Fiber Reinforced Polymer Systems Research Articles

Modeling and verification of response of RC columns strengthened in flexure with mechanically fastened FRP

One of the most important challenges exists in using of carbon fiber reinforced polymer(FRP) for strengthening of concrete members is the brittle behavior of RC members due to premature de-bonding of the layers. Mechanically fastened FRP (MF-FRP) systems are emerging as a promising method to prevent de-bonding of FRP strips and the prior brittle failure of strengthened RC members. The approach entails the use of fiber-reinforced polymer connected to the concrete substrate by means of steel anchors. In this research, a new anchor designed to delay de-bonding of FRP in reinforced concrete (RC) strengthened columns. Two reinforced concrete columns strengthened using the externally bonded conventional method and the new proposed method in comparison with unstrengthened RC column in the Laboratory. Specimens subjected to constant axial loading and cyclic lateral loading. Besides, numerical simulations performed using finite element models for nonlinear analyses of RC specimens: un-strengthened column; the column strengthened conventionally EB method and the column using new MF method.

Shape memory alloy-carbon fiber reinforced polymer system for strengthening fatigue-sensitive metallic structures

This paper provides an overview of the basic elements, course of development, experimental evaluation, and numerical simulation of a thermally activated shape memory alloy (SMA) and carbon fiber reinforced polymer (CFRP) composite system for fatigue repair or retrofit of metallic structures. Nickel titanium niobium (NiTiNb) SMA wires, which are able to generate 400 MPa recovery stress upon thermal loading and maintain that stress level at a wide range of temperatures, was adopted to apply compressive stresses near the crack. Monotonic bond behavior of single and multiple SMA wires to CFRP was investigated; the debonding onset load and maximum capacity were quantified. The fatigue behavior of patches consisting of multiple wires bonded to CFRP was studied. Results indicated that the system could maintain 80% of the recovery stress, after up to 2 million load cycles, so long as the maximum applied stress was below the debonding onset level. A fatigue strengthening system, using such multiple SMA wire system as underlay and CFRP patch as overlay, was applied to fatigue sensitive steel plates for fatigue life improvement evaluation. The average fatigue life of the patched steel plates was over 26 times longer than that of the unpatched plates tested at the same load range. Finally, a numerical framework was developed to simulate the fatigue crack growth in steel plates patched with such strengthening system and was validated by the experimental data. The findings suggest that the proposed system could be a promising alternative to traditional techniques for fatigue crack repair.

Coupled interface-based modelling approach for the numerical analysis of curved masonry specimens strengthened by CFRP

Aim of the present paper is to numerically study the bond behavior of curved masonry specimens externally strengthened by Carbon Fiber Reinforced Polymer systems (CFRP). A simple 1D-modeling approach is presented to this aim, where the coupled behavior between shear and normal stresses developing at the reinforcement/masonry interface level is specifically introduced to properly account for the role played by the curvature radius. The model is indeed enriched by the introduction of shear stress-slip laws able to account for the beneficial friction effect, when compression normal stresses develop at the interface level and the reduction of the slip strength corresponding to the de-cohesion in presence of normal stresses in tension. Considering some case studies derived from the current literature, consisting of shear-lap bond tests of curved masonry specimens characterized by different curvatures of the bonded surface and different strengthening configurations, the validation of the proposed approach is carried out. In particular, two modeling strategies are considered and critically compared: the first one, denoted as approach (A), where the presence of the mortar joints is neglected, and the second one, denoted as approach (B), where mortar joints are specifically introduced in the model. Finally, the results obtained by using the proposed simple approach are compared with those obtained from both sophisticated FE numerical models and theoretical formulas deduced from the current literature.

