Browsing by Subject "Shear modulus"
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Item Constitutive modeling of viscoelastic behavior of bituminous materials(2012-12) Motamed, Arash; Bhasin, AmitAsphalt mixtures are complex composites that comprise aggregate, asphalt binder, and air. Several research studies have shown that the mechanical behavior of the asphalt mixture is strongly influenced by the matrix, i.e. the asphalt binder. Therefore, accurate constitutive models for the asphalt binders are critical to ensure accurate performance predictions at a material and structural level. However, researchers who use computational methods to model the micromechanics of asphalt mixtures typically assume that (i) asphalt binders behave linearly in shear, and (ii) either bulk modulus or Poisson’s ratio of asphalt binders is not time dependent. This research develops an approach to measure and model the shear and bulk behavior of asphalt binders at intermediate temperatures. First, this research presents the findings from a systematic investigation into the nature of the linear and nonlinear response of asphalt binders subjected to shear using a Dynamic Shear Rheometer (DSR). The DSR test results showed that under certain conditions a compressive normal force was generated in an axially constrained specimen subjected to cyclic torque histories. This normal force could not be solely attributed to the Poynting effect and was also related to the tendency of the asphalt binder to dilate when subjected to shear loads. The generated normal force changed the state of stress and interacted with the shear behavior of asphalt binder. This effect was considered to be an “interaction nonlinearity” or “three dimensional effect”. A constitutive model was identified to accommodate this effect. The model was successfully validated for several different loading histories. Finally, this study investigated the time-dependence of the bulk modulus of asphalt binders. To this end, poker-chip geometries with high aspect ratios were used. The boundary value problem for the poker-chip geometry under step displacement loading was solved to determine the bulk modulus and Poisson’s ratio of asphalt binders as a function of time. The findings from this research not only improve the understanding of asphaltic materials behavior, but also provide tools required to accurately predict pavement performance.Item Development of constitutive models for linear and nonlinear shear modulus and material damping relationships of uncemented soils(2018-06-22) Wang, Yaning, Ph. D.; Stokoe, Kenneth H.; Cox, Brady R.; Rathje, Ellen M.; El Mohtar, Chadi S.; Gilbert, Robert B.; Wilson, Clark R.For many decades, hundreds of soil samples recovered from North America and other parts of the world have been tested in the Soil and Rock Dynamics (SRD) Laboratory at The University of Texas at Austin using combined resonant column and torsional shear (RCTS) equipment. Dynamic soil property relationships determined in this study include: (1) linear relationships of log G[subscript max] - log σ₀ and log D[subscript min] - log σ₀, and (2) nonlinear relationships of G/G[subscript max] - log γ and D - log γ. These relationships are important in geotechnical design, especially for geotechnical earthquake engineering and soil dynamics. More recently, higher resolution measurements have been made over shear strains ranging from about 1×10⁻⁶ to 0.3 %. Most recently, measurements of the initiation of pore-water pressure generation in nearly to saturated poorly-graded sands and low-plasticity silts have been conducted during torsional shear (TS) testing. A significant database has been developed involving all types of uncemented soil specimens tested in the SRD Laboratory since the late 1980's. The effects of soil type, index properties, density, confining state and strain level on the dynamic properties have been quantified through multivariable regression analyses performed on the database. Four sets of empirical models for log G[subscript max] - log σ₀, G/G[subscript max] - log γ, log D[subscript min] - log σ₀ and D - log γ relationships are presented in this study. These models are composed of simple equations that incorporate the key parameters controlling linear soil behavior as well as nonlinear soil behavior. The process used to develop the parameters in the models employed a residual analysis procedure in a staged manner. Both the mean values of the empirical models and the uncertainty associated with these values are presented. Those empirical models show significant improvements in accuracy and applicable range compared to previous models. The application ranges of these models and sparse portions of the associated database are also discussed for the sake of appropriate utilization and future updates. The influence of number of loading cycles, confining pressure and soil type (sand or low-plasticity silt) on the initiation of pore-water pressure generation during TS testing are briefly discussed. A preliminary model for the r[subscript u] - log γ relationship is presented. This model focuses on determination of the threshold strain at which pore pressure generation is triggered, γ[subscript t][superscript pp], and the early stages leading towards soil liquefaction.Item Dynamic properties of fine liquefiable sand and calcareous sand from resonant column testing(2015-05) Wang, Yaning, Ph. D.; Stokoe, Kenneth H.; Cox, Brady R.The study of the dynamic properties of two specific kinds of granular soils is performed using torsional resonant column testing. The sandy soils are: (1) liquefiable sand from Christchurch, New Zealand, and (2) calcareous sand from Puerto Rico. The effects of isotropic effective confining pressure, shear strain amplitude, void ratio, and total unit weight on the small-strain and nonlinear dynamic properties of both types of sand are presented and discussed. Empirical models from previous studies are examined to determine how well the models fit the test results.Item Dynamic properties of soils with non-plastic fines(2012-05) Umberg, David, 1987-; Stokoe, Kenneth H.; Rathje, Ellen M.The results from an experimental study on the dynamic properties of sand with nonplastic silt are presented. Combined resonant column and torsional shear equipment is used to evaluate the effects of confining pressure, shearing strain, frequency, and number of cycles of loading on the dynamic properties of silty sand. The goal of this study is