Carbon Fiber-reinforced Polymer Straps Research Articles

A novel winding–wedge anchorage for CFRP straps: Conceptual design and performance evaluation

Carbon fibre-reinforced polymer (CFRP) straps can be utilised as tension members in structures on account of their high-strength properties. The development of an effective anchorage device for CFRP straps that can fully utilise the strength of straps before anchorage failure is an ongoing technical challenge. To address such knowledge gap, a novel anchorage device termed winding-wedge anchorage that combines winding and wedge anchorage mechanisms is reported herein. For the winding anchorage component, different numbers of winding turns are applied. For the wedge anchorage component, three different types of wedge anchorage systems are investigated namely bonding, friction clamping, and friction non-clamping. Initially, the working mechanism, theoretical winding–wedge anchorage model (herein theoretical model), and design parameters are determined. Experimental and finite element analyses are then reported to assess the behaviour of the anchorage including failure modes and anchorage efficiencies. The accuracy of the theoretical model is also reported. It is concluded that the efficiency of the anchorage device is primarily dependent on the wedge anchorage component. Moreover, the bonding wedge anchorage component performed better than the other two wedge anchorage systems. The theoretical model and experimental results revealed that 1.5 winding turns results in an optimal interaction between the winding and wedge anchorages components.

Experimental Study of Fatigue and Fracture Behavior of Carbon Fiber-Reinforced Polymer (CFRP) Straps.

The hanger is one of the important components for through and half-through arch bridges. Conventional steel hangers are vulnerable to corrosion due to corrosive environments. Therefore, a new type of bridge hangers consisting of Carbon Fiber-Reinforced Polymer (CFRP) straps was developed recently. The CFRP straps are self-anchored, which is formed by layers-winding, and they have great advantages in corrosive environments such as high resistance to corrosion. In this study, the fatigue and fracture behavior of CFRP straps has been experimentally investigated. Firstly, the tensile testing of four CFRP strap specimens was conducted to investigate the static fracture behavior of CFRP straps, and three stages were observed, including delamination, cracking, and brittle rupture. Then, a fatigue test of thirty-nine specimens (four groups) was carried out to study the fatigue behavior of CFRP straps, where two types of pins, titanium alloy pin and CFRP pin, and two loading frequencies, 10 Hz and 15 Hz, were used. The number of cycles to failure, displacement, fatigue failure strain, outside surface temperature at the vertex of specimen, and scanning electron microscope (SEM) photographs were recorded and analyzed to investigate the fatigue behavior of CFRP straps. The experiment results show that the temperature development at the vertex of the CFRP strap varies obviously if different pins are used due to the different friction coefficients. In addition, the fatigue life of CFRP straps decreases significantly with the increase in loading rate for the titanium pin, while it only reduces slightly with the increase in loading rate for the CFRP pin.

Fretting Fatigue Performance of Unidirectional, Laminated Carbon Fibre Reinforced Polymer Straps at Elevated Service Temperature.

The fretting fatigue performance of laminated, unidirectional (UD), pin-loaded, carbon fibre-reinforced polymer (CFRP) straps that can be used as bridge hanger cables was investigated at a sustained service temperature of 60 °C. The aim of this paper is to elucidate the influence of the slightly elevated service temperature on the tensile fatigue performance of CFRP straps. First, steady state thermal tests at ambient temperature and at 60 °C are presented, in order to establish the behaviour of the straps at these temperatures. These results indicated that the static tensile performance of the straps is not affected by the increase in temperature. Subsequently, nine upper stress levels (USLs) between 650 and 1400 MPa were chosen in order to establish the S–N curve at 60 °C (frequency 10 Hz; R = 0.1) and a comparison with an existing S–N curve at ambient temperature was made. In general, the straps fatigue limit was slightly decreased by temperature, up to 750 MPa USL, while, for the higher USLs, the straps performed slightly better as compared with the S–N curve at ambient temperature.

STATIC TENSILE EXPERIMENT AND NUMERICAL ANALYSIS OF LAMINATED CFRP STRAPS

Pin-loaded laminated carbon fiber reinforced polymer (CFRP) straps are produced by winding unidirectional CFRP tapes around two separated pins, and the load is transferred by the pins. Axial tensile tests were performed to investigate the displacement and strain distributions in the individual layers, the crack propagation and the failure modes. A finite element model was developed and validated by the experimental results. Based on the experimental and numerical results, the effect of critical parameters on the load-bearing and failure mechanisms of the strap were analyzed. The analysis shows that the load transfer efficiency of CFRP straps was decreased by 20% compared to the straight strap. Brittle failure occurred at the intersection between the straight and the curved segments with the development of the delamination and longitudinal cracks. The curvature, strap thickness and friction coefficient had significant influence on the maximum stress of each layer, which always occurred at the intersection between the straight and the curved segments. The load bearing capacity and failure modes were greatly affected by the stress distribution in the curved segment even if the stress level was relatively low.

