Role of parent concrete characteristics and crushing technique on selectively demolished RCA quality

This study evaluates how parent concrete characteristics and crushing techniques influence the physical, microstructural, and mechanical properties of RCA. Four parent concretes from an end-of-life viaduct (beam, column, abutment-wall, and foundation) were characterized for cement paste microstructure and volumetric composition prior to recycling. Two crushing techniques were applied: a conventional high-energy Impact Crusher (IC) and the vibration-based Smart Liberator (SL). Feature importance analysis showed that RCA behaviour reflects a combined influence of processing technique and parent concrete. RCA produced from concrete with higher air-void content (≈4%) facilitated mortar detachment and exhibited the lowest water absorption (≈1.7%) and fastest saturation rate (∼3 days). Conversely, RCA derived from concrete with a dense, cohesive, and well-hydrated paste retained more attached cement paste, resulting in the highest water absorption (>7%) and slowest saturation (∼6 days). Despite exhibiting similar compressive strengths, these two parent concretes produced fundamentally different RCA morphologies and absorption behaviours, highlighting the governing role of W/C ratio–related properties, such as capillary porosity and hydration degree, together with paste volume and air-void content in determining RCA quality. The crushing technique also played an important role in defining coarse RCA morphology and surface texture. The IC produced angular particles with irregular surfaces and localized microcracking, especially in dense, low-porosity concretes. These stress-induced defects increased surface roughness and open porosity, raising water absorption and reducing abrasion resistance. In contrast, the SL applies selective attrition and shear along the aggregate–paste interface, removing attached mortar without fracturing the aggregate core and yielding smoother, denser particles with minimal internal damage.