Fiber-reinforced Polymer Thickness Research Articles

Behavior of FRP-interlayer-steel confined concrete columns subjected to eccentric compression

Fiber reinforced polymer (FRP)-interlayer-steel confined concrete (FISC) columns are proposed by bonding a filament-wound FRP tube with a concrete-filled steel tube (CFST) using grouting material to increase the corrosion resistance of CFST columns. FISC columns have high bearing capacity, benefiting from the dual confinement provided by both FRP and steel tubes on the core concrete. Despite this, the current research on the behaviors of FISC columns is limited, particularly regarding the confinement mechanism in FISC columns under eccentric compression. In this paper, a total of twenty short specimens were tested under eccentric compression, including seventeen FISC specimens, two FRP confined concrete-filled steel tubular (CCFT) columns, and one CFST specimen. Failure modes, load versus deflection curves and strain development for FISC specimens were presented and discussed, considering variations in eccentricity, FRP types, FRP thicknesses, and the ratio of FRP and steel confining indexes. Additionally, confining stresses of the FRP and steel tubes were obtained, revealing a non-uniform distribution of confinement on concrete along the sectional circumference, which decreased with increasing eccentricity. Furthermore, by regarding the concrete and grouting material as a whole unity and accounting for the influence of eccentricity, a sectional numerical analysis model for FISC columns was established and validated against test results, and the influences of steel thickness, FRP thickness, and concrete-grouting strength on the N-M correlation curves of FISC columns were investigated.

Bond strength prediction of externally bonded reinforcement on groove method (EBROG) using MARS-POA

The Externally Bonded Reinforcement on Grooves (EBROG) method represents an advancement in externally bonded reinforcement (EBR) techniques, specifically addressing the challenge of premature debonding often encountered in conventional Fiber Reinforced Polymer (FRP) applications directly bonded to concrete. This article introduces a novel and straightforward mathematical equation for predicting bond strength in the EBROG method using soft computing techniques for the first time. The study delves into the combined potential of the Multivariate Adaptive Regression Spline (MARS) model and the Pelican Optimization Algorithm (POA) for bond strength prediction in this method. The input parameters include FRP width, FRP thickness, elasticity modulus of FRP, concrete strength, groove length, groove width, and groove depth, while the output is EBROG bond strength. The study demonstrates exceptional accuracy with R2 values of 0.9629 for training and 0.9623 for testing, highlighting the model’s precision. The proposed bond strength prediction equation for the EBROG method undergoes validation against existing models, encompassing thirteen equations for EBR and a recent one specific to EBROG. Statistical metrics confirm the accuracy and reliability of the proposed equation. Notably, FRP stiffness emerges as the parameter with the highest relative importance, while groove width exhibits the lowest impact on bond strength.

Estimation of confined compressive strength of LRS‐FRP concrete specimens with computational intelligence

AbstractReinforced concrete structures deteriorate due to changes in temperature, corrosion, and attacks of sulfate and chloride contents. Retrofitting techniques like fiber‐reinforced polymer (FRP) jacketing, known for their strength and corrosion resistance, are increasingly used to strengthen and retrofit deteriorated structural elements. Large rupture strain (LRS)‐FRP composite, composed of polyethylene terephthalate and polyethylene naphthalate, both of which have high tensile strength and high strain at rupture have been used in the studies of many researchers. This research aims to develop a reliable and accurate machine learning (ML) model to estimate the compressive strength of LRS‐FRP confined specimens. A total of 303 LRS‐FRP confined specimens were gathered after a thorough literature review to develop ML models, utilizing the linear regression, support vector regression, regression tree, and artificial neural network (ANN) algorithms. Additionally, 44 analytical models (AMs) were used to compare the performance of the developed ML models. The results revealed that the performance of the developed ANN model was higher among all the ML and AMs. The R‐value and the mean absolute percentage error (MAPE) value of the developed ANN model were 0.9822 and 6.17%, respectively. The sensitivity analysis results show that the height of the specimens had the highest impact followed by the diameter of the specimen, the number of FRP layers and thickness, and then the tensile strength of LRS‐FRP. The ANN‐based mathematical expression is simple and easy to use to predict the compressive strength of the LRS‐FRP strengthened specimens.

