Carbon Fiber Reinforced Polymer Tendons Research Articles

Experimental Research and Analysis on Fatigue Life of Carbon Fiber Reinforced Polymer (CFRP) Tendons.

The fatigue life of carbon fiber reinforced polymer (CFRP) tendons was studied in this paper. A new wedge-type anchorage system was applied to the fatigue test of CFRP tendons and demonstrated an excellent fatigue resistance. In the test and analytical data, the fatigue stress ranged from 200 MPa to 800 MPa, and maximum stresses from 0.37 to 1.0 ( = ultimate tensile strength of CFRP tendons) were determined. The main work and results were that the stress range and stress level (maximum stress) were two key parameters affecting the fatigue life of CFRP tendons. A bilinear equation and a linear equation considering the fatigue life of CFRP tendons jointly affected by the stress range and the maximum stress were established. The error of predicted results and test results was 0.038 and 0.083, respectively, both representing good prediction accuracy. The predicted results of Whitney’s method showed that, at a 95% confidence level, when the stress range was 200 MPa, 400 MPa, and 600 MPa, the maximum stress limit of CFRP tendons, which were not broken in a fatigue test of 2 million times, was 63.9% , 53.0% , and 36.8% , respectively.

Static and Fatigue Behavior of Multitendon CFRP Cables with Integrated Anchorages

This paper presents a high-capacity anchorage system for multitendon carbon fiber–reinforced polymer (CFRP) cables and the associated construction technology. Four groups of cable anchorage systems used 37 CFRP tendons, each with a diameter of 4 mm, and extensively investigated their behavior through static and fatigue tests. In the static test, the synchronous performance of the cables was studied. The mechanical properties of the cables in the anchor zone were compared with the results from a finite-element (FE) analysis, and the anchoring efficiency also was evaluated. Furthermore, the fatigue behavior of cable anchorage system was verified. The relative displacement of each component, distribution of the shear stress, retention rate of the stiffness, and strength of the cables were discussed. The results indicate that the cable exhibits excellent synchronous tensile performance. The stress and displacement distributions of the cable in the anchor zone were in good agreement with the results from the FE analysis. The stress concentration was greatly reduced by the variable stiffness of the load transfer medium via layered casting technology, and the anchoring efficiency reached 97%. The cable anchorage system sustained 2 million cycles without any macroscopic cracks forming in either the cable or anchorage at a maximum stress of 0.45fs (the standard strength value) and a stress range of 200 MPa. The residual strength and stiffness of the cable reached 95% and 96%, respectively, compared with the initial behavior.

Effect of internal moisture content and dynamic mechanical analysis testing conditions on the Tg values of CFRP tendons

The high temperature and long-term performance of uni-directional pultruded carbon fibre reinforced polymer (CFRP) tendons in prestressed concrete relies on the integrity of the epoxy matrix and the degree of crosslinking. The dynamic glass transition temperature (Tg) obtained using Dynamic Mechanical Analysis (DMA) can be a useful indicator of the degree of cross-linking. However, variations in the epoxy plasticisation due to different initial hygrothermal equilibrium conditions in CFRP tendons can influence the measured Tg leading to inconsistent results. The dependency of Tg on the initial CFRP moisture content, and also DMA test parameters including the thermocouple position, the magnitude of static flexural strain, the static to dynamic strain ratio and the specimen geometry were investigated. Whereas the selected DMA test parameters had only a small influence on the Tg values, the initial moisture content of the CFRP tendon had a significant effect. A decrease in Tg of up to 38 °C was observed with an increase in average moisture content of up to 2.46%. Knowledge of the initial moisture content and the establishment of a comparative baseline are necessary to more accurately infer the crosslinking density and degree of cure of a resin from dynamic Tg results.

