Synergistic effects of fly ash and hooked steel fibers on strength and durability properties of high strength recycled aggregate concrete

To deal with the rising environmental issues, more attention is now being given to recycled-aggregate concrete to partly meet the demands of the construction industry. However, recycled concrete is inferior in mechanical and durability performance compared to its counterpart natural-aggregate concrete. In this study, an effective approach is presented to improve the strength and durability performance of recycled concrete using supplementary materials, namely fly ash and hooked steel fiber. Low calcium refined fly ash was used at 0%, 15%, and 30% by mass replacement of cement with three different doses of steel fiber (0%, 0.5% and 1%). Mixes were evaluated based on results of several strength (compressive strength, splitting-tensile strength, and flexural strength) and durability tests (water absorption, acid attack resistance, and chloride-ion penetration). The results showed that incorporating both steel fiber and fly ash in recycled concrete significantly improves its mechanical performance and the proposed concrete can be markedly upgraded. The strength testing revealed that a synergistic effect of 15% fly ash and 1% steel fiber upgraded the flexural, splitting-tensile, and compressive strength of recycled concrete by 73-78%, 39-50%, and 13-23%, respectively. Fly ash minimized the negative effects of both recycled coarse aggregates and steel fiber incorporation on water and chloride-ion permeability resistance of concrete. Additionally, both fly ash and steel fiber were remarkably useful in increasing the acid attack resistance of concrete.