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Double-skin tubular columns (DSTCs) are promising modern columns with concrete sandwiched between fiber-reinforced polymer (FRP) and steel skins. Typical concentrated and localized failure modes in DSTCs can be constrained or delayed when stiffened inner steel tubes are used. DSTCs with circular cross sections are better investigated in the literature than scarcely studied square shapes, especially with variations of stiffened inner steel tubes. This study examined the nonlinear confined concrete behavior in stiffened DSTCs with square-shaped glass FRP as outer skins and stiffened steel tube as inner skins. The documented test results on 26 DSTCs were used to formulate a new analysis-oriented, axial stress–strain model for confined concrete. Primary model-formulation parameters were evaluated using a genetic algorithm (GA)–driven error minimization. A detailed parametric study was then performed on square DSTCs by incorporating the proposed stress–strain constitutive relationship for confined concrete in nonlinear finite-element analysis. The variations used in the parametric study included the shape of the inner steel tube (circular versus square), unconfined concrete strength, and flat-stiffener geometry characteristics (i.e., quantity, configuration in terms of thickness versus the number of stiffeners for similar total cross-sectional area, and its cross-sectional dimensions). Analytical results were finally used to propose a new load-bearing equation for estimating the axial load capacity of square DSTCs having stiffened inner steel skins of circular or square shapes. The proposed formulation showed better accuracy (about 2% on average and 14% maximum disparity with refined ultimate axial load capacities of DSTCs used in this study) than the existing prediction models. [ABSTRACT FROM AUTHOR]

Iran is located in a seismically active region, which poses a significant threat of destructive earthquakes. Given the dense populations they support and their structural importance, schools demand resilience across various dimensions. In this study, we aimed to evaluate the seismic performance of schools in Khorramabad, Lorestan, Iran, by obtaining resilience curves. The research involved a thorough analysis of retrofitting options for a specific school, identified through a review of data from Lorestan province. Three retrofitting options were considered: (1) bracing, (2) friction dampers, and (3) utilization of fiber-reinforced polymers (FRP) in the columns. The structure was modeled and designed using suitable software before and after implementing the retrofitting options. Incremental dynamic analysis (IDA) was conducted using recommended ground motions (including 10 ground motions for near and far fields). Subsequently, a risk analysis of the study area was performed, and a comparison was made between damage and fragility curves under different conditions; this included an assessment of potential damage in future earthquakes. The results demonstrated the efficacy of the analysis method in evaluating the seismic performance of buildings. Last, the study highlighted the building's response under different earthquake intensities and confirmed that the maximum capacity determined closely corresponded to findings from other nonlinear analyses. This approach was proven to be highly effective in assessing a building's behavior and ultimate capacity in the face of earthquakes. [ABSTRACT FROM AUTHOR]

Steel plate girders are susceptible to web buckling due to slender web. This phenomenon creates an undesirable failure mode and limits their ultimate load capacity. The use of fiber-reinforced polymer (FRP) composite materials offers a good prospect of strengthening the slender web of plate girders to avoid web buckling. This paper presents the details of experimental testing of steel plate girders with nonrigid end posts. These girders were tested in three-point bending. The end shear panels of the girders were strengthened by glass FRP (GFRP) pultruded section stiffeners or glass fiber fabric composite. GFRP stiffeners were applied either vertically or along the compression diagonal in the shear panel. All strengthened panels failed by web shear buckling associated with a breakdown of the employed strengthening scheme. It was found that the shear capacity of the panel remained unchanged by replacing the steel stiffener with GFRP stiffeners in the end panel. The critical buckling load of the strengthened panels increased up to 44% compared to the unstrengthened panel. The shear capacities of the panels strengthened by the glass fabric composite and diagonal GFRP pultruded section were the highest and similar to each other. Both these panels resisted a 50% larger shear compared to the unstrengthened panel. The positions of the plastic hinges in all tested panels were the same due to the same ratio of panel length and depth of the web. [ABSTRACT FROM AUTHOR]

This paper presents a finite element (FE) model of reinforced concrete two-way slab strengthened using fiber-reinforced polymer (FRP) sheets. This model was validated against experimental data from the literature and it showed acceptable prediction accuracy. Although carbon-FRP (CFRP) is the most commonly used composite in repairing and strengthening reinforced concrete structures, it is important to consider other types of FRP composites such as the eco-friendly basalt-FRP (BFRP) and the newly developed polyethylene terephthalate-FRP (PET-FRP). Therefore, the validated FE model was utilized to perform a parametric study for slabs having different values of concrete compressive strength (ranging from 20 to 80 MPa) and strengthened with other types of FRP. The results show that CFRP provides the highest strength enhancement with a 34.5% increase in the ultimate load, while PET-FRP provides the lowest improvement with an increase of 11.2%, compared with unstrengthened slab. The results also show that the concrete compressive strength (fc') has moderate influence on the ultimate load. For example, increasing fc' from 20 MPa to 80 MPa increased the predicted ultimate load for CFRP-strengthened slab from 15% to 62%. The FE model provides a suitable prediction for the ultimate strength and deformability of the strengthened two-way slabs that helps in better understanding of the performance of strengthened slabs and allows engineers to optimize design parameters. [ABSTRACT FROM AUTHOR]

This study examines the pull-out behavior of timber elements connected using dowelled joints (type 1) and axially loaded screws (type 2), focusing on numerical predictions and experimental validation under various conditions. Different screw diameters, configurations, material properties, and corrosion resistance are examined. Pull-out tests demonstrated that type 2 connections, particularly using 10 mm and 14 mm screws, outperformed traditional dowelled joints in terms of load capacity. Numerical analysis based on Eurocode 5 showed close alignment with experimental results, particularly for screws with actual thread diameters of 5.2 mm and 6.2 mm, with minor deviations attributed to material variability. The finite volume method (FVM), implemented using the Geodict ElastoDict software, provided additional insights into the mechanical behavior of the connections. Corrosion resistance tests confirmed the long-term durability of fiber-reinforced polymer (FRP) components in saline environments, with negligible material degradation after 28 days of exposure. The 10-90-6 screw configuration was identified as the most efficient, balancing load capacity, displacement, and material cost. These findings highlight the effectiveness of both Eurocode 5 and FVM-based approaches for predicting screw behavior and emphasize the importance of accounting for material heterogeneity in timber connection design. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the effect of glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) strengthening materials on the static and dynamic behavior of reinforced concrete (R/C) beams subjected to progressive damage. Four identical beams, each strengthened with either GFRP or CFRP, are tested under a cyclic quasi-static loading pattern. Impact hammer tests are performed for undamaged states and various damage levels of the beams. The dynamic test data are analyzed using the Enhanced Frequency Domain Decomposition (EFDD) method to estimate the dynamic characteristics of the beams. In this context, the first three vibration modes in both vertical and horizontal directions are considered. Strengthening is applied to both pre-damaged and undamaged beams, enabling a comparison of their performance before and after the strengthening procedure. Beams strengthened with CFRP exhibit a higher load-bearing capacity and stiffness but also fail at lower displacement levels compared to those strengthened with GFRP, which demonstrate more ductile behavior. Furthermore, the modal frequency ratios indicate that the first vibration mode is more sensitive to damage than the second and third modes. This study highlights the effectiveness of both strengthening materials in enhancing the structural performance of both undamaged and damaged beams. [ABSTRACT FROM AUTHOR]

