Flexural-shear behavior of degraded RC beams strengthened with C-FRCM under the coupling effect of a corrosive environment and a sustained load

The paper presents an investigation on the flexural behavior of corroded reinforced concrete (RC) beam strengthened with Carbon Fiber Reinforced Polymer (CFRP) sheets. Different levels of corrosion are considered as a new method for classifying the corrosion levels of the corroded RC members. Twenty RC beams with five corrosion levels were fabricated by accelerated corrosion equipment, and then the CFRP were used to strengthen these members, considering the effect of different CFRP layers and different strengthened schemes, finally the static flexural load test was carried out. The test results show that the bearing capacity of the CFRP strengthening corroded RC beams is effectively enhanced, the crack width is restrained, and the flexural stiffness is improved. The ultimate flexural capacity of the specimens strengthened by one layer of CFRP sheet raises by 30% to 50% than that of the unstrengthened ones, with the increase of the corrosion level, the enhancement decreased. With the increase of the number of CFRP layers, the bearing capacity increases, while the increasing ratio reduces with the increase in the number of layers. For specimens with obvious cracks, the effectiveness of the strengthening method by bonding CFRP after cutting or chiseling off concrete method (RM1 or RM2) is higher than the method of bonding CFRP directly (DM). It is suggested that the corroded RC members with Level A, B or C1 can be strengthened by DM, while the other level members should be strengthened by RM1 or RM2. The formulae in current design code are used to predict the flexural bearing capacity of the RC beam strengthened with CFRP and their results are compared with experimental ones. A revised formula that can give a better prediction for multi-layer CFRP strengthening members is also proposed.

Externally bonded carbon fiber reinforced polymer (CFRP) composite laminates have been successfully applied to reinforced concrete (RC) beams and other structural elements for the purpose of increase load carrying capacity of such elements. This paper presents the experimental results on the flexural strengthening of reinforced concrete beams by CFRP laminates attached to the tensile soffit of the beams by epoxy adhesive. A total of six reinforced concrete beams having different degrees of strengthening scheme were tested to failure under transverse bending over an effective span length of 1900 mm. The increase of ultimate strength provided by the bonded carbon fiber was assessed by varying the layers of composite laminates. The results indicate that the flexural strength of beams was significantly improved as the layers of laminate increased. No de-lamination of the superimposed CFRP plates was observed from the test. However, de-bonding of CFRP laminates from concrete surface was observed for the case of multi-layer strengthening of beam. It is concluded that the attachment of CFRP laminates with edge strip plates has substantially influenced the performance of CFRP strengthened beams. The paper also highlighted the beams failure modes due to the different level of strengthening scheme.

The bearing capacity of reinforced concrete (RC) beam will be weakened by fire. It is necessary to strengthen RC beams after fire. The carbon fiber mesh (CFM) can be used to reinforce RC beams. In this paper, RC beams were exposed to varying temperatures, followed by reinforcement with varying layers of CFM. The influence of the heating temperature and the number of CFM layers on the flexural performance of RC beams was investigated. The results indicated that the cracking loads of RC beams were 18.2, 16.4, 16.3, and 15.5 kN when the RC beams were subjected to room temperatures, 150, 350, and 550 °C. Compared to the unreinforced beams at room temperature, the cracking loads of the RC beams were reduced by 9.89%, 10.44%, and 14.84%. As the quantity of CFM reinforcement layers rises, so does the ultimate bearing capacity. For example, when the temperature was 150 °C, the ultimate loads of the beams with one and three layers of CFM were increased by 20% and 31.76% compared to the reference beam. When the temperature was 350 °C, the ultimate loads of the beams with one and three layers of CFM were increased by 19.51% and 28.04% compared to the RC beam without CFM. When the temperature was 550 °C, the ultimate loads of the beams with one and three layers of CFM were increased by 20% and 26.67% compared to the RC beam without CFM. Fire-damaged RC beams can be strengthened by one layer of CFM and mortar if the temperature was below 350 °C. Fire-damaged RC beams can be strengthened by three layers of CFM and mortar if the temperature was below 550 °C. The mechanical properties can be obviously enhanced.

