The contribution of steel fibers to increase the ductility and service life in RC beams and slabs

Numerical and experimental analyses of RC beams and slabs reinforced with steel fibers and subjected to cyclic (4 Hz) four-point bending tests are carried out. The objective is to investigate the contribution of fibers to deflection control and increase of structural service life in elements with different inertia and height to length ratio. Fatigue causes a progressive damage in concrete structures due to microcracking diffusion and crack propagation, reducing the potential energy of the system, leading to increasingly ductility (larger deformations) and lower load carrying capacity. Experimental tests performed in the Hall of Technology at the Polytechnic School of the University of São Paulo demonstrated a reduction of vertical displacements of RC slabs reinforced with steel fibers (20 kg/m3 and 60 kg/m3). In contrast, RC beams reinforced with 20 kg/m3 of steel fibers did not show any improvement in ductility. The results can be attributed, among others, to the higher toughness of RC slabs observed in monotonic loading tests and the statistical data dispersion of plain and fiber reinforced concrete beams in terms of S-N curves, as demonstrated in [1]. In order to better understand the differences between the RC beams and slabs reinforced with steel fibers, numerical analyses were carried out in the commercial software Abaqus 3D aiming to analyze crack propagation and stress distribution during cyclic loading. Crack propagation leads to a transfer of concrete tensile stresses to the reinforced bars and steel fibers, as demonstrated in [2]. Failure will occur due to the loss of equilibrium (critical crack length) and depletion (overall energy dissipation). The numerical models were calibrated with static tests. The results of the cyclic models demonstrated the importance to correctly take into account the size effect (strain localization both in compression and tension). A cross sectional multi-layer analysis is performed to analyze the current design procedures techniques. A closed form damage approach based on [3] is incorporated in the analysis. The results demonstrated the reliability of the multi-section approach to predict the deflections of RC beams and slabs, providing important information for design purposes and fatigue investigation. 1 10th International Conference on Fracture Mechanics of Concrete and Concrete Structures FraMCoS-X G. Pijaudier-Cabot, P. Grassl and C.