Fiber-reinforced Polymer Profiles Research Articles

A novel ductile connection for FRP pultruded beam-to-column assemblies

The resistance, stiffness and ductility of the joints between Fibre Reinforced Polymer (FRP) members play a key role in ensuring the required structural performance of pultruded composite frames. Both bonded and bolted joints are characterised by poor mechanical performance due to brittle failure and low resistance respectively. Hybrid joints are stronger and more ductile, but are still affected by some criticisms such as (i) non-repairability, (ii) the presence of holes in the fibre-reinforced material, (iii) the difficulty of assembling a transverse beam into the joint, which is typically required for real 3D systems. To overcome such limitations, a novel ductile connection has been developed by combining ductile steel elements bonded to FRP members. The steel elements are bolted together and are designed to be weaker than the FRP profiles and adhesive. Experimental tests on beam-column assemblies have been carried out and the test results have shown that the investigated hybrid connection is characterised by adequate stiffness, resistance and high ductility. The damage is concentrated in the bolted steel elements, which can be easily replaced, confirming the repairability of the assembly.

Influence of seawater sea sand concrete environment on the long‐term tensile properties of pultruded basalt and basalt‐carbon‐hybrid fiber reinforced polymer profiles

AbstractTo reveal the durability of fiber‐reinforced polymer (FRP) profiles, which differ from FRP bars due to the clear variations in the internal structure of the fibers. This study experimentally investigated the tensile properties of FRP profiles in the seawater sea sand concrete (SSC) environment at 25, 45, and 60°C. Unidirectional basalt FRP profiles, multidirectional basalt FRP profiles, and basalt and carbon hybrid FRP profiles were considered. SEM analysis was performed to evaluate the corrosion mechanism. Results show that the strength degradation of FRP profiles accelerated with rising ambient temperature due to resin corrosion in hyperthermia. After 63 days of immersion in the SSC environment solution, multidirectional basalt FRP (BFRP) profiles exhibited more severe corrosion than unidirectional BFRP profiles (82.5% and 88.2%, respectively). The hybrid FRP profiles exhibited a strength retention of 86.4%, which was higher than that of multidirectional BFRP profiles due to the superior alkali resistance of carbon fibers compared to basalt fibers. In addition, prediction models of FRP profiles based on the Arrhenius relation were proposed with a high correlation coefficient of R2.Highlights Durability of FRP profiles in SSC environment was studied. Basalt and carbon fibers hybrid reinforced multidirectional polymer profiles. Corrosion mechanism of FRP profiles was revealed. Tensile property prediction model of FRPs in SSC environment was proposed.

Tensile strength and modulus of FRP pultruded composites at varying load angles relative to the pultrusion direction

Tensile strength and modulus are key material properties for the design of pultruded fiber reinforced polymer (FRP) composite members and connections. Pultruded FRP composites are orthotropic materials and thus both the tensile strength and modulus vary with the angle between the pultrusion direction and the applied force. In current design standards, the variation of material properties with load direction is addressed by prescribing the use of longitudinal properties for angles between zero and five degrees, and the use of transversal properties for angles greater than five degrees. To derive a more precise and representative equational relationship between load angle and tensile material properties, 228 coupons extracted from pultruded FRP profiles were tested in tension in accordance with ASTM D638-22 to determine the resulting average tensile strength and tensile modulus values. Coupons were taken from different profiles from different manufacturers, different section locations (e.g., web versus flange), as well as varying angles between the pultrusion direction and the applied force. The characteristic design value of the tensile strength and tensile modulus at each tested angle was determined in accordance with ASTM D7290-22. Using this dataset, a predictive relationship based on a generalized form of Hankinson’s equation was developed for both average and characteristic tensile strength and both average and characteristic tensile modulus as a function of three factors: 1) the differential angle between the pultrusion direction and the applied force, 2) the longitudinal tensile property (i.e., strength or modulus), and 3) the transverse tensile property (i.e., strength or modulus). The average relative difference between tested and predicted values using the developed equations varied between 4.4% and 8.3%, depending on the considered mechanical property.

