Experimental study of size effect on strain rate sensitivity of concrete-filled steel tubes

In recent years, concrete-filled steel tubular (CFST) structures have been commonly used in large-scale engineering projects. In such structures, the members are often of considerable size, however, the experimental study primarily focused on small-scale specimens due to limitations in the loading facilities. The size effect on the static and dynamic properties of CFST remains unclear. This study systematically investigates the influence of size effects on the strain rate sensitivity of CFSTs through quasi-static and Split Hopkinson Pressure Bar (SHPB) dynamic tests. Plain concrete specimens and CFST specimens with diameters of 35 mm, 50 mm, and 70 mm are used to characterize their size effect. The results reveal that under quasi-static loading, both plain concrete and CFST exhibit a distinct size effect, with the compressive strength decreasing as the diameter of specimens increases from 35 mm to 70 mm. However, under high strain rate loading, plain concrete exhibits higher strength with larger specimen size. In contrast, the compressive strength of CFST specimens shows minimal size dependence under dynamic loading. This difference is attributed to the radial inertial effect, which significantly enhances the apparent strength of plain concrete under dynamic conditions but is less pronounced in CFST specimens due to the confinement provided by the steel tube. Furthermore, the study decouples the pure strain rate effect and the radial inertial effect of the concrete material. These findings provide more accurate material strength for designing CFST structures against extreme dynamic loads.