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Concrete In Australia : September 2013
50 Concrete in Australia Vol 39 No 3 CONFERENCE TECHNICAL PAPER A maintenance strategy would then be developed on the basis of the updated measurements, although observed leakage, dampness, shrinkage cracks and corrosion of some structural steel work (base plates, bolts etc) was considered to require intervention much earlier. 5.0 MONITORING OF PATCH REPAIRS AND COATING SYSTEMS AT CHURCH STREET BRIDGE -- CARBONATION AND MOISTURE EFFECTS For Church Street Bridge, a total of ten embedded reference electrodes were installed in 2010 in cell walls and deck soffit at Cell 21 and Cell 27 within major patch repairs located in Span 3 (the southern span of the bridge), due to carbonation induced corrosion of the steel reinforcement (7). Some reference electrodes were also installed within the patch repairs adjacent to the interface with the parent concrete. At each location, an electrode was horizontally placed adjacent to a main steel bar at the same cover depth. It was required that the concrete over the steel not be disturbed by the installation. Calibration and Initial monitoring measurements were carried out upon installation, followed by a new set of measurements on two cells of the bridge deck, which was conducted in mid-December 2011. e monitoring of the corrosion of reinforcing steel is conducted through measurement of the potential of steel relative to the reference electrode adjacent to it. According to ASTM C876-99 a potential more negative than -350 mV CSE may indicate active corrosion in steel in concrete. It should be noted that ASTM C876-99 has been withdrawn in 2008 and the new version of the ASTM C876-09 does not contain any definite value to evaluate the possibility of corrosion based on the potentials. Nevertheless, changes in the potential indicate changes in the environment and/or electro-chemical condition of the steel. In each deck cell, there is a junction box containing terminals for the cables leading to the reference electrodes and reinforcing steel bars (Figure 13). All test results are presented in Table 3 and Table 4. e analysis of the measurements (if the value of -350 mV CSE potentials is used) indicates that the half-cell potentials of reinforcement in cell wall (Cell 21) and spandrel column (Cell 27) have become more positive than this value and indicate that the possibility of corrosion activity is less than 10%. It should be noted that the patch repairs are characterised by a trend of increase in resistivity with time as the repair material continues to cure and dry out. On the other hand some areas of the deck showed potentials progressively more negative than -350 mV CSE since the installation. e difference in these trends may be related to the differences in the moisture condition of concrete in the two locations. e deck concrete is likely to be wetter, leading to lower levels of oxygen and more negative potentials. is may not necessarily indicate corrosion activity. It should be noted that although a waterproofing membrane was applied to the deck during rehabilitation of the bridge, subsequent ground penetrating radar (GPR) investigation confirmed that the waterproofing system was being slightly undermined by moisture ingress and movement, particularly at the southern area of the bridge where the monitoring systems are located. A more detailed monitoring program in a drier period is needed to verify the likelihood of corrosion in the monitored areas of the deck and to generalise the results to the whole bridge deck. 6.0 MONITORING OF PATCH REPAIRS AND COATING SYSTEMS AT RACECOURSE ROAD BRIDGE -- CARBONATION AND MOISTURE EFFECTS A total of eight embedded reference electrodes were installed in a column and deck soffit within and at the periphery of major patch repairs of Racecourse Road Bridge in 2011 (Figure 14) due to carbonation induced corrosion of the steel reinforcement (8). e results of all the measurements are presented in Table 5 and Figure 15. All reference electrodes with the exception of R5 and R6 (columns) (refer Table 5) are embedded in the deck soffit which is susceptible to water seepage from the top, as influenced by the footpath and tram tracks. At present, the potentials of all the pairs are more negative than -350 mV (CSE), which indicate that the reinforcing steel in these locations is corrosion active by the criteria of ASTM C876-99. It should be noted that this standard was withdrawn in 2008, and that the revised new standard (ASTM C876-09) does not have such criteria. R3 and R4 (in the damp area of soffit) appear to have become more negative in the past 200 days; R5 and R6 (both in repaired area of column) have become less negative, and the other locations have been more or less Figure 7. Monthly mean temperatures at Project A (Source: www.bom.gov.au). Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Monthly Min Monthly Max Mean temperature (°C) 30 25 20 15 10 5 0 Month