Optimization Design And Performance Study Of Sprayable High Performance Concrete For Secondary Lining Of Tunnel

Compared with the traditional single-layer segment lining,the secondary lining has more excellent mechanical properties and durability,and can better adapt to the development trend of deep burial,superlong and large-section of today’s underground tunnels.In addition,traditional single-layer segments are exposed to more and more problems during use,such as segment water leakage,segment cracking,concrete carbonization,and steel bar corrosion.Secondary lining as the durability reinforcement of single-layer segment lining can be expected to be widely used in the future.However,in the secondary lining tunnels that have been constructed,the secondary lining layers are all made of ordinary concrete constructed by formwork.On the one hand,ordinary concrete has poor toughness,easy cracking and poor durability.On the other hand,the cast-in-place construction is also contrary to the concept of rapid construction of segment lining,which limits the promotion and application of secondary lining.In this paper,a kind of shotcrete with high impermeability and crack resistance was developed,which not only solves the shortcomings of poor durability of secondary lining materials,but also accelerates the construction speed of secondary lining structure.The main contents of the research are as follows:(1)Through a set of orthogonal experiments,the sprayable high performance concrete foundation ratio was determined as follows: water-binder ratio was 0.4,cement-bone ratio was 1/4,sand ratio was 50%,and fiber content was 0.1%.Then,based on the basic ratio,the effects of seven kinds of additional materials on the impermeability of concrete were compared and analyzed.The results show that appropriate amount of fly ash,silica fume,PVA fiber and nano-silica can comprehensively improve the impermeability of concrete under static water immersion,pressure water erosion and ion diffusion.The expansive agent increases the initial water absorption rate to a certain extent,but greatly reduces the water absorption rate,seepage height and chloride ion migration rate of the material.Shrinkage reducer and hydrophobic agent can reduce the water absorption of concrete under static water,but they are unfavorable to the resistance of concrete to chloride ion corrosion.(2)The effects of different external materials on early crack resistance and long-term shrinkage of concrete were studied.Fly ash aggravates early cracking of concrete,but it can reduce long-term drying shrinkage.Silica fume and nano-silica can cause serious early cracking of concrete and increase long-term shrinkage.PVA fiber significantly improves the crack resistance of concrete under plastic state,and has little effect on long-term drying shrinkage.Expansives and shrinkage reducing agents can inhibit the early and long-term shrinkage of concrete.Hydrophobic agent improves early crack resistance of concrete most obviously,but increases long-term shrinkage.(3)The influence of different external materials on the mechanical properties of concrete was studied.The addition of fly ash,expansion agent,shrinkage reducer and water repellent will weaken the compressive,tensile and flexural strength of concrete,and the degree of weakening of the mechanical properties by the expansion agent is relatively low.Silica fume can greatly enhance the mechanical properties of concrete.Nano-silica is conducive to the development of tensile and flexural strength of specimens,but reduces the compressive strength.PVA fiber can enhance tensile and flexural strength of mortar,but it has little effect on compressive strength.(4)The construction process of concrete pumping and spraying was simulated by FLUENT software.During the pumping process,the increase of the pumping flow of the concrete will lead to the increase of the pump pressure loss;the higher the yield stress and plastic viscosity of the material will also lead to the increase of the pressure loss,among them,the viscosity of the lubrication layer mortar has the greatest influence on the pump pressure loss;The change of pump pipe form has no effect on pump pressure loss.During the spraying process,when the concrete flow rate is constant,the increase of the air inlet pressure will lead to the acceleration of the concrete spraying speed,but it has little effect on the concrete dispersion;When the air inlet pressure is constant,the increase of the concrete flow will lead to the decrease of the concrete outlet velocity and the worse dispersion;the rheology of the concrete itself has little effect on the concrete outlet velocity and dispersion.(5)The effects of different interface roughness and interfacial agent on the interface impermeability and bonding properties were investigated.Increasing the interface roughness will lead to poor impermeability of the interface,but increasing the roughness can improve the splitting tensile strength.Applying cement paste,expansive cement paste or silica fume modified cement paste on the interface can not only improve the impermeability of the interface,but also enhance the bonding between new and old concrete.