Search

The anisotropic behavior of fiber-reinforced polymer composites, coupled with their susceptibility to various failure modes, poses challenges for their structural health monitoring (SHM) during service life. To address this, non-destructive testing techniques have been employed, but they often suffer from drawbacks such as high costs and suboptimal resolutions. Moreover, routine inspections fail to disclose incidents or failures occurring between successive assessments. As a result, there is a growing emphasis on SHM methods that enable continuous monitoring without grounding the aircraft. Our research focuses on advancing aerospace SHM through the utilization of piezoresistive MXene fibers. MXene, characterized by its 2D nanofiber architecture and exceptional properties, offers unique advantages for strain sensing applications. We successfully fabricate piezoresistive MXene fibers using wet spinning and integrate them into carbon fiber-reinforced epoxy laminates for in-situ strain sensing. Unlike previous studies focused on high strain levels, we adjust the strain levels to be comparable to those encountered in practical aerospace applications. Our results demonstrate remarkable sensitivity of MXene fibers within low strain ranges, with a maximum sensitivity of 0.9 at 0.13% strain. Additionally, MXene fibers exhibited high reliability for repetitive tensile deformations and low-velocity impact loading scenarios. This research contributes to the development of self-sensing composites, offering enhanced capabilities for early detection of damage and defects in aerospace structures, thereby improving safety and reducing maintenance expenses. [ABSTRACT FROM AUTHOR]

This study investigates the effectiveness of modified graphene (MGE) for developing novel structural adhesives and their application in concrete beam reinforcement. The research aims to reveal the mechanism of MGE and evaluate the effect of different reinforcement methods on the flexural performance of concrete beams. Two adhesive formulations – original structural adhesive (OSA) and functionalized graphene-based structural adhesive (FGSA) – were examined alongside two reinforcement methods: the grouting reinforcement method and the adhesive fiber-reinforced polymer (FRP) method. The results show that the increased spacing of the functionalized MGE flakes improved their dispersion in epoxy resin, enhancing cohesive forces through bonding interactions and leading to the enhancement of the basic mechanical properties of FGSA by 6% to 12%. Compared to OSA, the application of FGSA improved the flexural properties of concrete beams, with the degree of improvement depending on the reinforcement method, while the failure mode of the beams remained unchanged. In addition, the FGSA/FRP combination showed the most significant improvement in the fracture parameters (220% to 480%) compared to the unreinforced control, highlighting its efficiency and cost-effectiveness as a reinforcement method. These findings offer valuable insights into reinforcement strategies for concrete beams, addressing the limitations of structural adhesive bond strength and optimizing reinforcement techniques. [ABSTRACT FROM AUTHOR]

Previous research indicates that lightweight aggregate (LWA) concrete exhibits more brittle failure under compression than conventional normal density (ND) concrete. In contrast, angle-ply fiber-reinforced polymer (FRP) filament-wound tubes filled with ND concrete show significant improvements in strength and deformability compared to hollow tubes. This study evaluates the impact of these tubes on LWA concrete to mitigate its brittle behavior. Sixteen concrete-filled FRP filament-wound tubes (CFFTs) made of ±55° filament-wound glass FRP (GFRP) were tested under axial compression. The GFRP tube's mechanical properties and fiber winding angle of ±55° allowed for a compressive strength enhancement factor ranging from two to five for the CFFTs compared to unconfined specimens. Results also revealed substantial nonlinearity and deformability in the LWA-CFFTs due to the angle-ply tube orientation. An iterative analytical model predicted the CFFT's nonlinear response, validated by experimental data. The findings demonstrate that angle-ply FRP tubes effectively enhance the bearing capacity and deformability of inherently brittle unconfined LWA concrete. [ABSTRACT FROM AUTHOR]

The destructive earthquakes in the world clearly reveal the importance of the seismic behavior of the structures. The current study proposes and numerically investigates a systematic technique for the seismic assessment and retrofit of reinforced-concrete buildings according to both material strain limit approach and Quadrants assessment method. In this study numerical analysis were made for two three-dimensional building models, namely a single-storey 30 years old existing RC building in Koyna-Warna area (Zone-IV) in India, and an earthquake-resistant designed 4-story RC building. The structural analyses were performed for two 3-D models of the selected buildings are analysed. Nonlinear static adaptive pushover analysis method was used in Seismostruct software program for two different buildings. While one of the buildings is an existing building, the other is chosen as a new building. Structural characteristics and deficiencies of the investigated single-storey existing reinforced-concrete structure necessitated seismic evaluation. The deficiencies include pitched roofs, re-entrant corners, and structural distress such as structural element cracking and cover concrete spalling, among others. The single-story RC building is retrofitted with RC jacketing and fiber-reinforced polymer sheet wrapping, while the four-storey RC building does not need to be retrofitted based on the Quadrants assessment method. Also, the significant seismic design parameters are evaluated before and after the retrofit. The results show that the material strain limit technique when combined with the Quadrants assessment method makes a more efficient technique for seismic assessment and retrofit purpose. Also, the ultimate capacity and over strength factor of the retrofitted single-storey RC existing building are found 2.04 times and 1.82 times increased in X and Y direction as compared to the building without retrofit, because of the RC jacketing and FRP sheet wrapping. [ABSTRACT FROM AUTHOR]

This experimental study investigates the effectiveness of retrofitting fiber-reinforced concrete (FRC) beams using nano-graphene oxide (GO) and carbon fiber-reinforced polymer (CFRP) sheets. The primary goal is to evaluate the combined effect of GO and CFRP sheets on the mechanical performance of FRC beams. The experimental program involved preparing and testing multiple concrete beam specimens, with some retrofitted using GO and CFRP sheets. The principal results indicate that the integration of GO significantly enhances the compressive and tensile strength of concrete. Additionally, the application of CFRP sheets markedly improves the flexural strength and ductility of the beams. The retrofitted specimens exhibited higher load-bearing capacity and greater deformation before failure compared to control specimens. The significant conclusions drawn from this study are that the combined use of GO and CFRP sheets provides a synergistic effect, improving the overall mechanical performance of FRC beams. However, failure modes such as debonding and delamination of CFRP sheets highlight the need for optimization in bonding techniques and materials. The primary research outcomes demonstrate that retrofitting FRC beams with GO and CFRP sheets is a promising approach for enhancing the structural performance. This study underscores the potential of advanced materials in structural retrofitting and provides insights for future research to address observed failure modes and further improve the retrofitting techniques. [ABSTRACT FROM AUTHOR]

The first edition of ACI CODE-440.11 was published in September 2022, where some code provisions were either based on limited research or only analytically developed. Therefore, some code provisions, notably shear and development length in footings, are difficult to implement. This study, through a design example, aims at a better understanding of the implications of code provisions in ACI CODE-440.11-22 and compares them with ones in CSA S806-12, thereby highlighting a need for reconsiderations. An example of the footing originally designed with steel reinforcement was taken from the ACI Reinforced Concrete Design Handbook and redesigned with GFRP reinforcement as per ACI CODE-440.11-22 and CSA S806-12. A footing designed as per ACI CODE-440.11-22 requires a thicker concrete cross section to satisfy shear requirements; however, when designed as per CSA S806-12, the required thickness becomes closer to that of the steel-reinforced concrete (RC) footing. The development length required for a glass fiber-reinforced polymer-reinforced concrete (GFRP-RC) cross section designed as per ACI CODE-440.11-22 was 13% and 92% greater than that designed as per CSA S806-12 and ACI 318-19, respectively. Also, the reinforcement area required to meet detailing requirements is 170% higher than that for steel-RC cross section. Based on the outcomes of this study, there appears to be a need for reconsideration of some code provisions in ACI CODE-440.11-22 to make GFRP reinforcement a viable option for RC members. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) reinforcements have been used in versatile forms in recent construction practices to enhance durability performance and, consequently, to attain longevity of concrete structures. The shear strength of FRP-reinforced concrete (FRP-RC) beams holds significant importance in structural design. However, inherent analytical uncertainty exists concerning shear in concrete members due to the distinctive material characteristics of FRP bars compared to conventional steel reinforcements, such as their low axial stiffness and bond properties. This study aims to identify the shear-resistance mechanisms developed under combined actions between concrete and FRP reinforcements. To this end, the dual-potential capacity model (DPCM) was extended to FRP-RC beam members subjected to shear and flexure, and an attempt was also made to derive a simplified method. To validate the proposed approaches, a total of 437 shear test results from RC members incorporating FRP bars were used. Findings indicate that the proposed methods can provide an acceptable level of analytical accuracy. In addition, a significant shift in the shear failure mode of FRP-RC members with no stirrups was observed from the compression zone to the cracked tension zone as the FRP reinforcement ratios increased. Conversely, when FRP stirrups were added, the shear failure mode was mostly dominated by the compression zone. [ABSTRACT FROM AUTHOR]

