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Concrete In Australia : March 2012
Concrete in Australia Vol 38 No 1 43 was retained, thus indicating the integrity of the polyethylene curing. Tables 1, 2 and 3 show the test results for compressive strength, drying shrinkage and volume of permeable void (VPV/permeability) respectively. Table 1 shows that on the average, with the exception of the initial test panel, the minimum required 28 day compressive strength of 55 MPa was on the borderline of acceptability. Although it is considered that with more experience, further refinement of the mix design and manipulation of the alkaline activator dosage rates the strengths should be readily achieved on a consistent basis. e results in Table 3 indicate that the maximum limitations of drying shrinkage of 750 microstrain can be achieved. Table 3 however, indicates that the geopolymer concrete used in the precast footway panels was not able to achieve the VPV requirements of Section 610 for an equivalent 55 MPa concrete. e table indicates that the VPV results ranged between 19.5% and 21.7% for both cylinders and concrete cores far exceeding the maximum allowable limits of 12% for rodded cylinders and 14% for concrete cores for an equivalent concrete grade of VR470/55 as specified in Section 610. 4.1.1 Geopolymer precast footway panels -- Observations e general observations relating to the geopolymer footway panels trial can be summarised as follows: • As indicated in Table 1, strength development was initially an issue and as such acceleration of strength development was required due to the geopolymer mix design used and the urgency with which the mix design was developed at the time. • e casting bed was heated to a temperature range of between 18 °C and 35 °C with an even heat distribution following some initial problems. It was considered that the requirement for bed heating may be eliminated with improvements in raw materials and the geopolymer mix design. • e overall slump retention, discharge, kibble transfer and placement and consolidation under vibration were considered satisfactory. • Finishing was found to be somewhat difficult with coarse aggregate difficult to get down from the surface and paste was brought to the surface using an expanded mesh roller which was also forcing down the coarse aggregate. e mix was found to be stickier than conventional concrete and as such water spray was applied on the surface to facilitate finishing due to stiffness of the mix. Generally longer setting times were indicative of higher water content. Optimal finishing includes screeding then waiting as long as possible before final finish. Stipple finish was found to work better than broomed and was selected for finish of the panels. • With respect to curing, the precast practice to cover in polyethylene plastic was found to be sufficient. • e units stripped well and lifting was achieved within normal times of 16-20 hours, with lifting strengths in the order of 15 MPa to 20 MPa. 4.2 Insitu geopolymer concrete landscape retaining walls Construction of the insitu geopolymer concrete landscape retaining walls was undertaken utilising conventional techniques for formwork construction, concrete placement by pumping, compaction with a poker vibrator, finishing and curing with polyethylene plastic (Figure 4). e quality of the landscape retaining walls ranged from Figure 4. Construction of geopolymer concrete retaining walls using conventional techniques. Figure 5. Surface finish of geopolymer concrete retaining walls.