FRP-Retrofitting RC Bridges for Stabilization in Earthquakes

The purpose of this abbreviated paper is to analyze the applicability of FRP (fiber-reinforced polymer)-strengthening systems for retrofitting RC (reinforced-concrete) highway bridges to stabilize structural members under earthquake conditions. Three earthquakes in California, with multiple bridge failures, drove the requirement to improve bridge design and methods of structurally upgrading critical bridge components to withstand impacts of earth tremors. Retrofitting RC bridges by encasing and strengthening bridge bents with FRP-systems provides an effective method of mitigating the impacts of earthquake loading. This analysis considers types of retrofitting applications on bridge bents and the advantages of FRP systems to increase structural member axial capacity and ductility in columns and beams.

Shear behavior of concrete beams reinforced exclusively with longitudinal glass fiber reinforced polymer bars: Analytical model

A design approach based on the simplified modified compression field theory, but with the advantage of not requiring an iterative procedure, is proposed for evaluating the shear capacity of beams without including shear reinforcement. This model is capable of simulating beams flexurally reinforced with steel and fiber reinforced polymer (FRP) systems. To appraise the predictive performance of the proposed model, a database (DB) composed of 215 reinforced concrete beams without shear reinforcement is set. By applying the model to this DB, an average value for Vexp./Vana. of 1.05 with a COV of 24% is obtained, where Vexp. and Vana. are the shear capacity registered experimentally and obtained with the model, respectively. By applying the approach proposed by ACI Building Code to the DB, average and COV values of 0.91 and 42% are determined, revealing the higher predictive performance of the proposed model. This was even higher in the beams flexurally reinforced with FRP systems, since the proposed model conducted to average and the COV values of 1.0 and 22%, while 0.76 and 32% were obtained with the ACI approach. When proposed model was applied to the beams tested in the experimental program carried out, an average and COV values of 1.02 and 5.23% were determined.

Effects of Environmental Conditioning on the Bond Behavior of FRP and FRCM Systems Applied to Concrete Elements

This paper presents the results of an extensive experimental campaign of bond tests aimed to assess and compare the influence of several environmental conditioning factors (humidity and temperature) on the bond behavior of two different types of composites systems glued to concrete elements: a fiber-reinforced polymer (FRP) system made of a carbon sheet applied with epoxy resin and a polybenzoxozole (PBO) grid applied with a cement-based mortar, i.e., a fiber-reinforced cementitious matrix (FRCM) system. Several environmental conditions have been considered (partial immersion in water at 23, 30, and 40°C for short and long periods with and without further drying processes, exposure in air at 30 and 40°C) before testing the specimens according to two well-known setups for bond tests: a single push-pull shear test and a beam test. The experimental results were mainly analyzed in terms of failure modes and loads, showing a clearly negative effect of the conditioning factors for the specimens with the carbon fibre reinforced polymer (CFRP) sheet as the conditioning time increases because of the plasticization phenomena of the epoxy adhesive. Conversely, for the specimens with the PBO grid, the failure loads were slightly lower or even greater than the ones relieved for the reference specimens as the exposure periods increase, whereas in the case of short exposure, the bond strength reduced and the scattering of the experimental resulted increased.

Strengthening of shear-critical RC beams: Alternatives to externally bonded CFRP sheets

Extensive research has been conducted on strengthening of shear-critical RC beams. This previous research has helped to better understand the behaviour of shear strengthening systems and has improved their performance. However, there is still a potential to further improve upon the performance of existing shear strengthening systems. A cement-based composite system is an innovative strengthening system that shares many of the benefits of fiber reinforced polymer (FRP) systems (such as light weight, ease of installation and non-corroding) while overcoming some of the draw backs of epoxy-based FRP systems (including poor compatibility with concrete substrate, lack of vapour permeability and fire resistance). The current paper presents the results of an experimental study conducted to investigate the effectiveness of cement-based composite systems in comparison to an epoxy-based system (carbon fiber reinforced polymer, CFRP) to strengthen shear-critical RC beams. Two types of cement-based systems were investigated in this study: carbon fiber reinforced polymer (CFRP) grid embedded in mortar and carbon fabric reinforced cementitious mortar (CFRCM). The results showed that the cement-based systems (CFRP grid in mortar and CFRCM) perform better compared to the epoxy-based system (CFRP sheet) in terms of increase in shear capacity relative to ultimate strength of the strengthening systems. In addition, the CFRP grid embedded in mortar is the most efficient shear strengthening system due to its excellent bond to concrete.

