## 2 Experimental Methodology
### Test Results  
The impact solicitation of the specimens can be understood from the measurements taken by the load cells and the accelerometer. Exemplarily, the records of test RC-U0-IMP-1 are represented in Figure 4. Reactions measured at both supports are symmetrical in all tests, whereby the total reaction is represented. Both impact force and total reaction had a two-stage shape over time, first with a high-frequency peak followed by a second stage with a smaller frequency. The peak of the impact force was higher than that of the total reaction. A time gap of 2-3 ms was observed between the beginning of the impact event and the activation of the support reactions due to the time required for the wave propagation from  
the total reaction at both supports. It has to be noted that such a value differs from the maximum impact load due to the effect of inertia forces. The total reaction has to be used as a strength indicator rather than the impact load unless the effect of inertia forces is removed from the impact load, which in turn is equivalent to the total reaction [12; 42; 43]. Total reaction-midspan deflection diagrams are represented in Figure 6 for all tested series. The midspan deflection was calculated by double integration of the measured acceleration, in agreement with previous research [12; 43; 44; 45]. As observed in Figure 6, there is a shift of the origin of total reaction-deflection diagrams due to the fact that a deflection at the midspan already developed when the impact effect reached the supports (refer to Figure 4 as well). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Series** & **Test ID** & \(R_{\text{max}}\) **(kN)** & \(v_{\text{max}}\) **(mm)** & \(v_{\text{res}}\) **(mm)** & \(E_{\text{1}}\) **(kJ)** & \(E_{\text{2}}\) **(kJ)** \\ \hline \multirow{3}{*}{**RC-U0**} & RC-U0-IMP-1 & 325.6 & 59.7 & 59.7 & 0.82 & 0.75 \\  & RC-U0-IMP-2 & 209.5 & 67.8 & 67.8 & 1.32 & 0.46 \\ \multirow{3}{*}{**RC-U35**} & RC-U35-IMP-1 & 297.2 & 37.5 & 7.5 & 1.08 & 0.93 \\  & RC-U35-IMP-2 & 278.6 & 43.7 & 28.3 & 1.02 & 1.64 \\ \multirow{3}{*}{**RC-U55**} & RC-U55-IMP-1 & 263.8 & 38.7 & 38.7 & 1.04 & 1.57 \\ \cline{1-1}  & RC-U55-IMP-2 & 275.2 & 39.3 & 39.3 & 0.88 & 1.69 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of test results1. Figure 5: Crack pattern diagrams of tested specimens. From the crack patterns at failure (Figure 5), it is clear that all beam specimens presented extensive shear cracking. The conventional RC beams (series RC-U0) failed by shear with various diagonal cracks, multiple fragmentation, and cover spalling, whose induced debris is one the most undesirable hazards for humans and goods under structures subjected to impacts. The crack patterns of the beams strengthened with a HPFRC layer showed the development of two types of shear cracks (Figure 5): on the one hand, two diagonal cracks with an inclination of 45\({}^{\circ}\) or higher forming a shear plug at the midspan; on the other hand, two diagonal cracks in both shear spans with a shape similar to shear-flexure cracks typically developed in quasi-static tests of RC [12]. [PERSON] et al. [8] classified the former crack categories as type I (shear plug) and type II (diagonal cracks at shear spans), as schematized in Figure 7b. The most interesting difference between the failure pattern of the two strengthened series can be observed at the HPFRC layer: in series RC-U55, debonding occurred at the interface between HPFRC and RC, and the HPFRC between the two type II cracks was split out; meanwhile, the HPFRC in series RC-U35 was able to stay in contact with the RC and to develop a resisting mechanism similar to a stress ribbon able to retain the damaged RC part. This last mechanism can be very interesting for real structures, as it has the potential to protect humans and goods below the structure. Figure 6: Load-midspan deflection diagrams of tested specimens. The shape of the load-deflection diagrams of Figure 6 can be useful to understand the different behavior of tested series.