Fiber Reinforced Polymer Rebars Research Articles

FRP reinforced-concrete slabs: a comparative design study

Durability of concrete structures represents a major challenge today for both existing and new structures. Fibre-reinforced polymer (FRP) composite rebar offers a highly durable alternative to steel rebar. Research on the use of FRP rebar has been conducted over the past 40 years; however, its widespread application is yet to occur. This paper investigates the use of FRP as reinforcement, in particular the application to slab elements and associated performance at serviceability and ultimate limit state. Existing design guidance such as the ACI and CSA standards are reviewed and a number of slab designs with dimensions common in practice are undertaken. A third approach that follows the Eurocode design philosophy is also reviewed. The design of the different slab forms is assessed according to these codes, and results compared with the behaviour predicted by non-linear finite-element analysis. The relative costs of each resulting slab design are also discussed. The findings highlight differences in the relative accuracy of the different code recommendations and indicate the need for further research to exclude potentially non-conservative design. The work presented is intended to provide a contextual comparison of existing design approaches, which will be useful to practitioners involved in design of concrete structures with FRP rebar.

Interfacial debonding detection in fiber-reinforced polymer rebar–reinforced concrete using electro-mechanical impedance technique

For reinforced concrete structures, the use of fiber-reinforced polymer rebars to replace the steel reinforcement is a topic that is receiving increasing attention, especially where corrosion is a serious issue. However, fiber-reinforced polymer rebar–reinforced concrete always carries the risk of structural failure initiated from the debonding damage that might occur at the reinforcement–concrete interface. This study employed an electro-mechanical impedance–based structural health monitoring technique by applying lead–zirconate–titanate ceramic patches to detect the debonding damage of a carbon fiber–reinforced polymer rebar reinforced concrete. In the experimental study, a carbon fiber–reinforced polymer rebar reinforced concrete specimen was fabricated and it was subjected to a pullout test to initiate the debonding damage at the reinforcement–concrete interface. The impedance and admittance signatures were measured from an impedance analyzer according to the different debonding conditions between the reinforcement and the concrete. Statistical damage metrics, root-mean-square deviation and mean absolute percentage deviation, were used to quantify the changes in impedance signatures measured at the lead–zirconate–titanate patches due to debonding conditions. The results illustrated the capability of the electro-mechanical impedance–based structural health monitoring technique for detecting the debonding damage of fiber-reinforced polymer rebar–reinforced concrete structures.

Nonlinear Finite Element Analysis of FRP Strengthened RC Beams with Bond-Slip Effect

This paper investigates the structural behavior of fiber reinforced polymer (FRP) strengthened reinforced concrete (RC) beams by developing a new simple, efficient and accurate finite element model (FEM-B). In addition to the FRP, concrete and steel rebars, the adhesive and stirrups which have been generally ignored in the reported models from literatures are considered in the new models. At first, a finite element model (FEM-P) is developed assuming perfect bond between concrete, FRP and adhesive interfaces. Then the FEM-P model is expanded to form the FEM-B model by including the bond-slip effect between concrete, FRP and adhesive interfaces. The developed new finite element models (FEM-B and FEM-P) are validated against experimental results and demonstrate to be effective for the structural analysis of FRP strengthened RC beams. Furthermore, parametric studies are carried out to learn the effects of types and thickness of FRP on the structural behavior of FRP strengthened RC beams based on the FEM-B model. The research findings are summarized finally.

Mechanical properties and bond characteristics of different fiber reinforced polymer rebars at elevated temperatures

This paper presents the results of an experimental study on the effect of elevated temperatures on the mechanical properties of FRP bars and the bond behavior between FRP bars and concrete. Four types of reinforcement bars namely: basalt fiber reinforced polymer (BFRP), CARBON fiber reinforced polymer (CFRP), glass fiber reinforced polymer (GFRP), and steel bars of 10mm diameter were used. The results showed that the FRP bars suffered significant reductions in their mechanical properties upon exposure to high temperatures of up to 450°C at which the GFRP and BFRP melted and lost their total tensile strength capacity. At a critical temperature of 325°C, the FRP bars lost as high as (55% and 30%) of their tensile strength and elastic modulus, respectively. The percentage reduction in mechanical properties and bond strength was more pronounced in specimens with FRP bars than those with steel bars under elevated temperatures; the percentage reductions in bond strength between concrete and FRP bars reached as high as 81.5% after exposure to 325°C. Based on the experimental results, an empirical model was proposed to predict the post-heating bond stress-slip relationship between the FRP bar and the surrounding concrete: a good agreement was noticed between the experimental results and the proposed model.

