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Reinforced concrete buildings are prone to collapse during seismic events due to the brittle shear failure of non-seismic beam-column joints (BCJ). In this study, two one-third scale reinforced concrete (RC) frames incorporating various non-seismic details were tested under lateral cyclic loading. One of the RC frames was used as control, while the other was strengthened using externally bonded carbon-fiber-reinforced polymer (CFRP) sheets in a L-Shaped configuration with particular attention to anchorage to evade debonding. For the strengthening process, L-shaped CFRP sheets were bonded to the inner face of columns, extended on beams both above and below the joint up to a hinge length. To avert debonding, the L-shaped CFRP sheets were fully wrapped with CFRP sheets around the column, both near the joint and at the end of the sheet. The sheets were also wrapped around the beam, through two slots in the slab that were adjacent to the beam-column interface and at the far end of the sheet. Test results confirmed that the installation of CFRP sheets in an L-shaped configuration altered the brittle-shear failure mechanism of the beam-column joints to a ductile failure by repositioning the hinges away from the columns. Additionally, the proposed anchorage method successfully eradicated the debonding and peel-off of the CFRP sheets. Moreover, strengthening with the CFRP sheets in the L-shaped configuration enhanced the strength and ductility of the RC frame by 45% and 43%, respectively. According to the findings of this study, the application of L-shaped CFRP sheets proved effective in strengthening RC frame structures. [ABSTRACT FROM AUTHOR]

Geopolymer concrete is a new concrete technology that emerged as an alternative to conventional concrete by replacing traditional Portland cement with geopolymer binders. Strengthening of concrete structures using CFRP composites has become an essential technique last decades. Most of the studies were conducted on conventional concrete of Portland cement. In this research, an experimental and finite element analysis has been conducted to assess the bonding properties between geopolymer concrete substrate and CFRP composites using near-surface mounted strengthening system. The program consisted of concrete prisms bonded with NSM CFRP laminate and tested using single-lap shear test set-up with different bond lengths. Bond-slip behaviour has also been evaluated. The experimental results exhibited significant bonding properties, achieving an average bonding stress between 8.97 and 15.58 relative to the bonding length. The numerical analysis showed comparable results and a good correlation with that of experimental work.

Among many manufacturing industries, civil engineering sectors have been more involved in incorporating fiber reinforced polymer (FRP) composites. These composite materials have been selected as an appropriate solution for strengthening reinforced concrete structural elements because of their excellent tensile strength, high strength to weight ratio, and simplicity of implementation. This experimental study aims to evaluate the flexural behaviors of structural light-weight reinforced concrete (SLWC) one-way slabs strengthened with different patterns of CFRP. The proposed material in the current study is using pumice aggregate as a full replacement of natural coarse aggregate. Four structural light-weight concrete (SLWC) slabs with the dimensions of 1200 mm long, 450 mm wide, and 80 mm thick were cast and tested to failure. One slab has been taken as a control and the other samples are strengthened with five strips in one layer, ten strips in two layers and full wrap CFRP. The samples are tested under a four-point load bending test setup until failure. Each of the ultimate loads, mid-span deflection, cracking loads, crack patterns, and failure modes were well evaluated. The results showed that, strengthening with CFRP composites significantly increases load-carrying capacity. Strengthening with five strips, ten strips, and full wrap with CFRP increased the ultimate capacity by 115%, 138%, and 170% respectively and decreased mid-span deflection by 43%, 58%, and 55% compared to the reference specimen respectively.

Impact loads lead to the failure of structures and significantly diminish their operational lifespan. The necessity to enhance impact performance has shown gradual progress, resulting in utilising nano-fillers as an additional reinforcement within the matrix. Despite the significant number of studies that have been done on this unique hybrid material, there have only been a few reviews published that discuss the effect of production processes on mechanical properties and performance in these hybrid composites under projectile impact. There have been conflicting results obtained in experimental results from the literature. The disparity is related to the variation in dispersion, bonding states, and inconsistent fabrication processes. This work defines the pros and cons of carbon nanotube (CNT)-based composites along with a systematic representation of the development of CNT-reinforced composites under projectile impact using experimental, analytical, and numerical techniques. The potential of CNT reinforcement on fibre-reinforced polymers (FRPs) and its effect on mechanical properties have been discussed. Furthermore, different impact test setups are explored to determine the effective method to determine the impact performance of CNT-reinforced laminates. Moreover, the impact of surface treatment is discussed using different non-destructive methods, and the influence of CNT reinforcement is determined. In addition, mechanical and impact response with varying configurations of fibres is gathered from the available literature, and optimal design based on the required application is suggested. Also, analytical methods developed to determine the impact response of laminates are discussed to determine the parameters dominating the impact response of the laminate. This review will help researchers find the right combination of FRP materials for a given application.

Reinforced concrete buildings are prone to collapse during seismic events due to the brittle shear failure of non-seismic beam-column joints (BCJ). In this study, two one-third scale reinforced concrete (RC) frames incorporating various non-seismic details were tested under lateral cyclic loading. One of the RC frames was used as control, while the other was strengthened using externally bonded carbon-fiber-reinforced polymer (CFRP) sheets in a L-Shaped configuration with particular attention to anchorage to evade debonding. For the strengthening process, L-shaped CFRP sheets were bonded to the inner face of columns, extended on beams both above and below the joint up to a hinge length. To avert debonding, the L-shaped CFRP sheets were fully wrapped with CFRP sheets around the column, both near the joint and at the end of the sheet. The sheets were also wrapped around the beam, through two slots in the slab that were adjacent to the beam-column interface and at the far end of the sheet. Test results confirmed that the installation of CFRP sheets in an L-shaped configuration altered the brittle-shear failure mechanism of the beam-column joints to a ductile failure by repositioning the hinges away from the columns. Additionally, the proposed anchorage method successfully eradicated the debonding and peel-off of the CFRP sheets. Moreover, strengthening with the CFRP sheets in the L-shaped configuration enhanced the strength and ductility of the RC frame by 45% and 43%, respectively. According to the findings of this study, the application of L-shaped CFRP sheets proved effective in strengthening RC frame structures.

