CFRP-strengthened Beams Research Articles

Performance and failure analysis of carbon fiber-reinforced polymer (CFRP) strengthened reinforced concrete (RC) beams

At present, the strengthening and retrofitting of reinforced concrete (RC) beams using carbon fiber-reinforced polymer (CFRP) are receiving extensive attention. The use of strengthening technique for RC beams vary according to purpose. In general, this technique aims to increase load-carrying capacity, either through flexural or shear strengthening. The present study aims to investigate the load–deflection response of CFRP-strengthened RC beams through experimentation with five various strengthening configurations and two different types of surface treatment. The RC beams that were tested in this work were small scale. Results indicate that compared with control beams, CFRP-strengthened beams notably reinforced ultimate load and deflection, as well as ductility, because of the stiffness and grip provided by CFRP. Among the five configurations, the continuous U-wrapping and strip wrapping systems were proven effective for strengthening. A close interaction between internal stirrups and external CFRP strips, which will negatively affect the overall strength of the beams if the reinforcements were placed simultaneously along a similar section of the beam. The failure pattern of the strengthened beams considerably depends on strengthening configuration and internal steel reinforcement, which control the ultimate effectiveness of strengthening. In addition, the ultimate capacity of the beams was predicted in accordance with the ACI 440.2R-17 guidelines and was finally compared with the experimental ultimate loads of the CFRP-strengthened beams. Experimental results show that the predicted values were extremely conservative.

Finite element investigation of fatigue performance of CFRP-strengthened beams in hygrothermal environments

The effectiveness of reinforced concrete (RC) external bonded with carbon fiber reinforced polymer (CFRP) laminates is bound by the negative effect of hygrothermal environment and cycle load. In order to prove the durability of CFRP-strengthened RC beams in hygrothermal environment, this paper concentrates on a numerical and experimental research of the failure modes and fatigue life of CFRP-strengthened RC beams under the coupling action of hygrothermal environment and cyclic load. Considering that two main failure modes of CFRP-strengthened RC beams are debonding of CFRP-concrete interface and main reinforcement fracture, the failure criterion of interface debonding and main reinforcement are involved in finite element method (FEM). Based on the fatigue damage accumulation, the failure mode which occur first can be predicted. The results show that the main failure mode of CFRP-strengthened RC beams in indoor environment is main reinforcement fracture. With the increase of temperature and humidity, interface debonding become the main failure mode. The fatigue life corresponding to failure mode under different hygrothermal environments can be calculated, which is in good agreement with experimental results.

Composite Strips with U-Shaped CFRP Wrap Anchor Systems for Strengthening Reinforced Concrete Beams

The contribution of carbon fiber reinforced polymer (CFRP) wrap as an anchorage system of CFRP plates, used for flexural strengthening, is investigated. Eighteen concrete beams (100 × 150 × 1400 mm3) were cast and cured for 28 days before strengthening with both CFRP plates (1.2 × 50 mm2) and anchorage-used, U-shaped CFRP wraps. These were adhered on the top of the CFRP plates at different longitudinal and transverse extensions. The mechanical response of the concrete beams was examined under four-point loading including measurements of load versus deflection, bond stress versus slippage and strain in CFRP plates. Crack formation and failure modes were also monitored and characterized. End anchorage regimes, using 300-mm-long U-shaped segmented CFRP wraps, impart the best improvement to load capacity, rigidity, and toughness (up to 80, 57, and 40%, respectively) without harming tangibly ductility in terms of deflection at failure. Anchorage cases involving attaching continuous single or double layers of U-shaped CFRP wrap on the top of CFRP plates impart lower but tangible overall improvements in mechanical performance of CFRP-strengthened beams. A simple empirical model is developed to predict ultimate strain in CFRP plate based on present results before examining using available literature data. Moreover, the efficiency of the present technique is compared to that present in literature.

