Search Results (569)

Existing analytical models for Fiber-Reinforced Polymer (FRP)-strengthened timber beams are generally limited to individual strengthening techniques and cannot readily accommodate hybrid reinforcement systems. This study develops a generalized analytical model to predict the flexural capacity of timber beams strengthened with near-surface-mounted (NSM) FRP bars, externally bonded FRP sheets, or their hybrid combination within a unified theoretical framework. The model is formulated based on internal force equilibrium and strain compatibility, incorporating a constitutive model for timber with linear elastic tensile behavior and a bilinear compressive stress–strain relationship including post-peak softening. The generalized formulation can be readily adapted to different strengthening configurations through appropriate simplifications. The proposed model was validated against experimental results obtained from four-point bending tests on small-scale timber beams strengthened with NSM GFRP bars and externally bonded GFRP sheets. The analytical predictions showed good agreement with the experimental results, with differences generally ranging from 2% to 23%, demonstrating satisfactory predictive accuracy. The experimental results further showed that the hybrid strengthening system increased the flexural capacity of the timber beams by up to 84% compared with the unstrengthened control beams, while also improving stiffness, ductility, and overall structural response. Failure was primarily due to timber tensile rupture and longitudinal splitting, whereas the GFRP reinforcement remained effective without rupture, indicating efficient utilization of the strengthening system. The proposed generalized analytical model provides a practical and reliable design tool for predicting the flexural strength of timber beams strengthened with various FRP reinforcement configurations, thereby supporting the structural rehabilitation and sustainable retrofitting of timber structures. Full article

In this study, the seismic vulnerability of the Ferdowsi School building in Tabriz is investigated. The research began with comprehensive fieldwork, during which exploratory surveys and in-depth technical inspections of all structural components were performed. Experimental testing of prismatic masonry specimens was carried out to evaluate their mechanical characteristics, and the resulting properties were then incorporated as input parameters into the numerical model. The seismic vulnerability assessment was then carried out using nonlinear static (pushover) analysis, applying a lateral load pattern proportional to the first vibration mode of the structure. For numerical simulation, the building was modeled in the ABAQUS finite element software using the macro-modeling technique. The results of the nonlinear static analysis indicated that the building does not possess sufficient load-bearing capacity at the target displacement. Damage was primarily concentrated in the form of cracking in the masonry walls as well as in the dome-shaped sections of the roof, requiring the implementation of a seismic retrofitting scheme to enhance the structure’s seismic performance. To rehabilitate the structure, horizontal and vertical reinforced concrete beams were introduced as confining elements for the masonry walls and subsequently applied in the strengthening project. Furthermore, due to the presence of a domed roof at the first-floor level, it was strengthened using FRP composite materials to enhance tensile capacity and ductility. At the second-floor level, where the roof structure is made of timber elements, a steel cable system was employed to improve its strength and diaphragm action. As for the third-floor timber truss roof, the connections were upgraded and reinforced to provide reliable force transmission and to maintain the overall integrity of the structural system. Following the implementation of the retrofitting measures, the structural model was re-analyzed using nonlinear static analysis. The results demonstrated that the proposed strengthening scheme successfully increased the structural capacity up to the target displacement level and satisfied the intended performance requirements. In the final section of the paper, the implementation details of the retrofitting interventions, as well as the practical experiences gained during the implementation process, are presented and discussed. Full article

This study presents an approach to quantifying impact-induced damage severity in composites, focusing on synthetic carbon fibre-reinforced polymer (CFRP), natural flax fibre-reinforced polymer (FFRP) and hybrid fibre reinforced polymer (HFRP) composite of carbon and flax. The investigation aims to quantitatively characterise impact damage under energies ranging from 10 to 70 J through acousto–ultrasonics (AU) testing, proposing an efficient technique for evaluating the integrity of various FRP composites under in-service conditions. AU testing was performed at azimuthal angles of 0°, 30°, 45°, 60° and 90°, utilising acousto–ultrasonic waveform indices (AUWIs), such as wave velocity, peak amplitude, energy content, centroid frequency and skewness factor. The damage severity index is correlated with the damage mode. The findings establish that wave velocity is a reliable parameter for quantifying damage severity across all composite material types considered, with high adjusted R² values of 0.92 for CFRP, 0.89 for FFRP and 0.90 for HFRP. Peak amplitude also shows considerable sensitivity. Finally, this research highlights the limitations of traditional non-destructive evaluation (NDE) techniques and demonstrates the potential of combining multi-damage metrics with advanced imaging methods, such as X-ray micro-computed tomography (X-ray µCT) and scanning electron microscopy (SEM), to provide a comprehensive assessment of damage in various composite materials. The proposed methodology offers a promising approach for quantifying the impact damage severity in composite structures, as applicable to wind turbine blades, amongst other structural components. Full article

