## 1 Introduction  
The advantageous properties of light-weight, high tensile strength along the fiber direction, and durability under harsh environments have made fiber reinforced polymer (FRP) composites an advanced and competitive solution to repair and retrofit civil engineering structures [1; 2; 3]. There has been an increasing number of field applications using FRP bars and sheets to replace steel bars [1; 2; 3; 4; 5; 6; 7; 8; 9; 10] and to strengthen existing structures [11; 12]. FRP thin-walled profiles have also been put into field applications of newly built pedestrians, or to replace the old decks of small and medium span bridge superstructures [13; 14; 15]. Despite the excellent properties of FRP composites, they show specific disadvantages when compared with traditional construction materials (e.g., concrete and steel) [16]. FRP composites have a higher strength but lower modulus when compared with steel, which makes the structural design of FRP structures usually governed by rigidity and deformation [1]. Therefore, properly combining FRP with traditional materials to make economic and reliable structural components has been a more pressing research topic of late [17; 18]. Among the various combinations of FRP and concrete, the FRP-concrete hybrid deck/beam system is one of the structures that has been developed both in laboratory research and field applications [19; 20; 21; 22]. Herein, the definition of a FRP-concrete hybrid deck/beam is a deck or a beam consisting of thin-walled FRP profiles at the bottom that are connected to a solid concrete slab on the upper part. In the market of bridge decks and girders, the FRP-concrete hybrid deck/beam can result in an advanced and cost-effective solution by combining the reasonable stiffness and low cost of concrete with the light-weight, fast fabrication, high strength, and high durability of FRP composites [23; 24]. More importantly, after taking into account the lower cost of transportation, easier installation, and low or even no maintenance cost, the high initial cost of FRP can be compensated, which makes the hybrid system more competitive in the market for decks of different spans and girders for short span bridges [25; 26; 27; 28]. Similar to steel-concrete composite decks and beams, a FRP-concrete hybrid deck/beam needs strong composite action to obtain a high structural performance. Additionally, it has been revealed that strong composite action between FRP and concrete can maximize the use of the tensile strength of FRP and compressive strength of concrete [29; 30; 31]. Therefore, the shear connection along the FRP-concrete interface has been a research topic that has attracted more attention since 1994 [32; 33; 34; 35]. A simple way to improve the connection capacity and reduce the slip is to use strong and stiff connections such as polymeric adhesives [20], coated sands on FRP profiles [20], perforated FRP ribs [25; 32], and FRP shear keys [29; 34]. However, since FRP cannot be welded easily like steel, FRP-concrete interfacial connections usually have a lower shear capacity and slip modulus when compared with the connections in steel-concrete hybrid components [25; 29; 30; 31; 35; 36; 37; 38]. The connection types, for example, steel studs, perfobond ribs, and channel plates, in steel-composite decks and beams cannot be directly used for FRP-concrete hybrid decks/beams as the interfacial slip can cause notable flexural rigidity degradation and normal stresses increase at the FRP and concrete parts. Furthermore, the interfacial slip and separation can weaken the performances of FRP-concrete hybrid beams under fatigue [39], aggressive exposure [39], dynamic [40], and creep [40; 41] scenarios. Therefore, how to improve the composite action between FRP and concrete plays a pivotal role in the improvement of the structural design of a FRP-concrete hybrid beam. However, all of the aforementioned connections can entail extra costs due to the use of more materials and complicated installations.