# On Flexural Performance of Girder-To-Girder Wet Joint for Lightweight Steel-UHPC Composite Bridge
## 1 Introduction  
An accelerated bridge construction (ABC) concept has been proposed in recent years. ABC is a bridge design and construction method that takes full advantage of prefabrication bridge elements, and it has a minimum disturbance to the in-service traffic network of bridge constructions. However, connections are always the focus of attention for precast systems, and also a potential source of future maintenance. The common way for the construction of connections is to increase the reinforcement or to apply the post-tensioning system, while the above schemes extend the on-site construction time, as well as the cost and maintenance.  
In order to solve the above problems, it is necessary to improve the structure and material mechanical properties. Thus, a type of ultra-high-performance concrete (UHPC) girder-to-girder joint that makes bridges continuous is proposed in the present study. UHPC is a class of cement-based material, and previous research has demonstrated that it has the excellent properties of anti-freeze, anti-corrosion, and crack resistance performance [1; 2; 3; 4; 5], and it has been widely used in bridge construction [6; 7; 8; 9]. The use of UHPC material can reduce the bridge weight by about 37-54% [10; 11]. A large number of studies and applications have been done on UHPC deck-to-deck joints: [PERSON] performed a structural test on a full-scale pi-girder fabricated with the longitudinal joint. In the following year, a series of tests on field-cast UHPC connections between high-performance concrete (HPC) panels were conducted [12]. In 2013, [PERSON] et al. published a design guide for precast UHPC waffle deck panel systems and connections [13]. [PERSON] et al. gave the design and details of narrow jointsused in the Mackenzie River Twin Bridges [14]. [PERSON] published a report which provided detailed guidance on the design and deployment of field-cast UHPC connections, including connections of adjacent precast deck panel, precast deck panel to girder, adjacent box beams, and other longitudinal elements [15]. In China, [PERSON] presented a UHPC longitudinal joint flexural test, and the result showed that the cracking strength of the proposed joint could meet the design requirement [16]. [PERSON] [17] investigated the flexural behavior of a UHPC joint in seven UHPC slabs under negative bending moment, in which an innovative method using steel wire mesh was presented to enhance the interface between precast and cast-in-place UHPC at the joints.  
As for research of the seismic performance of UHPC joints, [PERSON] and [PERSON] investigated the UHPC-filled duct connections for accelerated bridge construction of reinforcement concrete (RC) columns in high seismic zones [18]. The above research and applications indicated that UHPC is feasible in prefabricated structures.  
However, reports on the treatment of the UHPC girder-to-girder joint that made the bridge continuous are rarely seen. Thus, in the present study, a type of girder-to-girder joint suitable for steel-UHPC composite bridges is proposed and tested. For accelerating bridge construction, the scheme of a fully precast lightweight steel-UHPC composite girder element is also presented. The composite element replaced the normal strength concrete deck of the steel-concrete composite girder with a thinner, tougher, and ultra-high performance concrete (UHPC) deck.  
The following section of the paper introduces the design concept of the joint in detail. In order to verify the design, a bending test was conducted on a large scale specimen with two joints (i.e., the first generation T-shaped joint and the traditional I-shaped joint) and is presented in the third section. Then, in the fourth section, the scheme is further optimized (named as the second generation T-shaped joint) and experimental verification was carried out. Finally, the calculation methods of deflection, crack width, and bearing capacity of the proposed design are presented.