Under earthquake excitations, reinforced concrete (RC) columns could be subjected to lateral drift reversals and a combination of axial forces, bending moments, and torsional effects. This paper investigates the behavior of glass fiber-reinforced polymer (GFRP)-RC columns under seismic-simulated loading, including torsion, which has not been studied previously. Seven large-scale circular GFRP-RC column-footing connections were cast and tested under various combined reversed cyclic loading configurations to examine the effects of torsion-bending moment ratio (tm), transverse reinforcement ratio, and concrete compressive strength. The test results revealed that increasing the tm reduced the lateral load capacity and deformability of the GFRP-RC column, but resulted in a more symmetric torque-twist relationship. Increasing the transverse reinforcement ratio mitigated core damage and provided additional support (for example, spiral turns) for torsion-induced tensile stresses. Moreover, increased concrete compressive strength bolstered torque capacity and torsional stiffness, while, under a tm of 0.4, it resulted in decreased twist capacity. When torsion was present, increasing the concrete compressive strength had an insignificant impact on the bending-shear response, differing from findings for GFRP-RC columns subjected to seismic loading without torsion. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) bridge columns often encounter complex combinations of loads, including flexural, axial, shear, and torsional forces, during seismic events, especially in the presence of geometric irregularities such as skewed or curved bridges, unequal spans, or varying column heights. Corrosion-related deterioration in RC structures spurred the adoption of glass fiber-reinforced polymer (GFRP) as a promising alternative to steel reinforcement. This study experimentally investigates the performance of GFRP-RC circular columns under cyclic loading, including torsion with different torsion-to-bending moment ratios (tm and longitudinal reinforcement ratios. The results showed that, with the same reinforcement ratios, the addition of torsion to cyclic bending and shear significantly altered the behavior of the GFRP-RC column in terms of mode of failure, load resistance, drift capacity, and energy dissipation. The inelastic deformability hinge shifted upward with increased tm. Higher tm accelerated stiffness degradation, while increasing the longitudinal reinforcement ratio enhanced lateral load, drift, and twist capacities. [ABSTRACT FROM AUTHOR]

Flexural strength and ductility of exclusively bonded or unbonded steel prestressed concrete (PC) members are well covered and documented in the literature and codes of practice. However, current design methods are limiting the use of hybrid (that is, a combination of unbonded and bonded steel and fiber-reinforced polymer [FRP]) tendons, particularly when using brittle material such as FRP tendons. In this paper, a general procedure for evaluating the nominal moment capacity and ductility of hybrid PC members was developed using the strain compatibility approach. The procedure is applicable for members with any combination of bonded or unbonded steel and FRP tendons. Using capacity design approach based on strain compatibility, the ductility performance of several hybrid systems with different parameters were compared. The parameters included, among others, the level of "net tensile strain" in the tension reinforcement at nominal strength adopted in ACI 318-19 as a measure of ductility, concrete compressive strength, and the newly defined hybrid prestressing ratio (HPR). HPR represents the ratio of the moment contribution of the unbonded tendons to the total moment capacity of the member with hybrid tendons. Nonlinear analysis was carried out for generating the entire load-deflection and moment-curvature responses of the different systems. The accuracy of the nonlinear analysis was verified by comparing it with available experimental data, and the analysis results were used to compare traditional curvature ductility measures of the various systems against the ductility measure specified in the ACI Building Code. A design example is provided in Appendix A to illustrate the use of the strain compatibility approach. [ABSTRACT FROM AUTHOR]

Interface shear transfer is vital to maintain the structural integrity of concrete composite elements. Therefore, shear connectors are provided at the concrete joint interface to maintain such integrity. Due to its high tensile strength and non-corrodible nature, glass fiber-reinforced polymer (GFRP) reinforcement can be used as shear connectors in composite elements, particularly those in harsh environments. Fifteen pushoff specimens were constructed and tested to failure. The specimen consisted of two L-shaped concrete blocks cast at two stages to provide the cold joint interface. The test parameters were the type, shape, and ratio of shear-friction reinforcement and concrete strength. It was demonstrated that GFRP-reinforced concrete RC) specimens with reinforcement ratios of 0.36% or more could resist the shear-friction stresses similarly to their steel-RC counterparts. Also, increasing the concrete strength increased the shear-friction capacity significantly. Moreover, the design model in the Canadian Highway Bridge Design Code resulted in very conservative predictions. [ABSTRACT FROM AUTHOR]

For the past several decades, there has been an ongoing academic challenge to understand the shear-transfer mechanism in reinforced concrete (RC) members, particularly in those with small shear span-depth ratios, also known as deep beams. Analytical uncertainty regarding shear strength inevitably increases when those deep members are strengthened in shear using externally bonded fiber-reinforced polymer (FRP) composites. This study aims to investigate the complex, interrelated effects of short shear span-depth ratios and FRP composites on RC deep beam members. To this end, the fundamental formulations of the dual potential capacity model (DPCM) are extended to RC deep members reinforced with externally bonded FRP composites. The proposed model can consider the various types of FRP composites, fiber bonding configurations, and fiber layouts, and various failure modes of concrete and FRP reinforcements are also reflected. A total of 131 shear test results of RC deep and short members with externally bonded FRP composites are carefully collected, and those are added to the existing database of RC slender members strengthened with FRP composites. On this basis, the proposed approach is verified by comparing test results with analysis results, and a reasonable level of analytical accuracy is achieved. The statistical data distribution of strength ratios between the test and analytical results is consistent across a range of shear span-depth ratios from approximately 0.7 to 4.0. Overall, the proposed DPCM approach provides a useful tool for analyzing the shear strength of RC deep beam members strengthened with externally bonded FRP composites. [ABSTRACT FROM AUTHOR]

AbstractIn this work, an attempt is made to produce a ductile construction system using natural fiber-based engineered cementitious composite (ECC) beams where the conventional longitudinal steel reinforcements are replaced using glass fiber-reinforced polymer (GFRP) bars. In total, sixteen medium-scale reinforced ECC beams were tested under an un-symmetric three-point bending configuration to generate shear failure. The parameters considered as a part of the experimental program include (a) type of longitudinal reinforcement (steel or GFRP) and (b) type of natural fiber (flax, hemp, kenaf and pineapple). In addition, a detailed analytical study was performed using the Architectural Institute of Japan (AIJ) method, and a modified equation was proposed for predicting the shear behavior of FRP reinforcement using ECC beams. Test results reveal that the use of GFRP reinforcements resulted in the reduction of peak load from 33.6% to 65.4% when compared to similar steel-reinforced R-ECC beams. However, the failure mode was highly ductile due to the excellent fiber bridging mechanism, multi-crack formation and effective stress redistribution at the crack tip. The predictions obtained from the modified AIJ method improved the accuracy of shear capacity FRP-reinforced ECC sections when compared to the experiments. [ABSTRACT FROM AUTHOR]