These elements are innovative and of interest to many researchers for the reinforcement of wooden elements. For the reinforced beam elements, the effect of the reinforcement factor, FRP and steel elastic modulus or FRP and steel arrangement of the reinforcement on the performance of the flexural elements was determined, followed by reading the load-displacement diagram of the reinforced beam elements. The finite element model was then developed and verified with the experimental results, which was mainly related to the fact that the general theory took into account the typical tensile failure mode, which can be used to predict the flexural strength of reinforced timber beams. From the tests, it was determined that reinforced timber beam elements had relatively ductile flexural strengths up to brittle tension for unreinforced elements. As for the reinforcements of FRP, the highest increase in load-bearing capacity was for carbon mats at 52.47%, with a reinforcement grade of 0.43%, while the lowest was for glass mats at 16.62% with a reinforcement grade of 0.22%. Basalt bars achieved the highest stiffness, followed by glass mats. Taking into account all the reinforcements used, the highest stiffness was demonstrated by the tests of the effectiveness of the reinforcement using 3 mm thick steel plates. For this configuration with a reinforcement percentage of 10%, this increase in load capacity was 79.48% and stiffness was 31.08%. The difference between the experimental and numerical results was within 3.62–27.36%, respectively. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) composites have gained increasing recognition and application in the field of civil engineering in recent decades due to their notable mechanical properties and chemical resistance. However, FRP composites may also be affected by harsh environmental conditions (e.g., water, alkaline solutions, saline solutions, elevated temperature) and exhibit mechanical phenomena (e.g., creep rupture, fatigue, shrinkage) that could affect the performance of the FRP reinforced/strengthened concrete (FRP-RSC) elements. This paper presents the current state-of-the-art on the key environmental and mechanical conditions affecting the durability and mechanical properties of the main FRP composites used in reinforced concrete (RC) structures (i.e., Glass/vinyl-ester FRP bars and Carbon/epoxy FRP fabrics for internal and external application, respectively). The most likely sources and their effects on the physical/mechanical properties of FRP composites are highlighted herein. In general, no more than 20% tensile strength was reported in the literature for the different exposures without combined effects. Additionally, some provisions for the serviceability design of FRP-RSC elements (e.g., environmental factors, creep reduction factor) are examined and commented upon to understand the implications of the durability and mechanical properties. Furthermore, the differences in serviceability criteria for FRP and steel RC elements are highlighted. Through familiarity with their behavior and effects on enhancing the long-term performance of RSC elements, it is expected that the results of this study will help in the proper use of FRP materials for concrete structures. [ABSTRACT FROM AUTHOR]

• A new analytical approach to predict the bond behaviour of mechanically anchored FRP-to-substrate joints is proposed. • The end anchorages of the bonded joints are simulated through a spring with a linear behaviour. • One unique function is used to predict all the stages that the interface undergoes until complete debonding. • The Finite Element Method is used to validate the proposed analytical approach. • The good accuracy obtained in all studied joints suggests that the proposed analytical solution covers a wide range of cases. Fibre-reinforced polymers (FRP) are often externally bonded (EB) to concrete, steel or timber structures for structural strengthening purposes. In the EB reinforcement system, the bond between materials is critical for the success of such a bonding system. However, the system is prone to debond at an FRP strain level much lower than its rupture value. For this reason, it is often necessary to use end anchorages in FRP-strengthened beams to delay or avoid this premature debonding of FRP from the beams. To better understand the debonding process of mechanically anchored FRP-to-substrate joints, the present work proposes a new analytical approach that considers an elastic end anchorage, which can simulate, through a spring, the slips developed in an end anchorage such as an FRP U-wrap jacket, FRP spike anchor, steel plate anchorage, among others. This new approach can also simulate the bond performance of FRP-to-substrate joints with no end anchorages by assuming that the stiffness of the end anchorage is zero. Expressions for defining the load-slip curves, FRP strains, interfacial slips, and bond stresses developed throughout the bonded length are derived and validated against the results from the Finite Element Analysis (FEA). In the end, the model was used to simulate several experimental tests on mechanically anchored FRP-to-substrate joints available in the literature. Despite its simplicity, the proposed analytical approach covers wider situations that no other known similar approach can deal with. [ABSTRACT FROM AUTHOR]

The principal objective of this study is to employ non-destructive broadband dielectric spectroscopy/impedance spectroscopy and machine learning techniques to estimate the moisture content in FRP composites under hygrothermal aging. Here, classification and regression machine learning models that can accurately predict the current moisture saturation state are developed using the frequency domain dielectric response of the composite, in conjunction with the time domain hygrothermal aging effect. First, to categorize the composites based on the present state of the absorbed moisture supervised classification learning models (i.e., quadratic discriminant analysis (QDA), support vector machine (SVM), and artificial neural network-based multilayer perceptron (MLP) classifier) have been developed. Later, to accurately estimate the relative moisture absorption from the dielectric data, supervised regression models (i.e., multiple linear regression (MLR), decision tree regression (DTR), and multi-layer perceptron (MLP) regression) have been developed, which can effectively estimate the relative moisture absorption from the dielectric response of the material with an R¬2 value greater than 0.95. The physics behind the hygrothermal aging of the composites has then been interpreted by comparing the model attributes to see which characteristics most strongly influence the predictions. [ABSTRACT FROM AUTHOR]

