Conventional Fiber-reinforced Polymer Research Articles

Experimental study on shear performance of PET FRP-strengthened RC beams without stirrups

The shear behavior of simply supported RC beams strengthened with polyethylene terephthalate (PET) fiber-reinforced polymer (FRP) was studied in this paper. The test variables include strengthening schemes (i.e., U-wrapping and full-wrapping) and FRP reinforcement ratios. A comprehensive and meticulous investigation was conducted on the failure mode of RC beams, shear force-deflection curves, the development process of shear cracks, the evolution of FRP strain, and the interaction between FRP and concrete. The shear capacity and deformation were increased by 42.5–123.2% and −16.34–96.89% using the fully wrapped scheme, respectively. The improvement in shear capacity was 42.5–72.55% when the U-wrapped scheme was adopted. The PET FRP fully wrapped beams exhibited a type of ductile shear failure. The strain of PET FRP in the fully wrapped specimen exceeded 4% without fracture, ensuring the deformation capacity after the peak load. The shear deformation caused the increase in post-peak mid-span deflection. The U-wrapped specimens showed debonding failure, with the debonding strain of PET FRP being more than that of conventional FRP. Finally, six design guidelines for PET FRP shear contribution were evaluated for both U-wrapped and fully wrapped schemes.

Sensitivity analysis of parameters affecting the seismic performance of RC columns strengthened by fabric-reinforced cementitious mortar

In recent years, fabric-reinforced cementitious mortar (FRCM) has emerged as a popular choice for strengthening reinforced concrete and masonry structures due to several advantages over conventional fiber-reinforced polymer (FRP) composites. Particularly, the enhancement of Reinforced Concrete (RC) columns using FRCM composites has garnered significant attention. While experimental investigations are crucial for assessing the effectiveness of FRCM, physical experiments are often resource-intensive and time-consuming. Therefore, this study seeks to investigate the impact of various design parameters on the performance of RC columns strengthened with FRCM under low-amplitude cyclic loads simulating earthquakes through Finite Element (FE) analysis. The FE model, incorporating columns and FRCM strengthening materials, was developed using the DIANA 10.5 program. To assess the reliability of the model, analytical results were compared with experimental findings from a previous study, focusing on lateral strength, hysteresis behavior, and failure modes. The validation outcomes demonstrated a reasonable correlation between the test and numerical results. Subsequently, a sensitivity analysis was conducted to explore the influence of input parameters, such as concrete compressive strength, fabric reinforcement quantity, longitudinal reinforcement ratio of the columns, and pre-axial loading levels, on the seismic performance of RC columns reinforced with FRCM. The findings of the sensitivity analysis were discussed in detail.

Bond behaviour at high temperatures between concrete and CFRP or steel strengthening bars applied according to the embedded through-section (ETS) technique

The embedded through-section (ETS) technique is an efficient shear-strengthening method for reinforced concrete (RC) structures, in which fibre reinforced polymer (FRP) or steel bars are bonded in holes predrilled in RC members. Compared to conventional FRP strengthening methods, ETS systems are expected to present better performance when exposed to high temperatures/fire, due to the insulation provided by concrete to both adhesive and FRP/steel components. Yet, the mechanical/bond properties of the strengthening materials are highly affected when the glass transition temperatures (Tg) of their components is attained/exceeded; however, there are no studies available about the influence of elevated temperatures on the performance of ETS systems. This paper presents an experimental study that addresses the effects of elevated temperatures on the bond behaviour of ETS systems – pull-out tests were conducted on steel rebars or sand-coated carbon-FRP (CFRP) rods bonded with epoxy adhesive on concrete cylinders at temperatures up to 90 °C. The results obtained confirm that the strength and stiffness of the bonded interfaces decrease when the temperature in the adhesive approaches/exceeds its Tg; moreover, with sand-coated CFRP rods, failure was also influenced by the bond between the sand coating and the CFRP rods core, and thus the average bond capacity was significantly reduced for temperatures below the Tg of the epoxy adhesive. Bond-slip relations were calibrated for different elevated temperatures for the steel/CFRP-concrete interfaces using data obtained from the pull-out tests; these bond laws can be used to simulate the mechanical behaviour of ETS-strengthened-RC members subjected to elevated temperatures.

