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Concrete In Australia : June 2014
42 Concrete in Australia Vol 40 No 2 FEATURE: FIBRE REINFORCED POLYMER 3.2 .4 Influence of fibre orientation To examine the influence of fibre orientation, a group of CFFT specimens were manufactured using an automated filament winding technique with FRP tubes having 0.6 mm total nominal fibre thickness. These specimens were 100 mm in diameter and 200 mm in height, and they were manufactured using a HSC with 81 MPa test day concrete strength. As explained previously, CFFTs manufactured with inclined fibres can display significantly different failure modes compared to CFFTs with fibres aligned in the hoop direction. Figure 10 illustrates this observation by presenting example specimens with fibres aligned at 45, 60, 75 and 88 degrees with respect to the longitudinal axis. It can be seen in this figure that CFFTs with fibres oriented at low angles (ie 45 and 60 degrees) exhibit a gradual ductile failure with no catastrophic failure observed. CFFTs with fibres oriented at 75 or 88 degrees displayed a distinct rupture of the FRP shell accompanied by an instantaneous loss of applied load, similar to behaviour observed from CFFTs with fibres aligned in the hoop direction. It can be seen from the axial stress-strain relationships presented in Figure 11(a) that fibre orientation has a significant influence on axial stress-strain behaviour of CFFTs. CFFTs with fibres oriented at 45 and 60 degrees show no clear signs of catastrophic failure and as such, experienced large axial strains. However, these axial strains were only experienced after substantial strength loss. CFFTs with fibres oriented at 75 and 88 degrees displayed ascending stress-strain curves indicating highly ductile behaviour. As mentioned previously, CFFTs with inclined fibre orientation had strain gauges installed on the FRP tube at the inclined orientation. Figure 11(b) presents a comparison of axial stress-orientation strain relationships for CFFTs with inclined fibres. It can be seen in this comparison that substantial differences exist between the relationships of specimens with different fibre orientations, with only the CFFTs with hoop oriented fibres developing orientation strains close to the fibre ultimate tensile strain of 0.029 reported by the manufacturer. This observation indicates that fibres used for FRP- confinement of concrete are most effective when aligned in the hoop direction, with fibre efficiency reducing significantly with an increase in fibre alignment with respect to the hoop direction. Figure 10: Failure of CFFT specimens with inclined fibre orientation: (a) 45 degree fibre angle; (b) 60 degree fibre angle; (c) 75 degree fibre angle; (d) 88 degree fibre angle. (a) (b) (c) (d) Figure 9: Influence of specimen end condition on axial stress-strain behaviour of: (a) CFFT specimens; (b) FRP-wrapped specimens. (a) (b) 0 20 40 60 80 100 120 0 0.01 0.02 0.03 0.04 AxialStress(MPa) Axial Strain End Plates No End Plates 0 20 40 60 80 100 120 0 0.01 0.02 0.03 0.04 AxialStress(MPa) Axial Strain End Plates No End Plates CIA.indb 42 CIA.indb 42 20/05/14 12:40 PM 20/05/14 12:40 PM