This paper aims to study the effect of Carbon Fiber Reinforced Polymer (CFRP) strengthening techniques on the fatigue limit of Reinforced Concrete (RC) beams. An accelerated fatigue method was utilized to find the fatigue limit based on the hypothesis of linear cumulative damage as determined by the Palmgren-Miner rule. Six RC beams with dimensions of 152.4 × 152.4 × 1,520 mm were tested under monotonic and cyclic loading using a four-point bending configuration: two non-strengthened RC beams, two RC beams strengthened with Near Surface Mounted (NSM) CFRP rods, and two RC beams strengthened with Externally Bonded (EB) CFRP sheets. Strengthened RC beams had less stiffness degradation and more energy dissipation when compared to the non-strengthened RC beam. The NSM CFRP technique demonstrated a better monotonic flexural strength than the EB CFRP technique. NSM CFRP rods and EB CFRP sheets increase the fatigue limit when compared to the non-strengthened RC beam by 19% and 33%, respectively.

Five reinforced concrete (RC) beams with epoxy-bonded carbon fiber reinforced polymer (CFRP) sheets were loaded symmetrically with sinusoidal dynamic loads simulating vehicles loads. One sustained loaded RC beam strengthened with externally bonded CFRP sheets and two control RC beams were experimentally investigated. Variables considered in this experimental program included compressive strength of the concrete, reinforcement ratio, bonded length, and load range. The experimental program included two parts: dynamic test and static test. Application of CFRP sheets to the bottom of five RC beams was conducted under dynamic loads and then the static test began after 174,000 dynamic cycles. The results demonstrate the feasibility of rehabilitating and strengthening damaged RC beams with CFRP sheets while the beams are under simulating vehicles loads. The static test program shows that the application of CFRP to RC beams results in increased strength and enhanced performance.

Combined effects of wetting–drying cycles and sustained load on the behaviour of FRP-strengthened RC beams

This research is devoted to investigate the behavior and performance of reinforced concrete beams strengthened with externally bonded Carbon Fiber Reinforced Polymer (CFRP) laminates under the effect of torsion. In this study a theoretical analysis has been conducted using finite element code ANSYS. Six previously tested beams are used to investigate reinforced concrete beams behaviorunder torsion, two of them are solid and the rest are box-section beams. Also, two beams are without CFRP reinforcement, which are used as control beams for the strengthened one, and the other four beams are strengthened with CFRP laminates with different number of layers and spacing. Numerical investigation is conducted on these beams, and comparisons between the available experimental results for these beams and numerical results from the current study are made. Conclusions from these comparisons are presented and discussed. An increase of about 15.6% in the ultimate torque for the solid beam and of about 9.8% in the ultimate torque for the box-section beam is observed after using the CFRP strips. A parametric study is carried out to study the torsional behavior of RC beams having different number of CFRP layers and concrete compressive strength; also U-wrap for the CFRP configuration is investigated.

Research on chloride ion permeability resistance of concretes bonded with CFRP sheets under dry-wet cycles

This paper presents the results of an experimental research work designed to study the flexural strengthening capacity and the modes of failure of Reinforced Concrete (RC) rectangular beams which are strengthened with external bonding of carbon fiber reinforced polymer (CFRP) composite materials to the tensile face of the RC beams. In total, seven beams were cast, one beam was preserved as control beam, two beams were strengthened without the application of preloading and the four beams were precracked and repaired with CFRP sheets. The main experimental parameters include the original damaged degree of RC beams, the amount of CFRP sheets, the longitudinal tensile reinforcement ratio, shear span to effective depth ratio, and the concrete cover thickness. All beams were tested in four-point bending over a span of 1500mm. Test results in the current study indicate that the effectiveness and flexural capacity of the CFRP strengthened beams. The flexure enhancement of the CFRP strengthened beams varied between 41% and 125% over the control beam. This study confirms that the CFRP sheets technique significantly enhances the flexural capacity of reinforced concrete beams. Finally, the contribution of CFRP sheets on the flexural capacity and rigidity of precracked and non-precracked RC beams is significant for the more longitudinal tensile reinforcement ratio.

Externally bonded carbon fiber reinforced polymer (CFRP) materials are well suited to the rehabilitation and reinforcement of civil engineering structures due to their high specific strength, specific stiffness and corrosion resistance. To probe the fatigue behavior of CFRP strengthened concrete structures, three point bending experiments of reinforced concrete (RC) beams strengthened with carbon fibre laminate (CFL) under constant amplitude loading were performed. The histories of midspan flexibility and bending stiffness of strengthened beams were recorded automatically. And the linear curve between fatigue strength and the logarithm of fatigue life was obtained. The failure modes go through concrete cracking, CFL debonding from concrete and steel bars yielding and fracture with increasing cycles of fatigue loading. Bonded CFL increases the ductility of strengthened RC beam and results in dense distribution of cracks compared with normal RC beam, and it’s bending stiffness at damage state as well. The fatigue damage evolvement shows three stages of nucleation, steady expansion and failure. Then the failure mechanism was studied and a cumulative damage model was proposed to describe the fatigue damage and fracture process of CFL strengthened RC beams under constant amplitude loading.

