Research On Shape-Change Mechanism For Crack Of Long Span Prestressed Concrete Box-Girder Bridge

The problem of excessive downward and cracking of long span prestressed concrete box-girder bridges in service has not been realized by the bridge engineers, and it needs to be solved urgently. In the past, people have made some achievements by analyzing and studying this problem on stress according to the custom, but not found the essential reason. This dissertation built a shape-change failure criterion for concrete and researched on the shape-change mechanism of cracking of the long span prestressed concrete box-girder bridges from studying the relationship between shape-change and failure of concrete material by holding the shape-changing failure essence of material.i. By investigating the typical cracks of some long span prestressed concrete box-girder bridges at home, this dissertation analyzed the load factors and deflection factors which induced to crack, pointed out that it was hard to explain the cracks on stress analysis by summarizing the existing research results at home and abroad, and put forward the way of shape-change to study the cracking problem of long span prestressed concrete box-girder bridges.ii. The shape-change is the change of shape and volume of solid material, can be expressed by strain tensor in the spatial rectangular coordinate system. This dissertation deduced the shape-change variable on the octahedral and in the Haigh-Westergaard coordinate system. It pointed out that the shape-change of concrete including instantaneous shape-change and creep shape-change, can be calculated according to nonlinear elastic orthotropic constitutive model and time-dependent constitutive model respectively. On the basis of summarizing spacial creep regularity of concrete under uniaxial, biaxial or triaxial stress, it built time-dependent constitutive model with single compliance function for creep based on creep Poisson’s ratio. On the basis of analysing the creep mechanism of concrete, it put forward the concept of spherical compliance function for creep and deviatoric compliance function for creep, and built time-dependent constitutive model with double compliance function for creep.iii. It pointed out that the failure essence of solid material was the shape and volume instability when the shape-change exceeded its bearing capacity, and discussed the shape-change failure theory of concrete material. It studied the relationship between shape-change and failure of concrete material, deduced the shape-change failure criterion from the stress spatial failure criterion for concrete, and built the shape-change failure criterion for concrete with consideration of its creep based on the strain spatial failure criterion directly built by test data.iv. To check the shape-change failure criterion for concrete, this dissertation put forward a test method which realized biaxial loading on cube concrete specimen with fixtures and uniaxial loading equipment and testing the spatial strain of the cube concrete specimen with ordinary strain gauge. The variation tendency of the test result of spatial strain with the change of stress was consistent with its computing result, but its value had some deviation when the stress was large. The shape-change failure index r / r can be caculated by the test result or the computing one of shape-change respectively. They were consistent when σ3 / σ3f was little, but had some deviation when σ3 / σ3f was large due to the test deviation of spatial strain. In general, the variation tendency of the shape-change failure index r / r with the change of σ3 / σ3f was consistent. When σ3 / σ3f ＜ 0.90-0.95, r / r ＜ 1, one can accurately judge that the concrete was not failed; when σ3 / σ3f ＞0.90, r /r grew quickly with σ3 / σ3f , and it exceeded critical value when closing to the bearing capacity point of concrete, one can accurately judge that the concrete was failed. So this test proved that the shape-change failure criterion for concrete in this dissertation was correct.v. A comparative study had been done on several usual shrinkage and creep model for concrete in this dissertation. Baded on considering factors, application scope and the quality of the predicted results, the GL2000 model was choosed to analyse the shrinkage and creep effect of long span prestressed concrete box-girder bridge. It put forward a method to calculate structure shrinkage effect with consideration of the non-uniform shrinkage of box girder section, and a method to calculate structure creep effect with consideration of the spacial creep of concrete. By the study on solar radiation temperature field and comparative study on temperature gradients model at home and abroad, the temperature effect of a long span prestressed concrete box-girder bridge was analysed.vi. A long span prestressed concrete box-girder bridge, for example, its structual effect was analysed using the method put forward in this dissertation with solid element, and the formation mechanism of web crack was explained well with the shape-change failure criterion for concrete.