## 3 Results and Discussion  
### Phenomenology of Resisting Mechanisms  
The brittleness and high tendency to develop shear cracking and fragmentation of RC beams under impact loading were confirmed by the results of RC-U0 series. Test results indicate that the impact response of strengthened specimens mobilized more complex resisting mechanisms than those reported for quasi-static loading scenarios. Previous researchers [46] explained that the contribution of the HPFRC layer to the shear capacity under quasi-static loading can be due, on the one hand, to the increase of the shear strength of the RC layer and, on the other hand, to additional shear resisting mechanism. The former can be explained by the capacity of the HPFRC layer to control the width of the critical shear crack, thereby increasing aggregate interlocking, as well as enlarging the compression zone depth, which in turn increases its capacity to carry shear stresses. The additional contribution was assigned by [PERSON] and [PERSON] [29] to the capacity of the strengthening layer for bending in double curvature along the interlayer debonding zone, as depicted in Figure 7a. It has to be noted that critical shear cracks under quasi-static loading can be assimilated to previously defined type II cracks. The present experimental results show that two types of shear cracks (I and II as defined previously, see Figure 7b) developed. Type I cracks opened immediately upon impact as a local effect, while type II cracks were due to flexure-shear interaction at the webs produced by inertia forces, as similarly reported by previous researchers [6; 8; 47]. From the video records of completed tests, the sequence of crack development and activation of internal forces are as follows: The impact load is firstly resisted by inertia forces in the midspan, before the activation of reaction forces. At this stage, the midspan zone is highly stressed and type I cracks develop. In the second stage, when reaction forces are activated and the impact force is cancelled, inertia forces are fully equilibrated by the reaction forces. Shear forces and bending moments in the second stage are responsible for type II cracks. Due to the similar crack patterns of all tested specimens, it seems that the formation of type I and II cracks was not affected by the presence of a HPFRC layer. Rather, the contribution of the stress ribbon mechanism of the HPFRC layer was activated upon the formation of such cracks and responsible for the retention of the damaged RC part of the beams and control of fragmentation. The crack pattern of RC-U35 series indicates that the full tensile capacity of the HPFRC was utilized by the stress ribbon mechanism, as the HPFRC layer shows visible macrocracks at an average distance of 180 mm to each other (Figure 5). In RC-U35 series, the stress ribbon mechanism was able to bridge the damaged zone between the two type II cracks. The full utilization of the tensile capacity of the HPFRC confirms conclusions made in previous research (see [14; 30; 31; 32; 33; 34]) on the positive strain-rate influence on the tensile properties of HPFRC in the impact domain. Such strain rate has been in the range of 1-30 s\({}^{-1}\) for specimens tested under the same conditions as those of the present paper [12]. These results also agree with the conclusions of [PERSON] and [PERSON] [13] for SFRC regarding the rate sensitivity of the pull-out mechanisms mobilized by straight steel fibers, showing better dynamic enhancement and lower fiber ruptures than hook-end fibers. In RC-U55 series, the full development of the stress ribbon by the HPFRC was limited by the interface debonding, which according to the videos recorded during testing, took place very soon upon impact. Debonding can be explained by the wave sequence sketched in Figure 7c: upon impact, a compressive wave propagates firstly from the top to the bottom of the beam at midspan. Thereafter, a tensile wave is reflected and, in case a weak layer is crossed by the reflected wave, debonding can occur.