Flexural design of a modular footbridge system with pretensioned carbon fiber reinforced polymer reinforcement

AbstractA common problem of concrete footbridges are corrosion damages of the steel reinforcement. The related loss of capacity as well as visual damages often require expensive and elaborate refurbishment or even reconstruction. To overcome these drawbacks, a modular footbridge system without steel reinforcement is developed. The application of noncorrosive carbon fiber reinforced polymer (CFRP) reinforcement is suitable for building slender constructions, which are durable and long‐lasting. For the elements of the modular bridge system CFRP reinforcement is applied as mesh fabrics and pretensioned tendons. To enhance the durability and reduce costs, high strength concrete (HSC) with a high density is applied. Another advantage of the high density is that no additional surface is required. The modular construction method allows for fast assembly and disassembly of the footbridge.In this paper, the dimensioning and flexural design of the modular footbridge system are presented. In a first step, the requirements regarding cross sections, spans, and level of prestressing are defined based on natural conditions and applied loads. The results are analyzed considering the realization of a modular construction. In a second step, the flexural design of the pretensioned footbridge is presented. In this context, a flexural design model for beams pretensioned with CFRP tendons based on Eurocode 2 is developed.

Effect of Temperature on Material Properties of Carbon Fiber Reinforced Polymer (CFRP) Tendons: Experiments and Model Assessment.

Material properties at elevated temperatures are important factors in the fire safety design and numerical analysis of concrete members strengthened with fiber reinforced polymer (FRP) composites. Most of the previous research mainly focused on tensile strength and elastic modulus in conventional steady state temperature tests. However, the transient state test method is more realistic for strengthening concrete structures. At the same time, the coefficient of thermal expansion of FRP composites is also one of the important factors affecting concrete members at elevated temperatures. This paper presents a detailed experimental investigation on the longitudinal thermal expansion deformation, and the mechanical properties of carbon FRP (CFRP) tendons with 8 mm diameter in the steady state and transient state. The results indicate that longitudinal deformation of CFRP tendons is negative at high temperature; in addition, the transient state test results of CFRP tendons are slightly higher than the steady state test results. The final part of this paper assesses the accuracy of different empirical models. Furthermore, a new equation calculating the properties of CFRP composites at elevated temperatures is presented with the numerical fitting technique, which is in good agreement with the experimental results.

Evaluation and prediction of carbon fiber–reinforced polymer cable anchorage for large capacity

Aiming to address the problems of stress concentration on conical wedge anchorage, a fiber-reinforced polymer cable anchorage with segmental variable stiffness of the load transfer medium was proposed. The key parameters that affect the anchorage behavior were investigated. The mechanical properties of the carbon fiber–reinforced polymer tendon and load transfer medium were tested. The failure mode, anchoring efficiency, stress, and displacement in the anchor zone were studied. The parameter optimization was performed using an experimentally verified finite element simulation. The parameters of the anchorage system with large capacity were evaluated. The results demonstrate that the compressive strength of the load transfer medium is the designed stress limit for the anchorage system. The cable does not slip or become damaged in the anchor zone, and the anchoring efficiency reaches 91%. The distribution of the shear and radial stress on the cable surface is smooth, and the stress concentration is greatly relieved. The result of the finite element simulation is consistent with the experimental values when the friction coefficient is 0.15, and the material and geometric parameters of the anchorage system with cable forces of 5000, 10,000, 15,000, and 20,000 kN are suggested. The geometric parameters of the anchor system with diverse cable capacity can be preliminarily designed based on the fitting equations.

Shear behavior of concrete beams pre-stressed with carbon fiber reinforced polymer tendons

This article presents experimental and numerical studies on the shear behavior of reinforced concrete beams pre-stressed with carbon fiber reinforced polymer tendons. A total of 23 beams were tested to analyze the failure mode and shear performance of pre-stressed concrete beams. Experimental results revealed that there were two typical shear failure modes, that is, shear compression failure and inclined compression failure. Next, the experimental and numerical results were used to explore factors influencing the failure mode and the shear behavior of the concrete beams, including the type of pre-stressing tendons, stirrup ratio, shear span–depth ratio, number of pre-stressing tendons, and their initial pretension levels. It is demonstrated that shear span–depth ratio and stirrup ratio are the two main determinants of the failure mode and shear capacity of the concrete beams pre-stressed with carbon fiber reinforced polymer tendons. However, the bonding conditions of the pre-stressing carbon fiber reinforced polymer tendons have no significant effect on the shear capacity of the pre-stressed concrete beam.

