Zhao Xiang

Graphene oxide (GO) has been verified to ameliorate the performance attenuation of cement-based materials exposed to high temperatures; however, its efficacy on cement-based materials containing recycled fine aggregates (RFAs) remains ambiguous. Therefore, this study assesses the macro/microperformance variations in mortar containing RFAs with diverse GO dosages (0~0.08%) after high-temperature exposure. The hydration products, pore structure, and microstructure of mortar containing RFAs and GO were analyzed using an X-ray diffractometer, mercury intrusion porosimetry, and scanning electron microscopy, respectively. The study finds that GO can reduce the mass loss rate of mortar exposed to high temperatures and enhance the residual compressive strength and flexural strength. The incorporation of 0.06% GO leads to a 10.4% increase in compressive strength and a 32.3% increase in the flexural strength of mortar containing 50% RFA after high-temperature exposure at 500 °C. Furthermore, the incorporation of GO refined the pore size and improved the densification of the interfacial transition zone (ITZ) of the mortar after high-temperature exposure. Taking into account the mechanical properties, water absorption, and microstructure of the mortar containing RFAs, a GO dosage of 0.04~0.06% could be recommended as the optimum engineering dosage.

Hanging tunnels are a unique type of highway constructed on hard cliffs and towering mountains, renowned for their steep and distinctive characteristics. Compared to traditional full tunnels or open excavations, hanging tunnels offer significant advantages in terms of cost and construction time. However, the engineering design and construction cases of such tunnels are rarely reported, and concerns about construction safety and surrounding rock stability have become focal points. Taking the Shibanhe hanging tunnel as a case study, this paper focuses on the stability of the surrounding rock during the excavation of limestone hanging tunnels using physical analog model (PAM) experiments and numerical calculation. Firstly, based on the similarity principle and orthogonal experiments, river sand, bentonite, gypsum and P.O42.5 ordinary Portland cement were selected as the raw materials to configure similar materials from limestone. Secondly, according to the characteristics of hanging tunnels, geological models were designed, and excavation experiments with three different sidewall excavation widths and rock wall slopes were carried out. The effects of these variables on the stress and displacement behavior of the surrounding rock were analyzed, and the laws of their influence on the stability of the surrounding rock were explored. Finally, numerical simulations were employed to simulate the tunnel excavation, and the results of the numerical simulations and PAM experiments were compared and analyzed to verify the reliability of the PAM experiment. The results showed that the vertical stress on the rock pillars was significantly affected by the sidewall excavation widths, with a maximum increase rate of 53.8%. The displacement of the sidewall opening top was greatly influenced by the sidewall excavation widths, while the displacement of the sidewalls was more influenced by the rock wall slope. The experimental results of the PAM are consistent with the displacement and stress trends observed in the numerical simulation results, verifying their reliability. These findings can provide valuable guidance and reference for the design and construction of hanging tunnels.