Bonding strength and interfacial properties between substrates of proposed underwater concrete technology subjected to casting interval variations

The casting interval between old and new substrates of self-protecting underwater concrete (SPUC) plays a critical role in determining interfacial bonding strength, particularly in marine and hydraulic structures where corrosion is a major concern. This study evaluates the shear strength and interfacial interactions between SPUC layers. Experimental work included shear strength testing under different parameters, such as underwater protecting agent (UPA) dosage, superplasticizer content, water-to-cement ratio, and fine aggregate proportion. Microstructural characteristics of SPUC, cured in both air and underwater conditions, were analyzed using backscattered electron (BSE) imaging and X-ray diffraction (XRD). Results show that casting interval strongly influences failure mode, with short intervals (≤0.5 h) achieving up to 84% of the air-cured shear strength, while intervals of 10–24 h caused a sharp strength reduction. Lower UPA dosages notably improved interfacial strength, and surface roughness increased significantly with shorter intervals. Further, increased flowability reduced the bond strength, whereas incorporating 40% coarse aggregate (1–2 cm) significantly enhanced bond cohesion. Lastly, the optimum mix proportions were identified for various intervals, providing practical guidance for enhancing underwater bonding performance. • The casting interval significantly influences the shearing strength of SPUC. • Surface roughness increases proportionally as the casting interval decreases. • Self-protecting underwater concrete (SPUC) exhibits superior performance compared to traditional underwater concretes. • SPUC delivers the same advantages as self-compacting concrete (SCC), but in underwater applications.