Behavior of slender square steel tubular columns filled with fresh concrete and demolished concrete lumps

Adding large-size demolished concrete lumps (DCLs) into the core of concrete-filled steel tubes (CFTs), a proposal by the authors and co-workers, has been demonstrated to be a practicable alternative means for old concrete recycling. This paper reports preliminary results of an experimental campaign newly implemented concerning the structural behavior of slender square steel tubular columns filled with DCLs and fresh concrete (FC). The replacement ratio of FC by DCLs and the load eccentricity were chosen as the two primary variables. The presented research focuses on the evaluation of the ultimate strength of the aforementioned columns. Design provisions in current codes of practice specified for conventional CFTs were used to check their applicability to the columns studied. The experimental outcomes indicate that: (1) the encasement by the outer square steel tube and the inherent cohesion between DCLs and FC ensured the integrity of inner concrete, thus the failure mode of the columns filled with FC and DCLs was similar to their counterparts filled with FC alone; (2) the initial stiffness, peak load, and post-peak response of the specimens appeared to be influenced only to a rather limited extent by the addition of DCLs; (3) the ultimate strengths of the concentrically and the eccentrically loaded specimens incorporating DCLs can be satisfactorily estimated by the design code Eurocode 4: 2004 and GB 50936-2014, respectively.