Carbon Fiber Reinforced Polymer Tendons Research Articles

Experimental Investigation of the Tensile Properties with Bending of CFRP Tendons in Suspension Bridges

Carbon-fiber-reinforced polymer (CFRP) has gradually become a new material to replace traditional steel due to its outstanding advantages. Because of its poor transverse stress performance, there is a reduction effect on the tensile strength in the bending state. To study the mechanical properties of CFRP tendons subjected to combined tension and bending at the saddle of a suspension bridge, a series of bond-type anchorages were made. Specimens with different diameters of CFRP tendons were tensioned on the device with different bending radius saddles. The test results revealed that the tensile properties were significantly affected by the severity of the bending of the CFRP tendons, including the failure mode, fracture force, and stress distribution. The highest reduction in fracture force was found at the bending radius of 3 m, of up to 38.05%. Furthermore, the tensile properties were also found to be influenced by the diameter of CFRP tendons. It was found that increasing the bending radius was more conducive to improving the performance of CFRP tendons with a smaller diameter. When the bending radius increased from 3 to 12 m, the efficiency coefficient (the ratio of the fracture force to the ultimate force) of D8, D10, and D14 increased by 11.21%, 7.74%, and 2.26%, respectively. Decreasing the bending radius leads to unevenness of the stress distribution and increasing the diameter of the CFRP tendon leads to brittleness and difficulties in anchoring, thus resulting in the decrease in the efficiency coefficient. In addition, the ratio of the bending radius to the tendon diameter was less than 2.4, the efficiency coefficient of the specimen was less than 80%, and the specimen mostly suffered shear failure. Furthermore, the finite element (FE) models validated by the test results were used to reveal the stress state and study the effect of contact friction on the properties of CFRP tendons. The FE results show that the CFRP tendons with a smaller bending radius presented higher shear stress concentrations. As the contact friction increased, the load-bearing capacity of CFRP tendons decreased significantly.

Short-Term Performance of New Composite Anchorage with Multiple CFRP Tendons

The relatively low shear strength of carbon fiber-reinforced polymers (CFRPs) makes it difficult to anchor CFRP tendons. Although many anchorages with CFRP tendons have been developed, few studies have been conducted on composite anchorages with multi-CFRP tendons. In this study, four groups of new mechanical clamping–bonding composite anchorages with multi-CFRP tendons were designed, and the stress of each component of the anchorage was analyzed to reveal the force transmission mechanism of the anchorage. Moreover, the short-term performance of the anchorages was experimentally investigated. The key parameters, including the elastic modulus of the bonding medium, preloading force, cone angle, and bond length, were further analyzed using the finite-element method. The results showed that the composite anchorage reliably anchored multiple CFRP tendons, and the efficiency coefficients of the four anchorages were greater than 0.95. The suggested minimum bond length was approximately 35 times the diameter of the CFRP tendon when anchoring fewer than five tendons. Finally, a sufficient preloading force could effectively reduce the initial slip, whereas increasing the cone angle and elastic modulus could reduce the final slip.

Experimental and theoretical analysis on the long-term performance of novel carbon fibre-reinforced polymer tendon composite anchorage

Carbon fibre-reinforced polymer (CFRP) has the advantages of light weight, high strength and superior corrosion resistance. In this study, the long-term performance of novel CFRP tendon composite anchorages was assessed, and the influences of the number of tendons, load level and over-tensioning was analysed. Then, the residual anchoring capacity of the anchorage after long-term performance test was determined. Finally, the slippage of CFRP tendons and the load loss were analysed based on the time strengthening theory. Results revealed that the composite anchorage had excellent long-term performance, and the six groups of specimens were intact after a long-term experiment. Over-tensioning significantly improved the long-term performance of specimens by reducing the load loss caused by the tendon slippage and the slippage of each measuring point. Furthermore, results of the long-term experiment showed that the residual anchoring efficiency of the remaining specimens was greater than 0.95, except for one group of specimens, because of incomplete axial tension. Eventually, the theoretical analysis results showed that over-tensioning can effectively reduce the load loss caused by the creep effect of the free section tendon. In summary, the novel composite anchorage displayed excellent reliability. This study provides a research case for the long-term performance of CFRP tendon anchorage and a basis for the application of the novel composite anchorage in practical engineering.

