| Summary: | Compound concrete (CC) can be cast by mixing fresh concrete (FC) with coarsely crushed demolished concrete lumps (DCLs) that have been obtained from a demolished structure. DCLs are larger than recycled concrete aggregates and are treated as a whole concrete replacement instead of a coarse aggregate replacement. This distinction implies that CC uses less Portland cement in comparison to FC, which reduces the embodied carbon of this material. A significant challenge for the broader application of CC is the lower compressive strength and ductility stemming from the weak interface between the DCLs and surrounding FC. However, the use of CC in concrete-filled steel tubes (CFSTs) and concrete-filled stainless-steel tubes (CFSSTs) has the potential to mitigate this issue due to the lateral confinement provided by the steel tube. Furthermore, the use of CFSST members is particularly favourable in corrosive conditions such as marine environments due to the material properties of stainless-steel.
The environmental benefits of using CC coupled with the lack of comprehensive research motivated the development of this research project at the University of Nottingham. The experimental investigation initially evaluates the compressive behaviour and strengths of 96 CC test specimens, including both 150 mm-sided CC cubes and 150×300 mm CC cylinders. A novel design equation that predicts the compressive strength of CC is derived using the data generated in this study and the data obtained from six previous studies in the literature. The proposed design equation is shown to be more accurate than the existing compressive strength equation proposed in previous studies. The experimental investigation also studies the compressive behaviour of circular CFST members, square CFSST members and rectangular CFSST members. The axial capacity of the CFST and CFSST members are predicted using the European Code (EC4), American Specification (AISC360) and Chinese Code (DBJ13-51). The continuous strength method is also used herein as a modification to EC4, which provides a more accurate prediction of the design strength of stainless-steel by modelling the strain hardening effect. Finally, the applicability of the considered design codes is evaluated by comparing the predicted axial capacities to the experimental failure loads.
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