Study on carbonation resistance and micromechanism of shotcrete in high geothermal tunnels

The internal structure of shotcrete is often damaged under variable high-temperature curing conditions. To study the carbonation resistance and microscopic mechanism of shotcrete in high geothermal tunnels, accelerated carbonation tests were performed under different initial curing conditions (40 °C, 60 °C, 80 °C, relative humidity 55%). The results show that with increasing curing temperature, the carbonation rate is faster due to the increase in microcracks and pores in shotcrete caused by variable high-temperature curing. It is speculated that 60 °C is a critical point. When the curing temperature exceeds 60 °C, the carbonation resistance of shotcrete more significantly decreases. A carbonation depth prediction model including curing temperature was established. With increasing carbonation age, the mechanical strength of shotcrete increases, but the splitting tensile strength decreases at a later stage. The carbonation reaction will produce CaCO3-based carbonation products and fill microvoids to make the shotcrete structure denser.