A Mechanical and Environmental Assessment and Comparison of Basalt Fibre Reinforced Polymer (BFRP) Rebar and Steel Rebar in Concrete Beams

Abstract This paper compares holistically the mechanical and environmental performance of basalt fibre reinforced polymer (BFRP) rebar against conventional steel rebar in concrete beams. This assessment involves material testing and life cycle assessment (LCA). The results show that BFRP tendons in reinforced concrete beams are stronger and lighter than steel with a better environmental profile and fewer embodied emissions, as fewer material and energy resources are required during production. The future market is expected to be within the precast industry, rather than in on-site construction, as precast BFRP concrete beams have approximately half the emissions of steel reinforced concrete beams.

Transient Thermal Tensile Behaviour of Novel Pitch-Based Ultra-High Modulus CFRP Tendons.

A novel ultra-high modulus carbon fibre reinforced polymer (CFRP) prestressing tendon made from coal tar pitch-based carbon fibres was characterized in terms of high temperature tensile strength (up to 570 °C) with a series of transient thermal and steady state temperature tensile tests. Digital image correlation was used to capture the high temperature strain development during thermal and mechanical loading. Complementary thermogravimetric (TGA) and dynamic mechanical thermal (DMTA) experiments were performed on the tendons to elucidate their high temperature thermal and mechanical behaviour. The novel CFRP tendons investigated in the present study showed an ambient temperature design tensile strength of 1400 MPa. Their failure temperature at a sustained prestress level of 50% of the design tensile strength was 409 °C, which is higher than the failure temperature of most fibre reinforced polymer rebars used in civil engineering applications at similar utilisation levels. This high-temperature tensile strength shows that there is potential to use the novel high modulus CFRP tendons in CFRP pretensioned concrete elements for building applications that fulfill the fire resistance criteria typically applied within the construction industry.

Acoustic emission monitoring and finite element analysis of debonding in fiber-reinforced polymer rebar reinforced concrete

The acoustic emission technique is widely used for mechanical diagnostics and damage characterization in reinforced concrete structures. This article experimentally investigated the feasibility of debonding characterization in fiber-reinforced polymer rebar reinforced concrete using acoustic emission technique. To this end, carbon-fiber-reinforced polymer rebar reinforced concrete specimens were prepared and they were subjected to pullout tests to study the interfacial debonding between concrete and reinforcement. Test results showed that the debonding failure between concrete and reinforcement was characterized by the total peeling off of the helical wrapping layer of the carbon-fiber-reinforced polymer reinforcement. The response of acoustic emission activity was analyzed by descriptive parameters, such as cumulative acoustic emission hits, amplitude, and peak frequency. The evolution of debonding failure is thus characterized by these acoustic emission parameters. The results demonstrated a clear correlation between the damage evolution of carbon-fiber-reinforced polymer rebar pullout and the acoustic emission parameters. In addition, finite element analysis was adopted to study the stress field during the pullout of the reinforcement. The simulation results agreed well with the experimental investigations.

