Carbon Fiber Reinforced Polymer Bars Research Articles

Flexural performance of RC beams strengthened with inorganic adhesive bonded NSM CFRP bar

Organic adhesives (e.g. epoxy) have been widely utilized in near-surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strengthening technique for reinforced concrete (RC) structural members. Their inevitable shortcomings, such as poor fire resistance, emission of toxic fumes, and incompatibility with concrete substrate, necessitate the advancement of eco-friendly inorganic adhesives. In this study, two types of inorganic adhesives, including ordinary Portland cement mortar (OPCM) and alkali-activated slag mortar (AASM), were adopted in the NSM CFRP bar strengthening method for RC beams. The inorganic adhesive type, bonding length, and additional anchorage device on CFRP bar were considered as parameters in the strengthening scheme. One control beam and four strengthened beams were fabricated and tested, followed by comparison of their failure modes, loading behaviour, and reinforcement strains. The results show that all the strengthened beams exhibit higher ultimate loads than the control beam. Comparing to the OPCM, the AASM is more effective in mitigating debonding of CFRP bar and enhancing the ultimate load of beams. Furthermore, it is essential to provide a sufficient bonding length for AASM bonded NSM CFRP bars to guarantee their strengthening effectiveness. The addition of anchorage device for CFRP bars can further enhance the synergistic action between CFRP bar and adhesive, leading to a further enhancement in flexural strength and CFRP bar strain. In addition, the flexural strengths of OPCM or AASM bonded NSM CFRP bar-strengthened beams can be accurately predicted by the equations provisioned in ACI 440.2R-2017.

Study on Bond-Slip Behavior between Seawater Sea-Sand Concrete and Carbon Fiber-Reinforced Polymer (CFRP) Bars with Different Surface Shapes.

The application of CFRP bar and seawater sea-sand concrete (SSSC) in construction can overcome the shortcomings in conventional reinforced concrete, such as corrosion induced by carbonation and chloride ingress. In this study, the bond-slip behavior between an SSSC cube and CFRP bar has been investigated, and different CFRP bar surface shapes have been considered. A total of 27 specimens (9 groups) were fabricated for a pull-out test, where three types of CFRP bar with different surface shapes were used: smooth regular bars, double-wrapped bars and ribbed bars. Bond strength, bond-slip curve, and failure mode have been presented and discussed. FE models have been constructed and validated by experimental results. The effect of concrete compressive strength and relative area of ribs on bond strength has been studied through numerical simulations. It is found that the bond strength increased with concrete compressive strength, and the ribbed bar had significantly higher bond strength than the smooth regular bar. Pull-out failure was observed when the cover-depth-to-bar-diameter ratio was no less than 4 and, otherwise, splitting failure occurred. In addition, a simple formula has been proposed to approximately evaluate the bond strength between an SSSC cube and CFRP bar and validated by experimental results, and analytical expressions for different bond-slip curves have also been developed.

Bond performance of FRP bars in plain and fiber-reinforced geopolymer under pull-out loading

Geopolymer-bonded fiber-reinforced polymer (FRP) bar is an emerging strengthening system for concrete structures, owing to the excellent geopolymer/concrete bond and the superior moisture and fire resistance of geopolymer matrix (compared to organic bonding adhesives). To apply this strengthening system, the bond performance between FRP bars and geopolymer should be properly understood. In this study, pull-out tests on 81 specimens with a carbon FRP (CFRP) bar embedded in plain and fiber-reinforced geopolymer were conducted. Key factors including the strength of geopolymer (different mass ratios of fly ash and slag), bar embedded length (5, 10, and 15 times the bar diameter), fiber volume fraction in geopolymer (0, 1%, and 2%), and bar diameters (3 mm, 6 mm, and 10 mm) were explored. The results showed that (1) an increase in geopolymer strength improves the bond strength; (2) increasing embedded length (from 5db to 15db) would reduce the average bond strength (by 20–30%); and (3) adding steel and PVA fibers in geopolymer had a marginal effect on the bond strength. Furthermore, a modified bond strength model of CFRP bars in the geopolymer mortar is proposed based on the current experimental results. The findings lay the groundwork for understanding the bond behaviour of FRP bars in geopolymer, which is critical for the successful application of geopolymer-bonded FRP bar strengthening system.