Fiber-reinforced polymer (FRP) composites are extensively used in aerospace, civil engineering, and defense applications because of their low density, high specific strength, corrosion resistance, and structural design flexibility. However, prolonged exposure to hygrothermal conditions, ultraviolet (UV) radiation, and thermo-oxidative environments can progressively damage these materials, leading to mechanical degradation and shortened service life. This review examines environmental aging in FRP composites at the levels of the polymer matrix, fiber/matrix interface, and reinforcing fibers. Representative predictive models, finite element methods, and experimental characterization techniques are summarized, together with the evolution of mechanical properties under different aging conditions. Hygrothermal degradation is mainly associated with moisture diffusion, matrix swelling, and interfacial debonding, whereas UV and thermo-oxidative aging are largely governed by photo-oxidation and thermally activated free-radical reactions. These processes may induce chain scission, crosslinking, matrix embrittlement, and interface damage. Under coupled environmental exposure, degradation is not simply additive because moisture transport, oxidation kinetics, and failure pathways may interact. Future research should emphasize multiscale characterization, anti-aging modification, interface engineering, protective coatings, and reliability-oriented lifetime prediction. Full article

Fiber-reinforced polymer (FRP) composites provide an effective strengthening solution for timber members because of their high tensile capacity, low self-weight, corrosion resistance, and practical applicability in rehabilitation works. Although FRP strengthening of timber beams has been widely investigated, most available experimental evidence concerns softwood and glued-laminated systems, whereas comparatively limited data are available for dense tropical hardwoods used in marine and waterfront infrastructure. This study investigates the flexural behavior of Azobé (Lophira alata) hardwood beams strengthened with externally bonded carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) laminates. The main contribution of this work is the application of externally bonded FRP strengthening to Azobé timber members intended for marina pontoon and related waterfront applications, where structural upgrading may be required to accommodate increased service loads. Mechanical characterization of the timber was first conducted through compression and tensile tests. Subsequently, nine beams were tested under three-point bending, including three un-strengthened reference beams, three GFRP-strengthened beams, and three CFRP-strengthened beams. The average ultimate load increased from 26.92 kN for the reference beams to 35.59 kN and 39.85 kN for the GFRP- and CFRP-strengthened beams, respectively. Statistical indicators, including standard deviation, coefficient of variation, standard error, confidence intervals, and two-sample t-tests, were included to account for the limited number of specimens and the natural variability of timber. CFRP exhibited the highest mean response within the present test series; however, the difference between the CFRP- and GFRP-strengthened beams is interpreted as an indicative experimental trend rather than a general statistical conclusion. No visible premature de-bonding was observed, and the strengthened specimens failed mainly by FRP rupture, suggesting bond engagement under the tested configuration. Nevertheless, bond behavior was not directly quantified using strain, slip, or interfacial measurements. The experimental results were also compared with analytical predictions based on the Italian guideline CNR-DT 201/2005 and with a simplified section-level interpretation. Overall, the findings indicate that externally bonded FRP laminates can provide a practical upgrading solution for existing Azobé timber members in marina pontoon and waterfront structures, while larger experimental series and direct bond/strain measurements are required for broader validation. Full article

