## 1 Introduction  
Concrete is a non-homogenous and brittle material, vulnerable to tensile forces and stresses, especially at high strength [1, 2]. In the search for concrete with improved qualities such as higher fracture toughness [3, 4], it was possible to overcome its fragileness by reinforcing the fibers embedded into the material to provide a better control of cracks [5]. The use of fiber reinforced concrete has gained popularity due to improvements in the fracture behavior of concrete [6, 7], and also to the successful utilization for the production of shotcrete, which has been used for reparation, rehabilitation, tunneling, stabilization, etc. [8].  
Steel fiber has been used for reinforcement in concrete structures because of its superior mechanical properties to each of the concrete ingredients. Steel fibers allow provision of the capability to bear higher load during the fracture process of concrete. Polymeric fibers have also been used as an alternative to steel fibers because of a relatively low cost and better construction behavior, having some useful properties such as deformability and corrosion resistance [5, 9]. However, due to their weaker mechanical properties than those of steel fibers, polymeric fibers do not provide higher mechanical performance than steel fibers when incorporated into concrete for reinforcement [10].  
For applications in marine infrastructures, use of steel fiber is not a good option due to its susceptibility to corrosion. In such cases, combined use of different types of fibers, for example a mixture of polymer and steel fiber to obtain corrosion resistance while maintaining the proper level of mechanical performance, can be considered. The works by [PERSON] et al. [2] and [PERSON] et al. [11] utilized three types of fiber combinations (polymer-ceramic, ceramic-steel, and steel-polymer) in order to find the best option for synergy in performance. It was reported that the effectiveness of fiber hybridization depends on factors intrinsic of the material and production process [3,12,13], but if not well designed, a reduction in some of the mechanical properties of concrete occurred.  
This work was initiated for the development of shotcrete that can be used for stabilizing marine underground tunnels. The amount of steel fiber needed to be minimized in order to reduce the risk associated with corrosion. Therefore, polyolefin fiber was chosen as a replacement material of steel fiber. Since the mechanical performance of concrete using polyolefin fiber is lower than that made of conventional steel fibers, a hybridization strategy was required to obtain both mechanical performance and resistance against corrosion induced by chloride ion penetration.  
Finding a proper mixing ratio between polyolefin and steel fiber for shotcrete and verifying it through evaluation of its mechanical performance, especially on its flexural strength and flexural toughness, would allow a hybrid fiber composed of steel and polyolefin fibers to be used successfully for shotcreting purposes. Different proportions of steel and polyolefin fibers were added for formulation of the high-strength shotcrete mixture. Considering earlier findings that beneficial enhancement on durability was obtained from fiber reinforced concrete with the addition of supplementary cementitious materials [14, 15, 16], 40 wt% of the Portland cement was replaced with ground granulated blast furnace slag. Finally, the chloride ion penetration resistance of the base concretes was quantitatively evaluated using standard test procedures.