Experimental study on lap behavior of CFRP indented bars in UHPC

The lap behavior of Carbon Fiber Reinforced Polymer (CFRP) indented bars in Ultra-High Performance Concrete (UHPC) were investigated based on pullout test under direct tension on 27 specimens in total. The test variables include splice length, UHPC strength and stirrup ratio. The lap failure mechanism of CFRP bar embedded in UHPC and the influence of test parameters on lap performance were analyzed. Test results showed that the failure modes, pull-out and rupture of CFRP bar, in those lap-spliced specimens were governed only by the splice length of the CFRP bar embedded in UHPC. The pullout failure was caused by the shearing off of indented ribs on the surface of the bars and the fracture failure was governed by the tensile strength of the bars. The bond strength of lap-spliced indented CFRP bars in UHPC is mostly dependent on the shear strength of indented ribs on CFRP bars, little on the UHPC strength and stirrup ratio. For CFRP bars with a 2480 MPa tensile strength and a 9.8 mm diameter (d), a splice length of 30d in UHPC can fully develop the tensile strength of the bar. A simplified equation for predicting the critical splice length of CFRP bars in UHPC was proposed. Compared with the results on anchorage of CFRP bars in UHPC, the critical splice length of CFRP bar in UHPC was 1.45 times the corresponding anchorage length.

Evaluation of residual bond behavior of CFRP and steel bars embedded in UHPC after exposure to elevated temperature

Carbon-fiber-reinforced-polymer (CFRP) bar with high tensile strength is expecting when used with ultra-high-strength-concrete (UHPC), but the reduction of mechanical properties of CFRP under thermal effects is an important issue. This study invesitigated the residual bond strength of CFRP bars in UHPC specimens after exposure to elevated temperatures. For comparison, the residual bond strength of steel rebar was also conducted after exposure to elevated temperatures. For this purpose, the pull-out specimens with dimensions of 150 mm × 150 mm × 150 mm were fabricated to investigate the effects of temperature at ambient temperature and after exposure to elevated temperature at 150 °C and 250 °C. As a result, the CFRP bars embedded in UHPC showed a reduction in the bond stress and these reducing effects is remarkable as the increase of the thermal temperature. Contrary to the test result of the CFRP bars, there was no significant difference in bond strength of the steel rebar in UHPC after exposure to elevated temperature, rather, the residual bond strength after exposure to the elevated temperature at 250 °C increased compared to ambient temperature by 9%. In order to quantitatively identify the initial bond slip behavior after exposure to thermal heating, the experimental results were compared with the modified BPE model and the CMR model, and each parameter was compared by the least square methods. The bond-slip model for CMR was more suitable than the modified BPE model in both the CFRP bars and steel bars, and the experimental results were compared with the previous research; the residual bond stress of CFRP bar in UHPC satisfies the criteria presented by ACI 440.6 M after the heating.

Evaluation of prestress relaxation loss and harping characteristics of prestressing CFRP systems

This paper presents partial results of an investigation about the use of prestressing carbon fiber reinforced polymer (CFRP) systems in prestressed bridge girders. First, stress relaxation tests were conducted on prestressing CFRP cables and bars and continued over a period of one year from the initial prestressing process. Stress relaxation loss found to be linearly related to logarithm of the time and invariant of the length of the specimen. The effect of CFRP anchor loss was also evaluated and used to develop equations for predicting prestress relaxation in both pre- and post-tensioning applications. Then, harping applications of CFRP systems under various harping angles and deviator sizes were experimentally investigated. Higher harping capacity for CFRP cable compared to bar was observed. Premature failure of the CFRP due to localized shear/contact stresses at deviaor location redulted in a significat harping capacity reduction. To maximize the contact surface and reduce the localized shear stress, new deviators were developed, which improved the results. Finally, a finite element (FE) model was developed and calibrated to better understand the harping behavior of CFRP. By performing parametric study on different harping profiles, jacking stress limitations were obtained for the use of CFRP in practical range of harping angles.

