Near Surface Mounted Technique Research Articles

Enhancing Flexural Strength of RC Beams with Different Steel–Glass Fiber-Reinforced Polymer Composite Laminate Configurations: Experimental and Analytical Approach

This study intended to measure the efficiency of different strengthening techniques to advance the flexural characteristics of reinforced concrete (RC) beams using glass fiber-reinforced polymer (GFRP) laminates, including externally bonded reinforcement (EBR), externally bonded reinforcement on grooves (EBROG), externally bonded reinforcement in grooves (EBRIG), and the near-surface mounted (NSM) system. A new NSM technique was also established using an anchorage rebar. Then, the effect of the NSM method with and without externally strengthening GFRP laminates was studied. Twelve RC beams (150 × 200 × 1500 mm) were manufactured and examined under a bending system. One specimen was designated as the control with no GFRP laminate. To perform the NSM method, both steel and GFRP rebars were used. In the experiments, capability, as well as the deformation and ductileness of specimens, were evaluated, and a comparison was made between the experimental consequences and existing standards. Finally, a new regression was generated to predict the final resistance of RC beams bound with various retrofitting techniques. The findings exhibited that the NSM technique, besides preserving the strengthening materials, could enhance the load-bearing capacity and ductileness of RC beams up to 42.3% more than the EBR, EBROG, and EBRIG performances.

NSM-CFRP rods with varied embedment depths for strengthening RC T-beams in the negative moment region: Investigation on high cyclic response

Throughout its service life, structural reinforced concrete (RC) encounters cyclic loads and the load amplitude might always vary, leading to a mixed high and low-cycle failure. This research investigates the high cyclic response of RC T-beams strengthened in the negative moment region using near-surface mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) rods. The distinctive aspect lies in exploring varied embedment depths, including the introduction of half-embedded configurations as an alternative NSM technique. The study comprehensively analyzes load-carrying capacity, failure modes, energy dissipation, ductility, stiffness degradation, and strain behavior, providing insights into the effects of loading rates on structural response. Through an experimental program comparing fully-embedded (BF-D) and half-embedded (BH-D) CFRP rods with a control beam (BN-D) under high-rate cyclic loading, significant increases in ultimate load capacity (21.23% for BH-D and 30.86% for BF-D) are demonstrated despite debonding occurrences. The findings highlight a trade-off between benefits (enhanced ultimate load capacities, improved energy dissipation, and increased stiffness) and drawbacks (reduced ductility and tendencies toward brittle behavior) under high loading rates. Furthermore, a rate-dependent material formula is developed and validated, predicting the flexural strength of the negative moment region in good agreement with experimental results. Finally, this research contributes practical solutions to RC element strengthening challenges and advances understanding of NSM-CFRP-strengthened beam, emphasizing the impact of loading rates on structural response.

Analysis of FRP-Strengthened Reinforced Concrete Beams Using Electromechanical Impedance Technique and Digital Image Correlation System.

Fiber-reinforced polymer (FRP) strengthening systems have been considered an effective technique to retrofit concrete structures, and their use nowadays is more and more extensive. Externally bonded reinforcement (EBR) and near-surface mounted (NSM) technologies are the two most widely recognized and applied FRP strengthening methods for enhancing structural performance worldwide. However, one of the main disadvantages of both approaches is a possible brittle failure mode provided by a sudden debonding of the FRP. Therefore, methodologies able to monitor the long-term efficiency of this kind of strengthening constitute a challenge to be overcome. In this work, two reinforced concrete (RC) specimens strengthened with FRP and subjected to increasing load tests were monitored. One specimen was strengthened using the EBR method, while for the other, the NSM technique was used. The multiple cracks emanating in both specimens in the static tests, as possible origins of a future debonding failure, were monitored using a piezoelectric (PZT)-transducer-based electromechanical impedance (EMI) technique and a digital image correlation (DIC) system. Clustering approaches based on impedance measurements of the healthy and damaged states of the specimens allowed us to suspect the occurrence of cracks and their growth. The strain profiles captured in the images of the DIC system allowed us to depict surface hair-line cracks and their propagation. The combined implementation of the two techniques to look for correlations during incremental bending tests was addressed in this study as a means of improving the prediction of early cracks and potentially anticipating the complete failure of the strengthened specimens.

