r - The Hong Kong Polytechnic University

Figure 9 Prediction of axial compressive behaviours of CFT columns using Xiao’s confinement model (Choi
and Xiao 2010b)
RECENT RESEARCH PROGRESS ON IMPACT BEHAVIORS
The author and his research team are recently conducting experimental studies on the mechanical behaviour of
confined concrete under impact loading and have made some progress. Using a gas gun facility, conventional
CFT and the proposed CCFT specimens under high-speed impact were tested and the results validated the
improved impact behavior of confined CFT [Shan et al. 2007]. Using the split Hopkinson pressure bar (SHPB)
equipment, concrete filled tubes under high strain rate axial compression were studied [Xiao et al. 2009].
Through the analysis of the testing data, the author defined the dynamic coefficient of the transverse
confinement and found it was within the range of conventional values obtained from static tests. Most recently,
CFT and CCFT stub columns were tested using a large-capacity drop-weight testing facility developed by the
author and his team. Figure 8 shows the differences between the failure patterns of CFT and CCFT specimens
under drop-weight impact. As exhibited in Fig.10(a), the CFT specimen had apparent local buckling in the tubes,
and the core concrete infill crushing was severe. However, there was little sign of local buckling in the tube of
the counterpart CCFT specimen, and the core concrete had only a few countable thin cracks identified after the
removal of the steel tube.
(a)
(b)
Figure 10 Comparisons of failure patterns: (a) CFT specimen, (b) CCFT specimen
The improved impact behaviour of using transverse confinement to the CFT is also clearly demonstrated in
Fig.11 by comparing the residual strain and impact energy relationships between CFT and CCFT specimens. In
Fig.11, the vertical axis shows a performance index as the ratio of the residual strain and the yield strain,
whereas the horizontal axis shows the impact intensity index defined as the impact energy divided by the static
yield energy. As shown in Fig.9, the difference between residual strains of CFT and CCFT specimens with two
layers of CFRP at relatively lower impact energy is small, however, becomes significant when subjecting to
larger impact energy. It is also shown that the increase of transverse confinement by using more CFRP layers,
the residual strains can be drastically reduced.
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