Fiber Reinforced Polymer Tendons Research Articles

A quantitative residual stiffness model for carbon fiber reinforced polymer tendons

AbstractIn this study, tension‐tension fatigue tests were conducted to investigate the residual stiffness degradation of carbon fiber‐reinforced polymer (CFRP) tendons. Different stress levels were used in the tests, and measurements of residual stiffness and the number of loading cycles were taken. Based on experimental data for CFRP tendons, a quantitative residual stiffness model was developed by modifying Yao's model. This model is applicable to various stress levels. To assess its accuracy and applicability, the predicted results of this model were compared with those of cited models from other researchers. The findings revealed a three‐stage degradation of residual stiffness in CFRP tendons under different stress levels. Furthermore, it was observed that the proportion of fatigue life accounted for by Stage III decreased with smaller stress ranges, while the proportion accounted for by Stage II increased. The proposed quantitative residual stiffness model was verified using both experimental and cited data.

Effect of Bond Condition on Cyclic Behavior of Post- Tensioned Concrete Beams with Carbon Fiber-Reinforced Polymer Tendons

Effect of Bond Condition on Cyclic Behavior of Post- Tensioned Concrete Beams with Carbon Fiber-Reinforced Polymer Tendons

Experimental Study on the Flexural and Shear Performance of Concrete Beams Strengthened with Prestressed CFRP Tendons

To investigate the influence of external prestressing with carbon fiber-reinforced polymer (CFRP) tendons on the flexural and shear performance of reinforced concrete beams, a total of six simply supported beams with a length of 3.5 m and two two-span continuous beams with a length of 7 m were studied. The influences of the prestressing level of CFRP tendons, the pattern of straight-line and flexural reinforcement of externally prestressed CFRP tendons, and the existence of initial cracks in the beams on the flexural bearing capacity of the reinforced beams were explored through modeling tests. The calculating equation for anticipating the ultimate flexural bearing capacity of concrete beams reinforced with externally prestressed CFRP tendons was modified based on the test results. The findings indicate that the flexural bearing capacity of the test beams was significantly enhanced through both flexural and straight-line reinforcement. Specifically, the ultimate flexural bearing capacity of the simply supported beams exhibited improvements of approximately 160% and 170%, respectively, whereas the ultimate flexural bearing capacity of the continuous beams was enhanced by approximately 113% through the reinforcement of flexural arrangement. Additionally, the shear resistance capacity of the concrete beams was notably improved through the flexural arrangement reinforcement, and the shear resistance capacity of the beams exhibited a substantial increase as the prestressing level of the CFRP tendons increased. The flexural moments in the vicinity of the support point of the reinforced beam exhibited an increase ranging from 41.7% to 112.5% when the stirrup strains started to escalate rapidly. Additionally, the rigidity of the test beams showed a close correlation with both the prestressing level and the existence of initial cracks.

Effect of initial pretension on dynamic bond behavior of an ultra-high performance concrete-filled anchorage for carbon fiber-reinforced polymer tendons

This paper is focused on the effect of initial pretension on the dynamic bond performance of the anchorage under impact loads. Drop-weight impact tests were conducted on 15 groups of specimens with four embedded lengths and various pretension ratios; the effects of pretension and embedded length on the bond strength and slip of the anchorage are discussed in the paper. The test results show that a higher pretension ratio led to severer surface damage on the CFRP tendons. With the increase of the pretension ratio, the mechanical interlocking between CFRP ribs and UHPC was improved, leading to an increased average bond strength. By contrast, the relatively large shear deformation and severe damage of CFRP ribs induced by pretension formed “initial defects”, which resulted in a lower residual bond performance during impact loading. Based on the test results, a pretension ratio-dependent strength retention function was proposed to quantify the shear damage effect of CFRP ribs caused by pretension; by incorporating this function, accurate formulas were developed for determining the dynamic bond strength of a pretensioned anchorage.

A Study on the Performance of Prestressed Concrete Containment with Carbon Fiber-Reinforced Polymer Tendons under Internal Pressure.

