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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 37 values could be compared with the conventionally used criterion for corrosion initiation, i.e. -350 mV Cu-CuSO4 electrode potential. e results are presented in Table 4. e results are graphically presented in Figure 7. Information supplied by VicRoads showed that the initial half-cell potential readings, after the hardening of the backfill in September -- October 2009, were very negative; the direct readings being -600 to -800 mV for the upstream wall and about -1000 mV for the downstream wall. Figure 7 shows that during the past few years, since October 2009, the potentials seem to have shifted to more positive values and have stabilised. e rapid change in the potentials during the first year may be partly due to loss of moisture from the concrete and partly due to the reference cells which gradually stabilised in the concrete. After the first year, the change in the half-cell potential has been small but more negative, and may be mainly due to the corrosion activity of the reinforcement. e present potentials are about -250 mV (CSE) for the top part of walls, and about to -350 mV (CSE) for the middle and bottom part of the walls, which may reflect the difference in the moisture content in these areas. e half-cell potentials of the reinforcement in the upstream wall (blended slag cement concrete) is more negative than that of downstream wall (geopolymer), which may indicate that the steel in the upstream wall has higher corrosion activity, although the differences in the moisture content and binder type may also be responsible. In any case, the risk of corrosion is relatively small at the half-cell potentials observed. 2.2 Petrographic examination of cores Petrographic examination was conducted on one core from the the downstream wall, where geopolymer concrete was used, and one from the upstream wall, where blended slag cement was used. A summary of the examination is presented below. 2.2.1 Core C10/2036-2 (downstream wall) e coarse aggregate is of the basic igneous rock type of fine- to medium-grained dolerite, and shows considerable variation amongst the various particles. e groundmass surrounds much larger olivine phenocrysts, which have been oxidised to iddingsite and exhibit a reddish brown colour. Some of these show alteration to other secondary phases. None of the varieties appear to contain siliceous mineral phases that could be considered reactive in high alkali concrete. e fine aggregate is graded quartz sand, ranging in size from 2--0.05 mm. Some quartz grains showed very mild undulatory extinction features, which indicates that the sand is probably innocuous in concrete but may react at very high alkali contents. e cementitious matrix of the geopolymer concrete appeared to be very compact and included numerous angular grains, typical of slag cement. It exhibited isotropic features under cross-polarised light and is well bonded to the aggregate particles. Some areas of the matrix included a considerable number of round air bubbles, typically 0.3--0.5mm in diameter. Some other areas of the matrix showed a few fine microcracks which run between fine aggregate particles and sometimes join microcracks at the periphery of coarse aggregate particles. e presence of microcracks may indicate reduced strength of the concrete, although the microcracking is not significant. 2.2.2 Core C10/2036-5 (upstream wall) e coarse aggregate is broadly the same as is Core 2036-2, but the fine- grained variety was not seen in the section examined. e crystal grains in the aggregate particles were larger, with olivine being the largest crystal size, and finely-divided pyroxene scattered throughout the ground mass. e fine aggregate component is quartz sand, and similar to that in Core 2036-2. e cementitious matrix is compact and isotropic under cross-polarised light, but contains fewer slag grains than the matrix in Core 2036-2, which could indicate Table 3. Approximate geopolymer mix design used in Swan Street Bridge retaining walls. Figure 6. Example of positioning of reference electrodes. Material Mass (kg/m3) Total binder 400 (VR400/40) Coarse aggregate 20 mm 450-500 14 mm 450-500 Fine sand 700-750 Fly ash NIL Ground granulated blast furnace slag Close to 400 GP cement Small if any Sodium silicate solution (SiO2/Na2O=2) NIL Sodium hydroxide solution NIL Sodium meta-silicate anhydrous (solid activator) 5-10% of binder Super-plasticiser NIL Air entraining agent (0.3%) 0.5-1.5 Water 192-235 Water/binder ratio 0.52-0.57 Slump 100-200 mm