Fatigue Behavior of Concrete Beam with Prestressed Near-Surface Mounted CFRP Reinforcement According to the Strength and Developed Length.

The prestressed near-surface mounted reinforcement (NSMR) using Fiber Reinforced Polymer (FRP) was developed to improve the load bearing capacity of ageing or degraded concrete structures. The NSMR using FRP was the subject of numerous studies of which a mere portion was dedicated to the long-term behavior under fatigue loading. Accordingly, the present study intends to examine the fatigue performance of the NSMR applying the anchoring system developed by Korea Institute of Construction and Building Technology (KICT). To that goal, fatigue test is performed on 6.4 m reinforced concrete beams fabricated with various concrete strengths and developed lengths of the Carbon Fiber Reinforced Polymer (CFRP) tendon. The test results reveal that the difference in the concrete strength and in the developed length of the CFRP tendon has insignificant effect on the strengthening performance. It is concluded that the accumulation of fatigue loading, the concrete strength and the developed length of the tendon will not affect significantly the strengthening performance given that sufficient strengthening is secured.

Prediction and optimization of stress distribution in bonded anchors for CFRP tendons

This study proposed a universal theoretical model for predicting the stress distributions in bonded anchors for carbon fibre reinforced polymer (CFRP) tendons based on the elasticity theory. The proposed model is adaptable for predicting the stress distributions of straight anchors as well as anchors with different conical angles, and it was validated by experimental results. A parametric analysis was conducted to examine the effect of the structural form of anchors, conical degree and bonding material on the stress distributions in the anchor zone based on the developed theoretical model. The results reveal that the resistance against the slippage of the tendon increases with respect to the conical degree. However, by having a high conical degree, serious stress concentrations as well as a high risk of shear and compressive damage appear in the anchor zone. The conical degree from 3° to 4° is suggested for the anchors with a conical length of no less than 200 mm. The straight segment at the free end in a straight-conical-straight anchor has little effect on the stress distributions whereas the straight segment at the loading end reduces significantly the stress concentration and even out the stress distribution in the anchor zone. It is suggested that the length ratio of the conical to the straight segment at the loading end be kept in between 4:1 to 2:1, which has a positive effect on the stress distributions by reducing the maximum stress as well as transferring the tensile load from the loading end towards the free end more efficiently. The bonding medium influences the stress distributions as well. A homogeneous bonding medium with a lower modulus produces better stress distributions in the anchor zone. An improvement in the stress distributions becomes more distinct with the application of a bonding medium with a gradually decreasing modulus from the free end to the loading end. A thicker bonding medium generates a more uniform stress distributions in the anchor zone, but the increase of the thickness has to be limited due to a larger deformation appearing simultaneously in the bonding medium.

Experimental Investigation on the Creep Property of Carbon Fiber Reinforced Polymer Tendons under High Stress Levels.

Carbon fiber reinforced polymer (CFRP) tendons are generally used as prestressing members to take full advantage of their high strength. Their creep property is one of the key factors influencing the reliability and safety of the structures, especially under sustained high stress. In this study, using a new wedge-type anchorage system, experimental research was carried out on the creep behavior of CFRP tendons under high stress levels from 0.69 to 0.85 fu. All the tests lasted for a duration of 1000 h. It was found that the creep coefficient tends to increase with the stress level. Compared to their static properties, the residual strength of CFRP tendons after creep tests is 4.54% lower while the after-creep elastic modulus is 6.99% higher. Through data analysis, a semi-logarithm linear relationship between the creep coefficient and time was established, and the creep coefficients at 1 million hours were extrapolated.

