Size effect in concrete structures based on nonlinear fracture mechanics

Abstract

In this dissertation, the size effect in concrete structures is investigated on the basis of nonlinear fracture mechanics, and new prediction models are suggested. In most structrual members with initial cracks (or initial notches), strength tends to decrease as the member size increases:a phenomenon known as size effect. In contrast to other structrual materiasl(wood, steel ect,), concrete shows the size effect with or without initial crack. Published experimental results indicate that even a large concrete member concrete member without initaial crack resists somestress, contrary to Bazant``s size effect law. In this study, the size effect problems are investigated particularly in the absence of initial crack, and the existing size effect law is modified for application to the stength evaluation of actual test specimens. It is carried out by two approaches, one is based on the concept of dissimilar initial cracks and the other is based on the fracture behavior of concrete under increasing stress field. With the theoretical modification through the two approaches, effect of the amximum aggregate size on the width of microcrack zone is also discussed. It seems to be reasonable to assume that the fractrue process zone width does not vary linearly according to the maximum aggregate size, as cracks occur at a narrowly strain concentrated region. On the other band, the characteritic length concept is newly introduced to apply the modified size effect models for the compression test cylinders with general height to diameter ratios. Though both the end restraints and the eneray release zone influence the size effect, major emphasis is placed on the characteristic lengh, as the latter determines the sixw of main crack zone while the former acts contradictory for the size effect. To verify the presented size effect models extensive experimental data available in the literatures are analyzed with comparison to the Bazant``s formula. The existing size law based on fractr...