Bonded Carbon Fiber-reinforced Polymer Plates Research Articles

Prestressed CFRP Plates and Tendon Strengthening of Steel–Concrete Composite Beams

This study aims to enhance the ultimate capacity and stiffness of steel–concrete composite beams through external strengthening with prestressed carbon fiber-reinforced polymer (CFRP) plates and post-tensioned CFRP tendons. A 3D finite element model was developed using ANSYS and validated using experiments. The impact of various parameters on the capacity of the beam was investigated, including the level of post-tensioning in the CFRP tendons, tendon profile, degree of shear connection, and beam load level when adding strengthening CFRP tendons. Results indicate that reinforcing composite beams with bonded CFRP plates using post-tensioning tendons with trapezoidal and parabolic profiles can increase maximum load capacity by 37% and 60%, respectively, while maintaining high stiffness. This study also indicates that the optimal strengthening conditions for the composite beam are when the beam is loaded up to 70% of its capacity and has a composite action degree of 100%.

Strengthening of a 19th-century roadway metallic bridge using nonprestressed bonded and prestressed unbonded CFRP plates

Abstract This paper summarizes the strengthening of a historically significant 122-year-old roadway metallic bridge using nonprestressed bonded carbon fiber reinforced polymer (CFRP) plates. Prior to the bridge strengthening, sets of laboratory tests were conducted to characterize the mechanical properties, glass transition temperature, and CFRP-to-steel bond strength of two different epoxy adhesives (linear and nonlinear) commonly used for structural retrofitting. Furthermore, the field application complexity, as well as the short- and long-term efficiency of the developed nonprestressed bonded system, was compared with the existing flat prestressed unbonded retrofit (FPUR) system. Short-term measurements, including sets of truck-loading tests performed before and after strengthening, demonstrated that the tensile stresses were reduced by approximately 15% and 44% in the bridge cross-girders after strengthening by the proposed nonprestressed bonded and FPUR systems, respectively. The long-term measurement results, obtained from a wireless sensor network (WSN) system installed on the bridge, revealed that because of the mismatch of the steel-CFRP thermal expansion coefficient, diurnal temperature changes can cause significant thermal-induced stress cycles in the nonprestressed bonded CFRP plate. These diurnal cyclic stresses must be considered in the design of bonded CFRP strengthening solutions.

Experimental Investigation on Fatigue Behavior of Wide-Flange Steel I-Beams Strengthened Using Different CFRP End Cutting Shapes

In recent decades, the application of carbon fibre-reinforced polymer (CFRP) composites for strengthening structural elements has become an efficient option to meet the increased cyclic loads, or repair due to fatigue cracking. The premature failure due to end-debonding is a key limitation to achieve high fatigue performance of strengthened steel beams with externally bonded CFRP plates. The objective of this study is to explore the reinforcing techniques using the CFRP in-plane end cutting shapes and the triangular spew fillet of adhesive at the tips of the plate to care for fatigue damaged of wide-flange steel I-beams due to end-debonding. Four in-plane CFRP end cutting shapes were chosen, namely: rectangular, semi-elliptical, semi-circular and trapezoidal. The application of the trapezoidal end shape was found to be the best configuration for delaying the end-debonding failure mode and high fatigue life compared to the other CFRP in-plane end cutting shapes. Applying the triangular spew fillets of adhesive significantly increased the end-debonding and steel beam fracture initiation life of the strengthened beams.

Effects of Lateral Bracing and Stiffeners on the CFRP Failure of Strengthened Steel Beams

In this paper, the effects of lateral bracing and web stiffeners on the Carbon Fibre Reinforced Polymer (CFRP) failure modes and buckling strength of the CFRP strengthened wide-flange steel I-beams are investigated experimentally. The study consisted of eight beams tested under static gradual load until failure. The main test variables were steel plate stiffeners, lateral bracings, and bonding of CFRP plates to beam soffits. The results showed that the use of steel plate stiffeners did not only prevent stress concentration below the point load, but it could also help to delay debonding of the externally bonded CFRP plate. The use of lateral bracing indicated a significant effect in preventing the CFRP splitting failure mode. In addition, the use of stiffeners with lateral bracing simultaneously, showed improvement in the in-plane flexural strength, stiffness and ductility of the CFRP strengthened I-beams.