Designing lightweight composite sandwich structures is challenging due to the conflicting objectives of minimizing structural weight and cost while satisfying strength and stiffness requirements. The optimization procedure becomes more complex when multiple discrete design variables and nonlinear material behavior are involved. This study presents a newly developed optimization methodology for a sandwich structure composed of Fiber Reinforced Polymer (FRP) laminated facesheets and an aluminum honeycomb core. To reduce the computational cost associated with repeated high-fidelity Finite Element (FE) analyses, a surrogate modeling strategy based on Artificial Neural Networks (ANNs) is employed to approximate the structural response. The applied dataset is generated using Monte Carlo simulation in which combinations of design variables are used as inputs, and the corresponding structural responses obtained from the analytical formulation are used as outputs for training the ANN surrogate model. The trained ANN model is integrated with a Multi-Objective Niching Memetic Particle Swarm Optimization (MO-NMPSO) algorithm to simultaneously minimize structural weight and material cost while satisfying constraints on facesheet strength, wrinkling, intra-cell buckling, deflection, core shear failure and structural thickness. The resulting Pareto-optimal solutions are validated through detailed FE simulations, demonstrating the reliability of the newly elaborated optimization framework. The results of the newly developed computationally efficient optimization procedure provide a diverse set of optimal design solutions for the investigated sandwich structure. Full article

Pultrusion is an efficient continuous manufacturing process for fiber-reinforced polymer (FRP) composites, but conventional open-bath impregnation has limitations such as resin exposure, quality variation, and resin loss. To overcome these limitations, closed-injection pultrusion (CIP) and short-pot-life resin systems have recently been introduced. However, the effects of processing variables on the quality and properties of composites manufactured using such resin systems have not been fully clarified. In this study, the effects of resin viscosity and pulling speed on the quality and mechanical properties of carbon FRP (CFRP) and glass FRP (GFRP) composites manufactured by CIP were investigated. CFRP and GFRP composites were fabricated at resin temperatures of 30 and 40 °C and pulling speeds of 300, 400, and 500 mm/min. The manufactured composites were evaluated in terms of void content, microstructure, hardness, and tensile properties. The results showed that increasing pulling speed increased void content and promoted macrovoids and locally poor impregnation, whereas the influence of resin temperature was relatively limited. Hardness, tensile strength, and elastic modulus decreased as pulling speed increased. These results demonstrate that CFRP and GFRP composites can be successfully manufactured by CIP using short-pot-life resin systems, and that precise control of resin viscosity and pulling speed is essential for achieving high quality and mechanical performance. Full article

Unsaturated polyester (UP) composites are widely utilized in engineering applications, including vehicle body structures, due to their ease of processing and good interfacial compatibility with natural fibers. However, the inherent brittleness of UP limits its performance under impact or tensile loading, as it exhibits minimal plastic deformation and is prone to crack initiation and propagation. In this study, bamboo fiber was incorporated into the UP matrix at various mixing ratios to enhance its crack resistance. After achieving uniform dispersion, the composites were subjected to a splitting tensile test to evaluate their crack resistance behavior. The results indicate that the composite containing 80% polyester exhibits the highest fracture toughness, with a crack resistance value of K_1C = 1.396 MPa·m^0.5. This value represents a 192.03% improvement compared with neat polyester (K_1C = 0.713 MPa·m^0.5). The enhanced crack resistance is attributed to the fiber bridging and energy-absorption mechanisms introduced by the bamboo fibers. These findings demonstrate the effectiveness of bamboo fiber reinforcement in improving the fracture performance of UP-based composites, highlighting their potential for use in lightweight structural applications. Full article

Basalt fiber-reinforced polymer (BFRP) composites are increasingly considered as a sustainable alternative to traditional FRP systems for the strengthening of reinforced concrete (RC) structures, owing to their favorable mechanical properties, durability, and lower environmental impact. This study investigates the effectiveness of externally bonded BFRP strips for the strengthening of RC beam–column joints, with particular attention to the influence of strengthening layout on the structural response. An experimental program was carried out on full-scale RC beam–column joint specimens subjected to monotonic loading with load–unload cycles of increasing amplitude. Each specimen was first tested in its original configuration to induce controlled damage and subsequently strengthened using BFRP strips arranged according to two different layouts. This approach enabled a direct comparison between the behaviour of pre-damaged and retrofitted specimens and allowed the contribution of the BFRP reinforcement to be clearly identified. BFRP strengthening markedly improves joint performance, enhancing strength, ductility, and energy dissipation while limiting stiffness degradation. The results underline the critical role of the strengthening layout in governing the effectiveness of the composite system, as well as the influence of substrate cracking in the activation of the BFRP reinforcement. Full article

