Carbon Fiber Reinforced Polymer Steel Research Articles

Effects of adhesive property and thickness on the bond performance between carbon fiber reinforced polymer laminate and steel

The bonding between CFRP (Carbon Fiber Reinforced Polymer) laminate and steel plays a key role in the strengthening effects of CFRPs on a steel structure. In this paper, the toughness of the adhesives and the bonding thickness were investigated on the CFRP-steel bond performance using a single lap shear test. The properties of the adhesives, brittle (linear elastic) or tough (nonlinear elastic, lower modulus, higher elongation), led to apparent differences in the ultimate load, stiffness, the shape of the bond-slip curve, and the stress distribution of the CFRP-steel bond. For the tough adhesive, an increase of the bonding thickness from 0.5 mm to 1.5 mm improves the fracture energy but reduces the interface stiffness. On the contrary, for the brittle adhesive, the effects of the bonding thickness are negligible. According to the test and literature results, key parameters of the bond-slip models of the tough adhesive and brittle adhesive in the case of the delamination of CFRP laminate were determined.

Bond-slip model of the CFRP-steel interface with the CFRP delamination failure

The bond-slip model of a carbon fiber reinforced polymer (CFRP)-steel bonded interface is crucial for predicting the bond behavior of CFRP-strengthened steel structures. CFRP delamination failure is often encountered in existing studies on the bond behavior between CFRP plates and steel plates. However, this failure mode has been investigated by few scholars, especially regarding the interfacial bond-slip model based on CFRP delamination failure. In this paper, a total of 16 single-shear joints were manufactured to investigate the effects of the adhesive types, the adhesive thicknesses and the CFRP types with different inter-laminar shear properties on CFRP-steel bonded interfaces. Strains and displacements on the surface of the CFRP-steel joint were measured by three-dimensional digital image correlation (3D-DIC) technology. The bond-slip model based on CFRP delamination failure was established for the first time by introducing the inter-laminar shear strength, the inter-laminar shear modulus and the inter-laminar shear energy dissipation of the CFRP plate. The results showed that the adhesive thickness and the inter-laminar shear properties of the CFRP plate had a significant effect on the bond-slip model based on CFRP delamination failure. Finally, the accuracy of the model was verified by comparison with experimental data.

Numerical analysis of normal concrete lining strengthening methods under different damage levels

A number of 1/5 scale normal concrete (NC), reinforced concrete (RC), Engineered cementitious composites (ECC) and reinforced ECC (R/ECC) tunnel lining specimens were tested under vertical loading applied at the mid-top of the crown and the mechanical behaviours were analysed. Four lining FE models were established and compared in detail with the test data. Subsequently, numerical analysis of NC lining strengthening methods for different damage levels was performed. The NC linings exhibiting different damage levels were strengthened by either RC arch, ECC arch, R/ECC arch, steel plate and carbon fibre reinforced polymer (CFRP) sheet. The mechanical behaviour of the retrofitted NC linings was analysed, and the strengthening effects were thoroughly evaluated. The results showed that the load-bearing capacity and ductility of NC linings retrofitted by R/ECC arch were better than NC linings strengthened by ECC arch or RC arch. The steel plate and CFRP sheet retrofitted NC linings resulted in lower load-carrying capacities compared with linings strengthened by arches. For the plate strengthening methods, when the damage level of the strengthened NC lining is lower, more pronounced strengthening effects could be attained compared with the highly damaged linings.

Experimental and Numerical Analysis of a Reinforced Wood Lap Joint.