This paper presents the behavior of unreinforced cylindrical concrete elements confined with a hybrid system, consisting of an ultra-high-performance concrete (UHPC) jacket and basalt fiber-reinforced polymer (BFRP) grids. For exploring the feasibility of the proposed strengthening scheme, a series of tests are conducted to evaluate material properties and to obtain results related to interfacial bond, load-bearing capacity, axial responses, and failure modes. To understand the function of the individual components, a total of 57 cylinders are loaded under augmented confining conditions, including plain cores with ordinary concrete (CONT), plain cores with UHPC jackets (Type A), and plain cores with UHPC jackets plus BFRP grids (Type B). By preloading the cores at up to 60% of the control capacity (60%fc') before applying the confinement system, the repercussions of inherent damage that can take place in vertical members on site are simulated. The compressive strength of UHPC rapidly develops within 7 days, whereas its splitting strength noticeably ascends after 14 days. The adhesion between the ordinary concrete and UHPC increases over time. While the Type B specimens outperform their Type A counterparts in terms of axial capacity by more than 18%, reliance on the BFRP grids is reduced with the growth of UHPC's strength and adhesion because of the interaction between the hardened UHPC and the core concrete. The adverse effects of the preloading are noteworthy for both types, especially when exceeding a level of 30%fc'. The BFRP grid-wrapping alleviates the occurrence of a catastrophic collapse in the jacketed cylinders, resulting from a combination of the axial distress and lateral expansion of the core. Analytical models explain the load-carrying mechanism of the strengthened concrete, including confinement pressure and BFRP stress. Through parametric investigations, the significance of the constituents is clarified, and design recommendations are suggested. [ABSTRACT FROM AUTHOR]

This paper presents a new approach for damage detection in thin plates by fusing variational mode decomposition and spectral entropy (VMD-SE). In this method, after the received signal is decomposed into some intrinsic mode functions (IMFs) by variational mode decomposition (VMD), the spectral entropy ratio of the first and last IMFs is calculated for optimizing the VMD's parameters and improving its decomposition performance. Moreover, the cross-correlation coefficient between the decomposed IMFs and the reference signal is computed to separate the desired IMF, which contains more damage information. Finally, the spectral entropy of the obtained IMF is calculated as an indicator for assessing the damage's severity. The comparative analysis of the simulated signal clearly shows that only the proposed method can successfully separate the damage-related and reference signals. To verify the VMD-SE method, damage detection of two different types of damage on aluminum and composite fiber-reinforced polymer (CFRP) plates is conducted by using this new approach. The experimental results demonstrate that the parameters of VMD affect greatly its decomposition performance, and the best parameters are selected. The results also indicate that the normalized spectral entropy monotonically increases when the diameter of the through-hole or the length of the scratch increases. In addition, the correlation coefficients of the fitting lines of the plates are larger than 0.998. The experimental results of aluminum specimens demonstrate that the damage's location has an influence on the normalized spectral entropy. At last, based on the linear relationship, the severity of damage in the fourth specimen is identified. The identification results demonstrate that the relative error of the aluminum and CFRP plates is less than 7.34%, which indicates that this new algorithm by fusing VMD and spectral entropy can detect the damage size in thin plates accurately and efficiently. [ABSTRACT FROM AUTHOR]

AbstractFiber-reinforced polymer (FRP) confining of reinforced concrete columns (RCCs) is a widely adopted strengthening technique in civil engineering. However, accurately predicting the axial strength and ductility of eccentrically-loaded FRP-confined RCC remains a challenge. In this study, finite element models (FEM) were employed in conjunction with a novel approach of gene expression programming (GEP)-probabilistic based modeling to develop predictive models for the axial strength and ductility of eccentrically-loaded FRP-confined circular RCCs. Extensive FEM simulations were conducted, considering various input parameters such as eccentricity-to-section diameter ratios, height-to-section diameter ratios, concrete cover to diameter, concrete compressive strength, FRP layer thickness, and reinforcement ratios. The study pursues three main objectives: first, to assess the stress–strain behavior of FRP-confined RCCs and analyze the effects of various input parameters on their axial strength, axial and lateral strains, and ductility; second, to develop mathematical equations using the GEP algorithm to estimate the axial strength and ductility based on input variables such as eccentricity-to-section diameter ratios, concrete compressive strength, FRP layer thickness, and reinforcement ratios; and third, to conduct probabilistic evaluation and analysis of uncertainties in the predictions resulting from the GEP models. The resulting database facilitated the development of data-driven models, achieving correlation coefficients of 0.943 and 0.947 for axial strength and ductility, respectively. External validation and sensitivity analyses demonstrated the superior performance and accuracy of the proposed models compared to existing literature models. Additionally, a semi- and full-probabilistic approach was implemented to account for uncertainties in input variables, yielding probability distributions for GEP-based models of axial strength and ductility. These distributions enable the assessment of associated risks and aid in informed decision-making for design and structural solutions. Overall, the developed models offer valuable tools for design purposes and contribute to advancing the understanding and optimization of FRP-confined RC column behavior under eccentric loading conditions. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) slabs are widely used in modern building structures due to their superior properties and ease of construction. However, their mechanical properties are limited by their punching shear strength in the connection region with the columns. Researchers have attempted to add steel reinforcement in the form of studs and randomly distributed fibers to concrete slabs to improve the punching strength. An additional strengthening method that consists of the application is a Fiber-Reinforced Polymer (FRP). However, current codes poorly calculate the punching shear strength of FRP-RC slabs. The aim of this study is to create a robust model that can accurately predict its punching shear strength, thus improving the analysis and design of composite structures with FRP-RC slabs. In this study, 189 sets of experimental data were collected and expanded using kernel density estimation (KDE), considering the small amount of data. Secondly, a punching shear strength prediction model for FRP-RC panels was constructed using XGBoost and compared with the model modeled by codes and researchers. Finally, a model explainability study was conducted using SHapley additive exPlanations (SHAP). The results show that kernel density estimation significantly improves the robustness and accuracy of XGBoost. The R-squared, standard deviation, and root mean square error of XGBoost on the training set are 0.99, 0.001, and 0.001, respectively. On the test set, the R-squared, standard deviation, and root mean square error are 0.96, 62.687, and 67.484, respectively. The effective depth of the FRP-RC slabs is the most important and proportional to the punching shear strength. This study can provide guidance for the design of FRP-RC slabs. [ABSTRACT FROM AUTHOR]

The present work sets out the evaluation of composite laminates through optimization objective functions. Genetic Algorithms (GA), as well as Simulated Annealing (SA), were performed to determine the optimal ply orientations of a fiber-reinforced polymer laminate for three given load cases: (1) in-plane loads, (2) combined moments, and (3) in-plane loads and combined moments. It searches the optimal orientation layup, which fulfills at the same time both the maximum strain criterion and the Tsai–Wu failure criterion. Construction of an objective function based on the strains and the stresses involves a minimization process to deformations and a maximization of the safety factor. For the first load case, the initial solution is [±45/0/90/±45]s, and the best solution is [(45/135)2/452]s. For the second load case, [–45/45/45/–45/0/90]s is the initial layup sequence, then, the best solution obtained is [(45/–45)2/452]s, where plies at 0° and 90° are not necessary even when axial loads are applied. For the third study case, the original layup sequence is [0/45/–45/45/0/90]s; meanwhile, the best solution calculated is [21/24/145/140/45/49]s. An interesting observation is that each pair of layers has a 5° gap. The simulations show that the qualitative results from the GA are better than the SA, but with a significantly higher computational cost. These computational tools are expected to be used as a reference guide for an optimal fiber configuration concerning the common orientations used when composite laminates are designed for a structural application. [ABSTRACT FROM AUTHOR]

