Structural response of thin-walled circular steel tubular columns filled with demolished concrete lumps and fresh concrete

A straightforward use of coarsely-crushed demolished concrete lumps (DCLs) mixing with fresh concrete (FC) has recently been suggested by the authors and co-investigators, with a desire to provide another option for recycling concrete waste. Due to their being free of strong, costly crushing, the DCLs termed herein are characterized by larger size than conventional recycled aggregates. Bearing in mind this salient feature, the proposed recycling approach is easy to follow: a pre-determined percentage of DCLs, along with FC of complementary amount, can be placed in layers into a hollow steel tube, thereby forming a new structural member named steel tubular column filled with DCLs and FC. Field practice has demonstrated constructability of the proposed columns, as well as their advantages of being low-impact and cost-effective. In this research, several aspects pertaining to the structural response of said columns under concentric and eccentric loadings are addressed. An experimental program was first carried out, which comprised 24 intermediately slender thin-walled circular steel tubular columns filled with DCLs and FC, accompanied by 12 reference columns filled with FC alone. Primary test variables were the replacement ratio of FC by DCLs, the eccentricity of loading, and the thickness of steel wall. It was observed that, despite a noticeable reduction on ultimate strength, the overall structural performance of the proposed columns was not dramatically compromised, even though as much as 40% of FC was replaced by DCLs. Subsequently, a three-dimensional finite-element model is developed, attempting to understand the effect of random spatial distribution of DCLs on the ultimate strength of the columns, with the aid of Monte Carlo simulation technique. Stochastic simulations indicate that heterogeneous locations of DCLs lead to variations in the ultimate resistance, but such effect is generally limited in a probabilistic sense. This implies that when designing the columns concerned, a homogenization-based estimate of the resulting compressive strength of concrete infill may be reasonable and adequate, irrespective of the locations of DCLs. To examine this, lastly, a code-based design discussion is made, involving comparisons between analytical predictions using design provisions and experimental results of 111 pertinent column tests. A unified, simplified and more robust recommendation for determining the axial and axial-flexural capacities of the proposed columns is accordingly suggested, relying upon a strength-reduction factor of 0.9.