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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 47 two downstream crosshead overhangs during the application of the repair material (ie D1, T1). A second reference electrode was installed in the two untreated control areas of the two downstream columns, about 400 mm from the underside of the crosshead (ie D2, T2). A third Ag/AgCl reference electrode was installed in the crossheads adjacent to the interface of the major patch repairs and the parent concrete (ie D3, T3). e purpose of this was to monitor the effects, if any, of the concrete repair on the adjacent unrepaired areas (Figure 1). is is to check the general expectation that repaired areas normally act as large cathodes with the adjacent unrepaired areas becoming anodic and thus prone to corrosion. If this is the case this monitoring will give some indication as to the timing of this. Reference electrodes D2, T2, D3 and T3 were embedded in mortar of similar resistivity to the original concrete. e locations of the resistivity measurements were also numbered accordingly and monitored at regular time intervals (Figure 3). Monitoring of potentials and resistivities commenced in July 1991. Additional testing over the years included chloride and pH testing, external half-cell potential and resistivity measurements and more recently corrosion rate measurements, as well as visual inspections and delaminations survey. Recent investigations and assessments revealed that although after almost 20 years of service a significant proportion of the concrete repairs and protective coatings have approached the need for re- intervention in this very aggressive marine environment, overall the combination of some repairs and associated coatings has proven to have performed in a reasonably satisfactory manner. In effect, it has been established that good quality conventional patch repairs in combination with good protective coatings can perform reasonably well and provide a service life of almost 20 years in harsh marine environments compared to a service life of some 5 years achieved in earlier years using lower quality procedures and materials. As a result of this ongoing monitoring over the near 20 year period, it may be concluded that both a polymer modified cementitious coating and an epoxy coating have been relatively effective in limiting the ingress of chlorides into the concrete piers particularly early on, despite their shortcomings which may be attributed to mixing, application, curing and other operational practices. e application of polymer modified cementitious repair materials in combination with the protective coatings has resulted in delaying the onset of macro-cell corrosion (incipient anode effects) in the neighbouring coated areas for a significant period of time, although such effects appeared to have manifested themselves in the past 3 to 5 years in a moderately accelerating manner. 3.0 MONITORING OF CORROSION MONITORING SENSORS AT PATTERSON RIVER BRIDGE SINCE CONSTRUCTION -- MARINE EXPOSURE Six macro-cell/galvanic current corrosion-monitoring sensors were installed within the cover concrete in the downstream column of Pier 2 during construction in 1994/95 (4), with three installed in the upstream face and three in the downstream face (Figure 4), to monitor the performance of the new bridge and the various durability provisions over time. e macro-cell or galvanic corrosion current flowing between the steel anodes and the carbon cathode is measured periodically. As the chloride ions penetrate the cover concrete, the steel anodes become active and corrosion currents are measured. e outer steel anodes were located within the first 15 mm of the cover concrete. ey were placed at 0.2 m, 0.7 m and 2.2 m above the top of the pile cap to basically measure the effects within the tidal, splash and atmospheric zones respectively. A dummy sensor was also cast in a more permeable concrete so that corrosion could be induced at a faster rate in order to complement the system (Figure 5). Monitoring is done on a frequent basis via a remote Figure 3. Resistivity of concrete for Tooradin Pier columns. 0 5 10 15 20 25 30 35 40 45 50 Jun-9 4 Oct-95 Mar-97 Jul-9 8 Dec-99 Apr-01 Sep-02 Resistivity (kW ·cm) 1-Up 1-Mid 1-Low 2-Up 2-Mid 2-Low 3-Up 3-Mid 3-Low Figure 4. Galvanic monitoring sensors; monitoring box-later converted to remote monitoring; dummy probe placed in more permeable concrete.