As the last barrier to preventing nuclear leakage, it is crucial to enhance the load-bearing capacity and cracking resistance of nuclear containment under internal pressure accidents. Currently, fiber-reinforced polymers are widely used in prestressing concrete structures because of their superior performance, but little research has been conducted on fiber-reinforced polymers in the field of nuclear power plants. In this paper, carbon fiber-reinforced polymer (CFRP) is used as a prestressing tendon material instead of traditional steel strands to study the damage mode of the new type of containment under internal pressure and the feasibility of using CFRP as prestressing tendons. In this study, a three-dimensional refinement model is established, employing ABAQUS 2020 software to analyze and quantify the pressure-bearing performance of nuclear containment with CFRP tendons and finally determine the reasonable range of CFRP tendons that can be used as a replacement. The research shows that the containment with CFRP tendons has an obvious strengthening effect in delaying the generation of cracks, restraining the speed of crack development, reducing the plastic damage of the steel liner, and improving the ultimate bearing capacity of the containment.

Effect of temperature on epoxy resin deterioration for fibre-reinforced polymer reinforcement in simulated concrete pore solutions

Fibre-reinforced polymer (FRP) tendons have excellent mechanical properties and corrosion resistance and are widely used in concrete structures; however, corrosion deterioration of FRP tendons is a serious problem in a highly alkaline concrete environment owing to the hygrothermal ageing effect of the resin. Accordingly, this study investigated the macro and micro properties evolution of epoxy resin in a simulated concrete pore solution at different temperatures. The results showed that the resin properties deteriorated significantly under long-term hygrothermal immersion. However, short-term high-temperature soaking caused some of the resin to undergo post-curing, improving its properties.

Verification of Composite Beam Theory with Finite Element Model for Pretensioned Concrete Members with Prestressing FRP Tendons.

Composite beam theory was previously developed to establish an analytical solution for determining the transfer length of prestressed fiber-reinforced polymers (FRP) tendons in pretensioned concrete members. In the present study, a novel finite element (FE) modeling approach is proposed to provide further verification of the developed analytical method. The present FE model takes into account the friction coefficients obtained from pull-out tests on the FRP tendons and prestressed concrete members. Convergence analysis of two numerical simulations with different mesh densities is carried out as well. The results demonstrated that the transfer length predicted by the fine FE model with a friction coefficient of α = 0.3 for high pretension is in good agreement with the measured values and the analytical solutions. The consistency between the analytical solution and FE simulation not only further proves the reliability of composite beam theory but also demonstrates the importance of the bond-slip relationship in predicting the transfer length of pretensioned concrete members prestressed with FRP tendons.

Damage Assessment of Segmental Concrete Beams with Different Types of Joints and Prestressed External Tendons

Precast segmental beams with prestressed external tendons have become increasingly common in bridge constructions owing to the merits of high-quality control and fast onsite construction with minimum environmental impact. Due to the nonlinear characteristics of the dynamic behavior of these segmental beams associated to the joint opening, the traditional condition monitoring and damage assessment methods based on vibration parameters either in time or frequency domain using the linear theory are unsuitable. To overcome this challenge, this paper proposes a damage assessment method through analyzing nonlinear structural dynamic responses for monitoring the conditions of segmental beams with different joint types and prestressed external tendons. Two damage indices are proposed to identify the occurrence of damage and to qualitatively indicate the damage severity. In the proposed approach, the measured dynamic responses of the segmental concrete beams under hammer impact are first adaptively decomposed into a finite number of mono-components by using variational mode decomposition (VMD) technique. Then, the instantaneous frequencies of the decomposed mono-components from the dynamic responses are obtained by conducting the Hilbert transform. Two damage indicators are defined for the damage assessment and for the identification of the damage location, respectively. Three four-span segmental beams with different types of joints (i.e. dry joints or epoxy joints) and prestressing tendon types (i.e. steel and carbon fiber reinforced polymer tendons) were constructed and tested under five loading levels with different damage severities. The feasibility and accuracy of the proposed approach are verified by comparing the identification results with the secant and initial stiffness extracted from the load–deflection curves at different loading levels. The effectiveness of these two damage indicators is also validated with the structural conditions observed in the tests. The results show that the proposed approach can successfully identify damage in segmental concrete beams.

