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Concrete In Australia : September 2013
54 Concrete in Australia Vol 39 No 3 CONFERENCE TECHNICAL PAPER instantaneous off-potential, which is at least 100 mV more negative than the depolarised potential. • Extended potential decay criterion -- A potential decay over a maximum of 72 hours of at least 100mV from the instant off potential subject to continuing decay and the use of reference electrodes (not potential decay sensors) for the measurement extended beyond 24 hours. • Absolute potential criterion -- An instant off potential (measured between 0.1 s and 1 s after switching the DC circuit open) more negative than -720 mV with respect to Ag/AgCl/0.5 KCl. • Absolute passive criterion -- A fully depolarised potential or a potential which is continuing to depolarise over a maximum of 72 hours after the cathodic protection system has been switched off which is consistently less negative than -150 mV with respect to Ag/AgCl/0.5 KCl. e initial monitoring results (Table 6) following energisation showed that the CP system satisfied criterion (a) and (b); that is "the 100 mV polarisation decay" a short period after its encapsulation within the reinforced concrete jacket, while the SCC was developing its strength and reducing its permeability. However, the results obtained in subsequent monitoring, namely some six months (Table 7) and twelve months (Table 8) after energisation did not comply with the CP criteria given above, although the S4-R4 electrode pair approached the "absolute passive criterion". It must be emphasised however, that the DAS galvanic anodes are encapsulated within high quality 25% fly ash concrete which will be characterised by high-strength and low- permeability and therefore a higher resistivity as the concrete continues to cure and dry out, thus preventing any moisture ingress. e fly ash component of the concrete continues to refine the microstructure of the concrete thereby reducing the amount of free water in the concrete. A good quality acrylic anticarbonation coating with some waterproofing capability has also been applied on the concrete jackets, thereby further inhibiting the ingress of moisture into the concrete. e sacrificial anodes cathodic protection for steel is based on the fact that the metal used as the anode has a more negative electrode potential, the corrosion of which is thermodynamically favoured and provides electrons to satisfy the demands of oxygen reduction of the steel (corrosion reaction). e output of galvanic anodes depends on the corrosion of the anodes in the concrete. e reduction in the current output in regard to the age of reinstated concrete after installation indicates that the anode corrosion is not rapid. However, this level of current may be what the reinforcing steel needs as this stage, as the high quality fly ash concrete in combination with the protective coating are providing adequate protection to the steel reinforcement. It is expected that as the combined effectiveness of the good quality concrete and coating diminishes over time the contribution of the DAS galvanic anodes in protecting the steel reinforcement will be increased. Table 5. Steel reinforcement potentials at Racecourse Road Bridge, CSE (mV). Figure 15. Racecourse Road Bridge half-cell potentials of rebar since the installation. Date Age (d) R1 Soffit R2 Soffit R3 Soffit R4 Column R5 Column R6 Soffit R7 Soffit R8 Soffit 15/11/2011 -455 -473 -655 -625 -387 -315 -601 -366 14/12/2011 29 -501 -489 -696 -653 -451 -424 -639 -464 14/06/2012 212 -592 -532 -900 -790 -439 -395 -522 -524 9/10/2012 329 -620 -537 -878 -781 -463 -430 -459 -494 Figure 16. Repair system of RC jackets and encased galvanic anode and junction box at Queen Street.