The energy absorption characteristics of composites are developed for applications likely to be subjected to sudden or prolonged impacts. In lightweight applications, the possibilities of Al 6061 composites are being explored with various known materials with similar properties. Three materials of low densities but high strength was combined: hexagonal honeycomb Al 6061 core was sandwiched with polyurethane foam and embedded within two glass fiber-reinforced polymer plates. L-9 orthogonal array Taguchi DOE was adopted for the design of the experiment varying the polyurethane mixture, core height, and face sheet thickness using the three-factor method. From the output of the Taguchi DOE, twenty-seven samples were fabricated. In order to determine the force-displacement relationship for each of the test samples, a three-point bending test was performed on a universal testing machine. Energy absorption (EA) and crushing force (CF) were determined from the force and displacement values. These values were then optimized to minimize the PCF while maximizing EA. Numerical analysis performed correlated with the results from experimental samples after testing. Since energy absorption is a measure of applied force and the resulting displacement on a material, the energy absorption characteristics were discussed. [ABSTRACT FROM AUTHOR]

The use of sustainable natural fibers as reinforcement in polylactic acid (PLA) composites offers a promising approach to developing environmentally friendly and renewable composite materials. This research article explores the impact of weave designs on various mechanical properties of fiber‐reinforced polymeric composite (FRPC) laminates. Four different weave designs, including 1/1 plain, 2/2 matt, 4/1 satin, and 3/1 twill, were developed using flax yarn obtained from the Linum usitatissimum plant. These weave designs were incorporated into composites by wrapping the flax yarn with PLA filament as the matrix, followed by compression molding. The study examines the static and dynamic mechanical properties of the subsequent flax/PLA composites, including flexural strength, Charpy strength, tensile strength, and viscoelastic characteristics like damping factor, loss, and storage modulus. The plain 1/1–reinforced composite exhibits superior tensile strength (12 MPa), tensile modulus (9800 MPa), and flexural properties (29 MPa) due to the higher number of intersection points in the weave design, while in the case of impact strength (6.7 kJ/m2), satin structure‐reinforced composite showed superior properties due to higher float length of warp yarns and lower intersection points in the weave structure. [ABSTRACT FROM AUTHOR]

GFRP-concrete-steel composite columns (GCS) are formed by combining different components with excellent mechanical properties. The GCS exhibits good corrosion resistance and can serve as a pile foundation platform for offshore wind turbines. An Artificial Neural Network (ANN) is utilized to swiftly predict the loading capacity of GCS columns with various geometrical parameters. The prediction error is primarily controlled within 10%. The thickness of GFRP tubes (tG), the inner diameter of GFRP tubes (DG,i), the thickness of steel tubes (ts), the outer diameter of steel tubes (DS,o), the strength of GFRP tubes (fG), the strength of concrete (fC), and the strength of steel tubes (fS) were used as independent variables to determine the key factors affecting the loading capacity of GCS columns. The impact of the number of input variables on the final prediction accuracy is analyzed. The proportional influence of different parameters on the loading capacity of GCS column is investigated using the Garson algorithm, among which, the thickness of the GFRP tube (tG) accounts for the largest proportion, approximately 24.57%. The suitability of the artificial neural network for predicting the loading capacity of GCS columns with various geometries is revealed. The results indicate that the ANN can be utilized for high-precision compressive strength prediction, and the thickness of the GFRP tube (tG) is the most influential factor on the loading capacity of GCS columns. [ABSTRACT FROM AUTHOR]

In recent years, additive manufacturing (AM), especially large-format additive manufacturing (LFAM), has gained momentum in the manufacturing industry. While LFAM offers benefits over conventional manufacturing processes, such as minimizing material waste and providing vast geometric freedom, assessing its sustainability remains challenging due to limited data, particularly on energy consumption. Most existing data pertain to small-scale or desktop AM and are not directly applicable to LFAM. In this study, we conducted real-time measurements of electricity usage for a type of LFAM known as big area additive manufacturing (BAAM), which typically uses fiber-reinforced polymer pellets as feedstock. We collected electricity usage data from fifteen printing jobs over two months in an industrial production setting. These data fill the existing gap and can be reused to enhance the community's understanding of LFAM electricity usage, support further research, and promote sustainable development in advanced manufacturing technologies. [ABSTRACT FROM AUTHOR]

Natural plant fibers (NPFs) have emerged as a sustainable alternative in the manufacture of composites due to their renewability and low environmental impact. This has led to a significant increase in the use of natural plant fiber-reinforced polymers (NPFRPs) in a variety of industries. The diversity of NPF types brings a wide range of properties and functionalities to NPFRPs, which in turn highlights the urgent need to improve the properties of fiber materials in order to enhance their performance and suitability. This paper provides insight into the processing mechanisms behind NPF fiber treatments, exploring how these treatments affect the mechanical, thermal and environmental properties of NPFRPs. It also offers a critical assessment of the advantages and disadvantages of physical, chemical, biological and nanotechnological treatments. The findings of our analysis provide a basis for the development of future treatments that aim to enhance the material properties of NPFRPs, thereby increasing their competitiveness with conventional synthetic fiber-reinforced polymers. Finally, a novel thermoplastic resin composite system, Elium–NPFRP, is proposed that embodies the principles of green development. The system has been designed with the objective of capitalizing on the environmental benefits of NPFs while simultaneously addressing the challenges associated with the integration of NPFs into polymer matrices. The Elium–NPFRP composite system not only exemplifies the potential of NPFs for sustainable materials science, but is also a practical solution that can be implemented in a diverse range of applications, spanning automotive components to construction materials. This has the potential to reduce carbon footprints and promote a circular economy. [ABSTRACT FROM AUTHOR]

Natural fiber-reinforced composites are composite materials composed of natural fibers, such as plant fibers and synthetic biopolymers. These environmentally friendly composites are biodegradable, renewable, cheap, lightweight, and low-density, attracting attention as eco-friendly alternatives to synthetic fiber-reinforced composites. In this study, natural fiber-reinforced polymer foam core layered composites were produced for the automotive industry. Fabrics woven from goat wool were used as the natural fiber. Polymer foam with expanded polystyrene (EPS) and extruded polystyrene (XPS) structures was used as the core material. During production, fibers were bonded to the upper and lower layers of the core structures using resin. The hand lay-up method was used in production. After resin application, the samples were cured under a heated press for 2 h. After the production was completed, the material was cut according to the standards (10-20-30 Joule), and impact and bending tests were conducted at three different energy levels. The experiments revealed that at 10 J, the material exhibited rebound; at 20 J, it showed resistance to stabbing; and at 30 J, it experienced penetration. While EPS foam demonstrated higher impact resistance in the 10 J test, it was found that XPS foam exhibited better impact resistance and absorption capabilities in the 20 J and 30 J tests. Due to the open and semi-closed cell structure of EPS foams and the closed cell structure of XPS foams, it has been concluded that XPS foams exhibit higher impact resistance and better energy absorption properties [ABSTRACT FROM AUTHOR]

Recently, the liquid composite molding technique (LCM) has been used for producing fiber-reinforced polymer composites, since it allows the molding of complex parts, presenting good surface finishing and control of the mechanical properties of the product at the end of the process. Studies in this area have been focused on resin transfer molding (RTM), specifically on the resin rectilinear infiltration through the porous preform inserted in the closed cavity neglecting the sorption effect of the polymeric fluid by the reinforcement. Thus, the objective of this work is to predict resin radial flow in porous media (fibrous preform), including the effect of resin sorption by fibers considering a one-dimensional approach. For correct prediction of the flow behavior inside the porous media, an advanced modeling approach composed of the mass conservation equation and Darcy's law is used, and the solution of the coupled equation is obtained. Transient results of the flow front location, velocity and pressure within the mold during the resin infiltration are shown, the effects of different parameters for resin (viscosity), reinforcement (sorption term, permeability and porosity) and process (injection pressure and injection radius) are analyzed, and an in-depth discussion is performed. [ABSTRACT FROM AUTHOR]

