Fiber Reinforced Polymer Systems Research Articles

Evaluation of Hybrid Fiber Multiscale Polymer Composites for Structural Confinement under Cyclic Axial Compressive Loading

Fiber reinforced polymer (FRP) confinement is recognized as the most promising technique for the strengthening and retrofitting of concrete structures. In order to enhance the performance of conventional epoxy-based FRP composites, nano filler modification of the epoxy matrix was implemented in the current study. In particular, the cyclic loading response of standard concrete specimens externally confined by epoxy-based natural and hybrid fiber reinforced polymer systems was investigated. The confinements were realized with sisal fiber reinforced polymer (SFRP) and hybrid sisal basalt fiber reinforced polymer (HSBFRP). Moreover, the effects of multiwalled carbon nanotubes (MWCNT) were also investigated. Three different specimen sets were considered for study: (i) unconfined specimens, (ii) epoxy-based FRP confined specimens and (iii) MWCNT incorporated epoxy-based FRP confined specimens. The specimens were tested in repeated compressive mode in loading-unloading cycles at increasing displacement levels. The test results revealed that FRP wrapping could enhance the mechanical behavior of unconfined columns in terms of strength and ductility. Moreover, it was evident that the mechanical properties of the epoxy matrix were enhanced by MWCNT incorporation. The developed epoxy-based FRP confinement containing MWCNT ensures improvement in axial strength by 71% when compared with unconfined specimens. The epoxy-based FRP confinement, with and without MWCNT, exhibited a high strain redistribution behavior around the concrete core. In comparison to the unconfined specimens, the confinement could increase the sustained axial strain from 0.6 to 1.4% using epoxy-based FRP confinement and to 1.6% with MWCNT incorporated epoxy-based FRP confinement. Further, an empirical model was developed to predict the ultimate axial stress of concrete columns confined externally with FRP jackets. The ultimate compressive strength obtained from the experimental study was compared with the proposed model, and the observed deviation was lower than 1%.

Experimental and statistical investigation on synergistic effect of nano based epoxy hybrid FRP on strength and durability of circular concrete columns

Strengthening and retrofitting of concrete structures using fiber reinforced polymer (FRP) wrapping is a promising technique in construction sector. Epoxy is the commonly used matrix which possess a higher rate of degradation under exposure to harsh environment. The evaluation of strength and durability of concrete cylinders confined by multi walled carbon nanotube (MWCNT) incorporated epoxy with hybrid sisal and basalt fiber composite systems exposed to various environmental conditions forms focus of this paper. Specimens are subjected to various environmental exposure such as elevated temperature, acidic, alkaline and sea water conditions. Two varieties of epoxy viz. neat epoxy and MWCNT modified epoxy systems are considered. Mechanical and durability properties are analyzed based on axial compressive behavior, stress strain response, visual inspection and modes of failure. The MWCNT incorporated epoxy based hybrid FRP confined specimens exhibited a strength reduction less than 10% when compared with unexposed confined specimens under aggressive environmental conditions, while unconfined specimens showed strength reduction by 40% when compared to unexposed unconfined specimens. The MWCNT modified epoxy based hybrid confinement showed an energy absorption of 6.24 times that of unconfined specimens upon chemical exposure. Ultrasonic Pulse Velocity test revealed efficacy of confinement system in protecting concrete core from a sudden failure, which in turn increases the compressive strength of system. A statistical analysis using ANOVA was employed to find significance of these factors and confirmed with experimental results. The effect of MWCNT incorporation is significant in FRP confinement and shows the possibility of the FRP system to be adopted as a major retrofitting material in alkaline and sea water environments.

Nano filler incorporated epoxy based natural hybrid fiber confinement of concrete systems: Effect of fiber layers and nano filler addition

