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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 49 year to obtain comparable results. Since 2010 the monitoring measurements have been made in the month of February each year. e monitoring consists of measuring the potentials of the installed reference electrodes against the reinforcement and the macro-cell ladders installed in the walls, slabs, and small bridges which are components of the overall complex of structures. With the exception of a small number, most reference electrodes were found to be functioning normally during monitoring. As indicated in Figures 8-11, compared with the results obtained in 2007 the half-cell potentials measured in the 2012 investigation were almost the same or slightly more negative. Particularly, compared with the data obtained in the previous two years, the half-cell potentials in walls have become slightly more negative. is indicates that corrosion activity may be commencing. e small scale of potential changes over the past few years indicates that the corrosion state of the reinforcement in the structures has been stable. e ongoing monitoring has established that for the retaining walls, steel corrosion is likely at 36% of the monitored locations and uncertain at 36% to 41% of locations for the two walls. However, the east wall showed more likely corrosion activity than the west wall and other parts of the overall complex. Seawater seeping through a number of locations at the east wall has also been observed. For the base slabs, steel corrosion is likely at 25% of the monitored locations; and uncertain at 67% of the locations. For the main structural slabs, steel corrosion has not yet started, but it is uncertain at 13% of locations. e percentage of corrosion active areas has increased compared to the data of 2011. e half-cell potential data for one of the macro-cell ladders, installed in the road base slab, indicates high risk of corrosion at the bottom of the slab. e data of macro-cell ladders installed within the main structural slab indicate that there is no corrosion risk at present. Comparatively, the corrosion activity in the overall complex of structures has not significantly changed over the past three years of monitoring, and at the present time is still considered very mild. However, the ladder sensor and the potentials measured in the vicinity of the road base showed a high possibility of chloride penetration and corrosion activity. In order to establish an initial base level of overall condition of the various structural components, the monitoring measurements conducted on the embedded electrodes and macro-cell ladders was further supplemented with a diagnostic assessment which also included determination of concrete properties and corrosion status of the reinforcement steel. e diagnostic assessment involved cover thickness determination, delamination testing, measurements of the half- cell potentials, and corrosion current density of reinforcement at selected areas of slabs and walls. Concrete cores were drilled from slabs and walls and laboratory testing was conducted on the cores for compressive strength, volume of permeable voids (VPV) and chloride profile. Chloride profiles determined from the cores showed that the chloride content is high at the surface and lower in the interior of the concrete. Comparatively, the chloride content in the retaining walls was found to be much higher than that in the base slabs. e chloride ingress profiles show that the threshold chloride content (0.4% by cement mass) has reached only 5 mm depth in the base slabs, and about 35 mm in the retaining walls (Figure 12). Neither of these has reached the depth of reinforcing steels. Generally the results of the diagnostic assessment were reflecting the results of the various monitoring sensors, and further diagnostic assessment was considered necessary over the next two years, including determination of the depth of chloride ingress which is the trigger for corrosion initiation. Table 1. Corrosion state of the reinforcing steel in Pier 2 downstream column. Table 2. Corrosion rate criteria for reinforcing steel. Location Corrosion current density, μA/cm2 Half-cell potential, mV CSE Resistance,(kΩ) (Resistivity, kΩ∙cm) 2001 2008 2001 2008 2001 2008 1.05 m above pile-cap 0.015 0.028 -136 -83 2.39 (33.7) 3.42 (48) 0.45 m above pile-cap 0.020 0.039 -107 -88 2.30 (32.4) 3.05 (43) 0.16 m above pile-cap 0.016 0.044 -117 -93 2.31 (32.6) 3.93 (55) Note 1: For steel, a corrosion current density of 1.0 μA/cm2 is equivalent to a corrosion rate of 11.6 μm/year. Corrosion current density, icorr (μA/cm2)1 Corrosion rate category < 0.1 no corrosion expected 0.1 to 0.5 low to moderate rate 0.5 to 1.0 moderate to high rate > 1.0 high rate Figure 6. Galvanic ladder type macro-cell sensor positioned within thickness of cover concrete.