Improving the physical, mechanical, and creep properties of wood fiber-reinforced polymer composites is crucial for broadening their application prospect. In this research, seven types of high-density polyethylene (HDPE) composites reinforced with different mass ratios of Masson pine (Pinus massoniana Lamb.) and Chinese fir [Cunninghamia lanceolata (Lamb.) Hook.] were prepared by a two-step extrusion molding method. The mass ratios of the two fibers were 60:0, 50:10, 40:20, 30:30, 20:40, 10:50, and 0:60, respectively. The surface color, density, dimension stability, bending, tensile, impact properties, dynamic mechanical properties, and 24 h creep properties at a 10% stress level of the seven composites were investigated. Additionally, the Rule of Mixtures (ROM), the Inverse Rule of Mixtures (IROM), the Hirsch models, and the improved model were employed to simulate the mechanical properties, while the Findley index model, the two-parameter index model, and the modified ExpAssoc model were employed to simulate the creep performance of the composites. This study revealed that as the proportion of Chinese fir wood flour increased, the mechanical properties of the composites gradually improved, the storage modulus showed an increasing trend, while the loss modulus decreased, and the overall creep strain of the composites increased. Among the various models, the modified model simulated the mechanical properties of the composites the best, while the modified ExpAssoc model simulated the creep behavior most effectively. [ABSTRACT FROM AUTHOR]

The glass fiber-reinforced polymer (GFRP) materials of wind turbine blades can be recovered and recycled by crushing, thereby solving one of the most perplexing problems facing the wind energy sector. This process yields selectively crushed wind turbine blade (SCWTB), a novel waste that is almost exclusively composed of GFRP composite fibers that can be revalued in terms of their use as a raw material in concrete production. In this research, the fresh and mechanical performance of concrete made with 1.5%, 3.0%, 4.5%, and 6.0% SCWTB is studied. Once incorporated into concrete mixes, SCWTB waste slightly reduced slumps due to the large specific surface area of the fibers, and the stitching effect of the fibers on mechanical behavior was generally adequate, as scanning electron microscopy demonstrated good fiber adhesion within the cementitious matrix. Thus, despite the increase in the content of water and plasticizers when adding this waste to preserve workability, the compressive strength only decreased in the long term with the addition of 6.0% SCWTB, a value of 45 MPa always being reached at 28 days; Poisson's coefficient remained constant from 3.0% SCWTB; splitting tensile strength was maintained at around 4.7 MPa up to additions of 3.0% SCWTB; and the flexural strength of mixes containing 6.0% and 1.5% SCWTB was statistically equal, with a value near 6.1 MPa. Furthermore, all mechanical properties of the concrete except for flexural strength were improved with additions of SCWTB compared to raw crushed wind turbine blade, which apart from GFRP composite fibers contains approximately spherical polymer and balsa wood particles. Flexural strength was conditioned by the proportion of fibers, their dimensions, and their strength, which were almost identical for both waste types. SCWTB would be preferable for applications in which compression stresses predominate. [ABSTRACT FROM AUTHOR]

This research advocates for the use of basalt fiber-reinforced polymer (BFRP) bars and recycled tire steel fibers to reinforce concrete beams. Six concrete beams were constructed using different volume contents of recycled tire steel fibers (0, 0.5%, 1.0%, 1.5%) and BFRP reinforcement ratios (0.48%, 0.75%, 1.08%). Mechanical properties tests were conducted to investigate the flexural characteristics and failure modes of beams utilizing the non-contact full-field strain displacement measuring technology–digital image (3D-DIC) technology. The recycled tire steel fiber (RTSF) improves flexural performance, which contributes to inhibiting crack propagation, reducing flexural deformation, and improving the first cracking and ultimate loading capacities. Under the same reinforcement ratio, in comparison to ordinary BFRP beams, the cracking load of BFRP-RTSF beams increased by 17.73%, 23.76%, and 42.94%, respectively, and the ultimate bearing capacity increased by 4.03%, 5.85%, and 13.21%, respectively. In addition, a modified calculation model of bearing capacity considering RTSF tensile strength is proposed. The predicted values of BFRP-RTSF beams match well with the experimental values. [ABSTRACT FROM AUTHOR]

This study investigates the tensile behavior of carbon-fiber-reinforced polymer (CFRP) and textile-reinforced mortar (TRM) under various design variables to enhance understanding and application in construction structures. TRM reinforced with CFRP grids is highly effective for strengthening existing structures due to its lightweight nature, durability, ease of installation, and corrosion resistance. The research aims to evaluate how design parameters such as the CFRP grid type, mortar matrix strength (influenced by the water-to-cement ratio), specimen length, and grid width affect TRM's mechanical properties. Through the direct tensile test using a universal testing machine, TRM specimens were subjected to load until failure, with data collected on stress–strain relationships, crack patterns, and strengths. Specimens included untreated CFRP grids (Groups KC, Q47, and Q85) and sand-coated CFRP grids (Specimens AQ47_7 and AQ85_7), each tested under controlled laboratory conditions. The results indicate that crack formation significantly influenced load transfer mechanisms within the specimens, with longitudinal strands bearing load as cracks propagated through the mortar matrix. The presence of sand-coated CFRP grids notably enhanced interfacial bond strength, leading to increased cracking strength and ultimate strength compared with their untreated counterparts. The findings underscore the importance of the surface treatment of CFRP grids for improving TRM performance, with implications for enhancing structural integrity and durability in practical applications. The results provide valuable insights into optimizing TRM design for better crack control and mechanical efficiency in infrastructure. [ABSTRACT FROM AUTHOR]

This study investigates the mechanical properties of Glass Fiber-Reinforced Plastic (GFRP) pipes in the circumferential direction using the split-disk method, with a focus on understanding the influence of aggressive environmental conditions. The split-disk method was used to determine the key mechanical properties, including the hoop tensile strength and modulus, and also the Poisson's ratio, which are critical for the performance of GFRP pipes under internal pressure. The experiment was conducted under controlled laboratory conditions, and the results were analyzed to assess the effects of exposure to aggressive environments (saltwater and alkaline solutions at 20 °C and 50 °C). The correlations between the UTS, elastic modulus, and Poisson's ratio highlight how GFRP pipes degrade under environmental exposure. As the UTS decreases, so do the stiffness and lateral deformability, with the most significant reductions occurring in chemically aggressive environments at high temperatures. Exposure to an alkaline solution weakens the GFRP pipes, with the strength dropping more sharply at higher temperatures, with the UTS decreasing by 21%. Saltwater exposure reduces the elastic modulus, especially at higher temperatures, with a 14% decrease, accelerating material degradation and reducing deformation resistance. An alkaline solution further lowers the modulus, with a 21% decrease at 50 °C, showing the lowest stiffness. Air exposure, in contrast, has a less severe effect, with the pipes retaining much of their mechanical integrity. These findings collectively suggest that environmental degradation affects the overall mechanical behavior of GFRP pipes, providing valuable insights for the design and maintenance of GFRP piping systems, particularly in industries where exposure to aggressive environments is common. This study underscores the importance of considering environmental factors in the material selection and design processes to ensure the long-term reliability of GRP pipes. [ABSTRACT FROM AUTHOR]

Shear strengthening of reinforced concrete beams using near-surface mounting (NSM) method with fiber-reinforced polymer (FRP) strips is more effective because of improved bond strength, better fire resistance and high maintainability. However, the surface preparation for NSM method is a difficult process where the beam–slab corner is not accessible by the rotary blade of the groove-making equipment. Consequently, the application of NSM method becomes more difficult to apply. Therefore, in this study, the effect of reducing in NSM length on the shear strength has been investigated by reinforcing only a part of the height, not the entire web of beam, referred to as the partial-length NSM method (PLNSM). Half scaled five RC T-beams were made and tested under symmetrical four-point static loading system. All except one was strengthened in shear in which the effect of reduced NSM length was balanced by inclining the partial-length NSM strips (IPLNSM). Furthermore, to mitigate the detrimental effect of reduced lengths of NSM strips, the retrofitting was enhanced by additional externally bonded reinforcement (EBR) method using CFRP sheets. The results showed that there was no significant negative effect of reduced NSM length on the strength of the strengthened specimens, and by providing inclined NSM strips, significant improvement in the strength was observed. Additionally, the hybrid approach combining the inclined partial length NSM (IPLNSM) and EBR method showed improvement in strength and deflection capacity. Lastly, when compared with the currently available design procedures, it was found that the available formulation can predict the design strength of PLNSM and IPLNSM reinforcement, thus making them a viable option for retrofitting reinforced concrete beams. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymers are crucial for insulating electrical equipment, necessitating accurate thermal property data for an effective thermal analysis. This case study uses a cost-effective method to thermally characterize a glass fiber-reinforced epoxy resin used in air-core reactor insulation. The approach simultaneously estimates temperature-dependent thermal conductivity (k) and specific heat (cp) for class H/180 insulation. By analyzing transient heat conduction in a 3D composite sample under vacuum and at various temperatures, the method optimizes sensor placement, enabling accurate property estimation with a single thermocouple. The estimated through-thickness thermal conductivity at room temperature deviates by less than 6% from standard guarded hot plate measurements. The method's reliability is confirmed by accurately retrieving the applied heat flux using the estimated properties and measured temperature data. The results are valuable for designing accurate simulation models to predict and manage the thermal behavior of air-core reactors, as implemented by GE Grid Solutions in Itajubá, Brazil. [ABSTRACT FROM AUTHOR]

