## 1 Introduction  
Steel girder bridges have been in use for more than 60 years in many countries, such as the United States, Japan, and European countries. The deterioration of these bridges is most evident in reinforced concrete (RC) slabs directly affected by traffic loads. These RC slabs need to be repaired or replaced to maintain the service life of these bridges [1]. In the replacement of these RC slabs, precast (PCa) concrete slabs are widely used to shorten the on-site construction time (as early traffic opening is necessary in many cases) and improve slab durability [2, 3, 4]. Figure 1 shows a loop joint connecting two precast concrete slabs for (a) conventional loop joint and (b) proposed narrow loop joint without transverse reinforcements in this study. A loop joint, such as that shown in Figure (a)a, is conventionally used to join PCa slabs on site in Japan [5, 6, 7, 8]. The force transfer occurring in a loop joint subjected to a flexural moment is schematically shown in Figure 2. The flexural resistance of a joint with a loop reinforcement depends on the bond stress of the straight part and the bearing stress of the curved part [9]. Therefore, the length of the straight part of the reinforcement should be sufficient to ensure bond capacity, and transverse reinforcements need to be added to the joint to improve the bearing capacity of the curved part of the reinforcement. According to the German Institute for Standardization (DIN) 1045 [10], the length of the loop reinforcement inside the joint should be more than 1.5 times the internal diameter of  
the curved part of the loop reinforcement. Therefore, the joint width should be 250 mm for 13-mm reinforcement, 300 mm for 16-mm reinforcement, and 350 mm for 19-mm reinforcement. Furthermore, tensile stress, i.e., splitting stress, occurs in the transverse direction due to the bearing pressure from the loop [11]. Transverse reinforcements are integrated in the conventional loop joint, as shown in Figure 2, to resist this splitting stress and provide a dowel effect for interlocking the loops. However, in this method, a relatively large width of the joint is needed, and considerable time and effort are consumed in constructing the joint due to the addition of the transverse reinforcements to the joint on site. These disadvantages affect the progress of on-site construction and should be addressed, especially when this method is applied to highway bridges, where early traffic opening is necessary in many cases. Several methods for joining PCa slabs on site have been developed in recent years to facilitate the arrangement of transverse reinforcements in the joint or reduce the joint width. [PERSON] et al. [12] and [PERSON] et al. [13; 14] proposed methods that use headed reinforcements instead of loop reinforcements in the joint. [PERSON] et al. [15] used high-strength fiber-reinforced cementitious composites to improve the bond strength between reinforcements without loop reinforcement, whereas [PERSON] et al. [16] considered a joint with a combination of shear keys and reinforcements with fiber-reinforced concrete. [PERSON] et al. [17; 18] proposed a joint structure that uses a perfobond strip with steel fiber-reinforced mortar. However, these new joints are still being tested, and their force transfer mechanisms and design methods have not been clarified. Moreover, some of these techniques are expensive because of the structural complexity and the materials used in the joint. This study proposes a narrow cost-effective loop joint that measures 200 mm in width, has D19 loop reinforcements, and does not have transverse reinforcements, as shown in Figure 1b, to address the disadvantages of the conventional loop joint. A cost-effective loop splice is used along with fiber-reinforced mortar to ensure force transfer in the joint without transverse reinforcements. The joint material can be reduced by 40% through  
Figure 1: A loop joint connecting two precast concrete slabs: (a) conventional loop joint; (b) proposed narrow loop joint without transverse reinforcements.