Development of a Pretensioning Anchorage for Sand-Coated CFRP Tendons: Modeling and Validation.

This paper presents a finite element (FE) analysis of an anchor for prestressing of sand-coated carbon-fiber-reinforced polymer (CFRP) tendons during the manufacturing of precast elements. This anchorage is temporary and removed after 2 to 7 days, when the pretensioning is released and the concrete is finally compressed. The investigated anchor consists of a conical metal barrel and three polymer wedges. The main tendon material properties are measured, compared with theoretical values and define the basis for the FE model. The latter considers both steps, pressing-in of the wedges and the subsequent loading of the tendon (diameter 7.5 mm). The relatively soft contact between polymer wedge and sand-coating is characterized experimentally and implemented with a pressure-overclosure condition. For the validation of the FE model, the strain distribution in the tendon is measured using fiber optical sensing. The therefore crucial process of fiber integration is described, and a novel approach is presented to uncover the optical fiber. The strain distribution of a sample with two anchors loaded in tension up to 80 kN is presented. The stress concentration at the front of the anchorage is highlighted. In addition, the finite element model is compared with the experiment, showing a good agreement of the strain distribution. A failure criterion according to Puck is finally implemented, which allows to assess potential fiber or interfiber failure.

Solution Uptake in Cylindrical Carbon-Fibre-Reinforced Polymer (CFRP) Tendons

Salt water exposure conditions relevant to carbon-fibre-reinforced polymer (CFRP) prestressed concrete structures in marine environments are investigated. The diffusion into relatively small diameter CFRP tendons can be a lengthy process so the prediction of the long-term moisture uptake using short-term experiments on thin films of epoxy would be advantageous. However, the fibre inclusions within a composite introduce complexities, and factors are typically required for correlation with pure epoxy specimens. Diffusion parameters based on moisture uptake result from CFRP tendons exposed to salt water solution at 20°C and 60°C are compared with those obtained using equivalent thin film specimens. The higher temperature is selected to accelerate the moisture uptake. It is found that the measured ratios of tendon and epoxy diffusivity were temperature dependent, and the combination of the higher temperature and salt solution leads to an increased propensity for moisture uptake in the tendon. Existing analytical models to predict the CFRP tendon diffusivity from that of a thin film of epoxy did not appear to capture the observed trends. However, predictions using a unit cell with a fibre interface zone suggest that this may be due to an increased diffusivity in the interphase region.

Cyclic behavior of polypropylene fiber reinforced concrete beams with prestressed CFRP tendons and nonprestressed steel bars

The main purpose of this paper is to evaluate the cyclic behavior of polypropylene fiber-reinforced concrete (FRC) beams with a hybrid reinforcement system of prestressed carbon fiber reinforced polymer (CFRP) tendons and nonprestressed steel bars. A total of six beams are tested, including four beams prestressed with CFRP tendons, one prestressed with steel tendons and one reinforced with steel bars. The experimental variables include the partial prestressing ratio, amount of CFRP tendons, and jacking stress level. The test results are presented in terms of failure mode, hysteretic response, flexural capacity, self-centering capability, ductility, stiffness degradation and energy dissipation. The results show that the CFRP prestressed beam failing due to concrete crushing exhibits much higher ductility and dissipated energy than the beam failing due to CFRP rupture. An increase in the partial prestressing ratio or a decrease in the jacking stress has a favorable effect on the ductility. A numerical moment–curvature analysis procedure is applied to perform a parametric analysis of the ductility and deformability of the hybrid FRC section. It is recommended that the reinforcing index should be sufficient to ensure compression failure, but should not be greater than 0.35 to ensure reasonable ductility and deformability.

