Reinforced concrete half-joint structures: Structural integrity implications of reinforcement detailing and deterioration

A half joint is a particular type of RC structure. Within the existing UK Highways Agency network alone there are 400+ concrete bridges with half joints. The advantages of this structural form include a level running surface along the bridge deck and the support spans, and precast beams can be easily lifted into place and supported during construction. However, a disadvantage inherent in this type of construction is that there are problems associated with leakage through the joint. This enables moisture to accumulate at the beam seats thereby increasing the propensity for the deterioration of the concrete and reinforcement steel. The situation is adversely compounded by limited access to the bearing seat joint which leads to both inspection and maintenance issues. A further complication is that the detail in the beam seat region represents a potentially sensitive structural detail. Some half-joint structural vulnerabilities may have been present from the outset e.g. in the initial design or construction, some may develop with time e.g. deterioration. Half joint details have come under intense scrutiny since the collapse of a section of the de la Concorde Overpass in Quebec, Canada in 2006. Five people were killed and six others injured. Thus, a key challenge is to understand the lessons to be learned from the Concorde Overpass Collapse, and the inherent vulnerabilities in half joint structures.
This project will provide improved analytical tools and experimental evidence for the more accurate assessment of half-joint structures, and will inform the management strategy for half-joint structures throughout the UK. Overly conservative strength assessments can result in unnecessary closures or vehicle weight limits, both of which are costly. Non-conservative strength assessments potentially put public safety at risk. The outcomes of this project are therefore of national, and international importance.
The behaviour of reinforced concrete structures in shear is notoriously complex and there is no generally accepted unifying theory. This presents a major difficulty in the analysis of half-joint structures. The current work will explore, compare, extend, and adapt existing theories in the context of half-joint structures. A further goal is to incorporate detailing deficiencies and/or deterioration within the analytical and experimental approaches to thereby enhance the predictive capability for structures in service. The load-sharing implications in non-compliant details and the sensitivity of the structural performance to a deterioration outcome, such as a loss of strength will be investigated. This approach reflects the type of information that would be collated as part of a destructive or non-destructive testing strategy, and provides a general, robust and repeatable framework from which others can benchmark their results.
Experiments on small and large scale specimens will be undertaken to provide the essential validation of the expected behavior. Small-scale tests will be used to characterise the bond anchorage behaviour in a representation of a deteriorated system. The large-scale tests will deliver an experimental database of half-joint structures with non-compliant details and determine the influence of strength reductions in the constituent materials. This will provide an important experimental evidence of the behavior of such structures, and highlight the most critical detailing and deterioration combinations. While the project has been inspired by, and will address, the difficult challenges faced by those who build, own and maintain reinforced concrete bridges, the development of the underlying fundamental scientific and engineering understanding will have far greater implications. A fuller understanding of the failure modes and sensitivities associated with complex reinforced concrete details will provide insight into the life-time performance of a wide range of existing infrastructure assets.

After the tragic collapse of the Concorde Overpass in Quebec, Canada, the appointed Commission of Inquiry made a number of recommendations for future practice including the need to 'Improve the knowledge acquisition process'. In particular, the Commission recommended that [1]' ... the Government ensure that there be an effective surveillance of scientific intelligence processes and knowledge involving academics and top-level practitioners; this will ensure that persons responsible for designing and maintaining structures, both in private practice and in Government services, be kept constantly informed of new developments and changes in standards and practices.' Although this statement was in a Canadian context, the message resonates globally. The project team assembled to deliver the proposed work includes international and national representation, private and public organizations, and academic and industrial partners. This combination of expertise will present a powerful mechanism in which to improve the knowledge acquisition process both within, and beyond the UK sector.
The academic merit will be of interest to researchers and practitioners across a range of disciplines including structural engineers, material scientists and analytical modelers. The developed tool(s) with which to assess half joint structures will provide invaluable information about the sensitivities within a given structure to detailing deficiencies and/or deterioration. The undertaking of novel experiments will provide an experimental database of non-compliant half-joint structures, provide an important experimental basis for the effects of a strength loss in the constituent materials (as a representation of deterioration), and highlight the most critical combinations. The outcomes will contribute towards the formulation of informed, evidence-based policy approaches for the assessment and monitoring of the UK's 400+ half joint structures. Most importantly, the research will also be applicable more broadly in the context of the determination of the residual strength capacity of existing reinforced concrete infrastructure.
The dissemination of the results through undergraduate and graduate teaching programmes will improve the awareness of future generations of engineers to the complexities hidden within our surrounding built environment and the urgent need for validated strategies, with a sound theoretical foundation, to assist with the ongoing challenges associated with an aging infrastructure asset profile. The skills gained by the post-doc working on the project will result in a range of advanced analytical and experimental expertise, the outcome of which will be a highly employable individual with a bright academic, or non-academic professional future.

[1] Commission of Enquiry into the collapse of a portion of de la Concorde Overpass, Government of Quebec, 2007.