Numerical investigation of the uplift performance of prestressed fiber-reinforced polymer floating piles

ABSTRACTFinite element analyses of prestressed fiber-reinforced polymer floating piles subjected to uplift force have been conducted in this paper. First, parameters of the modified BPE model (bond–slip model at the fiber-reinforced polymer–concrete interface) were calibrated using existing pullout testing data on fiber-reinforced polymer rebars embedded in concrete. Nonlinear spring elements were used in numerical modeling to characterize the bond–slip behavior at the fiber-reinforced polymer–concrete interface. A parametric study was performed to assess the influence of rebar diameter, fiber-reinforced polymer material, embedment length, and concrete strength on the mobilized bond stress. Upon the successful modeling of the pullout performance of fiber-reinforced polymer rebars in concrete, numerical models were developed to investigate the dependence of the uplift performance of floating piles on the prestress level, uplift force, fiber-reinforced polymer type, and compressive strength of concrete.

Modeling and mechanical characterization of braidtruded fiber reinforced polymer rebar

Traditional steel rebar used for concrete reinforcement can be susceptible to corrosion. One solution to this problem is to substitute fiber reinforced polymer (FRP) rebar in the place of steel. However, conventional unidirectional FRP rebars are limited by a linear elastic failure mode at relatively low ultimate strains. In this study, FRP rebar was developed to behave in a pseudoductile manner, where a progressive failure was achieved based on the composite architecture. Hybrid FRP rebar consisting of a unidirectional carbon fiber core encased in a braided aramid fiber overwrap was manufactured, and a model was developed to reflect the tensile response of the rebar based on constituent material properties and geometric parameters. Experiments were conducted in order to validate the model, which was successful in reflecting the pseudoductile tensile behavior exhibited by the rebar. POLYM. COMPOS., 39:1582–1593, 2018. © 2016 Society of Plastics Engineers

Parametric Study of Shear Strength of Concrete Beams Reinforced with FRP Bars

Fibre Reinforced Polymer (FRP) bars are being widely used as internal reinforcement in structural elements in the last decade. The corrosion resistance of FRP bars qualifies its use in severe and marine exposure conditions in structures. A total of eight concrete beams longitudinally reinforced with FRP bars were cast and tested over shear span to depth ratio of 0.5 and 1.75. The shear strength test data of 188 beams published in various literatures were also used. The model originally proposed by Indian Standard Code of practice for the prediction of shear strength of concrete beams reinforced with steel bars IS:456 (Plain and reinforced concrete, code of practice, fourth revision. Bureau of Indian Standards, New Delhi, 2000) is considered and a modification to account for the influence of the FRP bars is proposed based on regression analysis. Out of the 196 test data, 110 test data is used for the regression analysis and 86 test data is used for the validation of the model. In addition, the shear strength of 86 test data accounted for the validation is assessed using eleven models proposed by various researchers. The proposed model accounts for compressive strength of concrete (fck), modulus of elasticity of FRP rebar (Ef), longitudinal reinforcement ratio (ρf), shear span to depth ratio (a/d) and size effect of beams. The predicted shear strength of beams using the proposed model and 11 models proposed by other researchers is compared with the corresponding experimental results. The mean of predicted shear strength to the experimental shear strength for the 86 beams accounted for the validation of the proposed model is found to be 0.93. The result of the statistical analysis indicates that the prediction based on the proposed model corroborates with the corresponding experimental data.

Manufacturing and characterization of braidtruded fiber reinforced polymer rebar

Fiber reinforced polymer (FRP) rebar can serve as a noncorrosive alternative to conventional steel rebar for use in concrete reinforcement. By combining pultrusion and braiding into a single manufacturing process called braidtrusion, FRP rebar consisting of both unidirectional and off‐axis fibers can be produced. In this study, a variation on the braidtrusion process was introduced, utilizing an impregnation ring and dieless curing system to produce high quality FRP rebar in a continuous manner. The resulting rebar was characterized to assess the quality and consistency of the manufacturing process. Degree of cure, braid angle, cross‐sectional area, and fiber volume fractions were measured, and a general examination of the rebar microstructure was conducted. In addition, analytical equations based on previous works were presented to predict braid angle and cross‐sectional area of the resulting rebar based on manufacturing parameters. The predictions made using these equations were found to be in good agreement with the experimental results, and can be used to facilitate future design and manufacturing of FRP rebar. POLYM. COMPOS., 39:337–350, 2018. © 2016 Society of Plastics Engineers