Bond behavior of an ultra-high performance concrete-filled anchorage for carbon fiber-reinforced polymer tendons under static and impact loads

To investigate the bond behavior of an ultra-high performance concrete (UHPC)-filled bond-type anchorage for carbon fiber-reinforced polymer (CFRP) tendons, both static tensile tests and impact tests were performed on 20 groups of specimens. The anchorages with embedded lengths of 50, 100, 150 and 200 mm were subjected to the static tension and longitudinal impact loads with drop heights of 150, 300, 600 and 900 mm. During the tests, the longitudinal tension and slip of the anchorage were obtained, and the effect of anchorage length and loading rate on the bond behavior was discussed. The results show that in the static tests, slip failure occurred for the anchorages with lengths from 50 to 150 mm, and CFRP bar ruptured as the length raised to 200 mm. When subjected to impact loads, slip failure was observed for all specimens. The damages on the CFRP bar to UHPC interface under impact loads were slighter than those under static tension, and correspondingly the dynamic bond strength was approximately half of the static one owing to the weakened mechanical interlocking. With increasing the embedded length, the bond strength and slip increased due to the radial constraining effect of outside steel tube. Based on the experimental results, a strain rate-dependent function was proposed to quantify the retention of bond strength under impact loads, and prediction formulas were established for determining the dynamic bond strength and critical embedded length of the bond-type anchorage.

Transient analysis of a novel full submersible floating offshore wind turbine with CFRP tendons

Floating offshore wind turbines (FOWTs) to exploit steady and abundant wind resources have attracted a great deal of attention in recent decades. The deep sea is rich in wind energy, but wind farms in these areas face some extreme environmental conditions, e.g., typhoons. A conceptual anti-typhoon full submersible platform (FSP) with carbon-fiber-reinforced polymer (CFRP) tendons, combining the advantages of existing various types of floating wind turbine foundations is proposed. The stability and tendon stresses of the proposed FSP were assessed through a combination of ANSYS-AQWA and FAST, and a comparison with other traditional platforms was made to verify the performance superiority of the proposed FSP. Additionally, the effects of mooring system type on the stability of the FSP under extreme wind speed conditions were investigated. Numerical results showed that the displacements of the proposed FSP in the surge and heave directions were significantly reduced, but in the pitch direction, they were not much different from those of the tension-leg platform. A hybrid mooring system can significantly reduce the surge displacements of the platform, which is essential for a high power output from a FOWT. In general, the proposed FSP is a promising and competitive offshore wind structure.

Experimental investigation and damage evaluation of a novel bond type anchorage for carbon fiber reinforced polymer tendons

Experimental investigation and damage evaluation of a novel bond type anchorage for carbon fiber reinforced polymer tendons

Simulation Research on Continuous Concrete Beams Reinforced with External Prestressed CFRP Tendons.

This paper examines the effects of different loading patterns on the static characteristics of continuous concrete beams reinforced with external prestressed carbon fiber-reinforced polymer tendons (EPCFRPT) and qualitatively analyzes the results for two continuous concrete beams: SB-1 under symmetrical loading, and SB-2 under asymmetrical loading. Then, a finite element analysis model is introduced and calibrated by tests conducted at Southeast University and data collected from a literature review. Based on the FEA model, the initial prestress, cross-section area, and eccentricity of CFRP tendons as well as the steel reinforcement configuration were selected by a parametric study. The results indicated that the initial prestress and tendon cross-section area had the most influence on the tendon stress increment and the secondary bending moment of the middle support, while the reinforcement distribution and eccentricity of the tendons had little effect. The secondary bending moment had a linear positive correlation with the stress increment of tendons. These results allow a simplified equation for calculating the external load bending moment amplitude coefficient at ultimate to be proposed.

