## 1 Introduction  
In recent decades, terrorist attacks on buildings and infrastructure has become a global threat [1; 2; 3]. To protect civilian lives from possible terrorist attacks, civil infrastructure should provide resistance to extreme loads such as impact and blasts. Ordinary concrete, which is one of the most widely used construction materials, is well-known to be weak under such extreme loadings because of its poor energy absorption capacity and brittle nature [4; 5; 6]. The addition of fibers is one of the most effective methods to overcome this defect [7; 8; 9]. The principal benefit of incorporating random evenly distributed fibers into cementitious material is to resist and decay crack propagation [10; 11; 12]. The fiber contribution to concrete strength begins when concrete micro-cracks initiate and enhance the cracking behavior owing to bridging the cracked sections [13; 14; 15].  
The fibers used for concrete reinforcement are mainly steel fibers, carbon fibers, and polymer fibers [16; 17]. Among the polymer fibers, polypropylene (PP) and polyvinyl alcohol (PVA) fibers have attracted most attention due to the outstanding toughness for concrete reinforced with them [18; 19]. Concrete is a complex material with multiple phases which include large amounts of C-S-H gel at micron-scale size, sands at millimeter-scale size, and even coarse aggregates at centimeter-scale size. Thus, the properties of FRC will be improved at a certain level, but not whole levels if reinforced with only one type of fiber [20]. For instance, steel fibers are supposed to strengthen concrete at the coarse aggregate scale, while PP or PVA fibers are suitable for the fine aggregate-scale crack prevention, and carbon nanotubes are proven to improve the strength at the scale of cement grains [21]. In practice, hybrid fibers are incorporated in a common cement matrix, and the hybrid fiber-reinforced concrete (HFRC) can offer more attractive engineering properties because the hybrid composite derives benefits from each individual fiber and exhibits a synergetic response [16; 22; 23; 24]. In addition, HFRC shows improved structural behavior compared to conventional concrete, such as less spalling and scabbing under impact loadings [3; 25; 26; 27].  
Previously, most of the fiber reinforcement research has been carried out to examine tensile strength [28; 29], flexural strength [30; 31; 32] and drop-weight impact toughness [33; 34; 35; 36]. Only some work dealt with the blast or impact resistance performance affected by fiber content and type [9; 37; 38], since the extreme loading tests are costly and even dangerous.  
[PERSON] et al. [37] checked the hybrid steel FRC subjected to air-blast loading whereby the hybrid steel fibers may reduce the effect of spalling and scabbing. [PERSON] et al. [39] evaluated the repeated impact resistance of UHPFRC samples including plain straight steel fibers and hybrid steel-PVA and steel-basalt fibers based on the SHPB test machine. It was concluded that the use of hybrid steel-PVA fibers exhibit greater compressive toughness under repeated impacts than steel-basalt fibers. In addition, the hybrid fiber-reinforced UHPC samples exhibited much smaller cracks than the single steel fiber-reinforced UHPC samples because small-sized PVA and basalt fibers were able to effectively control the micro-cracks.  
This work, therefore, aims to investigate the blast-resistant performance of hybrid fiber-reinforced concrete against contact detonation. With reference to control specimens, the HFRC panels are assessed for the blast damage level. The damage coefficient and blast-resistant coefficient are then introduced to evaluate the HFRC panel ability against explosion. Finally, the hybrid effect index is introduced to determine the positive or negative roles played by PP and PVA-fiber.