Modeling of Bond of Sand-Coated Deformed Glass Fibre-Reinforced Polymer Rebars in Concrete

In this paper, 30 beam tests, 48 Losberg pullout tests and 6 standard pullout tests were conducted to investigate bond-slip response of sand-coated deformed glass fibre-reinforced polymer (GFRP) rebars in concrete. The specimen parameters varied were concrete strength, embedment length, bar diameter and test method. Two failure modes including pullout failure and splitting failure were observed. The experimental bond stress-slip (τ-s) curves of sand-coated deformed GFRP rebars in concrete in pullout failure included a micro-slippage branch, a slippage branch, a descending branch and a residual branch, while the τ-s curves in splitting failure included a micro-slippage branch and a slippage branch. Based on the tests, a new bond-slip constitutive model for sand-coated deformed GFRP rebars in concrete was presented. Compared with other available models, the model proposed in this paper can provide more reliable simulations of the test results.

FRP Composites and their Using in the Construction of Bridges

Traditional material for concrete reinforcement is steel. In recent times, non-metallic fibres have been intensively investigated and some of them used for construction of bridges. FRP composites are durable materials which are different from the steel reinforcement for their resistance to the electrochemical corrosion. With respect to steel, different mechanical behavior of non-metallic reinforcement, however, involves some drawbacks - namely the lack of thermal compatibility between concrete and FRP reinforcement. FRP composites belong among anisotropic materials. Their properties depend on the type, volume and alignment of the fibres, the matrix type, form and quality of the construction. The analysis of causes of failures on German bridges shows that reinforcement corrosion initiated by chlorides makes up 2/3 of all the failures recorded in the bridge construction. Corrosion of the reinforcement initiated by chlorides is the main cause of the loss of serviceability of bridge structures. In such an environment, fibre-reinforced polymer (FRP) reinforcement can fully replace the traditional steel reinforcement. The paper presents the mechanical properties and durability of different types of the FRP rebar's and their use in construction of bridges.

Shear behaviour of new beams made of UHPC concrete and FRP rebar

The primary objective of this study was to develop a new type of high-performance lightweight beam with improved performance over conventional reinforced concrete (RC) by adding fibre-reinforced polymer (FRP) reinforcing rebar to ultra-high-performance concrete with microfibre reinforcement (UHPC-SFR). This new type of beam was designed to be lightweight, to have high compressive and tensile strength, to be able to sustain large bending moments, and to be resistant to shear. The main objective was to verify the mechanical behaviour of the beams and to compare it with the behaviour of typical RC beams. An experimental program was designed to identify its failure modes and bending behaviour. The results indicate that the behaviour of such RC beams is comparable to typical RC beam behaviour to a certain extent. An analytical model for validating this concept is presented here, which is based on the typical material behaviour law hypotheses of nonlinear mechanical beam behaviour. The load–displacement and moment–curvature relationships predicted using this model were compared to the experimental results obtained for several large-scale specimens. The comparisons revealed a good correlation between the analytical and experimental results and illustrate the potential of these composite beams in civil engineering structures.

Optimized shear design equation for slender concrete beams reinforced with FRP bars and stirrups using Genetic Algorithm and reliability analysis

A lack of confidence in the shear design of concrete beams reinforced with Fiber Reinforced Polymer (FRP) bars may lead to excessive reinforcement and thus increase the overall cost of construction. In order to improve the accuracy of the current guidelines, two databases of slender (shear span to depth ratio>2.5) FRP reinforced concrete (RC) beams were developed from literature. The first set of data contains slender beams of 116 tests with only FRP rebar as longitudinal reinforcements whereas the second set contains 46 test beams reinforced with FRP as both longitudinal reinforcements and stirrups. The second database was used to evaluate the confinement effect of FRP rebar as stirrups. Thereafter a Genetic Algorithm was implemented to optimize the shear equations proposed in FRP design guidelines of ACI 440.1R-06, CSA S806-02, CSA S6-09, and Reineck’s tooth model. The optimized shear equations showed less scatter than the original equations as they could achieve an average Vtest/Vcalc close to 1.0 with a significant improvement in coefficient of variation (CoV) with the validation database compared to the original equations. It was observed that the concrete shear equation prediction could be improved if the concrete strength could be taken to a power value less than what is currently recommended in the guidelines. In the case of transverse shear equations, the inclusion of concrete strength improved the results of the optimized equations. The optimized shear equations were further calibrated by performing reliability analysis in order to use them for design purposes. The resistance factors for the shear design equations were calculated at a target reliability index, βT of 3.5 in order to achieve an acceptable level of structural safety.

