## 1 Introduction
[PERSON] and colleagues [24; 31] simplified [PERSON] et al. 's dog-bone solution and locally substituted the rebar with turned rebar segments connected by a butt head and lateral welding chords along a steel coupler. This strong connection system ensured plastic dissipation in the plastic hinge region only and the protection of the existing rebar (anchorages and existing undamaged rebar part outside the plastic hinge). In the present paper, a new repair strategy for severely damaged RC components is proposed to simplify the solutions presented in the literature. The strategy includes: (1) The removal of the external concrete part around the column in the plastic hinge region to permit the local rebar substitution; (2) the cleaning of the residual concrete from the rebar surface; (3) shaping the surface of the concrete core to improve the interlock between existing and new concrete parts; (4) substitution of the damaged longitudinal rebar parts with new shaped rebar segments, tested successfully by [PERSON] et al. [31]; and (5) constructing a concrete jacket by means of high-performance fiber reinforced concrete (HPFRC)with steel or polymer fibers to restore the removed concrete parts and to ensure seismic retrofitting (column shear strength and ductility improvements). The rebar substitution was performed as outlined by [PERSON] and colleagues [24; 31], and because this process is simple, it is effective, rapid, and adaptable to each specific intervention, allowing cost savings. The novelty of the proposed repair strategy is the use of steel or polymer fibers in the concrete to restore the removed concrete parts. For that reason, the new concrete jacket does not change the component cross-section dimensions after the repair. The fibers improve the compressive and tensile strength of the concrete due to the fiber tensile strength in the concrete cracks (fiber bridging effect) and increases the component durability by reducing the shrinkage cracking, spalling, and thermal cracking [45]. The concrete jacket used to restore the concrete parts improves the shear strength and the ductility of the repaired RC component [38; 39; 40; 41]. In fact, the fibers additionally contribute to the shear strength of the RC component by bridging the cracks and confining the concrete to a distributed transversal reinforcement, improving the ductility of the repaired component. The transverse steel is not necessary and therefore the concrete cast is simplified. The external FRP wrapping is not necessary, so the cost and time required for intervention can be considerably reduced. The repair solution can be performed in four-to-five-days, and it is also applicable for quick emergency responses after a strong earthquake for structures such as bridges that can be critical in the net of structures and infrastructure [1; 2; 3; 4; 5; 6; 7; 8]. The remainder of this paper is organized as follows. First, the results of the tests performed at the Fuzhou University (China) laboratory on the HPFRC concrete with steel or polymer fibers, used to repair the RC component concrete parts, and considering different fiber volume fractions, are presented. This study evaluates the concrete workability in casting the new concrete part and the variation in the concrete mechanical characteristics for different types and fiber contents to select the proper fiber content (Section 2). In particular, compression (Section 2.2.1) and bending tests (Section 2.2.2) are carried out to determine the concrete compressive and the tensile strengths and the load crack tip opening displacement (CTOD) curves, respectively. Next, equations are presented to determine the tensile strength of the fiber concretes based on the bending tests in Section 2.3 and the design equations, used to evaluate the shear strength of the repaired circular columns, are discussed in Section 3. A bridge, with a configuration typical of highway bridges around the world, is selected for a case study presented in Section 4.