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Concrete In Australia : June 2014
40 Concrete in Australia Vol 40 No 2 FEATURE: FIBRE REINFORCED POLYMER relative to the hoop direction. These specimens experience a slow fracture of the FRP shell as individual fibres progressively ruptured and caused tearing and splintering throughout the FRP tube, as seen in Figure 5(d). 3.2 Axial compressive behaviour of FRP-confined concrete columns 3.2 .1 Influence of confinement method To examine the influence of confinement method on axial behaviour, specimens with 152 mm diameter and 305 mm height were prepared as either CFFTs or FRP-wrapped specimens. These specimens were manufactured as either NSC or HSC, with test day concrete strengths of 35 and 65 MPa respectively, and they were confined by CFRP tubes having 0.234 mm nominal fibre thickness. Figure 6 presents a comparison of the axial stress-strain behaviour of specimens manufactured by different confinement methods. It can be seen in this figure that both NSC and HSC can exhibit highly ductile behaviour when confined by either FRP tubes or wraps. A comparison of these graphs reveals similar axial performance with only minor visible differences between CFFT and FRP-wrapped specimens with otherwise identical parameters. This observed similarity in axial behaviour of CFFTs and FRP-wrapped specimens was also reported in Vincent and Ozbakkaloglu (2013a) where a large database and in-depth assessment are presented. 3.2 .2 Influence of specimen slenderness The influence of specimen slenderness on axial compressive behaviour was examined by preparing 152 mm diameter CFFTs with H/D of 1, 2, 3 and 5, as shown in Figure 7. Both NSC and HSC mixes were used, with average test day compressive strengths of 55 and 115 MPa, respectively. All CFFTs were manufactured with Type 1 aramid FRP (AFRP), with material properties supplied in Table 1. NSC and HSC specimens were confined by FRP tubes having a total nominal fibre thicknesses of 0.4 mm and 1.2 mm respecitvely. Examples of the axial stress-strain behaviour of the NSC and HSC specimens can be seen in Figures 8a and 8b respectively. It can be seen in Figure 8 that H/D ratio has a significant influence on axial stress-strain performance for both NSC and HSC specimens. This comparison indicates that CFFTs manufactured with a H/D ratio of 1 display significantly higher (d) Figure 4: Specimen test setup: (a) specimen before testing; (b) technical illustration. (a) (b) (a) (b) (c) Figure 5: Example failure modes of FRP-confined concrete specimens: (a) full height rupture; (b) mid-section rupture; (c) top section rupture; (d) progressive failure of inclined fibre specimen. CIA.indb 40 CIA.indb 40 20/05/14 12:40 PM 20/05/14 12:40 PM