The utilization of environment friendly materials in the construction industry is essential for sustainable development. For the purpose of strengthening and retrofitting concrete structures, fiber reinforced polymer (FRP) composites have been predominantly used. In the area of structural strengthening, as an alternative to synthetic fiber reinforced composites, the application of natural fiber reinforced composites is of immense interest. This study discusses the mechanical properties of externally strengthened concrete cylinders using a natural hybrid fiber system with two layers of sisal fiber sheets followed by 1 to 3 layers of basalt fiber sheets. In addition, the effect of multi walled carbon nanotubes (MWCNT) incorporation in the epoxy matrix from 0.5 to 1.5 wt%, on the various properties are analyzed. The inclusion of MWCNT in the epoxy matrix improved the overall performance of the FRP significantly. The results show that there is substantial improvement in mechanical properties up to 1 wt% MWCNT content in composites, beyond which it is reduced due to agglomeration of nanofiller. The MWCNT incorporated epoxy based five layered hybrid sisal basalt FRP confined specimens showed an improvement of 114% and 427% in peak strength and strain values under axial compressive strength tests. Moreover, the axial compressive behavior, stress–strain response, energy absorption and ductility characteristics of the confined column specimens were evaluated. The major focus of the study is to assess the influence of basalt FRP layers on the ultimate load carrying capacity and performance of hybrid sisal basalt FRP (HSBFRP) confined specimens. Statistical analysis of variance (ANOVA) was carried out and revealed that the number of FRP layers and addition of MWCNT in epoxy significantly affect the overall performance. The addition of basalt fiber layers in hybrid sisal basalt FRP systems is found to improve the load carrying capacity and energy absorption characteristics of confined specimens. Therefore, HSBFRP confinement can be used in areas where moderate strength and greater ductility is commanded.

Ultimate torsional resistance and failure modes of FRP-strengthened reinforced concrete members: A nonlinear design model

Reinforced concrete (RC) structural members, especially flanged and spandrel beams, bridge girders, curved beams, and eccentrically loaded beams may be subjected to significant torsional loading. Despite extensive research on the strengthening of RC members, far too little attention has been paid to the torsional behavior of RC members strengthened with fiber-reinforced polymer (FRP) sheets. Therefore, a limited number of guidelines specified the design of externally bonded FRP systems in torsion. This study provides an important opportunity to advance the understanding of the torsional behavior of FRP-strengthened members by developing a nonlinear design model predicting the failure mode and ultimate torsional resistance of FRP-strengthened RC members subjected to torsion. Serious discussions and analyses of desired failure modes emerge by defining two balanced conditions between the components, and eight failure regions. The first balanced condition defined between FRP and concrete specifies the failure mode of the member, and the second balanced condition defined between steel and concrete is used for the analysis of steel reinforcement stresses. A clear design approach is proposed in this study to access the desired percentage of the externally applied FRP sheets based on the required torsional strength. Moreover, the ultimate stresses and strains of concrete struts, steel reinforcements, and FRP sheets are derived by incorporating the nonlinear behavior of concrete. To evaluate the validity of the design method, 70 FRP-strengthened specimens are collected from 11 previous studies; then, a comparative analysis is conducted among the experimental results, the estimations of the presented model, and the predictions of the commonly available guidelines for torsional strengthening. The excellent agreement between the test results and the presented model estimations confirms the validity and accuracy of the presented model.

Prediction of compressive strength and evaluation of different theoretical standards and proposed models of brick columns confined with FRP, FRCM, or SRG system

The strength capacity of confined masonry column is one of the topics that need to be studied. In this study, the efficiency of using different types of advanced composite (non-corrosive materials) such as fiber reinforced polymer (FRP), fiber reinforced cementitious matrix (FRCM), or steel reinforced grout (SRG) in confining masonry columns is investigated. A wide range of experimental database of masonry column specimens has been collected from the results that available in scientific literatures. Different theoretical standards and proposed models that used to predict the capacity of masonry columns confined with FRP and FRCM are evaluated based on collected experimental database. Since there is no standard code or specific proposed model for SRG system, the confined capacity of this system is predicted and evaluated using the FRCM proposed models. The justification of using these models is that both FRCM and SRG systems have the same concept of using inorganic material as a paste material. An index named “equivalent fiber reinforcement index (EFRI)” is proposed to capture the key factors that control the behavior of the confined masonry columns with different advanced composite. This index is used as reference parameter for the purpose of the comparison between different strengthening systems. As a result, all types of advanced composite presented a significant increase in ultimate capacity. Also, the behavior of the masonry columns was significantly dependent on the type of fabric used. Different modes of failure were reported, including crushing of masonry block, as well as a deboning of FRP from the masonry substrate and deboning or slippage of fabric within inorganic paste matrix. Compared with other models and standards, the American Concrete Institute Committee 440 (ACI 440) and American Concrete Institute Committee 549 (ACI 549) shows very good predictions for the confined capacity of masonry columns strengthened by FRP and FRCM or SRG respectively.

