Study On Crack Control Of Over-long Concrete Frame Structures

In recent years, all kinds of large-scale public structures, workshop structures, marts etc. appeared. Over-long concrete structures are widely used. Indirect stress due to concrete shrinkage and temperature variations often produces serious cracking in over-long concrete structures. Because expansion joints are commonly not permitted, reasonable design and construction measures must be adopted to meet the request of crack control.Although large numbers of over-long projects have been done successfully, design method is still behind project practices. Perfect and accepted design method of over-long concrete structures has not been found in any codes, specifications and reference books. This dissertation focuses on crack control of over-long concrete frame structures. On the basis of a series of theoretical analysis and study on material property of concrete, calculation of thermal stress, crack control measures, theory study of over-long concrete structures was deepened and suggestions on design and construction were proposed. The main content of the dissertation is summarized as follows.1. Experimental study on material property of concreteExperiments of concrete strength, elastic modulus, shrinkage, creep were done. The concrete tested is compensation-shrinkage concrete of grade C40 from large platform of Nanjing Olympic Sports Center. Characteristic of concrete strength and elastic modulus was analysized. Effect of compensation-shrinkage concrete was evaluated. The rule of concrete shrinkage and creep versus age was summarized. Using the shrinkage model and creep model of CEB-FIP MC 90 and ACI209, the test data were analysized and compared with model value. Appropriate models of shrinkage and creep were established and it provided references to material parameter of similar concrete.2. Study on thermal stress of different form concrete frame structuresShrinkage stress was compared with thermal stress. The error that shrinkage is equalized to temperature variations in calculation was evaluated. Thermal stress of concrete frame structures whose plane is rectangle, ring and arc was analysized. Characteristic of thermal stress of one-story and multi-story rectangular concrete frame structures was reviewed. Three methods of calculating elastic thermal stress were introduced. Thermal stress of concrete frame structures whose plane is ring and arc was emphasized and corresponding formula was derived. Cause of cracking and characteristic of crack were also analysized. Thermal stress of frame structures whose plane is arc was analysized and compared with that whose plane is ring. In addition, thermal stress of large platform of Nanjing Olympic Sports Center was analysized when beams and columns are connected rigidly. Solution of thermal stress was confirmed according to its characteristic.3. Thermal stress calculation of no-crack concrete frame structuresThermal stress was compared with load stress and characteristic of thermal stress was analysized. Calculation theory and methods of concrete creep were summarized. Among these methods, the method of age-adjusted effective modulus and elastic continuation and plastic flow theory were mainly introduced. The method of age-adjusted effective modulus can be used to calculate thermal and shrinkage stress under effect of concrete shrinkage and monotone temperature variations, and practical formula was brought forward. Thermal stress under seasonal temperature variations was calculated by elastic continuation and plastic flow theory. The maximum value of thermal stress was given versus different cast time of concrete.