Flexural strengthening of RC beams with externally bonded CFRP plate: experimental study on shear-peeling debonding

This paper reports on experimental investigations of externally bonded carbon fibre reinforced polymer (CFRP) plate debonding behaviour using beam specimens flexurally strengthened by CFRP plates induced by a relative vertical displacement imposed on the two sides of a shear crack in each beam under a four-point bending load. Three specimens, two of which were control specimens to confirm shear failure and one of which was a shear-deficient test specimen under high-level fatigue load, were strengthened with bottom-bonded CFRP plate and side-wrapped CFRP sheet. The variables investigated during the experimental programme were the displacement between the concrete substrate and the CFRP plate, the strain distribution of the CFRP plate and the local bond stress–slip relation. The test results showed the effectiveness of shear–peeling debonding in CFRP plates along with shear crack width opening based on fatigue load and local debonding with peeling off as an important factor affecting the fatigue life by shear failure in this case.

NDT evaluation of long-term bond durability of CFRP-structural systems applied to RC highway bridges

The long-term durability of CFRP structural systems applied to reinforced-concrete (RC) highway bridges is a function of the system bond behavior over time. The sustained structural load performance of strengthened bridges depends on the carbon fiber-rein- forced polymer (CFRP) laminates remaining 100 % bon- ded to concrete bridge members. Periodic testing of the CFRP-concrete bond condition is necessary to sustain load performance. The objective of this paper is to present a non-destructive testing (NDT) method designed to evaluate the bond condition and long-term durability of CFRP laminate (plate) systems applied to RC highway bridges. Using the impact-echo principle, a mobile mechanical device using light impact hammers moving along the length of a bonded CFRP plate produces unique acoustic frequencies which are a function of existing CFRP plate-concrete bond conditions. The purpose of this method is to test and locate CFRP plates de-bonded from bridge structural members to identify associated deterio- ration in bridge load performance. Laboratory tests of this NDT device on a CFRP plate bonded to concrete with staged voids (de-laminations) produced different frequen- cies for bonded and de-bonded areas of the plate. The spectra (bands) of frequencies obtained in these tests show a correlation to the CFRP-concrete bond condition and identify bonded and de-bonded areas of the plate. The results of these tests indicate that this NDT impact machine, with design improvements, can potentially pro- vide bridge engineers a means to rapidly evaluate long lengths of CFRP laminates applied to multiple highway bridges within a national transportation infrastructure.

Analyze of the interfacial stress in reinforced concrete beams strengthened with externally bonded CFRP plate

A theoretical method to predict the interfacial stresses in the adhesive layer of reinforced concrete beams strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) plate is presented. The analysis provides efficient calculations for both shear and normal interfacial stresses in reinforced concrete beams strengthened with composite plates, and accounts for various effects of Poisson`s ratio and Young`s modulus of adhesive. Such interfacial stresses play a fundamental role in the mechanics of plated beams, because they can produce a sudden and premature failure. The analysis is based on equilibrium and deformations compatibility approach developed by Tounsi. In the present theoretical analysis, the adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the reinforced concrete beam and bonded plate. The paper is concluded with a summary and recommendations for the design of the strengthened beam.

Fatigue design criteria for strengthening metallic beams with bonded CFRP plates

Different failure criteria used to prevent fatigue crack initiation in metallic members are described. A series of closed-form analytical solutions were developed to predict the fatigue resistance of metallic beams after strengthening with carbon fiber-reinforced polymer (CFRP) laminates. The model was used to determine Young’s modulus, pre-stress level and dimensions of the CFRP laminates such that the metallic detail is shifted from a ‘finite-life’ regime to the ‘infinite-life’ regime. This method is capable of predicting changes in both the stress range and the mean stress after strengthening. The results show that increasing the stiffness and pre-stress levels of the CFRP laminate can affect the fatigue life of the retrofitted member through different mechanisms. The latter preserves the alternating stress and decreases the mean stress level, whereas the former decreases both the mean and alternating stresses proportionally. To validate this model, a series of fatigue tests were performed on five steel beams, including one reference beam and four strengthened beams. The beams were strengthened with normal modulus (NM), high modulus (HM) and ultra-high modulus (UHM) laminates. Finally, a design example for the fatigue strengthening of a typical riveted metallic girder is presented. The developed analytical model was used to find the most effective fatigue strengthening solution under different cyclic load scenarios. Although the major focus in this paper is on steel members, it also describes others, such as wrought iron and cast iron.

