Prestressed Concrete Beam Fatigue Test

In recent years, the unbonded partially prestressed concrete (UPC) has been widely used in the engineering structure because of its convenience construction technology and good economic benefit. Since the deep fatigue researching on UPC is insufficient, the mechanism of the fatigue for UPC structure has been discussed in this paper, which comes from the subject at National Science Fund Research Project (the research on fatigue and durability of the unbonded partially prestressed concrete beams). The main works are presented as follows:1. The summary of the research on the domestic and foreign fatigue test of high strength and low relaxation PC steel strand was made. The results showed that: the cause to the fatigue failure of steel strand is the interfilamenta abrade of reinforcing wires; Main influencing factors of fatigue life of steel strand are free length of sample and loading frequency, low limit of fatigue stress, and stress amplitude of fatigue stress. Then, the allowable value of fatigue strength is put forward.2. Fatigue test on six UPC beams are made, in which design factors are prestressed degree, comprehensive reinforcement index, and equivalent stress level. Through the test, it is revealed that the fatigue failure processes can be divided into dead load crack, the development of fatigue crack, and fatigue fracture of steel bars.3,After systematic analysis on fatigue strength of the six UPC beams, it is found that the reason for fatigue failure of UPC is the fatigue failure of the tensile unprestressed steel bar, which has the maximal strain amplitude and the shortest fatigue life. The fatigue failure won't happen on prestessed steel bar and pressing area concrete, since their small strain amplitudes and the longer fatigue life which figured out is greater than 5 million times.4. Systematic analysis on main design parameters which influence fatigue strength of UPC beam has been carried out. The result indicated that the main design parameters which influence fatigue strength of UPC beam include prestressed degree, comprehensive reinforcement index, and equivalent strain level. The bigger prestressed degree is, the smaller strain amplitude to tensile unprestressed steel bar,steel strand,compressed steel bar and concrete are. As strain amplitude change of the unprestressed steel bar, the most influence parameter is comprehensive reinforcement index. With the comprehensive reinforcement index increacing, the strain amplitude of tensile prestressed steel bar decreased notablely. The maximal stress of tensile unprestressed steel bar and concrete are affected by equivalent strain level, while improving the equivalent level, the strain of steel bar and concrete increase sharply.5. Generally the shear fatigue failure won't occur on UPC beam, for the smaller stress of the stirrup in the UPC beams.6. Based on the test analysis and static analysis method of the UPC beam, the estimation method of fatigue life for UPC beams was given. And the fatigue life of UPC beam which calculated with the method agreed with the experimental data satisfactorily.