|Keywords:||reinforced concrete; CFRP; flat slabs; punching shear; load-deformation behavior; strengthening; prestressing; bonding; load history|
|Full text PDF:||http://infoscience.epfl.ch/record/204770|
Deformation-dependent punching shear resistance often constitutes the decisive design criterion for reinforced concrete slabs supported by columns. The increasing number of aging structures exhibiting insufficient punching resistance and detailing deficiencies, and undergoing changes of usage has resulted in a growing demand for strengthening against brittle punching failure. Hence, the research presented in this thesis aims at gaining a better understanding of the identified problems occurring especially in existing structures, such as pre-deformation. A further objective is the experimental and theoretical validation of the performance of a new strengthening concept. A detailed analysis is carried out of the structural behavior of centrically supported rotation-symmetric slabs subjected to punching. In the first part of this research project a literature review is conducted to characterize the available punching models for new slabs and discuss to what extent they are able to consider the problems linked with existing slabs. Benefits and limitations of current strengthening solutions are illustrated, suggesting local prestressing as a promising concept. In the second part such a concept is analyzed that has been developed to improve the system efficiency and punching resistance by immediately activating the post-installed shear reinforcement composed of carbon fiber-reinforced polymer (CFRP) composites, thus partially unloading the slab. An experimental campaign comprising sixteen slabs verifies that the installation of this strengthening concept leads to a more ductile system behavior and significant increase of punching resistance compared to a non-strengthened slab. In the third part the load-deformation behavior of slabs is analyzed and a modification of an analytical model applied to a slab sector is developed, considering the influence of shear on flexural behavior. An overestimation of the flexural capacity and consequently of the punching resistance is thus avoided. The agreement of the modified sector model results with experimental results is confirmed for a large number of experiments from literature. The model is also capable of predicting the load-rotation responses and punching resistances of slabs strengthened with prestressed CFRP straps. The fourth part of the research concerns the effect of pre-deformation, or load history, on the structural behavior of uniaxial members and biaxial slabs. Single unloading and reloading cycles, which reduce tension stiffening and result in additional deformations, leading to decreased concrete punching resistance, are investigated. In slabs prestressed CFRP straps can compensate the additional deformations caused by load history.