Carbon Fiber Reinforced Polymer Strengthening Research Articles

Anchoring the CFRP strengthening of concrete bridge decks: A comparison of methods

Debonding failures are a common problem in concrete bridge decks strengthened with adhesively attached carbon-fibre reinforced polymer (CFRP) strips. Accordingly, in this study, rectangular concrete slabs strengthened with CFRP have been experimentally evaluated to simulate the strengthening of T-beam and box girder slabs. The resulting static load data have been used to compare the effects of four different anchoring methods in terms of crack distribution, deflection, reinforcing steel strain curve, and CFRP strain distribution. The most suitable bridge deck strengthening anchoring method has been then identified and analysed using extant strengthening design methods. The results show that the most practical anchoring method is the use of open CFRP strips attached with concentrated adhesive. The findings of this study indicate that when strengthening T-girder bridges, more than two CFRP anchorage strips should be evenly spaced within the extension of the anchorage length, while for box girder bridges, even more evenly spaced strips should be used. This research and its conclusions can be used as a reference for the improved design of bridge deck strengthening.

Experimental study on fatigue behavior of butt-welded thin-walled steel plates strengthened using CFRP sheets

Abstract Welded steel joints in structures are susceptible to fatigue failure. In this context, advanced carbon fiber reinforced polymer (CFRP) possess a demonstrated potential for the fatigue strengthening of steel structures. However, limited information is available on CFRP strengthening of butt-welded joints, which are the most common joint types in steel structures. This paper presents a study on fatigue behavior of butt-welded thin-walled steel plates strengthened using CFRP sheets. A total of 34 specimens with both single-sided and double-sided strengthening were performed via constant amplitude tensile fatigue loading, and another four specimens were employed for hot-spot stress measurement. The effect of CFRP strengthening on fatigue behavior was investigated by varying the strengthening scheme (single-sided or double-sided) and the number of CFRP sheet layers. Test results showed that the fatigue life of butt-welded steel plates with CFRP strengthening increased by approximately one to ten times when compared to the fatigue life of butt-welded steel plates without CFRP strengthening. Triple-layered double-sided specimens seemed to exhibit the best effect on fatigue life; their fatigue strength at 2 million cycles increased by 34% compared to that of specimens without CFRP strengthening. Specific fatigue design S–N curves of butt-welded thin-walled steel plates with and without CFRP strengthening were proposed directly. The fatigue test results were used to calibrate a fatigue design approach for CFRP strengthened butt-welded steel plates. In addition, the effect of CFRP strengthening on stiffness degradation was also discussed.

Effect of CFRP Strengthening on Web of Steel I-Beam

This study is aimed to strengthen the structural steel I beam with carbon fiber reinforced polymers (CFRP) composites to its web to increase its shear strength capacity. Lightweight Beam LB-100 @ 5.1 Kg/m and LB-115@ 8.1 Kg/m is used for this purpose. CFRP have light weight and high-strength as compare to other types of FRPs, so it can be used for strengthening purpose instead of using conventional methods of repair such as replacement and other. The web portion of the I-beam is made rust free to achieve proper bonding between the web and fiber sheet. A web of the beam is strengthened by carbon fiber reinforced polymer (CFRP) strips in layers with the help of adhesive. The beam with CFRP is cured in air for 48 h before testing. Experimentation is done at the UTM of 100 Ton capacity. A three point bending tests were performed on various beams and results show that the load carrying capacity of a strengthened beam is increased by 20 to 35% as compared with control beam (non-strengthened) and the local failure of beam is also avoided. It is observed that the deflection of strengthened beam is also reduced as compared to a non-strengthened beam. So, this technique of strengthening can be used to improve the performances of various old structures in civil engineering which undergoes loss of strength due to corrosion.

