CFRP-strengthened Reinforced Concrete Beams Research Articles

Flexural performance of RC beams using hybrid anchored ​CFRP sheets with bond defects

Hybrid anchorage technique can effectively prevent end debonding and impede intermediate debonding of CFRP sheet in strengthened reinforced concrete (RC) beams. However, its effectiveness in the presence of bond defects at the CFRP-concrete interface is unclear. Therefore, this study conducted four-point bending tests on CFRP-strengthened RC beams, considering two factors: bond defects (with and without) and strengthening methods (externally bonded and hybrid anchorage). The results showed that bond defects can lead to premature intermediate debonding of the CFRP sheet, resulting in a 22.49% decrease in debonding load due to local stress concentration around them. Owing to the effectiveness of the hybrid anchorage method, the propagation of the intermediate debonding is significantly impeded, leading to a 4.32% and 21.61% higher in ultimate load and deflection, respectively. This is because the hybrid anchorage can control the distribution of local cracks and improve the overall stress distribution of CFRP. A finite element analysis was performed and compared against the experimental results which shows good agreement. Then, parametric analyses were conducted to further analyze the flexural behavior of the strengthened beams with bond defects. Finally, an analytical model was proposed to predict the flexural capacity of the hybrid strengthened beams with bond defects.

Effects of CFRP sheets on the flexural behavior of high-strength concrete beam

Abstract The aim of this study is to evaluate numerically the effects of carbon fiber-reinforced polymer (CFRP) sheets strengthening on the flexural performance of high-strength concrete (HSC) beam using ABAQUS 3D finite element (FE) modeling software. The developed FE models were verified against the experimental results found in literature. The FE models can accurately estimate the performance of CFRP-strengthened high-strength reinforced concrete (RC) beams. Subsequent parametric analysis was performed to assess the performance of CFRP-strengthened concrete beams considering various parameters including compressive strength of concrete, CFRP width, thickness, length, number of CFRP layers, and CFRP strengthening schemes. Based on the results of FE analysis. It was demonstrated that using HSC significantly enhances the performance of CFRP-strengthened RC beams. It was also confirmed that width, thickness, and layer number of CFRP sheets improve the flexural behavior of CFRP-strengthened HSC beams by increasing the ultimate loads and strain-hardening behavior of the specimens. The strengthening schemes contribute to delaying or inhabiting the debonding especially when the CFRP sheets are added along the bottom of the beams. It was demonstrated that using CFRP sheets U-wrapping contributes to the prevention or delay of debonding and increases the capability of resisting the stress imposed on the concrete. Therefore, installing the CFRP sheets at the bottom face of beam below the tensile reinforcement enhances the performance of CFRP-strengthened HSC beams.

A simplified fatigue model for CFRP-strengthened RC beams under the coupling action of a hot–wet/saline environment and cyclic loading

Reinforced concrete (RC) structures in subtropical coastal areas are subjected to the combined influences of temperature, humidity, and salt exposure. Previous studies on the fatigue failure of fiber-reinforced polymer (FRP)-RC structures did not consider the coupling effect of the environment and loading. In this study, a simplified hot–wet/saline environmental fatigue model was developed to consider the linear influence of environmental parameters, including temperature, humidity, and salinity. Then, a closed environmental box was self-designed to provide the temperature–saline environment. By simulating a subtropical coastal environment with typical salinity levels (0, 3, and 5 wt% NaCl) and temperatures (10, 35, and 50 °C), fatigue tests were conducted on 38 typical carbon fiber-reinforced polymer–RC beams. In assessing the simplified model’s predictive accuracy, the average errors for estimating the fatigue limits and lives in the validation groups with or without saline conditions were 4.1 % and 33.7 %, respectively. The simplified model indicated that higher unit temperature and humidity similarly reduced the fatigue limit and that higher salinity decreased the fatigue limit by approximately 7.2 times greater than temperature or humidity. This research provides valuable insights into the strength design and life prediction of RC structures in the offshore or subsea regions of subtropical coastal areas.