Experimental behaviour and design model of FRP-UHPC-steel tubular columns under monotonic axial compression

By integrating structural steel with Fiber-Reinforced Polymer (FRP) and Ultra-High Performance Concrete (UHPC), FRP-UHPC-steel tubular columns (FUSTs) emerge as innovative composite members that offer exceptional corrosion resistance and lightweight properties. FUSTs hold significant potential for use as thin-walled tubular columns working in harsh environments, such as wind turbines and high-voltage transmission towers. To obtain in-depth understanding of key parameters including the steel fiber ratio of UHPC, the specimen void ratio and the FRP thickness, this paper tested 24 specimens to evaluate their compressive behaviour, including 18 FUSTs and 6 UHPC-filled FRP tubes (UCFFTs). Experimental results showed that: (1) FUSTs demonstrated ductile behavior with significant strain enhancement and notable strength improvement; (2) the steel fibers in UHPC had marginal influences on the ultimate condition of FUSTs; (3) a larger inner void had a general effect to lead to more localized rupture for the FRP tube; (4) the FRP thickness was the predominant influencing factor on both the general shape and the ultimate point of the normalized axial stress–strain curves. Finally, a design model was proposed, which was able to capture the general shape of the axial load–strain curves, and could generate reasonably accurate predictions for the peak load.

Compressive behavior of concrete-filled FRP tubes with FRP solid waste as recycled aggregates: Experimental study and analytical modelling

The disposal and recycling of FRP (Fiber-Reinforced Polymer) solid waste pose significant challenges globally, with a recycling rate of less than 10 %. Current methods of handling FRP solid waste (FSW), including landfill disposal, incineration, chemical treatment, and thermal cracking, face limitations in achieving large-scale recycling and resource utilization. In civil engineering, FRP is widely employed for retrofitting structures and holds promise for new constructions. A new approach has emerged for recycling FSW by using it as aggregates in concrete, offering lower costs and reduced environmental impact. Research on using FSW aggregates in plain concrete shows that optimizing replacement ratios and aggregate shapes can enhance split tensile strength and toughness without significantly lowering compressive strength. To further improve the mechanical performance, this study investigated concrete-filled filament-wound FRP tubes with FSW aggregates (FSW-CFFTs) under axial compression. 35 specimens were tested to evaluate the influence of granular FSW aggregates (i.e., replacement ratio = 0 %, 20 %, 40 %, 60 %, and 80 %) and FRP thickness (i.e., 0 mm, 3 mm, and 5 mm) on their axial compression behavior. The experimental study revealed that: incorporation of FSW aggregates in unconfined concrete cylinders led to a notable decrease in Young's modulus and compressive strength, while concurrently increasing the peak compressive strain; as the replacement ratio of FSW aggregates increased, there was a tendency for the peak stress of FSW-CFFTs to decrease, while their ultimate axial strain tended to increase; compared FSW-CFFTs with unconfined concrete cylinders, the FRP confinement helped to mitigate the reduction ratio in axial compressive strength induced by the inclusion of FSW aggregates in the concrete, particularly at higher FSW aggregate replacement ratios; specimens with a thicker FRP tube exhibited higher secondary stiffness, along with higher strength and larger ultimate compressive strain. The modeling work indicated that accurately predicting axial stress-strain curves requires consideration of the bi-axial stress state of filament-wound FRP tubes.

