Bonded Fiber Reinforced Polymer Composites Research Articles

Effects of steel surface treatment with silanized carbon nanotubes on the bonding properties between steel and epoxy adhesive

ABSTRACT The interfacial bonding properties of steel and adhesives are the key factors affecting the application of external bonded fiber reinforced polymer composites to strengthen steel structures. The present study proposed a new method to treat steel surface with carbon nanotubes (CNTs) for modification of the interfacial bond strength between steel and an epoxy adhesive. CNTs were modified in ethanol-water mixture by silane and PVP (Polyvinyl Pyrrolidone) to improve the dispersity of CNTs. Steel plates treated through dip-coating or spray-coating with the modified CNTs suspensions were used for single lap joint tests. Compared to the control samples, the treated steel surfaces resulted in a significant increase in the interface shear strength (i.e., by 52.8%). The desirable enhancement is attributed to the existence of CNT between the steel and adhesives, which is connected to the steel surface through Si-O-Fe covalent.

A neuro-fuzzy approach to estimate the shear contribution of externally bonded FRP composites

The wide usage of fiber-reinforced polymer (FRP) composites as an external shear strengthening material have resulted in the development of various design guidelines. However, the accuracy of the design guidelines is a matter of concern since the shear behavior of the RC beam becomes more complex with the addition of FRP composites. On the other side, soft computing methods have been proved to be efficient enough to analyze complex systems. However, their application in structural engineering is limited. Therefore, in the current investigation, an effort has been made to evaluate the shear contribution of the EB-FRP composites with the help of adaptive neuro-fuzzy inference system (ANFIS). A total of 151 sets of data assembled from literature was utilized for the development and evaluation of the ANFIS model. The ANFIS predictions were validated against the obtained experimental results as well as with the estimations of six widely implemented design guidelines. The comparative study has shown that the ANFIS estimates are in decent agreement with that of experimental outcomes and the accurateness of the current ANFIS model is superior to the considered design guidelines. Finally, a parametric investigation was performed to explore the combined effect of various parameters as well as the impact of independent parameters.

Strengthening of RC Beams Using Externally Bonded Fibre Reinforced Polymer Composites

An experimental study is presented on the behaviour of reinforced concrete (RC) beams retrofitted externally with glass fibre reinforced polymer (GFRP) fabrics. Out of ten beams, one beam without GFRP and nine beams wrapped in various lay-up patterns with one, two, three and four layers of GFRP fabrics have been tested for flexure under two-point loading. Loads corresponding to the first crack/delamination and ultimate failure of the beams have been recorded and types of failure have also been observed. Load versus deflection graphs have been plotted at salient locations of beams. Thereafter, a critical discussion has been made with respect to increase in the flexural strength of retrofitted beams as compared to the control beam in order to explore the optimal use of GFRP fabrics for strengthening the RC beams. In addition, a design proposal has been developed in extension to IS: 456-2000 to predict the ultimate design strength of RC beams strengthened with fibre reinforced polymer (FRP) fabric sheets. Thereafter, the values of the experimental flexural strength of the strengthened RC beam have been compared with the corresponding values of design flexural strength computed from present formulation and ACI: 440-2R-08 for its validation. The results obtained from the present experimental study show that flexural strength of the strengthened RC beams increases with increase in number of layers for all lay-up patterns. However, the beams wrapped with two layers GFRP fabrics in the tension face and half of the both sides below neutral axis show superior performance with respect to flexural strength, ductility and economy.

Durability of FRP - concrete bonded joints in structural rehabilitation: A review

Fibre Reinforced Polymer (FRP) composites have been the source of an increasing interest in the field of rehabilitation, namely for repair and strengthening of reinforced concrete (RC) structures. One of the most effective techniques comprises externally bonded FRP composites, in which the efficacy of rehabilitation depends considerably on the integrity and durability of the bond established between the FRP and the substrate, which is provided by a structural adhesive. This paper presents a concise review of critical durability issues associated with the structural bond between FRP and concrete for rehabilitation purposes. A number of factors determining the durability of FRP-concrete bonded joints have been identified and grouped into the following categories: materials – adhesive and adherends, joint characteristics and in-service conditions. For each of those factors, the main findings obtained in previous studies are summarized and the aspects that need further investigation are outlined.

