## 2 Engineering Application and Experimental Design  
### Engineering Application  
The Second Ring Viaduct Expressway Project in Hohhot, China, has required an investment of about 300 million dollars, with a total length of 65.31 km. It was completed and opened to traffic in August 2015. In the construction of this project, advanced construction methods, excellent structural forms, and new building materials were applied. In collaboration with the China First Highway Engineering Co., Ltd. and Hohhot Urban and Rural Construction Committee, we applied ECC to the pilot project of expansion joints of a steel box girder bridge of the expressway to improve the status of frequent destruction of expansion joints during service. The steel box girder of the ECC expansion joint pilot project consisted of a single box, double chamber, and fully welded steel box girder, which were made from Q345q-E steel. The main line was a six-lane dual carriageway viaduct with a design speed of 80 km/h and a standard cross section of 13 + 3 + 13 m. The expansion joint structure of the pilot project is shown in Figure 1a. The expansion joint device was a modular bridge expansion device. The section form of the expansion joint area and the stress in the transition zone of the expansion joint under vehicular wheel load are shown in Figure 1b. The transitions strip material of the expansion joint was originally designed as steel fiber-reinforced concrete (SFRC). The upper part (thickness of 150 mm) of the expansion joint in the demonstration section was replaced by ECC, and the lower part was poured with the SFRC. The process of the ECC expansion joint pouring, vibration, and surface treatment is shown in Figure 2. In this project, four ECC expansion joints were completed, and the amount of ECC material used was about 10 m\({}^{3}\). The viaduct expressway has been in operation for four years. During this period, our team conducted many investigations and collected relevant data on the ECC expansion joints of the experimental demonstration section. As shown in Figure 3a,b, it was a steel fiber-reinforced concrete expansion joint. During the curing period, as shown in Figure 3b, many microcracks appeared prematurely on the concrete surface of the expansion joints, and the cracking phenomenon of the concrete was further aggravated under long-term vehicle loading. In less than one year, as shown in Figure 3a, a large area of visible cracks appeared. Relevant departments have made several repairs to the cracks in the SFRC expansion joints. The ECC expansion joint after four years of operation is shown in Figure 3c,d. Compared with the SFRC expansion joints, there were no visible cracks in the ECC expansion joints. Thus, the ECC expansion joint performed excellently. Figure 1: Details of expansion joint: (**a**) structural form and (**b**) section and force diagram. Engineered cementitious composite (ECC): engineered cementitious composite. Figure 3: Investigation and comparison: (**a**) steel fiber-reinforced concrete (SFRC) expansion joints after 8 months; (**b**) cracking during period in SFRC; (**c**) ECC expansion joints after 4 years; (**d**) no crack in ECC.2.2. Experimental Design  
Figure 2: Construction process of ECC expansion joint pouring: (**a**) pouring, (**b**) self-leveling after pouring, (**c**) vibrating, and (**d**) surface treatment. ECC/steel composite structure specimens were used in shear tests. As shown in Figure 4a,b, the specimens were composed of 50 mm-thick ECC and a 10 mm-thick alveolar steel plate, and the shear sections were \(100\times 100\) mm. The test parameters were as follows: S was related to the width of the post-poured ECC, W was related to the cross-layer width between the pavement and steel plate, and H was related to the thickness of steel bridge deck. S was 30, 40, and 50 mm; W was 10, 15, and 20 mm; and H was 5, 10, and 20 mm.