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The present paper addresses the influence of shape, size, and frictional characteristics of granular materials by utilizing X-ray computed tomography (CT) imaging and FEM. High-fidelity tri...

Because of the recent advancements in hardware and reconstruction algorithms, multiphase flow modeling in porous media is experiencing a shift toward using advanced imaging techniques such ...

Foamed warm-mix asphalt (WMA) has been widely accepted and used in the United States and many other countries around the world. However, several key concerns about WMA technology still need to be answered, including the major issue of moisture-induced damage. Because of the reduced production temperatures and the foaming process with water, moisture may be entrapped in pavements after compaction. The trapped moisture decreases the adhesion between asphalt binder and aggregates and the cohesion among asphalt binder, resulting in stripping and other forms of pavement distress. The neutron scattering technique provides a unique tool for the determination of the microscopic structure of asphalt and for the detection of the presence of moisture and its spatial distributions in asphalt. In particular, small-angle neutron scattering (SANS) in the wave vector transfer range from 0.003−0.5 A−1 is suitable to probe the spatial density fluctuations in the real space from 200−1 nm, which has a resolution se...

High spatial resolution (∼13.7 mm/pixel) neutron tomography was performed on partially water-saturated compacted silica sand specimens with two different grain morphologies (round and angular) at Helmholtz Zentrum Berlin using cold neutrons at the cold neutron radiography and tomography beam line. A specimen mixed with heavy water was imaged for contrast comparison purposes. Microfocus X-ray imaging was also performed on these specimens with slightly higher resolution (∼11.2 mm/pixel) using geometric magnification to locate the solid phase (silica particle boundaries) more precisely. Image processing was performed to remove unwanted gammas detected because of the gadox scintillator used for the high-resolution neutron imaging system. The visualization of solid, gas, and liquid phases for different grain morphologies is presented at the grain level. Using dual-modal contrast possible from simultaneous use of neutrons and X-rays, the authors introduce, for the first time, an improved ability to dist...

Because of the short construction season and cold winters in Alaska, the prestressed concrete decked bulb-tee bridge system is very popular. However, the concrete deck is an integral part of the bridge superstructure and cannot be easily replaced when it deteriorates. Obviously, there is merit in combining durable “premanufactured” fiber-reinforced polymer (FRP) composite deck with stiffer prestressed concrete girders in cold regions. However, the effects of long-term exposure to extreme temperature variations and various moisture conditions typical of cold regions on the performance of FRP composite materials are not fully understood. This paper summarizes the combined effect of low-temperature and deformation strain levels on the longitudinal modulus of glass fiber-reinforced polymer (GFRP) samples. The modulus of elasticity of GFRP laminate coupons was tested at various temperatures down to −31 °F ( −35°C ) by temporarily subjecting the samples to three strain levels of 1,000, 2,000, or 3,000 microstra...

Various aspects of the mechanical behavior of kaolin clay are discussed in light of experimental observations from a series of strain controlled true triaxial undrained tests performed on cubical kaolin clay specimens with flocculated and dispersed microfabric, using a fully automated flexible boundary experimental setup with real-time feedback control system. The laboratory procedures used to prepare flocculated and dispersed microfabric specimens are presented. Mercury intrusion porosimetry is used to evaluate the pore structure of these specimens. The influence of microfabric on the consolidation behavior of kaolin clay is evaluated based on the data obtained from K0 consolidation during constant rate of strain tests and the isotropic consolidation during true triaxial tests. Undrained tests on kaolin clay show that the following vary with microfabric of specimen: The shear stiffness, excess pore pressure generated during shear, and strength and strain to failure. For both microfabrics, the observed st...

The influence of geometric arrangement of platelets (microfabric) on the mechanical behavior of kaolin clay is investigated using lubricated end triaxial testing on solid cylindrical specimens. A series of compression and extension tests under drained and undrained conditions were performed on clay specimens with different microfabric for overconsolidated ratio values of 1 and 10. The solid cylindrical specimens with dispersed and flocculated microfabric were produced in the laboratory using slurry consolidation technique under K0 condition. Based on the experimental observations, it is evident that microfabric strongly affects the mechanical behavior of kaolin clay, such as its stress–strain relationship, effective stress ratio, shear strength, excess pore-pressure evolution, and volumetric response. The influence of confining pressure on clay specimens with dispersed and flocculated microfabric is also studied in this research. This study shows that the microfabric can change the basic nature of clay. F...

A series of combined axial-torsional tests were performed to study the 3D mechanical behavior of kaolin clay in an undrained condition. Using the digital image analysis technique, discrete local deformation of the surface of a hollow cylindrical specimen under loading was recorded. A linear interpolation method was used to generate a continuous deformation and strain field of the specimen based on the recorded discrete local deformations. Evolution of shear band was vividly visualized and recorded during the loading process for various inclinations of major principal stress. The theory of strain localization on continuous bifurcation was briefly reviewed and applied to the Mohr-Coulomb model, and a single hardening model incorporating the concept of loading-history-dependent plastic potential was developed by the writers. The largest critical plastic modulus and orientation of the shear bands were predicted by using the theoretical solution. Significant disagreement was observed between the experimental results and theoretical predictions related to the initial occurrence of strain localization and the inclination of fully developed shear bands.