In the last decade, synthetic fiber, as a reinforcing specialist, has been mainly used in polymer matrix composites (PMC's) to provide lightweight materials with improved stiffness, modulus, and strength. The significant feature of PMC's is their reinforcement. The main role of the reinforcement is to withstand the load applied to the composite. However, in order to fulfill its purpose, the reinforcements must meet some basic criteria such as: being compatible with the matrix, making chemical or adhesion bonds with the matrix, having properties superior to the matrix, presenting the optimal orientation in composite and, also, having a suitable shape. The current review reveals a detailed study of the current progress of synthetic fibers in a variety of reinforced composites. The main properties, failure modes, and applications of composites based on synthetic fibers are detailed both according to the mentioned criteria and according to their types (organic or inorganic fibers). In addition, the choice of classifications, applications, and properties of synthetic fibers is largely based on their physical and mechanical characteristics, as well as on the synthesis process. Finally, some future research directions and challenges are highlighted. [ABSTRACT FROM AUTHOR]

The research investigates the effect of using external fiber-reinforced polymer (FRP) –composites to strengthen the alkali-silica reaction (ASR)-damaged reinforced concrete (RC) beam-column joints. The nonlinear finite element analysis (NLFEA) method has been used to achieve the aims of this paper. Before experimenting, the B-C beam model has been verified following accredited previous experimental. Afterward, the experiment was extended to include the impact of the column’s axial load level and the ASR damage stage. For these purposes, the levels of the axial load were set at 0%, 25%, 50%, and 75%, while the stages of the ASR damage were: stage 0 (un-damaged), stage 1 (45 days), stage 2 (80 days), and stage 3 (120 days). Some of the models were strengthened with FRP, while the others were not. The evaluation of the models' structural performance was conducted by monitoring: the failure mode, distribution of stresses, degradation of stiffness, dissipation of energy, displacement ductility, pulling-pushing ultimate load capacity, and correspondent displacements, in addition to the horizontal load-displacement hysteretic loops and envelopes. The achieved results from this work indicated that the strengthened-with-FRP of ASR damaged RC B-C joint models showed an enhancement in their cyclic performance, as they showed: higher load capacity, more significant horizontal displacement, more displacement ductility, more dissipation of energy, and less secant stiffness degradation. It was also noticed that the higher the stage of the ASR damage, the better the efficiency of FRP composites. Further, strengthening the joint models with FRP transformed their failure mode from brittle mode to ductile, by forming a plastic hinge in the beam side at a column axial load exceeding 25%. On the other side, applying a column axial load below 25% only improved the ultimate deflection and corresponding axial load capacity.

This paper details the development and experimental assessment of a friction-type connector, designed to transfer shear flow between the top flange of a fiber-reinforced polymer (FRP) tub girder and a composite concrete deck for bridge applications. In contrast with previously used bearing-type connectors, this system relies on a deformed FRP surface to transfer shear via direct interlock with the concrete deck. The connector is materially efficient, simple to fabricate, can be used with lower-grade structural or stainless-steel fasteners, and provides a high degree of interface stiffness. Six compression-shear specimens were tested to assess the connector fatigue resistance and ultimate connection strength. Additionally, two short beam specimens were tested in three-point bending, one of which was subjected to fatigue loading. Based on the compression-shear tests and short beam tests, the connection exhibited strength exceeding that predicted by AASHTO for frictional concrete-concrete connections. The connection strengths were significantly greater than the factored demand required by AASHTO for a typical model FRP bridge girder. The cyclic loading of the connection in both compression-shear and beam bending showed that connection stiffness and strength do not significantly degrade, due to the application of 1 × 106 to 6 × 106 cycles of traffic-induced factored fatigue load. [ABSTRACT FROM AUTHOR]

Fiber Reinforced Polymer (FRP) composites are used extensively in aerospace, marine and civil engineering. High stiffness-to-weight and high strength-to-weight ratios make these materials suitable for modern wind turbine blade manufacturing industries. In the present study, glass, carbon and glass-carbon composite materials are studied to determine their tensile properties at elevated temperatures as this is an important design consideration for wind turbines operating in warm climates. Specimens were tested in a range of temperatures to investigate thermal effects on their mechanical, thermal and failure behaviors.Experimental and FEA results show that tensile strength and heat flow of specimens decrease with increasing temperatures. Hybridization of two materials changes failure mode behaviors. Tensile strength test data was analyzed using chi-square goodness of fit statistical model. Results were correlated by using linear regression analysis. Normal, lognormal and 2-parameter Weibull statistical approaches were used to quantify the degree of variability in tensile strength of specimens.

