Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1992. / Vita. Abstract. Includes bibliographical references (leaves 252-257). Also available via the Internet

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

No description available.

MDent, Faculty of Health Sciences, University of the Witwatersrand / PURPOSE The purpose of this in vitro investigation was to determine the effect of using either gold or titanium retaining screws on preload in the dental implant body-abutment complex. This preload is of vital importance for the long term success of the dental implant complex. Inadequate preload results in either loosening or fracture of the retaining screw, and is the most commonly occurring mechanical complication in implant supported/retained prostheses. Similar complications occur when excessive preload is applied to the retaining screws. These complications can result in unscheduled visits with costly and time consuming repairs for the clinician and patient. Routine maintenance protocols for implant supported prostheses range from biannually to five year visits to the dentist. Maintenance visits involve removal of the prosthesis facilitating cleaning of both the implant and prosthesis and inspection of retaining screws . This study sought to gain insight into changes in preload generation after repeated torque application to gold and titanium screws and to observe whether gold or titanium generated better preload. A maintenance protocol would be suggested if any observable pattern was noted. MATERIALS AND METHODS The test setup consisted of an implant body, a cylindrical transmucosal abutment and the retaining screws (gold or titanium). The implant body was anchored using a load cell. Transmucosal abutments were attached to the implant body using either a gold or titanium v retaining screw. A torque gauge was used to apply torque of 20Ncm, 32Ncm and 40Ncm to the retaining screws. This was undertaken to investigate the effect of gold or titanium on preload generated. The effect of applying torque beyond manufacturers recommended 32Ncm was carried out to see if greater preloads could be achieved. All components were from the Southern Implant system. RESULTS Gold retaining screws were found to achieve consistently higher preload values than titanium retaining screws. Preload values were not significantly different from the first to the tenth torque cycle. Titanium screws showed more consistent preload values, albeit lower than those of gold retaining screws. However due to possible galling of the internal thread of the implant body by titanium screws, gold screws remain the retaining screw of choice. Maintenance protocols suggest replacing retaining screws every 20 years. After ten torque cycles were applied to each screw there was an insignificant change in preload generated in both titanium and gold screws. This study was therefore inconclusive with regards to maintenance protocols. CONCLUSION Within the limitations of this study, gold retaining screws generated better preload than titanium. Torque application beyond manufacturers’ recommendations resulted in a more stable implant complex. Further investigation into repeated torque application to retaining screws is required, to determine ideal maintenance protocols.

published_or_final_version / Applied Geosciences / Master / Master of Science

No description available.

No description available.

Drystone walls have been used extensively around the world as earth retaining structures wherever suitable stone is found. Commonly about 0.6m thick (irrespective of height), there are about 9000km of drystone retaining walls on the UK road network alone, mostly built in the 19th and early 20th centuries, with an estimated replacement value in excess of £1 billion[1]. Drystone wall design is traditionally empirical, based on local knowledge of what has worked in the past. Methods vary from region to region, driven by both custom and the nature of the materials available. Design is not necessarily optimised, and includes unknown margins of safety. There is a recognised need for guidance on the assessment and maintenance of dry stone retaining walls, as no suchdocumentscurrentlyexist. Thisthesisdocumentstheconstructionofaseriesoffull-scaletestsdesignedto provide sufficient information to validate current theoretical and numerical analysis techniques. The development of a unique test rig is detailed, in addition to the testing regime and results from a programme of five 2.5m high drystone retaining walls. The walls were subjected to localised surcharging and foundation movements, recreating the conditions that many in-situ walls are subject to. Movements such as toppling, bulging and sliding were observed, and recorded using a broad range of instrumentation. This has provided high quality, quantitative data relating to the factors which influence these mechanisms, and their affect on wall stability. Also documented are the associated laboratory tests which have been conducted to determine the mechanical properties of backfill and the walls themselves, as well as the manner in which they interact together. To assist in the analysis of these full-scale tests, a limit equilibrium program has been developed. This package allows the rapid generation of a wall of any size and constructed with any materials. The limit equilibrium program has then been used in conjunction with the data from the full-scale and laboratory tests to analyse observed drystone wall behaviour. These include the phenomena of toppling, bulging, bursting, sliding and individual block rotation. In each case, the underlying causes of such movements have been determined, and the critical parametersidentified.

Segmental retaining walls (SRW), typically constructed along highways, have grown in popularity over the past decade. Manufacturers of SRW blocks have estimated the service life of a properly constructed wall to be approximately 75 years. However, there have been reports of SRW systems failing after only five years in service. Suspected causes of the SRW failures are freeze-thaw damage while exposed to deicing salts sprayed by snow plows from highways. The current standard test method used for evaluating the freeze-thaw durability of SRW blocks has several drawbacks and does not accurately replicate environmental exposure field conditions. The objective of this research is to develop and assess a new standard test method for evaluating the freeze-thaw durability of SRW blocks that obtains reproducible results and offers sufficient information on the freeze-thaw performance for SRW block manufacturers and state highway agencies (SHAs). The research completed a preliminary proof of concept test for the new freezethaw test method developed using small, commercially available SRW blocks to mitigate potential problems and establish appropriate test parameters. The testing produced results of freeze-thaw degradation that followed the same modes of failure that has been discovered during field evaluations. After the proof of concept test was completed, a series of freeze-thaw tests were conducted using sets of SHA approved and non-SHA approved SRW blocks. Three different manufacturers’ SRW blocks were evaluated. There was no significant freezethaw degradation of any of the blocks after 200 freeze-thaw cycles, so for two blocks, experiments were extended to 400 cycles using a twelve-hour freeze-thaw cycle. The modification of the test did not result in more rapid deterioration of the SRW blocks. The researchers found that the freeze-thaw durability test method developed herein is beneficial for determining the freeze-thaw performance of the lower quality specified blocks. The test method gives realistic results, which match typical deterioration modes that are common in field settings, in a timely manner. However, the test method for testing SHA quality SRW blocks takes longer times and may not be a reasonable test for such products.