Fiber-reinforced Polymer Confinement Research Articles

Eccentric compression behavior of concrete-filled steel tube columns strengthened by CFRP/steel strip

Concrete-filled steel tube (CFST) column structures have been widely applied in structural engineering because of their superior bearing capacity and ductility. During the service lives of CFST columns, due to the change in the service function, the increase in the load, and the corrosion of the steel tubes, some existing CFST columns face the strengthening demand of increasing bearing capacity and durability. In this study, two efficient strengthening methods for CFST columns were proposed, namely, winding carbon fiber-reinforced polymer (CFRP) or welding steel strips around the columns. A total of 11 specimens were tested to investigate the behavior of the strengthened CFST columns under eccentric compression according to parameters such as the height-diameter ratio, fiber-reinforced polymer (FRP) layer, eccentric distance, and steel strip thickness. The results indicated that the ultimate strength and initial stiffness decreased with increasing eccentric distance. The arrangement of the FRP and steel strips effectively improved the eccentric carrying capacity and ductility. The improvement in the ultimate load due to FRP confinement exceeded 34.31%, and that due to the steel strips exceeded 13.47%. Finally, considering the effect of the eccentric distance and height-diameter ratio on the behavior of the structure under compression, a model for predicting the bearing capacity under eccentric load was proposed and verified by a database that included various design parameters.

Evaluation of Hybrid Fiber Multiscale Polymer Composites for Structural Confinement under Cyclic Axial Compressive Loading

Fiber reinforced polymer (FRP) confinement is recognized as the most promising technique for the strengthening and retrofitting of concrete structures. In order to enhance the performance of conventional epoxy-based FRP composites, nano filler modification of the epoxy matrix was implemented in the current study. In particular, the cyclic loading response of standard concrete specimens externally confined by epoxy-based natural and hybrid fiber reinforced polymer systems was investigated. The confinements were realized with sisal fiber reinforced polymer (SFRP) and hybrid sisal basalt fiber reinforced polymer (HSBFRP). Moreover, the effects of multiwalled carbon nanotubes (MWCNT) were also investigated. Three different specimen sets were considered for study: (i) unconfined specimens, (ii) epoxy-based FRP confined specimens and (iii) MWCNT incorporated epoxy-based FRP confined specimens. The specimens were tested in repeated compressive mode in loading-unloading cycles at increasing displacement levels. The test results revealed that FRP wrapping could enhance the mechanical behavior of unconfined columns in terms of strength and ductility. Moreover, it was evident that the mechanical properties of the epoxy matrix were enhanced by MWCNT incorporation. The developed epoxy-based FRP confinement containing MWCNT ensures improvement in axial strength by 71% when compared with unconfined specimens. The epoxy-based FRP confinement, with and without MWCNT, exhibited a high strain redistribution behavior around the concrete core. In comparison to the unconfined specimens, the confinement could increase the sustained axial strain from 0.6 to 1.4% using epoxy-based FRP confinement and to 1.6% with MWCNT incorporated epoxy-based FRP confinement. Further, an empirical model was developed to predict the ultimate axial stress of concrete columns confined externally with FRP jackets. The ultimate compressive strength obtained from the experimental study was compared with the proposed model, and the observed deviation was lower than 1%.

Experimental and statistical investigation on synergistic effect of nano based epoxy hybrid FRP on strength and durability of circular concrete columns

Strengthening and retrofitting of concrete structures using fiber reinforced polymer (FRP) wrapping is a promising technique in construction sector. Epoxy is the commonly used matrix which possess a higher rate of degradation under exposure to harsh environment. The evaluation of strength and durability of concrete cylinders confined by multi walled carbon nanotube (MWCNT) incorporated epoxy with hybrid sisal and basalt fiber composite systems exposed to various environmental conditions forms focus of this paper. Specimens are subjected to various environmental exposure such as elevated temperature, acidic, alkaline and sea water conditions. Two varieties of epoxy viz. neat epoxy and MWCNT modified epoxy systems are considered. Mechanical and durability properties are analyzed based on axial compressive behavior, stress strain response, visual inspection and modes of failure. The MWCNT incorporated epoxy based hybrid FRP confined specimens exhibited a strength reduction less than 10% when compared with unexposed confined specimens under aggressive environmental conditions, while unconfined specimens showed strength reduction by 40% when compared to unexposed unconfined specimens. The MWCNT modified epoxy based hybrid confinement showed an energy absorption of 6.24 times that of unconfined specimens upon chemical exposure. Ultrasonic Pulse Velocity test revealed efficacy of confinement system in protecting concrete core from a sudden failure, which in turn increases the compressive strength of system. A statistical analysis using ANOVA was employed to find significance of these factors and confirmed with experimental results. The effect of MWCNT incorporation is significant in FRP confinement and shows the possibility of the FRP system to be adopted as a major retrofitting material in alkaline and sea water environments.