Numerical study on fracture behavior of CFRP-strengthened self-compacting concrete beams

This research is dedicated to the numerical investigation of the fracture behavior of notched self-consolidating concrete (SCC) beams which are strengthened by carbon-fiber-reinforced polymer sheets (CFRP). The beams have been constructed with an initial notch in the middle of span and reinforced with a CFRP sheet under the beam. Therefore, the load-carrying capacity of the specimens, shear stresses in CFRP, and crack mouth opening (CMOD) have been documented. Furthermore, specimens have been modeled using the finite-element approach and the nonlinear static analysis. The validity of the numerical modeling has been established through comparing the experimental results with those of numerical simulations. The effect of variation in different parameters such as sheet thickness, concrete mechanical properties, beam height and adhesive strength have been investigated via numerical analyses. The results show that in experimental and numerical models, the load-crack mouth opening (P-CMOD) curves have two peak load points. As the load increases, the first peak value is obtained, which is then followed by a descent in the load-carrying capacity. By further implementing the load, the second peak value is reached on account of the relatively high cohesive impact of the CFRP sheet. Moreover, it is observed that in P-CMOD curves the first and second peak points increase as the concrete strength and the beam height increase, though the rate of this growth is less for $ P_{2\max}$ . While the adhesive strength and thickness of the CFRP sheet have the most influence on the second peak load which indicates the performance of the CFRP sheet.

Contribution of externally bonded FRP shear reinforcement to the shear strength of RC beams

An experimental program of T-beams strengthened in shear by CFRP sheets has been developed with two longitudinal reinforcement ratios and different externally bonded CFRP shear strengthening amounts in a discontinuous or continuous U-shaped configuration with and without mechanical anchorages. The effectiveness of the external reinforcement without anchorages was low regardless the CFRP reinforcement ratio. There is an evidence of the interaction of the different shear strength components, concrete Vc, transverse steel Vs, CFRP sheets Vf that have been obtained from the registered data of the instrumentation. This interaction might explain that the ultimate shear force of some CFRP-strengthened beams was lower than the one of the control beam. There is a significant difference between the sum of the Vc, Vs, and Vf components and the total shear force. This difference is assumed to be carried out by the flange (including its internal transverse reinforcement), which represents a significant percentage of the total shear strength (with a mean value of 38%). Finally, the contribution of the CFRP has been obtained by different existing guidelines and has been compared to the experimental values.

Identifying CFRP strip width influence on fracture of RC beams by acoustic emission

Carbon fiber-reinforced polymer (CFRP) is a widely used material to strengthen deficient structures externally. While CFRP has numerous advantages, debonding is the main problem of CFRP-strengthened reinforced concrete (RC) beams as it causes sudden failure. Thus, to circumvent this problem, failure modes of an RC beam strengthened with CFRP need to be understood. There are various application types of CFRP strips and plates depending on their ease of application and contribution as a strengthening material such as side bonding and wrapping. Using CFRP as a strip is a cost effective way and is widely preferred. Strip width and spacing have a firsthand effect on the mechanical behavior of the strengthened element. Accordingly, in this study, AE parameter analysis and SiGMA analysis were applied for identification of CFRP strip width effect on fracture of RC beams having different CFRP widths. Within the scope of the experimental study, a reference beam and two CFRP-strengthened beams were tested under cyclic loading and monitored by AE. Both mechanical and AE results show that the behavior is not enhanced by increasing the strip width.

Fire Behavior of Full-Scale CFRP-Strengthened RC Beams Protected with Different Insulation Systems

In this paper, a series of experimental studies conducted to investigate the fire behavior of insulated full-scale carbon-fiber-reinforced polymers (CFRP)-strengthened reinforced concrete (RC) beams is presented. Four CFRP-strengthened RC beams, respectively insulated with a thick coating system, ultrathin coating system and calcium silicate board system, were tested under ISO834 standard fire exposure. The test results revealed that satisfactory fire endurance for CFRP-strengthened concrete beams can be obtained with the protection of the three systems. The major role of fire insulation materials is to delay the failure of adhesive in the early stage and reduce the performance degradation of concrete and internal reinforced bars after the bond failure of the CFRP–concrete interface. In addition, it was indicated that effective anchorages of CFRP and reasonable anchoring constructions of the insulation system played important roles in ensuring the fire-resistant capability of CFRP-strengthened concrete beams. Further, a detailed finite element model was developed as an alternative to the standard fire test. The predicted temperature and deflection results were in good agreement with the measured ones. Based on the case studies, insulation thickness, insulation thermal conductivity, CFRP amount and load ratio were proven to be the main influences of the fire resistance of insulated CFRP-strengthened beams.