Evaluation of Ultimate Axial Strain Models for Fiber-Reinforced Polymer-Confined Concrete Cylinders

Fiber-reinforced polymer (FRP) composite has been widely applied to reinforce and rehabilitate concrete columns in civil engineering. It is necessary to deeply understand the ultimate deformation (i.e., the ultimate axial strain εcc ) of FRP-confined concrete columns. A large database including different types of FRP (conventional FRP and large-rupture-strain FRP), concrete compressive strengths (19.4 MPa-169.7 MPa), and cylinder sizes (51 mm-508 mm), as well as various concrete types, is collected in this study. For this reason, several existing ultimate axial strain modes for FRP-confined cylinders have been evaluated via some statistical parameters based on the database. The verification results illustrate that the prediction accuracy of axial strain models is not good enough due to the intrinsic difficulty in predicting deformation and variability of data source, although the available models with better performance can achieve the average ratio between experimental and predicted strain value ( εccexpe/εccpred ) below 10%.

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%.

Vibration-based FRP debonding detection using a Q-learning evolutionary algorithm

The secured bonding between the externally bonded fiber reinforced polymer (FRP) and the host structure is critical to provide the composite action of the FRP strengthened structure. Conventional FRP debonding assessment is usually based on nondestructive testing methods, which have limited sensing coverage and thus cannot detect debonding far away from the sensors. In this study, the global vibration-based method is developed to identify the debonding condition of FRP strengthened structures for the first time. An FRP strengthened cantilever steel beam was tested in the laboratory. As debonding damage is non-invertible, a series of FRP debonding scenarios were specially designed by a stepwise bonding procedure in an inverse sequence. In each scenario, the first six natural frequencies and mode shapes were extracted from the modal testing and used for detecting the simulated debonding damage via the model updating technique. An l0.5 regularization is adopted to enforce sparse damage detection. A new Q-learning evolutionary algorithm is developed to solve the optimization problem by integrating the K-means clustering, Jaya, and the tree seeds algorithms. The experimental results show that the debonding condition of the FRP strengthened beam can be accurately located and quantified in all debonding scenarios. The present study provides a new FRP debonding detection approach.

Sustainable Fiber‐Reinforced Composites: A Review

AbstractSustainable fiber reinforced polymer (FRP) composites from renewable and biodegradable fibrous materials and polymer matrices are of great interest, as they can potentially reduce environmental impacts. However, the overall properties of such composites are still far from the high‐performance conventional glass or carbon FRP composites. Therefore, a balance between composite performance and biodegradability is required with approaches to what one might call an eco‐friendly composite. This review provides an overview of sustainable FRP composites, their manufacturing techniques, and sustainability in general at materials, manufacturing, and end‐of‐life levels. Sustainable plant‐based natural fibers and polymer matrices are also summarized, followed by an overview of their modification techniques to obtain high‐performance, multifunctional, and sustainable FRP composites. Current state‐of‐the‐art mechanical and functional properties of such composites are then surveyed, and an overview of their potential applications in various industries, including automobile, aerospace, construction, medical, sports, and electronics is provided. Finally, future market trends, current challenges, and the future perspective on sustainable natural FRP composites are discussed.

Test Methods for Characterizing the Properties of Fiber-Reinforced Polymer Composites at Elevated Temperatures.