Predictive models for the peak stress and ultimate strain of FRP confined concrete cylinders with inclined fiber orientations

It is proved that fiber orientation significantly affects the confinement of fiber-reinforced polymer (FRP) wrapped concrete members. The inclination of fiber relative to the hoop direction reduces confinement effectivity. In addition, most available models for peak stress and ultimate strain of confined concrete fail to provide accurate estimations for FRP-wrapped concrete in such cases. Accordingly, the main objective of the present study is to suggest modification factors that can be applied to the existing predictive models in the case of inclined fiber orientation and improve their prediction accuracy. For this purpose, a database of experimental results on FRP-confined concrete columns with inclined fiber orientation from previous studies have been collected. Then, by using an evolutionary algorithm, modification factors were proposed to incorporate the effect of fiber orientation. Applying these modification factors to the existing stress and strain models for FRP-confined concrete cylinders leads to a more accurate estimation of the behavior of FRP-confined columns with inclined fibers.

Experimental and Numerical Investigation of Axial Compression Behaviour of FRP-Confined Concrete-Core-Encased Rebar.

The axial compression behaviour of fibre-reinforced polymer (FRP)-confined concrete-core-encased rebar (FCCC-R) was investigated by performing monotonic axial compression tests on seven groups of FCCC-R specimens and three groups of pure rebar specimens. The research parameters considered were the FRP winding angle (0°, ±45°, and 90°), number of layers (2, 4, and 6 layers), and slenderness ratio of specimens (15.45, 20, and 22.73). The test results showed that FCCC-R's axial compression behaviour improved significantly compared with pure rebar. The axial load-displacement curves of the FCCC-R specimens had a second ascending branch, and their carrying capacity and ductility were enhanced substantially. The best buckling behaviour was observed for the FRP winding angle of 90°. The capacity and ductility of the specimens were positively related to the number of FRP-wrapped layers and inversely related to the slenderness ratio of the specimens. A finite element model of FCCC-R was constructed and agreed well with the test results. The finite element model was used for parametric analysis to reveal the effect of the area ratio, FRP confinement length, internal bar eccentricity, and mortar strength on the axial compression behaviour of FCCC-R. The numerical results showed that the area ratio had the most significant impact on the axial compression behaviour of FCCC-R. The confinement length of the FRP pipe and internal bar eccentricity had similar effects on the axial compression behaviour of FCCC-R. Both of them had a significant impact on the second ascending branch, with the post-peak behaviour exhibiting minimal differences. The influence of mortar strength on the axial compression behaviour of FCCC-R was observed to be minimal.

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.

FRP-confined rubber concrete with effect of strain rate: Tests and analysis-oriented stress–strain model

Rubber concrete is attracting more and more investigations and applications in recent years, due to its environmental advantages and lightweight performance. As the replacement by rubber particles can reduce the concrete strength, an efficient and lightweight solution for strengthening and repairing rubber concrete is to apply fiber-reinforced polymer (FRP) confinement. This paper is the first to consider the influence of strain rate on the stress–strain relationship of rubber concrete with FRP confinement. To develop a stress–strain model of FRP-confined rubber concrete under different strain rates, an experimental program with 36 FRP-confined concrete specimens was conducted in this work. In the experiments, rubber particles were used to replace 0–30 % fine aggregates by volume. Strain rates of 3.3 × 10-5/s, 3.3 × 10-4/s, and 3.3 × 10-3/s were used to simulate three different strain rate levels of quasi-static loading, fast loading, and seismic loading, respectively. The experimental results showed that the compressive strength of FRP confined rubber concrete obviously increased with the strain rate increases, and decreased with an increasing rubber replacement ratio. Based on the experimental results, this paper proposed analytical models to predict the lateral strain-axial strain relationship, peak strength, and peak strain for FRP confined rubber concrete considering the strain rate influence. Finally, an analysis-oriented stress–strain model of FRP confined rubber concrete under different strain rates was established. The predicted results of the analysis-oriented stress–strain model showed good agreement with the experimental data.

