Temperature Control Of High Speed Rail Box Girder During Prefabrication Process

ABSTRACT:At present, high-speed railway construction has been developed rapidly in China. Elevated bridge structure has a great proportion in high-speed railway, usually can reach70to80percent in a railway line. So the construction speed of bridge structure usually determinates the project cycle of a railway line, and directly influences the social benefits and economic benefits of project. The precast work of simply-supported box girder is most time-consuming and costly in bridge construction. During the prefabrication process, the result of temperature control plays a important role in prefabrication quality and cycle time of box girder. So it is very important to study the temperature control technology. Shorten construction period, enhance temperature control quicken construct speed and guarantee of durability receive much attention from colleagues all over the world.Based on the research of temperature control of high speed rail box girder during prefabrication process, this paper obtains the fast prefabricated and construction methods of simply-supported box girder on the premise of durability. The main research contents and research achievements are summarized as follows.(1) In order to shorten the precast construction period of simply-supported box girder in high-speed railway, according to heating up and cooling procedures in concrete conservation, the best steam curing process in simply-supported box girder concrete was initially identified. Method of installing cooling pipes in web of simply-supported box girder to cooling the girder concrete, inletting cooling water in prestressed ducts of simply-supported box girder, using ventilators in inner cavity of simply-supported box girder and other methods to quicken the cooling rate of girder concrete are brought forward. The fast cooling experiments in prefabricated site has certified the practicability of these fast cooling methods.(2) Using the finite element software ANSYS to calculate temperature field when simply-supported box girder in condition of natural curing、condition of using ventilators and condition of inletting cooling water in prestressed ducts respectively. The results show that the highest temperature of box girder concrete can reach66.52℃, the highest temperature difference is25℃, and the curing time has reached98hours under natural curing condition. The temperature of circulating water in method of inlet cooling water in precast prestressed ducts is better for25～15℃. In this interval, the highest temperature of simply-supported box girder decreases from natural curing’s66.52℃to57.88℃. The cooling rate is obviously quickened, and the biggest amplification is60%. The temperature difference of simply-supported box girder obviously decreases, and the last time of temperature difference than15℃approximately drops from natural curing’s60hours to9hours. Thus, it can be seen that the method of inletting cooling water in prestressed ducts has a good effect. The curing time of simply-supported box girder concrete is60hours with the method of inletting cooling water.(3) Temperature field in the condition of natural curing and quick cooling measures respectively were measured by field experiments, and detail dates in corresponding conditions were got. Comparing simulation results with test results we can obtained conclusions:The temperature-change curve of simulation agrees with test results very well. The highest temperature, the position of the highest temperature, the time of highest temperature and the cooling rate of simulation agree well, and the errors is less than5%. So we come to the conclusion that the thermal physical parameters and thermal boundary condition assumed in the paper were appropriate. They can reflect the reality situation, and can be used in next temperature field simulation analysis.(4)182concrete specimens were put in different position of the girder with curing steam in five construction sites, and were cured at the same time and in same conditions with girder concrete. Then the specimens’strength was test at a predetermined time. By the analysis of the experiment data, and combining with a large number of measured data by predecessors and the research of the influence of the highest temperature to later strength of the concrete development, the highest temperature of simply-supported box girder concrete can relax restrictions to65℃.(5) By using finite element method to analysis the simply-supported box girder concrete temperature stress field, and combining with the temperature cracks test results and research data in domestic and international, the range of temperature in China’s standard of15℃is too strict. It is better to adopt20℃, which adopts0.8as the maximum temperature gradient of safety coefficient.(6) The paper puts forward the summer and winter curing techniques which meet the standard requirements and recommended temperature index in this paper for simply-supported box girder concrete. It is not only the removal time is too long, but also the highest temperature and the temperature difference between inside and outside of the concrete are severely exceed the standard in the summer natural curing and winter steam curing techniques of simply-supported box girder. On this basis, the paper puts forward the summer and winter maintenance techniques which strict meet the specification limits. In the curing techniques, the curing time of simply-supported box beam concrete is reduced to56h in summer and81h in winter. The paper also puts forward the summer and winter maintenance techniques which meet the recommended temperature index in this paper. In the curing techniques, the summer maintenance time of simply-supported box girder concrete is no longer control by temperature index, and the curing time in winter is reduced to63h.