## 2 Experimental Methodology  
### Test Configuration and Set-Up  
Drop-weight testing has shown to be an effective experimental configuration to investigate the response of concrete structures under impact at a laboratory [37]. In the present research, three series of beam specimens were tested with the drop-weight testing machine of the Laboratory of Structures at the Technical University of Madrid (UPM, Spain). The experimental campaign is intended to evaluate the possibilities of a high energy-absorbing strengthening layer for RC beams without stirrups (i.e., with a high tendency to present brittle shear failure, local damage, and fragmentation under impact events).  
Tested specimens were 2000 mm long and had a rectangular cross-section of 125 mm (width) \(\times\) 250 mm (height). The longitudinal reinforcement consisted of 2 steel bars of 16 mm diameter at the top and bottom of the beams (\(g\) = 1.6%). A clear cover of 50 mm was considered between the bars and the beam edges in order to allow the installation of a strengthening layer on the tensile side. The test set-up and cross-sectional configuration of each series are plotted in Figure 1. No shear reinforcement was installed, except for two stirrups of 8 mm diameter on the supports to enhance anchorage of longitudinal bars. Reference series RC-U0 consisted of conventional RC beams, while series RC-U35 and RC-U55 included a 35- and 50-mm thick tensile layer of high-performance fiber-reinforced concrete, respectively, to simulate rehabilitation or strengthening of reference series. The reinforcement layout of reference series RC-U0 is a typical one of shear-critical RC members [12], which can be designed according to the most current shear models [38]. In series RC-U55, the interface between the conventional concrete and the strengthening material was placed at the level of the longitudinal steel reinforcement. Two specimens were tested for each configuration (i.e., a total of six impacts tests was completed).  
Impact tests were carried out with a three-point bending scheme, with a span length of 1600 mm between supports, as plotted in Figure 1. A steel mass of 200 kg was freely dropped to the top side of the midspan of specimens from a height of 1.75 m (impact velocity = 5.9 m/s). The striking end of the projectile and the supports were cylindrical with 29 mm radius. At both supports, two steel yokes prevented uplift of the specimens. The impact load and reaction forces were measured with HBM piezoelectric force washers suited for highly dynamic solicitations with a sensitivity of 4.3 pC/N (CFW-700 and CFW-190, respectively). The vertical acceleration at the specimens' midspan was measured with a PCB piezoelectric accelerometer of 5 mV/g sensitivity, placed at mid-height of the cross-section (125 mm from bottom). The midspan acceleration is a representative measurement of the beam response and allows integration to achieve midspan displacements as explained in Section 2.3. The measurements were taken at a sampling rate of 40 kHz with an HBM expressoDAQ module. A comprehensive description of the testing machine is reported elsewhere [12; 13]. An overview of the impact test configuration in the laboratory is given in Figure 2. It is noted that beam dimensions and test configuration are identical to those used in previous research of reinforced SFRC beams [12], except for the concrete cover, which was 40 mm, so that a comparative database including different cross-sectional schemes can be available for researchers.  
Figure 1: Test set-up and geometry of specimens (dimensions in mm).