Recent research trends focus on developing bio-based (derived from agricultural byproducts) fiber-reinforced polymer (FRP) composites for structural applications. Fire resistance is one of the key issues that need to be addressed for the use of these FRP materials in buildings. The thermal and mechanical properties of the constituent materials essentially determine the fire performance (and the fire resistance rating) of a structural member, and these properties vary with temperature. Further, the properties of composite materials such as the FRP are highly influenced by the composition and type of fibers and matrix, and these thermo-mechanical properties also vary significantly with temperature. Due to this variation, the fire resistance of FRP materials (both conventional and bio-based) poses a major concern for use in buildings. Currently, very few standardized test procedures are available for evaluating the high-temperature material properties of FRP composites. In this paper, a review of testing protocols and procedures for undertaking tests on FRP materials at various elevated temperatures for evaluating their properties is carried out. Recommendations are provided on the most suitable test methods, specimen conditions, testing regime, and other issues associated with testing at elevated temperatures. In addition, the applicability of the proposed test methods is illustrated through a case study on conventional FRP specimens. Further, the applicability of the recommended test procedures for measuring high-temperature properties of bio-based FRP composites is highlighted.

Reinforcement of new and existing reinforced concrete beams with fiber-reinforced polymer bars and sheets – A numerical analysis

Due to its durability and corrosion resistance, fiber-reinforced polymer (FRP) has become more readily adopted as reinforcement in new construction or existing concrete structures. With the increasing use of FRP reinforced members, the addition of new construction materials has been explored which requires investigation for their safe implementation. The significance of this study is to shed light on the response of newly-constructed and retrofitted simply-supported and continuous concrete beams reinforced with traditional FRP in various practical scenarios as compared to counterpart beams reinforced with steel bars and beams made with non-conventional materials. The parameters considered include the effects of environmental exposure, loading conditions, the addition of externally bonded FRP, and concrete type on the beams. Beams cast with normal-strength concrete and reinforced with steel and conventional FRP materials, such as carbon and glass fibers, were compared to beams cast with ultra-high-performance fiber-reinforced concrete and reinforced with basalt fibers. The results indicate that the use of these non-conventional materials can be advantageous in a number of scenarios by improving the beams’ performance in terms of load and displacement responses.

Flexural behavior of RC beams externally bonded with polyethylene terephthalate (PET) fiber reinforced polymer (FRP) laminates

This paper presents an experimental study on the flexural behavior of reinforced concrete (RC) beams externally strengthened with a unique and sustainable type of fiber reinforced polymer (FRP), named polyethylene terephthalate (PET). Compared to conventional FRPs, PET-FRP is characterized by its large rupture strain and low elastic modulus. The flexural behavior of RC beams strengthened with one and two layers of PET-FRP was compared to that strengthened with one carbon FRP (CFRP) layer and an unstrengthened beam specimen. Test results showed that the specimens strengthened with two PET-FRP layers and one CFRP layer displayed equal and significant strength enhancement of 47% compared to the unstrengthened beam. However, the former exhibited higher ductility than the latter by 33%. Lower strength enhancement was depicted in the specimen strengthened with one PET-FRP layer. Despite that, the ductility of the PET-strengthened beam was substantial, which even outweighed that of the unstrengthened beam by 9%. The results of this study showed that PET-FRP is a promising strengthening material that improves structural performance and provides a sustainable alternative to conventional FRP strengthening systems.

Confinement model for LRS FRP-confined concrete using conventional regression and artificial neural network techniques

Concrete confined using fiber-reinforced polymer (FRP) composites experience significant enhancements in strength and strain. For the seismic retrofitting of existing reinforced concrete (RC) structures, a large rupture strain (LRS) FRP (i.e., polyethylene terephthalate and naphthalate, denoted as PET and PEN respectively), with a larger rupture strain of more than 5%, is a promising alternative to conventional FRPs with a rupture strain of less than 3%. The majority of analytical models on the stress–strain behavior of FRP-confined concrete under axial compression have focused largely on concrete confined with the traditional FRP material. Analytical research on LRS FRP-confined concrete is, however, limited. Moreover, all existed stress–strain models were determined based on theoretical analysis and test data fitting. In this paper, the artificial neural networks (ANN) method is employed to build a confinement model directly from experimental data to predict the different components of the stress–strain response. A test database consisting of 226 axial compression tests on LRS FRP-confined concrete specimens is used. The test results, in terms of full confined stress–strain response, strength, strain, FRP rupture strain, and dilation response were investigated. Predictive expressions and practical ANN models for the strength, strain, and shape of an axial stress–strain response are provided. Existing models for LRS FRP-confined concrete were also evaluated. The results of the existing and proposed models report that the proposed methods achieve significantly better results.

