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Concrete In Australia : June 2014
Concrete in Australia Vol 40 No 2 39 2.2 Instrumentation and testing Axial deformations of the specimens were measured with four linear variable displacement transducers (LVDTs), which were mounted at the corners between the loading and supporting steel plates of the test machine. These deflection readings were used to calculate average axial strains along the heights of the specimens. In addition to the four full height LVDTs, a surface mounted LVDT cage was attached to the mid- section of each 152 mm specimen to monitor mid-height deflections of these specimens. The LVDT cage had an LVDT installed on each of its four sides and it was designed to be mounted directly on the FRP shell via surface screws. The mid-height LVDT cage had a gauge length of 175 mm and it was placed at equal distance from each end. In addition to the LVDTs, specimens were instrumented with a large number of unidirectional strain gauges to monitor strain development on the FRP shell. These strain gauges had gauge lengths of 5 mm to 20 mm and were aligned in the axial, hoop or fibre orientation direction. Examples of the instrumentation used to monitor the axial behaviour are shown in Figure 3. The specimens were tested under monotonic axial compression, using a 5000kN capacity universal testing machine. To ensure an even loading surface, each specimen end was either ground flat using a precision grinding machine or levelled with a thin layer of capping material before testing. For the majority of the specimens, the load was applied directly to the concrete core through the use of precision cut steel discs 15 mm thick and 150 mm in diameter, thereby avoiding the loading of the FRP shell in axial compression. A group of specimens were tested without the steel discs to examine the effect of loading both the concrete core and FRP shell in axial compression. The test setup and instrumentation are shown in Figure 4. 3.0 TEST RESULTS AND DISCUSSION Important findings from the experimental program are presented and discussed in this section. Initially a summary of specimen failure modes is presented followed by discussion on the influence of investigated parameters on the compressive behaviour of FRP-confined concrete columns. 3.1 Failure modes of FRP-confined concrete columns Typical failures of FRP-confined concrete specimens are shown in Figure 5 where it is evident that in all specimens, failure resulted from a rupture of the FRP tube. For specimens with fibres aligned in the hoop direction, FRP rupture was typically vertical and resulted in an instantaneous loss of applied load. The observed FRP rupture for specimens with height-to-diameter (H/D) ratio of 1 or 2 was characterised by either continuous rupture of the FRP tube from top to bottom or localised segmented rupture. Examples of these two failure modes can be seen in Figure 5(a) and (b) respectively. Specimens with higher height-to-diameter ratios (ie H/D = 3 or 5) were not observed to fail with a continuous rupture and as such displayed only localised segmented rupture, as displayed in Figure 5(c). Specimens manufactured with inclined fibres were found to have a tendency to exhibit gradual ductile failure as the fibre orientation was increased Figure 2: Manufacturing process for FRP-confined specimens: (a) impregnating carbon fibre sheet with epoxy resin; (b) wet lay-up on styrofoam mould; (c) AFRP tubes prior to concrete pour; (d) concrete filled AFRP tubes. (a) (b) (c) (d) Figure 3: Specimen instrumentation: (a) application of strain gauges; (b) example of strain gauges attached to FRP surface; (c) mid-height LVDT cage. (a) (b) (c) CIA.indb 39 CIA.indb 39 20/05/14 12:40 PM 20/05/14 12:40 PM