# On Flexural Performance of Girder-To-Girder Wet Joint for Lightweight Steel-UHPC Composite Bridge
## 3 Comparative Model Test: The First Generation T-Shaped Joint and the Traditional I-shaped Joint  
### Experimental Model  
As shown in Figure 3a, the specimen is 6.2 m long and 1.875 m wide, composed of two joints and three prefabricated steel-UHPC composite girder elements, termed as part A, part B, and part C. The T-shaped joint is employed to connect two prefabricated components of part B and part C, and the other one is the traditional I-shaped joint (from the side view, the joint is shaped like the letter \"I\") connecting part A and part B. Part A, part B, and part C are the so-called prefabricated components. The pre-cast UHPC panel is reinforced by 16 mm diameter longitudinal steel bars with a central spacing of 83.4 mm, and reduced to 41.7 mm in the post-cast joint parts. In the transverse direction, the steel bar is 8 mm in diameter, with spacing of 100 mm. The reinforcement layout is displayed in Figure 3b. The steel girders of the specimen were fabricated by Q235 (the steel with yield strength of 235 MPa). Figure 2: Bending moment envelops of common continuous beam bridge. In reference to Shao [19], under natural curing conditions, the curing time-dependent shrinkage strain is in the range of 200-900 \(\mu\epsilon\)[1; 20; 21; 22], while under the steam curing conditions, the temperature-dependent shrinkage strain is generally in the range of 622-900 \(\mu\epsilon\), and the strain will reduce to zero after curing [1; 23; 24]. Consequently, the shrinkage-induced cracking is easy developed in the interface between the prefabricated UHPC and the newly cast UHPC. To verify the effect of shrinkage on the cracking development, the shrinkage test was conducted on part A (natural curing) and part C (steam curing) with the same reinforcement ratio and joint width. The main fabrication process is shown in Figure 4: (1) fabricating the I-shaped steel girders; (2) deploying the steel bars in pre-cast parts (part A (PA), part B (PB), and part C (PC)); (3) casting UHPC for prefabricated components; (4) steam curing of part B and part C; (5) deploying the steel bars in post-cast joint parts (T- and I-shaped joints); (6) casting UHPC for the joint parts; (7) natural curing of part A and joint parts; and (8) completing the experimental model. It is worth noting that the process is identical to the actual construction sequence. Figure 3: Illustration of the comparative model test (units: mm): (**a**) specimen display; (**b**) reinforcement layout. The composition of the UHPC material was the same as that in reference [25] (Table 1). Two types of steel fibers were employed in the UHPC material, one was \(\Phi\)\(0.2\times 13\) mm straight fibers and the other was \(\Phi\)\(0.3\times 25\) mm with hook-ends. The content within and beyond the joint region were slightly different (as shown in Table 2). The mechanical properties, including compressive strength, flexural strength, and elastic modulus of UHPC, were tested by following the standard methods of MOHURD (Ministry of Housing and Urban-Rural Development) [26]. Six specimens are conducted for each test, and the average values are listed in Table 2.  
For the loading convenience of the experimental test, the specimen was inversely configured to produce a negative bending moment. The specimen was simply supported on the bottom supports, as shown in Figure 5a. In order to ensure that the two joints were under the same load, the specimen was loaded in the form of four-point bending, the hydraulic MTS (Testing systems, Mechanical testing, and Sensing solutions) actuator was employed to apply the loading to the model. Before loading, \begin{table}
\begin{tabular}{c c} \hline \hline
**Material** & **Mass Ratio** \\ \hline Cement (PO42.5) & 0.7 \\ Silica fume & 0.2 \\ Fly ash & 0.1 \\ Quartz sand & 1.1 \\ Quartz powder & 0.1 \\ Slag powder & 0.2 \\ Superplasticizer & 0.015 \\ Water & 0.2 \\ Steel fiber (volume ratio (\%)) & 2.0/2.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: UHPC Martials Proportion (in reference [25]).