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Premature debonding failure of fiber-reinforced polymer (FRP) laminate is a primary reason for an accelerated onset of low working stress in the FRP of reinforced concrete (RC) structures strengthened with externally bonded (EB) FRP. Hybrid-bonded (HB) FRP can effectively prevent the debonding failure of FRP, leading to a significant enhancement in the ultimate strength of the HB-FRP system. The HB-FRP method mainly relies on external positive pressure provided by the anchoring device to improve the interfacial bond strength. Therefore, the magnitude of the positive pressure determines the strengthening effect of the anchoring system. This paper first optimizes the existing HB mechanical anchoring device. The positive pressure exerted on the FRP was adjusted by varying the torque to the anchoring device. The optimized anchoring device was used to study the bond behavior of the FRP-to-concrete interface under different torques. The test results showed that the debonding stress could be regulated by adjusting the torque, leading to a change in failure mode of the FRP-concrete interface. With the increase of torque, the utilization rate of FRP and ductility increased. When the torque was higher than a certain threshold, the FRP ruptured, after which, the continuously applied torque had no significant effect on the FRP-concrete interfacial bond behavior. Based on the experimental results, an ultimate bond strength model of the FRP-to-concrete interface under different torques was developed. Compared to the experimental results, the model showed satisfactory accuracy.

The majority of existing cyclic stress–strain models for fiber reinforced polymer (FRP) confined concrete are applicable only to cases where post-peak strain hardening occurs. Cyclic model catering for strain softening is rare due to the lack of sufficient experimental data. Recent experimental tests on FRP-confined concrete cylinders involving strain-softening have identified new factors that have a significant effect on the cyclic behavior. Through an analytical study, a newly defined parameter, the effective confinement rigidity, is found to be a key factor governing the cyclic softening and hardening. By including the additional key factors and using the latest database with more strain-softening cyclic stress–strain curves, a stress–strain model of FRP-confined concrete subjected to cyclic loading considering both post-peak hardening and softening is proposed. Compared with the existing models, the proposed model can predict the cyclic behavior of FRP-confined concrete with better accuracy.

This paper presents an analytical model to predict the bond behavior of hybrid bonded (HB) fiber reinforced polymer (FRP) strengthened concrete structures considering different confining effects. The bond strength of HB-strengthened concrete structures subjected to uniaxial loading is attributed to the friction-type bond from steel plate and the chemical bond provided by epoxy-based resin. The friction-type bond is assumed to develop before any interfacial slip occurs, and remain constant when the interfacial slip is nonzero; for the chemical bond between FRP and concrete, a tri-linear bond-slip relation is adopted. The loading process of HB-strengthened structures consists of three stages, and the stage-wise solution to slip, bond stress, and axial force of HB FRP are derived based on a governing ordinary differential equation (ODE). In this study, the experimental program includes twelve HB-strengthened concrete specimens, using different confining effects (twisting moments on the steel bolts). Test results indicated that the ultimate pullout strength increases and the failure mode switches from debonding to FRP rupture as the twisting moment increases. Moreover, by comparison to three series of experimental results, the proposed model can precisely predict failure mode, ultimate strength, load-slip relation and bond stress distribution. The twisting moment on bolts is found to have the most significant improvement on the ultimate tensile strength. Finally, the critical twisting moment without yielding FRP rupture is derived.

Externally bonded fiber-reinforced polymer (FRP) to a degraded reinforced concrete structure is currently a very popular strengthening technology; however, premature debonding mainly caused by concrete crack propagation leads to the underutilization of FRP. Engineered cementitious composite (ECC) has an excellent crack control capability. When used in combination with FRP, ECC can effectively delay the debonding of FRP and enhance the bearing capacity and ductility of FRP-strengthened structures. For the safe and economic strengthening design, the bond-slip relationship of FRP-ECC-concrete needs to be properly understood. This paper investigated the bond behavior of FRP–ECC–concrete interface through a series of simple shear tests. The effects of ECC thickness, concrete surface treatment, and construction method on the bond behavior were evaluated. The deformation and strain data of FRP were collected and analyzed using a noncontact full-field strain measurement system VIC-3D (three-dimensional video image correlation system). The experimental results show that the addition of an ECC layer delayed the debonding of FRP and significantly increased the maximum strain of FRP. The bearing capacity, the ultimate slip, and the interface energy consumption capacity also improved significantly.

