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Concrete In Australia : September 2013
48 Concrete in Australia Vol 39 No 3 CONFERENCE TECHNICAL PAPER monitoring system. Readings were found to be very low and averaging less than 0.006 μA (Figure 5), with some very minor seasonal variations which are very similar to the data presented in a previous paper on this bridge some 4 years ago (4). is further highlights the effectiveness of the high concrete quality (low VPV--10.5%--12.5%<14%of maximum allowable in concrete cores as per section 610 (5)) and protective measures adopted for this bridge from the outset. e very low corrosion current results of the monitoring sensors are quite clearly supported by other data including the external corrosion rate measurements, the negligible chloride penetration into the concrete, and the subsequent inability to calculate chloride diffusion coefficients and hence time to corrosion initiation. e same western column of the middle pier (Pier 2 downstream column) was tested externally for the corrosion state of the steel reinforcement. e corrosion rate and half-cell potential measurements (which are obtained simultaneously) are summarised in Table 1. Comparing with the results of 2001, the corrosion rate has marginally increased, however it is still below 0.1 μA/cm2 which has been considered as the criterion for "no corrosion expected" as indicated in Table 2. On both occasions at all three locations the corrosion rate is negligible. is conclusion is confirmed by the half-cell potential, which is more positive than -200 mV CSE (copper- copper sulphate electrode potential). ere is essentially no difference in the corrosion activity at the three locations measured. An example of the output of the embedded sensors is given in Figure 5 together with results for the dummy sensor placed in more permeable concrete. e sensors also show negligible or no corrosion activity, which is in agreement with the results of the linear polarisation measurements. Based on these results no corrosion activity is expected in the concrete in the foreseeable future, for at least 20 years and beyond. is is the result of the high quality of the concrete and the surface coating applied soon after construction. 4.0 MONITORING OF CORROSION MONITORING SENSORS AT PROJECT A (VARIOUS STRUCTURES) SINCE CONSTRUCTION -- MARINE EXPOSURE e group of structures constructed during 2004/2005, are located in close proximity to seawater, and are prone to chloride induced corrosion of the steel reinforcement. Various parts of these structures have been fitted with reference electrodes and ladder type macro-cell electrodes (Figure 6) to enable monitoring of the half-cell potential and corrosion activity of the steel reinforcement (6). ese sensors have been installed as a proactive measure to be able to identify the onset of corrosion activity and thus afford the asset manager the opportunity to undertake a more detailed diagnostic assessment if required, and put into place timely preventive intervention to avoid corrosion-induced damage to the structures. is requires regular inspection and monitoring to prevent development of corrosion-induced defects in the structure by addressing minor problems before they become unmanageable. Some 25 monitoring points were installed in each of the two long walls: fifteen monitoring points in base slabs and ten monitoring points in other slabs and superstructure components. Seven macro-cell ladders were also installed in two major structural slabs. e initial monitoring measurements were made in March 2007 which comprised half-cell potential measurements. Later corrosion monitoring measurements were conducted between 2009 and 2012 to provide a comparison of results with previous measurement and checking differences in the output of the sensors. e measurements conducted in 2009 established that the half-cell potentials were influenced by temperature (Figure 7) and it was determined then that the monitoring measurement should be conducted at similar months of the Figure 5. Typical results of corrosion monitoring sensors (T), dummy sensor results (B). PATTERSON RIVER BRIDGE PATTERSON RIVER BRIDGE Dummy Probe. Current (μA) Current (μA) 0.050 0.040 0.030 0.020 0.010 0.000 -0.010 -0.020 -0.030 -0.040 -0.050 0.050 0.040 0.030 0.020 0.010 0.000 -0.010 -0.020 -0.030 -0.040 -0.050 A1 A2 A3 A4 A5 A6 27/8/96 21/5/97 2/7/98 23/10/98 7/3/99 17/3/00 12/2/00 19/8/01 5/4/02 19/1/03 29/10/97 21/7/98 4/1/99 12/12/99 28/8/00 15/5/01 30/1/02 15/10/02 Date of Reading Date of Reading