Recycled Concrete as a Replacement for Concrete-Filled Steel Tube Using Fly Ash and Hybrid Fibers

The replacement of traditional concrete with RC can effectively alleviate nonrenewable resource depletion, ecosystem damage, and environmental pollution caused by the large-scale use of concrete-filled steel tubes. However, the surface of construction waste aggregate is covered with old mortar. Its high water absorption and weak interfacial transition zone reduce the mechanical properties of RC and limit its application. In this study, the mechanical properties of RC were enhanced synergistically by using high-volume fly ash (FA) and fibers at both micro and macro levels. The physical filling and secondary hydration products of FA improved the interfacial bond between aggregate and cement matrix, and the three-dimensional anchoring effect of different fibers interwoven to form a mesh structure dispersed the internal stresses and retarded the plastic shrinkage. The mechanical property tests (single-factor and orthogonal tests) and multiple linear regression calculations showed that when the mass ratio of FA:polyvinyl alcohol fiber (PVA):kapok fiber (KF):flax fiber (FF) was 1:0.01:0:0:0.02, the mechanical properties of FA and hybrid fiber–reinforced recycled concrete (FFRC) was the best; the compressive strength was 31.7 MPa and the flexural strength was 11.1 MPa. The predicted values of the multiple linear-regression (MLR) model from orthogonal tests were in good agreement with the experimental values, and the model has high prediction accuracy and minimum residual standard deviation. In order to verify the practical application effect of RC, FFRC with excellent mechanical performance from the orthogonal test was filled into steel tubes for bidirectional eccentric tensile tests, and the tensile capacity demonstrated a 16.8% improvement compared with that of ordinary concrete–filled steel tubes. While ensuring improved mechanical properties, the chloride-penetration depth of FFRC was only 0.85 cm and flexural strength loss after exposure to high temperature was only 4.6%. Therefore, the durability performance of FFRC is better than that of ordinary concrete. Calculations indicated that the maximum carbon emissions reduction could reach 26.5% through the utilization of construction waste and FA. Overall, this work provides a proven strategy for the application of RC.