Effects Of Magnesia On The Thermal Creep Stress Of Concrete In Hydraulic Tunnel

Temperature stress is one of the common causes of concrete cracks in tunnel lining.A large number of penetrating temperature cracks will not only affect the construction progress of the whole water conservancy project,but also will cause erosion to the concrete once the lining forms a seepage channel,thus seriously reducing the integrity of the hydraulic tunnel structure,economy and service life,sometimes even threaten the safe operation of the whole project.In recent years,people pay more and more attention to the crack problem of tunnel lining concrete,and the corresponding research on temperature control and crack prevention measures is also deepening which includes not only the improvement of construction method,but also the optimization of structural design,improvement of concrete material performance and so on.The research shows that adding magnesium oxide expansion agent in concrete can effectively compensate the shrinkage stress caused by the shrinkage of concrete and the change of temperature,thus reducing the formation of cracks.Up to now,magnesium oxide concrete has been successfully applied in many restricted position of foundation of water conservancy engineering,even in the whole dam section.However,the application of magnesium oxide concrete to concrete lining in underground engineering is rare.Therefore,it is of great practical significance to apply magnesium oxide admixed concrete to underground engineering structure and to analyze and study its effect in improving the thermal stress of tunnel lining concrete.In order to find out the compensatory effect and law of magnesium oxide concrete on the thermal stress of lining structure,we carry out the second development of ANSYS UPFs in this paper.The strain increment caused by micro-expansion deformation of concrete after the addition of magnesium oxide is considered as part of the increment of in-situ volume deformation,and the calculation is carried out in the program.Combined with the simulation analysis of long-lasting temperature field and temperature creep stress field of a water conveyance tunnel project in LiaoNing Province,the top arch is studied.The influence of different pouring sequence on the lining structure and the variation of temperature stress of tunnel lining concrete with and without magnesium oxide.The simulation results show that: although the pouring sequence of the two construction methods is different,the influence of the same conditions on the temperature stress of tunnel lining concrete is not obvious.Under the condition of plain concrete construction,the temperature difference of tunnel lining is large during the operation period,combined with the effect of water pressure and other factors,it is easy to produce large tensile stress on the lining surface.Therefore,it is necessary to take certain temperature control measures for tunnel lining concrete.On the other hand,the addition of magnesium oxide expansion agent can effectively improve the stress of concrete,and to some extent counteract the tensile stress caused by temperature drop shrinkage and deformation.It is an effective temperature control and crack prevention measure.