Prediction of long-term durability of unidirectional/multidirectional basalt fiber- and hybrid fiber-reinforced polymer profiles under concrete environment

Recently, concrete composite structures based on a fiber-reinforced polymer (FRP) profile, which serves in the corresponding alkaline concrete environment have been widely used. Unlike FRP bars, the long-term durability of FRP profiles in concrete remains unclear, particularly for those with multidirectional (MD) or hybrid fibers, whose internal structure differs from that of FRP bars. This study focused on the tensile strength degradation of FRP profiles exposed to an artificial concrete environment solution at various ambient temperatures. Three types of FRP specimens were considered, including unidirectional (UD) basalt FRP (BFRP), MD BFRP and MD basalt-carbon hybrid FRP profiles. The results show that the alkaline environment of the concrete caused a decrease in the tensile strength of the FRP profiles, and the rate of degradation in strength significantly accelerated as the ambient temperature increased; however, the environment had no apparent effect on the elastic modulus. In addition, the strength degradation of MD FRP profiles was more significant than that of UD FRP profiles after corrosion due to insufficient impregnation of the fibers by the resin, which results in initial defects as well as increased intrusion of external moisture and corrosive media. Scanning electron microscopy (SEM) was employed to analyze the morphology and degradation mechanism of FRP profiles in a concrete environment. Moreover, a long-term prediction method for tensile strength retention of three types of FRP profile in arbitrary ambient temperatures is proposed based on the Arrhenius equation.

Feasibility of bolted connectors in hybrid FRP-steel structures

Due to the low weight and excellent durability of composite materials, Fibre Reinforced Polymer (FRP) decks mounted on steel superstructures are becoming increasingly common in engineering practice. Bolted joints are generally used to facilitate connections between an FRP deck and steel girders in road bridges. The connections are subjected to both high magnitude static forces as well as fatigue loading due to overpassing vehicles. With ever increasing traffic on both road and railway bridges, fatigue performance is of critical concern. Bolted FRP joints have been extensively researched in the past under static loading, but less is known about the fatigue and creep behaviour of such joints. Furthermore, little research exists on non-pultruded FRP profiles connected using bolted connections. Therefore, the objective of this research is to investigate connectors’ feasibility by means of static, fatigue and creep experiments on four different types of bolted joints comprising mechanical connectors and injection techniques. The study focuses on application in vacuum infused GFRP panels with integrated webs made of multi-directional laminates) connected to steel bridge superstructures. In addition, experimental results are used to validate Finite Element Analyses (FEA). Based on the obtained results, the novel injected steel-reinforced resin (iSRR) connector shows promising potential in hybrid steel-FRP bridges where good fatigue endurance of the connection, are required.

An experimental study of FRP truss side plate joint

Ultra-high performance concrete (UHPC) slab - fiber reinforced polymer (FRP) truss hybrid beam has been proved as an effective and advanced bridge system. However, the limited shear strength and the significantly lower transverse properties compared with its longitudinal counterparts result in low rigidity and load carrying capacity of bolted FRP truss joints, which governs the overall performance of the UHPC slab- FRP truss hybrid beam. A FRP truss side plate joint was proposed which integrates a steel U-shaped gusset plate (SUGP) to reinforce the traditional bolted joint. The proposed joint outperformed the conventional bolted joints in terms of the failure mode, load carrying capacity, and stiffness. The load carrying capacity of the proposed joint can be improved by up to 63% and the ultimate deflection was reduced by up to 33%. The effects of a variety of key design parameters, including adhesive, bolt diameter, pre-torque, and length and thickness of the gusset plate, on the performance of the SUGP joint were evaluated. Test results showed that: (i) the adhesive between the pultruded FRP profiles and the steel gusset plate could increase both the initial stiffness and ultimate load carrying capacity of the steel SUGP joint; (ii) length and thickness of the steel gusset plate did not result in notable difference in the behavior of the steel SUGP joint for the range of parameters investigated herein; and (iii) application of pre-tightening torque within the thread of the bolts slightly improved the load carrying capacity but significantly improved the ductility of the steel SUGP joint.