Limited information is known about the effects of bond strength degradation during installation on the bond's quality and performance between fiber-reinforced polymer (FRP) reinforcement and substrate material. This research study's primary focus is to investigate the efficiency of the external FRP composites in rescuing the structural performance and controlling the mode of failure of the reinforced concrete (RC) beam-column joint with different bond strength degradation percentages nonlinear finite element analysis (NLFEA). Firstly, the RC beam-column model was validated against the published experimental results and then was expanded to consider the effect of the degradation percentages in bond strength between concrete and FRP composite (0 % (Fully bond), 10 %, 20 %, 30 %, 40 %, 50 %, 60 %, 70 %, 80 %, 90 %, and 100 % (control or un-strengthened joint). The structural performance was evaluated in terms of failure mode, stress distribution, pulling and pushing ultimate load capacity and corresponding displacement, horizontal load-displacement hysteretic loops, horizontal load-displacement envelopes, displacement ductility, energy dissipation, stiffness degradation, and equivalent hysteretic damping factor. The NLFEA results showed that the FRP strengthening technique with bond strength degradation percentages less than 30 % enhanced the cyclic performance (higher load capacity, larger horizontal displacement, higher displacement ductility, higher energy dissipation, and slower secant stiffness degradation). Also, the utilized FRP method with bond strength degradation percentage less than 30 % performed well in eliminating any surface debonding or buckling in the FRP composite because of the proper lateral support provided for the strengthening sheets. Finally, the bond strength degradation percentage less than 30 % could significantly enhance the deficient joints' seismic performance under strong beam-weak column conditions by changing its behavior to a more ductile one, including the beam flexural hinging. Moreover, the relocation of a plastic hinge in the beam provided more lateral strength for the joint specimens.

Regarding the high interfacial shear stresses at end-plate, and end-plate cover separation in externally bonded methods, bar end-interfacial debonding and bar-end cover separation in near-surface mounted technique, a hybrid strengthening method called combined externally bonded near-surface method was proposed. In this method, the externally bonded and near-surface mounted techniques complement each other and mutually overcome their limitations. The purpose of this study is to numerically investigate the flexural strengthening efficiency of reinforced concrete beams with combined externally bonded FRP sheets and near-surface mounted FRP rods. The numerical analyses were conducted with finite element software ABAQUS 6.11, which can accurately simulate the experimental investigations on the flexural behavior of reinforced concrete beams strengthened with FRP composites. Validation of finite element simulation was confirmed first by making a comparison with the experimental study presented in the literature for both un-strengthened and strengthened beams with FRP materials. The verified model of the un-strengthened beam, which serves as a control beam, was used to simulate reinforced concrete beam strengthened with externally bonded FRP sheets and combined externally bonded near-surface mounted technique. The numerical results of mid-span bending moment deflection, ultimate bending moment, failure deflection, and ductility index were reported. Based on the results of this study, it is concluded that the developed finite element models for the externally bonded, near-surface mounted, and combined externally bonded near-surface techniques can be used by structural engineers as an alternative solution in design-oriented parametric studies of strengthened reinforced concrete elements. The performance of the combined externally bonded near-surface mounted technique was confirmed by making the comparison between the results of the intended method with other strengthening techniques.

This paper presents the efficiency of the external fiber reinforced polymer (FRP) composites in rescue the structural performance and controlling the mode of failure of the heated damaged reinforced concrete (RC) beam-column joint by nonlinear finite element analysis (NLFEA). Firstly, the RC beam-column model was validated against the published experimental results and then was expanded to consider the effect of column axial load level (0%, 25 %, 50 %, and 75 %) and elevated temperature (23 °C, 200 °C, 400 °C, and 600 °C) on the models with and without FRP composites. The structural performance was evaluated in terms of failure mode, stress distribution, pulling and pushing ultimate load capacity and corresponding displacement, horizontal load-displacement hysteretic loops, horizontal load-displacement envelopes, displacement ductility, energy dissipation, and stiffness degradation. The NLFEA results showed that the FRP strengthening technique of heated damaged RC beam-column joint with FRP composite enhanced the cyclic performance (higher load capacity, larger horizontal displacement, higher displacement ductility, higher energy dissipation, and slower secant stiffness degradation) and the efficiency of FRP composite increased with the heated damage level. Also, the FRP composite strengthening technique gave the ability to transform the joint-column regions mode of failure from brittle into a ductile mode of failure through the formation of plastic hinge in the beam only at a higher level column axial loads of more than 25 %. While the application level of column axial loads less than 25 % only enhanced just the ultimate axial load capacity and corresponding deflection.

This article describes a novel method for detecting flaws in curing FRP composite materials while they are being manufactured. Such a method can improve the efficiency of the manufacturing process by minimizing, or potentially eliminating, the need for post-manufacturing inspection. The method utilizes a Kalman filter, a heat conduction model, and surface temperature measurements from infrared thermography to identify likely locations of flaw and/or curing anomalies. Specifically, a methodology that compares a metric of the time-history of Kalman filter corrections at different spatial locations to identify anomalous curing behavior was developed. Several numerical studies were performed using a previously-validated model to determine the proficiency of the technique. Results of the verification studies indicated that the proposed method was effective at identifying resin-rich regions without any modification to the detection criteria, while identifying resin-deficient regions required a more lenient detection criterion. In the case of multiple flaws, the proposed method was always able to identify the flaw closer to the surface, regardless of flaw significance, while the deeper flaw was only identified when the flaw was more significant than the near-surface flaw. The proposed method demonstrates promise for passive IR thermography-based flaw detection performed during the manufacturing of FRP composites and can serve to both improve the efficiency of the manufacturing process and the quality of FRP composite parts. Further experimental studies are required for validation of the technique before it can be applied for industrial application. [ABSTRACT FROM AUTHOR]

Masonry structures, which are subjected to in-plane shearing, should be reinforced. The situation is often appearing in the regions, where masonry buildings are destroyed due to earthquakes. There are many methods of strengthened this kind of structures. One of them is reinforcement by FRP composites, which are linked to the masonry surface. It is not quite simple to select a place, where the composites should be to obtain the most effective work of behaviour. In the paper the topological optimization is used to meet this goal.

