| There are more Gobi and deserts in the northwestern region.With the construction of the expressway,most of the projects under construction are in a climate where the temperature difference is too low and the climate is cold.Prefabricated concrete box girder under this environment is affected by bad weather conditions,and it is easy to generate non-load cracks at early age.In severe cases,the entire beam will be scrapped,and minor cracks will bury potential safety hazards and reduce the service life of the structure.Therefore,it is necessary to study the cracking mechanism of box girder under this environmental condition and to prevent cracking of box girder.This article relies on Gansu Province's traffic construction science and technology project “Research on Crack Control Technology of Prefabricated Highway Box Girders in Dry and Cold Areas under the Condition of Large Temperature Difference in Gansu Province”.The background of Baiming Expressway and Dundang Expressway project uses the test chamber with the same proportion as the actual box girder.The beam model,from the angle of temperature difference and dry shrinkage that can easily cause concrete cracking,was tested and tested,crack resistance ring was used to observe cracking of concrete,and the distribution function of temperature field and temperature gradient of precast concrete box girder under large temperature difference and dry and cold environment was studied.The macro and micro performance tests and strength loss analysis of the concrete under the maintenance mode resulted in a steam curing system that complied with the environmental conditions.It was proved through the observation of the early crack shrinkage cracking of the concrete crack prevention ring and the theoretical analysis of the dry shrinkage stress under different maintenance methods.The rationality of conservation measures.Through the test test,the following conclusions are drawn:(1)Due to the influence of the environment,the vertical temperature difference of the box girder is relatively large,and a large self-tension stress due to a temperature difference near the flange of the box girder tends to cause cracks in the site.Through the fitted regression analysis of the measured temperature,the temperature gradient distribution function that is in line with the project location is obtained.(2)The conventional curing concrete in the large temperature difference and dry environment does not meet the 28 d strength requirement.Steam curing is the preferred measure to ensure the strength growth.However,the higher strength of concrete under high temperature steaming loses its strength,and its ability to resist cracking decreases,and the higher the temperature,the higher the density of pores,the larger the pore spacing coefficient of the concrete.(3)A reasonable four-stage steam curing system is: preconditioning temperature is 20~35°C,heating rate is no more than 10°C/h,constant temperature is 45~50°C,cooling rate is not more than 10°C/h.(4)Analysis of the concrete strain test at different steaming temperatures proves the rationality of the constant temperature of the selected steam curing.Through the observation of crack cracking ring dry shrinkage cracking and the calculation of dry shrinkage stress under different maintenance methods,it is concluded that the cracked resistance of concrete after steam curing under great temperature difference and dry cold conditions is good.Reasonable steam curing can delay crack resistance ring cracking time to avoid cracks in the actual box girder structure at early age.Through the research in this paper,it will provide theoretical and experimental basis for the early cracking of concrete box girder under the conditions of large temperature difference and dryness.It has important theoretical and reference value for regulating the process control of steam curing under special circumstances and improving the precast quality of box girder. |
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