Analysis of beams prestressed with hybrid (Steel/Carbon Fiber-Reinforced Polymer) tendons using the conservation of energy approach

Durability of post-tensioned concrete members is greatly enhanced by using Carbon Fiber-Reinforced Polymer (CFRP) tendons in lieu of, or in combination with, unbonded and bonded steel tendons. However, current design and analysis methods are limiting the use of hybrid (i.e., a combination of unbonded and bonded steel and CRFP) tendons since they are only applicable to either bonded or to the rather complicated post-tensioned unbonded steel tendons. This paper presents an analytical approach developed to predict the overall response of concrete beams prestressed with hybrid tendons. The model is based on a structural idealization that uncouples the concrete beam and the unbonded tendon behaving as a trussed-beam system. The approach utilizes equilibrium equations, compatibility of deflection between beam and tendon, and the conservation of energy approach (CEA) applied at various limit states. Additionally, a simplified CEA is also presented at characteristic points for various cases including unbonded as well as hybrid tendons. Results are validated using experimental tests available in the literature indicating that CEA is a versatile and rational procedure that can accurately predict the overall flexural behavior of beams with hybrid tendons. A simplified version of the CEA using hand calculations is also developed for directly analyzing such beams with comparably accurate results.

Effect of Tendon-Related Variables on the Behavior of Externally CFRP Prestressed Concrete Beams.

This work assesses the flexural performance of prestressed concrete beams with external carbon fiber-reinforced polymer (CFRP) tendons, focusing on tendon-related variables. A finite element analysis (FEA) method is verified. A numerical parametric analysis of prestressed concrete beams with external CFRP tendons is carried out. Four tendon-related variables are considered, namely, the area, initial prestress, depth and elastic modulus of tendons. The analysis shows that flexural ductility decreases as the tendon area, initial prestress or elastic modulus increases but is insensitive to the tendon depth. The ultimate tendon stress increment (Δσp) is influenced by all of the four variables investigated. JGJ 92-2016 (Chinese technical specification for concrete structures prestressed with unbonded tendons) significantly underestimates Δσp and, hence, is over-conservative for the strength design of these beams. An equation is proposed for calculating Δσp, taking into account all four variables investigated. An analytical model is then developed to estimate the flexural strength (Mu) of prestressed concrete beams with external CFRP tendons. The proposed analytical model shows good agreement with FEA, i.e., the mean discrepancy for Δσp is 0.9% with a standard deviation of 11.1%; and the mean discrepancy for Mu is -1.6% with a standard deviation of 2.1%.

Cyclic load tests and finite element modeling of self-centering hollow-core FRP-concrete-steel bridge columns

This paper presents experimental and numerical investigations of the seismic performance of a novel self-centering (SC) hollow-core (HC) fiber-reinforced polymer (FRP)-concrete-steel (SC-HC-FCS) bridge column. This new structure is fabricated by mounting external energy dissipators (ED) and applying unbonded post-tensioned (PT) basalt FRP (BFRP) tendon to the conventional HC-FCS column that consists of an outer FRP tube and an inner steel tube, with the space between filled with concrete. The SC-HC-FCS column combines the advantages of accelerated bridge construction and self-centering. The effects of the initial prestress force values and the configuration of the energy-dissipated aluminum bar on the column performance are studied. Based on the experimental and numerical results, it is found that the proposed SC-HC-FCS column shows adequate self-centering and energy dissipation capacities. However, it is required to properly select the configuration and material properties of the aluminum bar as energy dissipators to further refine the seismic resistance of the SC-HC-FCS column.

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.

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.

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.

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.

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.

Long-term mechanical behaviors of uncracked concrete beams prestressed with external basalt fiber-reinforced polymer tendons

In this study, an experiment was conducted on four uncracked concrete beams prestressed with external basalt fiber-reinforced polymer (BFRP) tendons or stress-relieved strands subjected to 300-day sustained loads. The experimental variables included tendon type, prestress level (40 kN and 50 kN), and concrete strength grade (C40 and C60). The effects of these variables on the long-term deformation and prestress loss of the beams were discussed and analyzed. Based on an existing approach using an age-adjusted effective modulus method (AEMM), a modified model was proposed for predicting the long-term deformation and prestress loss of concrete beams prestressed with external BFRP tendons. The results indicated that the prestress level had significant effects on the long-term deformation and prestress loss. The concrete strength grade significantly affected the long-term deformation, but had no effect on the prestress loss. The stress-relieved strands exhibited a prestress loss that was 16% greater than that of the BFRP tendons. The theoretical results based on the modified model were consistent with the experimental results. The results of this study contribute to the design and prediction of the long-term behaviors of concrete beams prestressed with external BFRP tendons.

Serviceability of Beams Prestressed with Hybrid (Steel/ Carbon Fiber-Reinforced Polymer) Tendons

Serviceability of Beams Prestressed with Hybrid (Steel/ Carbon Fiber-Reinforced Polymer) Tendons