Bond Durability Factor for Externally Bonded CFRP Systems in Concrete Structures

Lack of understanding and confidence in the long-term performance of externally bonded fiber-reinforced polymer (FRP) composites in concrete structures still inhibits their application in repair of aged structures. While synergistic effects of multiple exposure conditions can be severe, it is generally agreed that the single most significant issue with externally bonded FRP composites is their susceptibility to degradation when exposed to moisture. This research utilized small-beam three-point bending test specimens to study FRP-concrete bond performance when subjected to accelerated conditioning environments (immersion in water and exposure to high humidity at elevated temperatures). Bond strength retention (Rb) was determined by dividing the conditioned strength by the average control strength. Test results from the present research were combined with other test data to form a database of over 700 test results. By utilizing an apparent analogy of FRP-concrete bonded systems to adhesive anchors, a bond durability factor (BDF) that quantifies loss in bond capacity due to accelerated conditioning is determined equivalently as a characteristic test value for adhesive anchors. Given their resistance to the environment, only carbon-fiber-reinforced polymer (CFRP) composites were examined. For the purpose of the analyses, and based on available data, it was determined that all CFRP systems may be split into three categories: wet-layup without putty, wet-layup with putty, and precured laminate. BDF corresponding to wet-layup without putty was determined to be 0.60. BDF for wet-layup with putty was not established due to the observed sensitivity to increasing conditioning temperature. CFRP laminate specimens failed prematurely by composite rupture or at the adhesive-composite interface; BDF was not determined as long term durability data corresponding to FRP-concrete bond failure mode was not available.

Effect of Shear Resistance on Flexural Debonding Load-Carrying Capacity of RC Beams Strengthened with Externally Bonded FRP Composites

Debonding failure is the main failure mode in flexurally strengthened reinforced concrete beams by externally bonded or near surface mounted fibre reinforced polymer (FRP) composites. It is believed that FRP debonding will be initiated if the shear stress on the concrete-FRP interface reaches the tensile strength of concrete. However, it was found through experimental and analytical studies that the debonding mechanism of FRP composites has the potential of shear failure in combination with debonding failure. Moreover, the shear failure probably influences the debonding failure. Presently, there are very little experimental and analytical studies to investigate the influence of shear resistance of reinforced concrete (RC) beam on FRP debonding failure. The current study investigates and analyzes the effect of shear resistance on FRP debonding failure based on test results. The analytical results show that the shear resistance of RC beam has a great effect on flexural debonding load-carrying capacity of FRP-strengthened RC beam. The influence of shear resistance on flexural debonding load-carrying capacity must be fully considered in flexural strengthening design of RC beams.

Assessment of design procedures for the use of externally bonded FRP composites in shear strengthening of reinforced concrete beams

Shear strengthening of reinforced concrete (RC) elements with fibre reinforced polymer (FRP) composites is actually a research topic far to be completely solved. Existing shear strength models lead sometimes to overly non-conservative results and need to be validated with a sufficient number of experimental tests. The aim of this work is to assess some common design models for the prediction of the shear capacity of RC beams strengthened with externally bonded FRP composites. The research is based on a wide database collected from recent literature and is mainly focused on the combination of the basic codes for RC structures (without strengthening) with current models for FRP shear strengthened structures. The research significance of this work is due to the lack of information on the performance of the above models when combined with basic codes. Particular attention was dedicated on the consistency of the models for FRP strengthened structures with Eurocode 2 [12], since it assumes a method based on the truss model with variable inclination of the compressed strut. Lastly, some improvements of the model proposed by Pellegrino and Modena [41], which takes into account the interaction between steel and FRP contributions to the shear capacity, are proposed.

Assesment of Proposed Models for Determining Allowable FRP Tensile Strain for Mitigation of Debonding Failure in FRP-Strengthened RC Beams

intermediate crack-induced debondingis one of the most dominant failure modes in FRP-strengthened RC beams. Different code models or provisions have been proposed to mitigateintermediate crack-induced debondingfailure.However, these code models or provisions cannot mitigate this failure mode properly. Recently, mew models have been proposed.Out of all the existing proposed models, five typical ones are investigated in the current study. These main available proposed models for mitigating debonding failure of externally bonded fiber reinforced polymer composites appliedto concrete is evaluated based on data obtained from experimental programs. It is shown that all the evaluated proposed models exhibit a high level of dispersion, they are not suitable for limiting the allowable tensile strain at the initiation of debondingof FRP laminates.