Influence of Temperature on the Mechanical Performance of Unidirectional Carbon Fiber Reinforced Polymer Straps.

The performance of pretensioned, laminated, unidirectional (UD), carbon fiber reinforced polymer (CFRP) straps, that can potentially be used for example as bridge deck suspender cables or prestressed shear reinforcements for reinforced concrete slabs and beams, was investigated at elevated temperatures. This paper aims to elucidate the effects of elevated temperature specifically on the tensile performance of pretensioned, pin-loaded straps. Two types of tests are presented: (1) steady state thermal and (2) transient state thermal. Eight steady-state target temperatures in the range of 24 °C to 600 °C were chosen, based on results from dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). Transient state thermal tests were performed at three sustained tensile load levels, namely 10, 15, and 20 kN, corresponding to 25%, 37%, and 50% of the ultimate tensile strength of the pin-loaded straps at ambient temperature. In general, the straps were able to retain about 50% of their ambient temperature ultimate tensile strength (UTS) at 365 °C.

Experimental Behavior of CFRP Strap–Strengthened RC Beams Subjected to Sustained Loads

AbstractRC beams strengthened with transverse prestressed unbonded carbon fiber–reinforced polymer (CFRP) straps have shown substantial increases in shear capacity when subjected to short-term stat...

Fatigue and Durability of Laminated Carbon Fibre Reinforced Polymer Straps for Bridge Suspenders.

Steel cables and suspenders in bridges are at high risk of corrosion-fatigue and in some cases of fretting-fatigue in their anchorages. These factors greatly limit the service stresses of a specific cable system and involve expensive protection measures. In order to investigate the above limitations, the fretting fatigue behaviour of pin-loaded carbon fibre reinforced polymer (CFRP) straps was studied as models for corrosion-resistant bridge suspenders. Two types of straps were tested: small model straps with a sacrificial CFRP ply and large full-scale straps. In a first phase, five fully laminated and carbon pin-loaded CFRP model straps were subjected to an ultimate tensile strength test. Thereafter, and in order to assess their durability, 20 model straps were subjected to a fretting fatigue test, which was successfully passed by 4 straps. An S-N curve was generated for a load ratio of 0.1 and a frequency of 10 Hz. In a second phase, one full-scale strap was tested for its ultimate tensile strength and two full-scale straps were fatigue-tested. The influence of fretting fatigue loading on the residual mechanical properties of the straps was also assessed, and although fretting fatigue represented an important limitation for laminated CFRP straps, it could be shown that the investigated CFRP tension members can compete with the well-established steel suspenders.

Evaluation of grout materials for CFRP ground anchors with strap ends

An evaluation of cement- and resin-based grout materials for applications in carbon fiber-reinforced polymer (CFRP) ground anchor heads composed of high-strength grout and a CFRP strap was conducted. Uniaxial compression experiments were performed on cylinder specimens. Appropriate loading-control methods and boundary conditions were determined in order to obtain the softening branch of the compressive stress vs strain curves. The concrete compression model developed by Sargin was successfully applied to also describe the compressive grout behaviors. The results of finite element analyses of the CFRP ground anchor head and implemented grout model compared well with experimental results. A parametric study based on different grout softening behaviors showed that only limited stress redistribution occurred after crack initiation.

Pull-out behavior of CFRP ground anchors with two-strap ends

Pull-out experiments were performed on three carbon fiber-reinforced polymer (CFRP) ground anchors simulating their applications in different rock types which provided different confinement levels. The CFRP tendons comprised, on the ground side, a prefabricated conical anchor body of high-strength grout in which two CFRP straps were embedded. The anchors reached an average load-bearing capacity of 1384kN with final failure occurring in the CFRP straps. Grout failure was successfully prevented by an adequate grout selection and the installation of CFRP confinement rings to balance spreading forces at the strap ends. The confinement level provided by the surrounding media influenced the activation of the CFRP components in the anchor body by influencing the friction at the CFRP/grout interfaces.

Experimental and numerical investigation of tensile behavior of non-laminated CFRP straps

Tensile experiments were performed on seven-layer non-laminated carbon fiber-reinforced polymer (CFRP) straps. A corresponding finite element (FE) model was developed and validated by the experimental results. The anchoring of the outermost tape layer by fusion bonding to the adjacent inner layer led to a non-uniform stress distribution among the tape layers on the straight strap segment where the bonding occurred. Stress concentrations at the transition points from the curved to the straight tape segments were negligible compared to the effect of the fusion bonding on the stress level. The strap ultimate load was very sensitive to minor initial damage at the tape edges as well as to stress concentrations in the fusion-bonding area. The latter may lead to a premature failure in the anchoring zone. The innermost tape layer remained fully anchored only by friction up to strap failure.

