This study combined research available through literature with extensive experimental studies and substantial physical modelling to estimate the remaining ultimate life of large offshore reinforced concrete structures. Although much research has focussed on concrete degradation due to chloride ingress, corrosion of permanently submerged concrete is regarded as negligible due to the long-assumed apparent worst case of tidal or splash zone exposure. Around 350 specimens were tested with a further 200 exposed for further testing by future research groups. Specimens ranged in size from standard cubes to various beam lengths up to 1.5 metres, allowing for material and structural properties to be assessed. My original contribution to knowledge in the sector enhances the fundamental understanding of corrosion in subsea concrete, challenging the generally held belief of negligible corrosion. Results and modelling provides an improved ability to ultimately estimate the longevity of fully submerged offshore reinforced concrete. Throughout this thesis, the results from experimental works, carried out as a direct result of the lack of data or information in literature, are reported, assessed and then utilised to provide updated ultimate life estimations. With the current offshore concrete structures currently coming to the end of their service life, and the likelihood of further offshore development using concrete for the renewables sector, understanding the long-term degradation is vital in determining the most effective decommissioning and derogation options. The research carried out directly provides detailed information of the likely time-to-failure, allowing for an informed decision to be made on operational and decommissioning plans. Experimental work was carried out over four main phases; corrosion initiation due to bulk diffusion of chlorides (Phase I), corrosion propagation in low oxygen environments (Phase II), corrosion in statically and dynamically cracked sections (Phase III) and structural response of heavily corroded individual and lapped bar sections (Phase IV). Phase I work shows a marked difference between submerged exposures to seawater as opposed to NaCl solution, the unsuitability for accelerated testing with seawater and the likelihood of rapid initiation in offshore structures. Further experimental works through Phases II and III found that although exposed to low oxygen concentrations, reinforcement corrosion continued at significant rates. A variation between anode sizes on the reinforcement is noted, but critically the cross sectional area of the steel was still reduced, albeit in fewer locations. Corrosive products were visibly different, with fewer expansive products, if any, present. Additionally, this study further highlights the importance of cracking on corrosion, currently ignored by recent model codes, such as the fib Model Code 2010, up to 0.2mm crack width. A linear relationship was found between crack width and corrosion rates, with cracking above 0.1mm considered significant. The loss of cross sectional area due to propagation was determined for the given environment, and consequently further studies were initiated in an attempt to determine the relationship between this corrosion propagation and the reduced serviceability or ultimate life of concrete beams. Serviceability, defined by beam stiffness, was reduced due to bond loss along reinforcement. Most importantly, however, results prove that the loss of cross sectional area to be the critical influence on loss of ultimate life. Initial estimates on the remaining ultimate life of the large offshore structures support early rough work that the structures would last centuries. This thesis, however, has shown this is due to the ability of concrete structures with such large volumes of steel to continue to ultimately withstand loading at high corrosion percentages and not due to negligible corrosion, or long initiation periods, commonly suggested in submerged, low oxygen environments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:600765 |
| Date | January 2014 |
| Creators | Thistlethwaite, Christopher |
| Contributors | Jones, Martyn; Newlands, Moray |
| Publisher | University of Dundee |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://discovery.dundee.ac.uk/en/studentTheses/4819cd1e-a5ce-48c2-a982-874196b3e8d9 |
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