Effect of Thermal Distress on Residual Behavior of CFRP-Strengthened Steel Beams Including Periodic Unbonded Zones

This paper presents the residual behavior of wide-flange steel beams strengthened with high-modulus carbon fiber-reinforced polymer (CFRP) laminates subjected to thermal loading. Because the coefficients of thermal expansion of the steel and the CFRP are different, temperature-induced distress may take place along their interface. Periodic unbonded zones are considered to represent local interfacial damage. Five test categories are designed depending on the size of the unbonded zones from 10 to 50 mm, and corresponding beams are loaded until failure occurs after exposing to a cyclic temperature range of ΔT = 25 °C (−10 to 15 °C) up to 84 days. The composite action between the CFRP and the steel substrate is preserved until yielding of the beams happens, regardless of the thermal cycling and periodic unbonded zones. The initiation and progression of CFRP debonding become apparent as the beams are further loaded, particularly at geometric discontinuities in the vicinity of the unbonded zones along the interface. A simple analytical model is employed to predict the interfacial stress of the strengthened beams. A threshold temperature difference of ΔT = 30 °C is estimated for the initiation and progression of CFRP debonding. Multiple debonding-progression stages in conjunction with the extent of thermal distress appear to exist. It is recommended that high-modulus CFRP be restrictively used for strengthening steel members potentially exposed to a wide temperature variation range.

Cracking Behavior of CFRP Laminate-Strengthened RC Beams with Premechanical and Postmechanical Environmental Damage

The main objectives of this study are to investigate the effects of three types of predamage and postdamage on the cracking behaviors of carbon fiber-reinforced polymer (CFRP)-strengthened reinforced concrete RC beams and to develop a rational methodology for predicting the average stabilized crack spacing. The predamage is induced by either sustained loading only or by the combination of sustained loading and corrosion. The predamage involved a sustained loading with an anchor tightening system, an electrochemical process to accelerate the migration of chlorides from an external electrolyte into the tested beams, and a wetting–drying cycle process with a controlled current to accelerate the corrosion of the reinforcing steel bars in the tested beams. The postdamage was induced by wetting-drying cycles. A loading test was conducted to determine the cracking behaviors of stabilized flexural cracks in the CFRP-strengthened beams with or without damage. The crack patterns, crack spacings, and test beam widths were recorded and compared, and the related mechanism was discussed. It was found that after CFRP strengthening, the effect of predamage or postdamage on the crack spacing and width is not as distinct as in the unstrengthened cases. The sustained loading predamaged beam showed insignificant differences in crack spacing and width compared to beams without predamage. Subsequently, a model capable of evaluating the crack behaviors of CFRP-strengthened beams with or without damage was developed. The analytical approach is based on equilibrium and compatibility equations to elucidate the average stresses of concrete and the CFRP laminate of a CFRP-strengthened beam element.

Experimental and numerical study on the effect of repairing reinforced concrete cracked beams strengthened with carbon fibre reinforced polymer laminates

This paper presents experimental and numerical investigations on the effect of repairing cracks in reinforced concrete (RC) beams prior to strengthening them with carbon fibre reinforced polymer (CFRP) laminates. The experimental campaign comprised flexural tests on three types of full-scale RC beams with T-shaped cross-section: (i) two reference un-strengthened beams, (ii) two CFRP-strengthened beams previously loaded and cracked, and (iii) two CFRP-strengthened beams, previously loaded, cracked and repaired with epoxy resin. The repair and strengthening techniques consisted of respectively injecting the cracks with epoxy resin and applying CFRP laminates according to the externally bonding reinforcement technique. In the numerical study, the structural response of all beams tested was simulated using the finite element software Atena, which features a smeared cracked model constitutive relationship for concrete. A parametric study was carried out in which the influence of material parameters, namely the fracture energy, on the beams structural response was assessed. Experimental results showed that repairing cracks by means of epoxy injection before strengthening them with CFRP laminates provided a considerable increase of stiffness, but only a slight increase of ultimate strength, as failure was triggered by the debonding of the strengthening system at the anchorage zones. In the numerical study a very good agreement with experimental data was obtained. For the repaired and strengthened beams, such agreement was obtained by increasing concrete’s fracture energy when compared to that of the reference beams.

CFRP strengthening of timber beams recovered from a 32 year old quonset: Element and system level tests

This paper presents an experimental program for examining the behavior of Douglas Fir timber beams recovered from a 32year old quonset. Selected beams are intentionally damaged by creating a notch at midspan and strengthened with carbon fiber reinforced polymer (CFRP) composites. Two types of CFRPs are used with different types of epoxy adhesives. The test program consists of two phases incorporating 27 beams: the element-level test with six individual beams and the system-level test with seven beam-assemblies consisting of three beams per system representing an actual service condition. Such a unique experimental approach enables to study the load-sharing mechanism of CFRP-strengthened beam–floor modules. Test beams are simply supported and monotonically loaded until failure occurs. The beams strengthened with the CFRP exhibit a significant improvement in load-carrying capacity of up to 184% and in deflection ductility of up to 165% when compared to the unretrofitted control. Stress concentrations at the notch (simulated damage) are an important factor that influences the failure characteristics and ultimate capacity of the beams. CFRP-strengthening results in improved load-distribution among the constituent beams in the assembly despite the localized damage.

