Experimental Evaluation of Interface Bonding between Fly Ash–Based Self-Protecting Underwater Concrete and Steel Bar

Understanding the interface strength between steel reinforcement bars and new antiwashout concretes is essential for assessing the effectiveness of rebar confinement, particularly in marine environment and hydraulic structures where corrosion is a significant concern. This study investigates the pullout strength and interactions of interfacial confinement between rebars and self-protecting underwater concrete (SPUC). SPUC provides the advantages of self-compacting concrete (SCC) when being used underwater because it adapts to surrounding water conditions at various depths. The research involved conducting bond, compression, and washout tests, along with a range of parameters such as underwater protecting agent (UPA) dosage, superplasticizers, water to cement content, fine aggregate proportions, and fly ash. Further, the microstructural characteristics of the concrete samples, both in air and underwater, were analyzed using scanning electron microscopy (SEM). The results demonstrated that SPUC achieved bond strength up to 80% of its air strength. Notably, lower UPA levels significantly improved the interfacial strength between SPUC and steel bars. Additionally, the inclusion of fly ash enhanced the interfacial confinement between concrete and rebar by as much as 30%, despite its adverse impact on compressive strength. Furthermore, the arrangement of coarse aggregate interlocking around the perimeter of the rebar was found to significantly enhance bond efficiency, with an optimal content of 50% of sand, yielding the optimum bond strength results.