Overcoming the brittleness of sustainable compression-cast recycled aggregate concrete through steel spiral confinement

The loose and porous adhered mortar of recycled aggregates produces concrete with low strength and poor durability. The newly developed compression casting technology applies pressure to fresh concrete to squeeze out excess water and air in the recycled aggregate concrete (RAC) and force the cement paste into the old adhered mortar to strengthen it, thereby effectively solving the aforementioned problems. However, the significant increase in the compactness and compressive strength of the RAC is accompanied by a pronounced increase in compression brittleness. Hence, this study attempts to overcome the brittleness of sustainable compression-cast RAC through proper design of transverse reinforcement. To achieve this objective, the axial compression behaviour of steel spiral-confined RAC specimens was experimentally investigated. It is found that the compression casting approach almost doubled the compressive strength of the unconfined RAC and improved the ultimate axial stress of the steel spiral-confined RAC by up to 69.9 %. The steel spiral-confined normal-cast RAC exhibited typical compression failure accompanied by a stress–strain curve characterised by strain hardening, whereas the steel spiral-confined compression-cast RAC exhibited localised shear failure accompanied by a stress–strain curve characterised by strain softening. The strain-softening behaviour of steel spiral-confined compression-cast RAC can be significantly improved by reducing the spiral pitch. Based on the test results obtained in this study and in the literature, stress–strain models were proposed separately for the steel spiral-confined compression-cast RAC and normal-cast RAC. The stress–strain models exhibited a significantly higher prediction accuracy than the selected models in the literature.