Hybrid strengthening and flexural behaviour of post-tensioned laminated glass beams

The strengthening of glass for structural applications is challenging. EBR (External Bonded Reinforcement) reinforcement is often preferred, although limitations result from stress concentrations especially in the case of post-tensioning. This study explores the use of CFRP (Carbon Fiber Reinforced Polymers) and/or Fe-SMA (Iron-based Shape Memory Alloys) as reinforcement in hybrid and alternative configurations. Five full size laminated glass beams were tested in flexure, combining NSM (Near-Surface Mounted) and EBR techniques and exploring the efficiency of different alternative strengthening layouts (i.e. reinforcement material versus application technique) to overcome the current shortfalls of glass strengthening for structural applications. Even for lower reinforcement ratios, hybrid strengthening systems performed better and increased load-carrying capacity while delaying crack-induced deboning failure, when compared to conventional EBR systems. Residual strength ratio increased from 87 % to 160.8 % and ductility from 407 % to 971 %. The hybrid system combining NSM-CFRP and EBR-SMA reinforcements showed the best performance, increasing the initial fracture stress of glass and maintaining sufficient residual strength capacity, while allowing the safe post-tensioning.

Experimental and theoretical investigation of the structural behavior of reinforced glulam wooden members by NSM steel bars and shear reinforcement CFRP sheet

Abstract Reinforcing the wooden structural members is considered as a challenging matter to overcome the drawbacks of using wood in the construction field. The current investigation, therefore, presents an attempt to evaluate the effectiveness of utilizing steel bars as reinforcements in glulam timber beams using the near-surface-mounted (NSM) technique in conjunction with carbon fiber-reinforced polymer (CFRP) sheet wraps. A series of flexural testing was carried out until failure for both reinforced and unreinforced glulam members in a simple support system. Eleven specimens were examined in two groups to compare them with the control beam. Five reinforced glulam (RG) beams of the first group were reinforced with different schemes at tension and compression zones using the same bar size. The other five specimens of the second group were reinforced with shear reinforcement using fully wrapped strips made of CFRP sheet in addition to the same flexural reinforcement schemes as the first group. Each glulam beam has a span of 1.35 m and a rectangular section sized 85 mm × 175 mm. Based on experimental outcomes, theoretic modeling was provided to estimate the ultimate load capacity and bending rigidity of reinforcing glulam members. Though several theoretical predictions of flexural capacity were overstated when compared to experimental predictions, these disparities were frequently about 10%, confirming that the proposed theoretical model was accurate, and the mean of ultimate loading capacity and deflection between experimental and theoretical results were 1.01 and 1.09, respectively. Experimental results presented show that the RG beams performed much better than the unreinforced reference beams in terms of structural behavior, with improvements in ultimate load capacity ranging from 16 to 49%. On the other hand, the shear reinforcement for RG members slightly improved flexural performance, and the ultimate load capacity increased by 2–7%. Therefore, it was concluded that the NSM techniques using ordinary steel bars were effective ways in strengthening the glulam members in terms of flexural stiffness with increasing ultimate load capacity.

Flexural Strength of Damaged RC Beams Repaired with Carbon Fiber-Reinforced Polymer (CFRP) Using Different Techniques

In this study, an experimental program was developed to investigate the flexural behavior of pre-damaged reinforced concrete (RC) beams that had been repaired and strengthened using carbon fiber-reinforced polymer (CFRP) laminates under a monotonic load. Two techniques were used: externally bonded reinforcement (EBR) and near-surface-mounted (NSM) reinforcement, to repair and strengthen the tested beams. The experimental program involved casting and testing nine simply supported RC rectangular beams; one beam was considered as the reference beam and did not undergo additional strengthening, and the remaining beams were strengthened using CFRP laminates. These eight beams were divided into two main groups for the purposes of strengthening: beams for which the EBR technique was used, and beams for which the NSM technique was used. The primary variables observed in the EBR and NSM groups included four damage percentages obtained according to the preload (20, 40, 60, and 80%) from the ultimate load carried by the reference beam. The experimental results show that decreasing the damage percentage leads to an increase in ultimate strength from about 3.6% to 17.2% for the beams repaired using the EBR technique and from 27.6% to 57% for the beams repaired using the NSM technique; additionally, the NSM method was more effective than the EBR method in terms of the flexural strength and mode of failure. However, using CFRP laminates enhances the flexure capacity of strengthened RC beams.

