Ultrasonic Testing And Experimental Study On Damage Of Lining Structure Concrete After Tunnel Fire

After the tunnel fire,it will cause not only casualties and property losses,but also serious damage to the tunnel lining structure,it will reduce the load capacity of the lining structure and the use of the tunnel safety and it is very important to select a suitable detection method to assess the burning degree of the tunnel lining structure quickly and accurately.In this paper,based on the ultrasonic testing technology,the appearance characteristics,mechanical properties,microstructure changes,acoustic parameter changes and damage assessment methods of lining structure concrete after high temperature are studied.Here is the main work:(1)The appearance characteristics and variation of residual compressive strength of lining structure concrete after different heating temperature and constant temperature time were studied.In the range of room temperature to 800℃,the surface color of concrete test block changes from light to dark and then shallower with increasing temperature,and the surface crack length changes from thin and short to long and wide and finally froms a network connection.At the same time,the remaining compressive strength of concrete are downward with the fire temperature or constant temperature time growth,the effect of constant temperature time change on the strength is much lower than the fire temperature.(2)According to the scanning electron microscopic analysis of concrete samples of the lining structure,the results show that: with the increase fire temperature,the concrete cement paste appears discontinuity,and the hydration products gradually decompose,resulting in decreased of the overall compactness of concrete and the deterioration of mechanical properties.At 200℃,the cement paste structure of the sample is intact and without microcracks.At 400℃,the paste aggregate connection area is obviously loose,C-S-H gel is significantly decomposed,and there are a small amount of micro-cracks.When the temperature is 600℃,the slurry is loose and porous,the crystal water is lost seriously,the concrete burst and there are a lot of cracks.At 800℃,the cement paste is honeycomb,the hydration products are completely decomposed,and a large number of cracks and holes appare in the concrete.(3)According to the experimental study of ultrasound on the concrete block of the lining structure,the results show that: the change of constant temperature time has less effect on acoustic parameterson;when the fire temperature increases,the sound time of concrete gradually increases,and the amplitude and ultrasonic velocity gradually decreases.The correlation coefficient between heating temperature and ultrasonic velocity of concrete under different thermostatic time is more than 0.95,and the ultrasonic velocity can be used to estimate the fire temperature of lining structure concrete in actual inspection.(4)Based on the test data,the strength measurement curve are established by ultrasonic method,rebound method and ultrasonic-rebound comprehensive method,and the correlation coefficienst are all above 0.90.By comparing the residual compressive strength calculated by the strength measurement formula with the test value,the relative errors of the three methods are concentrated in 1%～14% 、1%～10% 、 1%～6%,it shows that the ultrasonic-rebound synthesis method can calculate the residual compressive strength of concrete in tunnel lining structure after fire in a more accurate way.