Investigation of repair techniques for deteriorated end regions of prestressed concrete bridge girders

Deterioration of the end regions of prestressed concrete bridge girders is commonly observed in the field when girders are exposed to chloride-laden water that has leaked through failed expansion joints. Because the deterioration is often localized to the end regions of the girders, reliable repair techniques can provide a means to extend girder service life, avoiding the need for immediate superstructure replacement. To evaluate different repair methods and identify key design considerations for end region repair, shear tests to failure were conducted on prestressed concrete girders extracted from a decommissioned superstructure. Three repair systems were evaluated: an externally bonded fiber-reinforced-polymer (FRP) system, a near-surface-mounted FRP system, and a concrete end block. Only the externally bonded FRP system successfully restored both the strength and initial stiffness of the girder. Although the other two methods were not successful, the tests on the repaired girders highlight important factors that must be considered when designing repairs or conducting further research. The tests also demonstrate that end region deterioration can cause significant (> 40%) reductions in strength, underscoring the importance of addressing such deterioration observed in the field.

High Temperature Performance of Concrete Confinement by MWCNT Modified Epoxy Based Fiber Reinforced Composites.

The conventional method of fiber reinforced polymer (FRP) wrapping around concrete columns uses epoxy as the binder along with synthetic or natural fibers such as carbon, glass, basalt, jute, sisal etc. as the reinforcement. However, the thermal stability of epoxy is a major issue in application areas prone to fire exposure. The current work addressed this major drawback of epoxy by modifying it with a nanofiller, such as multiwalled carbon nanotubes (MWCNT), and reinforcing it using basalt and sisal fibers. The effect of exposure to elevated temperature on the behavior of concrete cylinders externally confined with these FRP systems was analyzed. Three types of specimens were considered: unconfined; confined with sisal fiber reinforced polymer (SFRP); and confined with hybrid sisal basalt fiber reinforced polymer (HSBFRP) specimens. The test samples were exposed to elevated temperature regimes of 100 °C, 200 °C, 300 °C and 400 °C for a period of 2 h. The compressive strengths of unconfined specimens were compared with various confined specimens, and from the test results, it was evident that the mechanical and thermal durability of the FRP systems was substantially enhanced by MWCNT incorporation. The reduction in the compressive strength of the FRP-confined specimens varied depending on the type of the confinement. After two hours of exposure at 400 °C, the compressive strength corresponding to the epoxy-HSBFRP-confined specimens were improved by 15%, whereas a 50% increase in strength corresponding to MWCNT-incorporated epoxy-HSBFRP-confined specimens was observed with respect to unconfined unexposed specimens. The MWCNT-modified epoxy-incorporated FRP-confined systems demonstrated superior performance even at elevated temperatures in comparison to unconfined specimens at ambient temperatures.

Utilizing Underwater FRP System for Hydraulic Structures Application

The development of the use of advance composites is remarkable, especially in applications that require the materials used to be corrosion resistant. One of the most important of these applications is the structures built inside or under water, especially hydraulic structures. Constant exposure to wet makes materials such as concrete and steel threatened to the high level of corrosion and deterioration. The traditional methods for repairing proved insufficient over time, in addition to being expensive in some cases. Therefore, it became necessary to search for alternatives that achieve engineering efficiency and lower costs. One of these alternatives is the use of fiber reinforced polymer (FRP) system. Until recent years, it was not possible to use FRP system under water as a repair system for hydraulic structures without cofferdam. However, during this period, types of underwater cured resins were produced and underwater FRP system is the best choice for this type of structures. There are no many practical applications for the use of FRP in repairing of hydraulic structures, so it is possible to study its application in marine structure to show the possibility of use it in hydraulic structure. This research highlight the advantages and disadvantages of traditional method in repairing underwater structures. Also present project that considered the underwater FRP as a construction material used in the rehabilitation of deteriorated concrete pile. As a conclusion, using fibers with water activated resin can provide an economical solution for future repair and rehabilitation projects. Also the pre-preg system is efficient for repairing dry zones, while the wet layup system is very effective in repairing splash zones of damaged structural elements.