Finite Element Analysis of Flexural Strengthening of Reinforced Concrete Beams Using Near-Surface Mounted CFRP Rebars

Most of the standing structures today have already deteriorated or are on the verge of deterioration due to an increase in their age and harsh environmental conditions. For this very reason, the structures are in need of being upgraded or replaced. Retrofitting using near-surface mounted (NSM) fiber-reinforced polymer (FRP) rods has now emerged as a promising technique for strengthening the reinforced concrete members by increasing their flexural strength. This study aims to numerically investigate the efficiency of the NSM technique for flexural strengthening of RC beams by using finite element (FE) software, ABAQUS, and also evaluate the impact of variables such as concrete strength, embedded length of FRP rods, filler material used for the FRP rods, and the number of FRP rods provided. Validation of the finite element model was confirmed by first making a comparison with the experimental study presented in the literature for an un-strengthened beam. Thereafter, the validated model was used to simulate reinforced concrete beams strengthened with the NSM technique. The numerical results of the cracking moment, steel yielding moment, ultimate bending moment, and deflection at failure were reported and the impact of the variables was evaluated. The FE analysis results indicated that for the RC beams strengthened with NSM FRP rods, the flexural capacity significantly increased compared to the control beam, while the mid-span deflections of strengthened beams at failure decreased compared to the control beam.

NSM and hybrid techniques for retrofitting corroded RC beams using GFRP bars

This is an experimental study on the flexural performance of corroded reinforced concrete (RC) beams strengthened by three different methods using Glass Fibre Reinforced Polymer (GFRP) bars. Three methods of strengthening were used: the traditional method of replacing damaged concrete covers and installing GFRP bars, the near-surface mounted (NSM) method, and a hybrid method involving mechanical fasteners and the NSM method. In total, seventeen RC beams were constructed one of which was used as a control specimen, while the remaining sixteen were divided into four groups and were exposed to accelerated corrosion to reach a theoretical mass loss of 3.5 to 30% in tensile steels. Three groups of corroded beams were strengthened using the three methods mentioned, while the other group received no strengthening. Then, all the beams were tested under four-point loading to determine their load-deflection response, stiffness, ductility, failure mode, and strain of the GFRP bars. The results showed that the conventional strengthening method using GFRP bars was relatively efficient in restoring the load-bearing capability of corrosion-damaged beams. However, sufficient anchorage length for the GFRP bars was required to achieve high ductility. The NSM method effectively improved the load-bearing capacity of beams with low corrosion levels; however, increasing the corrosion level caused the strengthened beams to fail prematurely due to concrete cover separation. It was observed that the hybrid strengthening method improved the load-bearing capacity of corroded beams while maintaining their ductility by preventing premature failure due to concrete cover separation.

Experimental and numerical investigation on the flexural performance of RC slabs strengthened with EB/NSM CFRP reinforcement and bonded reinforced HSC layers

In this paper, one-way reinforced concrete (RC) slabs were strengthened using External Bonding (EB) and Near Surface Mounted (NSM) techniques and tested under flexural loading until failure. The area of internal and external reinforcement, shape of strengthening elements (bars, strips, and reinforced high strength concrete (HSC) layers) and strengthening method (EB and NSM) were the experimental tested variables. The experimental results showed significant increases in the load capacities of the upgraded slabs ranging between 133.8% and 264.5% compared to their corresponding un-strengthened slabs. Consequently, the NSM technique showed higher load efficiency than the EB, even though they had nearly the same strengthening properties and area. Compared to the control slab, using NSM strips instead of EB strips or NSM bars amplified the slab load capacity by about 51.7% and 33.2%, respectively, while using NSM HSC layer instead of EB HSC layer increased slab load capacity by 74.7%. Moreover, doubling the area of EB strips improved the load capacity by 10.3%, while increasing the internal steel area by 195% enhanced the slab load capacity by 21% to 48 %, depending on the strengthening method. However, depending on the distance between internal and external strengthening elements the cracks were initiated at the ends of the strengthening element causing the slab shear failure at dissimilar loads. Finally, a verified finite element (FE) model was built to study the effect of bond length, material, and area of the strengthening element on the slab behavior.

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.

