Effect of superplasticizer addition and aggregate pre‐saturation on the microstructure and micromechanical properties of recycled aggregate concrete

Abstract

Three concrete mixtures were prepared to investigate the effects of recycled aggregate pre‐saturation and superplasticizer addition on the microstructure, micromechanical properties, and mechanical performance of recycled concrete (RC): natural aggregate concrete (NAC) as reference, RC with recycled aggregates pre‐saturated for 24 h (RC‐SSD), and RC with oven‐dried recycled aggregates and additional superplasticizer (RC‐SP). After curing for 91 days, the compressive strength and dynamic elastic modulus were measured. Microstructural observations were conducted using scanning electron microscopy in backscattered electron mode. Threshold‐based image analysis was employed to quantify porosity distributions. Nanoindentation was used to evaluate indentation modulus (M) and hardness (H).

RC‐SSD exhibited higher interfacial transition zone (ITZ) porosity and lower M and H. In contrast, RC‐SP showed reduced porosity and improved M and H, approaching the levels of NAC. These improvements were reflected in the macroscopic properties, with RC‐SP achieving higher strength and modulus than RC‐SSD. The results demonstrate that superplasticizer addition improves RC performance by refining the ITZ and enhancing M and H, whereas RCA pre‐saturation adversely affects the interfacial region.