External Fiber-reinforced Polymer Tendons Research Articles

Open issues on the use of carbon composite tendons as prestressed reinforcement of concrete members

In the last decades, fiber-reinforced polymer (FRP) composites gained popularity in the field of structural engineering thanks to their high mechanical properties and limited corrosion in harsh environments. These composites are used as reinforcement of new concrete members as well as for strengthening of existing structures. For existing reinforced concrete (RC) structures, different techniques are available to increase the strength and stiffness of the existing elements. Among them, a promising technology consists in inducing to the element a bending moment opposite to that induced by the applied load by using external prestressed FRP tendons. This technique increases the load carrying capacity of the beams and induces concrete crack closure, thus reducing the exposure of steel bars to the environment.In this paper, a literature review is carried out to investigate the open issues that hinder the diffusion of prestressing made with composite carbon fiber tendons. According to the literature, the main issue is represented by the anchor system that should guarantee high stress level into the composite material avoiding local ruptures. Another open issue is the definition of the maximum stress applied to the composite with respect to its tensile strength. In fact, high levels of stress may induce premature composite failure. Finally, the exposure of CFRP bars to some harsh environmental conditions and high temperatures may negatively affect the reinforcement effectiveness.

Parametric Study on Steel–Concrete Composite Beams Strengthened with Post-Tensioned CFRP Tendons

A sustainable environment can be achieved by strengthening the existing building to avoid new construction and by replacing the construction materials with long-lasting sustainable materials such as a fiber-reinforced polymer (FRP). Using post-tensioned (PT) FRP systems has proven to be an effective technique in strengthening the structure and decreasing cracks and deformability. In this study, a 3-D finite element model was built to investigate the flexural behavior of composite beams strengthened with external PT FRP tendons. Limited research studied the use of FRP tendons to enhance the structural behavior of composite beams. This paper represents a comprehensive study of the effect of several parameters that control the design of the FRP tendons. Parameters such as PT level, tendon material, tendon length, degree of shear connection (DOSC), and tendon profile shape were considered under loading. The 3-D model’s correctness is validated using published experimental data. It was observed that of all FRP materials, carbon FRP is the best type for upgrading the beam strength, and it was recommended to use a 30 to 40% PT level. In addition, applying external PT over the full length of the beam increases the ultimate load capacity significantly. However, due to the difficulty of construction, it was recommended to use 90% of the beam span length since the difference in beam capacity does not exceed 5%. Finally, adding PT tendons with a trapezoidal and parabola profile to composite beams significantly increases the yield load and the beam capacity.

Optimization of anchorage and deviator for concrete beams prestressed with external fiber-reinforced polymer tendons

This paper focuses on the optimization of anchorage and deviator for concrete beams prestressed with external basalt fiber-reinforced polymer (BFRP) tendons. A composite-wedge (CW) anchorage for BFRP tendons was first developed. The dimensions of the wedge were optimized using finite element (FE) analysis. Additionally, a three-dimensional deviator-tendon FE model was established, and the ratio of the curvature radius of the deviator to the radius of the tendon (Rd/Rp) and the bending angle (θ) of the BFRP tendon were optimized. Furthermore, an experimental study on the flexural behaviors of a non-prestressed concrete beam and a beam prestressed with external BFRP tendons was conducted. The results show that the proposed CW anchorage achieved an anchor efficiency factor of 85%. The recommended lower limit value of Rd/Rp is 200, and the upper limit value of θ is 5°. The RC beam prestressed with external BFRP tendons exhibited enhanced flexural behaviors, compared with the non-prestressed RC beam. The BFRP tendons at the anchor zones and tendons at the deviators remained intact at the ultimate load of the beam. The results validate the effectiveness of the anchorage and deviator parameters.

A review of studies on concrete structures prestressed with external fiber reinforced polymer composites tendons

The studies on concrete structures prestressed with external fiber reinforced polymer (FRP) tendons are reviewed in the aspects of FRP tendon, key technology and structural component, in this review. The tensile properties and long-term behaviors of FRP tendon are firstly introduced. The design-oriented values of creep-rupture stress, relaxation rate and the limits of maximum fatigue stress and fatigue stress range are provided. The advantages and deficiencies of three main types of anchor for FRP tendon, and the methods of reducing the stress concentration on FRP tendon at anchor are elaborated. The newly-developed composite-wedge anchor is emphasized, which possesses an anchor efficiency coefficient exceeding 90%. Meanwhile, the curvature radius of deviator is recommended to be larger than 200 times of the radius the cross-section of FRP tendons, and the bending angle of FRP tendons should not exceed 5°, based on the experimental results on the mechanical properties of deviated FRP tendons. The experimental results of concrete beams prestressed with external FRP tendons are reviewed, including monotonic loading, sustained loading and cyclic loading. The design methodologies in the codes at home and overseas are introduced. The accuracies of the calculating methods in the codes are evaluated using the experimental data of forty-two beams, and the methods in the Chinese code GB 50608—2020 are validated to be accurate in the design calculation for concrete structures prestressed with external FRP tendons. This paper is expected to actively promote the popularization and application of concrete structures prestressed with external FRP tendons.