Structural performance of RC beams strengthened with hybrid bonded CFRP

Debonding at the interface between externally-bonded (EB) carbon fiber-reinforced polymer (CFRP) composites and the concrete limits the performance of CFRP-strengthened reinforced concrete (RC) beams. To mitigate the interface debonding and improve the material utilizing efficiency of CFRP, a hybrid-bonding (HB) technique was developed. HB CFRP system combines the adhesive bonding and mechanical fastening (i.e., anchoring the CFRP plate through bolted cover steel plate bonded on the surface of the CFRP plate). When interfacial debonding occurs, the friction between the steel cover plate and the CFRP plate and the reacted normal stress at the interface enhanced d the concrete aggregates interlocking and the bond strength of CFRP-to-concrete interface. Additionally, the normal stresses can be also activated through pre-tightening the anchors. Five HB CFRP-strengthened RC beams, two conventional EB CFRP-strengthened RC beams, and two un-strengthened RC beams were tested. Two different steel cover plates lengths and three different pre-tightening torques were adopted. The test results demonstrated that the HB technique improved the local bond behavior in the anchored regions, resulting in higher strength utilization efficiency of CFRP and increased load-carrying capacity of the strengthened beams. Such improvement was more significant when longer steel cover plates and higher pre-tightening torques were deployed.

Entire mechanical analysis of prestressed CFRP strengthened RC beams under different prestressed introduced methods

In order to clarify the effect of mechanical tensioning and SMA wire heating recovery on introducing prestress into CFRP sheet strengthened reinforced concrete (RC) beams, an experimental research on the bending performance of prestressed CFRP sheet strengthened RC beams was conducted. Based on the test results, a bending carrying capacity model for RC beams externally strengthened with prestressed CFRP sheets was proposed. The model provides calculation methods for the decompression moment, cracking moment, yielding moment, and ultimate moment, corresponding to different failure modes of the RC beams strengthened with externally bonded prestressed CFRP sheets. Four experimental beams were designed to verify the accuracy of the model with the prestresses of 100 MPa and 200 MPa. The results show that during the yield stage and strengthening stage, the loading-unloading stress-strain relationship curves of SMA wire under different prestrains are basically consistent. When the prestrain of SMA wire is 10%, the maximum recovery stress reaches 448.5 MPa. Under the same prestrain conditions, the maximum recovery stress of CFRP sheets was reduced by 37.8–39.5% when the prestress was introduced through heating recovery of SMA wires. The failure mode of mechanically tensioned prestressed CFRP sheet strengthened beams is the CFRP sheet debonding caused by mid-span bending cracks, while the failure mode of strengthened beams with prestressed CFRP sheet by SMA wire heating recovery is the CFRP sheet end debonding. The cracking moment and yield moment of the strengthened beams are significantly increased by two methods of introducing prestressing. The stiffness improvement of mechanically tensioned prestressed CFRP sheet strengthened beam is relatively large. While, the prestressed CFRP sheet strengthened beam by SMA wire heating recovery gradually experience end peeling failure of the CFRP sheet, and the prestressing effect does not effectively limit the development of cracks, resulting in limited stiffness improvement. The calculation results are in good agreement with the experimental results, proving that the proposed method for analyzing the entire bending process can be used to predict the bending mechanical properties of the prestressed CFRP sheet strengthened beams.

Numerical study on the torsional behavior of CFRP-strengthened RC beams

The present paper offers a numerical study on the behavior of reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP) under pure torsional loading. The failure mode of RC beams significantly changed in accordance with torsional loading. Therefore, the effect of strengthening RC beams by CFRP under torsion is a vital area of study. This paper attempted to fill the gap by conducting a numerical (Abaqus) study, whereby the numerical investigation was in line with the parametric for a previous study of an experimental program. The numerical model achieved agreement with the results of the experimental program, then, this model was used by applying CFRP, instead of GFRP. Additionally, the numerical study was completely in line with the parametric study in terms of sample dimensions, steel reinforcement, number of FRP layers, and width of FRP strips, for the aim of comparing results. The comparative evaluation in this study aims to contrast the behavior of RC beams strengthened by CFRP with the behavior of those strengthened by GFRP under pure torsion. Parameters such as ultimate torsional moment, and the maximum angle of twist were studied. The study revealed that strengthening with CFRP improved the ultimate strength and post-cracking stiffness, in addition to localizing the damage.