During the restoration of ancient wood structures, the original material of the structures should be kept as much as possible, so a spliced method by using lap joints is commonly used to repair ancient wood structures. This study studies the mechanical behavior of a lap joint which was reinforced with fiber composite materials or steels. An experimental and numerical analysis were performed to study the strength, stiffness and failure modes of the lap joints. The test results showed that the strengthening effect of sticking carbon fiber-reinforced polymer (CFRP) sheets is better than that of sticking CFRP bars or steel bars due to the better bonding conditions; therefore, the lap joint reinforced with CFRP sheets was further analyzed using a numerical approach. The strength and stiffness were enhanced by increasing the reinforcement ratio of CFRP sheets. The use of a 0.34% reinforcement ratio made the bearing capacity of the lap joint reach that of the intact beam. The numerical model agreed well with the experiments in terms of stiffness. By analyzing the numerical analysis results, the structural behavior of the lap joint was revealed. A numerical model can be used to predict the stiffness and behavior of spliced beams with lap joints of different sizes.

Flexural behaviour of reinforced concrete beams repaired using a hybrid scheme with stainless steel rebars and CFRP sheets

The repair of corroded reinforced concrete (RC) structures is a critical issue for structural designers and managers because the repair cost significantly dominates the life-cycle cost. Moreover, in chloride-containing environments (e.g. coastal environments), RC structures repaired with conventional methods may corrode again since the ingress of chloride ions, water, and oxygen into the concrete will not be completely restrained. In this paper, a novel repair method for corroded RC beams using stainless steel (SS) rebars and externally bonded carbon-fibre-reinforced polymer (CFRP) sheets is proposed. The flexural behaviour of a group of RC beams repaired using the proposed method is experimentally investigated. The test variables include the longitudinal reinforcement types (i.e. carbon and SS), shear reinforcement configurations, and CFRP sheet wrapping schemes. Experimental results demonstrate that a carbon steel (CS) reinforcement can be replaced with an SS reinforcement to reduce future repair costs since there is no significant difference in the flexural behaviour of beams reinforced with CS and SS rebars. The effectiveness of CFRP sheet wrapping schemes on the flexural performances of SS-RC beams is approximately identical to that of CS-RC beams. In addition, the present paper discusses the serviceability and ultimate limit states of RC beams retrofitted with SS rebars and CFRP sheets based on the current design codes for ordinary RC beams strengthened using CFRP sheets. The predictions are in good agreement with the experimental results.

Flexural Performance of Steel\u2013FRP Composites for Automotive Applications

The design of hybrid structure offers an attractive solution to enhance strength and structural stiffness as well as to achieve lightweight effect and cost reduction. The applications of steel–FRP (fiber-reinforced polymer) composites in transportation and civil engineering have been comprehensively reviewed. In order to apply hybrid structures to car body parts such as B-pillar, flexural performance of steel–FRP composites is investigated by means of three-point bending test in this study. An analytical model is deduced to calculate the initial stiffness, the bending load and the energy absorption of steel–FRP composites. Steel–CFRP (carbon fiber-reinforced polymer) and steel–AFRP (aramid fiber-reinforced polymer) composites are experimentally studied and discussed. The results demonstrate that the steel–FRP composites exhibit significantly higher load-carrying capabilities and initial stiffnesses along with larger energy absorptions in the bending process compared to the single steel sheet.

Experimental investigation on shear behavior of RC beams strengthened by CFRP grids and PCM

This paper presents the experimental results of four-point bending tests for reinforced concrete (RC) beams strengthened by carbon fiber reinforced polymer (CFRP) grids and sprayed polymer-cement-mortar (PCM). The reinforced arrangements of CFRP grids, stirrups and longitudinal steel rebars were selected as changing parameters. The maximum loads, mid-span deflections and strains for the PCM, longitudinal steel rebars, stirrups and CFRP grids were obtained. The mechanical behavior of CFRP grids and the cooperation work performance between CFRP grids and steel rebars were investigated emphatically. The research results indicated that the shear strengthening effect of the CFRP grid-PCM reinforcing layers bonded along both sides of RC beams was sufficient. The shear contribution of the vertical CFRP grid was similar to that of the stirrup, but the stress distribution of vertical CFRP grid was not uniform due to the resistant action of grid points. Moreover, the vertical CFRP grids bore the lateral extruding action with opposite directions in tension and compression zones due to the compressive and tensile behavior of horizontal CFRP grids, resulting in the complex stress state of the vertical grid which differs from the stirrup. There exists the cooperation work between CFRP grids and steel rebars in shear strengthening RC beams. So as to clarify the shear mechanism of RC beams strengthened by CFRP grids and PCM, the complex stress state of vertical CFRP grid, influence of the horizontal CFRP grid and the preconditions for the normal work of CFRP grid should be taken into account. The proposed shear capacity calculation formula based on truss-arch model could satisfy the design requirements.