Experimental Behavior of UHPC Shear Walls with Hybrid Reinforcement of CFRP and Steel Bars under Lateral Cyclic Load

A new kind of structural system, ultrahigh-performance concrete (UHPC) shear walls reinforced with carbon fiber–reinforced polymer (CFRP) and steel bars, is proposed in this study. CFRP bars were used in the web region of the shear walls to obtain favorable self-centering capacity, and steel bars in the boundary element were used to achieve high energy-dissipation capacity. Quasi-static tests of four shear-wall specimens were conducted. The self-centering capacity, energy-dissipation capacity, and strength and stiffness degradation of the specimens were analyzed. The tested UHPC shear walls exhibited favorable self-centering and good energy-dissipation capacity. The experimental results showed that the stress in the CFRP bars is the key factor to determine the effectiveness of CFRP bars. The authors proposed a formula for calculating the proper location of CFRP bars and a design axial-load ratio limit for UHPC shear walls with hybrid CFRP/steel reinforcement bars.

Impact of sulfate damage on the behavior of full-scale concrete bridge deck slabs reinforced with FRP bars

This paper is aimed to analyze the performance of reinforced-with-FRP, sulfate-damaged concrete bridge deck slabs under concentrated loads, using the nonlinear finite element analysis (NLFEA) method. For experimentation purposes, twenty-seven full-scale models have been prepared to simulate concrete bridge deck slabs, with a length of 3000 mm and a width of 2500 mm. The parameters of the study were: (i) type of reinforcement, as three types were tested: glass FRP (GFRP), carbon FRP (CFRP), and steel; (ii) bottom transverse reinforcement ratio (ρ = 0.38, 0.46, and 0.57); and (iii) sulfate damage level, which consisted of three levels: Level 0 (undamaged), level 1 (73 days), and level 2 (123 days). All of the slabs models were equipped with two parallel girders of steel as supporters. To be able to analyze the models' performance up till failure, the load exerted by sustained truck wheels (87.5 kN CL-625 truck) was simulated by subjecting each slab model to a monotonic single concentrated load, with a contact area of 600 × 250 mm, at the center of the models. The simulated models encountered a punching shear mode of failure. The analysis showed that the models that were strengthened with CFRP and GFRP bars exhibited a remarkable improvement in the models': ultimate load, elastic stiffness, post-cracking stiffness, elastic energy absorption, and post-cracking energy; whereas, there was less influence on the models' ultimate deflection, compared to the ones strengthened with steel.

Study on flexural behavior of coral concrete beams reinforced with CFRP bars

The rational use of coral aggregate in concrete production is of great significance to reduce the cost and period of island construction. However, using steel rebars in coral concrete structure is impractical given the corrodibility of steel in marine environment. To this end, this study experimented the use of carbon fiber reinforced polymer (CFRP) bars as a replacement for regular reinforcement, and 12 coral concrete beams reinforced with CFRP bars (CCBRC) specimens were experimentally researched. The failure mode, concrete strain, CFRP bar strain, mid-span deflection, maximum crack width and ultimate flexural capacity were studied, and an applicable design equation for this type of members was established based on the results. The results showed that the crack width reaching 1.5 mm is the main failure mode. The concrete strain of mid-span section conforms to the plane section assumption, and the load-CFRP bars strain curve is bilinear. In addition, the load level of beam specimen is low when it reaches the limit value of deflection and crack width in normal service, and the established formula can reliably predict the ultimate flexural capacity.

