## 1 Introduction  
Concrete structures might be subjected to low to heavy impacts during their service life. Either accidental (rock falls, tornado-debris) or human-induced (terrorist attacks) impact events can produce severe damage or even failure of concrete structures if the impact scenario has not been addressed in the design stage. As the concern of impact hazards has increased in recent years, the capacity of many existing concrete structures which were not designed against impacts is under discussion. In such situations, strengthening or retrofitting structural concrete members can be required [1; 2; 3]. It has been extensively reported that concrete structures subjected to impact exhibit a high sensitivity to develop brittle failures by shear or punching before yielding to longitudinal reinforcement, even in cases of structures designed to show a ductile response under quasi-static loading [4; 5; 6; 7]. The differences between impact and quasi-static behavior are mainly due to three factors [8; 9; 10]: (a) inertia forces under dynamic loading lead to a time-dependent distribution of sectional forces different from the one under quasi-static loading; (b) material properties of concrete and steel are affected by the strain rate; (c) impulsive loading and wave propagation produce local damage near the impact zone. Due to the former factors, low energy-absorbing shear failures become dominant for reinforced concrete (RC) beams. In addition, spalling of concrete cover and fragmentation can produce debris which can potentially damage goods or persons under the impacted structure. In order to improve the deformation capacity against impacts and avoid shear failures, different measures have been researched. On the one hand, external sand cushions or other protecting layers can be used to reduce the impact that reaches the structure [11]. On the other hand, the performance of RC can be enhanced with the use of fiber reinforcement. [PERSON] and [PERSON] [12] showed that steel-fiber reinforced concrete (SFRC) can improve the integrity and energy absorption capacity of shear-critical RC beams subjected to moderate-velocity impacts: A 0.5% volumetric amount of deformed or straight steel fibers significantly reduced fragmentation, increased energy absorption, and avoided shear failure in case of straight fibers, while a 1.0% fiber amount was able to avoid diagonal cracking failures and prevent flying fragments for both fiber types. [PERSON] and [PERSON] [4] needed a 1.5% content of hook-end fibers to avoid punching failure of impact-loaded RC slabs with failure controlled by flexure under quasi-static loading; failure was by punching under impact with smaller fiber contents. The fiber geometry (and not only the amount) can play a significant role in impact regime, as the dynamic properties of SFRC depend on the rate sensitivity of the pull-out mechanisms mobilized during crack formation and propagation. Thus, [PERSON] and [PERSON] [13] concluded that short straight fibers are more efficient than hook-end fibers in dynamic regime because of their higher ability to develop full pull-out without brittle fiber rupture. In addition, the rate sensitivity of tensile properties of SFRC decreases as the fiber content increases [14;15]. Synthetic fibers have also shown to perform satisfactorily under high strain rates [16], and the combination of steel and synthetic fibers can contribute to reducing local damages and enhancing energy absorption under drop-weight testing [17]. Due to the significance of crack bridging and energy absorption capacity to prevent brittle failures, the design of impact-resisting structures seems an attractive field for the application of high-performance fiber-reinforced concretes. Such materials present high compressive strength and significant energy-absorption capacity in tension.