Damage Assessment And Imaging Analysis Of Tunnel Lining After Fire Based On Ultrasonic Testing

With the continuous development and expansion of national tunnel construction,the number of various tunnel accidents is also increasing year by year.In tunnel accidents,the number and harm of tunnel fire accidents are extremely prominent,which affects the bearing capacity of the tunnel lining structure.Therefore,it is of great significance to carry out damage assessment and defect detection of lining structures after fire.Based on ultrasonic nondestructive testing technology,this paper studies the apparent characteristics,mechanical properties and internal defect imaging of tunnel lining concrete after fire.The main research work and conclusions are as follows:(1)Under the action of different fire temperature and fire time,the apparent physical characteristics and residual compressive strength of tunnel lining concrete are studied.Before 300 ℃,the test block is basically no different from that under normal temperature conditions;when the fire temperature reaches 500 ℃,the test block begins to crack,and as the temperature increases,the test block gradually becomes loose,and the surface cracks,resulting in some bones.material exposed.The residual compressive strength of the test block showed a decreasing trend with the increase of temperature and the prolongation of fire time.(2)The microscopic test was carried out on the concrete test block of the tunnel lining structure.The results showed that with the increase of the fire temperature and the prolongation of the fire time,the cement slurry gradually became loose and porous,and the hydration products such as CSH gel gradually decomposed.The overall density of the block is decreasing.The internal structure of the concrete lining test block at 200 ℃ and normal temperature is basically similar,and there are no obvious cracks and voids;at 400 ℃,the slurry and the aggregate begin to separate gradually,the CSH gel decomposes significantly,and small voids and cracks begin to be numerous.Appears;at 800℃,the hydration products inside the test block have been completely decomposed,and the crystals have completely disappeared.(3)The supersonic velocity of the concrete specimen of the tunnel lining structure decreases with the increase of the fire temperature under the same fire time condition.When the temperature reaches 600℃,the sound speed of the three fire exposure times drops by more than 45%,indicating that 600℃ is the significant point of the change of the sound speed value;when the fire time is 2h,3h and 4h,the residual compressive strength of the test block,The correlation between fire temperature and sound velocity is good,and a calculation model for evaluating the residual compressive strength of tunnel lining concrete after fire under different fire time conditions is obtained.(4)Using the synthetic aperture focusing imaging technology to detect the concrete test block of the tunnel lining structure,high-resolution two-dimensional and three-dimensional images of the test block can be obtained.It can effectively identify cavity defects with a depth of 5 cm from the near surface;it can detect cavity defects with a vertical depth of less than 10 cm,a diameter of 1 cm to 2 cm,and a diameter of1 cm to 3 cm,and the positioning error does not exceed 20 mm.Provide assistance for repairing concrete of tunnel lining structures after fire.