Potentiality of far surface-mounted reinforcement for flexural strengthening of reinforced concrete beam

ABSTRACT This paper explores the potentiality of a proposed strengthening technique named as Far Surface Mounted (FSM) reinforcement for flexural strengthening of RC beam. In this technique the reinforcement is mounted in the tension face of the beam away from the existing beam surface and cast new concrete to increase the flexural capacity of the beam. Results of total 14 tested beams are analysed and presented here in this paper. To evaluate the performance of the new strengthening technique, the load-deflection behaviours of the beams strengthened with new technique are compared with the results of control beam and another beam strengthened with Near Surface Mounted (NSM) reinforcement method. It was found that the beam strengthened with new technique has higher flexural capacity than the control beam and beam strengthened with NSM technique. The influence of shear key spacing and inter-surface locking behaviour are also explored. In most of the cases, it found that the final failure modes are the shear and deboning of the added layer of concrete for the beams strengthened with new technique.

Effect of main and NSM reinforcing materials on the behavior of the shear strengthened RC beams with NSM reinforced HSC layers and bars

It became common practice to utilize near-surface mounted (NSM) bars to improve the shear capacity of steel-reinforced concrete (RC) beams. Consequently, carbon fiber-reinforced polymer (FRP) is usually used to improve RC beams' shear performance. Conversely, the use of Glass FRP (GFRP) for enhancing the shear behavior of the steel-RC and GFRP-RC beams is still limited. This study used NSM GFRP or steel bars to strengthen RC beams reinforced in tension with steel or GFRP bars. In addition, a new NSM technique was conducted using high-strength concrete (HSC) layers reinforced with GFRP or steel bars bonded within grooves to enhance beams' shear capacity. The results of seventeen beams used in the tests showed that NSM HSC layers improved the shear performance of RC beams more than NSM bars. Also, the beams reinforced in tension with steel bars had higher shear efficiency and were stiffer than those reinforced with GFRP bars. Conversely, the shear efficiency of steel RC beams strengthened in shear with internal stirrups or NSM reinforcement (or both) increased by 142.8–211.7% compared to GFRP RC beams without internal and NSM shear reinforcement. Conversely, the shear capacity of the steel-RC beams strengthened in shear with internal stirrups or NSM reinforcement (or both) increased by 153.5–279.9% compared to the corresponding beam without GFRP RC beam without internal and NSM shear reinforcement. The numerical analysis using the ABAQUS program showed great effects of the tension reinforcement and the NSM materials on the strain of longitudinal bars, stirrups and NSM reinforcement.

Experimental study on flexural strengthening of RC beams with NSM technique by different orientation of CFRP laminate

This paper gives the brief description on the use of laminates manufactured using Carbon Fiber Reinforced Polymer (CFRP) as strengthening material for the purpose of assessing the flexural strength in RC beams using Near Surface Mounted (NSM) technique. The main objective of this work was to assess the strengthened RC beam by considering the orientation of laminate as the main priority in NSM strengthening. This study gives the brief description about the utilization of tensile strength of laminate and efficiency of adhesive for different orientation of laminate. The strengthening methodologies adopted in this research work are Near Surface Mounted Laminate with Vertical Orientation (NSMLV), Near Surface Mounted Laminate with Vertical Orientation having Anchorages (NSMLVA) and Near Surface Mounted Laminate with Horizontal Orientation (NSMLH). The dimension of beam used was 200 mm (Breadth) X 200 mm (Depth) X 2300 mm (Length) and the effective span of the beam was considered as 2100 mm. All beams were designed as per Indian design code requirement to operate in under reinforcement criteria. A total of eight beams were casted, out of which two were used as control beams and the remaining were strengthened with different NSM techniques. The groove size used for NSMLV and NSMLH technique were different. The premature debonding takes place in NSMLV technique by not utilizing the tensile strength of laminate to greater extent, to overcome this drawback CFRP fabrics were used as the anchorages in four critical locations where debonding is predominate and this method was expressed as NSMLVA technique. This work was mainly focused on improving the ultimate load carrying capacity by satisfying the serviceability requirement as per codal provisions. The four-point bending test was performed for all the test specimens. In comparison with the control beams, all strengthened beams showed improved ultimate loads by satisfying the serviceability requirement. The ultimate load shows an increase of 21.74 %, 40.22% and 71.74% for NSMLV, NSMLVA and NSMLH technique respectively compared to control beams. The Stiffness of beam was increased by 14% and 30% respectively for NSMLVA and NSMLH technique compared to control beams. NSMLH technique maximizes ultimate load, which is 71.74% higher, and fulfills the serviceability requirements. In comparison with NSMLV and NSMLVA, the NSMLH Technique is the most effective method of strengthening which bears debonding to maximum extent and increases ultimate load by greater margin.

