Near Surface Mounted Carbon Fiber Reinforced Polymer Research Articles

Shear strengthening of RC beams with prestressed NSM CFRP: Influencing factors and analytical model

This study proposes the application of prestressed near-surface-mounted (NSM) carbon-fiber-reinforced polymer (CFRP) technique in the field of the shear-strengthening of bridges for the first time. Fourteen reinforced concrete (RC) beams shear strengthened with prestressed NSM CFRP were tested under static load. The effect of the CFRP prestressing level, spacing, angle, and end-anchorage measures on the shear-strengthening behavior was evaluated. The experimental results demonstrate that the ultimate shear capacity of prestressed NSM CFRP shear-strengthened beams increased by 65–127% when compared to that of the reference beams, and the width of shear cracks was effectively suppressed. The failure mode of prestressed NSM CFRP shear-strengthened beams without end-anchorage measures was web concrete cover separation, which can be suppressed using a CFRP U-jacket and through-beam screw. Increasing the CFRP prestressing level and percentage enhanced the ultimate shear capacity and cracking resistance of the strengthened beams. However, an excessively high CFRP percentage and prestress level combination resulted in large shear crack angles and decreased the shear contribution of CFRP and concrete. Finally, an analytical model based on the modified compression field theory (MCFT) was proposed to predict the flexural-shear load response of strengthened beams, which was in agreement with the experimental results.

Flexural behavior of reinforced concrete beams strengthened with gradually prestressed near surface mounted carbon fiber-reinforced polymer strips under static and fatigue loading

The near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strengthening technique, combined with gradually anchored prestressed technique, is utilized to delay the occurrence of concrete cover separation (CCS) and enhance the ductility of reinforced concrete beams. The load-carrying capacity of fully prestressed beams and gradually prestressed beams are investigated under both static and fatigue loading conditions. The study focused on the effect of gradient prestress on flexural behavior of strengthened beams, analyzed the failure mode, characteristic load, ductility, and stress distribution at CFRP-concrete interface under both prestress and load conditions. Results indicate that gradually prestressed beams outperform fully prestressed ones in restraining crack development, delaying yield of tensile reinforcement, improving bearing capacity and avoiding CCS failure. Bearing capacity was significantly increased by up to 35.48%, while ductility was greatly improved by 100.33% with ultimate deflection for gradually prestressed beams compared to fully prestressed ones. The fatigue life of gradually prestressed beams, which experienced a transition from CCS failure mode to fatigue fracture of tensile reinforcement, was significantly extended. Additionally, their ductility at failure was also greatly enhanced, thus confirming the effectiveness of gradually prestressed NSM CFRP strengthening technique.

Factors affecting flexural properties of RC beams strengthened with gradually prestressed NSM CFRP strips

In this study, the gradually anchored technique is combined with the prestressed near-surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) strengthening technique to reduce the stress concentration caused by prestress transfer at the CFRP end section and improve the failure mode and bearing capacity of strengthened beams. Flexural performance tests are conducted on one fully prestressed and six gradually prestressed NSM CFRP-strengthened reinforced concrete beams under static loads. The influencing factors include the gradient prestress, CFRP bond length, concrete cover thickness, and steel reinforcement type. The failure mode, characteristic load, shear stress distribution at the CFRP end section, as well as the crack development under prestress and load are analyzed. The test results indicate that the gradually prestressed beam is superior to the fully prestressed beam in reducing the stress concentration at the CFRP end, thereby inhibiting the development of cracks at the CFRP end, improving the bearing capacity, and delaying the separation of concrete cover. The bearing capacity and ductility of the strengthened beams are significantly improved, with a maximum increase of 35% and 100.33% for them respectively, confirming the effectiveness of the gradually prestressed NSM CFRP strengthening technique. Finite element analysis is conducted to further investigate the effect of gradient prestress on flexural performance of strengthened beams and propose design suggestions.