Investigation of bond in concrete of GFRP and steel hybrid rebar

Fiber reinforced polymer (FRP) rebar is a great alternative to the conventional steel reinforcing bar because of its corrosion-free characteristics and high strength. This study introduces a newly developed hybrid FRP rebar which is composed of a conventional steel rebar inside and glass fiber reinforced polymer (GFRP) on the outer surface. The developed hybrid rebar takes advantage of overcoming the low elastic modulus from their composite action and durability as the outer GFRP surface resists corrosion. In addition, this hybrid GFRP rebar is also cost-effective. This study investigated the bonding characteristics between the newly developed hybrid FRP rebar and concrete. A direct pull-out test was conducted following the KS F 2441 testing method. In the test result, the bond strength and slip relationship were investigated for test variables. The rebar outer diameter was 16 mm, and inside steel rebar diameters were 10 and 13 mm with varying thickness of GFRP.

Bond performance of fiber reinforced polymer rebars in different casting positions

In the last two decades, fiber reinforced polymer (FRP) rebars have become attractive to civil engineers, due to their corrosion resistance and high strength‐to‐weight ratio. To take advantage of such properties, many research studies have been carried out on the mechanical and physical properties of FRP rebars in concrete, and critical issues have been discovered regarding the bond behavior of FRP rebars. In this study, an effort was made to investigate the effects of FRP rebar types, locations and concrete casting positions on the bond performance of FRP rebars embedded in concrete members. Experimental investigations were conducted using a total of 63 specimens, with seven types of reinforcing bars, and three different casting positions. According to the test results, the bond performance of FRP reinforced concrete was greatly influenced by FRP type and surface condition; while vertical casting showed higher bond strength than horizontal casting. In addition, the bond strength reduction ratio of FRP rebars cast in the horizontal direction was smaller than that of steel bars. POLYM. COMPOS., 37:2098–2108, 2016. © 2015 Society of Plastics Engineers