Approaches for the Statistical Assessment on Structural Joints of Pultruded Fiber-Reinforced Polymer Profiles Using Trend Analysis

Approaches for the Statistical Assessment on Structural Joints of Pultruded Fiber-Reinforced Polymer Profiles Using Trend Analysis

A Review of Fibre Reinforced Polymer Structures

This paper reviews Fibre Reinforced Polymer (FRP) composites in Civil Engineering applications. Three FRP types are used in Structural Engineering: FRP profiles for new construction, FRP rebars and FRP strengthening systems. Basic materials (fibres and resins), manufacturing processes and material properties are discussed. The focus of the paper is on all-FRP new-build structures and their joints. All-FRP structures use pultruded FRP profiles. Their connections and joints use bolting, bonding or a combination of both. For plate-to-pate connections, effects of geometry, fibre direction, type and rate of loading, bolt torque and bolt hole clearance, and washers on failure modes and strength are reviewed. FRP beam-columns joints are also reviewed. The joints are divided into five categories: web cleated, web and flange cleated, high strength, plate bolted and box profile joints. The effect of both static and cyclic loading on joints is studied. The joints’ failure modes are also discussed.

Characterizing the Structural Behavior of FRP Profiles-FRCM Hybrid Superficial Elements: Experimental and Numerical Studies.

Composite materials have been increasingly used to produce hybrid structures together with concrete. This system is commonly applied to bridges and roof structures. The main idea of the current research was to extend this approach by replacing the concrete with a fabric-reinforced cementitious matrix (FRCM) composite, resulting in a combination of composite materials. The main aim was to characterize the structural behavior of fiber-reinforced polymer (FRP) profiles and FRCM hybrid superficial elements. Two different prototypes of the hybrid superficial structural typology were tested to cover bidimensional and three-dimensional application cases of the proposed technology. After mortar cracking, the experimental results revealed a ductile response and a high mechanical capacity. A finite element model was implemented, calibrated, and validated by comparing numerical data with experimental results of the two prototypes. The output was a validated model that correctly captured the characteristic response of the proposed technology, which consisted of changing the structural response from a stiff plate configuration to a membrane type due to cracking of the FRCM composite part of the full solution. The suggested numerical model adequately reflected the experimental response and proved valuable for understanding and explaining the resistive processes established along this complicated FRP-FRCM hybrid structure.

Pressure-dependent bond stress-slip model for sand-coated FRP-concrete interface

The natural bond between FRP (fiber reinforced polymer) profile and concrete is quite weak, additional measures for improving the FRP-concrete interface should be taken to ensure the hybrid FRP-concrete members work compositely. There are several methods for interface improvement, and this paper focuses on the sand-coating method. Experimental tests including pull-out tests and push-out tests were conducted on sand-coated FRP-concrete interface. Test results revealed that the interfacial behavior included two stages. In the first stage, the concrete surrounding the plate failed in shear, which was similar to the adhesive bonding FRP-concrete interface. In the second stage, interfacial dilatation was induced and significant friction took place between fractured concrete surfaces. The adhesive-FRP interface and adhesive-sand aggregates interface were further damaged. Based on test results and numerical analysis, the variation of the interfacial dilatation and friction coefficient with the interfacial slip were determined. Finally, a pressure-dependent bond stress-slip model which was suitable for both active and passive confinement conditions was developed. This model can be implemented into commercial FE software and used for FE analysis of structural members. Comparisons with test results of the present paper and those from the literature indicate that the proposed model can provide reliable predictions.