Double skin tubular columns (DSTCs) filled by ultra-lightweight cement composites (ULCCs), as primary load-bearing elements, are a practical and reasonable structural asset in lightweight civil infrastructure. This DSTC-ULCC system combines the advantages of three materials, 1) outer fiber-reinforced polymer (FRP) 1–3 ply jackets, 2) filled ULCCs, and 3) inner steel tubes to achieve lightweight, high strength and superior ductility. This article presents an experimental study on the mechanical behavior of DSTCs with FRP and ULCCs under axial compressive loading . The experiment designed a total of 26 DSTCs that were all filled with ULCCs. The key parameters were the number of FRP layers and the inner steel tube properties, (i.e., thickness and diameter of the steel tube). The test results indicate that the properties of the inner steel tube not only influences the effective FRP rupture strain ratio but also the ultimate axial strain of confined ULCCs. This calls attention to the inward buckling behavior for larger void ratios; moreover, thinner steel tubes result in more axial deformation of ULCC-filled DSTCs. In addition, ULCCs showed a much lower elastic modulus compared with normal concrete, which helped cause the change of transition stress of ULCCs in DSTCs, and FRP confinement can effectively increase the elastic length of the stress–strain curves for ULCCs. The experimental results are subsequently compared with predictions from an FRP-confined ULCC model, which led to a new ultimate strain model proposed herein for ULCC-filled DSTCs. The comparison demonstrates that the model can provide reasonably accurate predictions of the stress–strain curves of ULCCs in hollow DSTCs.

This paper presents an experimental study on the axial stress–strain behavior of carbon fiber-reinforced polymer (CFRP)-confined seawater sea-sand recycled aggregate concrete (SSRAC) columns. Forty-two circular and square specimens were tested under axial compression, and the effects of the aggregate replacement ratio, CFRP thickness, and corner radius were investigated. The results indicated that, by replacing natural aggregates with recycled aggregates (RAs), the CFRP-confinement effectiveness can be enhanced, showing a higher enhancement ratio in the ultimate strength and strain. The effect of the corner radius ratio on the ultimate condition of CFRP-confined SSRAC is also more pronounced compared with CFRP confined natural aggregate concrete (NAC). This conclusion was also validated by the nonuniform hoop strain distribution for specimens with varying corner radius ratios. The difference in CFRP confinement efficiency caused by the recycled aggregate replacement and corner radius is accurately captured by the DIC (Digital image correlation) measurement system. By reasonably taking the coupling effect of RAs and the corner radius ratio into account for the confinement efficiency parameter, the proposed ultimate strength and strain model for CFRP-confined SSRAC demonstrates a satisfactory performance when compared with test results.

Monotonic stress–strain relationship of actively-confined concrete has been used as the base model to establish analysis-oriented stress–strain model of fiber reinforced polymer (FRP) confined concrete. This approach is based on the assumption that the axial stress and strain of FRP-confined concrete are the same as those of actively confined concrete under the same confinement pressure and lateral strain. In this study, an experiment was conducted to verify this assumption for concrete subjected to cyclic loading. A total of 31 actively confined and FRP-confined concrete cylinders were tested. The results indicate that this assumption is not applicable to concrete under cyclic loading; a gap was found between the envelop curves of the two types of confined concrete. In addition, the test results also reveal that confinement pressure significantly affects both reloading modulus and plastic strain which are the main factors controlling cyclic behavior of confined concrete.

A stress–strain model is proposed for fiber reinforced polymer (FRP) confined concrete subjected to cyclic compression. This model employs an existing monotonic stress–strain relationship as the envelope curve. The cyclic load paths are formed by using a new and simple algebraic function. Concrete strength and confinement stiffness ratio are selected as the main factors that govern the parameters of the unloading and reloading paths. Available experimental results are used to evaluate the model parameters. The model uses more appropriate mathematical forms and includes significant factors ignored previously by others which makes it not only more rational and accurate but also simpler than other existing models reported in the literature.

Engineered cementitious composite (ECC) exhibits good post-cracking resistance and ductility and its unique properties make it a viable material for increasing structural performance under severe loading conditions. To understand the bond behavior between the ECC and steel bar, direct pullout tests were conducted in this paper. A comprehensive analysis was carried out on effects of ECC strength, steel bar diameter, and fiber reinforced polymer (FRP) confinement on steel bar-ECC bonding behavior. Based on the experimental data, performances of existing bond slip models for steel bar-ECC are evaluated and a new model with improved accuracy is proposed. The proposed model considers the influence of FRP confinement on bond-slip behavior.