Study On The Deterioration Law Of Fracture Performance Of Single-sided Immersed Lining Under Freeze-thaw Cycles

The distribution range of permafrost and seasonal frozen soil is very broad both in the world and in China.The frozen soil is mainly distributed in the northern hemisphere,while the frozen soil area in China is mainly concentrated in the northwest,northeast,north and central China.A large number of tunnels are built in permafrost regions.In the northwest region,due to the large difference in temperature difference between day and night,the freeze-thaw cycle has a great influence on the performance of tunnel lining concrete,and it is also the main source of concrete material deterioration.Seasonal tunnel lining concrete is often subjected to freeze-thaw cycles on one side during service.Therefore,the fracture damage performance of concrete materials under single-sided immersion conditions is studied,and the microscopic fracture mechanism of materials under this condition is analyzed.The indoor test method is used to reveal the deterioration mechanism of lining fracture performance under unilateral freeze-thaw cycles,which provides theoretical guidance for the construction and maintenance of seasonal frozen soil tunnels.The main research contents and conclusions of this thesis are as follows :(1)According to the monitoring of the temperature and humidity environment near the tunnel,the law is summarized,and the freeze-thaw cycle mechanism of indoor test is formulated.Then the indoor freeze-thaw test is carried out,and the apparent characteristics and quality of concrete specimens are analyzed respectively.After the freeze-thaw test,the surface of the concrete specimen appears to fall off and slag after the freeze-thaw cycle.The quality of the specimen was tested.When the number of freeze-thaw cycles was small,the quality of the concrete specimen increased slightly.With the freeze-thaw test,the quality of the concrete specimen decreased.After 50 cycles of freeze-thaw,the quality of the concrete specimen lost 2 %.(2)The three-point bending fracture test was carried out on the concrete specimens after freeze-thaw cycle test.The fracture performance of concrete specimens was analyzed from the aspects of load-displacement,bending strength and fracture toughness.With the increase of freeze-thaw cycle,the ultimate load of concrete specimens gradually decreased,and the ultimate load decreased by 16.3 %after 50 freeze-thaw cycles.The bending strength also decreases with the increase of freeze-thaw cycles.Under the condition of single-sided immersion,the fracture toughness decreases with the freeze-thaw cycle,and there is a linear correlation between the two.(3)The direction of crack propagation affects the crack propagation time and propagation length,and the propagation direction affects the stress state of the specimen.When the propagation angle is large,the crack propagation time is long,the top and middle of the specimen are mainly compressed,and the bottom of the specimen,that is,the prefabricated crack tip,is mainly tensioned.When the propagation angle is small,the specimen is mainly compressed.The freeze-thaw cycle has a certain influence on the stress state of the specimen,especially when the number of freeze-thaw cycles is large.(4)The microscopic parameters were calibrated by the numerical simulation software PFC2D,and the fracture tests of three-point bending beams under the conditions of 0,10,20 and 50 freeze-thaw cycles were simulated.By comparing with the indoor test results,the PFC2D numerical simulation results are in good agreement with the indoor test results.