The damage process of composite materials, such as short fiber-reinforced plastics (SFRP), is complex. Therefore, it is necessary to accurately represent the damage process in fatigue life prediction. Herein, fatigue life prediction was conducted by combining the digital image correlation method, which is a non-destructive testing technique, with a convolutional neural network (CNN), using Xception as the network architecture. High prediction accuracy was obtained when training and testing were performed on the same SFRP specimens. In contrast, using different specimens for training and testing resulted in lower accuracy. This issue may be improved by increasing the number of specimens. The regions of interest in the model were visualized by Gradient-weighted Class Activation Mapping. Notably, the model indicated the breaking point as the region of interest from the early stages of the test. The breaking point was identified at an earlier stage by the CNN than by visual inspection, demonstrating the potential for a new method of damage observation. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer and metal-laminated structures are widely used in aerospace and military fields because of their excellent mechanical and physical properties. However, the materials are prone to processing defects such as burring and furry during processing. According to the mechanism of vibration cutting, the mixed-frequency vibration tool holder (MFVT) with low frequency and ultrasonic-combined vibration is designed and manufactured in this paper. The low-frequency vibration module (LFVM) and the ultrasonic vibration module (UVM) of the tool holder are designed theoretically and studied experimentally. The working performance and stability of the MFVT are tested and analyzed. The results show that the combination of ultrasonic vibration and low-frequency vibration of the tool holder can meet the requirements of design and tool holder. Among them, the exciting surface has the greatest influence on the motion curve of the LFVM. The amplitude of low-frequency vibration decreases with the increase of tool holder speed. When the UVM is running continuously, the temperature rise tends to be balanced. The amplitude and resonant frequency of ultrasonic vibration will decrease with the increase of operating temperature. The heat generated at the end of the cutter chuck accounts for the total heat generated by the horn 98.7% of the amount, the stable temperature reached 231℃. [ABSTRACT FROM AUTHOR]

The automobile industry is exploring the use of HP-RTM (high-pressure resin transfer molding) for mass-producing glass fiber-reinforced plastic (GFRP) chassis parts to achieve weight reduction. In this process, dry fabric is used as a material. However, the fatigue endurance performance of these parts deteriorates due to the bobbin threads in the dry fabric. This study aims to propose a laminating method to prevent the deterioration of the durability of GFRP chassis parts with bobbin threads. Specimens were fabricated with different fiber volume fractions (Vf) and bobbin thread positions. After manufacturing, static properties were evaluated to compare the physical properties of each specimen, and the stress for fatigue durability evaluation was determined based on these static properties. Finally, fatigue endurance performance evaluations were conducted to determine the optimal lamination conditions for GFRP chassis parts. The results showed that for the three laminating methods applied in this study, an increase in Vf not only enhanced the static properties but also reduced waviness, as confirmed through OM images and changes in the slope of the stress–strain curve. Additionally, fatigue endurance tests revealed that, along with the improvement in waviness, adjusting the position of the bobbin threads—which negatively impact durability—resulted in an increase in fatigue endurance performance from approximately 67,000 cycles to about 870,000 cycles, an improvement of approximately twelve times. [ABSTRACT FROM AUTHOR]

This research systematically compares the effectiveness of Carbon Fiber-Reinforced Polymer (FRP) and Fabric-Reinforced Cementitious Matrix (FRCM) in shear strengthening of reinforced concrete (RC) deep beams featuring web openings. Through a comprehensive experimental program, six RC beams were subjected to shear tests, considering variations in the number of layers for both FRCM and FRP, employing a U-wrapping configuration. Recorded parameters include load-deflection curves, ultimate strength, cracking patterns, failure modes, and strains in steel bars, allowing a comprehensive comparison between strengthened and un-strengthened RC beams. The study compares observed shear strengths from experiments with shear capacities predicted by proposed models for RC beams strengthened with FRCM and FRP, following codes such as ACI 440.2R-17, CSA S806-12, Eurocode 2, and ACI 549.4R-20. Increasing layers enhanced shear strengths and post-elastic stiffness. The presence of substantial openings led to early shear cracks and reduced strength. CFRP improved shear strength by 13.99% (1-layer) and 18.12% (2-layer), while FRCM strengthened layers by 20.2% (1 layer) and 29.3% (2 layers). FRCM outperformed in strength and stiffness, while FRP excelled in ductility and concrete confinement. Experimental and calculated results varied based on ACI, CSA, and Eurocode, with ACI providing consistent and accurate results. CSA's calculation surpassed experiments, emphasizing its consideration of effective fabric design strain. [ABSTRACT FROM AUTHOR]

A key challenge in the utilization of waste polyester fibers (PET fibers) is the development of fiber-reinforced composites with high filler content and the improvement of fiber length retention. Herein, the effects of a two-rotor continuous mixer and a twin-screw extruder on the structure and properties of waste polyester fiber composites were evaluated. The results revealed that the mechanical properties of the composites were improved significantly with increasing fiber content, especially when processed using the twin-rotor continuous mixer. This mixer facilitated the formation of a robust fiber network structure, leading to substantial enhancements in tensile strength, flexural strength, and heat resistance. Specifically, compared to those processed by the twin-screw extruder, with 60 wt% fibers content, the tensile and flexural strengths of specimens processed by the twin-rotor continuous mixer increase by 21% and 13%, respectively. The average fiber length in specimens processed by the twin-rotor continuous mixer was 32% longer than that in specimens processed by the twin-screw extruder, attributable to the lower shear frequency and the higher tensile ratio of the former. This blending technique emerges as an effective strategy, contributing significantly to promoting the development and practical application of waste textile fiber-reinforced polymer composites. [ABSTRACT FROM AUTHOR]

The purpose of this work is to test the effectiveness of strengthening timber structures by means of composite bars. This article presents the results of preliminary tests carried out on solid beams made of fir wood. The test specimens, which are classified as strength class C24, had dimensions of 7 × 17 × 330 cm. Beams were reinforced with 8 mm diameter basalt fiber-reinforced polymer (BFRP) bars. The bars were glued into grooved channels along the bottom surface. Epoxy resin was used as an adhesive. The strength tests were conducted in accordance with the requirements of EN 408+A1:2012. The four-point bending scheme was adopted. The tests were conducted in the following two series: unreinforced beams (A) and beams reinforced with composites (B). Five elements were tested in each series. The reinforcement resulted in an average increase in the bending moment value of 8.41%. The deflection value at failure increased by 19.77%. The work also includes an analysis of the failure mode and a ductility analysis. Further tests should be carried out using a higher reinforcement ratio. A higher reinforcement ratio should make the presented reinforcement configuration more effective. [ABSTRACT FROM AUTHOR]

Voids behind tunnel linings can be formed either during or after the construction phase, occurring due to inadequate backfilling, substandard workmanship, water erosion, or gravitational forces. Investigations into numerous tunnels in which collapses occurred while in operation have indicated that voids behind the liner constitute the primary contributors to these failures. Consequently, it is imperative to devise lining reinforcement strategies tailored to the specific conditions encountered in the field. Fiber-reinforced plastic (FRP) represents a viable alternative construction material that has been widely utilized in the reinforcement of concrete structures. It is essential to quantitatively assess the reinforcing effect of FRP grids when they are employed in the restoration of deteriorated tunnel linings, thereby facilitating the development of effective maintenance designs. In this study, we aimed to enhance the sensitivity analysis of the reinforcement method by evaluating the impact of voids through the analysis of bending moments and axial forces within the tunnel lining. The effects of voids based on the different locations in which they occur were explored numerically through an Elastoplast finite element analysis. The study involved simulating tunnel linings that had been reinforced with FRP grids and assessing the effects of such reinforcement in tunnels afflicted with various structural problems. Based on the outcomes of these simulations, the internal forces within the lining are scrutinized, and the efficacy of the reinforcement is appraised. [ABSTRACT FROM AUTHOR]