A novel packing-coupled stress-strain model for confined concrete

To develop low carbon footprint concrete (LCFC) with less CO2 emissions, various minerals (limestone, metakaolin) and industrial waste by-product (fly ash (FA), silica fume (SF), slag) were used to replace cement partially. By optimizing the packing structure, cost-effective, sustainable LCFC with superior strength, workability and durability performances could be achieved. Research studies on fiber-reinforced polymer (FRP) confined LCFC showed that the peak stress, strain at the peak stress and post-peak branch of confined LCFC was far different from that of confined conventional concrete (with cement as the cementitious material only) with the same concrete strength and confinement scheme. To look at this problem more scientifically, this paper conducted experimental and analytical studies about the stress-strain curves of FRP-confined LCFC. A total of 45 concrete column specimens consisting of 15 unconfined concrete and 30 FRP-confined concrete were tested under uni-axial compression. The studied parameters were the thickness of FRP, water- and FA/SF-to-cementitious material ratios. Based on the test results, a novel packing-coupled stress-strain model for confined LCFC has been proposed and verified.

Behavior of large-scale FRP-confined square RC columns with UHP-ECC section curvilinearization under eccentric compression

Implementing section curvilinearization (SC) by curving flat sides into curved sides before fiber-reinforced polymer (FRP) jacketing has been proposed to enhance the FRP confinement effectiveness of FRP-confined rectangular concrete columns. In this paper, ultra-high performance engineered cementitious composite (UHP-ECC) is proposed as the added concrete for SC, with an aim to enhance both bending and axial strengths of concrete columns given that the UHP-ECC can contribute to resisting tensile stresses for columns. Thirteen large-scale FRP-confined square reinforced concrete (RC) columns with SC using UHP-ECC were tested under eccentric compression loading to explore their eccentric compressive behavior. The test parameters included the rise-to-span ratio, the FRP thickness, the eccentricity and the surface roughness of the old concrete. The test results show that FRP confinement with SC using UHP-ECC can substantially enhance the peak axial load and the bending moment than those without SC, while the ultimate lateral displacement is marginally increased. An increase in the rise-to-span ratio leads to an increase in the peak axial load.

Constitutive Model of FRP Tube-Confined Alkali-Activated Slag Lightweight Aggregate Concrete Columns under Axial Compression

Fiber reinforced polymer (FRP), as a novel type of composite material, has been extensively employed in structural strengthening and composite structures. The FRP tube-confined alkali-activated slag lightweight aggregate concrete column (FRP-AASLAC) can effectively improve the utilization rate of slag, reduce carbon emissions, reduce structural self-weight, and improve structural ductility. Therefore, the axial compressive properties of FRP-AASLAC were studied in this paper. The influences of the type of FRP, FRP thickness and the content of lightweight aggregate on the failure modes, bearing capacities, deformation properties and constitutive relationships of FRP-AASLAC were revealed. The results indicate that the constitutive relationships of FRP-AASLAC show double broken line patterns without obvious softening sections. The restraining effect of FRP on lightweight aggregate concrete is higher than that on ordinary concrete as lightweight aggregate concrete has lower strength and more easily undergoes lateral expansion under external loads. Models for compressive strength, peak compressive strain and constitutive relationship for FRP-AASLAC are proposed.

Machine learning-based model for the ultimate strength of circular concrete-filled fiber-reinforced polymer–steel composite tube columns

This study introduces a machine learning (ML)-based model for predicting the ultimate strength of circular concrete-filled fiber-reinforced polymer (FRP)–steel composite tube (CFSCT) columns. The support vector regression (SVR), back propagation neural network (BPNN), and random forest (RF) ML algorithms were trained and tested based on an extensive dataset containing 305 data samples of circular CFSCT columns under axial loads from existing literature. Seven parameters were selected as input variables, and the ultimate load of the circular CFSCT columns under an axial load was selected as the output variable to develop the ML-based models. The ML-based models were evaluated by comparing them with four existing empirical models previously developed by researchers. The results indicate that the ML-based models are good alternatives to existing empirical models, considering their superior prediction accuracy and applicability. The coefficient of determination (R2) of the SVR model was 0.992, demonstrating better performance than the BPNN and RF models with R2 of 0.984 and 0.982, respectively. However, only the SVR and BPNN model are considered applicable for the ultimate load prediction since the RF model exhibited an overfitting problem. Moreover, the results of the comparison between the ML-based and empirical models were visualized to illustrate the superiority of the ML-based models over existing empirical models in prediction accuracy and dispersion. Finally, sensitivity and parametric analyses were performed to analyze the influence of the input variables on the output variable. The results indicate that concrete compressive strength, steel tube thickness, and FRP thickness positively affect the ultimate strength of CFSCT columns. This study can provide a reference for the design of CFSCT columns.

