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In the steel-FRP composite bar (SFCB) reinforced concrete filled in an outer FRP tube, the bond behavior of SFCB in such FRP-confined concrete should differ from that in unconfined concrete. This paper aimed to understand the local bond behavior of SFCB in FRP-confined concrete. Sixty pullout specimens with variables (including the concrete strength, concrete cover-to-bar diameter ratio, and FRP confinement stiffness) were tested. The test results showed that the bond failure mode of SFCB was changed from concrete splitting in unconfined-concrete to pull-out in FRP-confined concrete. The study revealed that the development of FRP confinement promoted significant bond growth after concrete splitting and meaningful post-failure ductility. Greater FRP confinement stiffness resulted in the more pronounced the bond strength enhancement. In addition, a bond-confinement relationship at the bond interface was developed. Accordingly, the enhancement mechanism of SFCB in FRP-confined concrete is clarified: 1) FRP confinement substituted the cracked concrete in resisting to the wedge-action of SFCB; 2) FRP confinement improved the strength and deformability of interface concrete in resisting the shear-action of SFCB; 3) FRP confinement improved the friction. Finally, a bond strength model with a simple form was proposed, which can provide an accurate prediction of the bond strength of SFCB in FRP-confined concrete based on a confinement factor and the existing model in unconfined concrete. • The bond behavior of steel-FRP reinforced bar in FRP-confined concrete is studied. • The enhancement mechanism of FRP confinement on the bond is revealed. • The bond-confinement interaction at the bar-concrete interface is developed. • The bond strength model for SFCB in FRP-confined concrete is established. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study is conducted to gain insight into the bond behavior of ribbed fiber-reinforced polymer (FRP) bar in FRP-confined concrete. Various parameters, including the ratio of concrete cover to bar diameter, concrete strength, and FRP confinement level, are considered in this study. The test results demonstrate the effectiveness of FRP confinement in changing the bond failure mode of FRP bar from the concrete splitting to the pullout and improving bond ductility and strength. The bond response of ribbed FRP bar in FRP-confined concrete is characterized by three distinct stages: uncracked stage, cracking enhancement stage, and cracking degradation stage. The bond mechanism within each stage is elucidated. The study reveals that the normalized bond strength is distinctly influenced by the ratio of concrete cover to diameter and FRP confinement level, while concrete strength exhibits a negligible impact. Finally, the study proposes a novel model for predicting the bond strength of the ribbed FRP bar embedded in both unconfined and FRP-confined concrete. • The bond behavior of ribbed FRP bar in FRP-confined concrete is studied. • The bond mechanism of ribbed FRP bar in FRP-confined concrete is elucidated. • The bond strength model for FRP bar in both unconfined and FRP-confined concrete is developed. [ABSTRACT FROM AUTHOR]

• Cyclically loaded elliptical concrete confined by FRP (FCC) is investigated. • Mechanism of non-uniform hoop strain distribution is clarified. • Cyclic behavioral characteristics of elliptical FCC are properly modeled. 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. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP)-steel tubed reinforced concrete (FSTRC) columns present promising engineering applications due to the reasonable integration of three constituent materials. Compared with the numerous studies on their performance under monotonic loading, relatively few studies have focused on their cyclic axial behavior. Moreover, the available models for plain concrete solely confined by FRP might be incapable of estimating the confined concrete behavior in cyclically loaded FSTRC due to the possible effect induced by the steel reinforcement in concrete, the additional steel tube confinement, and the girth gaps at column ends. To this end, this paper investigated circular FSTRC stub columns under monotonic and cyclic axial loading. The test results confirmed that the inclusion of reinforcement had noticeable effects on the unloading path since the tensile axial stresses in longitudinal steel bars slowed down the stiffness degradation of the inner concrete during unloading. For envelope cycles, the increase of the additional steel confinement level had a negligible impact on the plastic strain and the stress deterioration of the inner concrete. A confinement model was developed to estimate the full-range behavior of the FRP-steel dually confined reinforced concrete under cyclic axial loading with favorable accuracy. [ABSTRACT FROM AUTHOR]

The strain and damage self-sensing properties of carbon nanofibers (CNFs)/flax fiber-reinforced polymer (FFRP) laminates under tension were investigated via simultaneously measuring the changes of electrical resistance (ER) and acoustic emission (AE) signals. The piezoresistive mechanisms together with the damage evolution of CNFs/FFRP laminates were also explored. The results revealed that both ER and AE responses to tensile strains in CNFs/FFRP laminates could be segmented into three stages, which confirmed their good damage self-sensing ability. The isochronous and reversible electrical resistance responses to tensile strains proved the stability and repeatability of strain self-sensing capability for CNFs/FFRP laminates. Moreover, in-situ ER measurement is sensitive not only to new damages but also to existing damages, whereas AE signals are only sensitive to new damages. Therefore, adding a small amount of conductive CNFs into non-conductive FFRP laminates could provide an effective strategy to achieve the self-sensing ability in their strain and damage development. [ABSTRACT FROM AUTHOR]

