Article Severely Damaged Reinforced Concrete Circular Columns Repaired by Turned Steel Rebar and High-Performance Concrete Jacketing with Steel or Polymer Fibers Junqing Xue 1, Davide Lavorato 2,*, Alessandro V. Bergami 2, Camillo Nuti 2, Bruno Briseghella 1, Giuseppe C. Marano 1, Tao Ji 1, Ivo Vanzi 3, Angelo M. Tarantino 4 and Silvia Santini 2 1 College of Civil Engineering, Fuzhou University, Fuzhou 350108, China; [email protected] (J.X.); [email protected] (B.B.); [email protected] (G.C.M.); [email protected] (T.J.) 2 Department of Architecture, Roma Tre University, 00153 Rome, Italy; [email protected] (A.V.B.); [email protected] (C.N.); [email protected] (S.S.) 3 Department of Engineering and Geology, University of Chieti and Pescara, 65127 Pescara, Italy; [email protected] 4 Department of Engineering, University of Modena and Reggio, 41125 Modena, Italy; [email protected] * Correspondence: [email protected] Received: 25 August 2018; Accepted: 10 September 2018; Published: 15 September 2018 Abstract: A new strategy that repairs severely damaged reinforced concrete (RC) columns after an earthquake is proposed as a simpler and quicker solution with respect to the strategies currently available in the literature. The external concrete parts are removed from the column surface along the whole plastic hinge region to uncover the steel reinforcement. The transverse steel is cut away, and each longitudinal rebar is locally substituted by steel rebar segments connected by welding connections to the original undamaged rebar pieces outside the intervention zone. The new rebar segments have a reduced diameter achieved by turning to ensure plastic deformation only in the plastic hinge, protecting the original rebar and the welding connections. The connection is specifically designed to be effective and simple, and is directly realized on column reinforcement. Finally, the removed concrete is restored by a jacket built with high-performance concrete with steel or polymer fibers. The use of concrete with high volume fraction of polymer fibers to repair the column is investigated for the first time in this paper. This concrete was characterized by compression and flexural tests in the laboratory and its mechanical characteristics were compared with those of the concrete with steel fibers, which are being increasingly used in construction. The repair strategy was applied to two RC columns (1:6 scaled bridge piers), tested by asymmetric cyclic tests. The results show that the column strength, stiffness, and ductility were restored, and the energy dissipation capacity improved. The experimental evidence was investigated by fiber models in OpenSees. Keywords: seismic behavior; RC structure; repair; rebar substitution; retrofitting; high performance fiber reinforced concrete; concrete jacket; plastic hinge; energy dissipation; hysteretic damping 1. Introduction In seismic regions such as Italy, the evaluation of the capacity and fragility of structural and infrastructural systems subjected to seismic events [1–9], and implementing proper survey programs Appl. Sci. 2018, 8, 1671; doi:10.3390/app8091671 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1671 2 of 33 to monitor the evolution of their capabilities [10,11] are important for preventing severe seismic damage being observed in recent seismic events [12] by means of retrofitting interventions [13–31]. Repair strategies to restore or improve the seismic capacity of reinforced concrete (RC) components, usually seriously damaged after a strong earthquake [32–44], may significantly reduce the economic, social, and environmental impacts with respect to reconstruction. Repairs should be performed simply and quickly on construction sites to assure that the strategy is appropriate also in case of bridges that are strategic for the emergency response. In the literature, the repair of RC components is usually performed in case of modest damage (local concrete spalling or modest deformation of the steel reinforcement) when the deformed rebar can be straightened, and the cover can be restored by simple mortar patches. Finally, an external fiber reinforced polymer (FRP) is applied to improve the shear strength and the ductility of the component [33]. However, serious damage, which includes high deformation of longitudinal rebar in tension or in compression, steel reinforcement rupture, and concrete core crashing in compression, may arise: (1) In the plastic hinge region of the RC components designed by capacity design criteria after a strong earthquake; (2) in the most stressed zones of the RC components not properly designed or built for the seismic action after a moderate earthquake; and (3) along the RC component when the corrosion of the steel reinforcement is severe. Damaged rebar substitution is problematic because anchoring new rebar in the existing foundation is difficult, and new rebar may congest the existing steel reinforcement and complicate the concrete pouring to restore the removed concrete parts. The substitution of the damaged rebar segments only in the plastic hinge using new rebar segments may minimize these problems. The new segments can be connected to the existing anchorages and to the rebar part outside the plastic hinge region, and the use of rebar segments can limit the congestion of the steel reinforcement. The protection of the rebar connections then become one of the most important design targets. A review of the literature provided few solutions for the local substitution of the broken rebar. Therefore, new research efforts are necessary. These research studies presented different solutions to connect new rebar to the existing rebar. The rebar substitution performed by Cheng et al. [35] used special dog-bone bars connected by welding to the existing rebar and guaranteed plastic deformation along the plastic hinge only. This dog bone was a special piece built in the laboratory, so the proposed connection with the original rebar can be problematic on actual construction sites. Lavorato et al. [36] successfully tested a local rebar substitution using stainless steel rebar to improve the durability of the intervention. A new rebar connection system using asymmetric lateral welding showed possible failure in the case of high levels of rebar deformation. Rodrigues et al. [37] locally substituted the damaged rebar parts with identical rebar segments connected by different connection systems by welding. This resulting solution was effective, but the column core was entirely removed to ensure adequate proper work space, and the use of steel segment rebar with the same size and mechanical characteristics of the existing rebar may not guarantee plasticization in the plastic hinge only—away from the original anchorages of the rebar—because the strength of the new steel segments may be statistically higher than that of the original rebar. Lavorato and colleagues [24,31] simplified Cheng 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 Lavorato et al. [31]; and (5) constructing a concrete jacket by means of Appl. Sci. 2018, 8, 1671 3 of 33 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 Lavorato 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 [38–41]. 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