Influence Of High Geothermal Environment On Microstructure And Properties Of Tunnel Shotcrete

Tunnel is the main type of engineering structure in infrastructure.With the construction of more and more deep-buried tunnels under complex geological conditions,the influence of the high geothermal environment on tunnel shotcrete is becoming more and more prominent.The unidirectional heat conduction between the rock wall of a high geothermal tunnel and shotcrete leads to the gradient distribution of the internal temperature of shotcrete,which has different effects on the hydration reaction and microstructure of tunnel shotcrete,resulting in the uneven development of mechanical properties.In this paper,by simulating the geothermal environment,the temperature sensor is used to measure the internal temperature field of shotcrete and calculate the corresponding temperature gradient;The hydration products of shotcrete at different distances from the heat source were analyzed qualitatively and quantitatively by thermogravimetric analysis(TGA/DTG)and X-ray diffraction(XRD);Scanning electron microscope(SEM)was used to observe the micro-morphology of hydration products and the surface microstructure of shotcrete cement paste;The distribution of pore structure was measured by mercury injection method(MIP),and the flexural strength and compressive strength of different parts were tested accordingly.The main research results are as follows:(1)In the Simulated Geothermal environment,the internal temperature of shotcrete decreases with the increase of the distance from the heat source,showing a gradient distribution.The higher the simulated temperature,the more significant the change of internal temperature of shotcrete with curing age,and the greater the temperature gradient,average heating rate,peak temperature and equilibrium temperature,the earlier the temperature reaches the peak and equilibrium time.The calculation shows that in the highland hot tunnel with rock wall temperature of 80 ℃,the temperature cracking index of shotcrete is lower than the specification requirements,which can not ensure the durability and safety of shotcrete.(2)Under the influence of temperature gradient,the content of calcium hydroxide and Ettringite in shotcrete is gradient distribution,and the crystal morphology is also affected.In the early stage of hydration,the greater the distance between the interior of shotcrete and the heat source,the less the content of calcium hydroxide,the more the content of ettringite,the coarser the morphology and in the shape of needle and rod;In the later stage of hydration,the smaller the distance from the heat source,the lower the content of ettringite and the shorter the morphology.At the same time,it will also lead to the gradient distribution of microstructure and hydration degree of shotcrete.In the early stage,the smaller the distance from the simulated heat source,the rougher and looser the microstructure surface and the higher the degree of hydration;In the later stage,the greater the distance from the simulated heat source,the more flat and compact the microstructure surface,but it has no effect on the degree of hydration.(3)The development of pores in shotcrete is affected by the internal temperature gradient and presents a gradient distribution.Under the condition of 80 ℃ simulated heat source,when the curing age is 1 day,the smaller the distance between the interior of shotcrete and the heat source,the lower the total porosity of shotcrete,and the larger the most probable pore diameter and median pore diameter;When the curing time is 28 days,the smaller the distance from the simulated heat source,the higher the total porosity of shotcrete,and the larger the maximum possible pore diameter and median pore diameter.The pore development in shotcrete is also affected by the curing time.With the increase of age,the pores will deteriorate in the area close to the heat source,and the total porosity,the most probable pore diameter,and the median pore diameter will increase,but it will not affect the area away from the heat source.(4)When the simulated heat source temperature is high,the internal compressive strength and flexural strength of shotcrete show gradient distribution.When the simulated heat source is 60 ℃ and 80 ℃,the development of early mechanical properties of shotcrete is promoted.The early compressive strength and flexural strength of shotcrete are higher in the area with higher internal temperature;However,it has an adverse impact on the later mechanical properties.The higher the temperature,the lower the compressive and flexural strength.The geothermal environment at 40 ℃ does not affect the mechanical properties of shotcrete.