Study on pure-torsional size effect of CFRP-strengthened RC beams using a 3D meso-scale numerical model

In this paper, a 3D meso-scale numerical model for pure-torsional failure analysis of carbon fiber reinforced plastic (CFRP)-strengthened reinforced concrete (RC) beams is established. Three main aspects regarding the establishment of the numerical model are thoroughly considered, including the heterogeneity of concrete, the bond-slip relationship between steel bars and concrete, and the nonlinear interaction between CFRP and concrete. Effects of CFRP fiber ratio and span-height ratio on pure-torsional performances and size effect behaviors of CFRP-strengthened RC beams are analyzed. With the increase of CFRP fiber ratio, the cracks spacing decreases, the number of cracks increases, and the strain of CFRP decreases. With the increase of the span-height ratio, the inclination angle of the oblique crack decreases, and the strain of CFRP increases. It is found that CFRP-strengthened RC beams have an obvious torsional size effect. Furthermore, CFRP fiber ratio and span-height ratio not only have a coupling enhancement effect on the nominal torsional strength, but also have a coupling weakening effect on the size effect behavior. Based on the simulation results, a CFRP-related Torsional size effect law (SEL), which can quantitatively describe the influences of CFRP fiber ratio and span-height ratio on the size effect of nominal torsional strength of CFRP-strengthened RC beams, is established.

Numerical and experimental investigation on the flexural stiffness of RC beams strengthened with prestressed CFRP plate exposed to wetting/drying cycles

The load-deflection relationship is critical for the serviceability and structural durability of reinforced concrete (RC) structures strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates. This study aims to investigate the impact of prestressing levels and wetting/drying cycles (WDCs) on the flexural stiffness of prestressed CFRP-strengthened RC beams. A modified flexural stiffness prediction model was proposed by incorporating prestressing levels, interfacial bond slip, and softening after WDCs. Subsequently, four-point bending tests were conducted on 11 strengthened beams with different prestressing levels and WDCs to estimate flexural behavior. The results suggest that WDCs can adversely affect the flexural stiffness of the strengthened beams, particularly for the post-cracking stiffness of the non-prestressed specimens. However, prestressed strengthening can mitigate the stiffness degradation caused by WDCs. Moreover, a finite element (FE) model utilizing a traction separation model can accurately predict flexural stiffness. Finally, the flexural stiffness was computed using both the existing and proposed models and the proposed model provides the closest prediction of the flexural stiffness for CFRP-strengthened beams under WDC and prestressing.

Fire endurance tests of CFRP-strengthened RC beams with different insulation schemes

The fire performance of externally bonded (EB) carbon fiber-reinforced polymer (CFRP)-strengthened reinforced concrete (RC) beams is an important issue in practical strengthening design. The cable mechanism of CFRP sheets in a fire can improve the fire endurance of the strengthened beams. This paper presents a feasible method to activate the cable mechanism of EB CFRP sheets in the strengthened beams by using appropriate insulation schemes and a mechanical anchorage system. Seven beams were constructed: three were tested at room temperature, while the remaining four were subjected to the ISO 834 standard fire. The experimental results showed that the three insulation schemes could provide reasonably good protection for the specimens, and thus all of them achieved a fire endurance of 2.5 h. The combined use of fire insulation and a mechanical anchorage system could reduce the deflection responses, mainly due to the active cable mechanism of the EB CFRP sheets during fire exposure. A finite element (FE) model was then proposed to predict the thermal and structural responses of insulated CFRP-strengthened RC beams. The FE parametric results further indicated that different insulation schemes with various thicknesses and thermal conductivities had significant effects on the fire performance of the strengthened beams.

Fire Resistance Evaluation of Concrete Beams and Slabs Incorporating Natural Fiber-Reinforced Polymers.