Glass fiber-reinforced polymer (GFRP) composites offer a compelling solution to the durability degradation of reinforced concrete (RC) structures in harsh marine and de-icing environments. Hybridizing fiber-reinforced polymer (FRP) with conventional steel reinforcement synergizes the superior corrosion resistance of FRP with the high ductility of steel. However, the synergistic mechanisms of GFRP–steel hybrid reinforced columns confined by either GFRP or steel spiral stirrups under axial compression remain insufficiently quantified. This study systematically investigates the axial compressive performance of such structures through material testing, static axial compression tests on seven short column specimens, and advanced finite element (FE) modeling. The investigation focuses on the effects of the steel-to-GFRP area ratio and the spiral stirrup type. Experimental results reveal that spirally confined hybrid columns exhibit failure modes remarkably similar to conventional RC columns. The incorporation of GFRP bars significantly enhanced the ultimate load-bearing capacity, while the steel bars ensured the requisite ductility. Notably, a higher ultimate capacity was achieved at a steel-to-GFRP area ratio of 1:1 under steel spiral confinement, retaining a ductility index equivalent to 83.6% of a pure RC column. Furthermore, an ABAQUS-based FE model was developed and rigorously validated against experimental data, successfully capturing the failure progression and ultimate capacities across diverse parameters. Ultimately, based on the superposition principle, by quantifying the independent load-bearing contributions and synergistic interactions of the spalled concrete cover, confined core, and hybrid bars, this study derives a theoretical formula. The proposed model accurately predicts the axial compressive capacity of spirally confined hybrid columns, providing an analytical tool for resilient structural design. Full article

When conventional FRP composites are applied to confine rectangular concrete columns, strength enhancement is often limited due to the highly non-uniform lateral expansion of sections with a large aspect ratio (e.g., 2.0). High ductile FRP (HDFRP), a composite of glass fibers and polypropylene (PP) fibers, improves column strength while alleviating corner stress concentration in square sections, demonstrating its promising application potential for strengthening members with rectangular cross-sections. Yet existing studies on HDFRP have primarily focused on circular and square sections. To explore its applicability to rectangular cross-sections, this study conducted axial compression tests on HDFRP-confined rectangular short concrete columns (HDFRP-CRCC), investigating the effects of aspect ratio, corner radius, and FRP thickness on their mechanical behavior. The test results demonstrate that the HDFRP composite material can significantly enhance the overall strength and axial deformability of rectangular concrete columns, thereby effectively overcoming the limited strength enhancement associated with conventional FRP systems. Based on the experimental results, a design-oriented model is developed to offer theoretical support for the application of HDFRP in strengthening rectangular frame structures. Full article

This paper innovatively employs an epoxy-free composite layer with basalt fiber-reinforced polymer (BFRP) and engineered cementitious composite (ECC) to reinforce the two-way concrete slab structure. Five strengthened slabs and one reference slab were tested under biaxial bending moments with four-side simply supported conditions. The thickness of ECC (15, 25, 35 mm) and BFRP grid (1, 2, 3 mm) were selected as two main variables in the test program. The experimental results showed that the cracking and ultimate load of the strengthened slabs were substantially improved. Notably, the cracking pattern was shifted from diagonally concentrated cracks to discontinuous short cracks, with no apparent debonding of the composite layer. As the thickness of the BFRP grid and ECC increases, both the flexural capacity and stiffness improve, with decrease in the maximum deflection and effective utilization rate of steel reinforcement and BFRP grid at mid-span. Furthermore, a theoretical model considering different positional distribution of yield line was proposed to predict the bearing capacity of the strengthened slabs, with the calculated values aligned well with the experimental results. This research highlights the FRP–ECC composite as a robust reinforcement method for two-way slabs, and offers a good design-oriented reference basis in the field. Full article

