Experimental research on flexural behavior of existing reinforced concrete slab strengthened with post-installed H-shaped steel

Many aging concrete slabs in existing buildings may be faced with problems of reduced safety or inability to meet the present and future requirements, which are caused by factors such as material deterioration, being close to or beyond the design life, updated design requirements, poor maintenance, and changes in usage. Compared with demolishing existing slabs and reconstructing new slabs, adopting the strengthening measures can enhance the bearing capacity and safety of existing slabs, minimize the waste of resources, and save time. The current concept of sustainable development and a circular economy call for a fast and efficient method for strengthening the existing slabs. Steel-concrete composite (SCC) beams fully utilize the composite action of the steel beam and concrete slab, resulting in a significant improvement in the bearing capacity and stiffness of a structural member compared with those of non-composites. Inspired by such structural system, this study proposes a new method in which the existing concrete slab was strengthened with the post-installed H-shaped steel (ECSPH). Actually, the ECSPH is a two-time-processing composite member and its structural responses under loading such as failure mode, bearing capacity, stiffness, etc. may be different from the steel-cast-in-place concrete composite beams. At present, there are few studies concerning the behavior of ECSPH members, and the improvement degree in the bearing capacity of concrete slab with the use of such strengthening method is unknown. To fill this gap, four-point bending tests are conducted on 12 ECSPH specimens with variations in the connection method, bolt-planting depth, bolt space, and interfacial bonding material between the H-shaped steel and concrete slab. The structural responses of the ECSPH specimens, including the failure modes, load-deflection response, load-strain response, bearing capacity, stiffness, and ductility are measured and analyzed. The test results indicate that two types of bending failure and one type of shear failure occur in the ECSPH specimens. A significant slip occurs at the steel-concrete interface at the initial stage of loading for the ECSPH specimen with a through-bolt connection, which results in the inapplicability of the plane section assumption. In addition, considering safety stock and the difference in the composite action of concrete slab and H-shaped steel between the ECSPH specimens and the SCC beams with full shear connection, the bearing capacity results calculated using the plastic analysis methods provided in the design codes are reduced (the recommended reduction coefficient is 0.75), and a calculation method for bearing capacity of an ECSPH member is thus proposed.