Carbon–glass hybrid fiber-reinforced epoxy polymer (C-GFRP) winding pipes integrated with the advantages of light weight, high strength, corrosion resistance, and cost-effectiveness offer immense potential to mitigate corrosion issues in oil, gas, and water transportation pipelines. In this study, C-GFRP winding pipes underwent accelerated aging tests through immersion in distilled water at temperatures of 25 °C, 40 °C, and 60 °C for 146 days. Water absorption tests were conducted to investigate the water absorption behavior of only CFRP- or GFRP-side absorbed water. Bending tests were performed to assess the evolution of the pipes' flexural properties in two directions (GFRP or CFRP in tension). The results showed that the single-sided water absorption behavior adhered to the two-stage diffusion model. The diffusion coefficient, activation energy, and 146-day water absorption were all higher for the CFRP-side absorbed water compared to the GFRP-side absorbed water. The flexural strength and modulus of C-GFRP pipes were influenced by post-curing and resin hydrolysis/debonding. Initially, the flexural strength of CFRP in tension was higher than that of CFRP in tension. After 146 days of aging, the flexural strength of CFRP in tension was lower than that of CFRP in tension. Utilizing Arrhenius theory, the long-term lives were predicted for the flexural strength at temperatures of 5.4 °C, 12.8 °C, and 17.8 °C. The predicted lives of GFRP in tension were higher than those of CFRP in tension. [ABSTRACT FROM AUTHOR]

In this study, the short-beam shear strength (SBSS) retention of two types of glass fiber-reinforced polymer (GFRP) bars—sand-coated (SG) and ribbed (RG)—was subjected to alkaline, acidic, and water conditions for up to 12 months under both high-temperature and ambient laboratory conditions. Comparative assessments were also performed on older-generation sand-coated (SG-O) and ribbed (RG-O1 and RG-O2) GFRP bars exposed to identical conditions. The results demonstrate that the new-generation GFRP bars, SG and RG, exhibited significantly better durability in harsh environments and exhibited SBSS retentions varying from 61 to 100% in SG and 90–98% in RG under the harshest conditions compared to 56–69% in SG-O, 71–80% in RG-O1, and 74–88% in RG-O2. Additionally, predictive models using both artificial neural networks (ANNs) and linear regression were developed to estimate the strength retention. The ANN model, with an R2 of 0.95, outperformed the linear regression model (R2 = 0.76), highlighting its greater accuracy and suitability for predicting the SBSS of GFRP bars. [ABSTRACT FROM AUTHOR]

Milling parts made from glass fiber-reinforced polymer (GFRP) composite materials are recommended to achieve the geometric shapes and dimensional tolerances required for large parts manufactured using the spray lay-up technique. The quality of the surfaces machined by milling is significantly influenced by the temperature generated in the cutting zone. This study aims to develop an Artificial Neural Network (ANN) model to predict the temperature generated when milling GFRP. The ANN model for temperature prediction was created using a virtual instrument developed in the graphical programming language LabVIEW. Predicting temperature is crucial because excessive heat during milling can lead to several issues, such as tool wear and thermal degradation in the polymer matrix. The temperature in the tool–workpiece contact surface during the milling process was measured using a thermography technique with a ThermaCAM SC 640 camera (provided by FLIR Systems AB, Danderyd, Sweden), and the data were analyzed using the ThermaCAM Researcher Professional 2.8 SR-2 software. Experimental research shows that the cutting speed has a much more significant effect on the temperature in the cutting zone compared to axial depth of cut and feed speed. The maximum temperature of 85.19 °C was measured in the tool–workpiece contact zone during machining at a cutting speed of 75.39 m/min, a feed rate of 250 mm/min, and an axial depth of cut of 12 mm. This temperature rise occurred due to the larger contact area and heightened friction resulting from the abrasive characteristics of the reinforcement material. [ABSTRACT FROM AUTHOR]

An approach to detecting discontinuities in carbon fiber-reinforced polymers, caused by impact loading followed by compression testing, was developed. An X-ray sensor-based installation was used, while some algorithms were developed to improve the quality of the obtained low-contrast radiographic images with negligible signal-to-noise ratios. For epoxy/AF (#1) composite subjected to a "high-velocity" steel-ball impact with subsequent compression loading, it was not possible to detect discontinuities since the orientation of the extended zone of interlayer delamination was perpendicular to the irradiation axis. After drop-weight impacts with subsequent compression loading of epoxy/CF (#2) and PEEK/CF (#3) composites, the main cracks were formed in their central parts. This area was reliably detected through the improved radiographic images being more contrasted compared to that for composite #3, for which the damaged area was similar in shape but smaller. The phase variation and congruency methods were employed to highlight low-contrast objects in the radiographic images. The phase variation procedure showed higher efficiency in detecting small objects, while phase congruency is preferable for highlighting large objects. To assess the degree of image improvement, several metrics were implemented. In the analysis of the model images, the most indicative was the PSNR parameter (with a S-N ratio greater than the unit), confirming an increase in image contrast and a decrease in noise level. The NIQE and PIQE parameters enabled the correct assessment of image quality even with the S-N ratio being less than a unit. [ABSTRACT FROM AUTHOR]

Glass fiber-reinforced polymer (GFRP) composites exhibit restricted mechanical performance, notably in terms of interlaminar shear strength and fracture toughness, as a consequence of the propensity for fiber/matrix fracturing and delamination when subjected to exterior loading. This study elucidates the enhancement of GFRP composites' mechanical characteristics through the integration of multi-walled carbon nanotubes (MWCNTs). A solution dip coating method was used to deposit 0.3 wt% MWCNTs on the glass fiber fabrics to manufacture the MWCNT-modified GFRP composites. A comprehensive experimental investigation was undertaken to evaluate the impact of MWCNTs on the mechanical attributes of GFRP composites across varying thicknesses and layups. Flexural strength, interlaminar shear strength and fracture toughness were investigated through three-point bending, short beam shear and end notch flexural (ENF) tests, respectively. To further decipher the microstructural enhancement mechanisms of MWCNTs in GFRP composites, fractured surfaces post-ENF testing underwent examination using a field-emission scanning electron microscope. The results revealed that MWCNT-modified GFRP composites with a 4-mm thickness and unidirectional orientation displayed optimal mechanical properties, and the MWCNT-modified GFRP composites with a certain layering angle surpassed the mechanical performance of their unmodified, thinnest unidirectional GFRP counterparts. This research thereby presents engineers with a novel design strategy to address the challenges posed by intricate application scenarios, enhancing the versatility and resilience of GFRP composites in advanced applications. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer composites, reaching a production of approximately 2.56 million tons in 2023 in Europe, display unique properties, yet they are disposed of at their end of service by conventional methods such as landfill and incineration. Here, we review the recycling of fiber-reinforced polymer wastes in the construction industry, with emphasis on fiber-reinforced polymer composites, recycling methods, and applications of carbon and glass fiber polymer composites in civil engineering. Recycling methods include mechanical, thermal, and chemical techniques. Applications comprise the use in fine fillers, coarse and fine aggregates, macro-fibers, alkali-activated materials, geopolymers, asphalt composites, and cement composites. We discuss workability, mechanical properties including compressive, flexural and tensile properties, durability, and surface modification. Future applications include three-dimensional concrete printing, self-sensing cement composites, self-heating and energy harvesting cement composites, and electromagnetic shielding. We propose a waste management hierarchy, considering the source of composites and their intended applications, to improve circularity. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP)-confined concrete tubular columns have been extensively studied, where the design of the column is usually based on stiffness and strength. The ability to sustain large deformations without losing structural integrity is also critical in geotechnical engineering. This paper proposes a novel high-ductility tubular column incorporating glass fiber-reinforced polymer (GFRP). The exterior container of the column is a GFRP jacket, while the infill material is sand or cemented sand. Different specimens are prepared with different infill materials (i.e., sand and cemented sand), and the thickness of the GFRP jacket (i.e., 3 mm, 5 mm, and 7 mm). Axial compression tests are conducted to study the compressive behavior of this novel tubular column with two load types (i.e., load on infill and load on the tube). Optical Frequency Domain Reflectometry (OFDR) technique is adopted to monitor the hoop strain distributions along the height of the columns. Experimental results reveal that the presence of infill material could prevent buckling failure of the hollow tubes, while the confinement effect provided by the GFRP tube significantly enhances the bearing capacity and ductility of the infill material. The FRP-confined cemented sand column exhibits notably increased initial stiffness, but slightly decreased ductility and peak load in comparison to the sand-filled FRP column. With the increase in axial displacement, the stiffness of the infill sand core exceeds that of the cemented sand due to the compressive hardening behavior of the sand. Through the OFDR technique, the deformation profile of the column is precisely delineated, facilitating the assessment of the uniformity of the dilation behavior. Moreover, the analysis-oriented model is adopted to rationally predict the load-strain behavior of FRP-confined cemented sand columns. [ABSTRACT FROM AUTHOR]

