Synergistic Effects of Pre-wetted Recycled Sand as an Internal Curing Agent on Concrete Performance: The Role of Replacement Ratio and Saturation Degree

This study investigates the synergistic effects of recycled sand (RS) replacement ratio and pre-wetting degree on low water-to-binder ratio concrete. Experiments characterized the moisture absorption-desorption behavior of RS and measured the workability, compressive strength, internal relative humidity, autogenous shrinkage, and drying shrinkage of twelve concrete mixes. Results indicate that finer RS particles exhibit higher absorption-desorption capacity. Increasing the replacement ratio impaired workability, compressive strength, and shrinkage resistance, while a higher pre-wetting degree improved workability retention, enhanced compressive strength, and significantly reduced both autogenous and drying shrinkage. A synergistic optimum was achieved at 50% replacement with 100% pre-wetting, minimizing strength loss while effectively mitigating shrinkage. These improvements are attributed to the internal curing effect of pre-wetted RS, which sustains moisture release, delays internal humidity reduction, and promotes continued hydration. The findings support the use of pre-wetted RS as an efficient internal curing material in sustainable concrete.