Combined effects of recycled aggregate and fly ash towards concrete sustainability

The recycling of demolished concrete has been emerging as a sustainable solution to warrant the reduction of construction wastes, as well as to prevent the depletion of natural resources from growing construction demand. Nevertheless, some key factors that would affect the properties of recycled aggregate concrete have not been thoroughly investigated, such as the proportion of recycled aggregates, the moisture state of recycled aggregates, and the design compressive strength of concrete. In particular, little research was done on the combined effects of recycled aggregates and fly ash, popularly used as a partial substitution of cement. Given the concerns, this study investigates the effects of such factors on the mechanical and durability properties of recycled aggregate concrete. Eleven cases of concrete mixtures were tested for the rheological properties of fresh concrete, compressive strength, tensile strength, and chloride diffusion coefficient. In general, the higher ratio of recycled aggregates resulted in the better flowability of concrete. Also, the use of fly ash improved the flowability of recycled aggregate concrete. The strength test results showed that the higher ratio of recycled aggregates generally caused the lower compressive and tensile strengths of concrete. However, the cases with 30% recycled aggregates showed only slight compressive strength reductions. Similarly, the use of fly ash caused only small reductions in the compressive strength of recycled aggregate concrete. In contrast, the negative effects of recycled aggregates and fly ash were greater in the tensile strength than in the compressive strength. Lastly, the cases containing fly ash exhibited much higher resistance to chloride penetration, even in the cases with recycled aggregates.