Natural fiber-reinforced polymer composites are gaining attention for their environmental benefits, such as biodegradability and reduced carbon footprint, as well as the potential of the natural fibers to replace or partially substitute synthetic fibers in various applications. However, challenges, such as poor interfacial adhesion and moisture absorption, limit the effectiveness of natural fibers, such as pineapple nonwoven fabric (PNWF) as reinforcement materials in polymer composites. To address these challenges, this study aims to enhance the properties of PNWF through glycidyl methacrylate grafting via radiation-induced graft polymerization. A 22 factorial design was employed to assess the effects of absorbed dosage and monomer concentration on the properties of the grafted PNWF. Infrared spectroscopy and electron microscopy analyses confirmed successful grafting. The grafted PNWF exhibited improved thermal stability and mechanical properties. The resulting composites showed significant enhancements in tensile and flexural strength, specifically, with tensile strength increase ranging from 23.32 to 34.49 MPa and flexural strength from 39.14 to 54.59 MPa. Additionally, the tensile modulus ranged from 0.77 to 1.29 GPa, while the flexural modulus varied from 1.17 to 2.06 GPa. These findings highlight the potential of PNWF grafted with polyglycidyl methacrylate (PNWF-g-PGMA) as an effective reinforcement material for various applications. [ABSTRACT FROM AUTHOR]

Wind-turbine blades pose significant disposal challenges in the wind-energy sector due to the increasing demand for wind farms. Therefore, this study researched the revaluation of Raw-Crushed Wind-Turbine Blade (RCWTB), obtained through a non-selective blade crushing process, as a partial substitute for aggregates in Self-Compacting Concrete (SCC). The aim was to determine the most adequate water/cement (w/c) ratio and amount of superplasticizing admixtures required to achieve adequate flowability and 7-day compressive strength in SCC for increasing proportions of RCWTB, through the production of more than 40 SCC mixes. The results reported that increasing RCWTB additions decreased the slump flow of SCC by 6.58% per 1% RCWTB on average, as well as the compressive strength, although a minimum value of 25 MPa was always reached. Following a multi-criteria decision-making analysis, a w/c ratio of 0.45 and a superplasticizer content of 2.8% of the cement mass were optimum to produce SCC with up to 2% RCWTB. A w/c ratio of 0.50 and an amount of superplasticizers of 4.0% and 4.6% were optimum to produce SCC with 3% and 4% RCWTB, respectively. Concrete mixes containing 5% RCWTB did not achieve self-compacting properties under any design condition. All modifications of the SCC mix design showed statistically significant effects according to an analysis of variance at a confidence level of 95%. Overall, this study confirms that the incorporation of RCWTB into SCC through a careful mix design is feasible in terms of flowability and compressive strength, opening a new research avenue for the recycling of wind-turbine blades as an SCC component. [ABSTRACT FROM AUTHOR]

The external bonding system using CFRP composite has been extensively utilized for strengthening different structures worldwide. However, premature debonding in this strengthening technique is a critical failure that leads to the fiber not reaching its ultimate capacity. In order to enhance the capacity of the externally bonded (EB) FRP and to slow the premature debonding failure mechanism, numerous anchoring techniques have been applied to improve the bonding capacity. The externally bonded reinforcement on grooves (EBROG) technology is one of the strategies that have been recently developed to delay the debonding issue. Although extensive studies have been conducted in the literature on the EBROG method, most of these studies have been focused on the bonding characteristics of grooves in the longitudinal direction, and few studies on the effect of different designs and configurations (e.g., width, height, and spacing) in the transverse groove direction have been conducted using only CFRP fabric. In the present study, an experimental investigation was carried out to study the bond behavior of the externally bonded reinforcement on transverse grooves (EBROTG) technique on CFRP-to-concrete joints involving different parameters, including groove width, depth, spaces between grooves, and strain evolution with the corresponding bond stress–slip relationships using CFRP laminate. Twenty-four concrete prisms, divided into eight groups of three specimens, were tested using a single-lap shear test set-up. The results of testing proved that the EBROTG method furnished a proper anchor system and highly enhanced the bonding force of the tests. The increasing range of bonding strength in the specimens reinforced with the transverse grooving method ranged from 11 to 86% compared to the externally bonded reinforcement (EBR), reflecting the effect of different widths, depths, and distances between grooves. [ABSTRACT FROM AUTHOR]

The automotive industry is under growing pressure from regulatory agencies to improve the recyclability of its plastic components. Simultaneously, manufacturers are adopting natural fiber composites in vehicles to reduce their carbon footprint and decrease reliance on petroleum-based materials. This presents a challenge at vehicle end-of-life, however, as natural fiber-reinforced polymers are substantially more difficult to recycle than their unreinforced counterparts. This study investigated the development of a mechanical recycling process for paper fiber-reinforced polypropylene composites, focusing on the impact of injection molding parameters—specifically, injection temperature and rate—on the thermal, mechanical, and water uptake properties of the composites. The results showed that processing temperature had a greater influence on composite performance than injection rate, with some limited interaction effects between the two. Higher processing intensity damaged the paper fibers, increasing the number of nucleation sites and resulting in greater polypropylene crystallinity. These structural changes reduced tensile properties at higher intensities, while flexural properties improved. Objective function analysis was applied to identify optimal processing conditions, balancing these competing trends. Overall, the findings demonstrate that paper fiber-reinforced polypropylene composites can be recycled into automotive-relevant injection molding compounds using conventional plastic manufacturing techniques, though careful tuning of processing parameters is essential to achieve optimal performance. [ABSTRACT FROM AUTHOR]

Novel fiber-reinforced polymer (FRP) bar-reinforced ultrahigh-performance engineered cementitious composites (UHPECCs) have been proposed recently. This innovative composite structure aims to address two critical challenges: (1) the reduced stiffness in FRP-reinforced normal concrete stemming from the lower elastic modulus of FRP; and (2) corrosion concerns faced by steel-reinforced concrete structures. Despite various investigations into the long-term performance of glass fiber–reinforced polymer (GFRP) bars in concrete environments, typically conducted through immersion in simulated concrete pore solutions, the durability of GFRP bars within UHPECCs remains unexplored. The inherently reduced water permeability of UHPECCs raises expectations for the enhanced protection they will provide to internal GFRP bars. Therefore, this study explores the durability performance of GFRP bars embedded in UHPECCs. Three types of GFRP bars made of various matrices (polyester, vinyl ester, and epoxy) were employed. The embedded GFRP bars were immersed in an alkaline solution at room temperature and 40°C for a duration of up to 360 days. Their tensile properties were assessed after specified periods. The results indicated a more pronounced deterioration in GFRP bars made of polyester than bars made of epoxy and vinyl ester. UHPECC covers provided good protection for internal GFRP bars compared with ordinary concrete due to the reduced permeability of UHPECCs. A microstructural analysis revealed that bar deterioration was predominantly due to matrix hydrolysis. This phenomenon resulted in the efficiency of fiber stress transfer. Notably, fibers in UHPECC-embedded GFRP bars exhibited no evident degradation, whereas those in the bars without UHPECC covers displayed slight degradation. [ABSTRACT FROM AUTHOR]