Numerical Investigation on the CFRP Strengthened Steel Frame under Earthquake

Steel structures are commonly used in seismic regions of the world because of its strength and ductility. However, these structures are still prone to damage during an earthquake. With this risk of seismic damage, the strengthening of steel structures is a major concern in order to resist the dynamic loads resulted from earthquakes. This report investigates the potential for the use of Carbon Fibre Reinforced Polymer (CFRP) to strengthen the rigid steel frame under a real earthquake load. This research will be undertaken using Strand7, a finite element (FE) analysis software. To validate the accuracy of this research, the finite analysis results have been compared to the available experimental study by the Authors. First, both FE models of a five-story bare steel frame and CFRP strengthened steel frame has been developed. Then the predicted numerical results of bare steel frame and CFRP strengthened steel frame under earthquake excitation are compared. The results indicated an increase in the seismic performance of the steel structure due to the strengthened with CFRP. The CFRP strengthened steel frame showed 15% less tip deflection compared to bare steel frame. Further analysis on the strengthening capabilities of higher thickness CFRP was performed to assess the effect of the thickness of CFRP and the higher thickness CFRP showed better seismic performance compare to normal thickness CFRP by reducing 34.38% of tip deflection.

Bond performance of Steel-CFRP bar reinforced coral concrete beams

Steel-Carbon Fiber Reinforced Polymer (SCFRP) bar provides a new way to solve the shortcomings of poor ductility and low stiffness of FRP reinforced concrete members. In order to understand the bond performance between SCFRP bars and coral concrete, 39 pull-out tests of SCFRP reinforced coral concrete is performed. Different parameters such as the types, diameters and bond lengths of SCFRP reinforcements were investigated. The failure modes, failure mechanism, stress process and bond-slip relationship of the specimens were analyzed. The failure modes of steel bar pull-out can be divided into steel core pull-out before yielding and steel core pull-out after yielding. The concrete splitting failure modes can be divided into coral concrete splitting before steel core yielding and Coral concrete splits after steel core yielding. There are four characteristic points in the bond slip curve: elastic slip point, yield slip point, peak slip point and residual slip point. The bond slip model of SCFRP reinforced coral concrete was developed in this paper using coefficients of slip before steel core yields and slip after steel core yields are 0.86 and 1.0 respectively, which can describe the bond slip behavior between SCFRP and concrete more accurately.

Study on the Flexural Performance of Hybrid-Reinforced Concrete Beams with a New Cathodic Protection System Subjected to Corrosion.

Steel corrosion is considered as the main factor for the insufficient durability of concrete structures, especially in the marine environment. In this paper, to further inhibit steel corrosion in a high chloride environment and take advantage of the dual-functional carbon fiber reinforced polymer (CFRP), the impressed current cathodic protection (ICCP) technique was applied to the hybrid-reinforced concrete beam with internally embedded CFRP bars and steel fiber reinforced polymer composite bar (SFCB) as the anode material while the steel bar was compelled to the cathode. The effect of the new ICCP system on the flexural performance of the hybrid-reinforced concrete beam subjected to corrosion was verified experimentally. First, the electricity-accelerated precorrosion test was performed for the steel bar in the hybrid-reinforced beams with a target corrosion ratio of 5%. Then, the dry–wet cycles corrosion was conducted and the ICCP system was activated simultaneously for the hybrid-reinforced concrete beam for 180 days. Finally, the three-point bending experiment was carried out for the hybrid-reinforced concrete beams. The steel bars were taken out from the concrete to quantitatively measure the corrosion ratio after flexural tests. Results showed that the further corrosion of steel bars could be inhibited effectively by the ICCP treatment with the CFRP bar and the SFCB as the anode. Additionally, the ICCP system showed an obvious effect on the flexural behavior of the hybrid-reinforced concrete beams: The crack load and ultimate load, as well as the stiffness, were enhanced notably compared with the beam without ICCP treatment. Compared with the SFCB anode, the ICCP system with the CFRP bar as the anode material was more effective for the hybrid-reinforced concrete beam to prevent the steel corrosion.