Flexural strengthening of reinforced concrete beams using geopolymer-bonded small-diameter CFRP bars

In this study, the strengthening system for reinforced concrete (RC) members using geopolymer-bonded small-diameter carbon fiber-reinforced polymer (CFRP) bars was proposed and investigated for the first time. An experimental program was conducted to evaluate the efficiency of the FRP-reinforced geopolymer matrix (FRGM) system for the flexural strengthening of RC beams. A total of 11 RC beams were tested, including ten strengthened beams and one reference beam. The influences of three major factors on the strengthening performance were investigated, including the bonding method (geopolymer vs. epoxy), fiber reinforcement in the matrix (plain geopolymer vs. steel or polyvinyl alcohol fiber-reinforced geopolymer), and the number/diameter of CFRP bars (7Φ3, 2Φ6, and 2Φ10). It was found that the strengthened beams showed significantly higher stiffnesses, yield loads, and ultimate loads than the reference beam and the geopolymer-bonded FRGM layer showed similar strengthening efficiency with the epoxy-bonded FRGM layer. For the FRGM strengthening system, the use of short fibers in the geopolymer matrix further improved the crack control capacity. An analytical investigation was conducted to predict the load capacities of the strengthened beams. Finally, a practical application case was presented to demonstrate the feasibility of this method in strengthening the concrete superstructure at The Port of Zhanjiang (Guangdong, China). The findings lay the groundwork for using this FRGM system to repair reinforced concrete structures.

Flexural Strength of Concrete Beam Reinforced with CFRP Bars: A Review.

Conventional reinforced concrete (RC) structures are commonly associated with the corrosion of steel reinforcement. The application of carbon fiber reinforced polymer (CFRP) bars as flexural reinforcement has become a new promising option. This paper presents a state-of-the art flexural strength on concrete beams reinforced with CFRP bars. Concrete compressive and CFRP bar tensile strain, reinforcement ratio, types of surface treatment on CFRP bar and concrete compressive strength were identified as aspects of behavior. Significant findings in the literature had manifested all aspects of behavior that were affecting the flexural strength, deflections and crack characteristics of CFRP RC beams. In addition, the experimental result on 98 specimens of CFRP RC beams from the literature show that ACI 440.1R-15 and CSA S806-12 standards underestimate the ultimate flexural moment capacity of CFRP RC beams. On the other hand, Kara and Ashour predictions are more accurate with the experimental values. Moreover, hotspot research topics were also highlighted for further considerations in future studies.

Enhancement of Bond Performance of FRP Bars with Seawater Coral Aggregate Concrete by Utilizing Ecoefficient Slag-Based Alkali-Activated Materials

To effectively utilize marine resources on reefs or islands and to improve the bearing capacity and serviceability of fiber-reinforced polymer (FRP) reinforced seawater coral aggregate concrete (CAC) structures in marine environments, this paper investigates the applicability of using alkali-activated materials (AAMs) as substitutes for ordinary Portland cement (OPC) in FRP reinforced CAC structures. Three types of FRP bars, i.e., carbon-FRP (CFRP), glass-FRP (GFRP), and basalt-FRP (BFRP) bars, with different bond lengths (L = 50, 70, and 100 mm) were selected to determine the bond characteristics of FRP bars in alkali-activated seawater coral aggregate concrete (AACAC), as well as in cement-based CAC, which was chosen as the reference. Moreover, a scanning electron microscope (SEM) was employed to detect the microstructure characteristic at the interfacial transition zone (ITZ) between the coral aggregates and the paste matrix. The results indicated that the AACAC specimens contained a stronger mechanical bite force at the paste–aggregate interface and exhibited a higher splitting tensile strength (approximately 6.7% improvement) than those of the CAC specimens. Additionally, the ultimate bond strength and the initial slope of the bond–slip curves at the ascending branch (i.e., initial bond stiffness) were significantly improved by utilizing AAMs. Improvements of approximately 26.6%, 26.8%, and 16.9% were achieved in the bond strength for the specimens with CFRP, GFRP, and BFRP bars, respectively. It was concluded that the utilization of AAMs as alternatives for OPC was an effective method in improving the mechanical interaction at the paste–aggregate interface and promoting the anchorage capacity of FRP bars in CAC, which may represent a promising approach for applying AAMs in FRP reinforced CAC structures or members.