This paper defines a conversion factor included in the European technical specification for the design of fibre–polymer composite structures that considers the effects of moisture on the mechanical properties of fiber-reinforced polymer (FRP) composites exposed to natural weathering. The first part of the paper presents a field investigation about the durability of four pultruded glass fiber–reinforced polymer (GFRP) structures in service for 11–22 years in a Mediterranean climate. Their performance was evaluated by dynamic mechanical analysis and mechanical tests on small-scale coupons. The changes of thermophysical and mechanical properties showed low-to-moderate degradation. The second part of the paper presents the design-oriented methodology to define the moisture conversion factor. The results from the experimental study and those gathered from a literature data survey on the retention of mechanical properties of FRP materials after long-term exposure to natural weathering were analyzed. Based on these data and statistical considerations, a value of 0.85 was adopted for the natural weathering moisture conversion factor. [ABSTRACT FROM AUTHOR]

Carbon fiber–reinforced polymer–steel composite bars (C-FSCBs) and coral sea-sand aggregate seawater concrete (CSSC) are attractive choices as construction materials for island and atoll engineering construction. Understanding the bond behavior between the C-FSCB and CSSC is crucial for evaluating the mechanical properties of C-FSCB-reinforced CSSC structures. In this study, the bond–slip behavior between the C-FSCB and CSSC was experimentally assessed via pullout tests. The influence of various factors on the bond behavior was discussed, and the bond mechanism between the C-FSCB and CSSC was analyzed. The results indicate that, unlike steel bars, the low rigidity of fiber-reinforced polymer caused surface fiber stripping (i.e., shear damage) of the C-FSCB after interface slip, consequently reducing the squeezing fragmentation of concrete between the ribs. As the diameter and bond length of the C-FSCB increased, the bond strength decreased. Compared to specimens with C-FSCBs, the bond strength of specimens with steel bars of the same diameter increased by 17.9%. The degree of coarse aggregate fracture at the CSSC interface in the splitting failure was much higher than that of normal concrete. Based on existing research data, a formula for calculating the bond strength of C-FSCBs in CSSC has been established to determine the anchorage length of C-FSCBs, and the calculated values accurately predicted the test values. [ABSTRACT FROM AUTHOR]

This paper examines experimental data from 22 large-scale concrete-filled FRP tubes (CFFTs), each 900 mm in length and 300 mm in diameter, subjected to either concentric or eccentric compression. The study encompasses CFFTs composed of normal-strength concrete (NSC) and high-strength concrete (HSC), employing glass fiber–reinforced polymer (GFRP) tubes with fiber angles of 60° and 80°. A primary focus of this study is investigating the cracking behavior of GFRP tubes in CFFTs during loading, a durability/serviceability-related issue that has received limited attention in existing literature. Based on the test data, the study identifies two distinct types of cracking: tension-induced cracking under eccentric compression and hoop crushing under either concentric or eccentric compression. The paper delves into the underlying mechanisms behind these cracking phenomena and thoroughly explores the influence of variables such as fiber-reinforced polymer (FRP) tube thickness, fiber angle, and the type of concrete core on the observed cracking behavior. Meanwhile, the paper provides an in-depth analysis of the structural behavior of CFFTs, with a focus on aspects such as GFRP tube thickness, fiber angle, and the differences between NSC and HSC cores. These experimental results not only enhance the understanding of the behavior of large-scale CFFTs under concentric or eccentric compressions but also offer crucial insights into the differences in the compressive behavior of FRP-confined NSC columns and FRP-confined HSC columns. Additionally, the comparative analyses of local cracking behavior with the global behavior of CFFTs contribute vital information toward the development of a design method of CFFT columns in practice, with different considerations of the issues in the ultimate limit state and serviceability limit state (e.g., the corrosion protection capabilities of the GFRP tube in CFFTs). [ABSTRACT FROM AUTHOR]

Even though many experimental studies have evidenced that fiber-reinforced polymer (FRP)-confined concrete columns under compression might exhibit a postpeak strain-softening response, followed by a hardening behavior (a stress reduction–recovery response), most existing models are only applicable to confined columns developing full hardening behavior. Additionally, their applicability is limited to a certain column cross-sectional shape (circular or noncircular). Therefore, the present study is dedicated to the establishment of a new design-oriented stress–strain model, unified for FRP-confined circular/noncircular concrete columns. For this purpose, first, a new nondimensional confinement stiffness-based index is developed, below which the column response is transformed from a full-hardening behavior (Type A) to a postpeak strain softening–hardening one (Type B). A parabolic-linear stress–strain formulation is proposed for Type A columns by developing a new expression for the calculation of the linear hardening branch's slope. For the case of Type B, a new methodology is introduced for the simulation of the stress reduction–recovery response beyond the transition zone, whose main elements are calibrated by a large test data set. The dominance degree of the noncircularity effect of the cross section on both Type A and Type B cases was assessed and reflected in the key components of the proposed stress–strain models, based on nonlinear regression analysis. With these considerations, this model could accurately simulate the impact of the noncircularity effect on the stress–strain relationship of Type A and Type B columns, whose predictive performance is validated through a comparative assessment with existing models based on large test stress–strain results. [ABSTRACT FROM AUTHOR]

The significant influence of the geometric properties of the rebar surface on the bond behavior of fiber-reinforced polymer (FRP)-reinforced concrete (RC) members is not adequately covered in current design provisions. Twenty-four hinged-type glass FRP (GFRP)-RC specimens have been tested in the present study to investigate the effect of concrete strength, embedment length, confinement conditions, and surface geometric properties of GFRP rebars on the flexural bond behavior of the specimens. It has been observed that the bond strength between the GFRP rebars and the concrete can be increased (up to 15%) by altering the surface geometric properties of the GFRP rebars. The effect of surface geometry is more pronounced in higher-strength confined concrete specimens with an embedment length of GFRP rebars equal to five times the diameter (d) of the rebar as compared to the other tested specimens. Confinement reinforcement has also been observed to increase the bond strength of the tested specimens by 10%–18%. Furthermore, the effect of the test variables on the mode of failure and the postpeak behavior of the specimens under flexural loading has also been investigated in this study. The findings of the present study strongly demonstrate the necessity of a contribution factor that considers the surface characteristics of GFRP rebars in the estimation of the bond strengths of GFRP-RC members. An equation for bond strength estimation, which considers contributions from bar surface geometry, has also been proposed from the findings of this study. It is imperative that similar factors are incorporated into future versions of design provisions in order to integrate the contributions of bar surface geometries to the bond strength of GFRP-RC members. Practical Applications: The research findings highlight the importance of considering the geometric properties of fiber-reinforced polymer (FRP) rebar surfaces in the design of FRP-reinforced concrete (RC) members. The bond strength between the rebar and the concrete can be significantly increased and the overall performance of FRP-RC members can be improved by adequately selecting the surface geometry of FRP rebars. This study also demonstrates the influence of concrete strength, embedment length, and confinement conditions on the bond behavior of FRP-RC members. It is recommended to implement a contribution factor that considers the effect of the surface characteristics in design provisions. A new equation has been proposed for engineers and designers to consider the surface geometry in the calculation of the bond strength of FRP-RC members. Implementing the findings of the present study in future design provisions will enhance the safety and effectiveness of FRP-RC structures, through improved performance and durability in various applications. [ABSTRACT FROM AUTHOR]