The paper deals with the influence of the brittleness of glass fibers on the selected performance properties of the fibrous polymer composite. Understanding the fatigue behavior of fiber-reinforced plastics is desirable for exploiting their features in safe, durable, and reliable industrial components. Based on the proposed methodology, it is possible to assess the impact of material reuse on selected mechanical and rheological properties. To verify the methodology by experimental analysis, homopolymer PP reinforced with chemically grafted glass fiber (30 wt%) was selected. The proposed methodology was subsequently verified by experimental analysis and evaluated statistically. The morphology of the fracture surfaces was evaluated, and the fiber-polymer matrix adhesion was monitored at the interface of the fracture surfaces. Based on the measured and evaluated values and fracture surfaces, we can say that the brittleness of the fibers significantly affects the performance properties of the tested polymer composite. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer composites are prominent structural components in various industries such as aerospace, automotive, and wind energy. These materials are considered due to their high strength-to-weight ratio and relative ease of fabrication. However, fiber composites possess low electrical and thermal conductivities and are prone to impact-induced damage. Metallization of fiber-reinforced polymer composites has become an area of interest as a means to prevent abrasive and corrosive damage while also improving other physical properties including thermal and electrical conductivity. The possibility of using cold spray as a novel composite metallization approach has been investigated in this work. The significance of cold spray for metallization is due to relatively low process temperatures which effectively protect the underlaid substrate from potential temperature degradation. As a practical approach to further reduce the possibility of cold spray-induced damage, the present study explores the impact and failure mechanics of metal particles coated with a thin polymeric shell, hence the term polymer-coated metal particle. The thorough model-based analyses presented herein indicate that the so-called polymer-coated metal particles can be cold spray deposited without imposing significant damage to the composite substrate mainly due to the 'cushioning' effect of the thin polymer shell. The results discussed here also provide guidelines for the surface metallization of high-performance fiber-reinforced thermoplastic composites in practice. [ABSTRACT FROM AUTHOR]

Petroleum-based cured epoxy polymers, used widely in aerospace, marine, and automotive industries, pose environmental threats due to their toxicity. Therefore, developing alternative curing systems for epoxy resin is crucial. This study explores the use of bio-based phenalkamines as curing agents for epoxy resin to enhance the mechanical properties of polymer composites and fiber-reinforced laminates. The functional groups, morphology, and thermal properties of the composites were analyzed using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). Mechanical properties of two epoxy systems—EP-TETA (triethylenetetramine-cured) and EP-PA (phenalkamine-cured)—were evaluated according to ASTM standards. EP-PA exhibited approximately 318% greater elongation, indicating improved ductility, and 16% higher ultimate force and stress compared to EP-TETA, suggesting its suitability for load-bearing applications. Additionally, phenalkamine-cured epoxy resin composites with adequate thickness demonstrated excellent performance. Glass fiber-reinforced GEP-PA showed 5.3 times greater toughness and adhesion in end notch tests compared to GEP-TETA. These results offer valuable insights for material selection in aerospace and automotive applications. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymers (FRP) as an exterior reinforcement material are generally utilized to enhance the effectiveness of the current and new structures. FRP-confined concrete is characterized by its better mechanical properties. Accurately predicting the ultimate conditions and stress–strain responses of FRP-confined concrete make sense to achieve superior reliability and optimized functionality of structures. In this study, four prediction models based on machine learning, containing support vector regression (SVR), back-propagation neural network (BPNN), generalized regression neural network (GRNN) and extreme learning machine (ELM), were established, and their prediction performance were compared to achieve accurate prediction of the ultimate conditions of FRP-confined concrete. Moreover, a BPNN-based model to predict the stress–strain responses of FRP-confined concrete was proposed. A carefully evaluated and scrutinized database containing 384 FRP-confined concrete specimens under compressive load from various open-access sources was used to train these models. The results showed that these prediction models were more accurate in their predictions than the design-oriented models. Moreover, GRNN and SVR had superior prediction accuracy, followed by BPNN. The machine learning-based predictive models proposed in this study served as a valuable reference for the rapid prediction on the mechanical properties of FRP-confined concrete. [ABSTRACT FROM AUTHOR]

Concrete cover separation (CCS) is a common type of delamination failures that occurs at the plate end in reinforced concrete (RC) beams strengthened in flexure using fiber-reinforced polymer (FRP) materials. The effectiveness of FRP U-wrap anchorage has been demonstrated in mitigating CCS failure, resulting in increased utilization of FRP composites for strengthening purposes. The current study presents the development of a nonlinear three-dimensional finite element (FE) model using ABAQUS software in order to simulate the flexural behavior and debonding failure of FRP-strengthened RC beams end-anchored with vertical FRP U-wraps. The cohesive zone model (CZM) is used in the developed FE model to predict the debonding failure of the anchored beams, considering the influence of relevant parameters. The validation of the FE model was achieved through comparisons with experimental data from the literature, which demonstrated good agreement. A parametric study was then conducted to investigate the impact of the design parameters of FRP U-wrap on the behavior of the anchored beams. The examined parameters included the width, layout, and height of the vertical FRP U-wraps. The findings of the FE parametric study have revealed that to successfully prevent plate-end debonding, it is critical to not only use the required area of vertical FRP U-wrap but also carefully consider the U-wrap layout. The FRP U-wrap should be placed at the plate-end region covering the concrete tooth created by the shear cracks, and the U-wrap layout should consider the detailing recommended in this study. [ABSTRACT FROM AUTHOR]

In this study, a novel passive hybrid damper based on combinations of shape memory alloy (SMA), steel and glass fiber-reinforced polymer (GFRP) was introduced. The hysteretic properties of both SMA and steel and reversibility effect of both SMA and GFRP rods provide a superior passive damping device to dissipate large amounts of input excitation energy besides superior re-centering characteristics. A series of experiments to achieve material properties were carried out with the aim to incorporate accurate properties of triple materials utilized in analytical model. The non-dominated sorting genetic algorithm II was applied to evaluate the quantity of proportionality for each material used in the device, in terms of increasing energy absorption, improving reversibility and reducing manufacturing costs. Then, the optimal damper was constructed and tested based on results obtained from analytical modeling. The results showed the high efficiency of proposed hybrid damper to attain mentioned multi-objective goals. Subsequently, the application of damper in a prototype structure was investigated. The results of the analyses showed that the use of the damper has a significant effect on reducing seismic effects. Therefore, the damper can be used as a superior low-cost device in a retrofitting program for various types of structural systems in new and existing deficient buildings in earthquake-prone regions. [ABSTRACT FROM AUTHOR]

Instrumented impact testing and compression-after-impact testing are important to adequately qualify material behavior and safely design composite structures. However, the stresses to which fiber-reinforced plastic components are typically subjected in practice are not considered in the impact test methods recommended in guidelines or standards. In this paper, a test setup for investigating the impact behavior of composite specimens under plane uniaxial and biaxial preloading is presented. For this purpose, a special test setup consisting of a biaxial testing machine and a specially designed drop-weight tower was developed. The design decisions were derived from existing guidelines and standards with the aim of inducing barely visible impact damage in laminated carbon fiber-reinforced plastic specimens. Several measurement systems have been integrated into the setup to allow comprehensive observation of the impact event and specimen behavior. A feasibility test was performed with biaxially prestressed carbon fiber-reinforced plastic specimens in comparison with unstressed reference tests. The compressive-tensile prestressing resulted in lower maximum contact forces, higher maximum deflections, higher residual deflections and a different damage pattern, which was investigated by light microscopic analysis. Finally, the functionality of the experimental setup is discussed, and the results seem to indicate that the test setup and parameters were properly chosen to investigate the effect of prestresses on the impacts behavior of composite structures, in particular for barely visible subsequent damages. [ABSTRACT FROM AUTHOR]