Experimental and theoretical studies on reducing hot spot stress on CHS gap K-joints with CFRP strengthening

This paper presents an experimental investigation into reducing the hot spot stress on gap K-joints made of circular hollow sections (CHS) with carbon fibre reinforced polymer (CFRP) strengthening. A series of well-designed CHS gap K-joints were strengthened by using combinations of unidirectional and bidirectional CFRP sheets. Hot spot stress tests were conducted on the joints before and after strengthening. By comparing stress concentration factors (SCFs), it was found that the proposed CFRP strengthening scheme could effectively reduce the hot spot stress on CHS gap K-joints, thereby prolonging the fatigue life of joints. Generally, the maximum SCFs in the chord and brace decrease by an average of 20% and 15%, respectively. Furthermore, a theoretical analysis was conducted to develop formulas for the SCF reduction coefficient, which were verified by test results. SCF formulas were suggested for CFRP-strengthened CHS gap K-joints.

Numerical and Experimental Study on Deficient Short Steel Tubes Strengthened with CFRP Under Compression

In view of development and repair costs, support of structures is imperative. Several factors, for example, design and calculation errors, absence of appropriate installation, change of structures application, exhaustion, seismic tremor, fire and natural conditions diminish their strength. In such cases, structures have need of rehabilitation and restoration to achieve their original performance. One of the most up to date materials for retrofitting is carbon fiber reinforced polymer (CFRP) that can provide an amount of restriction to postpone buckling of thin steel walls. This paper provides a numerical and experimental investigation on CFRP strengthened short steel tubes with initial horizontal and vertical deficiency under compression. Ten square and circular specimens were tested to study effects of the following parameters: (1) position of deficiency, horizontal or vertical; (2) tube shape, square or circular; (3) CFRP strengthening. In the experiments, axial static loading was gradually applied and for the numerical study three-dimensional (3D) static nonlinear analysis method using ABAQUS software was performed. The results show that deficiency reduces load-bearing capacity of steel columns and the impact of horizontal deficiency is higher than the impact of vertical deficiency, in both square and circular tubes. Use of CFRP materials for strengthening of short steel columns with initial deficiency indicates that fibers play a considerable role on increasing load bearing capacity, reducing stress at the damage location, preventing deformation caused by deficiency and delaying local buckling. Both numerical and experimental outcomes are in good agreement, which underlines the accuracy of the models adopted.

Finite Element Modeling of Strengthened Beams Using CFRP

Deteriorating and damage of some structural elements influence negatively their strength and the bearing capacity. However, it is necessary to take some measures improve structural performance. In this context, the composite material is often used to strengthen the damaged parts. This paper presents a model to analyze the effect of carbon fiber reinforced polymer strengthening of beams under four point bending. Finite element software ANSYS 12.0 has been used for modeling the beams by conducting nonlinear static analysis. The SOLID 65 and SHELL 181 elements have been used to, respectively; model the 3D concrete beams and the composite layer. Constitutive properties of different components (concrete and FRP) have been incorporated. The predicted Finite element analysis results for the load-midspan deflection are compared to the measured experimental data. Close agreement was found between the predicted and measured results at all stages of loading for both models developed.

Behaviour of CFRP strengthened CHS members under monotonic and cyclic loading

Tubular hollow members, though used in many civil and mechanical applications, are highly vulnerable structural elements when subjected to cyclic loading. They have been used extensively in both onshore and offshore civil infrastructure where cyclic loading can result from earthquake, waves, currents and wind. Due to service loads increment, errors in design, effect of severe environments or loss of material properties, tubular hollow steel members may require to undergo strengthening to withstand both static and cyclic loads. In the present study, a series of experiments on bare and externally-bonded carbon fibre reinforced polymer (CFRP) strengthened CHS steel members subjected to monotonic and cyclic loading has been conducted to investigate the effect of CFRP strengthening technique on the structural behaviour of strengthened members. The results showed that the CFRP strengthening is effective to enhance the cyclic performance of CHS steel members by improving the moment capacity, moment degradation behaviour, secant stiffness, energy dissipation capacity and ductility compared to bare steel CHS members. In addition, the moment capacity of CHS members has been improved under monotonic loading due to the CFRP strengthening as well. Moreover, the impact of adhesive types on the structural response of the strengthened specimens was also investigated.

