Research On Degradation Law Of Shield Tunnel Segment Mechanical Properties Under Fire And High Temperature

Fire is one of the man-made disasters that seriously affects the safety of tunnel.Different from ordinary ground building fire,tunnel fire has the characteristics of fast heating rate,high maximum temperature and a wide range of hazards due to their space tightness.At present,the research on tunnel fire mainly focuses on the distribution of temperature field and the flow state of high-temperature flue gas with fewer research on the thermal mechanical coupling response of lining.Among the few research results,the simplified twodimensional elastic model is widely used,which can neither reflect the nonlinear characteristics of the lining at high temperature nor describe the three-dimensional stress state of the structure well.At the same time,due to the characteristics of rings and blocks,the thermal-mechanical coupling response of shield tunnel is more complicated in the process of fire.In view of this,this paper studies the degradation law of shield tunnel segments under the action of fire and high temperature by means of theoretical investigation,model test,mathematical regression and numerical simulation.With the help of concrete hightemperature mechanical test,uniaxial compression test of concrete after high temperature and reinforced concrete bond-slip test after high temperature,the deterioration rule of mechanical properties of concrete,the change rule of compressive stress-strain curve and the bond between steel and concrete under different temperature conditions were analyzed respectively.Meanwhile,with the help of the test results,according to the CDP model,the concrete high-temperature plastic model and the shield tunnel fire thermal-mechanical coupling refined numerical model are established.The same fire condition is simulated by elastic model and high-temperature plastic model respectively to verify the applicability of the model.The main research results and conclusions are as follows:(1)With the help of high-temperature mechanical test of concrete,the variation law of concrete compressive strength and splitting tensile strength with temperature was verified,and the strength formula was obtained by formula fitting method according to the test results.(2)Using the uniaxial compression test of concrete after high temperature,the compressive stress-strain curve of concrete under different temperature conditions is obtained,and the corresponding calculation formula is obtained by using the normalized equation and piecewise function fitting method,which provides parameter reference and support basis for the subsequent concrete high-temperature plasticity.(3)Design the experiment of reinforced concrete bond-slip ability after high temperature,obtain the bond-slip curve of steel bar and concrete under different temperature conditions,conduct theoretical analysis on the bond-slip failure mechanism of steel bar and concrete,and prove the bond between steel bar and concrete.Deterioration law of strength with temperature.(4)A concrete high-temperature plasticity and a refined numerical model has been established for thermal-mechanical coupling of shield tunnel fire,and an elastic model and a concrete high-temperature plasticity is used to numerically simulate a certain fire condition.The results show that using the elastic model can obtain accurate results and greatly reduce the calculation time of the model at room temperature.However,the calculation results of the concrete high-temperature plasticity are more in line with the mechanical response of the tunnel under fire condition,and are more applicable than the elastic model.(5)The influence of different thicknesses of fireproof boards on the high temperature mechanical properties of shield tunnel segment in fire was clarified,and the thickness of fireproof boards without damage to the segment were obtained.