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Concrete In Australia : June 2013
38 Concrete in Australia Vol 39 No 2 Cathodic prevention/protection in durability design of reinforced concrete structures U Kreher -- Senior Materials Technologist, Aurecon Australia, Melbourne A Vinnell -- Materials Engineer, Aurecon Australia, Melbourne I Solomon -- Technical Director, Transport Aurecon Australia, Melbourne e increasing cost of large civil infrastructure projects in terms both initial construction costs and ongoing maintenance costs has lead to a demand in many instances for 100 year or greater design life. is demand challenges current industry durability practices in terms of materials selection, design methodology, construction and standards. Particularly in aggressive marine environments, chloride induced reinforcement corrosion is likely to be a long-term concern. In this situation, extended design lifetimes are difficult to guarantee with traditional methods, quality control and good design alone. In some instances it is not possible nor practical to provide the necessary concrete cover or mix design needed for durability. Structure owners are now frequently requesting structures to be either designed with cathodic prevention, or designed with provision for a future cathodic protection system. ese methods provide additional reinforcement protection and assist to vastly extend the service life of these structures. is paper will discuss durability issues relevant to cathodic prevention and cathodic protection methods, including where each method can be applied, relevant standards, construction and design issues and ongoing maintenance requirements. e recent construction of an innovative cathodic prevention system to a new landmark heavy rail bridge will be used as a case study. e bridge incorporated pretensioned precast elements, cast insitu and post tensioning. is paper describes some of the limitations and challenges faced throughout design and construction of the cathodic prevention system including constructability, QC, installation, commissioning and ongoing maintenance. 1.0 INTRODUCTION With asset owners having tighter maintenance budgets, it has become more and more important to construct durable infrastructure which will require limited maintenance over a long service life. As a result, a service life of 100 years and more is now a common minimum requirement for critical assets such as bridges, embankments and tunnels, as well as key concrete structures of major water infrastructure projects such as inlet and discharge tunnels of desalination plants. For reinforced concrete structures, a 100 year design life is often achievable by good design if the environmental exposure is not severe and provided quality assurance (QA) requirements are diligently followed during construction. However, experience has demonstrated that these requirements are often difficult to meet, with the resulting risks exacerbated if the structure is exposed to severe conditions such as chloride containing water spray or located in a tidal and splash zone. is paper discusses the option of providing cathodic prevention or cathodic protection to these concrete structures at various stages of their life, including suitability of each option, advantages and disadvantages. It briefly describes the design of a girder and headstock cathodic prevention system for a rail bridge, and experiences made during the construction of the girders and the bridge. Further, installation of the cathodic prevention system and commissioning experiences are summarised. 2.0 MAIN STANDARD TO DESIGN FOR 100 YEARS -- AS5100.5 e main tool for the concrete designer to achieve 100 year design life for new structures is the bridge code Australian standard AS 5100.5 (2004). Main factors to be taken into account and resulting parameters for design are: • exposure classification, eg atmospheric, splash and tidal, and permanently submerged • concrete cover to reinforcement • concrete quality including compressive strength, cementitious content, total water to cementitious material ratio, supplementary cementitious materials, compaction, and durability properties such as chloride diffusion coefficient • crack control • curing. With the design code and state specifications adhered to, a 100 year life should be theoretically achievable if the quality control and assurance systems are fully complied with. e reality is however, that it is not always possible to achieve the required standard of workmanship and control. Furthermore, AS 5100.5 does not specifically consider the influence of supplementary cementitious materials or non- standard exposure environments. Ultimate concrete quality may be affected by variances in the concrete mix proportions, workability, level of compacting and the effect of ambient