This paper presents a numerical study to evaluate the fire resistance of concrete beams and slabs incorporating natural fiber-reinforced polymers (FRP). A validated finite element model was applied to carry out a series of numerical studies on fire-exposed reinforced concrete (RC) beams and slabs strengthened with conventional and bio-based FRP composites. The model calculates the temperature-dependent moment-curvature relationship for various segments of the member at each time step, which are then used to calculate the moment capacity and deflection of the member. The variables in the beams and slabs include different strengthening techniques (externally bonded FRP and near-surface mounted FRP), different fiber composites, and fire insulation schemes (uninsulated and insulated). The results from the study indicate that the bio-based FRP-strengthened RC members undergo a faster degradation in moment capacity and also experience higher deflections under fire exposure. This leads to a lower fire resistance in RC members with bio-based FRP composites compared to beams and slabs with conventional FRP-strengthened concrete members. The addition of fire insulation to the bio-based FRP-strengthened members can enhance their fire performance and help achieve the required fire resistance ratings for use in building applications. In this study, the NSM CFRP-strengthened RC beams were found to have a fire resistance of 3 h without any fire insulation; however, the bio-based FRP-strengthened beams required a layer of vermiculite-gypsum-based fire insulation material (of about 25 mm) to achieve similar fire resistance.

Vigas de concreto armado reforçadas com CFRP em situação de incêndio: modelo numérico

Abstract The Carbon Fiber Reinforced Polymer (CFRP) has excellent flexural performance on strengthening and rehabilitation of reinforced concrete (RC) structures. However, it is known that CFRP strengthening systems are very vulnerable to high thermal exposure. To provide a better understanding of its fire behavior and fulfill gaps in this field, this investigation proposes the development of an accurate three-dimensional (3D) Finite Element (FE) model capable of simulating the CFRP-strengthened RC beams flexural behavior at ambient temperature and under fire conditions. To achieve the goals, thermal and mechanical numerical simulations have been performed to validate the developed FE models. Experimental results reported in two parallel studies were used to model’s validation. The numerical model has been satisfactorily validated and the results had a good predictability with the experimental results in terms of thermal and mechanical behavior.

Effect of fatigue loading on flexural performance of NSM CFRP-strengthened RC beams under different service temperatures

This paper presents an experimental work to investigate the effect of fatigue loading and high service temperature on the flexural performance of near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP)-strengthened reinforced concrete (RC) beams. The experimental program included 11 beams, where the effect of the strengthening (CFRP) ratio (0.06 % and 0.18 %), fatigue load level (R1 = 0.57 and R2 = 0.38) and applied temperature (20 °C and 70 °C) have been considered. Experimental results on fatigue tests showed that high service temperature resulted in an increase in deflections and reduction in the stiffness of the specimens along the test. Moreover, the application of fatigue cycles with larger amplitude resulted in the failure of the specimens (by steel rupture), whilst specimens submitted to lower fatigue amplitude survived to the fatigue cycles. The NSM CFRP-strengthening helped to postpone the final failure of strengthened RC beams after steel reinforcement rupture during the fatigue test. Besides, damage due to fatigue derived in lower residual yielding and ultimate load in the post-fatigue test.

Time-dependent behavior of NSM CFRP-strengthened RC beams under different service temperatures

The use of fiber reinforced polymer (FRP) for flexural strengthening of reinforced concrete (RC) beams has become a popular strengthening technique. Significant amount of work is available on the short-term flexural behavior of RC beams strengthened with near-surface mounted (NSM) technique. However, their time-dependent flexural behavior, specifically under high service temperature, has not yet been addressed. This paper presents an experimental work to evaluate the time-dependent behavior of NSM carbon FRP (CFRP)-strengthened RC beams. The experimental program included 23 beams, where the effect of different parameters such as strengthening (CFRP) area, steel reinforcement ratio and applied temperature (20 and 50 °C) have been considered. Experimental results show that the effect of strengthening area is significant on the flexural short-term response of the beams, while minor effects are found on the time-dependent deflections. On the other hand, increasing the service temperature has no significant effect on the short-term tests, but it produces a large increase in the time-dependent deflection of the specimens. Finally, an analytical procedure for the prediction of time-dependent deflections, which is based on the age-adjusted effective modulus method (AEMM), is presented. Good agreement between the experimental results and analytical predictions on time-dependent deflections is shown.