Strengthening of structurally damaged wide shallow RC beams using externally bonded CFRP plates

Reinforced concrete wide shallow beams (WSBs) are commonly used in the joist flooring systems. The structural behavior of WSBs strengthened with carbon fiber reinforced polymer (CFRP) reinforcement was studied on isolated beams and as part of full-scale building. The effect of structural damage on the performance of WSBs flexurally strengthened with CFRP plates was investigated and presented in this paper. Eight full-scale WSBs were tested under four-point bending up to failure. Seven beams were strengthened with CFRP plates bonded to the soffit of the beams and one beam was unstrengthened serving as control. Prior to strengthening, the beams were subjected to different levels of damaging by preloading to 30-95% of the beams' flexural capacity. One beam was fully damaged by preloading to failure and repaired before strengthening by replacing the crushed concrete. The data showed that the pre-damaged strengthened beams exhibited ultimate capacities up to 8% lower than those of the undamaged strengthened beams. However, the load carrying capacities of pre-damaged strengthened beams were more than those predicted by ACI 440 design guide, fib Bulletin 14, and JSCE design recommendations. Both fib Bulletin 14 and JSCE design recommendations gave very conservative predictions with average ratios of experimental to predicted ultimate capacity of 2.02 and 2.35, respectively. More accurate predictions were obtained by ACI 440 design guide as the corresponding ratio was 1.24. These results indicate that strong confidence and reliability can be placed in applying CFRP strengthening to structurally damaged WSBs.

Long-term behavior of wide shallow RC beams strengthened with externally bonded CFRP plates

The use of fiber reinforced polymer (FRP) for flexural strengthening by adhesively bonding to the tension soffit of reinforced concrete (RC) beams has become a popular retrofit technique. Extensive research work is available in the literature on the short term behavior of RC beams strengthened with externally bonded FRP reinforcement. On the other hand, the long term behavior of FRP-strengthened beams has not yet been fully explored. This paper investigates the long term deflection and cracking characteristics of wide shallow RC beams strengthened with carbon FRP (CFRP) plates. Five full scale wide shallow beams were constructed: two beams were tested under static loading and three beams were tested under sustained load for a period of 600days. Three of the beams were strengthened with CFRP plates bonded to the soffit of the beams and two beams were unstrengthened serving as control. The long term beam deflection was compared with the predictions of the effective modulus approach considering two different concrete creep models. Beams strengthened with CFRP plates showed significant improvement in the short term deflection and crack width compared to the unstrengthened beam. The strengthened beams, however, did not show that much improvement in the long term behavior. The effective modulus approach was found to reasonably predict the additional long term deflection of the beams depending on the incorporated concrete creep model. Further, an analytical procedure for predicting the long term crack width of the beams was presented.

Analytical Analysis of the Interfacial Stress in Damaged Reinforced Concrete Beams Strengthened by Bonded Composite Plates

A theoretical method to predict the interfacial stresses in the adhesive layer of damaged reinforced concrete beams strengthened with externally bonded carbon fiber-reinforced polymer plate is presented. The adopted model is developed including the adherend shear deformations by assuming a linear shear stress through the depth of the reinforced concrete beam, while all existing solutions neglect this effect. In addition, in the present study the anisotropic damage model is adopted to describe the damage of the reinforced concrete beams. It is shown that the damage has a significant effect on the interfacial stresses in fiber-reinforced polymer damaged reinforced concrete beam.