The effect of principal stress rotation on the mechanical behavior of Kaolin clay is investigated using combined axial-torsional tests on hollow cylindrical specimens. The yielding behavior and failure criteria are found to be strongly dependent on the principal stress rotation angle (β) and plastic work. A unique plastic potential function determined solely by the current stress state is not sufficient to model the plastic flow observed in these experiments. Therefore, a single hardening elasto-plastic model that includes a loading-history-dependent plastic potential function is proposed for normally consolidated Kaolin clay subjected to principal stress rotation. A general methodology for incorporating history dependency in modeling complex elasto-plastic behavior of cohesive soils is presented along with comparisons of model predictions with experimental data.

A combined axial–torsional testing system was developed to investigate the effect of rotation of principal stresses on the three-dimensional mechanical behavior of Kaolin clay. Uniform and reproducible cohesive specimens having a specimen shape of a hollow cylinder were obtained using a two-stage slurry consolidation technique. Precise stress paths (triaxial compression to pure torsional shear to triaxial extension), corresponding to a fixed rotation of the major principal stress axis, were achieved by using the proportional-integral-derivative (PID) feedback control technique. Kaolin clay specimens were tested under a variety of stress paths associated with a constant principal stress rotation angle (β) under undrained conditions. Typical test results, such as effective friction angle, undrained shear strength, stress–strain relationship, pore pressure evolution, and stress paths are presented as a function of β . During shearing, the procedure to use advanced servo-hydraulic control (using PID algorithm...

The influence of intermediate principal stress on the mechanical behavior of overconsolidated kaolin clay is investigated using three-dimensional true triaxial testing on cubical specimens. A flexible boundary, true triaxial setup with a real-time feedback control system was used to test soil specimens under stress and strain-control modes. Undrained tests on kaolin clay show that the following vary with intermediate principal stress: the stiffness at small strains, excess pore pressure generated during shear, and strength and strain to failure. Failure occurred at peak deviator stress followed by shear band formations and localized bulging. Prior theoretical formulations of bifurcation and undrained instability support these experimental observations. Analysis of data in the octahedral plane indicates that kaolin clay follows a nonassociative flow rule, which is described by a constant third stress invariant failure criterion with von Mises plastic potential surface.

The shape of aggregates has an important influence on the behavior of civil engineering materials. Digital imaging techniques provide unique opportunities for describing these features in an automated fashion. Shape analysis of fine and coarse aggregate particles is investigated in this study using Fourier transform of digital images. The geometric signature of each shape is extracted by measuring the distance between its centroid and the boundary at constant increment of angles, and Fourier transforms are used to evaluate its spectral information. The number of highest amplitude harmonics required for accurate profile regeneration is evaluated. Shape of a given aggregate particle is reconstructed using inverse Fourier transforms considering a limited number of significant harmonics. A parameter that can quantify the error between regenerated and original profiles is proposed. Using this value, two shape parameters are defined to describe the overall shape and the ruggedness of a particle. A procedure of quantitatively describing the roundness/angularity of aggregate shape is presented and extended to three dimensions using orthogonal views.

A series of one-eighth-scale strain controlled model pressuremeter tests were performed using an automated flexible wall calibration chamber testing system. Undrained model pressuremeter tests were performed with and without the internal pore-water pressure measuring piezometer needles at radial strain rates ranging from 1 to 8%/min in a controlled laboratory setup in beds of kaolin clay prepared with known stress histories. Reference strength and deformation properties were determined using one-dimensional consolidation, isotropically consolidated undrained compression, and K(sub 0) consolidated undrained compression laboratory tests; and comparisons were made with the interpreted values from the pressuremeter test data. Interpretations of the undrained shear strength, initial shear modulus, shear stress-radial strain variation, limit pressure, and horizontal stress for each net pressuremeter curve were evaluated via various interpretation procedures, using the Marquardt-Levenberg algorithm. Based on the laboratory test data and the analytical interpretations, several conclusions were made regarding the effects of the rate of probe expansion on the deduced shear modulus, undrained shear strength, and limit pressure in the pressuremeter testing of clays.

An attempt has been made to implement artificial neural networks (ANNs) for modeling the stress-strain relationship of sands with varying grain size distribution and stress history. A series of undrained triaxial compression tests for eight different sands was performed under controlled conditions to develop the database and was used for neural network training and testing. The investigation confirmed that a sequential ANN with feedback is more effective than a conventional ANN without feedback, to simulate the soil stress­ strain relationship. The study shows that there is potential to develop a general ANN model that accounts for particle size distribution and stress history effects. The work presented in this paper also demonstrates the ability of neural networks to simulate unload-reload loops of the soil stress-strain characteristics. It is concluded from this study that artificial-neural-network-based soil models can be developed by proper training and learning algorithms based on a comprehensive data set, and that useful inferences can be made from such models.