With the increasing trend towards using concrete-filled steel tubes (CFST) in civil structures, understanding their mechanical properties under impact loads has attracted the interest of researchers. The dynamic properties of concrete confined by steel tubes are sizedependent. An experimental program was carried out to investigate the relation between the size and impact response of CFST sub-samples. High-strain-rate tests were conducted on specimens made from self-consolidating normal concrete confined by mild steel tubes. To take into account the stress uniformity and confinement effects in the specimens, various height-to-diameter ratios (H/D) and diameter-to-tube-wall thickness ratios (D/t) were considered. Dynamic Increase Factors (DIFs) were derived as the ratio of the material strength at high strain rate to those of the same size under quasi-static loading conditions. The results were compared to two sets of reference tests, namely unconfined concrete and hollow steel tube specimens of the same size and with the same boundary conditions. The results indicate the influence of H/D ratio, D/t ratio, and end-friction coefficient on the stress-strain distribution, dynamic compressive properties and failure modes of sub-scale concrete-filled steel tubes under impact load. The size-dependent behaviour of the CFST is found to be a function of the level of confinement the circumferential steel tube imposes on the concrete filling. Two expressions are proposed for predicting the DIF of yield stress for CFSTs: one considering the concrete-steel interaction relationship presented in Eurocode 4, and an empirical expression based on the Cowper-Symonds model for steel. The proposed rate-and size-dependent expressions show close correlations with experimental results.
Concrete is recognized for being a fire-resistant construction material. At elevated temperatures concrete can, however, undergo considerable damage such as strength degradation, cracking, and explosive spalling. In recent decades, reuse of fire-damaged concrete structures by means of developing techniques to repair the degraded material has gained interest amongst researchers. Autogenic self-healing methods such as re-curing in water has proven to partly restore the strength of concrete. The extent of restoration is dependent upon various parameters such as concrete type, exposure temperature, and post-fire curing conditions for example. The use of selfcompacting/consolidating concrete (SCC) has become common in the construction industry due to its high workability and low permeability. This paper presents the results of an experimental study aimed at investigating the improved mechanical properties of high temperature exposed SCC concrete by the autogenic self-healing phenomenon resulting from water re-curing. The residual mechanical properties including strength, modulus of elasticity and ultimate strain of the material upon application of different post-fire curing regimes are presented herein with special emphasis on the effect of thermal profile including exposure time, temperature and cooling rate. The experimental results confirm that the recovery of material properties in fire-damaged SCC concrete is contingent on the post-fire water curing conditions.
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