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, ...

In the context of flexural strengthening of concrete structures, fiber reinforced polymers (FRP) have been used mostly by two main techniques: Externally Bonded Reinforcement (EBR) and Near-Surface Mounted (NSM). Both strengthening techniques are applied on the cover concrete, which is normally the weakest region of the element to be strengthened. Consequently, the most common problem is the premature failure of the strengthening system that occurs more frequently in the EBR one. In an attempt of overcoming this weakness, another technique has been proposed, called MF-EBR -Mechanically Fastened and Externally Bonded Reinforcement, which uses multi-directional carbon fiber laminates, simultaneously glued and anchored to concrete. To compare the efficiency of NSM, EBR and MF-EBR techniques, four-point bending tests with RC beams were carried out under monotonic and cyclic loading. In this work the tests are described in detail and the obtained results are discussed.Additionally, to assess the performance of a FEM-based computer program for the prediction of the behaviour of RC beams strengthening according to these techniques, the beams submitted to monotonic loading were numerically simulated. 2 IntroductionOver the last two decades, extensive research has been developed on the strengthening of reinforced concrete (RC) structures with fiber reinforced polymer (FRP) materials. High stiffness and tensile strength, low weight, easy installation procedures, high durability (no corrosion), electromagnetic permeability and practically unlimited availability in terms of geometry and size are the main advantages of these composites [1,2].The most common techniques for applying FRP's are, in general, based on the use of unidirectional FRP's through the: (i) application of fabrics (in situ cured systems) or laminates (pre-cured systems) glued externally on the surface of the element to strengthen (EBR -Externally Bonded Reinforcement); (ii) insertion of laminates (or rods) into grooves opened on the concrete cover (NSM -Near-Surface Mounted) [2,3]. Epoxy adhesives are the most used to fix the FRP to concrete. The strengthening performance of these techniques depends significantly on the resistance of the concrete cover, which is normally the most degraded concrete region in the structure due to its greater exposure to environment conditions. As a result, premature failure of FRP reinforcement can occur and, generally, the full mechanical capacity of the FRP's is not mobilized, mainly when adopting the EBR technique. To avoid this premature failure complements have been applied to the aforementioned strengthening techniques, such as the application of anchor systems composed of steel plates bolted in the ends of the FRP, the use of strapping with FRP fabric or the use of FRP anchor spikes. In addition to the stress concentration that these localized interventions introduce in the elements to strengthen, they require differentiated and time consuming tasks that can compromise the competitiveness of these techniques.More re...

Recently, laminates of multi-directional carbon fiber reinforced polymers (MDL-CFRP) have been developed for Civil Engineering applications. A MDL-CFRP laminate has fibers in distinct directions that can be arranged in order to optimize stiffness and/or strength requisites. These laminates can be conceived in order to be fixed to structural elements with anchors, resulting high effective strengthening systems. To evaluate the strengthening potentialities of this type of laminates, pullout tests were carried out. The influence of the number of anchors, their geometric location and the applied pre-stress are analyzed. The present work describes the carried-out tests and presents and analyzes the most significant obtained results.

This paper presents design procedures for fibre reinforced polymer (FRP) systems inserted in the cover of concrete elements according to the near-surface mounted (NSM) technique. Such strengthening system depends greatly on their bond strength. Two existing design formulations to estimate the bond strength of NSM FRP systems in concrete are studied. A reliability analysis is conducted with the purpose of making the design formulations consistent with the partial safety factors philosophy, including the Eurocodes.Hence, the necessary probabilistic distributions are calibrated based on a large database of bond tests. The results presented herein show that the existing guidelines can be extended and adopted under the framework of the Eurocodes. However, mainly due to their limitations in addressing individually all the possible failure modes, the variability of the probabilistic distributions found are quite high, leading to high partial coefficients of safety.This work is developed within the framework of strengthening concrete 2 structures with fibre reinforced polymers (FRP). One of the most effective 3 techniques to do so consists on the insertion of FRP bars into grooves opened 4 on the concrete cover of the element to be strengthened. Typically, these 5 FRP bars are fixed to concrete with an epoxy adhesive. These procedures 6 are commonly designated as near-surface mounted technique (NSM). Despite 7 the progress that has been made in the past years, design formulations to 8 safely apply NSM FRP systems in the strengthening of concrete structures 9 are still incipient [1, 2]. 10 One of the most critical aspects regarding the NSM technique is related 11 to the bond behaviour of the composite system [3], i.e. the stresses transfer 12 between concrete and the FRP reinforcing bar. To better understand that 13 behaviour, extensive bond tests have been carried out worldwide. Despite 14 the existence of a manifold of test setups, those can be grouped in two main 15 types: (i) direct and (ii) beam pullout tests [1]. In this work only the first 16 type of pullout test setup is addressed as explained in further sections. 17 Considering the bond behaviour of a direct pullout specimen (see an ex-18 ample in Fig. 1), five local failure modes can be identified. Two have cohesive 19 nature and occur either within the adhesive layer binding FRP to concrete 20 (A) or into the concrete surrounding the groove (C). Other two failure modes 21 have adhesive nature since they occur in the existing two interfaces, namely, 22 Finally, if none of the previous four failure modes occurred, failure will hap-24 pen by FRP tensile rupture (F) [1]. 25 In a previous work, a database of each one of the referred two types 26 of bond tests was gathered [1]. Based on it, two of the most important 27 guidelines for the design of NSM FRP systems were tested. One guideline is 28 proposed for the design and construction of externally bonded FRP systems 29 for strengthening concrete structures by the American Concrete Institute [4] 30 referred in the p...

This paper introduces a numerical simulation tool using the Finite Element Method (FEM) for near-surface mounted (NSM) strengthening technique using fibre reinforced polymers (FRP) applied to concrete elements.