J. D. Frost

Three-Dimensional Image Analysis of Aggregate Particles from Orthogonal Projections

Digital image analysis provides the capability for rapid measurement of particle characteristics. When an image is captured and digitized, numerous measurements can be made in near real time for each particle. Usually, image analysis techniques treat particles as two-dimensional objects since only the two-dimensional projection of the particles is captured. In this study, three-dimensional analysis of aggregate particles that was performed by attaching aggregates in sample trays with two perpendicular faces is described. After the initial projected image of the aggregates is captured and measured, the sample trays are rotated 90 degrees so that the aggregates are now perpendicular to their original orientation and the dimensions of the aggregates in the new projected image are captured and measured. The long, intermediate, and short particle dimensions (dL, dI, and dS, respectively) provide direct measures of the flatness and elongation of the particles. Some other shape indexes can also be derived from t...

Quantification of damage parameters using X-ray tomography images

Damage in materials can be represented in many forms such as specific void or crack surfaces, the spacing between cracks, scalar representation of damage and general tensorial representation of damage. This paper presents methods to quantify the specific damaged surface area, the specific damaged surface area tensor, the damage tensor, the mean solid path among the damaged surfaces and the mean solid path tensor. The methods are general and use the reconstructed three-dimensional structure from tomography images and a virtual sectioning technique to obtain cross-sectional images needed for the quantification. The paper also presents the relation between the quantified damage parameters and their applications in mechanical modeling. This work is applied in particular to quantify the damage parameters of asphalt concrete specimens of the WesTrack project.

Microstructure Study of WesTrack Mixes from X-Ray Tomography Images

Asphalt concrete is a composite comprised of aggregates, air voids (AVs), and microcracks of various sizes and shapes. The property of asphalt mixes is not only affected by volume fractions of these components such as voids in mineral aggregates (VMA, the complementary volume fraction of aggregates) and AV content but also the size and spatial distributions of these components. However, conventional and Superpave volumetric mix designs specify only bulk values of VMA and AV, which may not be able to capture all the critical factors of a mix. The unexpected WesTrack mixes’ premature failure may reflect this limitation in design specifications. Results are presented of a study to evaluate statistically the spatial and size distribution of the void systems of the mixes and to find other possible parameters that might be better or complementary to the current parameters to characterize the mixture internal structure and performance. Void systems of three original WesTrack mixes were evaluated using X-ray tomography images and stereology methods. The spatial void content variation and void size distribution for six specimens of three mixes were quantified. Specific damaged surfaces (void surfaces were considered damaged surfaces because they do not transmit forces), damage tensor, mean solid path among damaged surfaces, and mean solid path tensor, which indicate damage level and interaction of the damage, were also quantified. It was concluded that coarsegraded mix is more severely inherently damaged and that this set of parameters gave a performance ranking of the three mixes consistent with field observations. Moreover, quantified tensors are applicable in continuum damage mechanics for fatigue and rutting modeling.

A Critical Assessment of the Moist Tamping Technique

Current field sampling techniques are not readily able to produce high-quality undisturbed granular soil specimens for laboratory testing at an affordable cost. Accordingly, numerous specimen reconstitution methods have been developed for use in the laboratory. Among these methods, moist tamping has the advantage that it is relatively easy to control the global specimen density achieved, even for loose specimens, although unfortunately the method has also been qualitatively shown to yield less uniform specimens. This paper describes the findings of a study that critically assessed the moist tamping method by measuring the forces applied during the tamping process and investigated the uniformity of specimens prepared with the method by quantitatively analyzing both X-ray and optical images.

Shear failure behavior of granular–continuum interfaces

Abstract Particulate–continuum interfaces (e.g. soil–concrete, soil–steel, soil–geomembranes) determine the behavior of many geotechnical structures including deep foundations, synthetic impervious liners, trenchless technologies, and an assortment of earth retaining structures. Therefore, proper understanding of the localized shearing mechanisms that govern their behavior is essential if geotechnical design is to improve. This paper presents a summary of recent research on the coupled effect of surface roughness and hardness on interface shear behavior and strength. This was accomplished through performing a series of laboratory tests on a selection of sand–continuum material interfaces and through discrete element modeling of particulate–continuum interfaces.

Automated Determination of the Distribution of Local Void Ratio from Digital Images

The frequency distribution of local void ratio is believed to be an important parameter, in addition to the void ratio, for describing the mechanical behavior of granular materials. Oda proposed a method to determine experimentally the distribution of local void ratio from 2-D plane sections. To date, implementations of Odas method have depended to varying extents on operator judgment to form polygons by joining the centers of gravity of all particles that surround a void. Furthermore, the studies have involved a significant amount of manual work in making the required measurements. This paper describes a fully automated implementation of the method, which uses high-level, image-processing techniques. The proposed method eliminates operator judgment and manual work and makes the determination of the distribution of local void ratio from 2-D plane sections both repeatable and efficient. The method is illustrated with measurements performed on synthetic and real images. The importance of correcting the images to account for factors such as thickness of segmentation lines is demonstrated. Measurements that confirm the stability of the proposed polygon network generation procedure are also presented.