New thermoplastic carbon fiber reinforced polymer rebars and stirrups

AbstractAttributed to its extraordinary strength and durability, the future of reinforcing and prestressing material in reinforced cement concrete (RCC) structures most likely belongs to carbon fiber reinforced polymer (CFRP). However, CFRPs are rarely employed in real structures till now, since the technology to manufacture CFRP rebars and stirrups is still evolving. This is especially applicable for manufacturing of rebars with surface profiles (ribs) and bent sections for stirrups. Unlike thermoset polymers, thermoplastic polymers are highly viscous prior to curing and can be melted and cured again. This creates possibilities for the production technology as well as recyclability. In this paper, new thermoplastic CFRP rebars and stirrups are introduced, which are manufactured in a novel way. The rips are engraved directly after pultrusion and the stirrups are thermoformed out of a straight and already profiled bar. A holistic approach is applied, which enables to evaluate the mechanical overall performance and to compare it with existing products in a reasonable way. This requires the introduction of new parameters, for example the fiber utilization rate. The novel CFRP reinforcement is investigated through tensile tests on bars and stirrups as well as bond tests. The bond performance is quite similar to existing products. Especially while considering how efficient the fibers are utilized in the CFRP bar, the tensile performance and the quality of the stirrups is excellent.

Utilizing alkali-activated materials as ordinary Portland cement replacement to study the bond performance of fiber-reinforced polymer bars in seawater sea-sand concrete

Alkali-activated materials (AAMs) are considered an eco-friendly alternative to ordinary Portland cement (OPC) for mitigating greenhouse-gas emissions and enabling efficient waste recycling. In this paper, an innovative seawater sea-sand concrete (SWSSC), that is, seawater sea-sand alkali-activated concrete (SWSSAAC), was developed using AAMs instead of OPC to explore the application of marine resources and to improve the durability of conventional SWSSC structures. Then, three types of fiber-reinforced polymer (FRP) bars, that is, basalt-FRP, glass-FRP, and carbon-FRP bars, were selected to investigate their bond behavior with SWSSAAC at different alkaline dosages (3%, 4%, and 6% Na2O contents). The experimental results manifested that the utilization of the alkali-activated binders can increase the splitting tensile strength ( ft) of the concrete due to the denser microstructures of AAMs than OPC pastes. This improved characteristic was helpful in enhancing the bond performance of FRP bars, especially the slope of bond-slip curves in the ascending section (i.e., bond stiffness). Approximately three times enhancement in terms of the initial bond rigidity was achieved with SWSSAAC compared to SWSSC at the same concrete strength. Furthermore, compared with the BFRP and GFRP bars, the specimens reinforced with the CFRP bars experienced higher bond strength and bond rigidity due to their relatively high tensile strength and elastic modulus. Additionally, significant improvements in initial bond stiffness and bond strength were also observed as the alkaline contents (i.e., concrete strength) of the SWSSAAC were aggrandized, demonstrating the integration of the FRP bars and SWSSAAC is achievable, which contributes to an innovative channel for the development of SWSSC pavements or structures.