Comparison of monotonic axial compressive behavior of rectangular concrete confined by FRP with different rupture strains

Axial compressive behavior of rectangular concrete might differ greatly when it is confined by fiber reinforced polymer (FRP) with different rupture strains. To address this issue, this study experimentally investigated and compared the monotonic axial compressive behavior of rectangular concrete confined by FRP with three rupture strains. Axial compression experiments of 28 rectangular concrete stub columns confined by FRP were carried out. The FRP utilized in this study included conventional FRP with small rupture strain, i.e., carbon FRP (CFRP) and glass FRP (GFRP), and large rupture strain FRP (LRS-FRP), i.e., polyethylene terephthalate (PET) FRP. The key test parameters were the rupture strain of FRP, the FRP thickness, and the cross-sectional aspect ratio. Failure modes, stress vs. strain behavior, axial stress and strain relationship, hoop rupture strain, the effect of FRP confinement stiffness ratio, and the effect of cross-sectional aspect ratio were analyzed, respectively. Results of this research showed that the rupture strain of FRP exhibited significant influence on the failure mode and the axial deformation of rectangular concrete confined by FRP. FRP strain reduction factors were decreased with the increasing rupture strains of FRP. This is because the rectangular concrete confined by FRP with larger rupture strains dilated more non-uniformly than its counterpart using smaller rupture strain FRP. In addition, based on the transition point and the ultimate condition, the axial stress vs. strain curves of rectangular concrete confined by FRP could be categorized into three types, including sufficient, moderate, and insufficient FRP confinement, respectively.

Compressive behavior and modelling of engineered cementitious composite (ECC) confined with LRS FRP and conventional FRP

A combination of core engineered cementitious composite (ECC) and external fiber-reinforced polymer (FRP) composites can solve the problems of insufficient ductility and durability of normal concrete (NC) in adverse environments and improve bearing capacity. However, existing studies on the performance of ECC strengthened by FRP are very limited—particularly the research on large rupture strain (LRS) FRP-confined ECC. In this study, an experimental investigation of LRS FRP and conventional glass FRP-confined ECC cylinders with different confinement levels under axial compression is presented. It is found that the application of external FRP jackets leads to a significant improvement in the compressive strength and deformation of the core ECC. LRS FRP-confined ECC exhibits a higher ultimate deformation than conventional FRP-confined ECC. The dilation behavior, axial stress–strain behavior, and energy dissipation capacity between FRP-confined ECC and FRP-confined NC under the same confinement level are compared, and the former exhibits a significantly larger ductility and energy dissipation capacity than the latter. The results also indicate that most existing models are unable to accurately predict the compressive behavior of FRP-confined ECC. To address this, a dilation model and an axial stress–strain model of FRP-confined NC are modified to extend their application to FRP-confined ECC.

Design considerations for prefabricated composite jackets for structural repair: Parametric investigation and case study

Prefabricated fiber-reinforced polymer (FRP) repair systems are becoming popular for structural repair because FRPs superior resistance to corrosion and weathering. Several parameters impact the effectiveness of this type of repair, including the structural continuity provided by the joining system and grout properties. This study numerically investigated the effects of these critical design parameters to generate new knowledge and understand fundamental structural behaviour of a prefabricated composite repair system. The results reveal that the effective transverse tensile strength of FRP joint should be as low as 20% of the jacket strength for structural repairs. Moreover, grout stiffness dictated the stress transfer between the core and outer jacket. Increasing the compressive strength of the grout delayed grout cracking and degradation, accompanied by a rapid increase in FRP joint strain. Lastly, numerical simulation of a corroded steel column in an actual bridge shows that the application of the FRP jacket improves the strength capacity by 320%, which is 85% higher than that of the conventional FRP repair system. This study provides useful information for optimally designing and effectively using prefabricated composite jackets for structural repairs.