Axial compression behavior of FRP-confined steel-reinforced columns with a flange-broadened cross-shaped steel

Due to the hybrid confinement effect and reforming effective confinement area, the composite action of square columns is enhanced by utilizing cross-shaped steel on fiber-reinforced polymer (FRP) tube-confined steel-reinforced columns. These novel hybrid columns are FRP-confined concrete-encased cross-shaped steel columns (FCCSCs). This study adopted flange-broadened cross-shaped steel to develop the potential advantages of FCCSCs. Twenty-two specimens were tested under monotonic and cyclic axial compression, and the test parameters were column form, FRP thickness, cross-shaped steel dimension, and slenderness ratio. FCCSCs with broad steel flanges exhibited high axial load capacity and axial deformability. In addition, increasing the thickness of the flange, web, or FRP tube resulted in stronger hybrid confinement and outperformed strength behavior in confined concrete. Slender FCCSCs presented lower axial capacity and higher lateral deflection. Finally, the calculation method for compressive load capacities of slender FCCSCs is proposed.

Experimental and Numerical Study on Seismic Performance of PEN FRP-Jacketed Circular RC Columns

This paper presents an experimental study on the seismic behavior of polyethylene naphthalate (PEN) fiber–reinforced polymer (FRP)-jacketed circular reinforced-concrete (RC) columns. A total of seven specimens were tested under constant axial load and cyclic lateral load. The key parameters studied were the thickness (one, two, and three layers) and type (PEN FRP and CFRP) of FRP jacket. Test results indicated that the control column failed by buckling of the longitudinal reinforcement and shortage of ductility, while concrete spalling and bar buckling were effectively inhibited by the application of external FRP jacket. It is also observed that PEN FRP-jacketed specimens had more additional strain capacity compared with CFRP-jacketed specimens at the final condition. The additional strain capacity can serve as a safety reserve for structures and make PEN FRP more suitable for strengthening large or noncircular columns. Furthermore, it is found that 1-ply PEN FRP and 1-ply CFRP had a similar strengthening effect and thus the design of PEN FRP-strengthened column can simply refer to the design method of CFRP-strengthened columns in current codes. Different FRP thickness and types had marginal impact on the hysteretic curves of the specimens, which was attributed to the low axial load ratio (0.15) and high slenderness ratio (5.25) adopted in this paper. Finally, based on a cyclic stress–strain model for longitudinal reinforcement, including buckling effect, developed by the authors in a previous study, the test columns were simulated by the Open System for Earthquake Engineering Simulation (OpenSee)s to achieve an in-depth understanding of the experimental findings and associated strengthening mechanisms. Further parametric analyses showed that considering bar buckling played a significant role in accurately predicting the hysteretic response of FRP-jacketed columns.

Cyclically loaded elliptical FRP-confined concrete: Behavioral characteristics and modeling

The previous studies on elliptical concrete confined with fiber-reinforced polymer (FRP) are mainly concerned with their monotonic compressive responses. By contrast, no research has been focused on their performance under cyclic axial loading. Motivated by this research gap, the behavioral characteristics of cyclically compressed elliptical FRP-confined concrete (FCC) were investigated in this research. The experimental parameters included the aspect ratio varying between 1.0 (circular section), 1.3, 1.7 and 2.0, the number of FRP layers, and the loading pattern (monotonic loading or two schemes of cyclic loading). The results revealed that plastic strains and stress degradation in elliptical FCC were confinement-independent parameters, regardless of confinement efficiency (affected by sectional geometry) and confinement level (governed by FRP thickness). This was similar to the case of cyclically loaded rectangular FCC (characterized by depth-to-width ratio). Moreover, the mechanism of non-uniform hoop strain distribution was dominated by two aspects, namely curvature radius-induced varying confinement stiffness and concrete cracking-induced strain localization. In contrast, the cyclic loading had a marginal effect on this distribution. Based on the monotonic model (considered as envelope curve) for elliptical FCC and the hysteretic rules originally for circular FCC, the complete compressive behavior of cyclically loaded elliptical FCC could be modeled with favorable accuracy.

