Working performance, strength characteristics and microstructural characteristics of SUHPC at different spraying locations in tunnel

Ultra-high performance concrete (UHPC) has been extensively used in the design and construction of engineering structural reinforcement due to its high strength, excellent durability, and other characteristics. This study aims to explore the workability, strength characteristics, and microstructural characteristics of sprayed UHPC (SUHPC) at different tunnel spraying locations. Flowability, setting time, and field spraying tests were conducted to analyze the effects of tunnel positions on the maximum single-pass spraying thickness, compressive strength, and micro-pore structure of SUHPC. The results indicate that the maximum single-pass spraying thickness of SUHPC decreases exponentially with the increase of tunnel arch angle. The values are 4.0 cm, 5.0 cm, 6.8 cm, and 9.10 cm at the arch crown, arch shoulder, arch waist, and side wall, respectively. The compressive strength of SUHPC at different curing ages shows a nonlinear declining trend with the increasing arch angle. Specifically, the 28-day compressive strength is 108.3 MPa at the crown and 124.1 MPa at the sidewall, with a strength reduction of 12.7%, while the strength reduction from the arch waist to the shoulder is only 2.7%. Combined results of XRD, SEM, and MIP reveal that the pore proportion of SUHPC increases with the rising arch angle. The porosity of the sidewall and crown is 8.81% and 10.08%, and the proportion of pores larger than 5000 nm increase from 17% to 29%. The maximum normal self-weight component of sprayed layers at the crown inhibits cement hydration of SUHPC. These results indicate that the decrease in compactness and the increase in large pores caused by substrate orientation are important reasons for the strength reduction. The empirical relationships for thickness and strength apply to the material system and spraying parameters used in this study and can provide references for thickness control and performance evaluation of SUHPC in tunnel strengthening.