Numerical study on bending response of precast segmental concrete beams externally prestressed with FRP tendons

This study numerically investigates the bending response of dry key-jointed precast segmental concrete girders/beams (PSCBs) prestressed with external fiber reinforced polymer (FRP) tendons by using commercial finite element analysis (FEA) software Abaqus/CAE. The experimentally-validated model was used to conduct an intensive parametric analysis with a focus on the second-order effect. There has not been a similar numerical study of PSCBs with external FRP tendons in the published literature yet. The results showed that due to the rectilinear rigid-body bending shape, the behavior of PSCBs with external tendons was similar to that with internal tendons only if placing the deviators next to the opening joints. The second-order effect on the beam’s behavior and the harping effect on the tendon stress at deviators became more obvious when the deviators were located away from the opening joints. Both the second-order and harping effect were proportionate to the beam’s displacement. Therefore, using a high reinforcing index (ω) or a low span-to-depth ratio (L/dp) could mitigate the second-order and harping effect at the ultimate stage because the ultimate displacement of the beam decreased when increasing ω or reducing L/dp. Commonly-used CFRP tendons (Young’s modulus Ep = 145 GPa) were found to be the optimum to replace steel tendons in PSCBs with external tendons because they offered the PSCBs similar strength and ductility compared to steel tendons. The use of high-modulus CFRP tendons (e.g. Ep = 200 GPa) improved the stiffness and strength of PSCBs but greatly reduced the beam’s ductility. Lastly, the analytical analyses showed that the existing models yielded unconservative estimations of the effective depth (dpu) and stress (fpu) of external FRP tendons at the ultimate stage in PSCBs.

Numerical Study of Using FRP and Steel Rebars in Simply Supported Prestressed Concrete Beams with External FRP Tendons.

This study aimed at examining the feasibility of using fiber-reinforced polymer (FRP) rebars instead of steel ones in prestressed concrete beams (PCBs) with external FRP tendons. By applying an experimentally validated program, numerical tests were performed on simply supported PCBs, with investigated variables including rebars’ type and area. Three types of rebars were considered, i.e., carbon, glass FRPs (CFRP, GFRP), and reinforcing steel. The ratio of tensile rebars ranged from 0.22% to 2.16%. The results indicated that the beams with CFRP rebars exhibited better crack mode and higher ultimate load than the beams with GFRP or steel rebars. GFRP rebars led to considerably higher ultimate deflection and tendon stress increment than steel rebars. In addition, several models for calculating the ultimate stress in unbonded tendons were assessed. An analytical model was also proposed to predict the tendon stress at ultimate and flexural strength in externally PCBs with steel and FRP rebars. The model predictions agreed well with the numerical results.

Experimental Evaluation of Connections in Hybrid Precast Concrete Bridge Truss Girders

An innovative, corrosion-free, precast, prestressed concrete truss girder has been developed for short- and medium-span, slab-on-girder bridges. The girder consists of top and bottom concrete flanges connected by precast vertical and diagonal members and made of fiber-reinforced polymer (FRP) tubes filled with concrete. The verticals and diagonals are connected, respectively, to the concrete flanges by means of glass FRP dowels and stud reinforcement made of corrosion-resistant steel or FRP material. The flanges are pretensioned with carbon FRP tendons. The deck slab is reinforced with corrosion-resistant steel bars in the bottom transverse layer and with glass FRP bars in the bottom longitudinal and the top layers. The girders may be posttensioned with external carbon FRP tendons to balance the slab weight and to provide continuity in multispan bridges. The new system has the advantages of light weight and enhanced durability. The light weight reduces the initial cost and allows for longer spans. The improved durability reduces the maintenance cost and extends the structure's life span. This paper describes the general details of the system and presents an experimental evaluation of its critical components, namely, the FRP tubes and the truss connections. Two types of FRP tubes and four types of connections were investigated. The results are presented of tests of eight connection specimens under static loading and four specimens under fatigue. The tests showed excellent performance of the connections when filament-wound tubes and continuous double-headed studs were used.