Parametric Study on Steel–Concrete Composite Beams Strengthened with Post-Tensioned CFRP Tendons

A sustainable environment can be achieved by strengthening the existing building to avoid new construction and by replacing the construction materials with long-lasting sustainable materials such as a fiber-reinforced polymer (FRP). Using post-tensioned (PT) FRP systems has proven to be an effective technique in strengthening the structure and decreasing cracks and deformability. In this study, a 3-D finite element model was built to investigate the flexural behavior of composite beams strengthened with external PT FRP tendons. Limited research studied the use of FRP tendons to enhance the structural behavior of composite beams. This paper represents a comprehensive study of the effect of several parameters that control the design of the FRP tendons. Parameters such as PT level, tendon material, tendon length, degree of shear connection (DOSC), and tendon profile shape were considered under loading. The 3-D model’s correctness is validated using published experimental data. It was observed that of all FRP materials, carbon FRP is the best type for upgrading the beam strength, and it was recommended to use a 30 to 40% PT level. In addition, applying external PT over the full length of the beam increases the ultimate load capacity significantly. However, due to the difficulty of construction, it was recommended to use 90% of the beam span length since the difference in beam capacity does not exceed 5%. Finally, adding PT tendons with a trapezoidal and parabola profile to composite beams significantly increases the yield load and the beam capacity.

COMPREHENSIVE DESIGN-ORIENTED MODEL FOR FRP-CONFINED CIRCULAR RC COLUMNS

Today, the use of fiber-reinforced polymers (FRPs) for the confinement of reinforced concrete (RC) columns is a state-of-the-art technic in the strengthening of existing structures. Thus far, various design models are present in literature to predict the stress-strain behavior and to allow for the ultimate limit state design of FRP-confined concrete. Often, the models proposed are scientific in nature only, they are just designed for plain concrete specimens, and they do not meet the demands necessary for practical use. Therefore, this paper presents a comprehensive design-oriented model for FRP-confined circular RC columns considering both the ultimate limit state (ULS) and the serviceability limit state (SLS). Beside equations to calculate the new ultimate confined concrete strength and the accompanying axial strain, the paper introduces an approach to generate a stress block simplifying the design procedure for columns subjected to combined compression and bending. Furthermore, an equation is presented to allow for the calculation of a combined partial safety factor taking into account the material safety factors of both the concrete of the column and the FRP system used. In order to realize a proper and coherent limit state method, the entire model uses characteristic material values and equations only. At last, the SLS is addressed with a simple approach limiting the maximal concrete compressive stress.

COMPREHENSIVE DESIGN-ORIENTED MODEL FOR FRP-CONFINED CIRCULAR RC COLUMNS

Today, the use of fiber-reinforced polymers (FRPs) for the confinement of reinforced concrete (RC) columns is a state-of-the-art technic in the strengthening of existing structures. Thus far, various design models are present in literature to predict the stress-strain behavior and to allow for the ultimate limit state design of FRP-confined concrete. Often, the models proposed are scientific in nature only, they are just designed for plain concrete specimens, and they do not meet the demands necessary for practical use. Therefore, this paper presents a comprehensive design-oriented model for FRP-confined circular RC columns considering both the ultimate limit state (ULS) and the serviceability limit state (SLS). Beside equations to calculate the new ultimate confined concrete strength and the accompanying axial strain, the paper introduces an approach to generate a stress block simplifying the design procedure for columns subjected to combined compression and bending. Furthermore, an equation is presented to allow for the calculation of a combined partial safety factor taking into account the material safety factors of both the concrete of the column and the FRP system used. In order to realize a proper and coherent limit state method, the entire model uses characteristic material values and equations only. At last, the SLS is addressed with a simple approach limiting the maximal concrete compressive stress.