to determine if relationships in the literature for sands and gravels are accurate for predicting the shear modulus and material damping characteristics of soil with nonplastic fines or if the incorporation of a fines content parameter improves predictions. This goal was primarily accomplished by reconstituting and testing samples of an alluvial deposit from Dillon Dam, Dillon, Colorado according to predetermined gradation curves with variable amounts of non-plastic fines. Among the findings of this investigation are: (1) soil parameters such as Cu and D50 can be related to dynamic properties of soils with up to 25% fines, (2) the effects of non-plastic fines on the small-strain dynamic properties of soils are not very pronounced for soils with less than 25% fines, and (3) an increase in the amount of non-plastic fines in uniform soils or soils with more than 25% fines generally results in lower values of small-strain shear modulus, higher values of small-strain material damping, and more linear G/Gmax - log([gamma]) and D - log([gamma]) curves. The effect of non-contacting, larger granular particles in a finer soil matrix is also investigated along with the impact of removing larger particles from laboratory samples.Item Field measurements of the linear and nonlinear shear moduli of cemented alluvium using dynamically loaded surface footings(2010-05) Park, Kwangsoo; Stokoe, Kenneth H.; Bay, James A.; Kallivokas, Loukas F.; Rathje, Ellen M.; Wilson, Clark R.In this dissertation, a research effort aimed at development and implementation of a direct field test method to evaluate the linear and nonlinear shear modulus of soil is presented. The field method utilizes a surface footing that is dynamically loaded horizontally. The test procedure involves applying static and dynamic loads to the surface footing and measuring the soil response beneath the loaded area using embedded geophones. A wide range in dynamic loads under a constant static load permits measurements of linear and nonlinear shear wave propagation from which shear moduli and associated shearing strains are evaluated. Shear wave velocities in the linear and nonlinear strain ranges are calculated from time delays in waveforms monitored by geophone pairs. Shear moduli are then obtained using the shear wave velocities and the mass density of a soil. Shear strains are determined using particle displacements calculated from particle velocities measured at the geophones by assuming a linear variation between geophone pairs. The field test method was validated by conducting an initial field experiment at sandy site in Austin, Texas. Then, field experiments were performed on cemented alluvium, a complex, hard-to-sample material. Three separate locations at Yucca Mountain, Nevada were tested. The tests successfully measured: (1) the effect of confining pressure on shear and compression moduli in the linear strain range and (2) the effect of strain on shear moduli at various states of stress in the field. The field measurements were first compared with empirical relationships for uncemented gravel. This comparison showed that the alluvium was clearly cemented. The field measurements were then compared to other independent measurements including laboratory resonant column tests and field seismic tests using the spectral-analysis-of-surface-waves method. The results from the field tests were generally in good agreement with the other independent test results, indicating that the proposed method has the ability to directly evaluate complex material like cemented alluvium in the field.Item Material study of the steel reinforced elastomeric bridge bearings(2015-12) Sun, Cong, Ph. D.; Helwig, Todd Aaron, 1965-; Engelhardt, Michael D.; Williamson, Eric B; Tassoulas, John L; Mear, Mark ESteel laminated elastomeric bearings are widely used in concrete bridges due to their low cost and long history of good structural performance. However, elastomeric bearings have not been used extensively in steel bridge systems. Compared to concrete bridges, steel bridge systems generally have longer spans and may have significant support skew and horizontally curved geometry that results in significant demands on the bearings at the supports to accommodate rotations and complex bridge movements from both thermal loads and daily truck traffic. For such bridges, more costly pot bearings are normally used. The research described in this dissertation was part of a larger study investigating the possibility of using elastomeric bearings in such higher demand applications. More specifically, the research in this dissertation investigated issues related to material properties of the elastomer in larger bearings designed for higher demand applications. This dissertation first introduces a new testing methodology, referred to as the Dual Shear Test (DST), which is able to measure the elastomer material response in shear for samples cut directly from of bearings with different dimensions. The proposed geometry of the DST specimen significantly reduces the cost and effort compared to the more conventional Quad Shear Test, and also allows the measurement of shear response at very large shear strain levels. Based on a systematic experimental study, the accuracy and reliability of this new testing methodology was demonstrated. Different hyper-elastic material models were investigated in this dissertation that can be used in finite element studies of elastomeric bearings. These models were calibrated based on the new shear test methodology. With these material models, DST results can be interpreted and entered into finite element models. Using the Dual Shear Test, four bearings of different dimensions were tested. The variability of the shear modulus at different locations within the bearings was investigated. These tests were conducted to address concerns that larger bearings may have greater variability in elastomer material properties throughout the bearing. These tests showed there is somewhat greater variability in shear modulus in larger bearings and thicker bearings, although this variability was not significantly larger compared to smaller bearings. Finally, this research also investigated how the shear modulus of the elastomer changes as the temperature decreases. Results of tests showed that the shear modulus increases significantly as temperature decreases. This effect can be significant when analyzing the behavior of bridge bearings under temperature variations.