Browsing by Author "Dr. Tasnim Hassan, Committee Member"

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Effect of Percentage Baghouse Fines on the Amount of Antistripping Agent Required to Control Moisture Sensitivity
(2002-11-19) Fischer, William Kevin; Dr. Tasnim Hassan, Committee Member; Dr. Ahktarhusein A. Tayebali, Committee Chair; Dr. James Nau, Committee Member
Moisture damage in asphalt pavement reduces the service life of the pavement as well as increase permanent deformation. Moisture sensitivity in asphalt concrete mixtures is often associated with high concentrations of fine aggregate particles. In this study, the effects of baghouse fines, a source of fine mineral aggregate, on moisture sensitivity were examined. Two types of baghouse fines with different gradations were used in various concentrations in the laboratory production of hot-mix-asphalt samples. To determine the effects of the various baghouse fines contents, testing was performed to determine the tensile-strength-ratio of the different mixes. In order to prevent moisture damage in asphalt pavements, additives are often used to alter the interaction between the asphalt binder and the mineral aggregate. These additives can change the molecular charge of the binder or reduce the viscosity of the asphalt cement. In order to determine the effectiveness of the anti-strip additive in preventing moisture damage, the tensile-strength-ratio was also determined for specimens containing various additive and baghouse fines contents. The results of the tests showed a reduction in retained strength for the specimens without additive as compared to the specimens containing additive, demonstrating the effectiveness of the additive in preventing moisture damage. To assess the rutting resistance of the various asphalt mixtures, an Asphalt Pavement Analyzer test was performed. Half of the laboratory compacted specimens were moisture conditioned and tested submerged, while the other half was tested dry. Results indicate an increase in rut depth with the removal of anti-strip additive from the mix, indicating the effectiveness of the additive in preventing moisture damage. Finally the specimens were tested in the Superpave Shear Test Machine. Frequency Sweep and Repeated Shear tests were performed for each mixture with half of the samples again conditioned. The Frequency Sweep test measures the shear modulus and phase angle over a number of frequencies. The results of this test showed that the average shear modulus declined with moisture conditioning for each mix. The Repeated Shear test subjects the specimen to repeated loading and measures the accumulated plastic strain over a number of cycles, which can be used to determine rutting resistance. The results of this test corresponded with the Asphalt Pavement Analyzer results, with the rutting resistance decreasing with the removal of anti-strip additive. Based on these results, it was concluded that a large concentration of baghouse fines can increase moisture sensitivity in asphalt pavement. It was also determined that the LOF 6500 anti-strip additive, in the 0.5 percent concentration, was sufficient to prevent moisture damage in mixtures with high concentrations of baghouse fines. Finally, the results showed an increase in stiffness related to increased baghouse fine content.
The Impact of Load History on Deformation Limit States for the Displacement-Based Seismic Design of RC Moment Frame Buildings
(2008-10-22) Vidot-Vega, Aidcer Linalynn; Dr. Jay Tu, Committee Member; Dr. Mervyn Kowalsky, Committee Chair; Dr. Tasnim Hassan, Committee Member; Dr. James Nau, Committee Member
The main goals of this research were (1) to study the relationships between material strain and deformation parameters such as curvature and drift for reinforced concrete moment frame structures and (2) to identify the load history effects on these relationships. Through the use of moment-curvature analyses, trends between strain and curvature for rectangular reinforced concrete sections were explored. Curvature-strain relationships that depend on axial load ratio and longitudinal steel ratio were developed from the moment-curvature analyses results. The curvature expressions were subsequently used to develop equations to compute interstorey drift based on strain limits for RC moment frames. The resultant equations can be used in performance-based design approaches such as direct displacement-based seismic design to compute target drifts and system displacements for prescribed limit states based on material strains. The interstorey drift equations were correlated against 54 frame building analyses using OpenSees with reasonable accuracy. To study the load history effects in the relationship between material strains and drift, the research was divided in two phases. The first phase considered RC column members subjected to symmetrical and asymmetrical cyclic reverse loading and seismic acceleration time histories. Column members with 2% and 4% longitudinal steel ratio and axial load ratio of 5% to 20% were analyzed. The second phase considered moment frame buildings subjected to seismic acceleration time histories. A total of five different moment frame models were analyzed. All the results were compared to monotonic (pushover) analyses. It was concluded that monotonic section analyses can be used to describe the envelope of seismic response since the relationship between strains and displacements/drift is minimally affected by different loading histories. It was also determined that a simple addition of the growth steel strain to the flexural tension strain is not correct, as previously thought.
