Prestressed 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.

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.

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.

NSM CFRP Prestressing Techniques with Strengthening Potential for Simultaneously Enhancing Load Capacity and Ductility Performance

AbstractThe ductility performance of reinforced concrete (RC) members strengthened with carbon-fiber-reinforced polymer (CFRP) reinforcement using near-surface-mounted (NSM) technique has a tendency to decrease with the increase of the prestress level applied to the CFRPs. Hence, in this study, first to assure a sufficient degree of ductility for these prestressed members, a methodology was proposed to determine the maximum prestress level that can be applied to the NSM CFRP reinforcement. Using this methodology, a simplified analytical formulation was developed to determine this maximum allowable prestress level. In the second part, after demonstrating the good predictive performance of a developed three-dimensional finite-element model, first the effect of partial unbonding of prestressed NSM CFRP reinforcement at midspan to increase the ductility performance of strengthened slabs was numerically assessed. Then, a new hybrid-partially bonded system, combining the fully bonded non-prestressed and partial...

Flexural strengthening of reinforced low strength concrete slabs using prestressed NSM CFRP laminates

The effectiveness of the Near Surface Mounted (NSM) technique with prestressed CFRP (carbon fiber reinforced polymer) laminates for the flexural strengthening of reinforced concrete (RC) slabs of low strength concrete was assessed. Four RC slabs were tested, a reference slab (without CFRP), and three slabs flexurally strengthened using NSM CFRP laminates with different prestress level of the ultimate tensile strength of the CFRP: 0%, 20% and 40%. The experimental program is described and the main results are presented and analyzed in terms of the structural behavior of the RC slabs, failure modes and performance of the NSM technique with prestressed CFRP laminates. The results show that prestressing CFRP laminates with NSM technique is an effective technology to increase cracking, service, yielding and maximum loads of RC slabs made by low strength concrete. By applying NSM CFRP laminates prestressed at 20%, the cracking, service and maximum loads have increased, respectively, 258%, 123% and 125%, when the corresponding values of the reference slab are taken for comparison purposes, while 400%, 190% and 134% were the increase when applying laminates prestressed at 40%. Using available experimental results obtained with the same test setup, but using RC slabs of higher strength concrete, it can be concluded that as minimum is the concrete strength as more effective is the NSM technique with prestressed CFRP laminates in terms of serviceability limit states. A numerical strategy was used to evaluate the load-deflection of the tested RC slabs and to highlight the influence of the percentage of CFRP, the percentage of tensile longitudinal bars and the elasticity modulus of the CFRP on the effectiveness of the NSM technique with prestressed CFRP laminates for the flexural strengthening of RC slabs, by performing a parametric study.

Analytical approach for the flexural analysis of RC beams strengthened with prestressed CFRP

The objective of this paper is to propose a simplified analytical approach to predict the flexural behavior of simply supported reinforced-concrete (RC) beams flexurally strengthened with prestressed carbon fiber reinforced polymer (CFRP) reinforcements using either externally bonded reinforcing (EBR) or near surface mounted (NSM) techniques. This design methodology also considers the ultimate flexural capacity of NSM CFRP strengthened beams when concrete cover delamination is the governing failure mode. A moment–curvature (M–χ) relationship formed by three linear branches corresponding to the precracking, postcracking, and postyielding stages is established by considering the four critical M–χ points that characterize the flexural behavior of CFRP strengthened beams. Two additional M–χ points, namely, concrete decompression and steel decompression, are also defined to assess the initial effects of the prestress force applied by the FRP reinforcement. The mid-span deflection of the beams is predicted based on the curvature approach, assuming a linear curvature variation between the critical points along the beam length. The good predictive performance of the analytical model is appraised by simulating the force–deflection response registered in experimental programs composed of RC beams strengthened with prestressed NSM CFRP reinforcements.