Bond characteristics between high/ultra-high strength steel and ultra-high modulus CFRP laminates

With the increasing applications of high and ultra-high strength steel (HSS/UHSS) in engineering structures, there is a need to address rehabilitation and strengthening of such steel grades using carbon fibre reinforced polymer (CFRP). The bond between HSS/UHSS and CFRP is vital to ensure the efficiency of the strengthening. The existing bond study was focused on mild steel as substrate, with very limited work on HSS up to the grade of S690. Since HSS/UHSSs are designed to undergo much higher loading in service, much higher shear stress is expected in the adhesive layer, leading to a higher chance of premature debonding. In this paper, the bond between HSS/UHSS plates and ultra-high modulus (UHM) CFRP laminates under static tensile loading is studied experimentally, numerically and theoretically. Both single-sided and double-sided schemes were adopted. The numerical simulation using LS-DYNA software package was implemented and a reasonable agreement with the experimental results is found. A theoretical bond model was developed to relate the bond strength to the imposed strain in the steel member outside the bonded region.

Fatigue behavior of prefatigued reinforced concrete beams rehabilitated with near surface mounted carbon fiber reinforced polymer reinforcement

AbstractThis paper investigates the influence of cyclic preloading on the fatigue behavior of reinforced concrete (RC) beams rehabilitated with near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) reinforcement. The study includes eight RC beams with dimensions of 152.4 × 152.4 × 1,521 mm. Strengthened and nonstrengthened specimens were tested under monotonic loading to obtain the flexural static capacity. Two strengthened specimens were tested as a reference under constant amplitude cyclic stress without prefatiguing. Four nonrehabilitated RC beams were prefatigued to simulate service loading conditions. Later, these RC beams were rehabilitated with NSM CFRP reinforcement and tested under the same cyclic stress range of the reference specimens. Mid‐span deflection, strain of tension steel rebars and CFRP rods, stiffness degradation, and energy dissipation were analyzed and evaluated. Experimental results show that the rehabilitated prefatigued RC beams had similar stiffness degradation and failure modes to the reference strengthened RC beams. The rehabilitated RC beams had an improvement in the fatigue responses when compared to the reference strengthened RC beams. Moreover, the postfatigue monotonic behavior of the rehabilitated RC beams showed an increase in the elastic modulus and a decrease in ductility.

Experimental study on the bond behavior between CFRP plates and steel substrates under fatigue loading

This paper reports an experimental study on the fatigue behavior of bonded interfaces between carbon fiber reinforced polymer (CFRP) plates and steel plates. A total of 20 single-shear bonded joints were tested under monotonic and fatigue loading. The digital image correlation (DIC) method was employed. The effects of the load ratio, adhesive thickness and adhesive type on the fatigue life were investigated. The failure mode, fatigue life, relative slip, load-slip curve, CFRP strain and interfacial shear stress are presented. The results indicated that the cohesive failure was the dominated failure mode for all specimens. With the increase of the loading cycles, the debond initiated firstly at the loaded end and then propagated gradually toward the free end, which was obviously reflected by CFRP strain and shear stress distributions. The hysteresis was observed in the load-slip curve, indicating the energy dissipation occurred in the interfaces. The fatigue life decreased dramatically with the increase of the load ratio. When the load ratio was the same, the fatigue life decreased with an increasing adhesive thickness, and the fatigue life of the specimen with Sikadur-30 was longer than that with Araldite-2015. However, when the load range was similar, the fatigue life increased with an increase in the adhesive thickness, and the fatigue life of the specimen with Araldite-2015 was longer than that with Sikadur-30. The distribution shapes of the shear stress for two types of the specimens were approximately trapezoidal and triangular under monotonic and fatigue loading.

