Research On Hydration Heat Temperature Field And Mechanical Properties Of Precast PC Box Girder Based On Moving Bottom Die

The construction speed of bridge engineering determines the total construction period of has the largest investment and the longest construction period and the application of high-performance concrete,the hydration heat temperature of concrete increases and the shrinkage of concrete increases.Cracks appear on the surface of the beam body.The temperature field of of concrete cracks.Therefore,shorten the cycle of precast box girder,make girder quickly,analyze the temperature field of hydration heat of precast box girder and ensure its durability,which is widely concerned by the world counterparts.This paper will be combined with the actual project research.Prefabricated box girder project of Shandong High-speed Jiwei Project,this paper mainly does the following work by referring to relevant literature and investigating the characteristics and construction difficulties of the project:1.Formed a set of precast PC box girder construction technology based on moving bottom die,namely: The moving bottom die is in place,the bottom web reinforcement is installed,the hydraulic side die is installed,the inner die is hoisted,the roof reinforcement is hoisted,the end formwork is installed,the concrete of the beam body is poured,the beam body is moved,the high temperature steaming is raised,the initial tension is drawn,the bottom die is removed from the beam(the moving bottom die returns to the moving bottom die is in place),the final tension grouting is pressed,and the beam is stored.It can effectively realize the highly intelligent loading,unloading,transportation and storage of box girder formwork,save land area,and effectively improve the production efficiency of precast box girder.2.Concrete samples were taken from the test beam at the construction site,and the mechanical properties of concrete at early age of box girder under different curing conditions were experimentally studied.The results show that the curing condition has a significant influence on the development of early age strength of concrete.When t≤3d,the strength of the current condition is higher than that of the standard condition.With the passage of age,the strength of the current condition is lower than that of the standard condition,and the strength difference decreases gradually.3.Combined with the theoretical analysis of the temperature field of concrete,MIDAS/Civil software is used to conduct three-dimensional simulation analysis of the temperature field of the hydration heat of precast box girder concrete,and the temperature law of the temperature field and stress field is summarized,and compared with the in-situ test data.The results show that the method of temperature control and crack prevention by finite element method is economical and effective.4.MIDAS/Civil software was used to study the factors affecting the distribution of temperature field,namely,using low water-binder ratio to reduce the thermal effect of cement hydration;When the temperature of concrete entering the mold increases,the time when the temperature of section appears the highest will be advanced,and the temperature of hydration heat of concrete will also increase with a large increase.The smaller the specific heat of concrete is,the faster the temperature of concrete in the early stage increases.The appearance of thermal peak inside concrete is advanced with the increase of thermal conductivity.5.MIDAS/Civil software was used to analyze the displacement and stress of the box girder under different working conditions.In the maintenance stage of precast box girder and standing girder,the beam body has a slight downward torsion due to its dead weight,which reaches a maximum of 3.01 mm.In the prestressed tension stage,the downward torsion caused by the dead weight is offset due to the prestress applied to the beam body.The subsequent tension makes the beam body present a certain reverse arch,and the maximum upward torsion reaches 29.55 mm,which plays a certain role in delaying the cracking of the beam body.The influence of temperature effect on the deflection of beam body cannot be ignored,among which the overall rise and fall of temperature is less,and the temperature gradient rise and fall of temperature is greater.Based on the above research,the solutions to control temperature crack are put forward.