Bond behaviour of CFRP tendons in UHPFRC

For a safe design of prestressed concrete members, the bond behaviour of prestressing tendons needs to be investigated. In general, for prestressing tendons, an a priori determination of the transfer length is essential. For thin concrete elements, the minimum thickness to provide a split-free transmission zone has to be determined. To realise thin concrete members, high performance cementitious composites are advantageous. Ultra-high performance fibre reinforced concrete (UHPFRC) provides high compressive and tensile strengths with ductile material behaviour. The application of non-corrosive reinforcement, e.g. carbon fibre reinforced polymer (CFRP), allows for filigree concrete elements with a thin concrete cover to only fulfil bond requirements. For the material combination of CFRP reinforcement and UHPFRC, the bond behaviour was investigated experimentally and theoretically. By means of pull-out tests local bond strengths were determined for different release states of jacking forces (i.e. lateral extension of the reinforcement). Tests on small-scale beams were conducted to determine transfer lengths and minimum dimensions of thin UHPFRC elements for two types of CFRP prestressing tendons. Simulations and further investigations on the bond behaviour were performed using non-linear finite element modelling as well as an analytical approach.

Performance of precast segmental concrete beams posttensioned with carbon fiber-reinforced polymer (CFRP) tendons

Precast segmental prestressed concrete beams (PSBs) have been widely used in many elevated highway bridge projects around the world. Steel tendons at joint locations, however, are vulnerable to corrosion damages, which cause deteriorations and in extreme cases lead to the collapse of the whole structures. This study experimentally investigates the use of carbon fibre-reinforced polymer (CFRP) tendons as an alternative solution for the PSBs to tackle the corrosion issue. Four large-scale segmental T-shaped concrete beams with internal bonded or unbonded tendons and dry or epoxied joints were built and tested under four-point loading. The test results indicated that CFRP tendons showed satisfactory performances therefore could replace steel tendons for the use in PSBs. All the tested beams exhibited excellent load-carrying capacity and ductility. Tendon bonding condition greatly affected the flexural performance of the segmental beams. Joint type had only a slight effect on the load-carrying capacity and ductility of the beams, but significantly affected the beams’ initial stiffness. Unbonded tendons experienced an evident reduction in the tendon strength at the ultimate stage as a consequence of the loading type, harping effect and joint opening. Both AASTHO-1999 and ACI 440.4R-04 predicted well the tendon stress, thus the load-carrying capacity of the beams with bonded tendons, however, the accuracy significantly reduced for the cases with unbonded tendons. Similarly, the codes did not well estimate the deformation capacity of the prestressed beams with unbonded tendons. An empirical formula is proposed to predict the deflections of beams with unbonded tendons, which yields very close predictions to the experimental results.

Bond Durability of Carbon Fiber–Reinforced Polymer Tendons Embedded in High-Strength Concrete

The structural performance of carbon fiber–reinforced polymer (CFRP) pretensioned structures is controlled by the bond between the CFRP tendons and concrete. The bond strength of CFRP sand-coated tendons can be affected by humid environments due to the porous epoxy matrix structure or by defects in the external sand coating layer of CFRP tendons, e.g., due to storage conditions. Pullout tests were carried out to assess the bond strength performance of sand-coated tendons embedded in high-strength concrete and immersed in water at either 23 or 40°C. Sand-coated CFRP tendons with two different core diameters of either 4.2 or 5.4 mm were studied. To assess the effect of the sand coating coverage on the bond, half sand-coated and uncoated tendons were also tested. An image processing technique was developed to help correlate bond strength variations with variations in the sand coating. An average difference of 24% between the bond strengths of the half sand-coated and full sand-coated tendons was recorded. A large scatter in the pullout results for the sand-coated tendons of diameter 5.4 mm was observed and this was attributed to the manufacturing process. There was no clear trend of bond strength degradation in the sand-coated tendons even after roughly 1.5 years of full immersion in water irrespective of the exposure temperature. However, an increase in the bond strength of the uncoated tendons and in the bond stiffness of all CFRP tendons was observed. This was felt to be the result of concrete autogenous shrinkage in high-strength concrete and the potential swelling effects of the tendons in a humid concrete environment. Analytical models are used to describe the bond stress-slip behavior, and their suitability for sand-coated CFRP tendons is studied.