Fatigue behavior of notched steel beams reinforced with bonded CFRP plates: Determination of prestressing level for crack arrest

This paper presents the findings of an experimental and theoretical study on the behavior of notched steel beams reinforced by non-prestressed and prestressed bonded CFRP (carbon fiber reinforced polymer) plates under cyclic loading, with specific attention to the effect of crack propagation and FRP-to-steel debonding on the fatigue crack growth (FCG) rate. A fracture mechanics based model is proposed in order to estimate the required prestressing level needed to arrest the crack propagation in the notched beams. Several strengthened beams are tested under different fatigue loading ranges and the experimental results show good agreement with the proposed fracture model. An optical image correlation system (ICS) is used to track the crack tip and observe the crack development pattern as well as to provide information about stress distribution at the vicinity of the crack tip. Experimental results show that the fatigue life of a beam reinforced by the prestressed CFRP plate increases more than five times of that of an identical beam reinforced by non-prestressed CFRP plate. Prestressing also significantly reduces the residual deflection during the FCG process. Furthermore, it is shown that as the stiffness of the FRP-to-steel bond joint reduces the FCG rate increases, while as the crack grows (in steel web) the FCG rate decreases.

DESIGN FORMULA FOR REHABILITATED ANGLE STEEL MEMBER USING CARBON FIBER REINFORCED PLASTIC PLATES

The authors have developed a tensile force strengthening method using bonded carbon fiber reinforced plastic (CFRP) plate to enhance the life of existing buildings.A rehabilitation technique for compressive force strengthening as well as tensile force strengthening is required to enhance the strength and deformation of steel structure members. This paper reports the results of compressive loading tests on an angle steel brace before and after rehabilitation using bonded CFRP plates. These results show enhanced compressive force carrying capacity and deformation capacity as follows. 1) Although buckling tests showed enhanced flexural rigidity, there was no increment of axial stiffness. 2) An elastic buckling formula using the experimental flexural rigidity from a three-point bending test, gave a conservative evaluation of load carrying capacity. 3) Taking into count the contribution of CFRP up to the elastic limit strain of the steel member, its load carrying capacity could be evaluated from the formula of Design Standard for Steel Structures of AIJ. 4) The CFRP must be bonded over the full length of the angle member.

Adhesive bonding in steel beams strengthened with CFRP

The strengthening of metallic beams with a bonded carbon-fibre-reinforced polymer (CFRP) plate has found many applications in bridge and building structures in recent years. The structural behaviour of the adhesive bonding in this technique is the most distinct element to be considered. In this paper, previous analytical, experimental and numerical studies under static and fatigue loadings by the present authors are reviewed. An S–N curve based on the peak interfacial stresses is proposed to predict the fatigue life to crack initiation of the retrofitted beams. An equation to predict the fatigue life during the crack propagation has also been developed. Finally, the geometric effects of the spew fillet and of the taper at the plate end of the retrofitted beam are discussed. In the conclusion, design recommendations for steel beams strengthened by bonded CFRP plates are detailed based on the findings obtained from the studies presented.

Performance in Fire of Small-Scale CFRP Strengthened Concrete Beams

When strengthening concrete members with fiber-reinforced plastic (FRP) materials the strengthening is, typically, undertaken to carry live load. This live load is assumed to remove itself from the strengthened member in the event of a fire. Thus, the fire performance of the FRP is not important. However, if the strengthening system was designed such that the FRP took some of the dead load, then the performance in fire would become important. In this series of tests, 24 reinforced concrete beams were cast. They were divided into eight sets of three. The sets were split into fire tested and control. In the control group were an unstrengthened control set, a set strengthened with bonded carbon FRP (CFRP) plates, and a set with bonded CFRP plates with bolted anchorages. In the fire-tested group were an unstrengthened control set, a set strengthened with bonded CFRP plates, a set with bonded CFRP plates with bolted anchorages, a set strengthened with bonded CFRP plates and a cementitous fire protection system...

Evaluation of Steel I-Beams Strengthened by Various Plating Methods

Flexural behavior of up-graded steel I-beams, IPE 160, with bonded/welded steel plates and bonded carbon fiber reinforced polymer (CFRP) plates/sheets were evaluated in this work. Different types of end anchorage (belted CFRP sheet, steel clamped, and end welded) were applied to enhance the efficiency of the bonded element. The present experimental results showed that, plate end anchorage increased markedly the efficiency of bonded steel and CFRP plates. CFRP sheet showed higher resistance to debond than CFRP plate. Two separate CFRP strips bonded below the lower flange of the I-beam is the best method for bonded CFRP plate to I-beams. Bonded/welded hybrid joint technique for attachment of the steel plate in tension side is the most efficient technique. All strengthened beams and control beam failed due to distortional buckling after the yield occurrence.

