## 2 High-Performance Fiber Reinforced Concrete to Repair RC Components  
### Concrete Mix Design  
The repair strategy involves restoring the removed concrete parts using a high-performance fiber reinforced concrete (HPFRC) jacket with steel or polymer fibers.  
The high-performance fiber reinforced concrete with steel fiber was labelled by the abbreviation UHS, whereas the high-performance fiber reinforced concrete with polymer fiber was labelled by the abbreviation UHP. Due to their durability, polymer fibers are interesting for structural use because they can guarantee their full characteristics during the entire structure lifespan, even if exposed to sun, wind, and rain.  
Different steel or polymer fibers volume fractions were studied to determine the optimal concrete mix for structural use in the repair of RC components. The high-performance concrete (HPC) premix consisted mainly of Portland cement, silica fume, crushed quartz, and sand. Similar to ultra-high-performance concrete (UHPC), if a coarse aggregate is used, the interfacial transition zone (ITZ) between the coarse aggregate and mortar is weak, which decreases the strength of the concrete. Therefore, coarse aggregate was not used in HPC to ensure that the HPC would have high compressive strength [47]. The concrete mix design included: 1 part ordinary Portland cement (P.O.) 42.5, 0.3 parts silica fume (30%, particle size 0.1-0.2 \(\upmu\)m, density 2.0 g/cm\({}^{3}\)), 1.2 parts sand from the Minjiang River (dimension of smaller particle equal to 0.63 mm and density of 2.6 g/cm\({}^{3}\)), 0.025 parts superplasticizers (polycarboxylate, PCE; reduction water up to 25%), and water. The water/cement ratio was 0.26 due to the use of the superplasticizers properly calibrated as a function of the fiber type and the fibers volume content to ensure the performance of the concrete (such as the ability to pass), similar to the performance a self-compacting concrete (SCC) in the fresh state.  
Three steel fiber volume fractions (1, 2, or 3%) were investigated to select the proper UHS to repair the column. The steel fibers were 13 mm long, had an equivalent diameter of 0.2 mm, ultimate tensile strength of 2000 MPa, and a modulus of elasticity of 200 GPa.  
After the study on the concrete with steel fibers, two concretes with polymer fiber volume fractions of 2% and 4% were studied to select the optimal UHP to repair the column. The 2% value was selected to compare the results of concrete with steel and polymeric fiber, whereas the 4% is the maximum value that ensures the workability and cast of the concrete in modest volumes.  
The polymer fibers were structural fibers iBETON produced by Istrice (Fili & Forme Srl, Modena, Italy) according the Italian National Unification association (UNI) standards UNI EN 14889-2 [48], designed to improve the durability and mechanical properties of concrete. The fibers high-volume fraction of 4% had not been previously investigated for structural use. These fibers were composed of polymer material with high density and resistance, 29 mm long, had an equivalent diameter of 0.77 mm, ultimate tensile strength of 520 MPa, and modulus of elasticity of 4.1 GPa.  
The concretes had a mix design that guaranteed optimal placement and consolidation under their weight without any vibration operation, excellent deformability, and were cohesive and able to avoid segregation or bleeding.