Experimental And Theoretical Study Of Mechanical Property On The Railway Beams In High Performance Fly Ash Concrete Bridges

With the development of bridge structure towards long-span, heavy load, light weight, high strength and durability, High Performance Concrete (HPC) has been the first choice of concrete bridge today. High performance fly ash concrete (HPFAC) is one of the alternative solutions. Because of specifications of relevant standards, the application of fly ash in prestressed bridge girder is limited, especially for railway prestressed bridge girder. Generally speaking, fly ash is not adopted in prestressed concrete girder, which obstructs its application. In this paper, based on the research of properties of high performance fly ash (HPFA), the experimental and theoretical studies of mechanical properties on the railway beams with different mixtures of HPFA are investigated, which is expected to make some breakthrough with regard to this limitation. The main works in this thesis are stated as follows:(1) The research of 2,000,000 fatigue test of model beams with different mixtures of HPFA investigates the performance data of C50-C80 under the fatigue load. Test results show that the fatigue properties of prestressed concrete beams with FIPFA are stable, and model beams of fatigue test do not crack, also, its remanent strain of accumulation is small under the fatigue load. The displacement and strain of beams remains linear with the increase of load, still in elastic stage. It is tested that the fatigue properties of HPC prestressed girders meet the requirements of railway code when 20～40% of cement is replaced equally by FIPFA. So in the railway construction, the fatigue performance of HPFAC has been guaranteed in the prestressed girders.(2) By measuring the remanent strain of accumulation of 18 model beams of 9 groups with HPFAC, a regression curve of the remanent strain of accumulation of concrete beams with FIPFA under the fatigue load is obtained. Preceding works basis, a full-range nonlinear analysis of fatigue damage with the principle of piecewise linearity on flexural member of prestressed concrete is studied. Calculated results indicate that the fatigue damage process of prestressed concrete beams can be presented with accepted accuracy; at the same time prediction for fatigue life can be much more reasonable.(3) Properties of the shrinkage and creep for one year on the railway model beams with different mixtures of HPFAC are studied in the experiment. The experimental results suggest that the creep of the concrete beams with 20%～40% HPFA develops faster at its early stage under long duration, but with the passage of time, later strength of concrete is increasing and the creep is becoming slower. Based on regression analysis of the experimental results over 300 days, its creep coefficient is between 1.68～1.72 and much smaller than its creep coefficient of 2.17 with HPC beams of the same strength but without fly ash, which indicates that the concrete beams with 20%～40% HPFA can diminish the creep of beams.(4) Based upon the experimental results of shrinkage and creep of eight prestressed concrete beams with different mixtures of HPFA ,a method of the shrinkage strain and creep coefficient deducing under the bridge working environment from short term test of concrete bridge is proposed. A modified formula of JTG D62-2004 is obtained for computing the shrinkage strain and creep coefficient of concrete bridge following the concept of JTG D62-2004 for the model of shrinkage and creep. The contrast between the theoretical analysis and the test results shows that the predicted value maintains a good accuracy. When the method is used, there is no need to implement material test for shrinkage and creep, thus it can avoid the errors that might be resulted from deducing the real shrinkage and creep of concrete under the bridge working environment through the test result obtained under the standard condition.(5) The damage experiment of prestressed model beams with different mixtures of HPFA concrete indicate that model beams with different mixtures of HPFA concrete can satisfy ultimate bearing capacity of railway bridges. And crack width of model beams is calculated with current specifications, which is in line with the experimental results. This illustrates that current specifications can be applied to calculate crack width of beams with different mixtures of HPFA concrete.(6) From the discoveries of the damage experiment of prestressed model beams with different mixtures of HPFA concrete, it is proved that with the increase of loading, the basic frequency of model beams decreases and the maximum width of crack increases. However, from initial loading to near destroying, the decreasing scope of the basic frequency around 2%-6% indicates that the change of stiffness is not obvious. But the maximum width of crack is around 9.5 times-19 times in comparison with the initial maximum width of crack, the decrease of structure rigidity is predominant, which is an obvious omen of destroyed girder. The experimental results also prove that the predication by only using the basic frequency to judge the destruction of prestressed concrete beams is unfeasible.(7) Finite element analysis model for concrete beams prestressed based on nonlinear beam element theory is established. This model involves material of beam element and geometrical nonlinearities. At the same time in the model, the effect of unbounded tendon transforms equivalent nodal loads of beam element, and the corresponding finite element program is designed. The results of model beams by the finite element program compared with the experimental results indicate that the program is verified and the analysis model can better predict a full-range nonlinear structure response of prestressed beams from loading to destroying.(8) Based on the experiments of HPFA properties and model beam fatigue and dead-load tests, 6 pieces of C50 HPC beam containing 25% HPFA were cast. And they were erected successfully on Xiyang River Bridge on Luo-Zhan railway. From its opening to transporting today, the appearance of girder is good, no crack has been found, its creep camber is smaller than ordinary concrete beam, and its serviceability of girder is stable.