This review paper presents the current progress in the development of resistance welding techniques for thermoplastic composites, with a particular emphasis on their application in hybrid joints, such as those involving thermosetting composites and metals. Resistance welding, a fusion bonding method, offers significant advantages over adhesive bonding and mechanical joining by eliminating the need for additional adhesive materials and enabling integration into automated manufacturing processes. The study highlights the unique benefits of resistance welding, including lower energy consumption compared to other methods and its compatibility with automated manufacturing, which can reduce production costs by up to 40%. Key findings from the literature indicate that resistance welding is particularly effective in achieving strong, durable joints for complex and large structures, such as those used in the aerospace industry. The review also identifies the main challenges associated with resistance welding, including temperature control, current leakage in carbon-fiber-reinforced polymers, and potential corrosion when using metal meshes. To address these challenges, various strategies are discussed, including surface treatments, the use of nanocomposites, and the integration of carbon nanotubes. The review concludes by emphasizing the need for further research to optimize welding parameters and to develop non-destructive testing methods for industrial applications, ensuring the reliability and long-term performance of welded joints. [ABSTRACT FROM AUTHOR]

This present research conducted an experimental study on the influence of different earthquake damages on the seismic performances of aged reinforced concrete (RC) beam-column joints retrofitted with basalt fiber-reinforced polymer (BFRP) sheets under cyclic loadings. Six beam-column joints included one benchmark specimen, one uncorroded specimen retrofitted with BFRP sheets, and four earthquake-damaged corroded specimens with the corrosion rates of 0% and 9% retrofitted with BFRP sheets. Test results demonstrated that the seismic performances of earthquake-damaged corroded specimens effectually improved after retrofitting with BFRP sheets, and the retrofitting effectiveness was significantly affected by the earthquake-damaged levels. The bucking of stirrups, spalling and crushing of concrete of retrofitted specimens were effectually postponed, while the ductility of corroded and uncorroded specimens under the earthquake damages increased after retrofitting with BFRP sheets. The total cumulative energy dissipation and final stiffness of retrofitted specimens considerably improved with the maximum increase of 22.3% and 14.3% comparing to those of benchmark specimen, respectively. Besides, the models for predicting the shear strength of RC beam-column joint cores considering the influences of earthquake damage, reinforcement corrosion, and FRP retrofitting were proposed. [ABSTRACT FROM AUTHOR]

The behaviour of lightweight concrete (LWC) edge column-slab connections reinforced with two types of flexural reinforcement glass fiber-reinforced polymer (GFRP) and steel bars with different ratios of shear reinforcement are investigated experimentally and analytically using ABAQUS software. The experimental protocol covered evaluation of eight slabs and edge columns measuring subjected to static loading. Slabs are evaluated as supported, free-standing at one edge, and the column connection is included. The slabs are divided into two groupings, with four slabs in each group. The first group was reinforced with steel flexural, while the second group was reinforced with GFRP reinforcement. Within each group, one slab lacked shear reinforcement, while the other was reinforced with a varying shear reinforcement ratio. A good agreement was found between the numerical FE model and experimental outcomes in most tested slabs. [ABSTRACT FROM AUTHOR]

Due to their advantages of high rupture strength and long service life, polymer fibers are often used for soil improvement. However, there is no consensus on how the mixing of discrete polymer fibers affects the stress–strain relationship of clays. In this study, a constitutive relationship of polymer fiber‒reinforced clay was established on the basis of the stress–strain relationship between clay and polymer fibers. The elastic–plastic unified hardening (UH) model was employed, and the fiber contribution was introduced based on the UH model. The constitutive relationship of polymer fiber‒reinforced clay considers the anisotropic distribution of the discrete fiber orientation and the relative sliding between the fibers and clay matrix. The model was verified by referring to the results of consolidated undrained (CU) and consolidated drained tests of typical polymer fiber‒reinforced clays in previous studies. A series of CU tests on rubber fiber‒reinforced clay were conducted to validate the model further. The ratio of the simulated results to the experimental results gradually approached 1 with increasing axial strain. The constitutive relationship of polymer fiber‒reinforced clay could provide satisfactory results. Practical Applications: Polymer fiber mixing increases soil strength and enhances the properties of problematic soils, which makes the problematic soils more valuable for engineering applications. Studies have shown that the fibers in the soil tend to be distributed horizontally after the compaction process. With the anisotropic distribution of fiber orientation considered, the authors established a numerical calculation method for the stress–strain relationship of polymer fiber‒reinforced clay. A major objective of this work was to allow the use of computerized numerical analysis methods when performing mechanical analyses of polymer fiber‒reinforced clay, which avoids the need to conduct a large number of shear tests. In this study, a series of consolidated undrained tests of rubber fiber‒reinforced expansive clay were conducted. With the data collected, the numerical calculation method for the stress–strain relationship of polymer fiber‒reinforced clay was verified, and the numerical results agreed with the test results better. [ABSTRACT FROM AUTHOR]

This study aimed to enhance the structural performance of a drift joint with a slotted-in steel plate using laminated timber made from small- to medium-diameter timbers. An analysis was conducted to compare and evaluate the structural performance and seismic behavior of laminated timber portal frames reinforced with fiber-reinforced plastic (FRP) laminates, such as glass fiber cloth (GFC), glass fiber reinforced plastic sheet (GS), and carbon-reinforced plastic sheet (CS), under low amplitude cyclic lateral loads, in comparison to unreinforced portal frames (UR). The reinforced portal frame, strengthened by FRP laminates, exhibited higher resistance to strength and stiffness strength degradation due to cyclic loading-induced degradation at low drift angles compared to the unreinforced frame affected by heterogeneous timber. The reinforced portal frame, particularly the GFC and GS, exhibited energy dissipation rates 32% and 17% higher than UR, respectively, making them the most effective in vibration damping. FRP laminates mitigated fiber direction cleavage from drift pin holes. The average maximum moment and average yield moment of the reinforced portal frame increased by 18% and 21%, respectively, due to the alleviation of joint failure. Finally, CS significantly affected the maximum moment, while GFC significantly influenced the yield moment. [ABSTRACT FROM AUTHOR]

Embedded fiber Bragg gratings are increasingly applied for in-situ strain measurement in fiber-reinforced plastics, integral to high-end aerospace equipment. Existing research primarily focuses on in-plane strain measurement, limited by the fact that fiber Bragg gratings are mainly sensitive to axial strain. However, out-of-plane strain measurement is equally important for comprehending structural deformation. The birefringence of fiber Bragg gratings shows promise for addressing this problem; yet, the strain transfer relationship between composites and optical fibers, along with the decoupling method for multi-directional strains, remains inadequately explored. This study introduces an innovative method for multi-directional strain measurement in fiber-reinforced plastics using the birefringence of a single-fiber Bragg grating. The strain transfer relationship between composites and embedded optical fibers was derived based on Kollar's analytical model, leading to the development of a multi-directional strain decoupling methodology. This method was experimentally validated on carbon fiber/polyetherimide laminates under thermo-mechanical loading. Its reliability was confirmed by comparing experimental results and finite element simulations. These findings significantly broaden the application scenarios of fiber Bragg gratings, advancing the in-situ measurement technology crucial for the next generation of high-end aerospace equipment. [ABSTRACT FROM AUTHOR]