Experimental investigation on the mechanical properties of a bond-type anchor for carbon fiber reinforced polymer tendons

Carbon fiber reinforced polymer (CFRP) has gained popularity in civil engineering applications due to its high tensile strength, light weight, corrosion resistance, and fatigue resistance. However, the anchorage of CFRP tendons has been a challenge due to the relative low transverse shear strength of CFRP. To study the mechanical properties of bond-type anchor for CFRP tendons, straight-type, inner-cone-type, and composite-type anchors were developed in this study. The bond-slip behavior of CFRP tendons inside anchors and multiaxial stresses of the steel sleeve were experimentally tested. The effects of anchor type and length on the anchorage performance of CFRP tendons were studied with the focus on the anchoring mechanism. Test results demonstrate that the composite-type anchor exhibits the most reliable load-transfer mode and can eliminate the notch effect occurred in the inner-cone-type anchor. The peak bond stress initially occurs at the loading end of the anchor and gradually moves to the free end of the anchor as the load increases. This is related to the gradual failure of chemical adhesive between the CFRP tendon and the colloid. Besides, bond-slip relationship of CFRP tendon inside anchor is nonlinear. The slippages of CFRP tendons are initially small but increase quickly with increasing load. When approaching failure, the slippages increased significantly. In addition, the slippage of the anchor with scattered-end tendon is smaller than that with non-scattered-end tendon, indicating that the anchor with scattered-end tendon has superior bonding property.

Experimental study on fatigue performance of adhesively bonded anchorage system for CFRP tendons

Abstract This study is concerned with the fatigue performance of adhesively bonded anchorage for carbon fiber reinforced polymer (CFRP) tendons subjected to cyclic loading. Series of monotonous and cyclic experiments under different stress amplitudes and loading frequencies were carried out in order to investigate the mechanical performance of bonded anchorage systems for CFRP tendons and to study the influence of cyclic loading on the anchoring performance. The fatigue damage and the relationships between damage, loading frequency and interfacial temperature rise of anchorage systems were analyzed. The results show that cyclic tension-tension loading is instrumental in enhancing the synergistic interaction between the anchorage components and it also stabilizes the entire anchorage system performance provided that the stress amplitude is kept lower than 10% of the ultimate tensile capacity of the anchorage systems and the maximum stress is less than 50% of the ultimate capacity of the anchorage system. The loading frequency influences significantly the temperature variation in the anchorage system. A high loading frequency may result in a sharp temperature rise at the early-stage and mid-stage of cyclic tension-tension loading, followed by a rather stable late stage. This phenomenon of temperature rise during fatigue tests may indicate the extent of fatigue damage in the anchorage system.

Flexural behaviour of precast segmental concrete beams internally prestressed with unbonded CFRP tendons under four-point loading

This study investigates the use of carbon fibre reinforced polymer (CFRP) tendons on precast segmental beams (PSBs) to tackle the corrosion problems which are likely to occur at joint locations of PSBs prestressed with steel tendons. Up to date, the use of CFRP tendons was extensively documented for monolithic beams while their application on PSBs has not been reported yet. Three precast segmental T-section beams including two beams with unbonded CFRP and one with steel tendons were built and tested under four-point loads in this study. The test results showed that CFRP tendons can be well used to replace the steel tendons on PSBs. The beams with CFRP tendons demonstrated both high strength and high ductility as compared to the beam with steel tendons. However, the stresses in the unbonded CFRP tendons at ultimate loading conditions of the tested beams were low, ranging from only about 66% to 72% of the nominal breaking tensile strength. The type of joints i.e. dry and epoxied, greatly affects the initial stiffness of the beams but has no effect on the opening of joints at ultimate loading stage. Moreover, a comprehensive examination on four existing code equations to predict the stress in the unbonded tendons showed that the four examined codes predicted well the stress at the ultimate loading condition of the unbonded steel tendons, however, they significantly under predicted those in the CFRP tendons. A modification in the strain reduction coefficient used by ACI 440.4R for predicting the stress increment in unbonded CFRP tendons of monolithic beams is therefore proposed for PSBs based on the experimental results.