Debonding Strength of Steel Beams Strengthened with CFRP Plates

This paper addresses the debonding strength of partial-length, adhesively bonded carbon fiber-reinforced polymer (CFRP) plates that are used to strengthen steel beams. Bonded CFRP plates tend to debond under static and fatigue loadings because of the very high stress field at the plate end. Such failures limit the application of CFRP plates. Static and fatigue tests show that the stress intensity factor governs the debonding strength. The steel/adhesive corner was the locus of debond initiation. The effects of the following parameters on stress intensity factors are discussed: plate thickness, plate modulus, bondline thickness, adhesive modulus, and adhesive spew-fillet angle. The stress intensity factors are calculated using the Betti’s law-based reciprocal work contour integral method (RWCIM). The parametric study results indicate that the stress intensity factors cannot be used to represent the severity of the corner as the adhesive spew-fillet angle (and singularity) changes. Therefore, the use of stress intensity factors as a failure criterion for the purpose of predicting debonding strength is limited to the same spew-fillet angle.

Fatigue performance of metallic beam strengthened with a bonded CFRP plate

This paper gives details of a fatigue test programme of a series of small-scale steel beams bonded with a carbon fibre reinforced polymer (CFRP) plate. Backface-strain technique was used to detect crack initiation and monitor crack growth. Results show that this technique could detect crack initiation and track the deterioration of the adhesive layer. The strains recorded show that crack initiates and grows in Mode I earlier than in Mode II in the bonded joints, and this finding agrees with the observations on double-lap joints published in literature. The strains recorded also show that the adhesive spew fillet at the end of the CFRP plate is beneficial to the fatigue performance, although the improvement is not significant. An S– N curve was developed from the test results. The curve correlates the maximum principal interfacial stress at the plate end to the number of cycles to crack initiation. This maximum stress can be calculated from the authors’ previous analytical work. The fatigue limit of the S– N curve was found to be about 30% of the ultimate static failure stress.

Behaviour under static loading of metallic beams reinforced with a bonded CFRP plate

Carbon fibre reinforced polymer (CFRP) plates have been employed widely in the repair and upgrading of bridges and buildings in recent years. When they are used in the upgrading of metallic beams, the weakest link is the adhesive bonding, and plate peeling is the most important failure mode due to the high stress concentration at the end of the plates. This paper presents the results of a study on the static behaviour of metallic beams bonded with CFRP plates. A total of 10 specimens were tested for different cases. The results show the influencing factors on the strength of the metallic beams reinforced by different types of CFRP plates, the effects of the tapers at the plate ends and of the spew fillets beyond the plate ends. The test results also validate the theoretical results of stress analysis. The use of longer and tapered plates is suggested as a solution for improvement.

Retrofit of a Three-Span Slab Bridge with Fiber Reinforced Polymer Systems—Testing and Rating

A 45-year old, three-span reinforced concrete slab bridge with insufficient capacity was retrofitted with 76.2- and 127-mm wide bonded carbon fiber-reinforced polymer (FRP) plates, 102-mm wide bonded carbon FRP plates with mechanical anchors at the ends, and bonded carbon FRP fabrics. The use of four systems in one bridge provided a unique opportunity to evaluate field installation issues and to examine the long-term performance of each system under identical traffic and environmental conditions. Using controlled truckload tests, the response of the bridge before retrofitting, shortly after retrofitting, and after one year of service was measured. The stiffness of the FRP systems was small in comparison to the stiffness of the bridge deck, and accordingly the measured deflections did not change noticeably after retrofitting. The measured strains suggest participation of the FRP systems, and more importantly, the strength of the retrofitted bridge was increased. A detailed 3D finite-element model of the original and retrofitted bridge was developed and calibrated based on the measured deflections. The model was used to predict more accurately the demands for computing the rating factors. The addition of FRP plates and fabrics led to a 22% increase in the rating factor and corresponding load limits. During a one-year period, traffic loading and environmental exposure did not apparently affect the performance of the FRP systems. The increased capacity and acceptable performance of the FRP systems enabled the engineers to remove the load limits in order to resume normal traffic. Future tests are necessary to monitor the long-term behavior of the FRP systems.