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Concrete In Australia : March 2012
46 Concrete in Australia Vol 38 No 1 colour in the interior and a light colour in the surface zone, due to oxidation in air (Figures 2 and 3). 6.0 RESULTS AND DISCUSSION 6.1 Half-cell potentials of reinforcing steel bars e reference was a manganese dioxide electrode in alkaline gel (Figure 6), which was installed adjacent to steel bars at nominated locations. e potential of each electrode was measured against a saturated calomel electrode (SCE). To compare the half-cell potentials with conventionally used criterion for steel corrosion, ie -350 mV copper-saturated copper sulphate electrode potential (CSE), the SCE values were converted to CSE. e monitoring of the steel reinforcement potentials within the retaining walls over the past two years is shown in Figure 7. Although in the initial period the potentials of both the upstream and downstream wall cells were highly electronegative, they have steadily shifted in a more positive direction and appear to have stabilised between -350 mV and -250 mV CSE which is a very positive sign in terms of long term passivity of the steel reinforcement within the geopolymer concrete. 6.2 Compressive strength of concrete e compressive strength tested on cores is shown in Table 4. e concrete of the downstream wall has a higher strength than that of the upstream wall. e more negative potentials for the downstream wall probably arise because the concrete is denser and slower to dry out. 6.3 VPV of concrete e VPV tested on cores is shown in Table 6. e VPV of downstream wall is better than that of the upstream wall. Note that 1% VPV separates two grades of concrete, eg minimum VPV is 16% for VR400/40 and 15% for VR450/50 (tested by cores). However, the VPV is too high considering the strength values obtained. is could have arisen due to the nature of the geopolymeric binder. e VPV requirements for different grades of structural concrete, stated in VicRoads specification Section 610, are yet to be validated with this type of binder. 6.4 Resistance to chloride penetration As it was originally expected that the two retaining walls would have the same concrete, only one core from the downstream wall was tested for chloride penetrability. Two test methods were used to assess the chloride penetrability, ie ASTM C1202 (Electrical indication of concrete ability to resist chloride ion penetration), and NT Build 443 Method. e results are presented in Table 7 and Table 8, respectively. e ASTM C1202 method ranks the concrete as having very low chloride penetrability. is may have resulted from the high slag content of the concrete, which would reduce the amount of ionic charges in the pore solution of the concrete, rather than from low porosity of concrete. As shown in Table 8, the NT Build 443 test method resulted in a very low chloride diffusion coefficient (D= 1.58 x10-13 m2/s). is result confirms the conclusion of ASTM C1202 test. e NT Build 443 test results and theoretical curve calculated using the diffusion parameters in Table 8 are presented in Figure 8. 7.0 INTRODUCTION OF GEOPOLYMER CONCRETE IN SECTION 703 (GENERAL CONCRETE PAVING) Geopolymer concrete has been specifically defined and incorporated into the November 2010 version of Section 703 for the construction of lower risk general paving works such as traffic islands, median slabs, bicycle paths, edgings, footpaths, shared paths, vehicle and pram crossings and other similar slabs or pathways on prepared bedding, as a direct equivalent product to Portland cement concrete. It is required to comply with the minimum 28 day compressive strength requirements for each strength grade ranging from 20 MPa to 32 MPa. Various requirements dealing with the constituent materials, manufacture, delivery, placement, compaction, finishing and curing of geopolymer concrete have been addressed. 8.0 SUMMARY e use of geopolymer concrete in major structural applications for items such as bridge beams, precast parapet barriers, piles, columns, crossheads, abutments and bridge decks to VicRoads Standard Specification Section 610 "Structural Concrete" needs to be investigated further and monitored carefully prior to venturing into full- Voltage (V) Initial Current (mA) Total Charge after 6 hours (C) Penetrability 60 31.7 648 Very low Duration of immersion (days) Initial Chloride Content (%) Chloride content at the boundary, Cs (%) Diffusion Coef cient (m2/s) 35 0.01 1.15 1.58 × 10-13 Table 7. Downstream retaining wall -- ASTM C1202 test results (chloride resistance) (Core testing after 7 months) 4. Table 8. Downstream retaining wall -- NT Build 443 test results (chloride diffusion) (Core testing after 7 months) 4.