Time-dependent assessment and deflection prediction of prestressed concrete beams with unbonded CFRP tendons

This paper presents the assessment of the time-dependent behavior and the prediction of the long-term deflection of concrete beams prestressed with internal unbonded carbon fiber reinforced polymer (CFRP) tendons. A numerical model for the time-dependent analysis of concrete beams prestressed with unbonded tendons is calibrated against experimental results. Parametric numerical simulations are then conducted on simply supported unbonded prestressed concrete beams subjected to long-term sustained loads to investigate the effect of using CFRP tendons instead of low-relaxation steel ones, the magnitude of the initial prestress, the loading conditions, and the quantity of the compressive reinforcing steel. The results show that the long-term prestress loss of beams with CFRP tendons is considerably higher than that of beams with steel tendons. Moreover, it is shown that increasing the quantity of compressive reinforcing steel leads to a substantial decrease in long-term downward deflection. A modification of the ACI 318-14 equation is proposed to predict the time-dependent deflection of prestressed concrete beams with unbonded FRP or steel tendons.

Structural analyses of extruded CFRP tendon-anchor system and applications in thin-walled aluminum/CFRP bridge

Pre-stressed carbon fiber reinforced polymer (CFRP) tendon system is developed. Extrusion method guarantees the strength of the CFRP tendon could be fully used in strengthening. The pre-stressed CFRP tendons are applied to strengthen aluminum beams and portable aluminum bridge to improve the flexural rigidity and load carrying capacity of these structures and the improvements are revealed by the experiments. Applying extruded CFRP tendon-anchor system, the load carrying ability of the pre-stressed aluminum bridge has an improvement of 25%. Structural models are proposed to investigate the pre-stressing effects, including the deflection and the load capacity under different design criteria. Finite element method (FEM) is applied to analyze the nonlinear behavior of the pre-stressed beam and reveal the plastic torsional buckling failure style. All these analyses agree well with the experiments and suggest accurate methods to design pre-stressed aluminum/CFRP structures.

Comparative study on piezoresistive properties of CFRP tendons prepared by two different methods

Abstract Carbon fiber reinforced polymer (CFRP) material is an exceptionally and light fiber-reinforced polymer that can be used as a smart sensor due to its remarkable piezoresistivity. This study presents a comparative study on piezoresistive characteristics of CFRP tendons manufactured by two different methods, i.e., the vacuum process method (VPM) and the brush process method (BPM). The tensile and resistance tests were conducted simultaneously on two groups of CFRP tendon specimens respectively by VPM and BPM to investigate their differences in initial resistances, piezoresistive performance and sensitivity. Test results showed that the initial resistance of the CFRP tendons prepared by the VPM is larger than those prepared by the BPM. The resistances of the CFRP tendon specimens by two methods increase uniformly with increasing strain. The resistance increases more quickly at the same strain increment for specimens prepared by BPM compared with those for VPM. Three stages can be observed according to the variation of fractional change in resistance (d R / R ) with tensile strain. The scope of d R / R varies from 0% to 55% for BPM specimens, which is much wider than that for VPM specimens, which fall within 8.5%. The result is that BPM specimens are more sensitive than that of VPM. The values of sensitivity ‘ K ’ obtained for BPM specimens varying from 22 to 28 are much bigger than that for VPM ranging from 2.7 to 3.5. For BPM specimens, the pre-tension process can tighten up the fibers to reduce crimp and significantly reduce the amounts of bubbles during the brush process. However, the specimens prepared by VPM without undergoing the pre-tension process showed a relative looser state and carbon fibers might crimp.

Test and numerical simulation of large angle wedge type of anchorage using transverse enhanced CFRP tendons for beam string structure

Carbon Fiber Reinforced Polymer (CFRP) has many superior material properties and can be used as prestressing tendons for prestressing structures instead of steel. A large angle wedge type of anchorage using transverse enhanced CFRP tendons is designed and tested for beam string structure. The results of laboratory tests and numerical analysis under static loading conditions of the prototype of anchorage are discussed. For a wedge with an angle of 10°, the 5-mm-diameter CFRP tendon was anchored successfully, but 7-mm- and 9-mm-diameter were not due to high pressure. The results of test revealed that the sleeve should ensure adequate roughness of internal and external surface, enough wall thickness and cutting width, and appropriate length to avoid some failure mode caused by components themselves. Based on the failure mode analysis of laboratory tests and numerical results, some details were suggested to improve the large angle wedge type of anchorage for applying to large diameter CFRP tendon.