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Concrete In Australia : June 2014
Concrete in Australia Vol 40 No 2 41 0 10 20 30 40 50 60 70 80 0 0.0025 0.005 0.0075 0.01 AxialStress(MPa) Axial Strain CFFT FRP-wrapped ultimate stress and ultimate strain compared to companion specimens. The higher performance of CFFTs with a H/D of 1 can be attributed to the end plate confining effect. It is well established that the friction at the loading surface supplies additional confining pressures (Rousakis, 2001; Tamuzs et al, 2006). These additional confining pressures are localised to regions close to the specimen ends and this effect reduces substantially as the distance from specimen end increases. A comparison of CFFTs with H/D = 2 to 5 in Figure 8 reveals similar ultimate axial stress performance. On the other hand, it can be seen that H/D ratio has a significant effect on the axial strain performance, with an increase in H/D ratio resulting in a reduction in ultimate axial strains. 3.2 .3 Influence of specimen end condition A comparison of axial stress-strain relationships for specimens with and without end plates can be seen in Figure 9, where CFFT and FRP-wrapped specimens are presented separately. All specimens were 152 mm in diameter and 305 mm in height, had concrete compressive strengths of 49 MPa and were confined by Type 2 aramid fibres with a total nominal fibre thickness of 0.6 mm. It is evident from the similar compressive behaviours of the specimens shown in Figure 9 that loading of the FRP jacket during testing does not influence the behaviour noticeably. However, upon close inspection it can be seen that the inclusion of end plates slightly increases the ultimate axial strain and decreases peak stress. As can be seen in Figure 9, this effect is most noticeable in CFFT specimens. Figure 7: CFFT specimens prepared with H/D ratios of 1, 2, 3 and 5. (b) Figure 6: Effect of confinement method on axial compressive behaviour: (a) NSC confined by 2 layers of CFRP; (b) HSC confined by 2 layers of CFRP. 0 10 20 30 40 50 60 70 0 0.005 0.01 0.015 0.02 AxialStress(MPa) Axial Strain CFFT FRP-wrapped Figure 8: Effect of specimen slenderness on axial compressive behaviour: (a) NSC CFFTs; (b) HSC CFFTs. 0 20 40 60 80 100 120 0 0.01 0.02 0.03 AxialStress(MPa) Axial Strain NSC tf= 0.4 mm H/D=2 H/D=5 H/D=1 0 25 50 75 100 125 150 175 200 0 0.01 0.02 0.03 AxialStress(MPa) Axial Strain HSC tf= 1.2 mm H/D=2 H/D=5 H/D=1 H/D=3 (a) (b) (a) CIA.indb 41 CIA.indb 41 20/05/14 12:40 PM 20/05/14 12:40 PM