New Model Derivation for the Bond Behavior of NSM FRP Systems in Concrete

In this study, two different methods of data mining are used to develop new formulations for predicting the maximum bond strength force of near-surface-mounted fiber-reinforced polymer (FRP) systems in concrete. The advantages of each method are employed to find the most important parameters in estimating the maximum bond strength. A comprehensive database is used to develop new formulations for the maximum bond strength. Several effective parameters like geometrical and mechanical properties of the FRP, geometrical properties of the groove, bonded length, and concrete strength are involved in prediction of the maximum bond strength. The multivariate adaptive regression splines algorithm is implemented to accomplish sensitivity analysis. The results indicate that the geometry and mechanical properties of the FRP are the most important parameters. Alternatively, the M5′ algorithm as a rule-based method is employed to develop a more practical and simple model for estimating the maximum bond strength. Comparison of the developed models and the most common design codes demonstrates the superiority of the models in terms of the accuracy. Furthermore, the safety analysis based on demerit points classification scale also confirms the reliability of the proposed formulations.

FE Model Predicting the Load Carrying Capacity of Progressive FRP Strengthening of Masonry Arches Subjected to Settlement Damage

The retrofitting of masonry structures subject to differential foundation settlements is both an important and a highly challenging practice. Especially in the consideration of historical monuments, this challenge requires a strategic balance between providing the necessary modifications to ensure public safety while maintaining the integrity of the original structure. The use of strategically placed composite materials such as fiber reinforced polymers (FRPs) provides the potential to remove this dilemma and both preserve heritage while introducing a modern level of safety. This work studies, from an advanced FE point of view, a progressive reinforcement strategy to both strengthen and control the failure mechanism for masonry arches with an existing state of damage induced from a vertical differential abutment settlement. A heterogeneous FE approach of a semi-circular block and mortar arch on settled supports is examined. In this model a damage plasticity behavior is assigned to the mortar joints to allow for the hinge formations. Then utilizing the Italian CNR Recommendations for externally bonded FRP systems and the Abaqus birth and death approach, FRPs are introduced and strategically placed onto the settled support model after the initial hinge development. Finally, the structural behavior of the reinforced and unreinforced models are examined for an applied horizontal acceleration at a fixed support displacement.

Strengthening Masonry Arches with Lime-Based Mortar Composite

In recent decades, many strengthening interventions on masonry elements were performed by using fiber reinforced polymers (FRPs). These advanced materials proved to be effective to increase the load-carrying capacity of masonry elements and to improve their structural behavior, avoiding the most critical failure modes. Despite the advantages of this technique compared to more traditional methods, FRP systems have disadvantages related to their low resistance to high temperatures, impossibility of application on wet surfaces, low permeability, and poor compatibility with masonry supports. Therefore, composite materials made of a fiber textile embedded in an inorganic matrix were recently proposed as alternatives to FRPs for strengthening historic masonry constructions. These composite materials are easier to install, have higher resistance to high temperatures, and permit higher vapor permeability than FRPs. The inorganic matrix is frequently a cement-based mortar, and the composite materials made of a fiber textile embedded in a cement-based mortar are usually identified as FRCM (fabric reinforced cementitious matrix) composites. More recently, the use of natural lime mortar as an inorganic matrix has been proposed as an alternative to cement-based mortars when historic compatibility with the substrate is strictly required, as in case of restoration of historic buildings. In this paper, the effectiveness of a fabric made of basalt fibers embedded in lime mortar matrix (Basalt-FRLM) for the strengthening of masonry arches is investigated. An experimental investigation was performed on 1:2 scaled brick masonry arches strengthened at the extrados with a layer of Basalt-FRLM and tested under vertical load. The results obtained are compared with previous results obtained by the authors by testing masonry arches strengthened at their extrados with FRCM and FRP composites. This investigation highlights the effectiveness of Basalt-FRLM in increasing load-currying and the displacement capacities of masonry arches. The Basalt-FRLM-strengthened arch exhibited higher displacement capacity when compared to arches strengthened with polymeric and cementitious matrix composites.