Analytical modelling and design of partially CFRP-wrapped thin-walled circular NCFST stub columns under axial compression

The partial CFRP (carbon fiber reinforced polymer) wrapping scheme is widely used for strengthening engineering practices to obtain a moderate improvement in strength, whereas detailed investigations on partially CFRP-wrapped thin-walled circular normal-strength concrete-filled steel tubular (NCFST) stub columns are seldom presented. This paper aims to conduct a series of numerical analyses to figure out the axial compressive performance of circular NCFST stub columns wrapped by CFRP belts partially. A nonlinear finite element (FE) modelling focused on the surface interaction as well as the CFRP and thin-walled hollow steel section (HSS) confinement effects on the concrete infill is developed via ABAQUS solver and the accuracy is validated against the existing experimental data. Following this, a systemic parametric investigation is developed to explore the impact of steel yield stress, concrete compressive strength, diameter: thickness ratio, CFRP wrapping ratio and number of CFRP layer, etc. on the axial compressive properties of the partially CFRP-wrapped thin-walled circular NCFST stub columns. Elaborate investigations on the surface contact action, stress-strain response, typical axial force versus axial shortening curve and failure modes are presented to better figure out the axial compressive behaviour of the type of composite column. A discussion on the effective wrapping scheme of the partially CFRP-wrapped thin-walled circular NCFST stub column imposed with axial compressive force is described as well. Finally, two types of design formulas in compliance with the unified theory and the tubed concrete method are proposed to assess the axial compressive resistance of the partially CFRP-wrapped thin-walled circular NCFST stub column. The research may provide a considerable and scientific basis for employing CFRP materials for the CFST structures. • The FE modelling of the partially CFRP-wrapped axially loaded thin-walled circular NCFST stub column was developed. • The failure modes, the contact stress, the stress-strain response and the effective wrapping scheme were explored. • Design formulas to predict the axial strength of partially CFRP-wrapped circular NCFST stub columns were proposed.

Retrofitting of different types of reinforced concrete beams after exposed to elevated temperature

The development of the concrete industry has facilitated the use of nonconventional concrete types, such as high-strength concrete (HSC) and self-compacting concrete (SCC). Normal concrete (NC) beams experience a significant reduction in their strength after fire exposure. An experimental program is designed to investigate the effect of different levels of temperature on the most common concrete types and the effect of different retrofitting techniques to restore the capacity of reinforced concrete beams. Thirty-one beams were casted using NC, SCC, and HSC. Twenty-eight beams were subjected to temperatures of 400 °C and 600 °C for 60 min and 120 min to evaluate the beam behaviour under indirect fire conditions. Carbon fibre reinforced polymer (CFRP) laminates, bonded steel plates, and concrete jacketing have been used for beam capacity retrofitting. The exposure time at a high temperature level had a significant effect on the behaviour of the different beams, especially those with NC, which underwent a dramatic reduction in strength after being subjected to 600 °C for 2 h. The SCC beams had an acceptable behaviour under elevated temperature conditions. All the retrofitting techniques had a positive effect on SCC and HSC beams, but the bonded steel plate and CFRP techniques did not produce a noticeable effect on NC at 600 °C. The reinforced concrete jacket had a satisfactory effect in the drastically deteriorated beams.

Interfacial bond-slip degradation relationship between CFRP plate and steel plate under freeze-thaw cycles

The bond-slip degradation relationship of a carbon fiber reinforced polymer (CFRP)-steel bonded interface is crucial for predicting the long-term behavior of CFRP-strengthened steel structure under a freeze-thaw (F-T) environment. A total of 16 single-shear bonded joints were tested to investigate the effect of the bond length and the F-T cycles on the CFRP-steel bonded interfaces. The strains on the CFRP surface and the relative slips between the CFRP plate and steel plate were measured by using three-dimensional digital image correlation (3D-DIC) technology. The main properties of the bonded joints were examined, including the failure mode, the ultimate load, the load-slip curve, the CFRP strain distribution and the bond-slip relationship. The degradation models for the bond-slip parameters were formulated by introducing the damage factors, and the interfacial bond-slip degradation relationships were subsequently developed. Finally, the proposed degradation relationship was validated through comparison with the experimental bond-slip relationships and the predicted relationships under different F-T cycles.

