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Concrete In Australia : June 2014
44 Concrete in Australia Vol 40 No 2 FEATURE: FIBRE REINFORCED POLYMER different methods becomes more significant with an increase in concrete strength. This outcome can be attributed to the increase in concrete brittleness associated with an increase in concrete strength, which alters the concrete crack patterns from heterogenic microcracks to localised macrocracks. This finding is in agreement with those previously reported in Ozbakkaloglu & Lim (2013) based on an investigation of an extensive test database. These observations indicate that the axial strain measurements of FRP-confined HSC are highly sensitive to the instrumentation arrangement and significant differences occur in the results obtained from different measurement methods. It is recommended, therefore, in the interpretation of the results of FRP-confined HSC specimens, due consideration be given to the influence of instrumentation methods. 4.0 CONCLUSIONS This paper has presented important findings from an ongoing investigation at The University of Adelaide to investigate the less understood column parameters on the axial compressive behaviour of FRP-confined NSC and HSC columns. Based on the results and discussion presented in this paper, the following conclusions can be drawn: • Specimens with a H/D ratio of 1 exhibit significantly higher ultimate axial stress and ultimate axial strain than companion specimens with H/D = 2 to 5 due to the beneficial influence of the end plate confinement effect. For specimens with a H/D ratio of 2 to 5, H/D ratio has no significant influence on ultimate axial stress; however, an increase in H/D ratio from 2 to 5 causes a decrease in ultimate axial strain. • Specimen end condition does not have a significant effect on axial stress-strain behaviour. It was observed that the inclusion of end plates slightly increased the ultimate strain and decreased peak stress, with this effect more noticeable in CFFTs than FRP-wrapped specimens. • The axial compressive behaviour of CFFTs is highly sensitive to the fibre orientation. Fibres used for FRP-confinement of concrete are most effective in enhancing the concrete compressive behaviour when aligned in the hoop direction. Fibre efficiency reduces significantly with an increase in fibre alignment with respect to the hoop direction. • An increase in FRP-to-concrete interface gap results in a slight decrease in ultimate axial stress, whereas it leads to a significant increase in ultimate axial strain. It was observed that the interface gap affects the ultimate axial stress and strain of NSC and HSC in a similar manner, suggesting that the influence is independent of concrete compressive strength. • In NSC specimens, axial strains determined from full height LVDTs and mid-height LVDTs are similar. However, axial strains obtained from strain gauges are much lower and they do not accurately capture the full height deformations of specimens. For HSC specimens, axial strains determined from full height LVDTs, mid-height LVDTs or strain gauges differ significantly from each other. This indicates HSC columns are highly sensitive to the instrumentation arrangement used in the measurement of these strains. Figure 14: Influence of axial strain measurement method on stress-strain curves of FRP-confined HSC. Figure 13: Influence of axial strain measurement method on stress-strain curves of FRP-confined NSC. CIA.indb 44 CIA.indb 44 20/05/14 12:40 PM 20/05/14 12:40 PM