## 2 Experimental Methodology
### Specimens' Preparation and Material Characterization  
The beam specimens were manufactured at the Laboratory of Structures at UPM. In order to ease the placement of the strengthening layer, the beams were manufactured upside down. The conventional concrete was supplied by a local contractor. For series RC-U35 and RC-U55, the thickness corresponding to the strengthening layer was left empty during concreting of the conventional concrete (i.e., no demolition works of the concrete cover were carried out). The upper surface of the RC layer was prepared with a slight treatment consisting of wire-brushing and water-pressure cleaning. Though a more exhaustive surface treatment (e.g., sand blasting) is typically recommended [26], it has been preferred to keep a less demanding interface closer to real conditions of structures strengthened on the bottom side. An additional method for application of bottom strengthening is by bonding a prefabricated laminate with epoxy adhesive [39; 40]. Before pouring the strengthening material, the surface was washed again and kept moist. Fabrication and pouring of the strengthening material was carried out 15 months after manufacturing the RC. The conventional concrete was designed for a characteristic strength of 30 MPa. The maximum aggregate size was 12 mm, the content of Portland cement I 42.5 R was 350 kg/m\({}^{3}\) and the water/cement ratio was 0.5. The average cylinder (150 mm \(\times\) 300 mm) compressive strength at 28 days was 38.1 MPa. At testing age, the average cylinder (150 \(\times\) 300 mm) compressive and indirect (Brazilian) tensile strength were 40.8 MPa and 2.3 MPa, respectively. The reinforcing steel bars had a characteristic yield and ultimate strength of 500 MPa and 575 MPa, respectively. The strengthening material consisted of a dry mortar premix of compact-reinforced concrete (CRC kindly supplied by Hi-Con A/S, Hjallerup, Denmark), which was mixed with water and fibers following the supplier's instructions. A 2% volumetric content of short steel fibers (\(l_{f}\) = 12.5 mm,  
Figure 2: Overview of an impact test at Technical University of Madrid, Spain (UPM). \(d_{f}=0.3\) mm, \(f_{y}=2950\) MPa) was used. The flow diameter in the slump flow test was larger than 550 mm, which allows classifying the mixture as self-compacting according to the French Association for Civil Engineering, AFGC [19]. The beams were tested 2 months after strengthening. The average cubic (150 mm) compressive strength of the strengthening material was 126.4 MPa and 135.4 MPa at 28 days and testing age, respectively. Flexural-tensile tests were carried out on un-notched and notched prismatic (\(100\times 100\times 500\) mm, with span length between supports of 420 mm in all cases) samples at the age of beam tests. Un-notched samples were tested in a 4-point bending scheme. The results are represented in Figure 3a in terms of load-midspan deflection and equivalent bending stress-midspan deflection diagrams. A detail of the pre-peak stage is also plotted. Very thin cracks at a separation of 12-16 mm were observed in the central part of the specimens before localization of a macro-crack under peak strength, thereby showing the strain-hardening stage. The equivalent bending strength was 14.1 MPa on average. Notched samples were tested in a 3-point bending configuration with a 5 mm depth notch at midspan. Load-crack width results are plotted in Figure 3b. The average tensile properties of the strengthening material were estimated from inverse analysis of flexural tests according to [PERSON] and colleagues [41] and the results are listed in Table 1. The free shrinkage strain was also measured in 2 cylinders of \(150\times 300\) mm, showing an average strain of \(-180\times 10^{-6}\) from manufacturing to testing age. Hereafter, the strengthening material will be referred to as high-performance fiber-reinforced concrete (HPFRC), as it has high compressive strength and tensile strain-hardening.