FLEXURAL CAPACITY ASSESSMENT OF BRICK AGGREGATED PRE-CRACKED RC BEAM STRENGTHENED WITH CARBON FIBER POLYMER

Ageing and improvements to design code has led to many existing RC structures made of locally available brick aggregates are now found structurally deficient and are in need of rehabilitation. This research emphases on flexural capacity assessment and investigation of failure modes of Carbon Fiber Reinforced Polymers (CFRP) strengthened brick aggregated RC beams. Flexural performance of the RC beam specimens are evaluated using four point bending method. Six RC beams (initially cracked) with CFRP strengthening were tested by varying (i) type of CFRP, (ii) reinforcing area, (iii) anchorage type; and (iv) number of CFRP layers. Two beams were tested as control specimens. Unidirectional carbon fiber sheet (Tow Sheet) and individually hardened continuous fiber strands woven into sheet form (Strand Sheet) were used. Simple flexure failure was obtained for unstrengthened RC beams while end plate and interfacial debonding were observed for the initially cracked CFRP strengthened RC beams. Strengthening of pre-cracked beams using Strand Sheet gave better performance compared to Tow sheet. Overall flexural strength improvement of CFRP strengthened beams varied from 12% to 34% with respect to unstrengthened beams depending on strengthening methods.

FLEXURAL CAPACITY ASSESSMENT OF BRICK AGGREGATED PRE-CRACKED RC BEAM STRENGTHENED WITH CARBON FIBER POLYMER

Ageing and improvements to design code has led to many existing RC structures made of locally available brick aggregates are now found structurally deficient and are in need of rehabilitation. This research emphases on flexural capacity assessment and investigation of failure modes of Carbon Fiber Reinforced Polymers (CFRP) strengthened brick aggregated RC beams. Flexural performance of the RC beam specimens are evaluated using four point bending method. Six RC beams (initially cracked) with CFRP strengthening were tested by varying (i) type of CFRP, (ii) reinforcing area, (iii) anchorage type; and (iv) number of CFRP layers. Two beams were tested as control specimens. Unidirectional carbon fiber sheet (Tow Sheet) and individually hardened continuous fiber strands woven into sheet form (Strand Sheet) were used. Simple flexure failure was obtained for unstrengthened RC beams while end plate and interfacial debonding were observed for the initially cracked CFRP strengthened RC beams. Strengthening of pre-cracked beams using Strand Sheet gave better performance compared to Tow sheet. Overall flexural strength improvement of CFRP strengthened beams varied from 12% to 34% with respect to unstrengthened beams depending on strengthening methods.

SHEAR CAPACITY PREDICTION FOR STIRRUP-CORRODED RC BEAMS STRENGTHENED WITH FRP

Corrosion of steel reinforcement represents one of the main causes of deterioration and degradation of reinforced concrete (RC) structures. Shear reinforcements (stirrups) as an outer reinforcement in RC beams are more susceptible to corrosion problems and damage. This paper describes an analytical procedure for predicting the shear capacity of stirrup-corroded RC beams strengthened in shear using FRP laminates. The procedure shows how to incorporate the effects of the damages due to corrosion of stirrups into the design equations. An experimental investigation has been conducted to provide experimental data on the shear capacity of RC beams with corrosion-damaged stirrups strengthened using carbon fiber reinforced polymer (CFRP) sheets. The experimental study comprised three beams of 200 mm wide, 350 mm deep, and 2800 mm long. The steel stirrups in the beams were corroded using an accelerated corrosion technique. After CFRP strengthening, the beams were tested in four-point bending under a simply supported span of 2400 mm. The shear capacity of the strengthened beams was predicted using the proposed procedure and compared with the experimental ones. Good correlations were found between the predicted and experimental shear strength of the beams.

Numerical study of full-scale CFRP strengthened open-section steel columns under transverse impact

In recent years, externally bonded fibre reinforced polymer (FRP) material has gained popularity as an efficient means of strengthening existing civil engineering infrastructure. In the case of steel structures, FRP has frequently been deployed to strengthen against static and fatigue loads. However, the understanding of the effect of impact load on strengthened structures is still in its early stages. In parallel with this, most of the existing studies on impact have concentrated on small scale elements. Thus, the study presented here is aimed at examining the effectiveness of carbon fibre reinforced polymer (CFRP) in strengthening full-scale steel I-section columns against impact load. To achieve this aim, a non-linear finite element model built using ABAQUS was validated against experimental tests and then used to simulate the strengthening technique. The model included failure criteria of all investigated materials (steel, CFRP and adhesive material). Various parameters including boundary conditions, preloading level, kinetic energy, impact location and impact direction were examined. The CFRP strengthening technique was found to be highly effective in preventing column failure whether by global buckling failure or transverse shear failure. In addition the strengthened columns exhibited a reduced axial and transverse displacement by more than 70% in many cases.

