Research Performance Improvement Of Shotcrete For Tunnel Portal In Cold Region

There are many tunnels along the Sichuan-Tibet Railway,which account for 70%of the total length of the line.In the process of tunnel construction in cold region,shotcrete in tunnel entrance section is easily affected by large temperature difference along Sichuan-Tibet railway and alpine climate.Even in line with the requirements of railway engineering construction specifications,according to field research,shotcrete still faces serious problems such as slow development of hydration speed and early strength,high rebound rate,serious drop phenomenon,and unguaranteed quality of initial support.In such harsh and complex conditions,shotcrete at the tunnel entrance will undergo a series of environments,such as low temperature,low temperature to large temperature difference（defined as temperature conversion environment）and freeze-thaw cycle.However,there is still a lack of research on the deterioration law and performance improvement of shotcrete under the series environment of tunnel portal section in cold regions.Therefore,this paper simulates the low temperature,temperature transformation and freeze-thaw environment,and takes nano-modification as a variable to study the mechanical properties,pore structure characteristics and durability of tunnel shotcrete from the micro,microscopic and macro scales,which has important theoretical significance and application reference value for guiding the construction of tunnels in cold regions.The specific research contents are as follows:（1）The investigation results of the environment along the Sichuan-Tibet railway tunnel were analyzed,and the environmental parameters of low temperature and large temperature difference faced by shotcrete in the construction process of tunnel portal section were clarified.The optimal dispersion mode and optimal surfactant of nanomaterials were explored,and the mix proportion of tunnel shotcrete suitable for low temperature environment was designed.（2）The development law of compressive strength and splitting tensile strength,hydration reaction characteristics and meso-pore structure of shotcrete under different nano-modification measures were studied by simulating the low temperature environment.The results show that under low temperature environment,nano-SiO₂ and nano-TiO₂ can greatly promote the early hydration reaction of cementitious materials,improve the meso-pore structure of concrete,reduce the pore connectivity of concrete,and improve the mechanical strength of concrete.The optimal modification dosage of nano-SiO₂ was 2%,which was 39%higher than that of the reference condition at 1d age.The optimal modification dosage of nano-TiO₂ was 1%,which was 25%higher than the 1d age of the reference condition.At the same time,excessive nanomaterials will weaken the improvement effect of mechanical properties of concrete.（3）The influence of nano-modification measures and pre-curing time on the strength of concrete was studied by simulating the temperature conversion environment.The micro-pore structure characteristics of concrete were analyzed,and the multi-factor mathematical model of concrete mechanical properties and micro-pore structure was established.The results show that compared with the low temperature curing environment,the compressive strength and bond strength of 28d age concrete under temperature conversion environment are reduced to varying degrees.Nano-material modification has an obvious effect on the improvement of concrete strength,among which 2%nano-SiO₂ alone has the most obvious effect.Compared with the reference condition,the increase ratio after 1 d pre-curing is up to34%.The 2%nano-SiO₂ modification measures can significantly improve the microstructure of concrete under temperature conversion environment.The regression effect of the mathematical model established by experimental data is remarkable,which can better describe the relationship between concrete strength and micro pore structure parameters under temperature conversion environment.（4）The frost resistance of concrete under different nano-modification measures was studied by rapid freeze-thaw method,and the freeze-thaw damage prediction model of concrete was established based on the relative dynamic elastic modulus.The results show that under the condition of freeze-thaw cycles,the relative dynamic elastic modulus and mass loss rate of all working conditions have different degrees of deterioration with the increase of freeze-thaw cycles.After modification with nano-SiO₂ and nano-TiO₂ alone,the meso-pores of concrete were improved to varying degrees after freezing and thawing,and the frost resistance was improved,among which the effect of 2%nano-SiO₂ alone was the most prominent.（5）The porous media model based on the lattice Boltzmann method at REV scale was established through CT scanning results and mercury injection test data.The accuracy of the lattice Boltzmann model was verified by using the generalized Poiseuille pipeline model driven by constant force,and the impermeability of concrete under temperature conversion environment was predicted.The results show that there is a significant positive correlation between flow velocity and flow rate in the process of concrete seepage.When the flow field reaches a steady state,the flow will preferentially converge to the channel with excellent path and good permeability to form the seepage dominant channel,and the flow velocity and flow in the central position of the seepage dominant channel are more concentrated than those around.The improvement effect of each modification measure was ranked as follows:2%nano-SiO₂>1%nano-TiO₂>2%nano-SiO₂+10%sulphate aluminate cement.