Ductile, durable, and reliable alternative to FRP bars for reinforcing seawater sea-sand recycled concrete beams: steel/FRP composite bars

The combination of seawater, sea-sand and recycled concrete aggregate (RCA) to make the so-called seawater sea-sand recycled concrete (SSRC) is a promising alternative to mitigate the shortage of raw construction materials, e.g. river sand, freshwater and crushed stone. Although equipped with anti-corrosion and high strength, the reduced ductility due to the linear elastic nature of conventional fiber-reinforced polymer (FRP) bars hinders their application scope in civil engineering. Inherited the advantages of FRP bars, steel fiber-reinforced polymer composite bars (SFCBs) are also characterized by good ductility and stable post-yield stiffness, which can potentially be ideal reinforcement for concrete structures made with un-desalted corrosive raw materials. This paper presents experimental and theoretical investigations on the flexural performance of SFCB reinforced SSRC beams. Test results indicate that the displacement ductility factor of almost all the SFCB reinforced SSRC beams are in a range from 3.7 to 4.9. The ratios of peak load-to-yielding load of SFCB reinforced SSRC beams varied from 1.5 to 3.1, indicating a remarkable post-yield behavior. Compared with the counterparts with natural aggregates, SSRC beams show slightly reduced flexural strength and comparable ductility. Theoretical models were proposed to predict the load-deflection responses of SFCB reinforced SSRC beams, which paves the way for the design of such members.

Experimental Study on the Adhesion of Basalt Textile Reinforced Mortars (TRM) to Clay Brick Masonry: The Influence of Textile Density

Textile Reinforced Mortar (TRM) composite systems are gaining consensus within the scientific and technical communities as a viable and advantageous alternative to the most conventional Fibre-Reinforced Polymer (FRP) composites. Due to the good compatibility both in terms of stiffness and vapor permeability between the inorganic matrix and the substrate, the TRMs appear to be particularly well suited for strengthening masonry members and enhancing their capacity to withstand tensile and shear stresses, such as those induced by seismic shakings. This paper aims to investigate the mechanical response of a TRM system featuring an internal reinforcement made of basalt fiber textile. Therefore, the paper reports the results of an experimental campaign carried out by single-lap shear bond tests on masonry substrate reinforced by TRM strips. Three different kinds of TRM have been taken into account, each one characterized by a variable number of fabric plies. The results show that, in all cases, TRMs fail prematurely due to debonding between fabric and matrix. However, the aforementioned premature failure is the main concern emerging from these test results, and further work is requested in reformulating the matrix composition towards enhancing their tensile strength and, hence, restraining the occurrence of fabric-to-matrix debonding.

Axial compressive behavior of polyethylene terephthalate/carbon FRP-confined seawater sea-sand concrete in circular columns

Since fiber-reinforced polymer (FRP) composites possess excellent corrosion resistance, they have great potential of use with seawater sea-sand concrete (SSC) for the construction of marine and coastal infrastructure. The existing research on SSC columns confined with FRP jackets, however, is rather limited. Most of the limited research has focused on the axial compressive behavior of SSC columns confined by conventional FRP jackets (made from carbon and glass FRP composites). This paper presents results from the first-ever experimental study on the behavior of axially loaded circular SSC columns confined by polyethylene terephthalate (PET) FRP jackets, which are composed of a new and more promising type of FRP composites with a bilinear stress-strain response and a large rupture strain. Parameters varied between the tests presented in this paper included the type and thickness of FRP jackets and the type of mixing water used for casting the SSC. A modified analysis-oriented model was developed in which a new dilation equation was proposed for the PET FRP-confined SSC. The reliability and accuracy of the analysis-oriented model were then examined by comparisons between the predicted results and the test results of the FRP-confined SSC.