Flexural behavior of full-scale, carbon-fiber-reinforced polymer prestressed concrete beams

Highway bridge beams are subjected to aggressive environments, temperature fluctuations, and millions of loading cycles throughout their service life. The combi­nation of these effects can result in the reduction of the service life of structural components. In the past decades, more-durable composite materials, such as carbon-fiber-reinforced polymers (CFRPs) have been implemented in concrete structures to address problems related to environmental durability. To date, prestress­ing applications of CFRP in beams have been mostly investigated for rectangular cross sections, which are not representative of the geometry used in modern highway bridges. In addition, the construction and detailing aspects of CFRP prestressed concrete beams have not been investigated outside of laboratory conditions. This paper describes an experimental investigation conducted on eight 40 ft (12 m) long AASHTO Type I prestressed concrete beams with 3 ft (0.9 m) wide composite con­crete decks and detailed identically to highway bridge beams in practice. Three beams were pretensioned with carbon-fiber-composite cables, four beams with CFRP bars, and one with prestressing steel. The beams were tested under monotonic and fatigue loading. All CFRP prestressed concrete beams were designed to fail due to the rupture of the prestressing CFRP. The CFRP prestressed concrete beams exhibit several desirable features of the typical steel prestressed concrete beams in terms of serviceability and strength.

Optimization of Fiber-Reinforced Polymer Bars for Reinforced Concrete Column Using Nonlinear Finite Element Algorithms

The ductility and strength of reinforced concrete (RC) columns could be noticeably improved by replacing steel bars with polymeric bars. Despite the previous research on RC columns, most of those studies focused only on the lateral load capacity of this structural member and were mainly costly experimental studies. However, this paper is concentrated on the previously occurred damages to the reinforced columns in the previous earthquakes. Subsequently, finite element analysis has been performed to examine 24 models including the various shapes of RC columns. In employing the plastic behavior of steel, carbon fiber-reinforced polymer (CFRP), and glass fiber reinforced polymer (GFRP) bars, the bilinear hardening has been considered. To capture both compressive and tensile behavior of the concrete, the concrete damage plasticity model has been implemented. Furthermore, the optimization technique is used for CFRP models to compare with other models. In this paper, the parameters of energy, seismic factor, stiffness, and ductility have been computed using the method proposed by the authors. This suggested method is considered to compare the results from each parameter. Finite element results of steel bars are compared with carbon and glass models. The results show the stiffness of models is improved by CFRP bars, while the energy absorption and ductility factor are enhanced with steel bars. Moreover, GFRP bars can enhance the seismic factor. The reduction of column stiffness to almost half would occur in some rectangular cross-section columns.

Stimulations study on composite reinforced concrete beams

Fiber-reinforced polymer (FRP) reinforcements have seen a significant rise in usage in concrete buildings over the past ten years, thanks to their superior corrosion resistance, high tensile strength, and non-magnetization characteristics. However, due to the low modulus of elasticity of the FRP materials and their non-yielding properties, significant deflection and broad fractures are seen in the FRP reinforced concrete components under consideration. As a result, the design of such members may be influenced by serviceability needs in many instances. Flexural stresses are applied to FRP reinforced concrete members in this study, which results in the deflection of the members. The current study focuses on the behavior of concrete beams reinforced with glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) bars of various surface geometries with different reinforcement ratios using nonlinear finite element analysis in ABAQUS. Three different cross-sections of CFRP and GFRP bars were scrutinized. From the finite element modeling it was observed that increasing reinforcement ratio of CFRP bars compared to steel and GFRP reinforced beams. Because of low modulus of elasticity causes GFRP reinforced concrete members to deflect more than steel or CFRP reinforced concrete members. It is observed that load bearing capacity of FRP reinforced beam are higher that steel reinforced beams.

Flexural Behavior of Two-Span Continuous CFRP RC Beams.