Charpy impact properties of uni-directional carbon fiber-reinforced polymer tendons with protective layers

The present study explored potential forms of impact protection for carbon fiber-reinforced polymer (CFRP) tendons. Charpy impact tests were performed on CFRP rods and strands under protections of polyvinyl chloride (PVC) and poly-ethylen (PE). In the experimental program, the impact damage pattern and absorbed energy of CFRP tendons with and without external protection were obtained. The results show that the energy absorption capacity of CFRP tendons was improved under the protection of surface layers; and PE layers exhibited the highest protection effectiveness among all protection forms in terms of the increased absorbed energy and caused additional structural weight. Besides, two empirical expressions were established to quantify the relationship between the thickness of PE layers and the energy absorption capacity of the protected CFRP tendons, which may provide a simple tool to estimate the thickness of the layer required for the protection of a specific CFRP tendon in practical applications.

Effects of High Temperatures on the Performance of Carbon Fiber Reinforced Polymer (CFRP) Composite Cables Protected with Fire-Retardant Materials.

In this study, the safe critical temperature that can be tolerated by CFRP tendons under normal working conditions was derived through tensile tests at room and high temperatures. Next, the times required to reach a safe critical temperature for CFRP cables protected with different types of fire-retardant materials of various thicknesses were determined through fire resistance tests, Finally, fitting the surface of the finite element simulation results allowed the establishment of the temperature rise calculation model of CFRP tendons under the protection of fire-retardant materials. The results showed that 300 °C can be regarded as the safe critical temperature. Both high-silica needled felt and ceramic fiber felt exhibited high fireproof performance. With an increase in the thickness of the fire-retardant material, the time for the CFRP tendon to reach the inflection point of the heating rate increased, and the safe fire resistance time increased exponentially. According to the HC temperature rise curve, the fire resistance time of CFRP tendons protected by 24 mm thick high-silica needled felt was 45 min, and that for CFRP tendons protected by 24 mm thick ceramic fiber felt was 39.5 min. Under the action of fire corresponding to the hydrocarbon temperature rise model, the safe fire resistance time of CFRP tendons protected by 45 mm high-silica needled felt or 50 mm ceramic fiber felt was more than 2 h, sufficient to meet the specification. The proposed model of fire resistance performance enables the determination of the thickness of the fire resistance material required to obtain different degrees of fire resistance for CFRP cables for structural use.

Pseudo-Ductility Through Progressive Failure of Multi-Layered Carbon-Fiber-Reinforced Polymer (CFRP) Prestressed Concrete Beams

The design codes/standards recommend using an over-reinforced beam section for FRP-reinforced structures to use the ductility of concrete to give adequate warning prior to complete failure. In contrast, the present study is based on the nonlinear response prediction of beams prestressed with carbon-fiber-reinforced polymer (CFRP) tendons and designed under-reinforced. The beam is designed here such that a progressive rupture of the CFRP tendons occurs, i.e. they rupture one after the other, introducing a kind of pseudo-ductility, which is enhanced owing to the additional inelastic energy resulting from the sequential rupturing of the tendons. A parametric study has been conducted on CFRP-prestressed concrete beams with different numbers of tendon layers, varying prestressing ratios and several cross sections moreover, a sacrificing warning rebar has been introduced for improving the ductility further. The findings showed that the prestressed concrete beam with multiple layers of CFRP tendons resulted in an improved ductile design as compared to the single-layered system. Additionally, grading of concrete in the beam section has been made to compensate for the reduction in the load-carrying capacity due to the layering of tendons. The pseudo-ductility of such functionally-graded beams has been found to be improved, with energy ratios as high as 78.7% and having comparable load-carrying capacity.

FLEXURAL BEHAVIOR OF STEEL FIBER-REINFORCED LIGHTWEIGHT AGGREGATE CONCRETE BEAMS PRESTRESSED WITH CARBON FIBER-REINFORCED POLYMER BARS