Bond between TRM versus FRP composites and concrete at high temperatures

The use of fibre reinforced polymers (FRP) as a means of external reinforcement for strengthening the existing reinforced concrete (RC) structures nowadays is the most common technique. However, the use of epoxy resins limits the effectiveness of FRP technique, and therefore, unless protective (thermal insulation) systems are provided, the bond capacity at the FRP-concrete interface will be extremely low above the glass transition temperature (Tg). To address problems associated with epoxies and to provide cost-effectiveness and durability of the strengthening intervention, a new composite cement- based material, namely textile-reinforced mortar (TRM) has been developed the last decade. This paper for the first time examines the bond performance between the TRM and concrete interfaces at high temperatures and, also compares for the first time the bond of both FRP and TRM systems to concrete at ambient and high temperatures. The key parameters investigated include: (a) the matrix used to impregnate the fibres, namely resin or mortar, resulting in two strengthening systems (TRM or FRP), (b) the level of high temperature to which the specimens are exposed (20, 50, 75, 100, and 150 °C) for FRP-reinforced specimens, and (20, 50, 75, 100, 150, 200, 300, 400, and 500 °C) for TRM-strengthened specimens, (c) the number of FRP/TRM layers (3 and 4), and (d) the loading conditions (steady state and transient conditions). A total of 68 specimens (56 specimens tested in steady state condition, and 12 specimens tested in transient condition) were constructed, strengthened and tested under double- lap direct shear. The result showed that overall TRM exhibited excellent performance at high temperature. In steady state tests, TRM specimens maintained an average of 85% of their ambient bond strength up to 400 °C, whereas the corresponding value for FRP specimens was only 17% at 150 °C. In transient test condition, TRM also outperformed over FRP in terms of both the time they maintained the applied load and the temperature reached before failure.

Effectiveness of externally bonded FRP systems in strengthening of RC structures in seismic areas

This study concerns the use of externally bonded fibre-reinforced polymer (FRP) materials in the restoration of existing buildings. The first part of the paper reports on the results of an experimental programme developed by the author and aimed at evaluating the cyclic response of previously damaged reinforced-concrete (RC) specimens repaired using carbon-FRP reinforcements. The test results showed that the adopted reinforcing systems were capable of giving repaired specimens a significant increase in strength and deformation capacity as compared with undamaged specimens tested without external reinforcement (control specimens). A remarkable degradation of stiffness was observed in the repaired specimens. In order to evaluate the influence of the presence of damaged columns, inelastic dynamic analysis of spatial structures was performed. The numerical results obtained indicate better seismic performance of structures containing repaired elements when compared with corresponding structures comprising undamaged members.

Bond-Slip Models for FPR-Concrete Interfaces Subjected to Moisture Conditions

Environmental related durability issues have been of great concerns in the structures strengthened with the fiber reinforced polymers (FRPs). In marine environment, moisture is one of the dominant factors that adversely affect the material properties and the bond interfaces. Several short-term and long-term laboratory experimental investigations have been conducted to study such behaviors but, still, there are insufficient constitutive bond models which could incorporate moisture exposure conditions. This paper proposed a very simple approach in determining the nonlinear bond-slip models for the FRP-concrete interface considering the effect of moisture conditions. The proposed models are based on the strain results of the experimental investigation conducted by the authors using 6 different commercial FRP systems exposed to the moisture conditions for the maximum period of 18 months. The exposure effect in the moisture conditions seems to have great dependency on the FRP system. Based on the contrasting differences in the results under moisture conditions, separate bond-slip models have been proposed for the wet-layup FRP and prefabricated FRP systems. As for the verification of the proposed model under moisture conditions, predicted pull-out load was compared with the experimental pull-out load. The results showed good agreement for all the FRP systems under investigation.

