Optimizing Steel Fiber and Nano-CaCO3 in Recycled Aggregate Concrete

Abstract

The use of recycled aggregate can reduce dependence on natural stone, while steel fibres may improve the mechanical performance of concrete containing recycled materials. This study evaluated the workability, compressive strength, and splitting tensile strength of concrete incorporating recycled stone aggregate, nano-calcium carbonate, and steel fibres. Concrete mixtures were prepared using a cement:fine aggregate:coarse aggregate proportion of 1:1.90:2.50 and a water-to-binder ratio of 0.46. Nano-CaCO₃ was added at 4% by weight of cement, and 30 mm steel fibres were incorporated at volume fractions of 0%, 0.45%, 0.90%, 1.35%, and 1.80%. Slump was measured in the fresh state, while cube and cylinder specimens were tested after 7 and 28 days of curing. The slump decreased progressively from 64 mm in the reference mixture to 37 mm at the highest fibre dosage, indicating reduced workability with increasing fibre content. The fibre-containing mixtures exhibited higher mean compressive and splitting tensile strengths than the reference mixture at both curing ages. At 28 days, the maximum reported increases were approximately 4.85% for compressive strength and 23.25% for splitting tensile strength. The highest measured strength values occurred at the 1.80% fibre dosage, although the compressive-strength difference between the 1.35% and 1.80% mixtures was small. The findings indicate that steel-fibre incorporation can improve the measured mechanical performance of recycled-aggregate concrete, particularly its splitting tensile strength, while reducing workability.