Strength enhancement due to FRP confinement for coarse aggregate-free concretes

Concrete strength can be effectively enhanced by external fiber reinforced polymer (FRP) confinement. However, it is found from the literature that the strength enhancement coefficient (k1) varies significantly, especially for different types of concrete. This paper presents an in-depth investigation of the variety of strength enhancement coefficients with concrete types. First, two state-of-the-art databases for FRP-confined UHPC and FRP-confined ECC specimens are constructed in this paper. From analyzing the databases, although the mechanical performances and components are different for UHPC and ECC, nonetheless, their strength enhancement coefficients are quite similar (k1 = 2.27 and 2.01, respectively). With the validation by an experimental program on FRP-confined mortar specimens, it can be concluded that the similarity of strength enhancement coefficients for FRP-confined UHPC, ECC, and mortar, which are much lower than that for normal concrete, is caused by the lack of coarse aggregates. The reasons are discussed and well explained by a simplified experiment with steel balls and shear-friction mechanism. Based on the research, this paper proposed a unified strength enhancement coefficient, i.e., 2.13, for all coarse aggregate-free concretes. As the existence of coarse aggregates is found to be the main governing factor for the strength enhancement coefficient, this paper provides a fundamental basis for future research on confinement for next-generation coarse aggregate-free cementitious materials, such as lunarcrete and 3D-printed concrete.

Hybrid FRP-concrete-steel double-skin tubular columns of varying slenderness ratios under eccentric compression

Hybrid fiber reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) are an emerging form of hybrid column with excellent mechanical performance and durability. Such columns consist of an outer FRP tube, an inner steel tube and a concrete infill between the two tubes. While extensive experimental and theoretical studies have been conducted on such columns, these studies have been focused on short specimens under concentric compression. This paper presents the results of a series of tests on hybrid DSTCs of varying slenderness ratios subjected to eccentric compression. The effects of FRP confinement stiffness, load eccentricity, and slenderness ratio on the behavior of hybrid DSTCs were examined. The test results showed that the test hybrid DSTCs exhibited excellent performance with great ductility. A theoretical model with incorporations of existing stress-strain models for FRP-confined concrete was then adapted to simulate the behavior of the test hybrid DSTCs. The comparisons showed that the direct use of a concentric loading stress-strain model led to underestimation of the deformation capacity of the test hybrid DSTCs. Compared with the other two models, the eccentric-loading stress-strain model previously developed by the authors' group provided more accurate predictions for the ultimate deformation of the test hybrid DSTCs.

Mechanical analysis and finite element modeling of FRP-ECC-HSC composite stub column under axial compression

A novel composite column, consisting of an outer fiber-reinforced polymer (FRP) tube, an inner high strength concrete (HSC) core and an engineered cementitious composite (ECC) ring, has been proposed and investigated. Compared with normal FRP-confined HSC column, the FRP-ECC-HSC composite column could exhibit superior deformability performance. In this study, mechanical analysis was firstly conducted on the composite column to investigate the interaction behavior among different components, i.e. HSC core, ECC ring and FRP tube. Modified analysis-oriented models were proposed for HSC and ECC under uniform FRP confinement. Furthermore, finite element (FE) models were also established with the use of accurate concrete damaged plasticity (CDP) model to simulate the behavior of the composite column, especially the ECC ring which is under non-uniform confinement. The material property data used in the FE models were generated by the modified analysis-oriented models. It is shown that the FE models can provide close predictions on the axial load-axial strain behavior and hoop strain-axial strain behavior compared with test results. Meanwhile, stress distribution over the composite column section as well as the influence of ECC ratio were discussed with the validated FE model.

