Shrinkage And Creep Properties Of Shotcrete For Tunnel Lining

The high-speed development of highway tunnel,the shotcrete plays an important role in the tunnel initial lining.As a long-term deformation behavior of shotcrete,creep has an important effect on the mechanical performance and durability of tunnel supporting structure.At present,there are few researches on its creep shrinkage property at home and abroad,and the relevant theories are not systematic.Therefore,based on the service environment of the tunnel,it is of great engineering significance to explore the creep law of shotcrete and provide theoretical support and design suggestions for the initial lining construction of the tunnel.In this paper,the shrinkage and creep tests of shotcrete with different creep stress levels were carried out to explore the law of early creep of shotconcrete,and the applicability of the existing creep calculation model was evaluated according to the test results.Shotcrete specimens were prepared by two different construction techniques,and the creep rule and microscopic mechanism of shotcrete were investigated by SEM-EDS,XRD and TG-DTG microscopic analysis methods.The results show that:(1)The creep law of shotcrete is similar to that of ordinary concrete,showing that the creep rate of shotconcrete is fast in the early stage and gradually slows down in the later stage.However,the creep degree of shotconcrete is larger than that of ordinary concrete,and the creep rate of shotconcrete in the early stage is faster,and the 28-day creep degree of shotconcrete is about 2.41 times that of ordinary concrete.When the stress level of shotcrete compression creep test is between 40% and 50%,it can reach the critical value of linear stage and nonlinear stage.(2)Compared with the shotcrete test block prepared by indoor mixing,the shotcrete specimen prepared by high-pressure injection has higher strength and smaller creep change in the later stage.Mainly due to the mixing molding of shotcrete,in the mixing process after adding accelerator,agglomeration phenomenon,resulting in uneven mixing,molding difficulties,defects,poor compactness.(3)Liquid accelerators enable AFt and C-S-H gels to be generated rapidly in the hardened slurry in the early stage,which are then bonded together with other hydration products to form a network structure.When there is insufficient sulfate in the matrix,the accelerators will promote AFt to be converted into AFm in the hardened slurry,increasing the porosity of the matrix and making the matrix loose.On the macro level,the cement matrix is fast hard and early strong.Compared with ordinary concrete,the strength of shotcrete in the later stage is reduced,and the shrinkage is larger.The difference of creep degree between shotcrete and ordinary concrete increases with the increase of age.(4)CEB-FIP 2010 creep model was used to calculate the creep degree of shotcrete at each age,which was consistent with the overall change law of shotcrete creep test,showing that the creep degree increased rapidly in the early stage and gradually slowed down in the later stage.The error of creep degree in each age is less than 10%,which indicates good applicability.ACI 209 model,GL 2000 model and B3 model all have more than 30% error in the long-term creep prediction of shotcrete,and the applicability is poor.