Performance of Concrete Confined with a Jute–Polyester Hybrid Fiber Reinforced Polymer Composite: A Novel Strengthening Technique

The strengthening and rehabilitation of concrete members is an important issue which arises worldwide. Carbon, aramid and glass fiber reinforced polymer (FRP) composites are mainly used for strengthening and rehabilitation. However, its use is limited on a small scale because of its high price, lack of availability and environmental impacts. The solution of this issue gives rise to the use of locally available natural fibers and low-cost synthetic fibers. This paper presents the experimental and analytical results of circular and square concrete columns confined with jute–polyester hybrid FRP composites. The main objective of this study is to evaluate the viability and performance of concrete confined with the hybridization of jute and polyester (FRP) composite sheets to utilize its superior properties. A novel hybrid technique has been applied for the wrapping of fiber sheets. The fiber sheets were applied in such a way that a uniform bond between the inner and outer layer was achieved. A total of 32 plain, standard size circular and square concrete specimens, externally wrapped with a jute–polyester FRP (JPFRP) composite, were tested under monotonic axial compressive loads. The result shows that JPFRP confinement increased the strength, strain and ductility index ranged between 1.24 and 2.61, 1.38 and 8.97, and 4.94 and 26.5 times the un-jacketed specimen, respectively. Furthermore, the wrapping has a significant effect on the low-strength specimens, having a circular cross-section. For high strength specimens, the post-peak stress-strain behavior was dominated by the outer polyester jacket because of its large rupture strain. Additionally, the test results were used to evaluate the existing strength-strain models derived for conventional FRPs. The models predicted values either underestimating or overestimating the compressive strength and strain of JPFRP-confined specimens. However, the strength models performed better than the strain models. The JPFRP wrapping significantly enhanced the strength, fracture energy, ductility index, and post-peak response. Therefore, JPFRP confinement can be used for a small-scale application, where little strength and high ductility is demanded. Moreover, it can be used to prevent the peeling of the concrete cover and moisture penetration into the concrete.

Experimental and computational modeling of effective flexural/tensile properties of microwave pyrolysis biochar reinforced GFRP biocomposites

This study focuses on developing high-strength biocomposites through the addition of biochar particulate reinforcement in the conventional fiber-reinforced polymer (FRP) composites. The novel biocomposites, consisting of E-glass fiber, microwave-pyrolysis synthesized biochar, and vinyl-ester matrix, were manufactured in a pultrusion process. The high specific-surface-area of biochar particles along with their nanoscale hardness/Young's modulus attribute up to 34% flexural strength gain compared to the conventional GFRP composites. This research presents results from biocomposites tensile and flexural testing, conducted following a design-of-experiments. Experimental results are assessed against computational values derived from a hybrid composite theory on the basis of Rule of Mixture theory, as well as ABAQUS finite element modeling solved through composite homogenization technique.

Strengthening concrete using phosphate cement-based fiber-reinforced inorganic composites for improved fire resistance

Fiber reinforced polymer (FRP) composites have been widely used for strengthening of concrete structures for many years. A critical concern for the FRP strengthening technology is the fire resistance. In this study, phosphate cement-based fiber-reinforced inorganic polymer (FRiP) composites were used to replace conventional fiber-reinforced polymer (FRP) composites to strengthen concrete beams for improved fire resistance. The performances of FRiP composites, FRiP-to-concrete interfaces and FRiP-strengthened concrete beams were examined both before and after fire exposure. It was demonstrated that the phosphate cement-based FRiP composites can significantly enhance the mechanical strength of the concrete beams. In addition, they exhibit much improved fire resistance as compared to the epoxy-based FRP composites.