Flexural behavior of hybrid FRP-recycled aggregate concrete-steel hollow beams

Hybrid fiber reinforced polymer (FRP)-concrete-steel (FCS) hollow members are a new form of structural members possessing superior mechanical behavior and durability. Such members consist of an outer FRP tube, one or more inner steel tubes, and the concrete filled between the tubes. Existing studies on such members have been mainly focused on their use as columns; limited studies have been conducted on their use as flexural members. Besides, the effect of shear connectors between the steel tube and the concrete on the behavior of hybrid FCS hollow flexural members has not been investigated in detail. This paper, therefore, presents the results of an experimental program on the flexural behavior of hybrid FCS hollow beams with a rectangular hollow cross section. Recycled aggregate concrete (RAC) was used for the purposes of addressing the serious issue of construction waste nowadays and capitalizing the advantage of FRP confinement in enhancing the performance of RAC. The test results showed that such hybrid FCS hollow beams are very ductile, and the shear connectors play a significant role on the behavior of such beams. Finally, a theoretical beam model based on conventional section analysis was proposed to predict the results of the test hybrid FCS hollow beams.

Effect of strain gradient on the stress–strain relationship of FRP-confined ultra-high performance concrete

In the case where strain gradient prevails, whether the stress–strain curves of confined ultra-high performance concrete (UHPC) obtained from concentric compression tests can offer sufficiently high accuracy is still open to question. This study aims to understand the effect of strain gradient on the stress–strain relationship of UHPC with fiber-reinforced polymer (FRP) confinement. The response of FRP-confined UHPC cylinders subjected to different levels of strain gradient was experimentally and numerically investigated. The stress–strain curves were then calculated by performing a numerical regression method to study the strain gradient effect. The results show that the ultimate axial strain and the slope of the second branch are the two parameters most affected by the strain gradient effect, with a maximum increase of 20.8% and a decrease of 24.2%, respectively, under the applied highest level of the strain gradient. The findings in this paper reveal that the effect of strain gradient on the first branch of the stress–strain curve of FRP-confined UHPC is negligible, but the effect on the second ascending branch depends generally on FRP confinement levels.

Role of size on the compression behaviour of hybrid FRP strengthened square RC columns – Experimental and finite element studies

Confinement of reinforced concrete (RC) columns through external bonding (EB) of fibre-reinforced polymer (FRP) composite becomes less effective in non-circular sections. The effectiveness of FRP confinement reduces with an increase in the size of the cross-section. Hence, it is essential to develop an effective FRP strengthening technique for large-sized square columns. Hybrid FRP strengthening combines the advantage of both near-surface mounting (NSM) and EB strengthening procedures for enhancing the overall performance of RC members under different load combinations. Sixteen square RC columns of various size ratios 1.0 and 1.5 were cast, strengthened and tested to understand the efficacy of the hybrid FRP technique. In addition to the tests, detailed nonlinear finite element (FE) models were developed and validated with the test results and data from the literature. Results show that the efficiency of EB and Hybrid FRP strengthening decreased with the increase in sectional ratio of square columns. Hybrid FRP strengthening improved strength and ductility for all size ratios compared to EB or NSM FRP strengthening alone. Using validated FE models, a parametric study was also conducted to understand the effect of EB and NSM reinforcement ratio and column size on the efficiency of Hybrid FRP strengthening.

Effect of Aggregate Size on the Axial Compressive Behavior of FRP-Confined Coral Aggregate Concrete.

Using locally available raw materials for preparing concrete, such as coral reefs, seawater, and sea sand, is conducive to compensating for the shortage of construction materials used on remote islands. Jacketing fiber-reinforced polymer (FRP), as passive confinement, is a practical approach to enhance the strength, ductility, and durability of such coral aggregate concrete (CAC). Rational and economical CAC structural design requires understanding the interactions between the CAC fracture process and FRP confinement. The coral aggregate size is the critical parameter of their interaction since it affects the crack propagation of CAC and FRP confinement efficiency. This study conducted axial compression tests on FRP-confined CAC cylinders with varying coral aggregate sizes and FRP confinement levels. The test results indicate that the coral aggregate sizes affected the unconfined CAC strength. In addition, the dilation behavior of FRP-confined CAC varied with aggregate sizes, showing that CAC with smaller coral aggregate featured a more uniform hoop strain distribution and larger FRP rupture strain. These coupling effects are epitomized by the variation in the transition stress on the stress–strain curve, which makes the existing stress–strain models not applicable for FRP-confined CAC. A modified stress–strain model is subsequently proposed. Finally, the practical and environmental implications of the present study are discussed.