Structural safety of pultruded FRP profiles for global buckling. Part 2: Reliability-based evaluation of safety formats and partial factor calibration

This two-part paper presents a study about the structural safety of pultruded fibre-reinforced polymer (FRP) profiles with respect to global buckling phenomena, which often govern their ultimate limit states design: flexural buckling, flexural-torsional buckling and lateral-torsional buckling. The first part [1] presented the resistance models for those buckling mechanisms, the probability models for model uncertainties of resistance, and a preliminary set of partial factors related to the model uncertainty, all consistent with the Eurocodes. Aiming at validating the resistance format proposed and that set of partial factors, this second part presents explicit reliability analyses of pultruded FRP profiles designed for the different global buckling failure mechanisms, so that the design equations proposed comply with the target reliability index recommended in EN 1990 for common structural elements. The reliability-based calibration followed similar methodology of documents that assessed the reliability of design with the Eurocodes, including the stochastic modelling of loads and geometry. Partial factor calibration was made through iterative reliability analyses on representative structural elements defined from the proposed design equations. This calibration process allowed defining (i) recommendations for partial factor γRd and (ii) the representative material property used to define γm for each buckling mechanism. The final part of the paper includes recommendations for future research, towards the validation of assumptions made in this study and the standardization of FRP materials.

Optimization of multi-directional fiber architecture for resistance and ductility of bolted FRP profile joints

In view of the low structural efficiency of bolted joints in pultruded fiber-reinforced polymer (FRP) profiles with mainly unidirectional (UD) fiber architecture, off-axis plies were considered as an effective way to improve bolted joint performance. An experimental study of bolted joints was conducted with laminates composed of UD basalt fiber fabrics laid-up at different ply angles and stacking sequences to optimize the lay-up with regard to the integral structural performance of FRP laminates and bolted joints. Furthermore, 3D finite element models of the joints were established to obtain a better understanding of the progressive joint response and failure. The results showed that laminates with off-axis plies could significantly increase joint resistance due to a shift of the failure mode from sudden shear-out to progressive bearing failure. The change of failure mode thus also changed the response from brittle to pseudo-ductile. The experimental and numerical results agreed well in both the failure mode and joint resistance, with one exception, i.e. the applied Hashin-Rotem failure criterion was not able to capture fiber tie-action resistances in the 90°-plies. An optimum stacking sequence and proportion of 0°-, 45°- and 90°-plies was obtained, which can serve as reference for applications in optimized FRP pultrusion.

Inspection, diagnosis and rehabilitation system for all-fibre-reinforced polymer constructions

In the past few decades, the construction industry has been increasingly using fibre reinforced polymer (FRP) materials, due to their advantages in comparison with traditional materials, especially in aggressive environments. However, the information regarding the long-term behaviour of FRP constructions under real service conditions is limited and scattered. In this paper, a system for the inspection, diagnosis and rehabilitation of FRP constructions is presented and validated, providing a standardized and systematic tool for the maintenance of these constructions during their service life. A classification system for the anomalies and most probable causes is first presented. To assist the diagnosis of the anomalies, several in situ non-destructive methods are included in the system, as well as various rehabilitation techniques, able to maintain and/or restore the functionality of FRP elements. Several correlation matrices are proposed, interrelating different anomalies, and relating them with probable causes, and diagnosis and rehabilitation techniques. The system is validated and calibrated through an extensive field campaign of inspections of 410 substructures in 31 facilities. Although developed for FRP profiles and moulded gratings of all-FRP constructions, the system can also be used to inspect FRP components of sandwich or hybrid structures. The implementation of this system supports the inspection activities of a proactive maintenance strategy and, at the same time, the standardization of inspections, resulting in an increase of the effectiveness and objectivity of the actions to be taken upon the occurrence of anomalies in FRP constructions.

