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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 45 Use of corrosion monitoring sensors to monitor the in-situ performance and intervention needs in reinforced concrete structures Fred Andrews-Phaedonos, Principal Engineer -- Concrete Technology, VicRoads Dr Ahmad Shayan, Chief Research Scientist, ARRB Group Pty Ltd Dr Aimin Xu, Senior Engineer, ARRB Group Pty Ltd It is well recognised that visual inspections alone are not sufficient to establish the in-situ condition of a concrete structure, or to ensure that appropriate maintenance and rehabilitation works are undertaken. Such inspections only identify damage or deterioration which has manifested itself with no indication as to the internal condition of otherwise visibly sound concrete. In addition, such inspections can only provide a superficial view of the condition of the structure at any given time and do not give a clear or comprehensive picture of corrosion risk and future maintenance needs. As such, diagnostic assessments, consisting of a number of physical, electrical and chemical techniques, are generally used in combination with visual inspection to obtain a better assessment of the condition of reinforced concrete structures already affected by corrosion induced deterioration. e information from these techniques is used collectively to ensure correct interpretation of results and subsequent diagnosis and prognosis. However, as an alternative to the combination of visual inspection and diagnostic assessment which is generally used to establish the cause, extent and degree of deterioration already apparent, corrosion monitoring sensors may be cast into new structures or installed in existing structures alone or as part of repairs, to provide the asset manager with real time information as to the current state and performance of the structure or remedial works. Monitoring sensors can provide early detection of initiation and/or propagation of corrosion, and if required, allow for an early diagnostic assessment as well as facilitate timely and cost effective preventative maintenance intervention as opposed to more disruptive and expensive rehabilitation. Such early detection can also allow the asset manager to pre-plan and invest in a timely manner and thereby minimise the number of costly interventions, mobilisations and disruptions to the travelling public. is paper presents a number of case studies where monitoring sensors have been built into new structures, as well as where sensors have been installed as part of concrete rehabilitation works with a view to monitoring their effectiveness and ongoing performance. 1.0 INTRODUCTION Monitoring sensors in the form of reference electrodes were first utilised by VicRoads for the monitoring of a number of impressed current and sacrificial cathodic protection systems applied to reinforced concrete bridges since the late 1980s. A number of silver/silver chloride (Ag/AgCl) reference electrodes were installed for monitoring the half-cell potential of the steel reinforcement, as part of remedial works undertaken in 1991/92 at Sawtells Inlet Bridge which is located in an aggressive marine environment southeast of Melbourne. Stainless steel pins were also installed for measuring the resistivity of concrete. e internal Ag/AgCl reference electrodes were connected by cables leading to monitoring boxes placed at the upper edge of the crossheads. Monitoring of potentials and resistivities commenced in July 1991 (1, 2, 3). In the mid-1990s six macro-cell/galvanic current corrosion monitoring sensors (ladders) were installed in the downstream column of Pier 2, as part of a multi-level durability strategy adopted for the new Patterson River Bridge (Nepean Highway over Patterson River) constructed in 1994/95 in a very aggressive marine environment. e monitoring sensors consist of an array of steel pieces acting as anodes and a carbon cathode. e macro-cell or galvanic corrosion current flowing between the steel anodes and the carbon cathode was measured periodically. As the chloride ions penetrate the cover concrete, the steel anodes in- turn become active and corrosion currents are measured (4, 5). Project A was located in a severe marine environment, comprising a number of major reinforced concrete structures including bridges and retaining walls and was completed in 2005. ese structures are potentially subject to chloride-induced corrosion, resulting from exposure to salt water. For this reason some seventy-five Ag/AgCl reference electrodes and seven macro- cell/galvanic current corrosion monitoring sensors (ladders) were installed at different locations within major bridge components and exposed retaining wall piles. e sensors were installed as a proactive measure to be able to identify the onset of corrosion activity and put in place preventive intervention to avoid corrosion-induced damage to the structures (6). A corrosion monitoring system using a number of embedded Ag/AgCl reference electrodes was installed during concrete repair works at both Church Street Bridge and Racecourse Road