On the service life modelling of Tasmanian concrete bridges

Various forms of concrete have been used for buildings since the time of Egyptian civilisation. The art of cement manufacture was however lost during the Middle Ages. The rediscovery of the art in the nineteenth century and the subsequent development of reinforced and prestressed concrete have seen concrete become the most commonly used construction material throughout the world. While concrete structures perform well in many situations, lack of durability in others over the last two or three decades in particular has emerged as a significant issue for asset owners internationally. While there is a number of mechanisms by which concrete deteriorates, chloride induced corrosion of bridge and marine structures and carbonation induced damage of buildings are perhaps the most significant. The various concrete deterioration mechanisms and, to a lesser extent, cover to reinforcement have been researched and reported extensively, particularly in the last twenty years. The research has however tended to focus on specific aspects of the durability performance of concrete structures. From an asset owner's perspective, it is the interaction of the various aspects of the deterioration processes that determines management strategies for affected structures and code and specification requirements for new structures. The literature relating to the interaction of the chloride and carbonation deterioration mechanisms and cover to reinforcement is however limited. The Tasmanian Department of Infrastructure, Energy and Resources is responsible for the management of a substantial bridge asset with a high proportion of its value in close proximity to salt water. The durability of its concrete structures is a significant management issue. A series of corrosion investigations and structural surveys have provided a substantial body of data which has been analysed and reported in this thesis to assist with the management of the asset and provide the basis for enhancements to specifications and codes. This work has highlighted the high variability in the parameters used to describe the durability related properties of in situ aged concrete and in cover to reinforcement. The high variability leads to high probabilities of corrosion initiation, particularly for chloride induced corrosion, that are reflected in the durability related performance of the bridge asset. A high and continuing demand for maintenance, rehabilitation and replacement of corrosion affected structures is indicated. The high variabilities also suggest that incremental changes to existing approaches to durability in aggressive environments will not achieve the improvements in performance required to increase materials related reliability to levels that are consistent with, albeit lower than, levels of structural reliability. Approaches to enhance the durability, reflecting the significant level of change required, of concrete structures are proposed.