Anti-cracking And Durability Of HPC For Wide Box Girder Of Super-long Span Hybrid Girder Cable-stayed Bridge

Four aspects of high performance concrete for wide box girder were investigated to solve the problem of crack in wide box girder of super-long span hybrid girder cable-stayed bridge, including the mix designing, crack prevention, durability and construction for temperature control and crack prevention. The effect of mineral admixtures, polypropylene fiber, shrinkage reducing agent and hydration heat reducing agent on the workability, mechanical properties, crack resistance and durability and their mechanisms were studied, and the simulation calculations of the temperature fields and stress fields for the prestressed concrete wide box girder were also carried out. Based on all above the results, the proportions of high performance concrete with good crack resistance and good durability were supplied, and the construction for temperature control and crack prevention was also supplied.Firstly, the composition of high performance concrete for wide box girder was investigated. The high performance concrete was prepared to meet the requirement of workability, mechanical properties, crack resistance and durability. What's more, the influence of polypropylene fiber, shrinkage reducing agent and hydration heat reducing agent on the workability and mechanical properties of high performance concrete were investigated.Secondly, the cracking sensitivity and volumetric stability of HPC for box girder were investigated by means of board device, temperature-stress test machine (TSTM), and testing hydration heat, adiabatic temperature rise, drying shrinkage ratio and creep. And the indexes to assess the cracking sensitivity of concrete for wide box girder were established based mainly on the TSTM. The results indicated that adding mineral admixtures decreased the adiabatic temperature rise, drying shrinkage ratio and creep, which improved the crack resistance of concrete. What's more, the research results demonstrated that the concrete exhibited less drying shrinkage and creep with polypropylene fiber, less drying shrinkage with shrinkage reducing agent, and less hydration heat with hydration heat reducing agent. The adding of polypropylene fiber, shrinkage reducing agent and hydration heat reducing agent was beneficial to improve crack resistance. The durability test results indicated adding mineral admixtures improved the frost resistance and chloride penetration resistance, but decreased the carbonation resistance. The predicated service life of HPC for boxer girder was more than five hundred years based on the carbonation resistance.The XRD, SEM and MIP methods were applied to determine the microstructure and composition of the cement-based materials with mineral admixtures, and the mechanisms were preliminarily analyzed, including the fly ash improving durability but decreasing drying shrinkage and creep, hydration heat reducing agent reducing hydration heat. The analysis revealed that the creep characteristics of fly ash cement were decided by the amount of hydration products, 'micro-aggregate effect' and combination of interface around fly ash. The concrete exhibited less specific creep in the stance of less amount of non-evaporable water, 'more micro-aggregate', and more combination of interface. The analysis also revealed the hydroxyl of hydration heat reducing agent was absorbent around the unhydrated cement particle, which prevent the cement particle touching from the water. So the process of hydration was delayed, and the peek value of hydration heat was decreased.Finally, the simulation calculations of the temperature fields and stress fields for the prestressed concrete wide box girder were carried out under different construction plans. Some factors which affected the coefficient of safety for crack prevention of concrete were investigated, including different casting temperatures, choices of whole amplitude construction or half amplitude construction, and arranges of cooling pipes or not. According to the simulation calculation results, the half amplitude construction with arrangement of cooling pipes was choose to prevent the crack. And some measures to control the maximum temperature and the difference between the inside and outside temperature in boxer girder were put out.