A Numerical Evaluation of Insulated CFRP-Strengthened RC Beams Exposed to Fire

There are mainly two approaches to improve the fire resistance of FRP systems. While the most common way is to protect or insulate the FRP systems, the other way is to use fibers and resins with better fire-performance. In this paper a numerical investigation for evaluating the fire behavior of insulated CFRP-strengthened RC beams is presented.The effects of external loading and thermal expansion of materials in both the structural and the thermal behavior of composite elements due to loading and elevated temperatures are taken into consideration in a finite element model. The validity of the numerical model isdemonstrated withthe results from an existing experimental study on insulated CFRP-strengthened RC beam. The conclusions of this investigation have been employed to predict the structural behavior of concrete structures successfully.

Probabilistic assessment of torsion in concrete beams externally strengthened with CFRP composites

This paper illustrates an analytical probabilistic study of concrete beams subjected to torsion that are strengthened with carbon fiber reinforced polymer (CFRP). Hii and Al-Mahaidi’s method is one of the best analytical models for evaluating the torsional capacity of CFRP strengthened reinforced concrete beams. The first-order reliability method is carried out to probabilistically assess the capacity of CFRP-strengthened beams. For this aim, the statistical characteristics of design variables and model errors have been considered, followed by the determination of the average reliability indexes of the strengthened beams. The effect of each design variable on the average reliability is also determined. The assessment shows that the Hii and Al-Mahaidi’s analytical model is unconservative. In order to correct the situation, a more relaxed set of resistance factors for use in a load and resistance factor design format are needed. These are then determined for two target reliability levels of \( \beta_{\text T} \) = 3.0 and \( \beta_{\text T} \) = 3.5. It is found that factors of 0.9200 and 0.8225 are needed for target reliability levels, \( \beta_{\text T} \) = 3.0 and \( \beta_{\text T} \) = 3.5, respectively. Values of 0.9 and 0.8 are suggested for use in real practice depending on the target reliability sought.

Experimental Research of Effect on Carrying Capacity of Normal Section of CFRP-Strengthened Beams for CFRP Debonding

By the experiment of 13 RC beam strengthened with CFRP, the paper mainly study the effect on the load carrying capacity of normal section of CFRP-strengthened beams for CFRP debonding. The results show that for the strengthened beams without any anchorage at plate-end, they lost load carrying capacity soon after CFRP debonded and quitted working, or went on carrying load like an ordinary RC beam. Besides, the formula for the effective CFRP strain when CFRP end debonded was proposed. For the strengthened beams with sufficient anchorage at plate-end, the strengthened beams should be simplified as simply supported unbonded concrete member after CFRP debonded. A reduction factor was adopted to modify the ultimate tension strain derived from the planar section assumption. The calculated results of ultimate carrying capacity were in reasonable agreement between the suggested formula and the test results.

Investigation on Flexural Performance of RC Beams Flexurally Strengthened by Side-bonded CFRP Laminates

It is an effective way to improve the flexural behavior of reinforced concrete (RC) members by externally bonded carbon fiber reinforcement polymer (CFRP) laminates on the soffit of the members. However, there is little investigation on flexural performance of RC beam flexurally strengthened by side-bonded FRP laminates. To investigate the flexural behavior of RC beams side-bonded CFRP laminates and the difference of RC beams strengthened by soffit-bonded and side-bonded CFRP laminates, a total of 8 CFRP-strengthened beams and 1 control beam were tested. The experimental results show that: 1) the first crack loads of RC beams strengthened by side-bonded CFRP laminates are much higher than that of RC beams strengthened by soffit-bonded CFRP laminates. The first crack loads of side-bonded CFRP laminates beams improved significantly; 2) Side-bonded and soffit-bonded CFRP laminates have almost the same effect on the flexural stiffness of RC beams strengthened with same quantity of CFRP laminates before tension rebar yielding. However, side-bonded CFRP laminates can affect crack width and crack pattern of the strengthened beams, and the pre-crack stage of RC beam by sidebonded CFRP laminates extended remarkably. 3) different to soffit-bonded CFRP laminates RC beams, side-bonded CFRP laminates cannot improve the first yielding and the ultimate load bearing capacity of RC beams.