Using data mining algorithms to predict the bond strength of NSM FRP systems in concrete

This paper presents the effectiveness of soft computing algorithms in analyzing the bond behavior of fiber reinforced polymer (FRP) systems inserted in the cover of concrete elements, commonly known as the near-surface mounted (NSM) technique. It focuses on the use of Data Mining (DM) algorithms as an alternative to the existing guidelines’ models to predict the bond strength of NSM FRP systems. To ease and spread the use of DM algorithms, a web-based tool is presented. This tool was developed to allow an easy use of the DM prediction models presented in this work, where the user simply provides the values of the input variables, the same as those used by the guidelines, in order to get the predictions. The results presented herein show that the DM based models are robust and more accurate than the guidelines’ models and can be considered as a relevant alternative to those analytical methods.

Fracture-based interface model for NSM FRP systems in concrete

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.In order to properly simulate the structural behaviour of NSM FRP systems there are three materials (concrete, FRP and the adhesive that binds them) and two interfaces (FRP/adhesive and adhesive/concrete) that shall be considered.This work presents the major details of a discontinuous-based constitutive model which aims at simulating NSM FRP interfaces implemented in the FEMIX FEM software. This constitutive model was adapted from one available in the literature, originally employed for fracture simulation in meso-scale analyses of quasi-brittle materials, which is based on the classical Flow Theory of Plasticity combined with fracture mechanics concepts. The most important features of the implemented constitutive model are the consideration of both fracture modes I and II and the possibility of performing 2D and 3D analysis.In the end, results based on FEM simulations are presented with the aim of investigating the soundness and accuracy of the constitutive model to simulate NSM FRP systems’ interfaces.

A Seismic Strengthening Technique for Reinforced Concrete Columns Using Sprayed FRP.

Conventional methods for seismic retrofitting of concrete columns include reinforcement with steel plates or steel frame braces, as well as cross-sectional increments and in-filled walls. However, these methods have some disadvantages, such as the increase in mass and the need for precise construction. Fiber-reinforced polymer (FRP) sheets for seismic strengthening of concrete columns using new light-weight composite materials, such as carbon fiber or glass fiber, have been developed, have excellent durability and performance, and are being widely applied to overcome the shortcomings of conventional seismic strengthening methods. Nonetheless, the FRP-sheet reinforcement method also has some drawbacks, such as the need for prior surface treatment, problems at joints, and relatively expensive material costs. In the current research, the structural and material properties associated with a new method for seismic strengthening of concrete columns using FRP were investigated. The new technique is a sprayed FRP system, achieved by mixing chopped glass and carbon fibers with epoxy and vinyl ester resin in the open air and randomly spraying the resulting mixture onto the uneven surface of the concrete columns. This paper reports on the seismic resistance of reinforced concrete columns controlled by shear strengthening using the sprayed FRP system. Five shear column specimens were designed, and then strengthened with sprayed FRP by using different combinations of short carbon or glass fibers and epoxy or vinyl ester resins. There was also a non-strengthened control specimen. Cyclic loading tests were carried out, and the ultimate load carrying capacity and deformation were investigated, as well as hysteresis in the lateral load-drift relationship. The results showed that shear strengths and deformation capacities of shear columns strengthened using sprayed FRP improved markedly, compared with those of the control column. The spraying FRP technique developed in this study can be practically and effectively used for the seismic strengthening of existing concrete columns.

Modelling beam-column joints and FRP strengthening in the seismic performance assessment of RC existing frames

The recent seismic events demonstrated that the high vulnerability of existing RC structures is often related to the joint subassemblies shear failure which may lead to the collapse of the entire structural system. However, most of the available computer programs do not properly account for this aspect. This paper deals with the numerical seismic assessment of RC structural systems designed without proper seismic details in the joint panel and the benefits of the FRP local strengthening. A new modelling strategy has been developed to account for the joint nonlinear behaviour and the fiber reinforced polymer (FRP) strengthening in the finite element method (FEM). Several case studies were selected for the model validation. At the subassembly level, the model predictions were compared with recent experimental tests on full-scale beam-column joints with and without FRP strengthening. At the structure level, a case study building damaged in the joints during the L’Aquila earthquake and then retrofitted with FRP systems was analyzed. The predicted structural performances and structural damages were compared with observational data. Furthermore, the benefits of the joint FRP strengthening on the global seismic performances were numerically assessed.

