Bond performance and mechanisms of sulphoaluminate cement-based UHPC for reinforcing old concrete substrate

Many essential reinforced concrete structures exposed to marine environments have experienced severe corro-sion since their construction due to their aggressive surroundings. Thus, one of the major problems facing reinforced concrete structures nowadays is their repair and reinforcement. Compared to conventional ordinary Portland cement (OPC) concrete, ultra-high performance concrete (UHPC) is commonly used for strengthening existing concrete stuctures due to its excellent mechanical properties and durability. However, UHPC made with Portland cement can not show high strength early, which may not meet the needs of some special projects that require to show early strength performance of the concrete. Since concrete with early strength performance has relatively little research in bonding porperties between new and old concrete, further research is need. Sul-phoaluminate cement (SAC)-based material is an effective repair material. However, the commonly-used SAC-based repair material has a relatively high water to cement ratio and large porosity, leading to poor performance and a higher risk of seawater penetration. This paper investigates the use of SAC-based UHPC with low water to cement ratio and porosity as a repair material for OPC concrete substrate, providing a reference for the rein-forcement of reinforced concrete structures that are exposed to the marine environment. Considering the shrinkage of the repair material, this paper uses cylinder specimens and prism samples to compare the bond performance. The bond performance between SAC-based UHPC and OPC substrate using common interfacial agents is studied, and bond strength testing and SEM analysis are carried out. The results indicate that SAC-based UHPC has beneficial mechanical properties, including compressive and flexural strength, at an early age, which makes it possible to use the SAC-based UHPC as repair material for concrete structure. In addition, bonded specimens with no bond agents at the interface show a better bond performance than that of samples with different interfacial agents, and SEM analysis of the interface transition zone verifies the results of the bond strength test.