight concrete. The different test series investigated effects from lightweight and normal weight concrete used for the girder/slab combination and the quantity of shear reinforcement provided across the interface. The test results were compared to the results predicted by current design equations. A structural reliability analysis was performed and the test-to-predicted statistics were used to define LRFD resistance factors and quantify the probability of failure. The current design equations were found to be conservative and more conservative for lightweight concrete than for normal weight concrete. / Master of Science

No description available.

<p dir="ltr">Reinforced concrete elements are vulnerable to sliding against each other when shear forces are transmitted between them. Shear-friction is the mechanism by which shear is transferred between concrete surfaces. It develops by aggregate interlock between the concrete interfaces while reinforcement crossing the shear interface or normal force due to external loads contributes to the shear resistance. Current design provisions used in the United States (ACI 318-19, AASHTO LRFD (2020), and the PCI Design Handbook (2017)) include design expression for shear-friction capacity. However, the value of the reinforcement yield strength input into the expressions is limited to a maximum of 60 ksi. Furthermore, the concrete strength is not incorporated into the primary design expressions. These limits cause the potential contribution of high-strength reinforcement and high-strength concrete in shear-friction applications from being considered. Therefore, a research program was developed to investigate the possibility of improving current shear-friction design practice and addressing these current limits.</p><p dir="ltr">Specifically, an experimental program was conducted to evaluate the influence of high-strength reinforcement and high-strength concrete on shear-friction strength. In addition, a statistical analysis was performed using a comprehensive shear-frication database comprised of past tests available in the literature. The experimental program consisted of two phases. Phase I included 24 push-off specimens to study the influence of the yield strength of the interface reinforcement (Grade 60 and Grade 100) and the number and size of interface reinforcing bars (6-No.4 and 4-No. 5 bars) with three different interface conditions (rough, smooth, and shear-key). Phase II included 20 push-off specimens with rough interfaces to investigate the influence of the yield strength of the interface reinforcement (Grade 60 and Grade 100) and concrete strength (target strengths of 4000 psi and 8000 psi). The influence of these two variables was observed over a range of reinforcement ratios (ρ = 0.55%, 0.83%, 1.11%, and 1.38%).</p><p dir="ltr">The test results showed that the overall shear-friction strength was the greatest for rough interface specimens, followed by specimens detailed with shear keys. The smooth interface specimens had the lowest strengths. The results of both phases of the experimental program indicated that the use of high-strength reinforcement did not improve shear-friction capacity.</p><p dir="ltr">Furthermore, the results from the Phase II tests showed that increasing the concrete compressive strength led to increased shear-friction capacity. The test results from the experimental program were analyzed and compared with current design provisions, which demonstrated room for improvement of current design practice.</p><p dir="ltr">Following the experimental program, a comprehensive shear-friction database was analyzed, and multilinear regression was used to create a model to predict shear-friction strength. Factors were then applied to the model to provide acceptable design expressions for shear-friction strength (less than 5% unconservative estimates). The database was used to evaluate the factored model and current design provisions.</p><p dir="ltr">The research outcomes, especially the expressions for shear-friction strength that were developed and that include consideration of the concrete compression strength, along with the shear-friction tests demonstrating the lack of strength gain with the use of Grade 100 reinforcement, provide valuable information for the concrete community to help direct efforts toward improving current shear-friction design practice.</p>

Evaluations of stress history and the geostatic state of stress of soils are ascertained on the basis of field geophysical measurements that provide paired complementary types of shear waves. It is well-established that multiple types of shear waves occur in the ground due to their directional and polarization properties. The shear wave velocity (Vs) provides the magnitude of small strain stiffness (G0) which depends on effective stress, void ratio, stress history, and other factors (cementation, age, saturation). Herein, this study examines a hierarchy of shear wave modes with different directions of propagation and particle motion from in-situ geophysical tests (HH, VH, and HV) and laboratory bender element data. A special compiled database from well-documented worldwide sites is assembled where full profiles of stress state, stress history, and several paired modes of Vs profiles have been obtained from crosshole tests (CHT), downhole tests (DHT), and rotary crosshole (RCHT). Reference profiles of the lateral stress coefficient (K0) are available from direct in-situ measurements (self-boring pressuremeter, hydrofracture, and push-in spade cells). Stress history is documented in terms of yield stress ratio (YSR) from consolidation testing and careful engineering geology studies. A methodology is developed that relates both the YSR and K0 to stiffness ratios obtained from directional shear wave velocities. In further efforts, means to extract reliable shear wave profiles from continuous downhole testing via a new GT autosource and seismic piezocone testing are outlined and applied to results from three test sites in Windsor/VA, Norfolk/VA, and Richmond/BC. A driving impetus to this research involves the geologic conditions at the US Dept. of Energy's Savannah River Site (SRS) in South Carolina. Here, the overburden soils in the upper 60 m depths consist of very old Miocene and Eocene sediments, primarily layered deposits of sands, clayey sands, silty sands, and interbedded clays which exhibit an apparent and unusual stress history profile. Special geologic conditions include the dissolutioning of old calcareous sediments (Santee Formation) at depths of 40 to 50 m below grade, similar to karstic limestone deposits. As a consequence, caves, voids, and infilled soft soil zones occur within the soil matrix at these elevations, probably resulting in localized collapse of the overlying soil column. Based on conventional laboratory and in-situ test data conducted during geotechnical investigations at SRS, available interpretative relationships for assessing the soil stress history and geostatic stress states show scattered and inconsistent results. Complications abound in the systematic assessments of these geomaterials due to effects of very old ageing, cementation, desiccation, and diagenesis, as evidenced by unusual in-situ shear wave velocity profiles that decrease in magnitude with depth, as measured by CHT and DHT. Based on the findings of this study, it is recommended that a new set of shear wave velocity measurements be made at SRS to obtain HH waves (and complementary VH waves) needed for an independent assessment of YSR in the upper soil column.

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.