• Rectangular FRP-concrete-steel double-skin tubular columns (DSTCs) are tested. • Four parameters are considered to study the axial compressive behavior of DSTCs. • The axial stress-strain curves of confined concrete are approximately trilinear. • Increasing aspect ratio has positive effect on the behavior of confined concrete. • A design-oriented three-stage stress-strain model of rectangular DSTCs is proposed. This paper presents the experimental results of monotonic axial compression tests on eighteen glass fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular stub columns (DSTCs) with a rectangular outer glass FRP tube and an elliptical/rectangular inner steel tube, as well as eight rectangular glass FRP-confined hollow stub columns (FCHCs) with an elliptical inner void. The effects of cross-sectional aspect ratio, FRP thickness, void area ratio and the shape of inner steel tube (i.e., elliptical and rectangular) on the axial compressive behavior of confined concrete core in rectangular DSTCs are considered and analyzed. The test results demonstrate that rectangular DSTCs possess excellent bearing capacity and ductility. Additionally, the axial stress-strain curves of confined concrete core in rectangular DSTCs are approximately trilinear and show obvious post-peak softening phenomenon because of the non-uniform and insufficient confinement. Furthermore, the larger aspect ratio, the thicker FRP tube, the higher void area ratio, as well as elliptical inner steel tube (compared with rectangular) have positive confinement effect on the axial compressive behavior of confined concrete core in rectangular DSTCs. Based on the analysis of experimental data, a design-oriented three-stage stress-strain model (considering post-peak softening behavior) of confined concrete in rectangular DSTCs with elliptical inner steel tube is proposed. Through the comparison, it is found that the predictions calculated by the proposed model accord well with the experimental results. [ABSTRACT FROM AUTHOR]

• Elliptical FRP-confined high-strength concrete stub columns are axially loaded. • Increasing aspect ratio always has adverse effect on behavior of inner concrete. • Typical stress–strain curves of confined concrete are categorized into four types. • Threshold for sufficient confinement of elliptical FRP-confined concrete is given. • A design-oriented model for elliptical FRP-confined concrete is developed. Elliptical fiber-reinforced polymer (FRP)-confined concrete members have gained increasing attention due to their aesthetical characteristics and structural efficiency. This paper investigated the behavior of 24 elliptical FRP-confined concrete stub columns under axial compression. Effects of the amount of FRP confinement, the cross-sectional aspect ratio, and the use of high-strength concrete were discussed. The test results indicated that the stress and strain capacities of the confined concrete were enhanced with the increasing amount of FRP confinement. By contrast, the increasing sectional aspect ratio and the use of high-strength concrete had an adverse effect on the inner concrete's axial behavior due to the increasingly non-uniform distribution of FRP confining pressure. In general, the typical stress–strain curves of elliptical FRP-confined concrete were categorized into four types. Furthermore, using the concept of equivalent diameter, the threshold for sufficient confinement of elliptical FRP-confined concrete was determined by the equivalent confinement stiffness ratio to ensure an overall ascending behavior. After addressing some key issues, a design-oriented model for elliptical FRP-confined concrete with a relatively wider range of concrete strength (from 32.6 to 72.4 MPa) was developed herein with more satisfactory accuracy than existing models. This model with a simple form could be naturally reduced to a widely accepted model for FRP-confined concrete with circular sections. [ABSTRACT FROM AUTHOR]

• Bond durability of GFRP bars in concrete under immersion and load was investigated. • Degradation mechanism of GFRP bars in concrete was clarified. • Sustained load could accelerate the degradation of the bond performance. • Bond strength after 50 years was predicted according to fib Bulletin 40. In this paper, the bond durability of glass fiber-reinforced polymer (GFRP) bars in seawater sea-sand concrete (SWSSC) under the coupling effect of seawater immersion and sustained load was experimentally investigated. The test parameters included bar diameter (12 and 16 mm), immersion time (90, 180, and 270 days), and sustained load condition (loaded and unloaded). Test results showed that for the specimens with a small bar diameter, the bond strength had a slight growth at the initial stage of exposure mainly due to the water swelling of the bar, and then decreased due to the bond degradation. For the specimens with a large bar diameter, the bond strength increased with the increase of immersion time, which was determined by the increasing compressive strength of SWSSC. The sustained load reduced the bond strength of the specimens with a small bar diameter due to its acceleration of bond degradation, but had little effect on the bond strength of the specimens with a large bar diameter. The degradation mechanism was the rib deterioration of the bars caused by seawater immersion. Moreover, the sustained load could accelerate the degradation. The predicted bond strength retention of the specimen with a bar diameter of 12 mm under the coupling effect of the marine environment (moisture saturated and the mean annual temperature of 25–35 °C) and sustained load is 52% after 50 years of service. [ABSTRACT FROM AUTHOR]

The bond between FRP bars and concrete, the foremost performance for implementation of such reinforcements to corrosion-free concrete structures, is still unsatisfied due to the weak nature of duplex film in the interface. The existing approaches show low efficiency in improving the microstructures and bond between FRP bars and concrete. To address these issues, this paper provided a new approach for improving the bond between FRP bars and concrete by incorporating nanomaterials, as well as explored the modifying mechanisms and established the bond-slip models. For these purposes, the pull-out test, scanning electron microscope observation, as well as energy dispersive spectrometry analysis were performed. The experimental results demonstrated that the presence of nanomaterials increased the ultimate bond strengths between glass/carbon FRP bars and concrete by up to 16.2% and 37.8%, while the corresponding slips decreased by 28.7% and 35.4%, respectively. Such modification effects can be attributed to the optimized intrinsic composition and the reduced pore content of hydration products in the interface, especially in the duplex film, through the nanomaterial enrichment and nano-core effects. The bond-slip relationship between FRP bars and concrete with nanomaterials can be accurately predicted by the mBPE model. [ABSTRACT FROM AUTHOR]