Study On HPC For Long-span Continuous Box Girder Constructed With The Movable Scaffolding System

Movable scaffolding system (MSS) is quite a promising method in the construction of long-span bridge,as it has many advantages such as high performance-price ratio and high efficiency. To master the preparation technology for high strength anti-crack pumping concrete in long-span box girder constructed with MSS is of great importance in promoting the application of MSS in the bridge contraction of China. This paper studied the concrete for long-span box girder constructed with MSS,considering the situation in the 62.5m long box girder of Guangzhou Huangpu major bridge.The main content studied include the optimum mix design for HPC of long-span box girder constructed with MSS;the relatinship between setting time and early age strength;the shrinkage property and durability of concrete in long-span box girder constructed with MSS; the temperature stress develepment and distribution law of long-span box girder constructed with MSS which were studied with the finite element software ANSYS and temperature-stress test machine.The study results are as follows:(1) According to the dense skeleton packing theory, a mix design method for long-span box girder constructed with MSS was proposed.The high performance anti-crack concrete that has high early age strength(R₃≥45MPa) and young's modulus(E₃≥3.45×10⁴MPa) and satisfies the demand of prestress tensioning on the 3^rd day in long-span box girder was successfully designed with this method. (2) The setting time and early age strength of different mix design was tested,and their match relationship was revealed.The contradic between the demand for long setting time and high early age strength was resolved by the dense skeleton packing method and the stdudy of the relationship between setting time and early age strength. (3) A new evaluating method that uses the temperature-stress test as well as flat slab cracking tests and elliptic ring test for anti-cracking behavior of concrete in long-span box girder constructed with MSS was proposed. The index stress storage can reflect the effect of both temperature stress and tensile strength in the early age on anti-cracking behavior of concrete of different mix design and composing materials . It is proper for the concrete in long-span box girder constructed with MSS to have the stress storage excesses 30%.The replasing of certain amount additives for cement improves the stress storage of concrete and reduces its cracking risk. There is an optimum value for the amount of additive to make the stress storage have the largest value,and the possibility for cracking is the lowest,this optimum value is 15% for fine slag. (4) The finite element software ANSYS can imitate the the temperature and stress field of long-span box girder constructed with the movable scaffolding system acurately.The highest temperature and stress in box girder locates at the place where the web plate and the upper plate join,the largest compressive stress happens several hours before the highest temperature, which is much lower than the compressive strength of the concrete,the largest tensile stress happens at the time when the temperature equals the environmental temperature.The application of the research results in Guangzhou Zhujiang Huangpu bridge indicate that the designed concrete for C50 concrete satisfies the demand of prestress tensioning on the 3rd day,improves the engineering quality,reduces the construction period and engineering cost,prevents cracking from happening ,and the engineering application effect is satisfactory.