Bond strength is the key factor to determine the strengthening performance of fiber-reinforced polymer (FRP) and concrete, so it is important to establish a bond strength prediction model. To solve the problem that the parameters considered are not comprehensive enough and the accuracy needs to be improved, multivariate adaptive regression splines (MARS) and wavelet neural network (WNN) methods are proposed to estimate the bond strength of concrete confined by FRP. A total of 749 specimens and ten input variables collected from the literature were used to develop the MARS and WNN models. To verify the prediction performance of the proposed models, an improved particle swarm optimization method based on the shrinkage factor method was used to calibrate ten empirical models. The recalibrated empirical models were then compared with MARS and WNN models. The root mean squared error (RMSE), mean absolute error (MAE), coefficient of determination (R2), root relative squared error (RRSE), mean absolute percentage error (MAPE) and integral absolute error (IAE) values of MARS and WNN models were 3.454, 2.635, 0.917, 0.289, 0.153 and 0.127; 3.277, 2.516, 0.925, 0.274, 0.164 and 0.122, respectively, for the training datasets and 3.454, 2.701, 0.94, 0.245, 0.153 and 0.121; 3.652, 2.751, 0.933, 0.259, 0.184 and 0.123, respectively, for the testing datasets. While the RMSE, MAE, R2, RRSE, MAPE and IAE values of the training set and test set of the ten recalibrated empirical models are 7.081–11.293, 4.386–6.994, 0.410–0.701, 0.571–0.807, 0.215–0.306 and 0.212–0.312, respectively. The results show that the accuracy of MARS and WNN models is much higher than that of the ten recalibrated empirical models. [ABSTRACT FROM AUTHOR]

This attempt covers an investigation of cutting temperature at interfaces of Fiber Metal Laminates (FMLs) made of glass fiber-reinforced polymer (GFRP) stacked with an Al2020 alloy. GFRP/Al/GFRP and Al/GFRP/Al composite stacks are both investigated to highlight the effect of stacking arrangement on thermal behavior within the interfaces. In a first test series, temperature history is recorded within the metal/composite stack interfaces using preinstalled thermocouples. In a second test series, a wireless telemetry system connected to K-type thermocouples implanted adjacent to the cutting edge of the solid carbide drill is used to record temperature evolution at the tool tip. Focus is put on the effects of cutting speed and stacking arrangement on the thrust force, drilling temperature, and delamination. From findings, the temperature histories show high sensitivity to the cutting speed. When cutting Al/GFRP/Al, the peak temperature is found to be much higher than that recorded in GFRP/Al/GFRP and exceeds the glass transition point of the GFRP matrix under critical cutting speeds. However, thrust force obtained at constitutive phases exhibits close magnitude when the stacking arrangement varies, regardless of cutting speed. Damage analysis is also discussed through the delamination factor at different stages of FML thickness. [ABSTRACT FROM AUTHOR]

This study systematically investigates the enhancement effects of different fiber-reinforced polymer (FRP) materials on the axial compressive performance of concrete. Through experimental evaluations of single-layer, double-layer, and composite FRP reinforcement techniques, the impact of various FRP materials and their combinations on concrete's axial compressive strength and deformation characteristics was assessed. The results indicate that single-layer CFRP reinforcement significantly improves concrete axial compressive strength and stiffness, while double-layer CFRP further optimizes stress distribution and load-bearing capacity. Among the composite FRP reinforcements, the combination with CFRP as the outer layer demonstrated superior performance in enhancing the overall structural integrity. Additionally, numerical analyses of the mechanical behavior of the reinforced structures were conducted using ABAQUS 2023HF2 finite element software, which validated the experimental findings and elucidated the mechanisms by which FRP influences the internal stress field of concrete. This research provides theoretical support and empirical data for the optimized design and practical application of FRP reinforcement technologies in engineering. [ABSTRACT FROM AUTHOR]

In this study, the transverse shear strength (TSS) retention of two types of new-generation glass fiber-reinforced polymer (GFRP) bars, namely ribbed (RB) and sand-coated (SC) bars, was investigated under alkaline, acidic, and marine conditions in both high-temperature and laboratory environments for up to one year. The ribbed GFRP bars exhibited no notable reduction in strength under ambient conditions after 12 months, but under high-temperature conditions (60 °C), they showed TSS reductions of 10.6%, 9.7%, 11.1%, and 10.9% for exposure solutions E1, E2, E3, and E4, respectively. The sand-coated GFRP bars showed slight strength reductions under ambient conditions and moderate reductions under high-temperature conditions (60 °C), with TSS reductions of 22.5%, 29.0%, 13.0%, and 13.7% for the same solutions, highlighting the detrimental effect of high temperatures on the degradation of the resin matrix. Comparative analyses of older-generation ribbed (RB-O1 and RB-O2) and sand-coated (SC-O) GFRP bars exposed to similar conditioning solutions for the same duration were also performed. In addition, linear regression and artificial neural network (ANN) models were developed to predict strength retention. Models developed using linear regression and ANNs achieved coefficients of determination (R2) of 0.69 and 0.94, respectively, indicating that the ANN model is a more robust tool for predicting the TSS of GFRP bars than is the conventional linear regression model. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymers (FRPs), which are resistant to corrosion, are used as reinforcement material for concrete. However, the flexural behavior of concrete members reinforced with FRPs can vary depending on the properties of FRPs. In this study, the flexural behavior of one-way concrete slab specimens reinforced with a new grid-type carbon-fiber-reinforced polymer (CFRP) (carbon grid) manufactured by bonding pultruded CFRP strands to an adhesive was investigated. The experimental results indicated differences in the load–deflection relationships of the specimens depending on the carbon grid reinforcement amount. Specimens in which the carbon grids were over-reinforced or reinforced close to the balanced reinforcement ratio reached the maximum load due to concrete crushing and exhibited ductile failure. The specimen under-reinforced with the carbon grid exhibited brittle failure. Specimens with carbon grid reinforcement close to a balanced reinforcement ratio exhibited maximum loads ranging from 0.43 to 0.61 times the calculated flexural strength, which resulted in becoming 0.86–1.00 lower in the specimens with a wider width of the CFRP strands. This study proposes coefficients to estimate the stiffness of carbon-grid-reinforced concrete flexural members after cracking. Applying these coefficients resulted in stiffness calculations that reasonably simulated the behavior of the specimens reinforced with carbon grids after crack formation. [ABSTRACT FROM AUTHOR]

