Bond-slip response of steel fibers after exposure to elevated temperatures: experimental program and design-oriented constitutive equation

This study aimed to evaluate the effect of elevated temperatures on the bond-slip behavior of hooked-end steel fibers. A total of 180 pullout specimens were tested in post-cooling conditions using a double-sided pullout test with multiple embedded fibers for target temperatures between 25 and 750 °C. Results proved that the bond strength significantly increases for temperatures up to 450 °C, and drastically decreases for temperatures of 600 and 750 °C. The contribution of hooks reduced with temperature and is negligible for temperatures higher than 600 °C, while the fiber-matrix frictional interaction seems to improve for all temperatures evaluated. A temperature-sensitive constitutive equation that allows simulating the bond-slip behavior of hooked-end steel fibers is proposed and its suitability confirmed through a numerical model.The authors would like to thank the Institute for Technological Research (IPT) and its foundation (FIPT) for their financial and institutional support through the New Talents Program (Grants #N.01/2017 and #N.01/2018). This work was also partially supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) [Grant # 305055/2019-4 (Antonio Domingues de Figueiredo) and Grant # 310401/2019-4 (Luís A.G. Bitencourt Jr.)]. The authors would also like to thank the researchers Ph.D. Renata Monte (USP), MSc Priscila Rodrigues Melo Leal (IPT), and Eng. Tiago Haddad Marum (USP) for the technical contributions that improved the quality of this work.Peer ReviewedPostprint (author's final draft

A finite element approach for predicting the ultimate rotation capacity of RC beams

This paper presents a numerical approach to model the complex failure mechanisms that define the ultimate rotational capacity of reinforced concrete beams. The behavior in tension and compression is described by a constitutive damage model derived from a combination of two specific damage models [1]. The nonlinear behavior of the compressed region is treated by the compressive damage model based on the Drucker-Prager criterion written in terms of the effective stresses. The tensile damage model employs a failure criterion based on the strain energy associated with the positive part the effective stress tensor. This model is used to describe the behavior of very thin bands of strain localization, which are embedded in finite elements to represent multiple cracks that occur in the tensioned region [2]. The softening law establishes dissipation energy compatible with the fracture energy of the concrete. The reinforcing steel bars are modeled by truss elements with elastic-perfect plastic behavior. It is shown that the resulting approach is able to predict the different stages of the collapse mechanism of beams with distinct sizes and reinforcement ratios. The tensile damage model and the finite element embedded crack approach are able to describe the stiffness reduction due to concrete cracking in the tensile zone. The truss elements are able to reproduce the effects of steel yielding and, finally, the compressive damage model is able to describe the non-linear behavior of the compressive zone until the complete collapse of the beam due to crushing of concrete. The proposed approach is able to predict well the plastic rotation capacity of tested beams [3], including size-scale effects