Flexural-shear strengthening of reinforced concrete T-beams subjected to asymmetric loading using CFRP system combined by NSM strips and U-wraps

This paper presents a developed three-dimensional (3D) finite element model (FEM) of flexural and shearstrengthening of reinforced concrete (RC) T-beams using a CFRP (Carbon Fiber-Reinforced Polymer) system combined by near-surface mounted (NSM) strips and U-wraps subjected by asymmetric loading. The 3D FEM was verified by experimental results. Concrete was simulated by a total-strain-based rotating smeared crack model. NSM CFRP strips and CFRP U-wrap sheets were modeled as shell elements. Steel reinforcement and NSM CFRP strips were embedded in concrete, corresponding to a perfect bond. U-wrap sheets were modeled as a shell element and bonded to the concrete surface using an interface element, considering appropriate bond properties. Four RC T-beams from an experimental work were analyzed, and predictions of the applied load-displacement curve and failure mode were presented to demonstrate the accuracy of the developed finiteelement analysis. It is shown that the finite element results of T-beams agree well with the experimental behavior in terms of applied load versus displacement and failure mode. Moreover, a parametric study was conducted to examine the influence of some design-oriented parameters such as concrete compressive strength, amount of longitudinal tension reinforcement, amount of stirrups, multiple layers of externally bonded (EXB) CFRP sheets, and flange width.

Flexural repairing of heat damaged reinforced concrete beams using NSM-BFRP bars and NSM-CFRP ropes

The introduction of Basalt Fiber Reinforced Polymer (BFRP) bars marks a significant milestone in the construction industry's pursuit of advanced and sustainable materials. The application of near-surface mounted (NSM) BFRP bars presents a promising, emerging approach for restoring heat-affected reinforced concrete (RC) elements. This study investigated experimentally and theoretically the flexural performance of rectangular RC beams subjected to a temperature of (650 °C) for 3 h, followed by repairing using NSM-BFRP bars and NSM carbon fiber-reinforced polymer (CFRP) ropes. Among the twelve designed RC beams, seven were repaired with NSM-BFRP bars and three with NSM-CFRP ropes to address flexural deficiencies. The primary focus areas included failure modes, ultimate capacity, and the recovery percentage of original flexural capacity. Results showed that recovering percentages ranging from 88.2 % to 127 %, beams repaired using NSM-BFRP bars gave almost the same behavior as those repaired using NSM-CFRP ropes. Theoretical outcomes agreed well with experimental findings, in alignment with the ACI 440.2R-08 guidelines.

Performance of Postfire Reinforced Concrete Beams Retrofitted with External Bonded and Near-Surface Mounted CFRP: Experiments and Analyses

Fire imposes negative effects on reinforced concrete (RC) members, shifting postfire structures onto dangerous situations. This study aims at carbon fiber-reinforced polymer (CFRP) retrofit of postfire RC beams to recover their mechanical properties, providing technical information for structural engineers in practice. Toward this aim, experiments were performed for 16 RC beams, which were divided into four groups exposed to 30, 45, 60, and 75 min of ISO 834 fire. After exposure to fire, these beams were strengthened by external bonded (EB) and near-surface mounted (NSM) techniques using a similar amount of CFRP for comparison. These specimens were then loaded until failure. The results are compared with one another and compared with control specimens. It is found that both EB and NSM CFRP retrofitted beams failed in the form of peeling off of concrete, whereas no rupture of CFRP was observed. CFRP retrofit significantly changes the behavior of postfire RC beams, being brittle or exhibiting low ductility. The effectiveness of EB and NSM CFRP techniques is similar and it decreases with the increase in fire duration. CFRP retrofit increases the yield load-carrying capacity by 27.5%−40.9% for 30−60‐min postfire RC beams and recovers the yield load-carrying capacity for 75-min postfire RC beams. CFRP retrofit significantly increases the ultimate load-carrying capacity, whereas its increasing levels depend not only on the CFRP retrofit but also on the fire duration. Both CFRP retrofitting techniques successfully recovered the yield stiffness of postfire RC beams. The moment capacity of CFRP retrofitted postfire RC beams was theoretically analyzed, and a prediction model was proposed. The proposed model is simple, but it accurately predicts the moment capacity of retrofitted postfire beams; thus, it can be useful for structural engineers in practice.

Employing Carbon Fiber Reinforced Polymer Composites toward the Flexural Strengthening of Reinforced Concrete T-Beams.

