In steel fiber reinforced composites materials, fiber and matrix are bonded together through a weak interface. The study of this interfacial behavior is important for understanding the mechanical behavior of such composites. Moreover, with the outcome of new composites materials with improved mechanical properties and advanced cement matrices, such in the case of steel fiber reinforced self-compacting concrete, the study of the fiber/matrix interface assumes a new interest. In the present work, experimental results of both straight and hooked end steel fibers pullout tests on a self-compacting concrete medium are presented and discussed. Emphasis is given to the accurate acquirement of the pullout load versus endslip relationship. The influence of fiber embedded length and orientation on the fiber pullout behavior is studied. Additionally, the separate assessment of the distinct bond mechanisms is performed, by isolating the adherence bond from the mechanical bond provided by the hook. Finally, analytical bond-slip relationships are obtained by back-analysis procedure with an interfacial cohesive model.

This paper presents a review of current knowledge on the bond behavior of fiber reinforced polymer (FRP) systems inserted in the cover of concrete elements, commonly known as the near-surface mounted technique (NSM). In the first part, by studying the physics of the phenomenon, the typical failure modes, the most common bond tests and two of the most important design guidelines for FRP NSM systems are introduced. In the second part, a database of bond tests composed by 431 records is presented and the accuracy of existing design guidelines is assessed with this data. Lastly, the formulations proposed by these design guidelines are recalibrated based on the experimental results in the database.Keywords: FRP; NSM; Bond; Review grooves have vertical and parallel sides, square and rectangular bars explore better this grooves' geometry since a more uniform adhesive thickness is achieved. Moreover, with the use of round bars, split of the groove filling cover may occur due to the existing stresses perpendicular to the FRP. In the case of square and rectangular bars this normal stress component acts mainly towards the groove lateral concrete.Comparing square and rectangular bars, the latter maximize the ratio of surface to cross-section area, minimizing the bond stresses for the same tensile force in the FRP. Other advantage of using 3 rectangular bars is related with the simplicity of opening the grooves: a single saw cut is normally enough for obtaining the groove while with round/square bars two saw cuts and removal of the concrete in between are usually required. The main disadvantage of rectangular bars is the need for a deeper groove to provide the same reinforcement area.In terms of the adhesives used to bond FRP bars to concrete, epoxy adhesives are the most common, even though some researchers have used cement mortar [4,5]. In general, cement based adhesives have lower mechanical strength and higher curing time. On the other hand, they present better performance when subjected to high temperatures.The most recent comprehensive review on the NSM technique was published in 2007 [6]. In order to provide a wider overview of the technique, it was not focused on the bond. Moreover, since then, a manifold of experimental works focusing on bond performance of FRP NSM systems have been developed. Hence, the scope of this work is to provide a review on the bond behavior of FRP NSM systems in concrete. This review includes, in the first part, an introduction to the typical observed failure modes, the most commonly used bond tests and two of the most important design guidelines. In the second part of this paper, a database of 431 bond tests is presented, the accuracy of the design guidelines is tested and several modifications to these guidelines' formulations are proposed. FRP NSM technique Failure modes at structural levelConsidering a reinforced concrete element strengthened in bending (and/or shear) with a FRP NSM system, six failure modes combining different stress states on the three intervening materials (concrete, ...

Reinforcing bars made of Glass-Fiber-Reinforced Polymers (GFRPs) are more and more common as internal reinforcement of concrete structures and infrastructures. Since the design of GFRP reinforced concrete members is often controlled by serviceability limit state criteria (i.e. deflection or crack width control), an accurate knowledge of the GFRP-concrete bond behavior is needed to formulate sound design equations. Furthermore, bond laws currently available and widely accepted for conventional steel rebars cannot be straightforwardly applied for GFRP ones. Hence, an experimental program consisting of 36 pullout bending tests was carried out to evaluate the bond performance between GFRP bars and steel fiber reinforced self-compacting concrete (SFRSCC) by analyzing the influence of the following parameters: GFRP bar diameter, surface characteristics of the GFRP bars, bond length, and SFRSCC cover thickness. Based on the results obtained in this study, pullout failure was occurred for almost all the specimens. SFRSCC cover thickness and bond length plaid important role on the ultimate value of bond stress of GFRP bars. Moreover, the GFRP bars with ribbed and sand-coated surface treatment showed different interfacial bond behaviors

The present work resumes the experimental and numerical research carried out for the development of a numerical tool able of simulating the tensile behaviour of steel fibre reinforced self-compacting concrete (SFRSCC). SFRSCC is assumed as a two phase material, where the nonlinear material behaviour of SCC matrix is modelled by a 3D smeared crack model, and steel fibres are assumed as embedded short cables distributed within the SCC matrix according to a Monte Carlo method. The internal forces in the steel fibres are obtained from the stress-slip laws derived from the executed fibre pullout tests. The performance of this numerical strategy was appraised by simulating the tensile tests carried out. The numerical simulations showed a good agreement with the experimental results.