Near Surface Mounted Fiber Reinforced Polymer Strips Research Articles

Bond behavior between near-surface mounted fiber reinforced polymer strips and concrete using nonlinear finite element: a parametric study

A nonlinear finite element (NLFE) model is proposed to predict the bond behavior between concrete and near-surface mounted (NSM) fiber-reinforced polymer (FRP). The model is validated, based on the results from an experimental program carried out by the present authors, before being employed to generate a wide range of data considering key parameters, such as grooves spacing (50–200 mm), and embedment length (50–300 mm), and fiber type (basalt and carbon) of the NSM FRP strips as well as concrete's strength (25–55 MPa). The impact of steel reinforcement is also evaluated. The findings demonstrate a significant impact of NSM FRP strips' type, embedment length, and grooves' spacing on bond behavior. The higher-strength grade for concrete imparted noticeable improvements in bond characteristics, whereas the presence of steel reinforcement seems to enhance the pull-out force slightly by as much as 15% yet reduce bond ductility. The NLFE cracking patterns are used to help understand the influence of different parameters on bond failure mode between the NSM FRP strips and concrete.

Bond strength model for near-surface mounted (NSM) FRP bonded joints: Effect of concrete edge distance

As one of the mainstream topics in the research field of infrastructure application of FRP composites, the near-surface mounted (NSM) fiber-reinforced polymer (FRP) strengthening technique has attracted an increasing attention over the last decade and become an effective alternative to the externally bonded FRP strengthening method. The bond strength (i.e., maximum force that can be developed in the FRP reinforcement) of NSM FRP bonded joints has been extensively studied by worldwide researchers, and several bond strength models have been established. All the existing bond strength models, however, are not able to consider the detrimental effect of insufficient concrete edge distance (i.e., the distance between the groove and the nearer edge of the concrete). To clarify such detrimental effect on the bond strength between NSM FRP strip and concrete, a numerical parametric study, employing a three-dimensional meso-scale model developed by the author for NSM FRP bonded joints, is conducted in the present study. Based on the results from the parametric study, reduction factors are formulated to consider such detrimental effect and a bond strength model incorporating the reduction factors are proposed. The accuracy of the proposed bond strength model is verified with a large test database containing 86 specimens.

Flexural Strength of RC Beam Strengthened by Partially De-bonded Near Surface-Mounted FRP Strip

This paper presents an experimental work to study the flexural strength of reinforced concrete (RC) beams strengthened by partially de-bonded near surface-mounted (NSM) fiber reinforced polymer (FRP) strip with various de-bonded length. Especially, considering high anchorage capacity at end of a FRP strip, the effect of de-bonded region at a central part was investigated. In order to check the improvement of strength or deformation capacity when the bonded surface area only increased without changing the FRP area, single and triple lines of FRP were planned. In addition, the flexural strength of the RC member strengthened by a partially de-bonded NSM FRP strip was evaluated by using the existing researchers’ strength equation to predict the flexural strength after retrofit. From the study, it was found that where de-bonded region exists in the central part of a flexural member, the deformation capacity of the member is expected to be improved, because FRP strain is not to be concentrated on the center but to be extended uniformly in the de-bonded region. Where NSM FRP strips are distributed in triple lines, a relatively high strength can be exerted due to the increase of bond strength in the anchorage.

Experimental Evaluation of Static Cyclic In-Plane Shear Behavior of Unreinforced Masonry Walls Strengthened with NSM FRP Strips

An experimental study was conducted to assess the effectiveness of strengthening unreinforced-masonry (URM) shear panels with near surface-mounted (NSM) fiber-reinforced polymer (FRP) strips. A total of 23 wall panels (5 URM and 18 reinforced) were subjected to vertical precompression combined with either monotonic or increasing reversing cycles of in-plane lateral displacement under fixed-fixed boundary conditions. Two wall aspect ratios (height/length) and six different reinforcement schemes were tested. The experimental program was designed to produce diagonal cracking in the URM specimens and hence investigate the effectiveness of the various reinforcement schemes in controlling this failure mode. This was achieved for the aspect ratio 1 wall panels. The study revealed that the FRP strengthening was effective in improving the ultimate load, displacement capacity, ductility, and energy dissipation compared with the URM response. For the aspect ratio 0.5 panels, base sliding failures dominated the experimental program, making it difficult to fully assess the effectiveness of the various reinforcing schemes.