Concrete-Filled-Large Deformable FRP Tubular Columns under Axial Compressive Loading

The behavior of concrete-filled fiber tubes (CFFT) polymers under axial compressive loading was investigated. Unlike the traditional fiber reinforced polymers (FRP) such as carbon, glass, aramid, etc., the FRP tubes in this study were designed using large rupture strains FRP which are made of recycled materials such as plastic bottles; hence, large rupture strain (LRS) FRP composites are environmentally friendly and can be used in the context of green construction. This study performed finite element (FE) analysis using LS-DYNA software to conduct an extensive parametric study on CFFT. The effects of the FRP confinement ratio, the unconfined concrete compressive strength ( ), column size, and column aspect ratio on the behavior of the CFFT under axial compressive loading were investigated during this study. A comparison between the behavior of the CFFTs with LRS-FRP and those with traditional FRP (carbon and glass) with a high range of confinement ratios was conducted as well. A new hybrid FRP system combined with traditional and LRS-FRP is proposed. Generally, the CFFTs with LRS-FRP showed remarkable behavior under axial loading in strength and ultimate strain. Equations to estimate the concrete dilation parameter and dilation angle of the CFFTs with LRS-FRP tubes and hybrid FRP tubes are suggested.

Review of Current Design Guidelines for Circular FRP-Wrapped Plain Concrete Cylinders

With the widespread use of fiber-reinforced polymer (FRP) composites in the construction sector as a strengthening technique, the development of design guidelines for the field application of externally bonded FRP systems is ongoing in Europe, Japan, Canada, and the United States. The main goal of this study is to evaluate the current seven international design guidelines and four other design models for the prediction of confined concrete compressive strength of FRP-wrapped plain concrete cylinders against the experimental results of a large database of 812 specimens reported in the literature. The results clearly show that the reliability of predictions of confined concrete compressive strength of FRP-wrapped plain concrete cylinders by the design guidelines significantly varies for different ranges of unconfined concrete compressive strength. For example, the gain in confined concrete compressive strength of FRP-wrapped low- and medium-strength concrete cylinders is larger than that of high- and ultrahigh-strength concrete cylinders. Furthermore, a simplified model for the prediction of design/characteristic–confined concrete compressive strength is developed based on the design-assisted-by-testing approach. The developed simplified model accounts for the variation in confinement effectiveness for different ranges of unconfined concrete strengths.

Experimental Investigation on FRC Beams Strengthened with GFRP Laminates

The external bonding of fibre reinforced polymer (FRP) to reinforced concrete (RC) members has become a popular method of retrofitting/strengthening concrete structures in recent years. Extensive research has been conducted pertaining to RC beams strengthened with FRP laminates. However, the experimental studies on fibre reinforced concrete (FRC) beams strengthened using externally bonded FRP system are limited. The purpose of this research is to investigate the behaviour of steel fibre reinforced concrete (SFRC) beams strengthened with glass fibre reinforced polymer (GFRP) laminates. The beam specimens were incorporated with 1.0% volume fraction of short-steel fibres randomly distributed throughout the section. The beam cross-section was 150 mm wide and 250 mm deep and to a length of 3000 mm. All the beams were tested until failure. The study parameters of this investigation included service load, ultimate load, ductility, crack width and failure modes. Beams tested for this investigation consisted of reference (RC) beam, GFRP laminated RC beam, SFRC beam, and GFRP laminated SFRC beam. The test results showed that the SFRC beams strengthened with GFRP laminates exhibited better performance.