Analysis Of Long-term Crack Mechanism For Bending Reinforced Concrete Members

The calculation of the crack width in concrete structures is intricated for the complexity of generation and development of crack. Today, there are numerous models for calculation of crack width have been presented at home and abroad, which diferent factors are considered in models and the expressions of the models are not the same. Based the characteristics of the cracks occurred in flexural reinforced concrete members, the study on the calculation model and methods for short-term and long-run crack width is completed in this paper. The works finished are following.Firstly, a brief introduction on the mechanism and computation theory of cracks in concrete structures, analytical methods of shrinkage and creep effect and the up-to-date research for long-run crack width caused by creep is accomplised. Both the advantages and disadvantages of diferent models are discussed.In order to overcome the shortcoming of expression of the rebar stress between the cracks by trigonometric functions in analytical models, a cubic curve to describe the rebar stress is presented. The cubic faction expressed by the maximiun stress in crack section can be obtained by resolving the undetermined coefficients.According to the change of neutral axis position in the cross-section between cracks, the state of concrete stress in the gravity center of rebar is divided into three typical patterns. The change rules of concrete stress and its expressions according to the three patterns are obtained by the method of undetermined coefficients,which laid the foundation for a more accurate calculation of crack width.Based on the three calculation model of concrete stress in the gravity center of rebar, combined with the rebar stress expression by cubic curve form, computation formulas of various modes for short-term crack width are obtained. computation formulas for long-term crack width of three patterns are obtained through the introduction of research achievement of stress redistribution caused by creep and shrinkage between the cracking section and uncracked section. Finally, the obtained formulae on calculating crack width are developed, and the comparison of the theorical results by presented formulae with the test results accomplished by Nanjing institute of Technology is fulfilled showing that there exists certain deference between the results, which provides the information for further study on calculation of crack width in flexural reinforced concrete members.