Strengthening RC square columns with UHP-ECC section curvilinearization and FRP confinement: Concept and axial compression tests

In this paper, a novel column strengthening technique incorporating section curvilinearization (SC) using ultra-high performance engineered cementitious composites (UHP-ECC) and fiber-reinforced polymer (FRP) confinement is proposed and validated. Eleven large-scaled FRP-confined square reinforced concrete (RC) columns with SC using UHP-ECC as the additive material were prepared and tested. The main parameters investigated in this paper included the FRP thickness, the rise-to-span ratio, and the strength of the additive concrete. Test results show that the SC technique with FRP jacketing and UHP-ECC as the additive material enhances the load-bearing capacity of the RC columns by 52% with a 17% section increment. By assessing the existing design-oriented stress–strain models of confined concrete in curvilinearized square columns (CSCs) using the test results obtained in this study, it reveals that the current models could underestimate the ultimate strength of the confined concrete in CSCs with a two-layer FRP wrap, and therefore further research on the improvement on the stress–strain model of confined concrete in CSCs becomes necessary.

Analytical bond strength of deformed bars in concrete confined with transverse reinforcement and FRP

To ensure the long-term stability and safety of concrete structures, it is critical to accurately analyze the internal transverse reinforcements and externally bonded fiber-reinforced polymer (FRP) confinement to bond performances of deformed bars in concrete. However, no prediction models are available for bond strength and the bond-slip response with double confinement (transverse reinforcement and FRP). To address these issues, the present study proposes a predictive model to evaluate the bond strength of deformed bars in concrete under double confinement. The focus is on determining the internal pressure at the reinforcement/concrete interface provided by the transverse reinforcement and FRP at various stages of concrete cracking. The integral absolute error (IAE) of 12.0% for the 93 experimental data from the training dataset shows that the proposed model outperforms the other models. A general model for a bond-slip response involving double confinement is proposed and validated, along with a design recommendation to predict the minimum FRP thickness and bond length for seismic reinforcement.

Investigating the Bond Strength of FRP Laminates with Concrete Using LIGHT GBM and SHAPASH Analysis.

The corrosion of steel reinforcement necessitates regular maintenance and repair of a variety of reinforced concrete structures. Retrofitting of beams, joints, columns, and slabs frequently involves the use of fiber-reinforced polymer (FRP) laminates. In order to develop simple prediction models for calculating the interfacial bond strength (IBS) of FRP laminates on a concrete prism containing grooves, this research evaluated the nonlinear capabilities of three ensemble methods-namely, random forest (RF) regression, extreme gradient boosting (XGBoost), and Light Gradient Boosting Machine (LIGHT GBM) models-based on machine learning (ML). In the present study, the IBS was the desired variable, while the model comprised five input parameters: elastic modulus x thickness of FRP (EfTf), width of FRP plate (bf), concrete compressive strength (fc'), width of groove (bg), and depth of groove (hg). The optimal parameters for each ensemble model were selected based on trial-and-error methods. The aforementioned models were trained on 70% of the entire dataset, while the remaining data (i.e., 30%) were used for the validation of the developed models. The evaluation was conducted on the basis of reliable accuracy indices. The minimum value of correlation of determination (R2 = 0.82) was observed for the testing data of the RF regression model. In contrast, the highest (R2 = 0.942) was obtained for LIGHT GBM for the training data. Overall, the three models showed robust performance in terms of correlation and error evaluation; however, the trend of accuracy was obtained as follows: LIGHT GBM > XGBoost > RF regression. Owing to the superior performance of LIGHT GBM, it may be considered a reliable ML prediction technique for computing the bond strength of FRP laminates and concrete prisms. The performance of the models was further supplemented by comparing the slopes of regression lines between the observed and predicted values, along with error analysis (i.e., mean absolute error (MAE), and root-mean-square error (RMSE)), predicted-to-experimental ratio, and Taylor diagrams. Moreover, the SHAPASH analysis revealed that the elastic modulus x thickness of FRP and width of FRP plate are the factors most responsible for IBS in FRP.

