Assessing Pre-Tensioned Reinforcement Corrosion within the New Zealand Concrete Bridge Stock

Abstract

Precast pre-tensioned concrete bridge construction became common in New Zealand in the 1950s and a large number of pre-tensioned concrete bridges were constructed between 1953 and 1980. These bridges do not meet today’s durability requirements and as such many are at risk of chloride induced pre-tensioned reinforcement corrosion. This deterioration can be difficult to detect and has immediate structural implications, so identification of at-risk structures is critical for bridges to achieve their required service lives. This study aimed to expand on previous research into the deterioration of pre-tensioned I-Beam bridges, to obtain an accurate assessment of the severity, prevalence and distribution of corrosion risk to all of New Zealand’s pre-tensioned prestressed concrete bridge assets. The NZ State Highway pre-tensioned bridge stock was analysed, and pre-tensioned bridges were categorised by construction era and beam type. The number of bridges in each category was assessed, and standard drawings and typical details are presented for common bridge types in New Zealand. A computer based distribution and exposure classification tool was developed that draws from international best practice asset management techniques and allows remote assessment of both individual bridges, and of the bridge stock as a whole. The exposure classification of each pre-tensioned concrete bridge on the State Highway network was remotely estimated using the tool, and these results were used to further categorise the bridge stock. A representative sample of 30 bridges was selected and subjected to a non-destructive testing regime. Trends identified in the inspections were applied back to the other bridges using the construction eras and beam types identified in this thesis. A key objective of this study was to identify pre-tensioned bridges in the wider bridge stock that were likely to be at risk of chloride induced corrosion, and recommend that they each be subjected to more thorough investigations to quantify and manage the corrosion risk to each structure. The tools developed in this thesis were intended to be used by asset managers and bridge inspectors in future assessments of New Zealand pre-tensioned bridges. The construction eras and beam types developed can be used to identify standard bridge plans and other design documents relevant to a given bridge, and to quickly discern general information about the structure. The distribution and exposure classification tool can be used to remotely assess bridges using road level and satellite photography. This thesis provided insight into the condition and durability of the New Zealand pre-tensioned concrete bridge stock. It is anticipated that the information presented will be used to identify at-risk bridges nationwide so that inspection schedules and mitigation or remediation works can be designed and performed effectively.