The application of fiber-reinforced polymer (FRP) composites as piling materials in harsh environments has gained popularity due to their high corrosion resistance. FRP composites can be fabricated using different types of epoxy resin matrices and fibers. This study aims to investigate the interface behavior between sand and FRP materials with varying levels of hardness, with a particular emphasis on the abrasive surface wear of FRP. Monotonic interface shear tests (under normal stresses of 50, 100, 200, and 400 kPa) and interface shear tests repeated 20 times (under normal stresses of 200 and 400 kPa) are performed. The local surface roughness of the FRP plates is measured for tested samples under both monotonic and repeated loadings using laser scanning to evaluate the accumulated abrasion effect. The results of monotonic tests indicate that under a given shear displacement and normal stress, the samples with softer FRP plates exhibit higher interface friction angles and more pronounced dilative behavior. Following repeated tests, the interface friction angles of softer FRP specimens decrease, while the surface roughness of the FRP plates gradually increases. However, for the softest FRP plate, its surface is severely damaged after repeated tests under high normal stress levels, leading to unstable changes in the test results. [ABSTRACT FROM AUTHOR]

Enhancing the strength and ductility of metallic materials simultaneously is crucial for numerous industrial applications, yet it remains a formidable challenge due to the typical trade-off between these two properties. This study introduces an innovative approach to surmount this challenge by employing a composite bar design that leverages necking inhibition mechanisms for simultaneous improvements in both strength and ductility. The composite bars, comprising aluminum cores reinforced with spirally wound fiber-reinforced polymer (FRP) strands, were fabricated in various configurations to investigate different necking behaviors. Through uniaxial testing, the composite bars exhibited notable increases in both strength and ductility, attributed to the strategic design of the FRP winding angle and FRP content. This design effectively modulates the necking behavior, thereby enhancing the composite bars' mechanical properties. Analysis of the strain distribution further elucidated the role of the spiral FRP strands in necking prevention. The composite bar design method outlined in this study offers a viable strategy for enhancing the mechanical performance of metallic materials, significantly reducing the risk of abrupt failure under high loads and deformations. [ABSTRACT FROM AUTHOR]

Hybrid double-skin tubular columns (DSTCs) are a promising form of composite columns in which the concrete is sandwiched between inner steel and outer fiber-reinforced polymer (FRP) tubes. A modified form of DSTC incorporating stiffened steel tubes demonstrates more potential benefits than the standard unstiffened design. The inclusion of stiffeners in the steel tube enhances composite interaction among column components, effectively delaying local buckling deformations and potentially improving the behavior of confined concrete. The present study performs both experimental and analytical investigations to measure the impact of stiffeners on the confined concrete behavior of square-circle (SC) configured specimens, considering various column slenderness ratios. Thirty DSTC specimens were experimentally tested with variations in steel stiffener properties, including number, configuration (maintaining a similar combined area), thickness, and width; and unconfined concrete strength. The test results showed a substantial improvement in axial load capacity in the stiffened SC-shaped specimens, reaching up to approximately 37% in this study. Subsequently, the test data were utilized to propose a stress–strain confined concrete model specific to SC-shaped specimens with varying slenderness ratios. This proposed model was employed for calibration study using detailed finite-element (FE) analysis, demonstrating excellent agreement with the test data. A parametric investigation was then conducted through FE analysis to optimize the rib-stiffener properties. Finally, the findings from both the experiments and the FE analysis were utilized to develop a simplified equation that predicts the axial load capacity of stiffened SC-DSTCs. [ABSTRACT FROM AUTHOR]

In the first part of the paper, a critical review of the fundamental knowledge of the debonding mechanism of externally bonded fiber-reinforced polymer (FRP) composites from a quasi-brittle substrate (such as concrete and masonry) is presented. Then, the paper focuses on five selected research topics. Some of these topics have reached a quite mature state. However, the authors build upon their research carried out in the last 20 years and the work published in the literature to present new perspectives or clarify aspects that have not yet reached a shared consensus. Among these five topics, the following two topics are discussed more extensively and advances in their knowledge are proposed: (1) the specific features of the interfacial law (interfacial shear stress versus slip relationship) and its shape required to obtain a finite length of the stress-transfer zone and (2) the relationship between the bond capacity in direct shear tests and the force in the FRP strip corresponding to the full debonding of FRP-strengthened beams. [ABSTRACT FROM AUTHOR]

Hybrid fiber-reinforced polymer (FRP)–concrete–steel double-skin tubular members (DSTMs) yield much higher strength-to-weight ratios and much better corrosion resistance than traditional reinforced concrete members. The steel tube and the FRP tube in DSTMs act as propping and formwork for concrete casting, thus significantly reducing construction efforts. DSTMs have been extensively studied as columns and beams. This paper presents a study aimed at understanding the benefit of using the double-skin tubular cross section in arches. Five double-skin tubular arches (DSTAs) with a span length of 3.6 m were fabricated and tested to investigate their behavior. The test program examined the effects of section load eccentricity, eccentricity of the steel tube position in the section, thickness of the FRP tube, and diameter of the steel tube on the behavior of the DSTAs. All the DSTA specimens displayed ductile behavior. The DSTAs were found to provide a significantly higher ultimate load than corresponding steel–concrete composite arches of a similar cross section. A higher cross-sectional load eccentricity was found to reduce the ultimate load of a DSTA. A numerical model was also developed in OpenSees (2019) version 3.2.2 to predict the behavior of the DSTAs. The numerical predictions were found to agree with the experimental results reasonably well. [ABSTRACT FROM AUTHOR]

Steel-reinforced concrete (RC) columns typically exhibit significant residual displacements and low postyield stiffness when subjected to seismic loads, which may complicate subsequent repairs. This paper investigated the effects of different reinforcement methods on the seismic performance of concrete columns by using glass fiber–reinforced polymer (GFRP) and basalt fiber–reinforced polymer (BFRP) bars. Five samples were subjected to low-cycle horizontal loading tests to compare the corresponding seismic performance indices. The results showed that fully FRP-reinforced concrete columns had a lower energy dissipation capacity and ductility coefficient than RC columns but had other clear advantages in terms of ultimate displacement and a postyield stiffness increase by 100%. Compared to RC columns, the concrete columns with half FRP bars exhibited comparable energy dissipation, greater ultimate displacements and ductility coefficients, and a 60% increase in postyield stiffness. Incorporating FRP bars significantly reduced the residual deformation of FRP-reinforced concrete columns compared to RC columns under the same loading cycle, enabling structural repairs and continued use after an earthquake. Practical Applications: During the 2008 Wenchuan earthquake, many traditional reinforced concrete structures were severely damaged, causing significant economic losses and threatening lives and property. Related investigations verified that the deformation of reinforced concrete columns was the most serious under the same displacement level, and adding fiber-reinforced polymer materials greatly reduced the deformation of the concrete columns, which can improve the reparability of the structure after an earthquake. Additionally, the peak load-carrying capacity of fiber-reinforced polymer materials is similar to that of reinforced concrete columns, giving the former a certain application value. In the coastal areas of traditional reinforced concrete structures, because of corrosion, which leads to a significant reduction in the service life of the environment, it is proposed that the introduction of fiber-reinforced polymer materials with excellent corrosion resistance will largely improve the service life of the corrosive environment of the components while ensuring similar seismic performance. [ABSTRACT FROM AUTHOR]

The load-bearing behavior and the performance of composites depends largely on the bond between the individual components. Conventional grid-like textile reinforcement structures with thin and smooth yarn structures transmit forces primarily by an adhesive bond with the surrounding matrix. A sufficient load transmission is not possible. Thick, pultruded rebars made of fiber-reinforced plastics can be profiled by subtractive (e.g. milling) or additive (e.g. wrapping) techniques in order to create a rip-like structure and increased shear bond. Yet the discontinuous fiber course results in material inefficiency. A newly developed profiling technique allows a tetrahedral profiling of the complete roving structure, yet considering its anisotropic properties. In the article, we present this approach, and the first results from single yarn tensile and pull-out tests of single, double and triple plied profiled rovings in concrete and asphalt matrix. Thus, the highest bond is achieved in the brittle concrete matrix. Plied rovings with strong tetrahedral profiles show up to 600% higher bond stress compared with rovings with circular profiles, while maintaining high tensile properties. However, splitting-induced failure of the reinforced test specimens occurs, making plied profiled rovings favorable for high concrete cover and less brittle matrixes; for example, asphalt. The reinforced asphalt specimens show at −10°C similar bond properties, but at 30°C the bond decreases by 80%. In summary, the study shows that bond properties of profiled rovings are superior to conventional circular rovings, and promise a high material efficiency for use in concrete and asphalt reinforcements. [ABSTRACT FROM AUTHOR]