Zhuravskyi O. Strength, crack resistance and deformation of reinforced concrete slabs under complex loads

The dissertation is devoted to the solution of an important scientific and technical problem of calculation of plate combined-reinforced constructions with steel fiber and prestressed reinforcement in two directions. The subject of research is the stress-strain state of biaxially pre-stressed steel-fiber- concrete (SFС) slabs under transverse loads. In Chapter 1 of the dissertation highlights the current state of the problem of taking into account the complex stress-strain state when calculating the SFС of plates with transverse bending. This analysis of studies of prestressed slabs. Methods of calculation of reinforced concrete slabs at joint action of bending moments and longitudinal forces are analyzed. On the basis of the performed analysis the purpose and tasks of researches are formulated. The Chapter 2 is devoted to experimental and theoretical studies of the strength and deformability of the SFС under compression and tension. A method for determining the parameters of the diagram «σ-ε» during compression and tension of the SFС is proposed. Also the technique and results of experimental researches of shrinkage and creep of SFС at uniaxial and biaxial compression are resulted. A method for determining the relative deformations of transient and long-term creep of SFС under uniaxial and biaxial compression has been developed. The criterion for determining the boundary of the transition of transient creep SFС to long creep is proposed. In Chapter 3, the stress-strain state of biaxially prestressed SFС slabs after the transfer of compressive forces from reinforcement to concrete is investigated. Prestress losses in biaxially prestressed SFС plates caused by shrinkage and creep of SFС have been experimentally determined. A method for determining the prestress losses caused by instantaneous deformations of SFС (at the time of release of reinforcement from stops on concrete), as well as transient and long-term deformations of creep SFС. The obtained diagrams of creep and prestressing losses in the reinforcement showed that the smallest losses occurred in biaxially compressed plates in comparison with uniaxially compressed plates. The data of stresses and strains in the reinforcement before the tests on the transverse load are given. In Chapter 4, the bearing capacity and crack resistance of SFС plates under the action of a uniformly distributed load are investigated. The nature of the destruction of experimental samples is analyzed and the efforts of their destruction are determined. A method for calculating the bearing capacity and crack resistance of biaxially prestressed SFС plates based on the deformation method has been developed. The calculation method is performed using improved dependencies of current regulations. The criterion of crack resistance is the achievement of the ultimate stretched fibers of the SFС tensile tensile deformations, which are equal to . This takes into account the complete diagram of the SFС under compression, the work of reinforced concrete in the stretched zone. The effect of biaxial compression of concrete in the compressed zone using the appropriate coefficient is also taken into account. In Chapter 5, investigates the deformability of biaxially prestressed SFС plates during transverse bending. The existing methods of calculation of deflections of bending elements without cracks and with cracks according to the current norms and Eurocod are analyzed. An algorithm for calculating the curvature and deflection of SFС plates with transverse bending is proposed. The calculations are performed according to the algorithm for determining the bearing capacity with fixing the magnitude of the curvature at each step of loading. The results of experimental studies of deflections of prototypes, which showed good agreement with theoretical calculations, are presented. In Chapter 6, proposes a general algorithm for calculating the bending elements of rectangular cross-section, reinforced with ordinary and pre-stressed reinforcement, as well as steel fiber. The calculation method is based on the deformation theory of calculation of reinforced concrete structures taking into account the complete diagram «σ-ε» for concrete and reinforced concrete under compression. The technique makes it possible to calculate biaxially prestressed plates.