Control And Macro-micro Analysis Of Steam-curing Temperature Of Concrete Box Girder In Dry-cold And Large Temperature Difference Area

Along with the deep integration of the western part of China into the "one belt and one road" construction,the highway and railway have developed rapidly.Because of the harsh climate in Northwest China,such as dry and cold,large temperature difference,the durable concrete structure has been widely used.Among them,prefabricated components are more and more used in construction projects because they are not affected by season and climate and can speed up the construction progress on site.In this paper,the macro and micro properties of the precast box girder concrete under the steam curing system are analyzed under the dry and cold climate conditions.The effects of the preconditioning parameters(pre-curing period,heating period,constant temperature period,cooling period)on the compressive strength,microstructure and chloride ion resistance of the concrete blocks are studied.The reasonable pre-curing time,heating rate,constant temperature and constant temperature are obtained.Intermediate and cooling rate.The main research work includes the Baiming and Dundang high-speed projects and laboratory tests.The concrete compressive strength of prefabricated box girders is analyzed by rebound instrument on site,and the compressive strength of test blocks is analyzed by testing machine;the compressive strength of test blocks is analyzed by testing machine in laboratory test,and the core drilling,cutting and testing blocks of box girders are sampled,cut and tested in site.The micro-structure of concrete was analyzed by RapidAir,AutoPore IV9500 and X-ray diffraction phase analyzer,and the chloride ion permeability resistance of concrete was analyzed by electric flux analyzer.The influence of steam curing on compressive strength,micro-pore structure and chloride ion penetration resistance of autoclaved concrete at different ages was analyzed by sampling.The effects of cement varieties,fly ash content and mineral powder content on the compressive strength of concrete blocks under steam curing system are analyzed.The thermal analysis model of box girder is established by using ANSYS finite element software,and the temperature gradient of concrete box girder in situ and finite element model is compared and analyzed.The results are as follows:(1)Under the steam curing system,the best combination suggestion is that the pre-curing period is 4-6 hours,the heating rate is 20℃/h,the constant temperature time of the test block is 8 h(the actual box girder constant temperature time is 100 h),the constant temperature is 50℃,and the cooling rate should be less than 15℃/h.(2)Under the condition of steam curing temperature(45-50)℃ and humidity 95%,the compressive strength of box girder measured by rebound method in field test and that of test block measured by press in laboratory are both higher than those of the other three groups,and meet the design strength requirements.The frequency of chord length of pore is higher in the parameters of pore structure analyzed by pore analyzer,and the content of air in concrete is smaller.Among the parameters of pore structure,the compressive strength of concrete has a good correlation with the fractal dimension;X-ray diffraction phase analyzer analysis shows that steam curing accelerates the hydration process;and electric flux analyzer analysis shows that chloride ion penetration resistance is better.Therefore,the optimum temperature for steam curing is 45-50 ℃.(3)Finite element Ansys establishes the field solid box girder model,analyses the temperature field distribution of concrete box girder in the whole process of steam curing,and draws the conclusion that the hydration heat of box girder has little effect on the longitudinal direction,but greater influence on the transverse direction;the heat transfer process of concrete box girder is related to the thickness of components;the field measured value and the temperature change of solid concrete box girder obtained by finite element software Ansys establishes the model There are similarities.(4)Through orthogonal test design,it is concluded that the optimal level combination is level 2 of factor 1,Level 2 of factor 2 and level 2 of factor 3.That is to say,the performance of concrete under steam curing system can be improved when the cement label is 52.5,the content of fly ash is 10%,and the content of mineral powder is 10%.