Reinforced concrete structures are exposed to various loads under critical environmental conditions, which might lead to the deterioration of the structures prior to their designed service life. Hence, to improve the serviceability of the reinforced concrete (RC) structures and meet the design requirements, the structures are strengthened using carbon fiber reinforced polymer (CFRP) composites. The application of CFRP is done using two major techniques, namely externally bonded (EB) reinforcement and near surface mounted (NSM) techniques. The purpose of this study was to examine and compare the behavior of RC T-beams that had been flexurally reinforced utilizing EB and NSM techniques. Six full-size RC T-beams were evaluated, including a reference beam and four beams that had been flexurally reinforced using laminates made of EB and NSM CFRP. The findings of the experimental tests reveal that, when using the same quantity of CFRP, the beams strengthened with NSM laminates outperformed those strengthened with EB laminates in terms of ultimate load.

Impact of sulfate attack on flexural behaviour of CFRP strengthened concrete beams

The effect of exposing concrete beams (150 × 250 × 1600 mm), retrofitted with parabolic and straight profiles of near-surface-mounted (NSM) carbon-fibre-reinforced polymer (CFRP) ropes, to mild and moderate stages of cyclic sulfate treatment (CST) is investigated as a part of an ongoing comprehensive study. A total of 16 beams were used to evaluate structural performance for strengthened beams before and after exposure to CST for about 4 and 11 months when free concrete expansions of about 2008 and 8690μ strain, respectively, were attained. Mechanical tests on concrete cylinders showed increased brittleness of concrete as a result of the CST, with 6% and 15% reductions in compressive strength after exposure to mild and moderate stages of CST, respectively. The load capacity and stiffness for beams exposed to the mild stage of CST were slightly decreased, yet were noticeably increased for those exposed to the moderate stage of CST, owing to the significant precompressive stresses induced in concrete. In contrast, rotational ductility and toughness were progressively decreased as the treatment progressed. Generally, concrete beams, strengthened with curved and straight NSM CFRP ropes, cannot retain their overall structural performance under moderate levels of sulfate attack.

Shear strengthening of fire-damaged reinforced concrete beams using NSM CFRP laminates

The aim of this study is to evaluate the shear strengthening of fire-damaged reinforced concrete (RC) beams with carbon fiber-reinforced polymer (CFRP) laminates using the near-surface mounted (NSM) technique. An experimental program involving 13 RC beams subjected to heating for 60, 90, and 120 min in accordance with the standard fire curve presented in International Organization Standardization (ISO) 834 is presented. After cooling to room temperature, the beams are strengthened with NSM CFRP laminates at spacings of 75, 150, and 200 mm. During heating, explosive spalling of concrete occurred in the samples. The NSM CFRP system can efficiently strengthen the shear of the beams; in most cases, it recovers the initial shear capacity of fire-damaged RC beams and increases it considerably. A smaller spacing between the laminates results in greater load capacities and stiffness, and a spacing of 75 mm causes the RC beams to fail owing to peeling-out in the strengthening region.

Effect of Slenderness Ratio on Confined Reinforced Concrete Columns Using NSM-CFRP Rope

The need for the flexible near-surface-mounted carbon fiber–reinforced polymer (NSM-CFRP) material has recently been used as a strengthening material for structures. Therefore, this experimental work and finite element analysis studies were carried out to assess the behavior of confined rectangular columns using NSM-CFRP rope. Nine RC columns with three slenderness ratios (17.78, 24.44, and 28.89) including the spacing between ropes were investigated under concentric compression load. The obtained results revealed that the NSM-CFRP significantly contributed to the load capacity of RC columns. Furthermore, the role of the spacing between ropes in improving the load capacity of confined rectangular RC columns was observed. The load capacity increased by decreasing the spacing from 200 to 100 mm between ropes by 133.1%–149.7%. The improvement in the load capacity of columns when the slenderness ratio decreases were recorded. The percentage of load capacity was approximately 124.5%–139.7% and 112.3%–133.1% when using slenderness ratios of 24.44 and 28.89, respectively. The failure mode for all strengthened specimens was dominated by spalling and crushing of concrete followed by NSM-CFRP rope rupture. A numerical model was developed using ABAQUS software for NSM-CFRP rope–confined columns accounting for material nonlinearities. The results showed a good agreement between the experimental work and numerical model results.