Flexural Behavior of Insulated Concrete Sandwich Panels using FRP-Jacketed Steel-Composite Connectors

This study presented the experimental and theoretical results of insulated concrete Sandwich panels with innovative dumbbell-shaped steel fiber-reinforced polymer (FRP) composite bar (SFCB) connectors under flexural load. The influences of FRP thickness and raised thickness of dumbbell ends of connectors, axial compression ratio, the content of vitrified microspheres in the wythes, and loading direction were discussed. The increase in thickness of a glass FRP (GFRP) jacket on the hybrid bar from 2 to 3 mm led to increased initial cracking load, ultimate load, and flexural stiffness of the Sandwich panels by 75, 49, and 16%, respectively. The increase in the thickness of dumbbell ends from 4 to 6 mm led to increased initial cracking load, ultimate load, and flexural stiffness of the Sandwich panels of 18, 46, and 9%, respectively. The incorporation of vitrified microspheres in concrete wythes resulted in a significant increase in the load-carrying capacity of Sandwich panels but decreased ductility. Increased axial compression ratio from 0/1 to 0.2/1 contributed in improving the crack resistance and ultimate loads of Sandwich panels. Further increase of the axial compression ratio to 0.4/1 led to crushing failure of concrete wythe ends. Specimens under negative loads had higher ultimate loads than the counterparts under positive loads. Three-dimensional finite-element (FE) models were developed to simulate Sandwich panel flexural behavior and numerical results compared with the test data. Then, the verified FE model was used to analyze the influence of the arrangement of dumbbell-shaped SFCB connectors. Increased connector spacing from 550 to 650 mm was found to have an insignificant influence on load-deflection responses. Moreover, analytical solutions for deflection of Sandwich panels under combined axial-flexural load were obtained, in which the effect of slipping between the facade and structural wythes and shear deflection were considered. The predicted deflection at the ultimate load agreed well with the test results. This study provided a theoretical basis and design reference for FRP-jacketed steel-composite connectors in applications of Sandwich wall panels with large insulation layer thickness.

Axial Compression Behavior of FRP Confined Laminated Timber Columns under Cyclic Loadings

Fiber reinforced polymer (FRP) has been proved to be effective to improve the structural strength and ductility for column structures. An experimental study was conducted to investigate the compressive performance of FRP confined glued-laminated timber (GLT) and cross-laminated timber (CLT) columns. A total of 60 column specimens of two dimensions in height using different FRP types, FRP thickness, and laminate types were tested under cyclic axial compression loads. This study focuses on the compressive capacity and ductility of the new FRP composited timber structure. For this purpose, a loading protocol was designed, including a force-dependent pre-load and an amplitude-increasing displacement-dependent cyclic compression load. The results showed that the ultimate compression load of specimens was considerably promoted by the FRP sheets. Wrapping FRP sheets led to an average improvement of 29% and 24% for the FRP confined CLT and GLT specimens, respectively, compared to the initial stiffness of unreinforced specimens. Using the FRP sheets, the energy dissipation capacity of CLT and GLT specimens was increased by 358% and 266%, respectively. In general, GLT specimens had a higher energy dissipation rate compared to the CLT specimens, while CLT specimens showed a better potential for sustained energy consumption if confined with sufficient FRP sheets.