Seismic Retrofitting of Traditional Masonry with Pultruded FRP Profiles

This paper presents a study on the potentiality of seismic retrofitting solutions with pultruded Fiber Reinforced Polymer (FRP) profiles. This material can be used in connected frames providing lightweight, corrosion-free and reversible retrofitting of masonry buildings with the moderate requirements of surface preservation. In a hypothetical case study, an experimental program was designed; monotonic shear tests on a half-size physical model of the sample wall were performed to assess the structural performance before and after retrofitting with a basic frame of pultruded Glass Fiber Reinforced Polymer (GFRP) C-shaped profiles, connected to the masonry by steel threaded bar connections. During the tests, the drift, the diagonal displacements in the masonry and the micro-strain in the profiles were measured. The retrofitted system has proven very effective in delaying crack appearance, increasing the maximum load (+85% to +93%) and ultimate displacement (up to +303%). The failure mode switches from rocking to a combination of diagonal cracking and bed joint sliding. The gauge recordings show a very limited mechanical exploitation of the GFRP material, despite the noticeable effectiveness of the retrofit. The application seems thus promising and worth a deeper research focus. Finally, a finite element modelling approach has been developed and validated, and it will be useful to envisage the effects of the proposed solution in future research.

Static performance of a new GFRP–metal string truss bridge subjected to unsymmetrical loads

A unique lightweight string truss deployable bridge assembled by thin-walled fiber reinforced polymer (FRP) and metal profiles was designed for emergency applications. As a new structure, investigations into the static structural performance under the serviceability limit state are desired for examining the structural integrity of the developed bridge when subjected to unsymmetrical loadings characterized by combined torsion and bending. In this study, a full-scale experimental inspection was conducted on a fabricated bridge, and the combined flexural–torsional behavior was examined in terms of displacement and strains. The experimental structure showed favorable strength and rigidity performances to function as deployable bridge under unsymmetrical loading conditions and should be designed in accordance with the stiffness criterion, the same as that under symmetrical loads. In addition, a finite element model (FEM) with a simple modeling process, which considered the multi segments of the FRP members and realistic nodal stiffness of the complex unique hybrid nodal joints, was constructed and compared against experiments, demonstrating good agreement. A FEM-based numerical analysis was thereafter performed to explore the effect of the change in elastic modulus of different FRP elements on the static deformation of the bridge. The results confirmed that the change in elastic modulus of different types of FRP element members caused remarkable differences on the bending and torsional stiffness of the hybrid bridge. The global stiffness of such a unique bridge can be significantly enhanced by redesigning the critical lower string pull bars using designable FRP profiles with high elastic modulus.

Experimental Investigation of the Natural Bonding Strength between Stay-In-Place Form and Concrete in FRP-Concrete Decks/Beams

The Fiber Reinforced Polymer (FRP)-concrete hybrid deck/beam is a structural system that combines the durable thin-walled FRP composite profiles and the cost-effective concrete by interfacial shear connections. The interfacial slip can reduce the composite action, thereby causing a degradation of flexural rigidity and capacity. Therefore, using stay-in-place (SIP) forms is a simple way to fully utilize the natural bonding between FRP and concrete, which plays a pivotal role in the structural design of FRP-concrete hybrid decks/beams. This paper presents an experimental study on the natural bonding provided by the SIP forms and the in situ cast concrete. First, four comparative push-out test specimens revealed that the use of SIP forms could improve the ultimate shear capacity of steel bolts by 11.1%. Moreover, it could provide an initial stage with nearly zero slip. The average natural bonding strength of FRP-concrete was evaluated as 0.27 MPa, which agreed well with previous tests in the literature. Second, the beam specimen also confirmed that there was a load response stage with nearly zero slip along the FRP-concrete interface when SIP forms were used as the permanent form. Third, the strain measurements on the steel bolts, FRP profile, and concrete revealed that the failure of the natural bonding was a brittle process. Finally, the flexural response of the FRP-concrete hybrid beam was analytically modeled as three distinct stages, namely the full composite action stage, the slipping stage caused by a natural bonding decrease, and the partial composite action stage.