This paper investigates the feasibility of replacing steel bars with carbon-fiber-reinforced polymer (CFRP) bars in continuous reinforced concrete (RC) beams. A numerical model is introduced. Model predictions are compared with the experimental results that are available in the literature. A comprehensive numerical investigation is then performed on two-span CFRP/steel RC beams with ρb2 = 0.61–3.03% and ρb1/ρb2 = 1.5, where ρb1 and ρb2 are tensile bar ratios (ratios of tensile bar area to effective cross-sectional area of beams) over positive and negative moment regions, respectively. The study shows that replacing steel bars with CFRP bars greatly improves the crack mode at a low bar ratio. The ultimate load of CFRP RC beams is 89% higher at ρb2 = 0.61% but 7.2% lower at ρb2 = 3.03% than that of steel RC beams. In addition, CFRP RC beams exhibit around 13% greater ultimate deflection compared to steel RC beams. The difference of moment redistribution between CFRP and steel RC beams diminishes as ρb2 increases. ACI 318-19 appears to be conservative, and it leads to more accurate predictions of moment redistribution in CFRP RC beams than that in steel RC beams.

New type of CFRP reinforcement and technique for the flexural strengthening of RC balconies

The present work assesses the potentialities of a new carbon fibre reinforced polymer (CFRP) bar, applied according to a hybrid technique for the flexural strengthening of cantilever type reinforced concrete (RC) structures, such as the case of RC balconies. The CFRP bar is formed by an inclined extremity that is inserted into a hole according to the embedded through section (ETS) technique, a part that is applied according to the near-surface mounted (NSM) technique, and a transition zone between these two parts. The anchorage conditions of the ETS part of the CFRP bar allow limiting the strengthening intervention to the cantilever zone of the balcony, with an almost null intrusion to the interior of the building.The effectiveness of this CFRP reinforcement and technique was assessed by testing 9 double-sided RC balcony prototypes, in a total of 18 tests. The inclination of the ETS part of the CFRP bar (15° and 30°), the concrete strength class (C25 and C35) and the ratio of existing flexural tensile reinforcement (ρls=0.28% and 0.44%) were the variables investigated in terms of flexural strengthening performance. The results showed that the strengthening effectiveness has increased with the decrease of the inclination of the ETS part of the CFRP bar. A reinforcement ratio of 0.17% of CFRP bars with an extremity at 15° inclination, has doubled the load-carrying capacity of the corresponding reference prototypes.The applicability of the most recent formulation of the fib bulletin 90 for the flexural strengthening of RC elements with NSM CFRP reinforcements was assessed for this type of application. The ratio between maximum load registered experimentally and predicted with fib formulation was 0.95 with a standard deviation of 0.08. However, this high level of predictive performance requires the effective tensile strength of the transition zone of the CFRP bar to be known, which is still a critical aspect of the current generation of CFRP bars. A numerical approach was developed, capable of estimating the force–deflection, the strains and stresses in the constituent materials and evolution of stiffness degradation of the strengthened structures during the loading process, and its high predictive performance was also demonstrated by simulating the experimental tests carried out.

FLEXURAL BEHAVIOR OF FRP BARS AFTER BEING EXPOSED TO ELEVATED TEMPERATURES

This research study investigates the flexural behavior of fiber reinforced polymer (FRP) bars after being subjected to different levels of elevated temperatures (100, 200 and 300°C). Three types of glass FRP bars (ribbed, sand coated, and helically wrapped) and one type of carbon FRP bars (sand coated) were used in this study. Two testing scenarios were used: a) testing specimens immediately after heating and b) keeping specimens to cool down before testing. Test results showed that as the temperature increased the flexural strength and modulus of the tested FRP bars decreased. At temperatures higher than the glass transition temperature (Tg), significant flexural strength and modulus losses were recorded. Smaller diameter bars showed better residual flexural strength and modulus than larger diameter bars. The immediately tested bars showed significant strength and modulus losses compared to bars tested after cooling. Different types of GFRP bars showed comparable results. However, the helically wrapped bars showed the highest flexural strength losses (37 and 60%) while the sand coated bars showed the lowest losses (29 and 39%) after exposure to 200 and 300℃, respectively. The carbon FRP bars showed residual flexural strengths comparable to those recorded for the GFRP bars; however, they showed lower residual flexural modulus after being subjected to 200 and 300℃.