## 3 Experimental Results and Discussion
### Engineering Application  
Engineering application tests were carried out at a diversion tunnel in the Xinjiang Uygur Autonomous Region, China. The tunnel has a maximum buried depth of 2268 m and encounters a complex geology characterized by folding, faults, and variable rock masses, posing challenges such as large deformation and rock bursts. In some sections, the deformation intrusion reaches 15 cm. This project evaluates the use of sprayed UHPC for tunnel lining to reduce thickness, construction time, and associated risks, and to address spatial constraints.  
During on-site testing, PC, SF, WRT, MS, and PP fibers were mixed for 180 s in a mixer at a rotation speed of 95 rpm to create a pre-mix. At the designated tunnel section, the pre-mix was fed by a spiral conveyor into the mixer. Water and a SP were added, and the mixture was stirred at a rotation speed of 24.5 rpm for 360 s. The mixture was then placed in a hopper and pumped to the nozzle for spraying at a pumping rate of 4.9 m\({}^{3}\)/h and a pumping distance of 42 m. As shown in Figure 6f, the spraying sequence involved a single application between two steel arches, with a thickness of 12 cm. During the spraying  
Figure 5: (**A**) Pore microstructure characteristics of sprayed UHPC and cast UHPC. (**B**) Fiber microstructure in sprayed UHPC and cast UHPC.  
process, the air pressure was set at 0.8 MPa, and the air volume was 11.6 m\({}^{3}\)/h. The nozzle used had a diameter of 5 cm and a length of 50 cm. The dosage of the accelerator increased from the arch waist to the arch top, being 3%, 3%, and 5.2%, respectively. Additionally, the distance between the nozzle and the initial lining surface was maintained at 1.3-1.7 m (as shown in Figure 6g,h). As shown in Figure 6j, overly close spraying caused significant surface flatness discrepancies. Increasing the spraying distance to >1.3 m and controlling the pumping rate at 5 m\({}^{3}\)/h achieved a spray surface flatness of less than 3 mm, meeting the requirements (Figure 6k). It should be noted that during the spraying process, the dosage of the accelerator should be carefully controlled to avoid excessive amounts. An excessive dosage would lead to an increase in the rebound rate, which is not conducive to construction quality and efficiency. After the spraying was completed, the surface was treated and the rebound measurement was carried out during non-construction periods.  
The results show that the sprayed UHPC exhibits excellent workability, with a slump-flow of 601 mm. The mechanical properties are shown in Figure 7, and the results indicated that the strength reaches 1.5 MPa at 1 h and 3.9 MPa at 1.5 h after spraying, which is 1.25-3 times higher than conventional shotcrete (0.5-1.2 MPa). By 6 h, its strength reaches 10 MPa, which is 2.5-5 times the 6-h range of ordinary mixtures, demonstrating superior early age performance. The sprayed UHPC demonstrated a progressive strength development, achieving 20 MPa at 16 h, 25 MPa at 20 h, and ultimately reaching 30 MPa within 24 h (i.e., the strength of ordinary sprayed concrete at a 28-day curing age), the designed strength of 60 MPa is achieved within 48 h, with a rebound rate of less than 5%. The high early strength of the sprayed UHPC offered timely and effective support, reduced material waste, and enhanced economic benefits.  
Figure 6: Schematic illustration of integrated sprayed UHPC construction process flow and performance testing in tunnel support applications.