Comprehensive Study on Using Externally Bonded FRP Composites for the Rehabilitation of Reinforced Concrete T-Beam Bridges

This paper describes a synthesis of findings pertaining to rehabilitation of concrete T-beam bridges with externally bonded fiber-reinforced polymer (FRP) composites from a Pennsylvania Department of Transportation District 3 (PennDOT D3) project, with the purpose of answering common questions of concern mainly by state DOT engineers and officials. A method for selecting applicable candidate bridges for suitability of repair with externally bonded FRP composites is described. Three levels of repair are identified, Level 1 (contract), Level 2 (contract/department force), and Level 3 (department force). From this classification, a candidate bridge was selected for a contract repair project. Field and laboratory testing of existing bridge materials is described. Prerepair tests included ultrasonic pulse velocity and rebound hammer on beam concrete, compressive strength tests on deck concrete cores, carbonation tests for both beam and deck concrete, scanning electron microscope–energy dispersive X-ray spectroscopy (SEM-EDX) analyses for beam and deck concrete, and tension tests of the extracted reinforcing steel. Structural analysis was on the basis of AASHTO specifications. Finite-element (FE) modeling was performed to determine existing capacity, and the FE model was calibrated by testing of the bridge by using applied truck-loads. The FRP design was based on strengthening the bridge to sustain an HS-20 AASHTO truck loading. The FRP-repair system was designed on the basis of current American Concrete Institute design guidelines. Repair work and post construction load testing were completed. Supporting full-scale lab studies were conducted to evaluate the most effective concrete substrate repair method and FRP strengthening scheme for laboratory damaged concrete beams by accelerated corrosion, to assure better long-term performance under static and fatigue loads. Results from the rehabilitated bridge and supporting testing were used to develop draft PennDOT design standards and construction specifications and to apply lessons learned to the design and constructability of nearly 1,000 concrete T-beam bridges in Pennsylvania.

Evaluation of Allowable FRP Tensile Strain in Main Codes for Mitigation of Debonding Failure in FRP-Strengthened RC Beams

Intermediate crack-induced debonding is one of the most dominant failure modes in FRP-strengthened RC beams. Different code models and provisions have been proposed to mitigate intermediate crack-induced debonding failure. Out of all the existing code provisions and models, six typical ones are investigated in the current study. These available guidance for mitigating debonding failure of externally bonded fiber reinforced polymer composites applied to concrete is evaluated based on data obtained from experimental programs. It is shown that the current recommendations are inadequate to effectively mitigate intermediate crack-induced debonding in flexural members.

Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio

The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP (Vf). In all, 14 tests were performed on 4,520-mm-long T-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1) the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2) the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3) the effect of FRP strip location with respect to internal transverse-steel location; (4) the effect of FRP strip width; and (5) the effect of internal transverse-steel reinforcement on the CFRP shear contribution.

FRP-strengthened RC slabs anchored with FRP anchors

An abundance of tests over the last two decades has shown the bending capacity of flexural members such as reinforced concrete (RC) beams and slabs to be enhanced by the bonding of fibre-reinforced polymer (FRP) composites to their tension face. The propensity of the FRP to debond, however, limits its effectiveness. Different types of anchorages have therefore been investigated in order to delay or even prevent debonding. The so-called FRP anchor, which is made from rolled fibre sheets or bundles of lose fibres, is particularly suitable for anchoring FRP composites to a variety of structural element shapes. Studies that assess the effectiveness of FRP anchors in anchoring FRP strengthening in flexural members is, however, limited. This paper in turn reports a series of tests on one-way spanning simply supported RC slabs which have been strengthened in flexure with tension face bonded FRP composites and anchored with different arrangements of FRP anchors. The load–deflection responses of all slab tests are plotted, in addition to selected strain results. The behaviours of the specimens including the failure modes are also discussed. The greatest enhancement in load and deflection experienced by the six slabs strengthened with FRP plates and anchored with FRP anchors was 30% and 110%, respectively, over the unanchored FRP-strengthened control slab. The paper also discusses the strategic placement of FRP anchors for optimal strength and deflection enhancement in FRP-strengthened RC slabs.