Fiber-reinforced polymer (FRP) composites are widely used to strengthen reinforced concrete (RC) columns due to their high strength, durability, and ease of installation. Accurate prediction of the axial capacity of CFRP-strengthened concrete columns is essential for reliable structural design. Yet conventional empirical models often exhibit limited accuracy due to the complex interactions among structural parameters. This study develops a deep learning-based model to predict the axial capacity of CFRP-wrapped RC columns using a database of 469 experimental tests collected from published studies. A deep neural network (DNN) was optimized using the Optuna hyperparameter tuning framework and k-fold cross-validation to enhance model accuracy and robustness. Model performance was evaluated using statistical indicators, including R², RMSE, MAE, MAPE, and the a20-index. The results show excellent predictive performance with R² values approaching 0.99 and an a20-index of 0.98, demonstrating strong agreement between predicted and experimental results. Comparisons with the ACI 440.2R-17 and CSA S806-12 design codes indicate that the proposed DNN model provides significantly improved prediction accuracy, with lower errors. The developed approach offers a reliable and efficient tool for estimating the axial capacity of CFRP-strengthened concrete columns. Full article

This study is an experimental study on flexural strengthening of reinforced concrete beam where three types of epoxy-bonded jacketing systems are used (glass fiber-reinforced composite (GFRC, S1), jute fiber-reinforced composite (JFRC, S2), and hybrid fiber-reinforced composite (HFRC, S3)) and an unjacketed control beam (S0). All the specimens were subjected to displacement-controlled three-point bending to measure the enhancement of strength, stiffness, and energy absorption using mass-normalized (TPM) and synthetic-content-normalized (TSM) performance indices. Jacketing compared to control also raised the maximum load from 11.80 N to 17.10 N for GFRC (+44.9%), to 14.64 N for JFRC (+24.1%), and to 14.89 N of HFRC (+26.2%). The energy taken up rose from 38.44 J (S0), 152.50 J (S1, +297%), 95.32 J (S2, +148%), and 132.79 J (S3, +245%). Flexural strength was also increased to 56.26 MPa (S1), 43.54 MPa (S2), and 51.38 MPa (S3) and yield strength was raised from 10.43 MPa (S0) to 26.40 MPa (S1), 16.84 Mpa (S2), and 23.05 Mpa (S3). The increase of flexural modulus between S0 (4871.33 MPa) and S1 (12,322.34 MPa), S2 (7862.61 MPa), and S3 (10,759.57 MPa) showed the enhancement of the stiffness. Mass-normalized performance showed great overall efficiency in the case of GFRC and HFRC, with TPM = 3.70 and 3.60 J/kg, respectively, and synthetic-content efficiency was higher in the case of JFRC, with TSM = 9.66 J/kg, which is the advantage of low-synthetic reinforcement in energy-based performance. In general, the suggested jacketing systems have a great influence on flexural responsiveness and power absorption, whereby GFRC and JFRC offer maximum capacity and stiffness, respectively, and the greatest efficiency per unit synthetic material, respectively. In terms of novelty, the paper is one of the first to measure the sustainability-based performance of an epoxy-bonded GFRC, HFRC, and bio-based JFRC jacketing, comparing the results in terms of synthetic-content efficiency (TSM) and mass-normalized indices, which reflect the energy absorption benefits per unit of synthetic material. Full article

With the depletion of river sand and the rapid expansion of marine infrastructure, seawater–sea-sand concrete (SSC) has attracted increasing attention due to its low cost and sustainability. However, the high chloride content in SSC accelerates steel corrosion. This significantly limits its use in conventional reinforced concrete structures. In recent years, the rise in FRP–steel composite confinement has offered a new solution to this durability bottleneck. Based on this background, scholars have proposed a new type of FRP–steel composite tube confined seawater–sea-sand concrete (FCTSSC) column. This paper reviews the research progress on SSC, CFST, FCFST, and FCTSSC. The latter systems are developed based on the former. The results show that advanced FCTSSC columns exhibit strong synergistic confinement between the FRP and the steel tube when compared with CFST and FCFST. This synergy enhances the bearing capacity, ductility, and post-peak behavior of SSC. Both external and internal FRP configurations can reduce the brittleness and expansion of SSC. They also effectively restrain local buckling of the steel tube. Existing studies mainly focus on short columns. Research on intermediate slender and slender columns remains limited. This includes structural behavior, rational design models, and long-term durability. Finally, future research directions are proposed to support the practical application of FCTSSC in marine engineering. Full article