Test on fatigue repair of central inclined cracked steel plates using different adhesives and CFRP, prestressed and non-prestressed

This paper investigates the fatigue repair of 15 central inclined cracked steel plates using different repair materials and prestress levels. Thirteen steel plates were repaired with three kinds of carbon fiber reinforced polymer (CFRP) and two kinds of adhesive in different configurations (single-sided, double-sided, non-prestressed and prestressed). Two unrepaired steel plates were used as the reference. Fatigue performance was evaluated using the fatigue life and the failure mode. It was found that when the tensile stiffness of distinct CFRP was identical, the fatigue lives of different specimens in the same configuration were alike regardless of the failure mode. Nevertheless, different CFRP and adhesives contributed strongly to the failure mode. Fatigue lives demonstrated that double-sided repair was more efficient than single-sided repair, while prestressed repair had the highest repair efficiency. When prestressed, higher prestress level led to greater fatigue life improvement. Besides, prestress levels measured from CFRP strain and steel plate strain were compared; the latter was found to be more consistent with the tested fatigue lives.

Experimental Study on Interfacial Bond Behavior between CFRP Sheets and Steel Plates under Fatigue Loading

Using carbon fiber reinforced polymer (CFRP) composites for enhancing the fatigue behavior of the steel structures will be an important application. As the most critical part, the fatigue behavior of the CFRP-to-steel bonded interface directly determines the strengthening effect of steel structures reinforced by CFRP. In this paper, a series of CFRP-to-steel double-shear specimens are performed in order to study the interfacial bond behavior between CFRP and steel under fatigue loading. Two parameters are considered: the upper bound value and the lower bound value of the fatigue loading. An analysis of test results indicates that the crack development rate increases with the increment of the stress ratio or stress level and the crack development process includes two phases: crack stable development phase and debonding failure phase. A calculation model is put forward to describe the relationship between the crack development rate and the stress level. Besides, it can be obtained from the test results that the fatigue lives of the specimens decrease with the increment of the stress level. The empirical formula of S-N curve based on the form of single logarithm formula is proposed and the fatigue limit under the experimental conditions in this paper is determined to be 0.343 by computational analysis.

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.

Flexural Assessment of Blast-Damaged RC Beams Retrofitted with CFRP Sheet and Steel Fiber

This study presents the effects of blast-induced local damages on the flexural strength of blast-damaged and repaired specimens. In the experimental program, blast-damaged specimens were repaired with steel fiber reinforced cementitious composite (SFRCC) as well as carbon fiber-reinforced polymer (CFRP) sheets and tested for flexural strength measurements. The test parameters included shear reinforcement (amount and spacing), steel fiber content (0, 1.0 vol%), and retrofitting with CFRP sheets. The test results indicated that the use of higher amounts of stirrups demonstrated insignificant benefits in preventing local damages. However, it was shown that the use of small-diameter steel bars for stirrups with small spacing could decrease the local damages more effectively compared to the large-diameter steel reinforcement. For the residual strength of the damaged specimens, the specimens using more stirrups could resist over 60% of their original flexural strength. CFRP retrofitting showed insignificant enhancement in ductility of intact, damaged, and repaired specimens. However, it distributed the blast load and protected debris scattering. The addition of steel fibers results in increased ductility and enhanced blast resistance against local damages. All specimens, excluding control specimen, that repaired with SFRCC showed higher flexural strength to their original strength. Therefore, it can be concluded that replacing damaged concrete cover with SFRCC is adequate for repairing the blast-damaged RC members.