Long-term behavior of existing low-strength reinforced concrete beams strengthened with carbon fiber composite sheets

This paper presents the results of an experimental study of the short- and long-term behavior of low-strength reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP). A numerical analysis model was developed and verified for the calculation of the long-term deflection and maximum allowable long-term load of such RC beams. A parametric study was also conducted and it was found that the maximum allowable long-term load of a CFRP-strengthened beam was dominated by the deflection of RC beam when the cubic compressive strength of concrete was less than a certain value. For concrete of higher strength, the maximum allowable long-term load was dominated by the stress levels of the steel bars. It was also found that the yielding load of the strengthened beams increased significantly with areas strengthened by CFRP sheets and steel bars, while the maximum allowable long-term load only increased slightly.

Strengthening of Concrete Beams Having Shear Zone Openings Using Orthotropic CFRP Modeling

A finite element reinforced concrete model has been analyzed by the author with ANSYS 9 finite element program for both unstrengthened and CFRP-strengthened beams using concrete element model 25x25x25mm and discrete and smeared steel distribution with openings exist. The CFRP has been modeled using Solid46 element, which has orthotropic properties. The deflection results have been compared with an experimental and other finite element model which are performed by Mohamed [4], in which using 100x42.5x42.5 mm concrete element, smeared steel distribution with the same opening sizes, and modeling CFRP with ANSYS 5 finite element program using Link10 element which has a uniaxial properties. These results show that the author's model is much better than the Mohamed's [4] model comparing with the experimental one. A parametric study has been done on the proposed model for obtaining the maximum strains values for concrete and steel at failure loads, for different opening sizes and comparing them with the experimental one. This study show a good agreement between the proposed and experimental model results for strains values which indicate the efficiency of the proposed model for analyzing the unstrengthend and strengthened reinforced concrete beams.

Experimental Investigation on Effective Bond Length of Flexurally CFRP-Strengthened RC Beams Failed by Intermediate Crack Debonding

The models to calculate the effective bond length determined by single-lap or double-lap shear tests are more suitable for the end-debonding failure; and the improved models based on the simply shear test models to calculate the effective bond length of flexural CFRP-strengthened beams failed by intermediate crack debonding (IC debonding) are also lack of the support of experimental data. To investigate the effective bond length of flexural CFRP-strengthened beams, a total of 9 flexurally CFRP-strengthened beams were tested. The experimental results show that the existing models are very conservative for calculating the effective bond length in flexural FRP-strengthened RC beams failed by IC debonding. Based on the experimental results, this paper presents a model to calculate the effective bond length of flexural FRP-strengthened RC beams failed by IC debonding, and a model to calculate the plastic hinge length at the shear-flexure span.

Numerical Investigation of Creep Effects on FRP-Strengthened RC Beams

Numerical analysis using a finite-element model was performed to simulate and investigate the long-term behavior of two RC beams with similar steel reinforcement, cast from the same batch of concrete. One beam was a plain RC beam and the other beam was strengthened using carbon fiber-reinforced polymer (FRP) strips. The deflections of both beams have been monitored for 5 years after loading. The finite-element model included both creep of concrete and viscoelasticity of the epoxy adhesive at the concrete-carbon FRP (CFRP) interface. The results of the finite-element analysis are compared to experimental observations of the two beams. The finite-element analysis was found to be able to simulate the long-term behavior of the CFRP-strengthened beam and help us understand the complex changes in the stress state that occur over time.

Shear Strengthening of T Cross Section Reinforced Concrete Beams by Near-Surface Mounted Technique

With the purpose of evaluating the influence of both the percentage and inclination of the carbon fiber-reinforced polymer (CFRP) laminates on the effectiveness of the near-surface mounted technique for the shear strengthening of reinforced concrete T beams, an experimental program was carried out, using three percentages of laminates and, for each one, three inclinations: 90, 60, and 45°. The CFRP-strengthened beams had a steel stirrup reinforcement ratio (ρsw) of 0.1%. The highest CFRP percentage was designed to provide a maximum load similar to the one of a reference beam reinforced with ρsw equal to 0.24%. Although these beams have had a similar maximum load, the beams with CFRP presented higher stiffness. Laminates at 60° was the most effective shear strengthening configuration, having provided a maximum increase in the load capacity of 33%. The contribution of the CFRP strengthening systems was limited by the concrete tensile strength. Below certain spacing between laminates, a group effect occurs due to the interference between consecutive concrete failure surfaces, leading to the detachment of “two lateral walls” from the underlying beam core.