Photonic Bandgap Fibers For Transverse Strain Sensing
(2009-02-22) Van Vickle, Patrick Stephen; Dr. Kara Peters, Committee Chair; Dr. Tasnim Hassan, Committee Member; Dr. Larry Silverberg, Committee Member; Dr. Jeffrey Eischen, Committee Member
This research examines the change in bandgap characteristics of Photonic Bandgap (PBG) fibers under transverse loading for applications such as fabrication and service life monitoring of composite structures. Photonic Bandgap (PBG) fibers rely on Bragg reflection conditions in the plane of optical fiber crosssection and therefore offer great potential as transverse strain sensors which are insensitive to axial loading and temperature variations. A numerical study of the effect on the bandgap in PBG fibers under transverse loads is thus performed in this dissertation. First the fundamental equations for lightwave propagation in classical stepindex fibers, microstrucured holeyfibers and PBG fibers are reviewed. The behavior of each for sensing purposes is also discussed. The structural deformation and electromagnetics modeling of a PBG fiber is then performed using the Finite Element Method (FEM) because this method offers the ability to examine arbitrary fiber configurations, specifically through deformation where the fiber is no longer circularly symmetric. The FEM models were run for both uniaxial crush loads and uniform pressure loads for both silica and a doped PMMA material targeting strains up to approximately 6% at the boundary of the fiber. The results showed that degradation of the bandgap occurs with loading and that axis specific loading information may be obtained in fibers whose material normal and shear Pockelâ€™s constants differ by approximately 50% or more, although the exact difference required is not known. In the case of the PMMA uniform pressure load it was determined that the combination of loading and fiber characteristics may cause the bandgap to switch modes which may interfere with actual sensor implementation and should be avoided. The cross-section of the fiber studied was not rotationally symmetric which resulted in nonsymmetric optical output from the uniform pressure case. While fibers of this construction are likely to not be rotationally symmetric by design, the actual manufacture of the fibers results in a cross section that more closely approximates this condition.
Seismic Performance of Steel Bridge Bent Welded Connections.
(2009-09-14) Cookson, Kendra Ann; Dr. Mervyn Kowalsky, Committee Chair; Dr. James Nau, Committee Member; Dr. Tasnim Hassan, Committee Member
The objective of this research is to evaluate the seismic performance of steel bridge bent welded connections. Little is know about these types of systems with regard to their application in seismic regions such as Alaska. The focus of this research is specifically related to use of these structures in the State of Alaska. The research included both an experimental and analytical portion. The experimental portion consisted of four full-scale sub assembly bridge bents. The current practice in Alaska, a fillet weld, was evaluated as well as two additional weld configurations. The two additional weld configurations are a complete joint penetration weld with a reinforcing fillet and a simple complete joint penetration weld. All four test units were tested under reverse cyclic loading applied by a hydraulic actuator. The analytical portion consisted of applying the concepts of direct displacement based design in order to evaluate the test results. The results of the experiments show the current practice in Alaska is not adequate for the level of seismic intensity throughout the state. The complete joint penetration weld with reinforcing fillet was able to achieve moderate displacement capacity with the possibility of being used in some of the lower seismic regions of the State. The simple complete joint penetration weld was only able to achieve a low level of displacement capacity.