A new hybrid methodology according to near surface mounted carbon fiber reinforced polymer technique for the flexural strengthening of reinforced concrete beams

The objective of this paper is to propose a new hybrid methodology according to near surface mounted (NSM) technique, using carbon fiber reinforced polymer (CFRP) reinforcement for the flexural strengthening of reinforced concrete (RC) beams. This NSM hybrid flexural strengthening technique combines non-prestressed and prestressed CFRP laminates in the same application in order to provide a good balance in terms of load-carrying and ultimate displacement capacity to the strengthened elements. An experimental program composed of six RC beams was carried out to assess the benefits of this NSM hybrid technique when compared to the use of non-prestressed or prestressed NSM CFRP laminates (NSM prestressing technique). For this purpose, the performance of both techniques in terms of crack width, prevailing failure mode, ultimate displacement capacity, energy absorption, and load-carrying capacity of the strengthened beams was assessed. The experimental tests were also simulated by executing advanced 3D nonlinear finite element analysis. Moreover, the potentialities of other configurations for the NSM hybrid technique by adopting different non-prestressed CFRP reinforcement ratios were numerically assessed executing a parametric study, and the relevant results are presented and discussed.

Flexural Behavior of Stone Slabs Reinforced with Prestressed NSM CFRP Bars

A technique for reinforcing stone slabs with prestressed near-surface mounted (NSM) carbon fiber–reinforced polymer (CFRP) bars is proposed and investigated in this research. Six stone slabs were tested under monotonic loading. Among these slabs, one served as a control slab and was not reinforced, and two were reinforced with one or two NSM CFRP bars, and each bar was prestressed to 30% of the bar’s ultimate strength. The remaining three were reinforced with three NSM CFRP bars that had been prestressed to 0, 15, or 30% of the bar’s ultimate strength. The test results indicate that reinforcement with either prestressed or nonprestressed NSM CFRP bars shifted the failure mode of the stone slabs from abrupt fracture to a more ductile flexural failure. A remarkable improvement in the flexural response was obtained when the stone slabs were reinforced with prestressed NSM CFRP bars. In addition to exhibiting a higher cracking load, the stone slabs that had been reinforced with prestressed NSM CFRP bars exhibited cracks with a smaller width, and the strength utilization of the CFRP reinforcement was much higher compared to that of the nonprestressed reinforced slabs. An analytical model was developed based on a sectional analysis method to predict the flexural response of stone slabs reinforced with prestressed NSM CFRP bars. The predicted values show good agreement with the experimental results.

Flexural behavior of stone beams reinforced with prestressed NSM CFRP bars

This paper is devoted to experimentally evaluate the efficiency of prestressed near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) bars for reinforcing stone beams to improve their flexural performance. A total of nine stone beams, classified into three test sets (Set A, Set B, and Set C) according to the beam span and the cross-sectional dimensions, were reinforced with prestressed NSM CFRP bars and tested under monotonic loading. In each test set, one bare stone beam was also tested for comparison. In Set C, the reinforced stone beams were strengthened with CFRP-strip wrapping in the shear span to increase the shear capacity. The test parameters included the amount of NSM CFRP reinforcements and the prestressing force. It was found from the test results that the use of prestressed NSM CFRP bars shifted the failure of stone beams from abrupt fracture to a more ductile mode and the load capacity was significantly increased. Failure of the reinforced stone beams in Set A and Set B occurred either by slip of the CFRP reinforcement or CFRP debonding or flexural–shear failure. The reinforced stone beams with CFRP-strip wrapping in Set C failed in the flexural–shear mode accompanied by rupture of the CFRP strips. The load–deflection responses of the reinforced stone beams exhibited a three-stage behavior which was similar to that of the reinforced concrete (RC) beams. The CFRP-strip wrapping effectively enhanced the shear capacity of the reinforced stone beams and avoided the abrupt failure when the first crack occurred in the shear span. For the stone beams reinforced with a same amount of NSM CFRP reinforcements, the increase in the prestressing force resulted in an increase in the load capacity accompanied by a reduction in the deformation capacity when the failure was governed by CFRP debonding.

Effectiveness of prestressed NSM CFRP laminates for the flexural strengthening of RC slabs

An experimental program was carried out to investigate the effect of the prestressed Near Surface Mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) laminates on the behavior of reinforced concrete (RC) slabs. Four RC slabs were tested, a reference slab (without CFRP), and three slabs flexurally strengthened using NSM CFRP laminates with different prestress level of the ultimate tensile strength of the CFRP: 0%, 20% and 40%. The experimental program is described and the main results are presented and analyzed in terms of the structural behavior of the RC slabs, failure modes and performance of the NSM technique with prestressed CFRP laminates. The results show that prestressing CFRP laminates with NSM technique is an effective solution to increase cracking, service, yielding and maximum loads of RC slabs failing in bending. By applying 20% of prestressed NSM CFRP laminates, the service and ultimate loads have increased, respectively, 55% and 136%, when the corresponding values of the reference slab are taken for comparison purposes, while 119% and 152% were the increase when applying 40% of prestressed laminates. A numerical strategy was used to evaluate the load–deflection of the tested RC slabs, and a good agreement between experimental and numerical results was obtained.