Effect of high temperature on bond strength of near-surface mounted FRP reinforcement

This paper presents results from an experimental study on the effect of temperature on bond strength and modulus of near-surface mounted (NSM) fiber-reinforced polymer (FRP) strengthened concrete. 36 NSM FRP specimens, fabricated using different types of epoxy adhesive and FRP reinforcement, were tested to evaluate bond strength in 20–400°C temperature range. Results from these tests indicate that bond strength and modulus decrease significantly in 20–200°C temperature range, and only retain 20–30% of their original values at 200°C. NSM FRP strips and rods possess negligible bond strength beyond 400°C. Data from tests also indicates that NSM FRP system exhibits similar bond stress–slip response at both room and high temperatures. However, the peak value (bond strength) and the slope (bond modulus) are much lower at high temperatures. Data from the tests is utilized to propose empirical relations for variation of bond strength and modulus of NSM FRP strengthening system with temperature.

Design formula to evaluate the NSM FRP strips shear strength contribution to a RC beam

This paper presents the closing step of a synthesis process aiming at deriving, from a previously developed more complex model, a simple design formula to evaluate the shear strength contribution provided by a system of Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) strips to a Reinforced Concrete (RC) beam. The self-contained and ready-to-implement set of analytical equations and logical operations is presented along with the main underlying physical-mechanical principles and assumptions. The formulation proposed is appraised against some of the most recent experimental results and its predictions are also compared with those obtained by the two previous and more sophisticated versions of the same modeling strategy. Monte Carlo simulations are carried out in order to appraise the sensitivity of the NSM shear strength contribution prediction to the value assumed by the input parameters.

Three dimensional mechanical model to simulate the NSM FRP strips shear strength contribution to a RC beam: Parametric studies

This paper presents the results of a parametric study carried out by means of a mechanical model recently developed to simulate the shear strength contribution provided by a system of Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) strips to a Reinforced Concrete (RC) beam throughout its loading process. That model, developed fulfilling equilibrium, kinematic compatibility and constitutive laws of both materials, concrete and FRP, and local bond between themselves, takes into consideration the possibility that the NSM strips may fail due to: loss of bond (debonding), concrete semi-conical tensile fracture or strip tensile rupture. It also takes into consideration: (a) interaction between progressive force transferred by bond to the surrounding concrete and its tensile fracture and (b) bi-directional interaction among adjacent strips placed on the two sides of the strengthened beam cross-section web. In the first part of the paper attention is focused on the bond-based behavior of a single NSM FRP strip mounted on a concrete prism. The influence of each geometrical–mechanical parameter on the peak force transferable through bond stresses to the surrounding concrete, excluding the possibility of either concrete fracture and strip rupture, is analyzed. In the second part of the paper attention is focused on the comprehensive behavior of a single NSM FRP strip mounted on a concrete prism. The influence of each geometrical–mechanical parameter on the peak force transferable to the surrounding concrete, also including the possibility of both concrete fracture and strip rupture, is analyzed. The third part of the paper aims at assessing the influence of each geometrical–mechanical parameter on the maximum shear strength contribution provided by a system of NSM FRP strips to a RC beam. The results of these studies are presented along with the main findings.

Theoretical Model and Computational Procedure to Evaluate the NSM FRP Strips Shear Strength Contribution to a RC Beam

This paper presents a computational procedure to evaluate the shear strength contribution provided to a reinforced concrete (RC) beam by a system of near-surface mounted (NSM) fiber reinforced polymer (FRP) strips. This procedure is based on the evaluation of (1) the constitutive law of the average-available bond-length NSM FRP strip effectively crossing the shear crack, and (2) the maximum effective capacity it can attain during the loading process of the strengthened beam. Because of complex phenomena such as (1) interaction between forces transferred through bond to the surrounding concrete and the concrete fracture, and (2) interaction among adjacent strips, the NSM FRP strip constitutive law is largely different than the linear elastic one characterizing the FRP behavior in tension. Once the constitutive law of the average-available bond-length NSM strip is reliably known, its maximum effective capacity can be determined by imposing a coherent kinematic mechanism. The self-contained and ready-to-impl...