Experimental assessment of the thermo-mechanical bond behavior of NSM CFRP with cement-based adhesives

Cement-based materials, which are incombustible and show low thermal diffusivity, show great potential as a valid alternative to polymeric matrices for bonding near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) systems for the strengthening of reinforced concrete (RC) structures. The performance of these materials in different loading conditions, including high temperature exposure, still needs to be better explored. This paper presents a study on the characterization of the bond behavior using cement-based adhesives and CFRP strips with innovative surface treatment. The CFRP strips used in this study were manufactured specifically with sand-treated surfaces to improve the bonding performance between CFRP and cement-based adhesive by taking advantage of the better grip of increased roughness surfaces. The results of the pull-out tests in ambient conditions, as well as exposed to elevated temperatures, are presented and discussed. The effect of using sand particles of different sizes, adhered with two different resins on the CFRP strips surface embedded in different cement-based matrices, was compared. The obtained results showed that by applying a sand treatment on the surface of CFRP strips, the maximum pull-out force was up to 12 times higher than when using smooth surfaced strips. Pull-out tests showed that, by using the NSM sand treated CFRP strips and CFRP bars with cement-based adhesive, an average bond strength up to about 9 MPa and 11 MPa, respectively, could be achieved. Thermo-mechanical pull-out tests showed the proposed strengthening system is able to withstand up to 30 min of heat exposure before bond failure.

Shear behavior of composite stone beams reinforced with NSM longitudinal CFRP bars

Employing near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) bars in the tensile zone of bare stone beams can effectively improve their flexural performance. Meanwhile, shear failure could occur when excessive amount of flexural reinforcement is adopted. Seven composite stone beams were tested to study the shear behavior of composite stone beams reinforced with NSM CFRP bars. The test variables included the shear span to depth ratio and the reinforcement ratio. Digital Image Correlation (DIC) technology was used to capture the full-field strain of the beams. The results indicated that the composite stone beam exhibited ductile failure mode with visible deflection and multiple cracks formed in both constant bending zone and flexural-shear zone. With the decrease of shear span to depth ratio, the cracking load and shear capacity increased linearly but the deformation capacity decreased. For the same shear span to depth ratio of 2.5, the composite beam with a higher CFRP reinforcement ratio observed higher shear load-carrying capacity. DIC results demonstrated the shear force was mainly provided by the arch action before failure. A shear-resistance theoretical model based on Rankine's failure criteria, sectional mechanical analysis and experimental observations was proposed and verified with the current and previous test results.

Influence of Stirrup Spacing on the Strengthening and Rehabilitating of RC T-beams Using Near-Surface Mounted Carbon-Fiber-Reinforced Polymer Strips

This paper aims to investigate the effect of using different configurations of near-surface mounted carbon-fiber-reinforced polymer (NSM-CFRP) strips on the shear strength of strengthened and rehabilitated reinforced concrete (RC) T-beams with different internal shear stirrup spacing. The internal shear stirrup spacing was 50 and 150 mm. The NSM-CFRP strips were at an inclination of 45° and spaced at 75 and 150 mm. A total of eight beams were tested in this study: two beams without NSM-CFRP as control beams for the purpose of comparison; three beams were strengthened by NSM-CFRP; and three beams were rehabilitated by NSM-CFRP. The experimental shear capacities were compared with the theoretical values predicted by the ACI 440.2R-17. The results indicated that the use of NSM-CFRP strips enhanced the shear capacity for all beams compared to their corresponding control beams. The enhancements in the shear capacity increased with the decrease in the spacing of the internal shear stirrups and NSM-CFRP strips. The ACI 440.2R-17 was conservative in predicting the theoretical shear capacities.

Damage features extraction of prestressed near-surface mounted CFRP beams based on tunable Q-factor wavelet transform and improved variational modal decomposition

Damage detection of prestressed near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) beams has attracted much attention over the last few decades due to the dramatic increase in the proportion of prestressed NSM CFRP beams in the civil infrastructure. Although many studies have demonstrated the effectiveness of piezoelectric transducer-based damage detection techniques, one of the most reported problems is the variation in signal vibration characteristics caused by external noise, which limits the effectiveness of detection and quantification using traditional basic damage indicators. This study proposed a novel signal processing method of damage feature extraction in a noisy environment based on the tunable Q-factor wavelet transform with improved variational modal decomposition, which is aimed to expand the applicability of the piezoelectric transducer-based damage detection techniques in the damage diagnosis of the prestressed NSM CFRP beam. The proposed method is validated by a numerical case study of an analog signal and an experimental study of NSM CFRP beams. The results show that the tunable Q-factor wavelet transform and improved variational modal decomposition can be employed to remove the environment noise and extract the damage feature of the NSM CFRP beam, which helps in the damage assessment of the NSM CFRP beam.

