| The proportion of bridges on passenger dedicated railway lines is high in China. The number of the simply supported whole-span box girders such as 32m and 24m etc. gets several million holes, and accounts for 90% in the total bridge structure types. The concrete of simply supported whole-span box girder belongs to mass concrete, the technology of temperature control and cracks prevention of which is an important problem in structure design and construction. Moreover, the advanced High Performance Concrete (HPC) was used to guarantee the working life of simply supported box girders to 100 years, in which the influence factors of hydration heat temperature field are complex and is easy to form temperature cracks that bring great threat to the construction quality if proper temperature controlling measures are not taken. In this paper, the temperature distribution rule on hydration heat, temperature control and crack prevention technology of mass concrete in the course of precasting the monolithic simply supported box girder were presented and discussed.(1) Based on the construction experiences in beam fabricating yard of Jing-jin intercity dedicated railway line, the construction technology of precasting the 32m simply supported box girders was designed, and the problem of construction quality control was analyzed. The results indicatd that the key problem in construction quality control of box girders lies in controlling the hydration heat which possibly leads to temperature cracks.(2) The entity model of 32m simply supported double-track whole-span box girder was established and the temperature filed of hydration heat in the construction of precasting box girders was simulated with the ANSYS software. The results indicated that the hydration heat temperature of HPC increased slowly after pouring because of adding High Range Water-reducing Agent, and that the section thickness of box girders had important effects on the distribution of the hydration heat temperature field. The temperature is higher at the end cross section because of the thicker concrete, and the webs are easy to bring to temperature cracks because of the maximum temperature difference between inside core and surfaces. The other influence factors of the hydration heat temperature of HPC were analyzed and discussed such as the concrete casting temperature, the cement content and the fly ash content.(3) The reasonable curve of steam curing temperature was established according to the construction technology of steam curing about the 32m simply supported double-track whole-span box girder of Jing-jin intercity dedicated railway line. The ANSYS software was used to calculate the temperature filed in the process of steam curing. The results indicated that the temperature difference between the inside core and surfaces of concrete decreased significantly and could be controled within 6℃at the webs, upper plate and bottom plate due to the steam curing technology. And thus the temperature cracks could be controled easily. The controlling measures of the temperature cracks were presented in the full-span box girders in the process of steam curing.(4) The technical countermeasures of temperature monitoring were presented in the whole construction of precasting simply supported whole-span box girders according to the former simulation results and the field construction experiences. The crack prevention technical measures of HPC were studied and discussed from the three factors such as the selection of raw materials, the design of concrete mix and the technology of precasting construction.The suggested methods of pre-estimating the hydration heat temperature field before the concrete are poured and implementing temperature monitoring in the whole construction of precasting have great theoretical and practical significance in guiding construction better and preventing temperature cracks, and can provide references for the similar projects. |
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