중공형 GFRP 보강근의 인장성능 실험연구

섬유복합체 (Fiber Reinforced Polymer, FRP)는 비강도가 높고, 비부식성 재료라는 특징을 가지고 있어서 건설 분야에서 철근을 대체할 수 있는 보강근 재료로 인식되고 있다. 몇몇 유리섬유 복합체 (Glass FRP, GFRP) 보강근이 상용화되어 있지만 GFRP는 철근에 비해 가격이 비싸고 상대적으로 낮은 탄성계수와 취성 파괴 특성 때문에 다소 경쟁력이 떨어진다. GFRP 보강근의 재료가격을 낮출 수 없다면 사용된 재료의 성능을 최대로 하여 보강근의 성능을 높이는 것이 상대적인 가격을 낮추는 방법이 될 수 있다. 일반적으로 FRP 보강근의 직경이 커질수록 인장강도는 감소하는 것으로 알려져 있다. 이의 원인 중 하나는 보강근이 인장을 받을 때 외력이 중앙에 위치한 섬유에 충분히 전달되지 못하여 외측에 위한 섬유들만이 인장에 저항하기 때문이다. 따라서 본연의 역할을 수행하지 못하는 섬유는 제거함으로써 보강근의 단가를 낮추면서 보강근이 소정의 성능을 발휘하도록 한다면 가격대비 성능이 최적화된 FRP 보강근을 제작할 수 있다. 본 연구에서는 직경 19 mm의 GFRP 보강근에 대해 단면 내에 중공이 존재하는 경우 중공비율에 따른 인장특성의 변화를 실험적으로 관찰하였다. 중공이 없는 GFRP 보강근 세 개, 네 가지 중공비율에 대해 각각 여섯 개의 GFRP 보강근 시편을 준비하여 인장실험을 실시하였으며 결과 분석을 통하여 인장특성 변화를 도출하였으며 이를 바탕으로 최적의 중공비율을 제안하였다. Fiber-reinforced polymer (FRP) has been generally accepted by civil engineers as an alternative for steel reinforcing bars (rebar) due to its advantageous specific tensile strength and non-corrosiveness. Even though some glass fiber reinforced polymer (GFRP) rebars are available on a market, GFRP is still somewhat uncompetitive over steel rebar due to their high cost and relatively low elastic modulus, and brittle failure characteristic. If the price of component materials of GFRP rebar is not reduced, it would be another solution to increase the performance of each material to the highest degree. The tensile strength generally decreases with increasing diameter of FRP rebar. One of the reasons is that only fibers except for fibers in center resist the external force due to the lack of force transfer and the deformation of only outer fibers by gripping system. Eliminating fibers in the center, which do not play an aimed role fully, are helpful to reduce the price and finally FRP rebar would be optimized over the price. In this study, the effect of the hollow section in a cross-section of a GFRP rebar was investigated. A GFRP rebar with 19 mm diameter was selected and an analysis was performed for the tensile test results. Parameter was the ratio of hollow section over solid cross-section. Four kinds of hollow sections were planned. A total of 27 specimens, six specimens for each hollow section and three specimens with a solid cross-section were manufactured and tested. The change by the ratio of hollow section over solid cross-section was analyzed and an optimized cross-section design was proposed.

Design for shear strength of concrete beams longitudinally reinforced with GFRP bars

In this paper, a model for the evaluation of shear strength of fibre reinforced polymer (FRP)-reinforced concrete beams is given. The survey of literature indicates that the FRP reinforced beams tested with shear span to depth ratio less than or equal to 1.0 is limited. In this study, eight concrete beams reinforced with GFRP rebars without stirrups are cast and tested over shear span to depth ratio of 0.5 and 1.75. The concrete compressive strength is varied from 40.6 to 65.3 MPa. The longitudinal reinforcement ratio is varied from 1.16 to 1.75. The experimental shear strength and load-deflection response of the beams are determined and reported in this paper. A model is proposed for the prediction of shear strength of beams reinforced with FRP bars. The proposed model accounts for compressive strength of concrete, modulus of FRP rebar, longitudinal reinforcement ratio, shear span to depth ratio and size effect of beams. The shear strength of FRP reinforced concrete beams predicted using the proposed model is found to be in better agreement with the corresponding test data when compared with the shear strength predicted using the eleven models published in the literature. Design example of FRP reinforced concrete beam is also given in the appendix.

Design for shear strength of concrete beams longitudinally reinforced with GFRP bars

In this paper, a model for the evaluation of shear strength of fibre reinforced polymer (FRP)-reinforced concrete beams is given. The survey of literature indicates that the FRP reinforced beams tested with shear span to depth ratio less than or equal to 1.0 is limited. In this study, eight concrete beams reinforced with GFRP rebars without stirrups are cast and tested over shear span to depth ratio of 0.5 and 1.75. The concrete compressive strength is varied from 40.6 to 65.3 MPa. The longitudinal reinforcement ratio is varied from 1.16 to 1.75. The experimental shear strength and load-deflection response of the beams are determined and reported in this paper. A model is proposed for the prediction of shear strength of beams reinforced with FRP bars. The proposed model accounts for compressive strength of concrete, modulus of FRP rebar, longitudinal reinforcement ratio, shear span to depth ratio and size effect of beams. The shear strength of FRP reinforced concrete beams predicted using the proposed model is found to be in better agreement with the corresponding test data when compared with the shear strength predicted using the eleven models published in the literature. Design example of FRP reinforced concrete beam is also given in the appendix.