Concrete canvas is a composite material made with three-dimensional spacer fabric, which is manufactured by impregnating the fabric with cementitious materials. Concrete canvas has weaknesses in its mechanical properties including tensile strength. Strengthening concrete canvas with Fiber-reinforced polymer (FRP) materials can effectively improve its mechanical properties, particularly its tensile strength. Therefore, this study focuses on examining the impact of using FRP polymers to enhance the tensile properties of concrete canvas. Three types of fibers, namely Glass Fiber Reinforced Polymer (GFRP), Carbon Fiber Reinforced Polymer (CFRP) and Aramid Fiber Reinforced Polymer (AFRP; known as a Kevlar fiber) in various layer configurations, both warp and weft, were utilized to reinforce concrete canvas under tensile loading. The test results demonstrate that the use of FRP fibers significantly enhances the mechanical properties of concrete canvas. Specifically, single-layer glass fibers lead to considerable growth and reinforcement with carbon and Kevlar fiber layers results in more than five to seven times improvement in tensile strength compared to control concrete canvas. [ABSTRACT FROM AUTHOR]

Because of their improved properties, FRP composites are vastly used in the strengthening of aged concrete infrastructures. However, it has been observed that their performance is highly compromised when exposed to high temperatures, as expected during fire incidents, which critically affects FRP–concrete bond behavior, hence affecting the overall efficiency of the strengthening system. This paper critically presents the available literature concerning the degradation of bond strength between FRP systems with concrete substrates due to increased temperatures. Both analytical and numerical bond–slip models developed for the prediction of bond strength degradation under such conditions are reviewed. A generally confirmed fact is that exposure to high temperatures, especially those reaching glass transition temperature (Tg) for epoxy adhesives, leads to bond degradation. Therefore, cement mortar-bonded CFRP textiles display better performance in fire endurance. This present paper also utilizes machine learning algorithms for the prediction of bond strength under elevated temperatures based on an experimental database of 37 beams. The nonlinear relationships and variable interactions in the developed model provide a reliable method for the estimation of bond strength with reduced extensive experimental testing, where the critical role of temperature in bond behavior is identified. This paper emphasizes the use of advanced predictive models to ensure the durability and safety of FRP-strengthened concrete structures in thermally challenging environments. [ABSTRACT FROM AUTHOR]

The study focuses on developing a fiber-reinforced composite that would exhibit good mechanical properties and climate resistance, and fire safety parameters would surpass commonly used fiber-reinforced polymers. The subject of development is a polysiloxane thermoset matrix reinforced with basalt fibers, which is processed by partial pyrolysis at 650 °C after curing. The heat release rate test showed virtually zero heat released, and the toxicity test showed only a very low amount of carbon monoxide. The flammability test showed no ignition, no radiation, and no glow. Composites for mechanical tests were prepared in three variants differing in the distribution of reinforcement. Due to the intended use of the composite for thin-walled panels or shells of buildings, the mechanical properties were compared in identical tests with fiber cement plates. The flexural strength of the composites was 3 to 10 times, and the impact energy was 10 to 100 times higher than the values measured on fiber cement, depending on the type and orientation of the composite. The flexural strength measured after 240 freeze–thaw conditioning cycles is higher than fiber cement by 1.3 to 2 times. The climate resistance of the composite should be the subject of further development. [ABSTRACT FROM AUTHOR]

Evaluating the mechanical response of fiber-reinforced composites can be extremely time-consuming and expensive. Machine learning (ML) techniques offer a means for faster predictions via models trained on existing input–output pairs and have exhibited success in composite research. This paper explores a fully convolutional neural network modified from StressNet, which was originally used for linear elastic materials, and extended here for a non-linear finite element (FE) simulation to predict the stress field in 2D slices of segmented tomography images of a fiber-reinforced polymer specimen. The network was trained and evaluated on data generated from the FE simulations of the exact microstructure. The testing results show that the trained network accurately captures the characteristics of the stress distribution, especially on fibers, solely from the segmented microstructure images. The trained model can make predictions within seconds in a single forward pass on an ordinary laptop, given the input microstructure, compared to 92.5 h to run the full FE simulation on a high-performance computing cluster. These results show promise in using ML techniques to conduct fast structural analysis for fiber-reinforced composites and suggest a corollary that the trained model can be used to identify the location of potential damage sites in fiber-reinforced polymers. [ABSTRACT FROM AUTHOR]

Terahertz (THz) computed tomography (THz CT) exhibits the potential to provide a wealth of data, surpassing that of THz tomographic imaging in applications such as detecting embedded defects, particularly defect evolution within a glass fiber-reinforced polymer. To realize high-resolution THz CT, a systematic approach guided by wave propagation simulation was employed. First, the front wave of the THz beam was fine-tuned to realize a beam diameter of < 2 mm. To mitigate the strong refractive effect and minimize Fresnel reflection loss, a refractiveindex-matching material was fabricated and utilized as a rounded enclosure for samples with sharp corners. To further improve the reconstruction resolution, a flat surface enclosure was applied to collect all incident beams at the detector. To realize comparable results to those of full-angle CT, a limited-angle CT approach was implemented, and the frequency range 0.1-3 THz was used in the image reconstruction study. The experimental and simulated images were used to validate the findings, and conductive and non-conductive defects measuring 200 µm were successfully visualized. Additionally, a custom-built user interface enabled us to visualize the field spatial distribution of the THz beam with respect to frequency. [ABSTRACT FROM AUTHOR]

Bubbled Reinforced Concrete (RC) slabs have gained popularity in recent years as a practical construction method that eliminates unnecessary concrete in the center, thereby reducing the dead weight of the structure. This study provides a systematic framework to compare the performance and capabilities of one-way bubbled concrete slabs reinforced with Glass Fiber Reinforced Polymer (GFRP) bars and embedded steel I-sections. Four one-way concrete slabs, each with a length of 2,600 mm and a rectangular cross-sectional area of 600 mm in width and 150 mm in depth were employed. These slabs were reinforced with Glass Reinforced Plastics (GRP) rebar at the same reinforcement ratio and tested by two-point bending to failure. Different parameters such as specimen type (solid or bubbled slabs) and internal reinforcement were achieved using steel I-sections in two different shapes (2 and 4 pcs of steel I-sections), where 4I-section shape with a cross-sectional area equivalent to 2I-section shape, channel shear connectors, and bent steel bars (10 mm diameter) were used to improve the shear resistance. The results showed that bubbled slabs experienced a higher range of deformations (including deflection, strains, and cracks) by about (28%-88%) and a 15% decrease in ultimate load capacity compared to solid slabs. On the other hand, the use of steel I-sections as internal reinforcement significantly improved the specimen performance compared to unreinforced slabs (Steel Slab (SS) and Bubbled Slab (BS), respectively). Deflection was reduced by approximately 52% and 87% at the same load level, ultimate load capacity increased by approximately 121% and 179%, and flexural stiffness increased by approximately 197% and 272% at the same load level. [ABSTRACT FROM AUTHOR]

Structural engineers are increasingly favoring pultruded Glass Fiber-Reinforced Polymer (pultruded GFRP) composite for its lightweight, corrosion resistance, and high strength properties. The utilization of the GFRP material in reinforced concrete structures is not yet well-defined due to a lack of scientific evidence. The study focuses on the structural performance of composite beams made of encased GFRP sections and rebar encased in reinforced concrete. This study highlights the structural significance of embedding GFRP sections in concrete beams composed of GFRP rebars and normal reinforced concrete. To achieve this goal, five different specimens were tested and analyzed under two points of static loading. The experimental program consisted of one reference beam, without the encased GFRP I-section, and four hybrid beams. The study involved installing two types of shear connectors on composite beams with GFRP I-sections to analyze their impact on shear capacity and slide resistance. The experimental findings revealed that encasing the composite beams with GFRP improved their load-bearing and energy dissipation capabilities. Additionally, the shear connectors enhanced the ultimate capacity and eliminated slipping failures. Therefore, there was a strong agreement on the numerical results demonstrating the significance of this work. [ABSTRACT FROM AUTHOR]