An unbonded prestressing anchorage system for carbon fiber-reinforced polymer (CFRP) tendons was introduced. Eight lightweight aggregate concrete (LWC) beams prestressed with unbonded CFRP tendons which used CFRP auxiliary reinforcements and one fabricated using normal weight concrete (NWC) were tested under a four-point bending condition. The influences of concrete type, of prestressed degree and, of clear span length on the cracking moment, on the moment-deflection response and, on the crack width were analyzed. The existing deflection and crack width models for unbonded FRP prestressed beams with steel auxiliary reinforcements were modified from the view of equivalent axial stiffness. The applicability of the modified models was evaluated based on the test results. It is shown that the serviceability limit state of the beams is mainly governed by the crack width limit. The ratios between the moment at crack width of 0.5 mm and the ultimate moment range from 41% to 52%. With the same total reinforcement amount, the increase in the prestressed degree helps delay the cracking of the specimens. Increasing the amount of the auxiliary reinforcement and of the unbonded prestressed tendons both improve the beam stiffness and restrain the propagation of the cracks. The modified Cheng’s and modified Meng’s deflection models provide accurate deflection predictions for LWC beams with unbonded prestressed and auxiliary CFRP reinforcements.

Optimal Design of a Novel Composite Anchorage for Carbon-Fiber-Reinforced Polymer (CFRP) Tendons.

In this study, we proposed a novel composite anchorage that considers the anchoring performance and dimension simultaneously. The design concept of this composite anchorage was first introduced, followed by comparison with the traditional inner-cone bond-type anchorage and traditional composite anchorage through theoretical and experimental methods. Then, a parametric study was conducted to determine the influence of different parameters on the anchoring performance, and the optimal design parameters were recommended according to the finite element (FE) and test results. Finally, the practicability of the optimal design parameters were validated through experiments on the anchorage with multiple CFRP tendons. Results showed that the novel composite anchorage could improve the anchoring performance compared with the traditional inner-cone bond-type anchorage by promoting increased anchorage efficiency by 60.4% and, with an ideal failure mode of tendon rupture. Moreover, the novel composite anchorage had smaller dimensions and avoided the presence of a vulnerable position at the junction of the mechanical and bond parts compared with the traditional composite anchorage. In addition, a group of optimal design parameters of this composite anchorage with a pre-tightening force of 130 kN, an inclinational differential angle of 0.1°, an inclination angle of 2.9°, and an embedded length of 30 d~40 d were proposed. The composite anchorage with five CFRP tendons designed with the proposed parameters failed with the rupture of the tendons and exhibited an anchoring efficiency of 1.05. This result showed that the optimal parameters were suitable for this novel composite anchorage to grip multiple tendons. This study can provide an experimental and theoretical basis for designing large-tonnage anchorage for multiple FRP tendons used as hangers or cables in real bridges.

Shear behavior of innovative high performance joints for precast concrete deck panels

Precast concrete deck panels (PCDPs) are gradually applied in accelerated bridge construction (ABC) to achieve the goal of the fully prefabricated bridge. The joints between PCDPs are vulnerable under complicated stress state. The mechanical properties, feasibility of construction, and durability of joints are the focus to be investigated. This study proposed an innovative joint for PCDPs with high performance materials, including ultra-high performance concrete (UHPC) and carbon fiber-reinforced polymer (CFRP). The shear behavior of the novel joint was investigated on full-scale PCDPs via the static monotonic experiment. In addition, the 3D finite element numerical models were established to study the shear behavior, failure mechanism and influencing parameters of the proposed joints in PCDPs. Furthermore, the analytical models to predict the shear capacity of the proposed joints were developed. The results show that the proposed joints exhibit more reasonable failure mode and possess superior deformation ability compared to conventional joints. The initial stress and anchorage distance of CFRP tendons have obvious effect on the shear capacity. The proposed analytical model is able to accurately evaluate the shear capacity of the high performance joints for PCDPs. This study offers a novel joint type and provides guidance for the design of PCDPs.