Some variation in the topological distribution of fibers inside the matrix phase of fiber reinforced polymers (FRP) is inevitable. Such irregularities can accelerate moisture diffusion and adversely affect the life of FRP. This paper presents a hierarchical technique for characterization of clustered microstructures and their transient moisture diffusion response. The clustering descriptors are derived for different fiber volume fractions (dilute to dense) for the quantitative definition of a given fiber matrix architecture. The metrics are normalized to remove dependence on volume fraction. The microstructures are analyzed for Fickian moisture diffusion. Suggested descriptors show a good correlation with transient diffusion response in relation to saturation time. The results can be used to predict the time-dependent moisture diffusion response of FRPs for any given fiber volume fraction.

The effects of FRP retrofitted concrete specimens with couple of notches on the fracture behavior have been investigated during an experimental and analytical test program in this study. This paper represents the fracture characteristics and parameters for three point bending tests on two distinguish retrofitted and plain condition for both intact and notched (couple notches) specimens. The experimental test results find out in the laboratory tests indicated that for intact concrete specimens, there would be approximately 285% increase in ultimate flexural strength tests. The experimental results also represents that for the tested couple notched concrete specimens, there might be approximately 318% increase in ultimate flexural strength. Based on the analytical study, it is found that the near failure behavior of the notched specimens have been significantly improved using FRP retrofit of such specimens. The system`s global energy balance and failure load prediction of FRP debonding are the couple of consideration by a developed fracture mechanics based model which made by energy dissipation major mechanisms characterizing while debonding. Model verification is provided using previous researches experimental data from literature. In addition, fracture mechanics parameters were found out for three point bending test in this paper for better understanding on fracture behavior and fracture properties of intended specimens.

This study presents a crack phase-field approach for anisotropic continua to model, in particular, fracture of fiber-reinforced matrix composites. Starting with the variational formulation of the multi-field problem of fracture in terms of the deformation and the crack phase fields, the governing equations feature the evolution of the anisotropic crack phase-field and the balance of linear momentum, presented for finite and small strains. A recently proposed energy-based anisotropic failure criterion is incorporated into the model with a constitutive threshold function regulating the crack initiation in regard to the matrix and the fibers in a superposed framework. Representative numerical examples are shown for the crack initiation and propagation in unidirectional fiber-reinforced polymer composites under Mode-I, Mode-II and mixed-mode bending. Model parameters are obtained by fitting to sets of experimental data. The associated finite element results are able to capture anisotropic crack initiation and growth in unidirectional fiber-reinforced composite laminates. [ABSTRACT FROM AUTHOR]

Fiber Reinforced Polymer (FRP) composites are being increasingly used as replacements for metals in engineering applications. Though these composites are manufactured in near-net shape, machining is often necessary for integration and assembly. The most common machining operation performed on these materials is drilling. A commonly observed defect in drilling is edge defects which include incomplete fiber cutting. This report discusses a model which estimates edge defects during FRP drilling. Results predicted by the model are compared to experimental observations and possible techniques to characterize defects in FRP drilling are discussed.

The paper presents initial results of a new approach to FRP composite bridge construction that is presently being developed and tested in Poland. The concept combines lightweight concrete with FRP composites to create a durable highly optimised structure. The paper describes the bridge system itself and presents the research results on its development. The basic design is presented together with research results on its development: FEM analysis and a range of static test results of full-scale bridge beam experiments. The paper finishes with some test results of a full scale bridge that was constructed near Rzeszow in December 2015.

Abstract Conventional high-strength FRP materials suffer from brittle failure at low strains. This study proposes an innovative concept for achieving high-strength, large-deformation and tensile-behavior designable FRP composites. The proposed composite consists of FRP skins, inner cores and twisted knots to develop desired nonlinear behaviors. As the main load-carrying elements, FRP skins provide the ultimate strength and will squeeze inner cores under tensile loading. Inner cores are used to shape skins and determine the amounts of shaped skins to be unfolded by controlling core deformations. Core configurations are expected to be carefully designed and then realized by 3D printing technologies. Twisted knots are used to resist the opening stress at the core edges. Experimental results obtained from sixty tests validate this concept and identify the impacts of core configurations, i.e. brace thickness & angle, shell thickness, core span & height and core number, on the tensile behavior of the composite. A typical test failed in skin fracture developing a considerable stress (772 MPa or 78% FRP material strength) and strain (0.041 or 401% FRP ultimate strain). Test results also provide valuable data for further designing the proposed composite to feed increasingly updated demands. [ABSTRACT FROM AUTHOR]

Abstract This paper extends the applications of a novel and fully automated multi-scale computational homogenisation framework, originally proposed by the authors (Ullah et al. (2017)) for unidirectional and 2D-textile composites, to 3D-textile composites. 3D-textile composites offer many advantages over 2D-textile composites but their highly complicated and unpredictable post-cured geometries make their design very challenging. Accurate computational models are therefore essential to the development of these materials. The computational framework described in this paper possesses a variety of novel features which have never been tried for this class of composites and can potentially help to fully automatise and improve their design process. A unified approach is used to impose the representative volume element boundary conditions, which allows convenient switching between linear displacement, uniform traction and periodic boundary conditions. The computational framework is implemented using hierarchic basis functions of arbitrary polynomial order, which allows one to increase the order of approximation without changing the finite element mesh. The yarns' principal directions, required for the transversely isotropic material model are calculated using a potential flow analysis along these yarns. This feature is very useful for 3D-textile composites and can accurately determine fibres' directions even in the case of very deformed yarns. A numerical example from literature consisting of a 3D-orthogonal woven composite is used to demonstrate the correct implementation and performance of the developed computational framework. Also, the developed computational framework is used to perform a comparative study of the homogenised mechanical properties of five 3D-textile composites with different yarn architectures. [ABSTRACT FROM AUTHOR]