Modification of epoxy binder with multi walled carbon nanotubes in hybrid fiber systems used for retrofitting of concrete structures: evaluation of strength characteristics

The rapid development in infrastructural facilities necessitates an efficient approach for the repair and retrofitting of concrete structures and, confinement method using fiber reinforced polymer is a promising one. The commonly used carbon and glass fibers for confinement poses environmental and performance issues. The present study addresses these two major aspects by considering natural fibers along with modification of epoxy binder to impart ductile behavior ie., to investigate the effectiveness of multiwalled carbon nanotubes (MWCNT) incorporated synthetic and natural fiber reinforced polymer (FRP) systems as the external confinement. MWCNT is incorporated in 0.5–1.5wt.% in epoxy nano and epoxy multiscale and there is significant enhancement in tensile and fracture properties of the composites up to 1wt.%, beyond which it declined due to agglomeration. Various strength tests were performed with sisal, basalt, carbon and hybrid sisal-basalt FRP systems with different FRP layer thickness on plain concrete cylinders. From the test results it is outlined that external confinement with MWCNT incorporated FRP improved the axial load-carrying capacity, energy absorption and ductility of concrete with respect to that of control specimens. Compared with unconfined specimens, those strengthened with MWCNT modified hybrid FRP wraps containing sisal and basalt fibers recorded increments of 114% and 87% in their load-carrying capacity and energy absorption, due to the intrinsic rigidity of hybrid fibers and epoxy modification. Furthermore, the outcomes indicate that MWCNT incorporated hybrid sisal-basalt FRP confined specimens exhibited superior properties and the low strength of natural FRP confinement compared to artificial FRP can be improved by epoxy modification. The outer jacketing resisted abrupt and catastrophic failure to a great extent.

Flexural Behavior of Reinforced Concrete Beams Strengthened with Composite Carbon Fiber-Reinforced Polymer System

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Advanced and Simplified Modeling Approaches for the Study of the Bond Behavior of FRP Systems on Curved Masonry Substrates

Curved masonry structures externally strengthened by Fiber Reinforced Polymer (FRP) systems exhibits failure mechanisms that emphasize a local bond behavior particularly influenced by the curved geometry of the substrate and the position of the strengthening (i.e. at the intrados or extrados). Indeed, together with tangential stresses, normal stresses in tension or compression also arise by leading to a combined mode I–mode II behavior of strengthening system at the reinforcement/masonry interface level. In recent studies, the Authors proposed different modeling approaches for FRPs applied to curved masonry structures. In particular, both micro-modeling detailed approaches and simplified approaches were generally proposed. The present paper critically analyzes these models by underlining the main differences among them, the assumptions and their ability to reproduce specific phenomena experimentally observed.

Numerical Investigation of RC Beam Using Externally Bonded Fibre Reinforced Polymer Laminate Using Ansys Software

Due to its exceptional properties such as good tensile strength, serviceability, corrosion resistant, and light weight, externally bonded Fibre reinforced polymer (FRP) is increasingly being utilized to support reinforced concrete (RC) structures. De-lamination of FRP-plate with concrete cover owing the most prevalent type of failure is due to different stress concentration at the bonded plate’s end (stresses at end). The behaviour of the FRP-concrete interface is an important component in preventing debonding failures in RC structures. As a result, for the safe and cost-effective design of externally bonded FRP systems, a deeper understanding of FRP behaviour for concrete interfaces is necessary. The primary goal of this research is to investigate the behaviour of the FRP-concrete interface. Finite element (FE) simulations utilising the ANSYS Finite Element software are used to investigate the interaction behaviour. For the parametric investigation, FE simulation models are built to investigate the impact of various variables on the binding behaviour of FRP and concrete surfaces by altering the width and thickness of the FRP laminate. Different thickness of FRP 2 mm, 4 mm, 6 mm and different widths 75 mm and 100 mm are taken. Based on the results the multi variable nonlinear regression and multi variable logarithmic Finite element (FE) simulations utilising the ANSYS Finite Element software are used to investigate the interaction behaviour. For the parametric investigation, FE simulation models are built to investigate the impact of various variables on the binding behaviour of FRP and concrete surfaces. Regression study is conducted out, and the best fit for the parameters under evaluation is found. The above analysis results are compared with the published work for validation.