Preparation of Epoxy Impregnated Sand Coupons for Image Analysis

The study of sand structure using image analysis is an emerging area in soil mechanics research. Quantification of parameters such as the local void ratio distribution or the orientation of sand known to directly affect the behavior of sand masses is possible. However, accurate quantitative measurement requires high-quality specimen coupon surfaces for imaging. A comprehensive procedure for preserving sand specimens with epoxy resin and preparing specimen coupon surfaces for image analysis using a variety of grinding and polishing procedures is presented. Factors affecting the accuracy of subsequent image analysis measurements are discussed. Example images obtained using the proposed procedures are presented to demonstrate their effectiveness and potential.

NONINVASIVE MEASUREMENT OF PERMANENT STRAIN FIELD RESULTING FROM RUTTING IN ASPHALT CONCRETE

Asphalt concrete is a bonded granular material composed of aggregates and asphalt binder. The chemical, physical, and mechanical properties of these constituents are different. Under typical repeated tire loads, the aggregates do not deform but instead rigidly translate and rotate within the binder. Rutting is the surface manifestation of the internal-structure evolution in asphalt concrete under repeated wheel loading. A procedure to quantify the permanent strain field of asphalt concrete underlying the rutted zone produced by an accelerated wheel testing device is described. This new, noninvasive procedure has been automatically implemented in a computer program named MATCH, which was developed to compare the locations of particles before and after testing and therefore to permit the displacement of individual particles to be tracked. Pattern recognition based on each particle’s cross-sectional area, perimeter, and aspect ratio was used to match the particles. Application of the new procedure to measure the permanent strain field of an asphalt concrete specimen in a Georgia loaded wheel tester is described. The reliability of the procedure for automatically matching the particles in successive images was confirmed by manual procedures. It was found that the permanent strain and the mastic/solid area ratio evolution measured in this application demonstrated strong localism. Dilatancy and contraction were quite significant locally but insignificant globally. The strains in the mastic were much larger than the global or macrostrain. The proposed procedure has significance in the future study of micro-macro properties of bonded granular materials and how factors such as aggregate size, shape, and distribution affect the global response.

Load–Deflection response of transversely isotropic piles under lateral loads

In general, pile materials are assumed to be isotropic during the analysis of the load–deflection response of piles under lateral loads. However, commonly used materials such as reinforced concrete and timber as well as potentially promising new pile materials such as fiber reinforced polymers are typically transversely isotropic materials. Experimental studies have shown that transversely isotropic materials have a high ratio of section longitudinal modulus to the section in-plane shear modulus (Ezz/Gxz) compared to the value for isotropic materials. The high modulus ratio leads to a more significant shear deformation effect in beam bending. To account for the shear deformation effect, the Timoshenko Beam Theory has been adopted in deriving the solutions for the load–deflection response of transversely isotropic piles under lateral loads instead of the Classical (Euler–Bernoulli) Beam Theory. The load–deflection responses depend on the shear effect coefficient, the lateral soil resistance, the embedment ratio, and the boundary conditions. The deflection of the pile, if the shear deformation effect is considered, is always larger than if it is neglected. Copyright

Measurement of Relative Surface Roughness at Particulate-Continuum Interfaces

The shearing behavior at particulate-continuum interfaces is influenced strongly by the continuum surface roughness. To date this surface has been characterized with absolute roughness parameters that represent various aspects of the spatial distribution of surface features. However, for particulate-continuum interfaces, the effect of continuum surface roughness on interface behavior is itself dependent on the particulate material characteristics. This interaction suggests that the surface roughness be characterized in relation to the contacting particulate material. Filtering algorithms such as low-pass and high-pass filters are frequently used to isolate components of a surface profile such as waviness and roughness. However, these filters consider peaks and valleys of a surface profile equally and hence implicitly assume each feature has the same effect on the interface behavior. An extensive analysis that traces the centroids of different diameter particles as they traverse across surfaces is presented herein. The resulting observations include: (1) peaks and valleys on a surface affect particulate-continuum interactions differently; (2) conventional surface roughness parameters do not account adequately for the characteristics of the contacting particulate material; and (3) conventional filters incorrectly isolate surface features of interest in particulate continuum interfaces. A framework that allows for these issues to be considered in evaluating the relative roughness of surfaces is presented.