Nonlinear Finite Element Analysis of Concrete Columns Confined by Carbon Fiber Reinforced Polymer Sheets

In the past few years, the area of structural retrofitting became a hot topic for research. Providing additional confinement to reinforced concrete columns will result in increasing their axial load capacity. In this paper, the axial behavior of rectangular reinforced concrete columns confined by Fiber Reinforced Polymer (FRP) sheets is investigated using Nonlinear Finite Element Analysis (NLFEA). Experimental studies from the literature dealing with rectangular reinforced concrete columns wrapped with FRP sheets tested under uni-axial load are used to build and verify two Finite Element (FE) models. The proposed FE model showed good agreement with experimental results. The models were used to conduct a parametric study to investigate the effect of FRP thickness, FRP sheets spacing, and core concrete strength on the behavior of both plain and reinforced concrete columns. Furthermore, a new empirical model to predict the behavior of confined rectangular concrete columns is developed. The results of the parametric study showed that the effect of FRP confinement decreases with the increase of the compressive strength of concrete used. Increasing the number of layers (i.e., thickness) will enhance the strength; however, after 4 layers there is no significant improvement in strength gain. Large spacing between FRP sheets in partial confining configuration results in nonsymmetrical damage in concrete.

Behavior of unconfined and polyethylene terephthalate (PET) FRP-confined coal reject concrete under compression

Fiber-reinforced polymer (FRP) confinement has been proposed to mitigate the detrimental effects of coal rejects concrete (CRC) in the present study. Polyethylene terephthalate (PET) FRP jacket was adopted to confine CRC as a new type of novel column members, which are referred to as PET FRP-confined coal reject columns (PFCCRs). PFCCRs are often sustainable and cost-effective structural columns as the PET FRP composites are generally made from recyclable PET waste and coal rejects are industry by-products. A total of 49 columnar specimens, consisting of 21 CRC specimens and 28 PFCCR specimens were tested under uni-axial compression to understand the mechanical behavior of unconfined and PET FRP-confined CRC. The main parameters evaluated in this study were the volume replacement ratio of coal rejects and the FRP thickness. The test results indicated that the unconfined CRC specimens exhibited a significant reduction in compressive strength with the increase in replacement ratio, whereas the ultimate axial strain was enhanced. Compared with plain CRC specimens, PFCCRs had excellent deformation capacity and compressive strength with a strain hardening stress-strain response. The maximum normalized ultimate axial strain and axial stress of most of the PFCCR specimens are over 30 and 7, respectively. The stress reduction segment will occur only when the FRP confinement is insufficient. Furthermore, the test results were compared with the predictions by using existing analysis-oriented and design-oriented models for LRS FRP-confined concrete to verify their accuracy.

Stress-strain behavior and design-oriented model for FRP spiral strip-confined concrete

Partial fiber-reinforced polymer (FRP) wrapping has become an attractive strengthening technique for reinforced concrete (RC) columns. Apart from FRP full wrapping strengthening technique, FRP partial wrapping with FRP spiral strips leads to some other advantages such as better fire resistance. However, existing study on FRP spiral strip is limited and the confinement mechanism of FRP spiral strip wrapped concrete columns remains unclear. To this end, axial compression tests on 28 concrete columns wrapped with FRP spiral strips are conducted and the test results are presented in this paper. Results show that the ultimate axial stress and the slope of the post-peak linear branch of the axial stress–strain curve are significantly affected by the proposed confinement effectiveness factor. The desired confinement effectiveness factors for specimens can be achieved by varying the key confinement parameters (i.e., spiral angle, width and layers of FRP strips). It is found that the increases in FRP strip width and thickness lead to enhancements in both ultimate axial stress and strain of the FRP spiral strip-confined concrete, whereas a decrease in the spiral angle has a more pronounced effect on the ultimate axial stress enhancement than the ultimate axial strain enhancement. A new design-oriented stress–strain model is proposed for FRP spiral strip-confined concrete based on the new test database assembled in this study. The predictions corroborate reasonably well with the test results, demonstrating the accuracy and robustness of the proposed model.