Single Fibre Pull-Out Creep Tests
The results of the single fibre pull-out tests are shown in Fig. 6 with (a) the individual results and (b), the average responses. The results are further analysed by
Fig. 6 The individual results of the single fibre pull-out creep tests (a), and (b), the average responses
Fig. 7 The total pull-out creep at 240 days plotted against the load level
plotting the total pull-out creep against the applied load level in Fig. 7. It is clear that the pull-out creep is linear to at least 50 % of the load level and then increases non-linearly with an increase of the load level. This indicates that an additional mechanism causing this time-dependant pull-out is possibly activated at loads higher than 50 %.
To investigate this additional mechanism, micro CT scans were taken of the samples at different load levels after 240 days. The focus area was the matrix around the hooked-end of the fibre and the micro CT images are shown in Fig. 8. It is clear from the images that at both 50 and 85 % of the load a gap formed above the hooked-end which is the effect of the pull-out creep. No micro-cracking occurred at the 50 % load, therefore further supported the previously mentioned postulation that the pull-out creep is formed by some form of ITZ crushing and/or
Fig. 8 Micro CT images of a 50 and 85 % specimen after 240 days. The gaps above the hooked-end is indicated by “F” and the micro-cracking is indicated by “G” collapsing. At the 85 % load level micro-cracking is clearly visible, therefore identifying a second mechanism which causes the fibre pull-out creep. It should be noted that this behaviour is probably not only dependent on the load level, but also on the matrix properties, e.g. w/b ratios, curing etc.