Hybrid fiber-reinforced polymer (FRP)–concrete–steel hybrid double-skin tubular columns (DSTCs) consist of an outer FRP tube, an inner steel tube, and a layer of concrete filled between the two tubes. Previous studies have demonstrated their structural performance compared with conventional concrete columns. However, their durability performance, particularly in aggressive environments, is not well understood. Therefore, this study aims to investigate the durability of hybrid DSTCs subjected to a wet–dry cyclic environment for up to 2 years. The time-dependent behaviors, including axial load–strain (axial and hoop) curves, ultimate load, and ultimate axial and hoop strains, are tested and discussed with regard to the aging time. The test results indicate that the ultimate load of hybrid DSTCs with a 6-mm glass fiber–reinforced polymer (GFRP) tube continuously increased by 15.1% after 2 years of exposure primarily owing to the increase in concrete strength. By contrast, the ultimate load of hybrid DSTCs with a 3-mm GFRP tube increased by 15.2% after 1 year of exposure and increased by 11.6% after 2 years of exposure. The results indicate that the reductions can be attributed to the degradation of the GFRP tubes, especially for a thinner GFRP tube. In addition, a design-oriented stress–strain model for concrete in DSTCs is verified against the test results. [ABSTRACT FROM AUTHOR]

This paper presents a framework for assessing the reliability of fiber reinforced polymer (FRP)–strengthened reinforced concrete (RC) beams in flexure using stochastic nonlinear finite-element (SNFE) analysis and k-w-means clustering, based on active-learning kriging Monte Carlo simulation (AK-MCS), in which spatial variations in the concrete and bond material properties are considered. A computer algorithm was developed to augment commercially available nonlinear finite-element (FE) analysis software and automate the process for conducting the SNFE clustering-based AK-MCS analysis. The k-w-means clustering was based on the U learning function to provide multipoint enrichment to improve convergence of the stopping criteria by allowing parallel computation of the SNFE models. Parametric analysis indicated the accuracy of the reliability prediction of the examined member and proved the efficiency of the proposed analysis in reducing the number of calls to SNFE models compared with data in the existing literature, when using probability-based stopping criteria. Practical Applications: The quality of the FRP-to-concrete bond is affected by the integrity of the concrete at the interface, which varies across the dimensions of the strengthened member, causing added uncertainty in predicting the structural response, and hence the reliability of the FRP-strengthened member. This study proposes a computationally efficient approach to assess the reliability of FRP-strengthened concrete members by considering the spatial variation in the concrete properties (compressive strength, tensile strength, bulk modulus) and the quality of the FRP-to-concrete bond (shear and normal bond strength) by using an adaptive machine-learning technique. The proposed framework may be utilized by engineers to design FRP-strengthening systems for concrete members experiencing variation in the concrete properties due to poor quality control or active deterioration. [ABSTRACT FROM AUTHOR]

The low elastic modulus of glass fiber–reinforced polymer (GFRP) materials used in civil engineering may lead to insufficient structural stiffness in GFRP–aluminum space truss structures, limiting their ability to meet the service limit state requirements. To enhance flexural stiffness, a prestressed carbon fiber–reinforced polymer (CFRP) tendon system was developed and demonstrated. Full-scale three-point bending tests were performed to evaluate the flexural response of GFRP space truss girders, both with and without CFRP tendons. Four prestressing schemes were investigated, revealing the effect of the tendon system in enhancing stiffness. A simplified, design-oriented theoretical model using the equivalent continuum method and the force method was developed to aid structural design calculations. The model's formulas account for variable joint stiffness and equivalent shear deformation, enabling accurate stiffness evaluations. Parametric analyses were conducted on the prestress level, the girder-to-tendon stiffness ratio, and the geometric parameters of the CFRP tendon system. The results indicated that the four prestressing schemes enhanced the flexural stiffness and reduced the internal forces, validating the effectiveness of the novel prestressed FRP space truss structure. The proposed model accurately describes the prestressing enhancement mechanism and offers theoretical support for structural design. [ABSTRACT FROM AUTHOR]

This paper presents the proposed indeterminate strut and tie (IST) method to analyze fiber-reinforced polymer (FRP)-reinforced concrete deep beams with stirrups. The solution of the IST method does not require the assumption of reinforcement yielding and is capable of reflecting the material nonlinearity of concrete. The essential features of the IST methodology for glass FRP-reinforced deep beams are discussed and then analyzed in detail. These features include modeling concrete compressive response, softening concrete compression behavior in struts, and obtaining the depth of compression nodes. For each of these features, several approaches are discussed. The proposed IST methodology is validated by comparing the analysis results with the experimental results, and recommendations for the most suitable modeling methods are proposed. [ABSTRACT FROM AUTHOR]

One of the main barriers to the acceptance and adoption of fiber-reinforced polymers (FRPs) in structural engineering is the brittle failure mode and limited ductility caused by the elastic-brittle behavior of FRP materials. Introducing a compression-yield (CY) system is a promising way to improve the ductility of the structures. This study adopted perforated steel cylinders as CY systems. Six perforated steel cylinders with different parameters were tested, and the applicability of the existing constitutive model on the perforated steel cylinders was verified. A design approach for perforated steel members on the structures was proposed. Based on the proposed design approach, ultralightweight cement composite (ULCC)-filled carbon fiber-reinforced polymer (CFRP)-wrapped steel tube arches with perforated steel cylinders were designed and experimentally investigated. The ULCC-filled CFRP-wrapped steel tube arches with CY systems achieved high ductility while maintaining favorable bearing capacity. A model was then proposed to evaluate the damage degree of the structures based on the longitudinal strain in the CY system. The damage development of the structures can be detected based on the measurement of the CY system, which enables early warning to be provided before structural failure. [ABSTRACT FROM AUTHOR]

This experimental investigation is the first of its kind to evaluate the effects of a combination of long-term loading and marine exposure on the structural response of carbon fiber–reinforced polymer (CFRP) and glass fiber–reinforced polymer (GFRP) tubular composites infilled with seawater and sea sand-based ultrahigh-performance concrete (UHP-SWSSC). The specimens were conditioned under natural seawater exposure at 40°C, 3,000 h of sustained loading, and a combination of the two. Long-term strains indicate that the composite tubes (especially GFRP) experienced swelling due to marine exposure, even under sustained load, suggesting gradual detachment of the tubes from the ultrahigh-performance concrete (UHPC) core. Considerable hoop strength and stiffness of the confining fiber-reinforced polymer (FRP) were necessary to restrain the dilation of UHPC, which was mechanically improved by sustained loading and seawater exposure. Lateral confining pressure and stiffness of FRP correlated with improved confined UHPC properties. The FRP tubes utilized their full material capacity only at a sufficiently high level of confinement to efficiently restrain the UHPC core. Despite the increase in compressive strength, marine exposure reduced the ductility of the FRP–UHPC composites, particularly in those containing GFRP. The gradual stress redistribution from the inner concrete to the outer FRP tubes facilitated better utilization of FRP confinements after creep (with or without seawater exposure). [ABSTRACT FROM AUTHOR]