## 8 Conclusions  
A rapid repair strategy was presented and applied, on a 1:6 scale, to severely damaged RC columns of bridge pairs. The strategy included the removal of the concrete and the steel transverse reinforcement in the plastic hinge region to expose all the longitudinal rebar, the use of a turned rebar segment to substitute the damaged part of the longitudinal rebar, and the building of a concrete jacket using HPFRC with steel or polymer fibers to restore the concrete parts and to improve the seismic performance of the column (ductility and shear strength). For the first time, a HPFRC with polymer fiber was designed using a high volume fiber fraction (4%), which was investigated for structural use. This material presented good mechanical characteristics. The material ductility was at least comparable with that of HPFRC with steel fibers, with a modest reduction in the strength for compression, but was still acceptable for structural use given the base high strength of the concrete matrix. Good strength in tension and very good ductility in tension and compression were observed. This material can be a valid alternative to the use of HPFRC with steel fibers because the polymer fibers ensured greater durability, lower weight, and cost savings. Further experimental studies are necessary to better investigate the reduction of the compressive strength observed for concrete with polymer fibers probably due to: (1) The lower Young's modulus of the polymer compared to that of the matrix; and (2) the earlier cracking related to the interfacial transition zone (ITZ) effect, and to the hydrophobic nature of polymer fibers that could produce a void between the fiber and the concrete matrix. Cyclic tests were carried out on two RC columns (the ASB columns) that were severely damaged and were repaired following the proposed strategy using HPFRC with steel (2%) or polymer fibers (4%). The results showed that the repair strategy can be applied with success in a short time (four-to-five-days) on the RC columns. The use of the HPFRC jacket permitted considerable cost and time savings because new transverse steel and external FRP reinforcement were not necessary. The shear strength of columns can be predicted correctly by the code equations proposed by CNR DT 204/2006 [55], and additionally for hollow circular sections by using a proper shear area. The shear strength of the concrete jacket can ensure adequate shear strength due to the steel or the polymeric fiber contributions. The shear strength of the original core, which is usually not repaired because the repair is difficult, provides a contribution. However, this contribution is uncertain and further research efforts are necessary. For that reason, neglecting the uncertain contributions of the column core can improve the safety of the prediction. The reduction in the diameter of the new rebar segments by turning guarantee the proper plastic deformation along the plastic hinge region only, without rebar ruptures, which also occurred under high displacement ductility (>6); and the reduction in the stresses on the original anchorages, which are used to anchor the new rebar segments. The length of the plastic hinge was smaller than the theoretical length because the rigid rotations, due to the strain penetration, moved away from the anchorages and were reduced by the turning rebar. The connection among the new segments and the existing undamaged rebar parts in the columns, realized by a simple steel coupler and butt head and lateral welding joints, was efficient because the connections were intact at the end of the tests. The repair strategy restored the strength, secant, initial tangent stiffness, and ductility of the column. The stiffness recovery was obtained without performing resin injection in the cracked concrete core, but was probably due to the reduction in the base rotation due to the strain penetration. The jacket with steel (2%) or polymer fibers with a high fiber fraction (4%) showed the same recovery and improvement capacity.