Strengthening of plain concrete cylinders with hybrid composites based on basalt and sisal fiber systems

Strengthening and rehabilitation of concrete structures are becoming popular in the construction industry using synthetic fiber reinforced polymers (FRP). However, it has a major drawback due to their economic and environmental impacts. The study focuses on investigation of strength and ductile behavior of concentrically loaded hybrid sisal basalt FRP (HSBFRP) confined concrete structures. A combination of natural sisal and basalt fibers were proposed as the hybrid system. Confined concrete cylinders with different layers of hybrid FRP systems were subjected to various strength tests and test results proved that hybrid FRP confinement intensify the axial load-carrying capacity compared to unconfined specimens. A relative performance analysis of hybrid sisal-basalt FRP confinement with basalt and sisal individual FRP confinements was performed. From the test results it was noticed that the ultimate load carrying capacity increased by 63% along with a fracture energy of 23.17 MPa when the cylinders were confined with 5 layers of hybrid confinement. When compared with unconfined specimens the ductility index attained a maximum value of 4.31 when confined with multiple layers of sisal and basalt FRP wraps. It was proved that with the concept of hybridization in confinement, there was a significant enhancement in fracture energy and ductility index for concrete, thus preventing catastrophic failure.

Preliminary Investigation of an Approach to Improve Water Impermeability in Concrete with Externally Bonded FRP Systems

Good bond and water impermeability in fiber-reinforced polymer (FRP) bonded/coated systems are essential if the durability of FRP-rehabilitated concrete structures is to be ensured. In addition, water impermeability is required in some special FRP applications, such as the strengthening of underground water pipes. So far, there has been no method in the literature guaranteeing water impermeability in FRP strengthening works. This paper studies the feasibility of using a waterproof coating as the initial primer on cementitious materials before applying externally bonded FRP. In this preliminary investigation, by examining the two most important indicators (i.e., pull-off bond tests and water penetration tests), it was found that the use of an initial waterproof layer in the proposed FRP bonding system did not influence the pull-off bond strength but significantly improved the system's water impermeability. It is, therefore, suggested that an initial waterproof layer can be included in method statements for externally bonded FRP systems to upgrade the effectiveness and durability of FRP systems.

Novel Truss Analogy Approach to Analyze Reinforced Concrete Deep Beams Strengthened with Externally Bonded FRP Systems

AbstractA new analysis approach is devised herein to predict the response of reinforced concrete deep beams strengthened in flexure, shear or shear-flexure with fiber-reinforced polymer (FRP). The ...

Analysis of Columns Strengthened using Fibre Reinforced Cementitious Matrix Jackets

Abstract: Maintenance, repair and strengthening of existing concrete structures, either reinforced or prestressed,are important activities of civil engineers. Nowadays different techniques are available for the strengthening. Various techniques were adopted for strengthening RC structures, namely, steel plates, external post tensioning, externally bonded Fibre-Reinforced Polymer (FRP), and near- surface-mounted FRP systems to increase shear and flexural capacity. During the last few decades, strengthening of concrete structural elements by fibre-reinforced polymer has become a widely used technique. But it has several disadvantages due to the epoxy resin like debonding of FRP from the concrete structure, unstable nature of the epoxy at higher temperatures etc. To overcome this, an upgraded system was introduced as an alternative for FRP known as Fibre Reinforced Cementitious Matrices (FRCM). The objective of this paper is to investigate the feasibility of Fibre-Reinforced Cementitious-Matrix materials as an alternative external strengthening technique for RC members. Columns with circular geometry were wrapped with different fibre materials using cementitious matrix. The analysis was done using ANSYS software. Keywords: RC columns, FRCM, Strengthening, fibre, ANSYS

Cost and Effectiveness of Fiber-Reinforced Polymer Solutions for the Large-Scale Mitigation of Seismic Risk in Reinforced Concrete Buildings.

Recent seismic events have demonstrated that the high vulnerability of existing reinforced concrete (RC) buildings is mainly due to a lack of proper seismic detailing and the employment of poor-quality concrete. The reconstruction process following the 2009 L’Aquila earthquake highlighted that strengthening these buildings using solutions based on fiber-reinforced polymers (FRPs) can be both efficient and cost-effective. Indeed, their light weight, ease of installation, and the availability of specific guidelines and standards strongly supported their use in design practices, where they were the strengthening technique employed the most. This paper analyses and discusses the data on the actual cost and effectiveness of FRP solutions for seismic strengthening of existing RC buildings. To this end, the large database relating to the L’Aquila reconstruction process was used to select 130 RC buildings strengthened with FRP systems or FRPs combined with other techniques. Details of direct costs, including at the member level, and the types and percentages of strengthened members are analysed for both local and global strategies. This study thus provides readers with valuable data for use in cost-benefit analyses of FRP systems schemes to mitigate seismic risk at large-scale.