Postfatigue Monotonic Behavior of RC Beams Strengthened with Prestressed NSM CFRP Strips: Ductility Evaluation

AbstractThe flexural behavior of fatigued reinforced concrete (RC) beams strengthened using prestressed near surface mounted (NSM) carbon fiber–reinforced polymer (CFRP) strips was investigated. Five RC beams were tested; one beam served as a control unstrengthened beam, while the other beams were prestressed to strain values corresponding to 0, 21, 40, and 60% of the CFRP ultimate tensile strain. The beams were subjected to fatigue load conditions for 3 million cycles. Thereafter, the postfatigue monotonic flexural behavior of the beams was compared with identical unfatigued beams. Test results showed that the fatigue loading had a minor effect on the bond performance at the concrete-epoxy interface of the RC beams. In general, fatigue loading did not affect significantly the postfatigue monotonic performance of the tested beams in terms of yield and ultimate loads and deflections, as compared with identical unfatigued beams tested in static conditions. The ductility of the tested beams was also evaluate...

Reseach on Flexural Behavior of Reinforced Concrete Beams Strengthened with Prestressed Near Surface Mounted CFRP Tendons

The effectiveness of strengthening reinforced concrete (RC) beams with prestressed near- surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods has been investigated. The RC beams are tested under monotonic loading. The stressing process, failure capacity, flexural capaticy, displacement ductility and deformation are systematically studied. The text results show that the new strengthening technique with prestressed NSM CFRP rods can enhance the first-crack load, steel-yielding load of beams compared to the control beam. It is worth noting that the prestress can limit the crack width all the way up to failure, reduce the midpoint deflection, and improve the service performance of the strengthened beams. The strengthening method offers an alternative way to existing strengthening method. The results provide a reference for engineering application.

Evaluation of the transfer length of prestressed near surface mounted CFRP rods in concrete

The transfer length of a prestressed near surface mounted (NSM) fiber reinforced polymer (FRP) rod is the distance over which the rod must be bonded to the epoxy to develop the prestressing force in the rod. The transfer length is intended to provide bond integrity for the strengthened concrete member. This paper presents experimental results and an empirical equation to estimate the transfer length of prestressed NSM Carbon FRP (CFRP) rod in concrete beams. Twenty-two reinforced concrete specimens were strengthened with NSM CFRP rods. Two types of CFRP rods were used: spirally wound and sand blasted rods. Four prestressing levels were used: 40%, 45%, 50% and 60% of the tensile strength of the CFRP rod. The strain behavior in the CFRP rod was monitored by gauges mounted on the CFRP rod along the length of the beam. The test results showed that the transfer length of the prestressed NSM CFRP rod was about 35 times the diameter of the CFRP rod. The maximum bond stress of the CFRP rod in epoxy was found to range from 11 to 16 MPa for the sand blasted rods and from 12 to 23 MPa for the spirally wound rods. An empirical expression based on curve fitting of the measured data was proposed to predict the prestressing stress in the CFRP rod along the length of the beam.

Flexural strengthening of RC beams with prestressed NSM CFRP rods – Experimental and analytical investigation

The effectiveness of strengthening reinforced concrete (RC) beams with prestressed near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods was investigated. Four RC beams (254 mm deep by 152 mm wide by 3500 mm long) were tested under monotonic loading. One beam was kept un-strengthened as a control beam. One beam was strengthened with a non-prestressed NSM CFRP rod. Two beams were strengthened with prestressed NSM CFRP rods stressed to 40% and 60% of the rod’s ultimate strength. The test results showed that strengthening with non-prestressed NSM CFRP rod enhanced the flexural response of the beam compared to that of the control beam. A remarkable improvement in the response was obtained when the RC beams were strengthened with prestressed (40% and 60%) NSM CFRP rods. An increase up to 90% in the yield load and a 79% in the ultimate load compared to those of the control beam were obtained. An analytical model was developed using sectional analysis method to predict the flexural response of RC beams strengthened with prestressed NSM CFRP rods. The proposed model showed excellent agreement with the experimental results.