Experimental study on performance of CFRP-strengthened RC beams with geopolymer and epoxy

PurposeIn this paper, to obtain shear and bending performance of carbon fiber-reinforced polymer (CFRP)-strengthened beams bonded by geopolymers, the effects of impregnated adhesive types, strengthened scheme, CFRP layer and pre-cracked width are investigated, and the performance of CFRP-strengthened beams is validated by the establishment of Finite Element Models (FEMs).Design/methodology/approachIn this paper, static loading test and finite element analysis of epoxy-CFRP-strengthened (ECS) and geopolymer-CFRP-strengthened (GCS) were carried out, and the bearing capacity and stiffness were compared, the results show that GCS reinforced concrete (RC) beam is feasible and effective.FindingsThe bearing capacity, crack distribution and development, load–deflection curves of GCS RC beams with different pre-crack widths were investigated. The reinforcement effect of geopolymer achieves the same as epoxy, effectively improving the ultimate bearing capacity of the beam, with a maximum increase rate of 28.9%. The failure mode of CFRP is broken in the yield failure stage of GCS RC beam with reasonable strengthening form, and the utilization rate of CFRP is improved. CFRP-strengthened layers, pre-cracked widths significantly affect the mechanical properties, and deformation properties of the strengthened beams.Originality/valueCompared with ECS RC beams, the bearing capacity and stiffness of GCS RC beams are similar to or even better, indicating that GCS RC beam is feasible and effective. It is a new method for CFRP-strengthened beams, which not only conforms to the concept of national ecological civilization construction, but also provides an economical, environmentally friendly and excellent performance solution for structural reinforcement.

Fatigue Life Prediction of CFRP-Strengthened RC Beams with Flexural Crack under Hygrothermal Environments

The durability of reinforced concrete (RC) beams strengthened with carbon fiber-reinforced polymer (CFRP) is a worldwide concern in structural engineering. As an important part of the strengthened beam, the performance of the CFRP–concrete interface under hygrothermal environments is a delicate problem. In this paper, the fatigue behavior of CFRP-strengthened RC beams is analyzed by a theoretical model. In the model, CFRP–concrete interface degradation under hygrothermal environments is involved. Since interface debonding and rebar fracture induced by intermediate cracking are two typical failure modes, the damage models of rebar and the CFRP–concrete interface are established. Based on the theoretical model, the failure mode of CFRP-strengthened RC beams can be predicted, and fatigue life can be determined. The results showed that IC debonding is more likely to occur under hygrothermal environments. The accurate prediction of failure modes is essential for fatigue life prediction.

Finite Element Analysis of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer Sheets

This paper presents outcomes of extensive research conducted to study the behavior of reinforced concrete (RC) beams strengthened with CFRP sheets and subjected to shear and flexural loading. A 3-dimensional Finite Element(FE) modelling was carried out using ABAQUS software program to simulate the flexural and shearing behavior of a number of RC beams strengthen with CFRP. Moreover, the results of the numerical simulation were compared with actual results of experimental physical modelling of RC beams subjected to the same loading conditions for validation purpose.Appreciable number of sets of control RC beams and CFRP strengthened RC beams for flexural and shear were investigated. All beams were rectangular in cross section. To accurately describe the behavior of these tested RC beams, different material constitutive models were used in the simulation. Concrete damaged plasticity model was used to model concrete;a linear elastic perfectly plastic model was used to model both reinforcing steel and stirrups, while a linear elastic isotropic model was used to simulate the behavior of CFRP material. Moreover, the interface between concrete and CFRP material was accounted for and was simulated using a tie perfect bond model while steel reinforcement is bonded with concrete as anembedded element in ABAQUS. A careful comparison between the results of the experimental tests and those of the numerical simulation showed that the proposed FE model is able to describe the behavior of reinforcedconcrete beams strengthened with CFRP in terms of crack pattern and load deflection curves to an appreciable level of accuracy. Generally, the results demonstrate that numerical approach can be used to deeply investigate thebehavior of RC beams strengthened with CFRP sheets. It was also concluded that the numerical approach was able to capture significant aspects of structural behavior of the tested beams such as strain distribution in CFRP, which is often not easily captured experimentally or analytically.