Abstract The estimation of the long-term durability of adhesively bonded interfaces between Fiber Reinforced Polymers (FRP) and concrete substrates is crucial because degradation potentiates FRP premature debonding. One of the main reasons for mistrusting the use of FRP composites is the premature debonding phenomenon, which, associated to degradation, has been preventing their widespread use. In this research work, an analytical model is proposed that introduces ageing to estimate the effects of degradation of Glass (G) FRP externally bonded to concrete. Cycles were used to experimentally accelerate ageing of beam specimens, namely, (i) salt fog cycles; (ii) wet-dry cycles with salted water; (iii) temperature cycles between −10 °C and +30 °C; and (iv) temperature cycles between +7.5 °C and +47.5 °C. Based on the experimental results obtained and a corresponding bond-slip curve, the analytical model predicts the complete debonding process between FRP composites and a substrate. Consequently, the temporal evolution of the degradation of the bonded interfaces can be calculated and compared with the initial situation prior to exposure. The effects of the environmental conditions are reported and compared. [ABSTRACT FROM AUTHOR]

• Vibration-based method: changes in structural frequencies utilized to estimate the residual fatigue life. • Comparison of theoretical and machine learning algorithms such as Support Vector Machines (SVM) and ANN. • Check prediction accuracy with different input schemes, including single-mode and multi-mode frequencies. Fiber reinforced polymer (FRP) structures may experience cumulative fatigue damage during their service life which can lead to structural failure. This paper focuses on predicting the residual fatigue life of FRP structures using vibration parameters. The relationship between residual fatigue life and natural frequencies is examined through modal testing and fatigue measurements on FRP beam specimens. Two prediction methods; semi-empirical models and machine learning (ML) algorithms, are utilized. The semi-empirical models are derived from existing "residual stiffness" models based on the relationship between bending stiffness and flexural frequencies. ML algorithms Support Vector Machine (SVM) and Artificial Neural Network (ANN), are developed for fatigue life prediction. Experimental validation is performed using measured frequencies during fatigue testing of FRP beams. The ML algorithms can use multimode frequencies unlike single mode of semi-empirical models. The verification results show that the ML algorithms can be used to predict the residual fatigue life with the selection of the appropriate mode of frequency. The results show that ML algorithms outperform single-mode frequency inputs, and the use of higher modes of measured frequencies improves the precision of fatigue life prediction. An inverse algorithm based on SVM exhibits higher prediction accuracy and stability, even with limited training samples. [ABSTRACT FROM AUTHOR]

The growing need for the reinforcement of timber beams (required due to deterioration or damage to the material or change of use) has led to the development of new methods of reinforcement with modern materials. In the recent years the use of fibre reinforced polymers (FRP) as reinforcement materials for structures has been made possible thanks to the increased availability and lower costs. This paper presents FRP bars as products for strengthening timber structures. Strengthening timber with glass, carbon and basalt FRP can provide better features of timber beams, such as improved load capacity, rigidity and ductility. Also, the paper describes the theoretical model developed in order to predict the flexural capacity and flexural stiffness of timber beams reinforced with FRP bars.

Delamination is represented to be the most prevalent failure in composite structures. The use of composites in the manufacturing sector plays a very important role in the industry in general. Moreover these materials have unique characteristics when analyzed separately from constituents which are a part of them. In this paper, a partially ecological composite was made, using natural fibers as reinforcement (bamboo fiber), in the polyester resin matrix to form a composite, seeking to improve the mechanical behavior among its class of materials. The characteristics of a composite material are determined by how it behaves while machining, Drilling is the most predominant machining process because of its cost effectiveness when compared with other processes. Obviously delamination is the major problem that is focused by many researchers while selecting drilling as the machining process in polymeric composites. This research mainly emphasizes on the critical parameters by varying its speed, feed, and diameter of the cutting tool, their contribution to delamination was analyzed. Reduced delaminations were identified by varying the speed and feed rate.

Abstract The present study utilized piezoresistive carbon nanotube (CNT) fibers for the structural health monitoring of composites. In particular, diversified schemes including i) Pre-casted CNT fiber patches embedded into GFRP composites, ii) CNT fibers stitched to one glass fabric of the composites during the fabrication procedure, and iii) CNT fibers attached to the top of the composites, were proposed for incorporation of CNT fibers to the composites. The fabricated samples were subjected to cyclic tensile loadings with constant and increasing amplitudes to assess their piezoresistive characteristics. All types of samples showed sharp increases and decreases in the electrical resistance values, reflecting the applied stress. They also showed good sensing integrity under extended cyclic loadings as well as against loading up to the point of failure. Moreover, the fabricated composites exhibited tensile strengths comparable to those of the control samples, demonstrating feasibility to be used as a sensor of composite structures. [ABSTRACT FROM AUTHOR]

