Date of Award
Master of Science
Civil, Construction, and Environmental Engineering
David J. White
The application of 3-dimensional volumetric imaging using non destructive evaluation procedures opens doors in researching geomaterials, allowing study of internal characteristics. X-ray computed tomography (CT) is one such procedure which is especially well suited to geomaterials due to the high density contrasts often found between solid materials and their surrounding environments and/or pore spaces. Although X-ray CT has been applied to geomaterial research in the past, there is a lack of tested X-ray CT methodologies which can be readily applied to many important research tasks. Methods developed and applied herein concern the analyses of soil particle movement during application of stress and pore space characterization in terms of size, shape and pore continuity.;The general objectives undertaken in this research were to: (1) Develop a methodology which can be used to quantify soil particle directional movements and rotations during incremental loading. (2) Apply the above methodology to cylindrically shaped triaxial samples of poorly graded aggregate under incremental strain. (3) Develop a methodology whereby high quality cross sectional maps of density variation can be produced and void spaces correlated to hydraulic conductivity in terms of pore size, shape and continuity. (4) Apply the above pore space characterization methodology to cylindrical samples of Portland cement pervious concrete (PCPC) mix designs.;In order to obtain the goals, specialized methods to conduct X-ray CT scans and new post processing techniques were developed for both digital image processing and data reduction.;It was found that PCPC can be successfully imaged using X-ray CT and that the measurements can be filtered to remove artifacts associated with X-ray CT scanning. Blurring at the edges of scanned images can be mitigated by shielding the sample during the scanning process with a ceramic or cement material around its perimeter. Resulting X-ray CT cross sections can be divided into pore space and solid space regions by correlating a cutoff image voxel intensity value to known void ratio distributions within sample volumes. Void space cross sections produced can be post processed to remove grainy edges and further remove ring artifacts.;A specialized methodology for computing three dimensional particle movements was conceived and consisted of fixing three identifiable segments of lead solder to the particle. X-ray scanning conducted at incremental strain intervals allowed tracking of these markers. The alignment of markers to one another created vectors which could be tracked for quantifying particle movement including rotations. The movements of particles were computed and displayed as a function of position according to height and radial distance within the radially symmetric samples.;Results of these methods show that in the case of PCPC samples, larger pore size, pore shape (perimeter/area), void ratio, and pore continuity are traits of samples which demonstrate higher permeability. Also, regions of significantly higher permeability were found at the edges of samples which were determined to be due to boundary effects resulting from sample preparation within a cylindrical mold.;Key findings from tracking aggregate particle movements within triaxial samples includes the following: (1) Particle movements are generally concentrated in bulging regions within samples. (2) Average aggregate grain motion becomes increasingly horizontal in direction until a point just before peak deviator stress is achieved after which the averaged inclination of radial to vertical displacements becomes predictable. (3) Higher confining pressures result in less horizontal particle movement but greater rotations.;These research results bring greater understanding to the relation of pore characteristic to permeability and particle movements under triaxial stress. More importantly, though, are the versatile methods developed and tested in this research which may be useful to a variety of applications in analyzing geomaterials. Implementation of these methods has the capability of further quantifying, comparing, and understanding systems relevant to civil engineering to an extent only limited by the imagination and technical capability of researchers.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Halverson, Clinton, "Characterization of geomaterials with X-ray computed tomography (X-ray CT)" (2008). Retrospective Theses and Dissertations. 15309.