Behavior of CFRP-strengthened RC beams with web openings in shear zones: Numerical simulation

The passing of service ducts and pipes often necessitates web openings in reinforced concrete (RC) beams. The strength and stiffness of the beam are reduced due to such web opening(s). Existing experimental research has shown the possibility of utilizing externally bonded Carbon Fiber-reinforced polymer (CFRP) to compensate for the strength loss of beams. However, there have been few finite element (FE) methodologies for predicting and estimating the performance of such RC beams. This paper uses nine different FE models to simulate a CFRP strengthened web opening using ABAQUS. This paper evaluates RC beam performance with web openings in the shear zone and proposed reinforcement of CFRP. Nine RC beams with openings, numerical tested under 2-loads. The models were constructed using the ABAQUS software by FE to simulate RC beams with a CFRP strengthened by two rectangular web openings. This paper studies the sequential effect of the different fundamental parameters on the strengthened RC beams' overall stiffness and shear strength response, achieving optimum utilization of the strengthened technique in load-bearing capacity and potential deflection. Results have shown no need to strengthen small web openings in the RC beams' shear zone. Beams with large openings in the shear zone need strengthening with CFRP; the beam's efficiency has been significantly improved using 2-layers of CFRP sheets around the web opening and a single-layer CFRP strip in the flexural zone bottom the chord.

Strengthening effect of prestressed near-surface mounted CFRP bar system according to material properties of aged reinforced concrete beams

The flexural performance improvement of age-deteriorated reinforced concrete (RC) beams by a near-surface mounted carbon fiber-reinforced polymer (NSM CFRP) bar system was experimentally investigated. Ten 6.4 m long RC beams were fabricated and tested in four-point bending using different concrete compressive strengths to distinguish between old and new concrete, different steel reinforcement ratios to reflect deterioration with age, and different quantities of prestressed and non-prestressed CFRP bars. The results indicated that the ultimate strengths of the NSM CFRP-strengthened RC beams were up to twice that of the un-strengthened control beam, and the strengthening effect increased with the material properties of the RC beam. A finite element model of the strengthened RC beam was constructed and verified against the experimental results, then used to conduct a parametric study of the influence of the concrete compressive strength and steel reinforcement ratio on the strengthening effect of the prestressed NSM CFRP bar system.

EFFECT OF SHEAR-SPAN RATIO ON SHEAR FAILURE AND SIZE EFFECT OF CONCRETE BEAMS WITHOUT WEB REINFORCEMENT STRENGTHENED BY CFRP

The shear-span ratio has an important influence on the crack development and on the failure mode of reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP). However, there were few studies on the effects of its shear strength and size effect. A mechanical analysis model for shear failure of reinforced concrete beams strengthened with CFRP was established by using three-dimensional numerical meso-scale simulation method, considering the meso-heterogeneity of the concrete and the interaction between the CFRP and concrete. Based on the verification of the rationality of the meso-scale method, the influence mechanism and law of the shear-span ratio on the shear failure and size effect of CFRP-strengthened RC beams were simulated and analyzed. The results show that: the shear-span ratio has a great influence on the shear failure mode of the strengthened beam, and the larger the shear-span ratio, the closer the beam is to the cable-stayed failure with better ductility. The shear-span ratio had better shear capacity for CFRP-strengthened beams and the influence on the size effect of shear strength was small. The shear-span ratio has a greater influence on the CFRP shear contribution in the strengthened beam. The larger the shear-span ratio, the better the shear effect of CFRP on strengthened beams. The beam reinforcement effect of the shear-span ratio (λ = 2.5) is most effective.

Flexural Performance of Reinforced Concrete Beams Strengthened with Carbon Fiber-Reinforced Polymer (CFRP) under Hygrothermal Environment Considering the Influence of CFRP–Concrete Interface

As an important component, the bond behavior of carbon fiber-reinforced polymer (CFRP)–concrete interface for a reinforced concrete (RC) beam is very significant. In this study, a theoretical model was established to analyze the flexural behavior of CFRP-strengthened RC beams, and the CFRP–concrete interfacial bond–slip relationship under hygrothermal environment was unified into one model. Two failure criteria corresponding to two types of failure modes, i.e., concrete crushing and intermediate crack (IC)-induced debonding, were developed. Through the theoretical model, the flexural behavior of deflection, interfacial shear stress distribution and ultimate load of a CFRP-strengthened RC beam under hygrothermal environment were obtained and predicted. Moreover, the theoretical model was verified by test results. The results showed that the hygrothermal environment had a significant impact on the CFRP–concrete interface behavior. Compared with the control beam without hygrothermal environment pretreatment, the deflection and ultimate load of the strengthened RC beam decreased by 51.9% and 20%, respectively.