In Southern European countries several existing reinforced concrete (RC) buildings were designed before the introduction of modern seismic codes and thus they may be potentially vulnerable to horizontal loads. Recent seismic events have also shown that RC buildings designed without specific seismic provisions can be subjected to meaningful damages or even collapse during moderate-to-strong earthquakes. In this framework, straightforward methodologies for a preliminary and suitable seismic assessment and retrofitting of existing RC buildings are required, along with reliable and effective seismic rehabilitation techniques. In this study, a simplified displacement based procedure using non-linear static analyses is applied to obtain a preliminary estimation of the overall inadequacy of an under-designed four-storey RC frame and to propose suitable retrofitting interventions based on different rehabilitation strategies. To this aim, accurate numerical models are developed to simulate the seismic response of the RC frame in the original and retrofitted configurations. The effectiveness of three different retrofitting solutions countering the main structural deficiencies of the RC frame is evaluated through the displacement based approach. Then, non-linear dynamic analyses are carried out to assess and compare the seismic performance of the RC frame in the original and retrofitted configurations. A combined use of different approaches may represent a valuable tool to accurately address the retrofitting interventions and to assess their effectiveness in order to reduce the seismic vulnerability of poorly designed RC buildings. [ABSTRACT FROM AUTHOR]

Due to lack of research data on the rates of deterioration of FRP properties under a harsh environment exposure, it was pointed out in the design guidelines that the durability of FRP needs to be further developed. Therefore, in this study, 48 FRP samples were tested under wet-dry cycles exposure. The effect of wet-dry cycling times on the failure modes, tensile strength, and the probability distribution of different FRP (GFRP and CFRP) composite specimens were investigated. The experimental results showed that the wet-dry cycles have a significant adverse influence on the tensile strength, have a certain adverse effect on the elongation, and a very limited influence on the elastic modulus of FRP. According to the experimental results, a probability analysis was conducted on the degradation of tensile strength. Five widely used test methods were adopted to verify the possible distribution types of tensile strength, and a reliability index β was then calculated. Subsequently, the effects of the design tensile strengths of ACI-440, TR-55, GB 50608-2010, GB 50367-2013, European Fib Bulletin 14 and Italian CNR guidelines on the β were investigated. The investigation illustrates that only the design value of the TR-55 code can guarantee sufficient long-term safety of a CFRP composite, whereas all the six codes cannot guarantee the long-term safety of a GFRP composite and the partial safety factors in these codes are still not conservative. Therefore, a more conservative safety factor was suggested. Moreover, the design value of tensile strength needs to be further conservative when the standard deviation of the load is large. [ABSTRACT FROM AUTHOR]

The effective stress transfer between the fiber reinforced polymers (FRP) and the steel substrate is crucial for the successful retrofit of existing steel structures with FRP composites. However, there are no standard tests for FRP-to-steel interfaces, wherefore different test configurations have been used in recent years to assess the bond behaviour in these interfaces. The present study shows that the choice of test configuration is highly important and leads to different transfer stresses between the FRP and steel composites and consequently, has a direct influence on the strength of the bonded joint. Therefore, it is important to understand the debonding process that occurs in each test and avoid misinterpretations, erroneous analyses and dangerous characterizations of the interfacial behaviour of these interfaces. The current study presents a new analytical approach for the prediction of the debonding of FRP-to-steel interfaces when double-lap pull or double-strap tests are used. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer composites offer inherent advantages over traditional metallic materials in a number of different ways; however, these materials are also highly susceptible to impact damage. In this paper, we explore the response of FRP (fiber reinforced polymer) composites under impact conditions that could result in their rupture or catastrophic failure. The work performed was aimed at developing a general, data-driven equation for initially-stressed, flat, composite plates that would differentiate between impact conditions that would result in only a hole or crack and those which would cause catastrophic plate failure or rupture. If this equation were to be subsequently shown to also model the rupture/non-rupture behavior of, for example, composite overwrapped pressure vessels, then it could also be used to appropriately tailor the design parameters and/or operating conditions of such pressurized tanks. [ABSTRACT FROM AUTHOR]

An emerging application of fiber reinforced polymer (FRP) composites is to replace steel cables for cable-supported bridges, enhancing the service lives, reducing maintenance requirements, and increasing the bridge span. Vehicle collisions on the cable for cable-supported bridges have been regarded as a concern since FRP composites are sensitive to impact loads. The present article reviews the impact performances of FRP composites and cables. Aside from the common influencing factors, such as the impact energy and fiber or matrix types, the impact performances of FRP cables are particularly affected by the pretension load, cable tonnage, cable forms, impact position, and anchorage system. Experimental or numerical investigation of the affecting factors as well as enhancement and protective methods are needed to guide the anti-collision design of FRP cables in practical applications. However, high cable manufacturing and testing costs, property sacrifice in material design, and lower computational efficiency are possible challenges in future studies. [ABSTRACT FROM AUTHOR]

This paper presents some advanced FE analyses conducted on a masonry church damaged by the recent Emilia earthquake, occurred on 20 th -29 th May 2012. The Nativity of the Virgin Mary parish, located in the province of Ferrara (Italy), was subjected to a post-earthquake strengthening intervention, which consisted of introducing steel profiles, rigid diaphragms and repointing technique for the damaged masonry material. The earthquake highlighted a series of pre-existing structural vulnerabilities. A probable mechanism involving the detachment of the façade was also observed due to the presence of vertical cracks. Three FE models were created in order to investigate the seismic behavior of the church in the original, retrofitted and proposed FRP-retrofitted configurations. The mechanical properties of the material were adapted from National Technical Code recommendations for existing masonry constructions by introducing a macro-scaling homogenization technique. The performed numerical analyses highlighted the seismic vulnerability of the church in the original and retrofitted configurations. A strengthening intervention performed by means of FRP strips was numerically investigated. The results obtained by using the three different FE models were compared and the use of FRPs was found to be a quite reasonable strengthening intervention, ensuring a considerable seismic upgrading. [ABSTRACT FROM AUTHOR]