Assessment of the Predictive Performance of Existing Analytical Models for Debonding of Near-Surface Mounted FRP Strips

The near-surface mounted (NSM) strengthening technique, in which pultruded fiber reinforced polymer (FRP) plates or bars are adhesively bonded into grooves cut in the concrete cover, has proven to overcome several of the drawbacks of externally bonded FRP plates on reinforced concrete flexural members. However, as research on the NSM technique is relatively recent, the theoretical foundation for its application is not yet as complete, and some of the debonding mechanisms are not yet fully understood. This paper presents the main findings of a literature review aimed at identifying the existing analytical models to predict the debonding strength of reinforced concrete flexural elements retrofitted with NSM FRP strips, and of a thorough assessment of the adequacy of such models by comparison with all relevant published experimental data. It is concluded that for debonding of NSM FRP strips induced by flexural (or flexural-shear) cracking or the critical diagonal shear crack of the reinforced concrete member, the currently available models capture the behavior of the debonding mechanism and provide reasonably conservative estimations of the debonding strength. However, for debonding induced by stress concentrations at the strip ends, existing models do not fully capture the debonding behavior and as such, are shown to be excessively conservative.

New Approach for Modeling the Contribution of NSM FRP Strips for Shear Strengthening of RC Beams

This paper presents the main features of an analytical model recently developed to predict the near-surface mounted (NSM) fiber-reinforced polymer (FRP) strips shear strength contribution to a reinforced concrete (RC) beam throughout the beam's loading process. It assumes that the possible failure modes that can affect the ultimate behavior of an NSM FRP strip comprise: loss of bond (debonding); concrete semiconical tensile fracture; mixed shallow-semicone-plus-debonding; and strip tensile fracture. That model was developed by fulfilling equilibrium, kinematic compatibility, and constitutive law of both the adhered materials and the bond between them. The debonding process of an NSM FRP strip to concrete was interpreted and closed-form equations were derived after proposing a new local bond stress-slip relationship. The model proposed also addressed complex phenomena such as the interaction between the force transferred to the surrounding concrete through bond stresses and concrete fracture as well as the interaction among adjacent strips. The main features of the proposed modeling strategy are shown along with the main underlying physical-mechanical concepts and assumptions. Using recent experimental data, the predictive performance of the model is assessed. The model is also applied to single out the influence of relevant parameters on the NSM technique effectiveness for the shear strengthening of RC beams.

Bond Model of NSM-FRP Strips in the Context of the Shear Strengthening of RC Beams

Gluing of fiber reinforced polymer (FRP) strips by a structural adhesive into thin slits cut in the cover of reinforced concrete (RC) beams' lateral faces is a shear strengthening technique that is gaining increasing attention of the FRP community. Nonetheless, certain aspects related to the failure modes affecting the behavior at ultimate of such near surface mounted (NSM) strips need to be clarified. Recent findings have spotlighted that besides debonding and tensile rupture of the strips, another possible failure mode should be considered, which involves the fracture of concrete in the shape of semicones along the strips. Thus, a comprehensive analytical model accounting for all of those failure modes was recently developed for predicting the NSM contribution to RC beams shear capacity. Despite its consistency with experimental recordings, that model still needs improvement since some aspects related to the interaction between the force transferred by bond to the surrounding concrete and its tensile fracture still need to be clarified. The need to improve that analytical model led us to further address the issues regarding the debonding failure mode affecting the behavior, at ultimate, of a NSM FRP strip. A new local bond stress-slip relationship is proposed and closed-form equations to be implemented in that analytical model are derived and appraised on the basis of some of the most recent experimental results available in the literature.