## 1 Introduction
Among them, a differentiation between high-performance fiber-reinforced cement composites (HPFRCC) and ultra-high performance fiber-reinforced concrete (UHPFRC) has been established as a function of compressive strength (a limit of 120-150 MPa is used by different standards [18; 19; 20]) and other properties. The tensile capacity of high-performance materials is achieved by two behavioral stages: pre-peak strain-hardening and post-peak softening. The strain-hardening stage is achieved by multiple microcracking before macrocracking localization [21] and is responsible for higher energy absorption than conventional SFRC, which only has post-peak softening. A satisfactory impact response has been observed for reinforced beams when the conventional concrete is replaced by a high-performance material (R-HPFRCC of R-UHPFRC). [PERSON] and [PERSON] [22] compared conventional RC beams without stirrups to analogous R-HPFRCC specimens with 1.6% short steel fibers by volume; while the former failed by shear upon a single impact, the latter were able to resist five impacts with plastic hinge formation at the midspan and without shear cracking at the webs. [PERSON] et al. [23] showed that R-UHPFRC (2% short steel fibers) beams without stirrups were able to resist a number of subsequent impacts showing a crack pattern with multiple very thin flexural cracks. [PERSON] and [PERSON] [24] subjected R-UHPFRC plates to subsequent drop weight impacts, demonstrating that failure changed from punching after three impacts for 0% fiber content to flexure after 7, 9, and 18 impacts for 1%, 2%, and 3% content of short steel fibers, respectively. Nevertheless, a massive use of high-performance materials in the form of R-HPFRCC or R-UHPFRC members presents the disadvantage of high cost (mainly due to the high amount of fibers, typically from 2% to 6% [21; 25]). In addition, it is not a plausible solution for existing structures needing retrofitting. Rather, a more local application of the high-performance material as a strengthening layer can be a solution for impact-critical RC members. Such a configuration has shown to be efficient for rehabilitation or new construction of bridges [26], where researchers [27; 28] have demonstrated that a UHPFRC layer not only contributes to the bending capacity but also to the shear strength of the composite member [29]. The application of HPFRCC or UHPFRC to dynamic scenarios can be favored by the positive strain-rate influence on the material properties, as other studies [14; 30; 31; 32; 33; 34] have concluded that the tensile properties (peak strength, strain-hardening stage, and fracture energy) are kept in the high strain-rate domain (1-100 s\({}^{-1}\)). The rate sensitivity of UHPFRC in bending has also been verified in impact tests [35; 36]. At the structural level, to the authors' knowledge, only [PERSON] and [PERSON] [1] have carried out impact testing on composite UHPFRC-RC slab strips. They studied two configurations with the UHPFRC layer on the compressive and tensile sides of the specimens, respectively. For the specimens with the UHPFRC on the compressive part, the strengthening layer provided good protection against crushing and spalling on the impact zone, but such a configuration did not take advantage of the most remarkable properties of the strengthening material (i.e., strain-hardening and crack-bridging in tension). For the specimens with the UHPFRC on the tensile part, the cantilever test configuration and the limited measuring equipment were insufficient to fully explore the impact response of the UHPFRC-RC system. In the present paper, the impact performance of shear-critical RC beams strengthened with a thin layer of high-performance fiber-reinforced concrete on the tensile part is studied. Strengthening solutions are necessary for existing structures because even RC members designed to develop a ductile flexural failure mode under quasi-static loading might have shear failures under impact loads [4; 5; 6; 7].