Compression behavior of confined circular reinforced concrete with spiral CFRP rope with different slenderness ratios

NSM-CFRP strip cannot be wrapped around the column, the necessity for flexible near-surface mounted carbon fiber reinforced polymer (NSM-CFRP) material has recently arisen. Under concentric compression stress, twelve circular reinforced concrete columns (RC) with three slenderness ratios (17.3, 23.8, and 28.1) in addition to the spacing between ropes were investigated to assess the behavior of confined RC columns using NSM-CFRP rope. The results showed that the load capacity of RC columns was increased by using the NSM-CFRP rope and decreasing the spacing between the NSM-CFRP ropes. Furthermore, when the slenderness ratio is decreased, the load capacity of columns increases. Specimens with heights of 800 mm, 1100 mm, and 1300 mm reinforced with 150 mm spacing between CFRP ropes had a greater increase in capacity (3.3 times, 1.6 times, and 1.97) than control specimens of each height. The confinement technique using spiral CFRP rope at 150 mm spacing has the greatest impact on the load-carrying capacity of the RC column. Additionally, confining the RC column with spiral CFRP rope at a 150 mm spacing enhanced the bearing load capacity of the RC column with the lowest slenderness ratio more than the other slenderness ratios. Finally, when the slenderness ratio increases, the bearing load capacity and axial strain corresponding to the maximum axial stress decrease. The slenderness ratio had the greatest impact on the behavior of the RC column when there was a higher amount of CFRP rope or when the spacing between CFRP ropes was the lowest.

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.

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.

Fatigue behavior of RC slabs flexurally strengthened with prestressed NSM CFRP laminates

AbstractSome reinforced concrete (RC) structures requiring strengthening intervention are subjected to cyclic loads during their lifetime. To evaluate the effect of this kind of loading on the behavior of flexurally strengthened RC slabs with prestressed near‐surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminates, an experimental program formed by 10 RC slabs was carried out. Four specimens were submitted to monotonic loading and the remaining were submitted to 1 million cycles at a frequency of 1 Hz. Two levels of fatigue loading were applied and, for each one, three prestress levels for the CFRP were tested (0%, 20%, and 40%). Failure during the fatigue loading was occurred only in one specimen (at 752,000 cycles) with a maximum fatigue stress level of 92% (percentage of the maximum load carrying capacity of the equivalent slab tested up the failure under monotonic static loading). The other specimens, submitted to a maximum fatigue stress level between 46% and 82%, did not fail during the fatigue loading process. After 1 million cycles, the NSM strengthened slabs with 0%, 20%, and 40% of prestress presented an increase of 20%, 18%, and 25%, respectively in mid‐span deflection. In general, the strains in the CFRP laminates marginally increased during the fatigue loading. The results of the postfatigue monotonic tests until failure showed that the damage accumulation due the fatigue cycles neither affected the ultimate load capacity of the strengthened NSM CFRP slabs nor the failure mode in which the CFRP rupture.

The behavior of concrete beams strengthened with NSM CFRP strips: a numerical parametric study

A non-linear finite element (NLFE) model was developed to predict the flexural response of concrete beams, strengthened with near surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. The model was employed for predicting the influence of a wider spectrum of key parameters, concrete compressive strength (25–55 MPa), NSM FRP strips number (2–4) or spacing (20–35 mm), bond length (100–450 mm), FRP type (carbon, glass, and basalt) and loading pattern (four-point, three-point, and distributed load), upon the performance of these beams. The results showed a clear improvement in the performance of the CFRP-strengthened concrete beams when made at a higher-strength class of concrete. Using a higher number of NSM FRP strips of the same cross-sectional area contributed to enhancing the mechanical performance noticeably. Moreover, the model developed was capable of capturing the negative impact of using a lower spacing for a constant number of NSM CFRP strips. The impact of the FRP strips' type varied upon the mechanical property predicted and the strength grade of concrete used in the casting of the beams. The load resistance of NSM FRP-strengthened specimens was the highest under three-point load configuration, followed, in sequence, by distributed and four-point loads, respectively.

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.