## 1 Introduction  
The strengthening and retrofitting of existing aged buildings has been a research interest for decades and has received extensive attention from researchers all over the world [1, 2]. The enlarging cross-section method has the characteristics of a simple process, reliable force, and low cost of reinforcement. It is widely used to repair and strengthen structures such as beams [3] and columns [4, 5]. However, the enlarging cross-section method reinforcement was mostly made of denser ordinary concrete in existing column reinforcement projects. This results in a significant increase in the reinforced structure's self-weight, which would have a certain impact on the mechanical properties of the strengthened structure. All-light ceramsite concrete has lightweight and high-strength characteristics, making it 25-30% lower in weight compared to ordinary concrete of the same strength [6, 7]. The use of all-light ceramsite concrete as a material for strengthening engineered structures allows for a lower increase in the self-weight of the reinforced structure. In addition, all-light ceramsite concrete has better thermal insulation, heat insulation, sound insulation, fire resistance, seismic resistance, and no alkali-aggregate reaction, along with other advantages because of the special structure of shale ceramics particles [8, 9, 10]. [PERSON] and [PERSON] [11, 12] carried out experimental studies of encased ceramsite concrete outsourced H-beam and covered ceramsite concrete outsourced T-beam columns. The results showed that the encased ceramsite concrete has good bonding performance with the steel cross-sections. Beam and column bearing capacity, stiffness, and ductility are greatly improved. Thus, if the load-bearing performance of the all-light ceramsite-concrete-strengthened engineered structure could be guaranteed, the application of the enlarging cross-section method for strengthening engineered structures could be broadened. This would be of great significance in promoting the development of engineered structural reinforcement. The enlarged cross-section method of column strengthening is a combination of new-to-old concrete cross-section of columns. Therefore, better shear transfer between the new-to-old concrete is essential for the reinforced column to exert its own load-bearing capacity. Established studies have demonstrated that the synergy between new-to-old concrete depends mainly on the treatment and construction of the bonding surface [13]. When adding interfacial agents and increasing roughness are used at the interface of new-to-old concrete, they all significantly affect the bond between them [14, 15, 16]. For the former, the most common epoxy resin interfacial agent was used as an example, which could enhance interfacial adhesion by any fiber [17]. Existing findings pointed out that when epoxy-type interface agents are used for concrete of different ages and compositions, their tensile properties [18] and shear properties [19] show greater improvement than other interface agents on the bond surface of new-to-old concrete. In addition, epoxy-based interfacial agents can exert a sizeable cohesive force under different humidity and other environmental factors [20, 21]. For the latter, [PERSON] [22] found through an experiment that the bond strength between new-to-old concrete increased approximately linearly as the roughness of the interface grew. Furthermore, the ways to increase roughness (grooving [23, 24], hand-brushing [25], sand-blasting [26, 27], wrapping [28], drilling [29], and hand-chiseling [30]) also proved to affect the bond strength of the new-to-old concrete. With intensive research and extensive reinforcement works, post-installed rebar had become a new and effective method to improve the ultimate bearing capacity of new-to-old concrete specimens [31]. A reasonable and scientific setting of the location of the post-installed rebar would change the failure of the specimen interface from brittle failure without reinforcement to ductile failure.