Evaluation of Equivalent Bending Stiffness by Simplified Theoretical Solution for an FRP–aluminum Deck–truss Structure

A hybrid Fiber-Reinforced Polymer (FRP)–aluminum spatial deck–truss structure that incorporates an innovative structural form and advanced extrusion-type FRP profiles has the advantages of a lightweight, high-bearing capacity, and faster installation. In this study, a design-oriented investigation was conducted with a simplified theoretical solution to conveniently evaluate the equivalent bending stiffness of the above-mentioned unique structure. The simplified theoretical solution was derived using the equivalent continuum beam method based on the homogenization concept and shearing equivalence principle. The theoretical prediction enabled a direct format and simple calculation process convenient for engineers in terms of the calculation and design. The theoretical solution was experimentally and numerically calibrated to ensure that the formulae have the satisfactory accuracy. The accuracy of the predicted bending stiffness exceeded 90%. The derivation procedures and formulae further indicated that the FRP web diagonals played a key role in resisting the global bending stiffness of the unique structure, and thus, the shear deformation should be considered in the structural design in terms of stiffness-driven. When compared with the conventional calculation method for obtaining the bending stiffness of steel solid-web beams and planar trusses, the proposed method was more accurate and applicable for the unique hybrid structures.

Experimental Analysis of Failure Mechanisms in Masonry-PFRP Profiles Connections

Fibre-reinforced polymer (FRP) profiles, with their low density, high durability, and ease of construction, are particularly suitable for the retrofit of traditional masonry structures, particularly historic constructions in seismic zones. However, a critical aspect of this new technology application is the connection between FRP profiles and masonry walls. So far, no research studies are available on this subject. The authors carried out a preliminary experimental campaign on different connection systems between masonry and pultruded glass-fibre-reinforced polymer (GFRP) profiles. The note presents the immediate results of this study, focusing on the performance and collapse mechanisms; the study may contribute to the development of an effective connection system between masonry and FRP profiles to be adopted in the retrofitting of existing building with juxtaposed FRP frames.

Determination of critical loads for global buckling of axially loaded pultruded fiber-reinforced polymer members with doubly symmetric cross sections

This article proposes a new closed-form equation to determine the reduction factor for global buckling of concentrically loaded pultruded fiber-reinforced polymer struts based on the Ayrton–Perry formula and observed initial out-of-straightness of pultruded fiber-reinforced polymer members measured by other researchers, which makes the original solution recommended by Eurocode 3 easy to be used to predict the global buckling loads of doubly symmetric pultruded fiber-reinforced polymer members subjected to axial compression. The influence of the geometric imperfections of pultruded fiber-reinforced polymer profiles is considered in this new closed-form equation. Validation of the solution including the parameter of the reduction factor for global buckling of pultruded fiber-reinforced polymer columns is performed by comparison with published experimental evidence. In addition, compared with the five closed-form solutions available in the literature, this solution exhibits higher accuracy in predicting the global buckling capacity of concentrically loaded pultruded fiber-reinforced polymer struts with doubly symmetric cross sections. The solution implemented into the new reduction factor equation for global buckling of pultruded fiber-reinforced polymer members can be conveniently used by structural engineers at the preliminary engineering design stage for accurately assessing the reliability and safety of composite structures under concentric compressive loading.

Residual stiffness of bonded joints for fibre-reinforced polymer profiles

Abstract In this paper we present an experimental study on the behaviour of samples concerning double lap joints made of glass fibre-reinforced polymer composite (GFRP). According to a multistep displacement/force control procedure, a data driven approach is performed with the aim of investigating the behaviuor of adhesive joints between GFRP profiles at service conditions focusing on the non-linearity of the interfacial damage as the number of cycles increases. The present analysis has been performed regardless of the consideration of material/geometric non-linearities, which affect, instead, the failure load or the buckling limit. The final results provide a database for sketching a predictive rule to be used for a direct evaluation of the loss of stiffness of the joint.