Study On Some Theoretical Problems Of Concrete Box Girder Bridge Based On Life-Cycle Design

The life-cycle performance of concrete bridge varies with external factors, such as load effect, reinforcement bar corrosion, concrete carbonation and cracks, et al. The research project "Cracking mechanism and control measurement of crack in bottom plate of prestressed concrete continuous box girder bridge" has been supported by the scientific and technological plans of Zhejiang communications department. According to the project, this paper focuses on the life-cycle design method for concrete box girder bridge. The cracking mechanism in construction stage, the load-environment coupled effect in service stage and the time-dependent carrying capacity of box girder are investigated in this thesis. The primary work and achievements are as follows.(1) The cracking mechanism and parameters study are discussed according to numerical analysis of local and integral module of box girder. The exaggerated transverse stress by the downward radial force leads to the crack in bottom plate. In addition, the shortage of shear capacity due to the ducts and unreasonable reinforcement play crucial roles in the cracking of the bottom plate.(2) Transverse shearing crack is one of the most common crack during the construction of the box girder bridge, as well as longitudinal cracking, concrete cover spalling and vertical tensioning crack. And design methods for cracking control are proposed according to the crack patterns.(3) Nominal elastic module of corroded reinforced bar and corrosion factor are presented to illustrate the deformation process of steel bar during the service of bridge. According to the existing corrosion rate module, the corrosion factor of bars are deduced and compared. And the deformation characteristics are generally divided into acceleration type and degeneration one. Then, the time-dependent constitutive relation of steel bar is advanced.(4) The approach for load-environment coupled effect analysis of concrete bridge is proposed, and the finite element equation for separated element and combined element are deduced respectively. In order to take into account the effect of cracks, the ratio between crack strain and concrete cover depth is used to control the initial corrosion time of cracked element on some assumptions. The above approach is implemented in FORTRAN language, and tested by a reinforced concrete beam test.(5) Based on the characteristic of crack plane, the constitutive relation of cracked reinforced concrete is deduced according to the microscopic study of concrete crack. For the convenience of three dimensional problem, yield criterion and loading and unloading criterion are discussed. By compared the numerical result with experiment ones, the promoted method are tested, and well agreement is obtained.(6) Combined with project case, the load-environment coupled effect and the time-variable capacity of a continuous box girder bridge are studied by the promoted method. The numerical results indicate that the bridge has preferable overloading capacity after 100 years’ service. And from the capacity degeneration curve, early in the service is the fastest-changing period due to rapidest development of concrete strength. During the middle period of the service, the degeneration of capacity becomes stable, because of the stabilized concrete strength and the lower corrosion rate. And in the late time, the capacity has a sharp deterioration caused by various materials degradation.