Effect of CFRP strengthening properties with anchoring systems on P-I diagrams of RC panels under blast loads

Several experimental studies had been carried out on using fibre reinforced polymer (FRP) to repair and strengthen reinforced concrete (RC) panels under blast and impact loads. However, the information related to the capacity increment of RC panels with the FRP strengthening measures is not widely given. This study aims to investigate the dynamic response characteristics and lateral load carrying capacity of one-and two-way RC panels strengthened with carbon fibre reinforced polymer (CFRP) by using LS-DYNA software. The accuracy of the numerical model is verified with the available testing data. The calibrated numerical model is utilised to conduct a series of simulations to investigate the effect of CFRP strength, CFRP thickness and bond strength on the RC panel resistance with and without anchoring against blast loads. The numerical outcomes are used to develop pressure-impulse (P-I) diagrams that divided into three damage levels defined according to the UFC-3-340-02 specifications. Furthermore, empirical equations are proposed to easily construct the P-I diagrams that provide a quick damage assessment for CFRP strengthened panels.

Carbon Fiber Reinforced Polymers Used for Strengthening of Existing Reinforced Concrete Structures

The paper deals with some aspects regarding the behavior of modern and efficient solutions - for rehabilitation of reinforced concrete framed structures. The paper is devoted to experimental studies on carbon fiber reinforced polymer (CFRP) systems used as strengthening solution for reinforced concrete (RC) frames assumed as existing structures, which were tested as un-strengthened and as (CFRP) strengthened structures. Single span and single story frames (scale 1:2) were designed and detailed according to the Romanian design codes from 1970s under which seismic design was inadequate. The RC design and the magnitude of applied forces were ensuring the failure mechanism, of non-strengthened RC frames, by plastic hinges at columns ends. Then the columns were strengthened by using CFRP materials. The experimental program emphasized some important aspects regarding the behavior and failure by debonding of CFRP strengthening materials applied to RC frames. The analysis of experimental data and theoretical values showed up an increase of resistance and stiffness achieved by strengthening.

Utilization of magnetic water in cementitious adhesive for near-surface mounted CFRP strengthening system

Cement-based adhesive (CBA) is used as a bonding agent in Carbon Fibre Reinforced Polymer (CFRP) applications as an alternative to epoxy-based adhesive due to the drawbacks of the epoxy system under severe service conditions which negatively affect the bond between the CFRP and strengthened elements. This paper reports the results of, an investigation carried out to develop two types of CBA using magnetized water (MW) for mixing and curing. Two magnetic devices (MD-I and MD-II), with different magnetic field strengths (9000 and 6000 Gauss) respectively, were employed for water magnetization. Different water flows with different water circulation times in the magnetizer were used for each device. Compressive and splitting tensile strength tests of the magnetized CBA (MCBA) were conducted for different curing periods (3. 7, 14, 21 and 28 days) using MW. It was found that MW treatment increases the strength of CBA. The highest strength was obtained for MCBA samples when MD-I was used at a low flow rate (F = 0.1 m3/hr) for 15 mins of circulation time (T). The latter was found to positively affect MCBA properties when T was increased from 15 min to 60 mins. Prediction of the compressive and tensile strength values are also studied in this paper using genetic programming, the models showed good correlation with the experimental data.