Optimisation of shear strengthened reinforced concrete beams

External prestressed carbon fibre reinforced polymer straps can be used to strengthen shear-deficient reinforced concrete structures. For an efficient shear retrofitting system, the optimum combinations of parameters such as the number of straps, strap locations, strap stiffness and initial strap prestress need to be identified. The modified compression field theory and the shear friction theory have previously been applied to carbon fibre reinforced polymer strap strengthened beams. As implemented, both of these methods are iterative. Particle swarm optimisation and genetic algorithm stochastic optimisation methods were used to reduce the computational cost associated with the shear strength evaluation and also to search the design space for carbon fibre reinforced polymer strap strengthened beams. An initial comparison across several test functions showed that the preferred optimisation algorithm depended on the characteristics of the design space. When applied to a reinforced concrete case study, the genetic algorithm was better for searching the shear friction theory shear strength design space that was characterised by several peaks. However, for the smoother modified compression field theory shear strength evaluation space, and for the design space for the carbon fibre reinforced polymer strengthened beams calculated using either the modified compression field theory or the shear friction theory, the particle swarm optimisation converged more quickly and accurately. The optimised solutions reflect the assumptions within the underlying evaluation methods.

Rectangular Reinforced Concrete Beams Strengthened with CFRP Straps

AbstractShear-deficient reinforced concrete (RC) structures can be effectively strengthened using external prestressed carbon fiber–reinforced polymer (CFRP) straps. Due to the presence of the external elastic straps, a strengthened beam can continue to carry significant load beyond the stages of crack plane slipping and internal-shear stirrup yielding, and the concrete is subjected to high tensile strain levels. As a consequence, the concrete material models play a significant role in the context of modeling such behavior. The modified compression field theory (MCFT), which is a widely accepted shear theory for unstrengthened RC structures, incorporates the details of the stress-strain behavior of concrete. The MCFT also considers compatibility as a governing factor, which facilitates the inclusion of the strap system into the MCFT formulation. In the current study, modifications were investigated to model CFRP strap retrofitted RC beams associated with either uniform or nonuniform strap spacings. An exp...

Carbon Fiber-Reinforced Polymer Punching Reinforcement and Strengthening of Concrete Flat Slabs

Three different carbon fiber-reinforced polymer (CFRP) systems were used to reinforce new or strengthen existing concrete slabs against punching. Untensioned CFRP plates or only very low prestressed CFRP straps could not prevent brittle punching failure. Strap prestressing of at least 15% of the tensile strength led to a ductile two-peak slab response although the CFRP material systems were brittle. The ultimate load of the unreinforced slabs could be increased by 77 to 118% via a redistribution of forces from the concrete to the strap system. An empirical model was established that is able to estimate the first peak load of all the applied CFRP systems. It takes into account the parameters CFRP plate or strap strength, CFRP geometry, CFRP prestressing level, and punching load of the unreinforced slab. A cantilever strut-and-tie model was established to estimate the second peak load. Both models provide acceptable accuracy.

Precracked Reinforced Concrete T-Beams Repaired in Shear with Prestressed Carbon Fiber-Reinforced Polymer Straps

The results of an experimental and numerical investigation involving unstrengthened reinforced concrete (RC) T-beams and precracked RC T-beams strengthened in shear with prestressed carbon fiber-reinforced polymer (CFRP) straps are presented and discussed. The results provide insights into the influence of load history and beam depth on the structural behavior of both unstrengthened and strengthened beams. The strengthened beams exhibited capacity enhancements of 21.6 to 46% compared to the equivalent unstrengthened beams, demonstrating the potential effectiveness of the prestressed CFRP strap system. Nonlinear finite element (FE) predictions, which incorporated the load history, reproduced the observed experimental behavior but either underestimated or overestimated the post-cracking stiffness of the beams and strap strain at higher load levels. These limitations were attributed to the concrete shear models used in the FE analyses.

Axial and Flexural Performance of Square RC Columns Wrapped with CFRP under Eccentric Loading

AbstractThe majority of studies on fiber-reinforced-polymer (FRP) strengthened concrete columns deal with columns of a circular cross section. However, most concrete columns in the field have square or rectangular cross sections and resist eccentric loads. This paper presents the results of an experimental study on the performance of carbon-fiber-reinforced-polymer (CFRP) wrapped square reinforced concrete (RC) columns under eccentric loading. The influence of the number of CFRP layers, the magnitude of eccentricity, and the presence of vertical CFRP straps were investigated by testing 16 specimens. The specimens had the dimensions 200×200×800 mm and round corners with a radius of 34 mm. Twelve specimens were tested as columns and four specimens as beams. The results of this study showed that CFRP wrapping enhanced the load-carrying capacity and ductility of the columns under eccentric loading. Furthermore, the application of the vertical CFRP straps significantly improved the performance of the columns ...

