Study On Formation Mechanism And Crack Control Technology Of Temperature Variation Crack In G7 High Speed Wumu Section Under Large Temperature Difference

The Beijing-Xinjiang Expressway(G7 Expressway),as the second important route into Xinjiang in the national motorway network,is of great importance in improving the construction of the national and Xinjiang motorway network and promoting the economic construction of the northwest region.Affected by the drought and cold environment with large temperature difference,the semi-rigid base of pavement is prone to temperature variation crack,which seriously endangers the structural durability and operational safety of G7 expressway.Based on the field investigation and laboratory test,this paper studies the temperature variation crack using the existing theory,and finally puts forward the disposal technology of temperature variation crack induced by large temperature difference.The main research results include:(1)The structural layer diseases of G7 expressway Wumu pavement are mainly temperature-variant transverse cracks,and the crack forms of the surface road section and the base road section are mainly through cracks.The crack rate and pavement mass fraction of the base section are 0.52%～1.00%and 85～89 respectively.The crack rate and pavement mass fraction of the surface section are 0.30%～0.64%and 80～90 points,respectively.The two temperature-variable cracks have significant similarity in the cracking shape and strike dip angle.According to the cracking form of the crack profile,it can be inferred that the main disease of the pavement is the temperature-variable crack.(2)In order to clarify the influence of large temperature difference on damage and cracking of base layer,the mechanical indexes of drill core samples are obtained through indoor tests.The test results show that the damage and cracking of semi-rigid base is the most serious in freezing-thawing environment,and the peak stress drop is 50.73%,which is 2.8 times and 3.3 times of that in low temperature environment and high temperature environment respectively.The dynamic modulus decreased by 76.73%,which is 1.2 times and 1.0 times of that in low temperature environment and high temperature environment respectively.The increase of peak strain is 29.53%,which is 1.6 times and 2.9 times of that under low temperature and high temperature respectively.(3)SEM,EDS and NMR are used to analyze the pore evolution characteristics before and after the temperature cycle.It is found that the test temperature and the number of cycles significantly affected the hydration reaction.In the freezing-thawing environment(-20℃～20℃),the hydration products in the base decrease rapidly,the content of which decreased by 65.11%,and the harmful porosity increased by 18.19%.At high temperature(60℃～20℃),its content decreased by 47.17%,and the harmful porosity increased by 5.72%.At low temperature(-5℃～30℃),its content decreased by 54.08%,and the harmful porosity increased by 7.5%.(4)According to SEM and EDS research,the influence of temperature gradient and cycle number on the hydration reaction of the base layer is reflected in the temperature contraction mechanism of the base layer.The research shows that the shrinkage and cracking of temperature-variable cracks are induced by the joint action of temperature shrinkage effect and dry shrinkage effect of semi-rigid base.Based on this idea,the shrinkage stress formula of semi-rigid base is derived,and the calculation formula of crack spacing and width is further derived.Combined with the dynamic modulus data obtained in laboratory test,it is estimated that the crack spacing and crack width are 15m-25m and 1cm-5cm in semi-rigid base under different large temperature difference environments.(5)Taking crack spacing and crack width as important crack control indexes,this paper puts forward the disposal technology of semi-rigid base mortise and tenon joint structure with cold and high temperature resistance,conducts in-depth analysis on its action mechanism and construction technology,and conducts evaluation and analysis on the crack control effect of this technology combined with on-site monitoring data.The results show that the tenon and mortise reserved joint structure laid 50m apart has an ideal crack control effect on temperature-variable cracks.