Size dependency and boundary effects on the mechanical properties of concrete filled steel tubes under impact load

With the increasing trend towards using concrete filled steel tubes (CFST) in civil structures, understanding their mechanical properties under impact loads has gained interest among researchers. Dynamic properties of concrete confined by steel tubes are size dependent. Insight into the effect of specimen size and end boundary conditions on experimental data is essential for accurate prediction of
concrete characteristics under high strain rate loading. This paper presents the results of an experimental study aimed at investigating the dynamic properties
of concrete filled steel tubes. Using servo-hydraulic testing machines a series of high strain rate tests is conducted on sub-size specimens, made from self-consolidating normal concrete, confined by mild steel tubes. To take into account the stress uniformity and confinement effects in the specimen, various
length to diameter ratios (L/D) and diameter to tube wall thickness ratios (D/t) are considered. Moreover, the boundary effect due to end friction is investigated through use of different surface roughness at the specimen-loading plate interfaces. The results are compared to two sets of reference tests, namely quasi-static loading tests and high strain rate tests on unconfined concrete specimen of the same size and same boundary conditions. The results indicate the influence of L/D ratio, D/t ratio, and end friction coefficient on stress-strain distribution, dynamic compressive properties and failure mode of sub-scale concrete filled steel tubes under impact load.