Compressive behavior and analysis-oriented model of FRP-confined engineered cementitious composite columns

A total of 18 specimens of fiber-reinforced polymer (FRP) confined engineered cementitious composite (ECC) were tested under axial compression in this study. Both monotonic loading and cyclic loading were considered. Effects of different FRP materials, glass FRP (GFRP) tube and carbon FRP (CFRP) jacket, as well as different FRP thicknesses were investigated. All the specimens exhibited the typical strain hardening behavior, indicating the effective enhancement of compressive strength and strain of ECC under FRP confinement. Failure modes, stress–strain behavior and dilation behavior were presented and analyzed. The FRP-confined ECC showed large ultimate axial strains, demonstrating the superior deformability and ductility performance. Design equations were developed to predict the ultimate compressive strength and strain. Based on the test results obtained from this study and other test results collected in the literature, an analysis-oriented model was proposed to predict the overall compressive stress–strain behavior of FRP-confined ECC columns.

Size effect on compressive behavior of FRP-confined engineered cementitious composites (ECC)

• A first-ever experimental study on the size effect of GFRP- and LRS FRP-confined ECC columns was conducted. • The influences of specimen size, FRP confinement level, and FRP type were examined. • A new size-dependent ultimate stress model of FRP-confined ECC was established. The reliable design of the high-performance column made of fiber reinforced polymer (FRP)-confined engineered cementitious composites (ECC) necessitates a clear understanding of the size effect under axial compression. This paper presents the compressive behaviors of glass FRP (GFRP) and large rupture strain (LRS) polyethylene terephthalate (PET) FRP-confined ECC columns. The specimen size, confinement level, and FRP type were the variables. Five diameters of 100, 200, 300, 400, and 500 mm were designed to cover a wide size range. Results showed that both the compressive strength and ductility of ECC were substantially improved with FRP confinement. Compared with GFRP-confined ECC, PET FRP-confined ECC exhibited higher ductility. The key stresses, namely initial peak stress, post-peak ravine stress, and ultimate stress decreased with increasing specimen size. Nevertheless, the specimen size had a marginal influence on the key strains and dilation behavior. After evaluating the existing size-dependent ultimate strength models of FRP-confined concrete, a new expression for predicting the ultimate strength of FRP-confined ECC was introduced, which was shown to fit well with the test results.

A new cyclic model for FRP‐confined rubberized concrete

AbstractThis article proposes a new constitutive model for the cyclic behavior of rubberized concrete confined with fiber‐reinforced polymer (FRP) sheets. The model is calibrated with experimental results from 18 confined rubberized concrete (CRuC) cylinders tested in cyclic compression. The cylinders had 60% total aggregate volume replacement with recycled tyre rubber. Parameters investigated include the type of confining material (Carbon or Aramid FRP) and number of layers (two, three, or four). The results indicate that using FRP confinement leads to a strong (up to 100 MPa) and highly deformable (axial strains up to 7%) rubberized concrete that can be used in structural applications. The proposed constitutive model predicts accurately the material response under cyclic loading and can thus be used for design/analysis of highly deformable components made of FRP CRuC. This article contributes toward the development of advanced constitutive models for FRP CRuC, thus promoting the wider use of recycled materials in construction industry.

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.

Axial performance of grouted C-Shaped concrete block masonry columns jacketed by carbon and glass FRP

To overcome the relatively low ductility and capacity of masonry elements, the confinement technique may be an effective alternative for improving their performance, especially the columns. Fiber Reinforced Polymers (FRP) jacketing is a potential approach has benefits in terms of strength enhancement and gaining ductile behavior. Therefore, in this study, two types of FRP were used to improve column axial capacity and ductility: carbon and glass FRP. The effect of FRP wrapping on the compressive behavior of concrete masonry columns was investigated experimentally. Fully grouted scaled masonry columns constructed using C-shaped concrete blocks were wrapped with FRP jackets and tested under concentric compressive load until failure. For this purpose, 18 half-scale masonry prisms under variations in FRP jacketing and FRP thickness were developed and tested in six sets. The effect of FRP confinement on the unconfined masonry columns was evaluated. The CFRP and GFRP jacketing increased the peak strength and ultimate axial strain. Confinement improves post-peak performance by softening the descending stress–strain relationships. Increasing FRP jacket thickness improved the axial strain, strength, and ductility of masonry columns. The CFRP wrap gives 10 ∼ 16% more strength than GFRP. Ultimate strain enhanced by 76 % and 57 % for CFRP and GFRP respectively.

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.