Time-Dependent Behavior of RC Beams Retrofitted with CFRP Straps

A retrofitting technique that uses prestressed unbonded carbon-fiber-reinforced polymer (CFRP) straps to provide additional shear capacity has previously been shown to be successful under short-term static loading conditions. The current study explores the longer-term behavior of this retrofitting technique through two experiments (a sustained load and a cyclic load experiment) and the development of a model based on the modified compression field theory. The experiments indicated that the strain in the CFRP straps changes with time due to changes in the load sharing with the concrete (caused by creep) and the steel stirrups (caused by yield of these elements). The predictive model was initially validated against static experimental results before being applied to the longer-term experiments. The model predicts the trends in behavior well although it is conservative in its estimates of strap strain. The model was then used to determine the influence of stirrup yielding, the load level before and after retrofitting, and the duration of loading on the CFRP strap strains. The initial results suggest that the largest increases in long-term strap strain will occur when the straps are installed early in the structure’s service life although further experimental validation is required.

Modeling of an Unbonded CFRP Strap Shear Retrofitting System for Reinforced Concrete Beams

A retrofitting technique has been developed that uses carbon fiber-reinforced polymer (CFRP) straps to increase the shear capacity of reinforced concrete beams. The vertical straps are not bonded to the beam but are instead anchored against the beam, which makes this technique potentially more effective than bonded FRP retrofitting techniques. However, it also means that models for bonded FRPs are not appropriate for use with the straps. Instead, a model based on a shear friction approach has been developed where the strain in the straps is calculated based on a term that accounts for the effects of prestress and additional strain in the strap due to shear crack opening. The model can either consider the shear reinforcement to be smeared along the length of the beam or discrete elements. The “smeared” model was checked against an experimental database consisting of rectangular, T-, and deep beams, both in terms of predicted capacity and predicted strain in the straps. Overall the smeared model predicted t...

Efficient CFRP Strap Configurations for the Shear Strengthening of Reinforced Concrete T-Beams

A prestressed carbon fiber-reinforced polymer (CFRP) strap retrofitting system has been found to significantly enhance the shear capacity of existing reinforced concrete beams. In previous studies, the CFRP straps were supported on metal pads placed on the top and bottom of a beam necessitating top surface access. The goal of the current work was to develop a system where the straps were installed from underneath a slab without compromising the strengthening efficiency. A series of T-beam experiments was conducted where the CFRP straps were inserted through holes that were drilled from below the flange, thereby avoiding the need for access to the top surface. The depth of penetration of the CFRP straps into the compression flange, the concrete strength, the CFRP strap spacing, the presence of holes in the compression flange, and the size of the loading pads were all found to affect the shear performance. Using the most successful installation technique, the resulting CFRP strengthened beam failed at a loa...

Strengthening of RC T-section beams with low strength concrete using CFRP composites subjected to cyclic load

Various methods are developed for strengthening reinforced concrete beams against shear. Nowadays, external bonding of different composite members to RC beams was very popular and successful technique internationally. This study presents test results on strengthening of shear deficient RC beams by external bonding of carbon fiber reinforced polymer (CFRP) straps. Six RC beams with a T-section were tested under cyclic loading in the experimental program. Width of the CFRP straps, arrangements of straps along the shear span, and anchorage techniques that were applied at the ends of straps were the main parameters that were investigated during experimental study. Shear deficient beams with low strength concrete were strengthened by using CFRP straps for obtaining ductile flexural behavior. The test results confirmed that all CFRP arrangements improved the strength, stiffness and energy dissipation capacity of the specimens significantly. The failure mode and ductility of specimens were proved to differ according to the CFRP strap width and arrangement along the beam. Experimental results were compared with the analytical approaches that were suggested by ACI-440 Committee Report.

Improving shear capacity of RC T-beams using CFRP composites subjected to cyclic load

Various methods are developed for strengthening reinforced concrete beams against shear. Nowadays, external bonding of various composite members to RC beams was very popular and successfully technique internationally. This study present test results on strengthening of shear deficient RC beams by external bonding of carbon fiber reinforced polymer (CFRP) straps. Six RC beams with a T section were tested under cyclic loading in the experimental program. Width of the CFRP straps, arrangements of straps along the shear span, and anchorage technique that were applied at the ends of straps was the main parameters that were investigated during experimental study. Inclined CFRP straps were bonded along the shear spans of shear deficient beams for strengthening against shear by using epoxy. Arrangements and width of the inclined CFRP straps were the main parameters that were changed among the specimens. The test results confirmed that all CFRP arrangements improved the strength and stiffness of the specimens significantly. The failure mode, and ductility of specimens were proved to differ according to the CFRP strap width and arrangement along the beam. Experimental results were compared with the analytical approaches that were suggested by ACI-440 Committee report.