Externally bonded laminates based on high strength fibers nowadays are often used to strengthen the structures. It is possible, that some of those strengthenings are subjected to direct insolation. The paper presents the dangers appearing with the rise of temperature caused by the infrared solar radiation. Heating conditions were planned on the basis of exposition of concrete samples during the summer months in the southern Poland. The maximum recorded temperatures in the adhesive layer reached 65 °C and it was therefore 20 °C higher than the glass transition temperature of commercially available adhesives based on epoxy resin. Laboratory tests included a group of fifteen reinforced concrete beams in the real scale, seven of them were strengthened with externally bonded CFRP strip, seven with SRP tape. They were tested in various temperature conditions, from 20 °C to 80 °C. Beams were heated from the strengthened side with the use of linear infra-red radiators and when the temperature of the adhesive layer reached the predetermined value, beams were loaded to failure. Noticeable impact of temperature appeared from about 50 °C. In all cases failure was followed by delamination. Differences in behavior of CFRP and SRP strengthened beams were observed. Delamination of CFRP strip appeared in unexpected way, without any specific symptoms, while SRP failed with grater deflection and lower mid-span strains compared to the beam tested at room temperature. At temperatures above 65 °C appears significant bearing capacity degradation. It means that CFRP strengthening, which could be subjected to direct sun exposition, should always be protected by thermal insulation. [ABSTRACT FROM AUTHOR]

This study presents experimental testing on a 13 m long glass-fibre epoxy composite wind turbine blade. The results of the test were used to calibrate finite element models. A design optimisation study was then performed using a genetic algorithm. The goal of the optimisation was to minimise the material used in blade construction and, thereby, reduce the manufacturing costs. The thickness distribution of the composite materials and the internal structural layout of the blade were considered for optimisation. Constraints were placed on the objective based on the stiffness of the blade and the blade surface stresses. A variable penalty function was used with limits derived from the blade test and the structural layout of the turbine. The model shows good correspondence to the test results (blade mass within 6% and deflection within 9%) and the differences between test and model are discussed in detail. The genetic algorithm resulted in five optimal blade designs, showing a reduction in mass up to 24%. Structural modelling in combination with numerical search algorithms provide a powerful tool for designers and demonstrates that the reader can have confidence in the claimed potential savings when the reference blade models are calibrated against physical test data. [ABSTRACT FROM AUTHOR]

Due to lack of research data on the rates of deterioration of FRP properties under a harsh environment exposure, it was pointed out in the design guidelines that the durability of FRP needs to be further developed. Therefore, in this study, 48 FRP samples were tested under wet-dry cycles exposure. The effect of wet-dry cycling times on the failure modes, tensile strength, and the probability distribution of different FRP (GFRP and CFRP) composite specimens were investigated. The experimental results showed that the wet-dry cycles have a significant adverse influence on the tensile strength, have a certain adverse effect on the elongation, and a very limited influence on the elastic modulus of FRP. According to the experimental results, a probability analysis was conducted on the degradation of tensile strength. Five widely used test methods were adopted to verify the possible distribution types of tensile strength, and a reliability index β was then calculated. Subsequently, the effects of the design tensile strengths of ACI-440, TR-55, GB 50608-2010, GB 50367-2013, European Fib Bulletin 14 and Italian CNR guidelines on the β were investigated. The investigation illustrates that only the design value of the TR-55 code can guarantee sufficient long-term safety of a CFRP composite, whereas all the six codes cannot guarantee the long-term safety of a GFRP composite and the partial safety factors in these codes are still not conservative. Therefore, a more conservative safety factor was suggested. Moreover, the design value of tensile strength needs to be further conservative when the standard deviation of the load is large.

In this study, a work-of-fracture method using a three-point bend beam (3PBB) specimen, which is commonly used to determine the fracture energy of concrete, was adapted to evaluate the mode-I fracture and durability of fiber-reinforced polymer (FRP) composite-concrete bonded interfaces. Interface fracture properties were evaluated with established data reduction procedures. The proposed test method is primarily for use in evaluating the effects of freeze-thaw (F-T) and wet-dry (W-D) cycles that are the accelerated aging protocols on the mode-I fracture of carbon FRP-concrete bonded interfaces. The results of the mode-I fracture tests of F-T and W-D cycle-conditioned specimens show that both the critical load and fracture energy decrease as the number of cycles increases, and their degradation pattern has a nearly linear relationship with the number of cycles. However, compared with the effect of the F-T cycles, the critical load and fracture energy degrade at a slower rate with W-D cycles, which suggests that F-T cyclic conditioning causes more deterioration of carbon fiber-reinforced polymer (CFRP)-concrete bonded interface. After 50 and 100 conditioning cycles, scaling of concrete was observed in all the specimens subjected to F-T cycles, but not in those subjected to W-D cycles. The examination of interface fracture surfaces along the bonded interfaces with varying numbers of F-T and W-D conditioning cycles shows that (1) cohesive failure of CFRP composites is not observed in all fractured surfaces; (2) for the control specimens that have not been exposed to any conditioning cycles, the majority of interface failure is a result of cohesive fracture of concrete (peeling of concrete from the concrete substrate), which means that the cracks mostly propagate within the concrete; and (3) as the number of F-T or W-D conditioning cycles increases, adhesive failure along the interface begins to emerge and gradually increases. It is thus concluded that the fracture properties (i.e., the critical load and fracture energy) of the bonded interface are controlled primarily by the concrete cohesive fracture before conditioning and by the adhesive interface fracture after many cycles of F-T or W-D conditioning. As demonstrated in this study, a test method using 3PBB specimens combined with a fictitious crack model and experimental conditioning protocols for durability can be used as an effective qualification method to test new hybrid material interface bonds and to evaluate durability-related effects on the interfaces.