To address the issue of easy shearing damage of Carbon Fiber Reinforced Polymer (CFRP) in enhancing the axial compressive performance of CFRP and Ultra‐High‐Performance Concrete (UHPC) strengthened concrete columns, two methods, prestressed CFRP and UHPC with spiral stirrups, were employed for composite strengthening of reinforced concrete (RC) columns. A total of one unstrengthened column and eight strengthened columns were designed and fabricated to validate the effectiveness of the proposed methods. The axial compressive performance and bearing capacity of each specimen were analyzed by considering parameters such as single or composite strengthening method, presence of spiral stirrups in UHPC, and application of pre‐stressed CFRP. The results show that, compared with any single strengthened specimen, the ultimate bearing capacity of the composite strengthened specimen is greater than the sum of the corresponding single strengthened specimens, and the bearing capacity of the prestressed CFRP with spiral stirrup UHPC composite strengthened specimen is the most significant, reaching 235.63%. By incorporating spiral stirrups in the UHPC jacket, the phenomenon of uneven fragmentation during the failure of the strengthened column is improved. This helps prevent premature shearing damage of CFRP and enhances the fracture strain and effective utilization of CFRP. Additionally, prestressed CFRP effectively restrains the lateral deformation and crack development of the core concrete in the UHPC jacket, fully utilizing the high compressive strength of UHPC. This further enhances the ultimate bearing capacity and ductility of the specimens. Based on the experimental phenomena and strain of each material, the failure mechanism of prestressed CFRP‐spiral reinforced UHPC composite‐strengthened columns is proposed. Finally, a unified bearing capacity calculation formula for single and composite strengthened columns is established, based on the theory of confined concrete strength and the assumption of strength increment superposition. The formula is validated with experimental results from relevant literature, showing small errors in the calculated results and indicating good applicability of the formula.

Cracking behavior of sea sand RC beam bonded externally with CFRP plate

Experimental study on rehabilitation of corrosion-damaged reinforced concrete beams with carbon fiber reinforced polymer

In the recent construction industry, Fiber reinforced polymers (FRPs) have been considered to be an innovative material to repair and strengthen damaged structures. It is because FRPs have many beneficial characteristics, such as corrosion resistance, a high tensile strength-to-weight ratio, non-conductivity and design flexibility. As a demand of FRPs has increased, many researches on behavior of the structures which were externally strengthened with FRPs have been conducted. However, researches on time-dependant behavior of the structures have not been conducted yet. In order to provide improved serviceability to reinforced concrete (RC) members, the behavior of the RC members strengthened with FRPs under sustained loads should be investigated. This paper presents a series of long-term experiments and deformation-recovery experiments. For the long-term experiments, three RC beams were fabricated and two of the beams were strengthened with a carbon fiber reinforced polymer (CFRP) plate and a glass fiber reinforced polymer (GFRP) plate respectively. The beams were placed under sustained loads for about 550 days. After the 550 days, all of the beams were unloaded for the measurement of deformation recovery. The deflection and strains of rebar and FRP reinforcements were measures for about 60 days. As the result of long-terms experiment, the beams strengthened with CFRP plate showed a better performance in terms of deflection and strains of rebar and CFRP plate. Moreover, the beam with CFRP plate showed a higher deformation recovery and residual strength than the other beams.

In this paper, in order to estimate efficacy, creep recovery, and residual strength of Fiber Reinforced Polymers (FRPs) strengthened Reinforced Concrete (RC) beams, long-term flexural experiments and static flexural experiments were carried out. For the long-term experiments, the beams were strengthened with a Carbon Fiber Reinforced Polymer (CFRP) plate and a Glass Fiber Reinforced Polymer (GFRP) plate respectively. The beams were placed under sustained loads for about 550 days. After the 550 days, all of the beams were unloaded for the measurement of deformation recovery. The deflection and strains of rebars and FRPs reinforcements were measures for about 60 days. As the result of long-terms experiment, the beams strengthened with CFRP plate showed a better performance in terms of deflection and strains of rebars. And the strengthened RC beams were very effective in terms of deflection control. Furthermore, the strengthened beams have shown immediate deformation recovery. Through the static flexural experiments, it was shown that the CFRP strengthened beam had high residual strength. It seems that the sustained loads did not affect bond and residual strength of the beams.