Shear strength components of adjustable hybrid bonded CFRP shear-strengthened RC beams

This paper presents an experimental study on shear behavior of full-scale rectangular reinforced concrete (RC) beams under shear-strengthening of carbon fiber-reinforced polymer (CFRP) U-strips. The test involved five RC beams with transverse reinforcement, including one control beam, one shear-strengthened by externally bonded (EB) CFRP U-strips only, and the remaining three strengthened in shear by a hybrid bonded (HB) CFRP system under different normal pressures applied on the CFRP U-strips. The transverse reinforcing steel bars and CFRP U-strips were extensively instrumented with strain gauges, which enabled the accurate quantitative assessment of shear strength contributions from CFRP (Vf), stirrups (Vs) and concrete (Vc) during the whole loading process of a shear test. Further, by comparing Vs and Vc for beams with and without CFRP strengthening, the interactions can be evaluated and clarified. In addition, key features of shear behaviors, e.g. failure mode, load-deflection curves, shear capacity and ductility, shear crack angle, and strain development and distribution of stirrups as well as CFRP strips, are presented, compared and analyzed.

Experimental bond behaviour of a CFRP strengthening system for concrete elements at elevated temperatures

The bond between carbon fibre reinforced polymer (CFRP) laminates and concrete is very affected when exposed to elevated temperatures. The knowledge of its mechanical response at elevated temperatures is fundamental in view of the increasing use of these systems on the strengthening of concrete structures. This paper presents the results of an experimental investigation on the behaviour of the bond between concrete and a CFRP strengthening system at elevated temperatures. 21 single lap shear tests (SST) on concrete blocks strengthened with CFRP laminates were carried out. The laminate was assembled using the externally bonded reinforcement (EBR) technique. The SST tests were conducted under steady state conditions, first the specimens were heated to temperatures of 40 °C, 65 °C, 90 °C, 115 °C, 140 °C, 165 °C and subsequently, after temperature stabilization, the loading was increased up to failure of the specimen. Reference tests at ambient temperature have also been carried out. The results showed that approximately 40% of the bond strength was lost for temperatures near to the glass transition temperature (Tg) of the adhesive. However, despite the loss of bond strength with the increasing temperatures, a significant residual strength retention was still noticed for temperatures higher than the Tg. The bond between the concrete and the CFRP laminate was completely lost for temperatures of 165 °C.

Research on Arch Effect and CFRP Strengthening Effect of RC Beams with Corroded Stirrups

In many cases the corrosion of steel reinforcement is considered to be the main reason that leads to the deterioration of the bond between steel reinforcement and concrete and causes the failure of members. However, stirrups with smaller diameter are more likely to get rusted and it is more difficult to find corrosion expansion cracks on the surface of concrete. Then the function of stirrups to restrict longitudinal bar from slipping will be weakened, which alters the mechanical properties and failure mode of corroded Reinforced Concrete (RC) beams. In this paper, the influence of corrosion of stirrups and the effect of strengthening corroded RC beams with Carbon Fiber Reinforced Polymer (CFRP) sheets were studied. The corrosion rate for each group varied between 0% to 18% which represented mass loss of the longitudinal steel reinforcement on the tension side. The longitudinal reinforcement of beams in Group A was not wrapped by the stirrups in order to simulate the situation that the stirrups were seriously corroded and lost their function. Beams of Group B were corroded control beams. Beams of Group C were corroded and then repaired by applying longitudinal CFRP sheets at the bottom and additional U-shaped CFRP sheets around the cross-section. Test results showed that the corrosion of stirrups had a detrimental effect on strength as well as the bond stress between steel bars and concrete. The mechanical mode of experimental beams tended to act like an arch. The greater the corrosion rate was, the more obvious the arch effect was. In addition, combining longitudinal and U-shaped CFRP sheets enhanced the ultimate load by an average of 56.5% of corroded RC beams. However, because of the longitudinal cracks, the strengthening effect of CFRP was much diminished when the corrosion rate was particularly high.

