Mechanism And Control Of Cracking In Concrete Curved Box Girder Bridges

Concrete curved box girder bridges are widely used in highway and urban grade separation hub for its advantages in mechanical characteristics and cost. However, the crack problems of these bridges significantly exposed for complexity of mechanical characteristics of curved bridges, possible overload and low tensile strength of concrete. Taking inspected reinforced concrete box girder bridges in a highway grade separation hub as the background, the mechanism and control of cracks in these bridges is explored, and the related research about durability is completed. The primary research conclusions are listed as follows.1) According to the distribution of typical cracks of background bridges, analysis and summary of crack feature for reinforced concrete continuous curved box girders is done. Then, the relationship between parameters of curved bridges and statistics of cracks is researched, and load is preliminary determined as the initial cause of crack diseases. Some 2D and 3D finite element models are created to analyze mechanism of typical cracks. Cracking models of curved box girder bridges and nonlinear behaviour after cracking are emphatically researched. The calculation results show the curved box girder bridge cracks and its force phase changes from elastic range to elastic-plastic range under general loads, which is corresponded with cracking in concrete box girders. Besides, results of parameter analysis show measures such as suitably arranging bearings, reasonably setting pre-eccentricity distance of bearings and increasing reinforcement ratio could improve mechanical behavior of curved box girders.2) In terms of different crack width calculation theories, comparative analysis of crack width formulas of different codes for design of concrete bridges is completed. Then it is recommended to properly amend the parameters of the formula in existing code (JTG D62-2004), and the crack widths predicted by amended formula agree well with the measured values3) Given the limitation of selecting crack width as the unique parameter to evaluate cracking in concrete members in current codes, crack area is chose as the second parameter to establish a new assessment method for cracking in concrete bridges based on crack statistics, and its practical application is done.4) General questions for durability of concrete members and basic principles and steps of durability design are discussed. The impact of cracks on durability of concrete is analyzed from both macro and micro (test) level. And crack control measures based on durability design of concrete box girder bridges are proposed.5) In accordance with life cycle theory and conceptions of fuzzy mathematics, membership function for impact of cracks on residual service life of concrete structure is established and the residual service life for concrete bridges with cracks is calculated. Besides, framework of concrete bridge maintenance strategy based on performance control is proposed. Then case studies of maintenance strategies for a simply supported concrete box girder bridge are analyzed and applied.6) A crack control guideline for design and construction of concrete box girder bridges is draw up based on comprehensive summary of principles and codes of bridge design and construction, which can be referenced.