Analysis of Multi-Tendon Friction-Based Composite Anchorage Device for CFRP Cables and Its Anchorage Mechanism

Carbon fiber-reinforced polymer (CFRP) cables are anticipated to be employed in larger, longer, and more durable structures in the engineering field. However, its anchorage devices and mechanism should be appropriately developed and improved. At present, mainly relying on the adhesive force, most anchorage devices may lose their efficiency because of adhesive aging and failure or the slip of an individual tendon. A friction-based composite anchorage device with an integrated bearing of inner cone filler (i.e., load transfer media (LTM)) bonding and a single extruding anchor is proposed, and the anchorage mechanism is examined for Φ7 CFRP cables of strength 2400 MPa. Firstly, sufficient conditions for anti-slip failure of CFRP tendons in the anchorage zone are derived by assuming uniform LTM bonding. The obtained results reveal that the smaller inner pore size of the barrel leads to higher efficiency. Additionally, the maximum efficiency depends on the friction coefficient of the contact surface, the inner cone angle of the barrel, and the diameter and quantity of the CFRP tendons. The necessary conditions for the safety of the CFRP tendon anchorage zone are carefully obtained based on the Tsai–Wu failure criterion. It is concluded that the compressive stress of CFRP tendons in the anchorage zone should gradually increase from the load-bearing end to the no-loading end. Additionally, the relations among the anchorage efficiency coefficient and the CFRP tendon diameter d, the anchorage length l, the dip angle of LTM external conical surface α, and the friction angle β are derived based on the equivalent failure principle. The CFRP cables of four specifications (i.e., with Φ12, Φ19, Φ37, and Φ121 tendons) are designed under theoretical guidance, and eight static tests are carried out for more verification studies. The test results indicate that the anchorage efficiency coefficient of designed anchorage devices can be over 90%, and even up to 96.8%. Further, the failure modes are divergent destruction, which verifies the reliability of friction-based anchorage devices and provides a solid theoretical foundation for the design and engineering applications of CFRP cables.

Fatigue prediction model of ultra-high-performance concrete beams prestressed with CFRP tendons

In the present paper, a comprehensive study on the flexural fatigue behavior of ultra-high-performance concrete (UHPC) beams prestressed with carbon-fiber-reinforced polymer (CFRP) tendons is reported. A total of two UHPC beams prestressed with CFRP tendons were experimentally investigated. On the basis of the fatigue constitutive model of the materials, a fatigue prediction model (FPM) was developed to simulate the flexural fatigue evolvement of the beams. The strain and stress in UHPC and CFRP tendons were calculated by the sectional stress analysis. The influence of steel fiber was considered in the formulae for the crack resistance and crack width, and the midspan deflection was calculated using the sum of deflection before cracking and increment after cracking. The obtained test results were used to verify the FPM. A parametric study was then conducted to analyze the fatigue development of such component, and a formula to predict the flexural fatigue life of UHPC beams under different fatigue loads was proposed.

Shear behavior of ultra-high-performance concrete beams prestressed with external carbon fiber-reinforced polymer tendons

Shear behavior of ultra-high-performance concrete beams prestressed with external carbon fiber-reinforced polymer tendons

Flexural Behavior of UHPC Beams Prestressed with External CFRP Tendons

This paper presents an experimental investigation on the flexural behavior of ultra-high-performance concrete (UHPC) beams prestressed with external carbon fiber-reinforced polymer (CFRP) tendons. A total of eight T-shaped beam specimens were fabricated and tested, and the effects of the effective prestressing stress, partial prestressing ratio, deviated angle, and loading condition on the flexural behavior were analyzed. The experimental results indicate that the fully prestressed beams experienced a brittle failure, and the shear capacity of these beams was mainly controlled by the effective prestressing stress in CFRP tendons and the ultimate tensile strength of UHPC, whereas the partially prestressed beams failed in a ductile manner. The presence of internal steel reinforcement could significantly improve the flexural capacity and deformation ability. Thus, internal reinforcements should not be omitted from UHPC beams with CFRP tendons. A higher effective prestressing stress resulted in enhanced cracking load and flexural capacity. The deviated angle enhanced the utilization efficiency of high strength CFRP tendons. The loading condition exerted a slight influence on the flexural behavior of the specimens. Moreover, a method considering the effect of steel fibers was proposed and verified to predict the flexural capacity of UHPC beams prestressed with external CFRP tendons.