Design and testing of an adhesively bonded CFRP strengthening system for steel structures

In the framework of sustainable development policies, it is essential that infrastructure owners can rely on effective repair or strengthening solutions, designed and tested in relevance to actual service conditions. In the case of steel structures, fatigue damage is a major concern that can significantly affect the lifespan of the structure, and so far, there are very few operational methods capable of preventing fatigue cracks in the field. Adhesively bonded carbon fiber reinforced polymer (CFRP) composites are being successfully applied to the rehabilitation of concrete structures for more than two decades, and they are currently receiving much interest for the strengthening of steel elements, but mainly for curative purpose after severe damage has occurred.In the present study, which is part of a European project called FASSTbridge, a specific CFRP system has been developed as a preventive method against fatigue damage of steel structures. The proposed system consists of a commercially available ultra-high modulus (UHM) CFRP composite plate compatible with the stiffness of the host steel structures, which is bonded to the steel support using a novel hybrid epoxy/polyurethane adhesive. A first part of the paper presents the main specifications that should be adopted in the design of CFRP strengthening systems applied to steel structures, and that were identified from an extensive literature survey. These specifications have guided the development of the polymer adhesive and the choice of a peculiar commercial CFRP plate in the preliminary phase of the project. Experimental characterizations were then conducted (i) on the formulated hybrid polymer adhesive to optimize its curing schedule and check the previous specifications are fulfilled, and (ii) on CFRP reinforced steel specimens in order to verify the effectiveness of the proposed strengthening system. This experimental program involved both short term and durability tests that were performed by different laboratories. Such an inter-laboratory study made it possible to verify the performances of the developed strengthening system and to assess the influence of installation parameters and environmental conditions.

Prestressed CFRP-strengthening and long-term wireless monitoring of an old roadway metallic bridge

This paper presents an application of prestressed carbon fiber-reinforced polymer (CFRP) plates for the strengthening of metallic girders of a roadway bridge in Melbourne, Australia. The study also describes the application of a wireless sensor network (WSN) system for long-term structural monitoring of the retrofitted bridge girders. A flat prestressed unbonded retrofit (FPUR) system was developed to apply prestressed CFRP plates to the steel girders of the Diamond Creek Bridge (122 years old), which is subjected to daily passenger and heavy vehicles. The first section explains the results of sets of static and fatigue tests performed in the laboratory to examine the efficiency of the proposed FPUR system prior to its installation on the bridge. The second section presents details of different aspects of the CFRP strengthening of the bridge girders, fatigue design criteria, and layouts for short- and long-term monitoring. For the short-term measurements, the bridge was loaded with a 42.5-ton semi-trailer before and after strengthening. The CFRP plates were prestressed up to approximately 980 MPa (≈38% of the CFRP ultimate tensile strength), which resulted in about 50% reduction in the maximum tensile stress in the bridge girders. The third section discusses the development, installation, and preliminary results of the WSN system used to monitor the pre-stress level in the CFRP plates. The results of the short- and long-term measurements in this study show that the FPUR system is very effective for flexural and fatigue strengthening of bridge girders. Finally, a set of recommendations for long-term structural monitoring is provided.

Experimental investigation on the CFRP strengthening efficiency of steel plates with inclined cracks under fatigue loading

Employing advanced material such as carbon‐fibre‐reinforced‐polymer (CFRP) in tension fatigue strengthening of aged roads and railway bridges have shown a great capability of arresting or delaying crack initiation and/or propagation in steel structures. However, it is not clear whether the fatigue behaviour and the CFRP strengthening efficiency is the same when the cracked-steel elements exhibit a state of complex loading. The aim of this paper is to investigate the fatigue behaviour of the CFRP strengthening of steel plates with central initial inclined cracks with a focus on the effect of the CFRP properties. The initial slit-like cracks were oriented to introduce a state of combined action of tension (mode-I) and shear (mode II) stresses at the crack tips. The key parameters in this study are the mixed‐mode (shear to tension stresses) ratio, the crack-starter length ratio (initial crack length to the plate width), patching configurations, and mechanical properties of the composite material. All the test specimens were artificially notched with central cracks of different damage levels. This study covered the fatigue performance of steel plates strengthened with two configurations of composite materials of different tensile stiffness (high modulus CFRP sheets, and normal modulus CFRP plates). Furthermore, the efficiency of strengthening systems of different fibre orientation relative to the initial crack angle was investigated. The outcomes of this study are extending the current knowledge of the CFRP strengthening to its applications on metal plates contain defects subjected to mixed-mode fatigue loading.