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Concrete In Australia : September 2013
Concrete in Australia Vol 39 No 3 55 8.0 MONITORING OF GEOPOLYMER CONCRETE RETAINING WALLS In order to obtain a greater understanding of the practical potential of geopolymer concrete, in 2009 VicRoads undertook a small number of trials including the in-situ construction of two landscape retaining walls at a bridge over the Yarra River (Figure 17) (10, 11). Construction of the in-situ geopolymer concrete landscape retaining walls was undertaken utilising conventional techniques for formwork construction, concrete placement by pumping, compaction with a poker vibrator, and finishing and curing with polyethylene plastic. In order to monitor the long-term performance of the geopolymer concrete and enable monitoring of the corrosion state of the reinforcing steel, three MnO2 half-cell reference electrodes were installed at the centre of each of the in-situ walls adjacent to the steel reinforcement at three different levels along the height of the wall (Figure 17). Initial measurement of the potentials of the steel reinforcement against the reference electrodes commenced a few weeks after construction in 2009, and subsequently monitored on a regular basis. e initial half-cell potentials readings after the hardening of the geopolymer concrete were very negative, namely in the order of -600 to -800 mV for upstream wall and about -1000 mV for the downstream wall, reflecting the initial quality of the two walls. e half-cell potential of the steel in concrete, however, appeared to be stabilising over the following six months after construction with the potentials having shifted to more positive values by about 200 mV, as shown by the results of the monitoring system incorporated in the walls (Figure 18). Further measurements on the embedded reference electrodes in 2011/2012 showed that the half-cell potentials of both wing walls have become more positive since the 2010 measurements (Figure 18), and are stabilising between -350 mV and -250 mV (CSE) which based on conventional criteria, it is unlikely that corrosion of the steel is taking place. ese values may become even more positive, at least in some areas of wing walls, indicating that the corrosion risk is not significant at present. is is in agreement with results of very low penetrability to chloride ions (ASTM C1202) and very low chloride diffusion coefficient determined using the NT Build 443 test method. It should be noted however, that the VPV (volume of permeable voids) values to AS 1012.21 did not comply with the criterion of a maximum value of 16% for structural concrete of VR400/40 grade as set out in Section 610 (5). Nevertheless, it is argued that the higher VPV is not due to larger interconnected pore volume, but due to additional loss of water from the gel- like materials included in the geopolymer. It is likely that an excess amount of sodium silicate (which releases water as part of the chemical reaction) was used in the geopolymer formulation, which was not fully assimilated into the geopolymer binder and caused the high VPV. It is considered that further refinement of the geopolymer concrete mix design with the use of compatible water reducers and superplasticisers to reduce the amount of water in the mix will significantly reduce the VPV of the geopolymer concrete. 9.0 CONCLUSION Corrosion monitoring sensors may be cast into new structures or installed in existing structures alone or as part of repairs, to provide the asset manager with real time information as to the current state and performance of the structure or remedial works. Monitoring sensors can provide early detection of initiation and/or propagation of corrosion and therefore facilitate early diagnostic assessment and Figure 17. Finished painted geopolymer concrete wall and installed reference electrodes. Figure 18. Monitoring of reinforcement potentials -- Bridge over Yarra River west retaining walls.