Flexural behavior of strengthened concrete beams with multiple retrofitting systems

Due to material deterioration, structure age, changes in building function, fault design, and decreased structural reliability brought on by natural catastrophes, it may require to strengthening the reinforced concrete (RC) beams. The entire carbon footprint is increased by the usage of building materials like cement. Therefore, restoring/improving their overall functionality may be accomplished more sustainably by strengthening and repairing damaged members. In this study, a series of RC beams were tested in three-point bending to determine the ability of various strengthening techniques to improve the beams' flexural capacity. Three strengthening techniques were proposed: External bonding with glass fiber-reinforced polymer (GFRP) laminates, near-surface mounted (NSM), and U-jacket. The experimental study compared the various strengthening methods in terms of ultimate loads, crack loads, maximum deflections, gains in ductility, and energy absorption. The finite element program (ABAQUS) was used for the numerical computations because it is capable of properly simulating experimental research on the flexural behaviour of reinforced concrete beams. Compared to all techniques, the strengthening using GFRP in the different techniques (GFRP laminate, NSM, and U-jacket) propagation of cracks before failure occurred in the RC beam. The sample B3-St. plate-NSM, which used the NSM technique, had the highest peak load of 77.2 MPa compared to all samples and a 36.64% increase over the control sample B0-Control. Moreover, the sample B3-St. Plate-NSM has improved stiffness which contributed to reducing the deflection at peak load by 21.12% than the control sample. The strengthening system using steel plate as NSM showed the lowest cost ($/m`) among the other strengthening systems. The specimen B1-GFRP, which used the GFRP technique, had the highest toughness of 1099.2 kN.mm by an increase of 40.25% over the control sample. Finally, the results obtained by using numerical FE models show good agreement with the experimental results.

Bond of FRP rods in damaged RC beams strengthened with NSM technique

In strengthening reinforced concrete (RC) elements with composite materials, the role of adherence between concrete and FRP assumes great importance as it guarantees the effectiveness of the consolidation method. In this paper the bond mechanisms between FRP rods and concrete second the Near Surface Mounted (NSM) technique have been analyzed by experimental results of investigation and a theoretical linear model. The experimental investigation consists of direct pull-out tests on RC elements with circular GFRP and CFRP rods inserted into a groove. Through pull-out tests main parameters of bond have been presented. The experimental results are compared with those obtained by the theoretical model.

Deep embedment and NSM techniques for shear strengthening of reinforced concrete slender beams with cFRP ropes

Shear failure is catastrophic and occurs usually without advance warning, thus it is desirable that the beam fails in flexure rather than in shear. Many existing reinforced concrete beams are found to be deficient in shear strength and need to be strengthened. Reinforced concrete beams can be strengthened against shear failure by using polymer composites reinforced with fibres in many different techniques. This study aims to the most efficient shear strengthening of beams with carbon fibre reinforced polymer (cFRP) between near-surface mounted (NSM) technique and the deep embedment (DE) technique. Reinforced concrete beams with dimensions 15x15x70 cm, with the use of classified C30/37 concrete were constructed, each of which was reinforced with 3Ø10 B500c steel bars, only under bending. The specimens were tested in shear under two-point bending loading conditions, after having been strengthened with NSM or DE technique with cFRP rope by various reinforcing arrangements. As main conclusion, it appears that the NSM strengthening technique gives lower values than DE strengthening technique in correspondingly schemes.

Seismic behavior of RC columns strengthened with near-surface-mounted aluminum alloy bars and CFRP wraps

This paper proposed a near-surface-mounted (NSM) strengthening technique by using aluminum alloy (AA) bars and carbon fiber-reinforced polymer (CFRP) wraps to improve the seismic behavior of square reinforced concrete (RC) columns. Eight strengthened concrete columns with square cross sections of 250 mm × 250 mm and 1600 mm shear span were tested under axial compression and reversal cyclic loading to investigate the effects of NSM reinforcement ratios (0.36–1.29%), layers of CFRP wraps (0 and 3), and axial compression ratios (0.1 and 0.4) on the seismic behavior. The failure mode, hysteretic response, load-bearing capacity, ductility, stiffness degradation, and energy dissipation capacity of those column specimens were analyzed. Results indicated that the lateral load-bearing capacity, stiffness, and cumulative energy dissipation capacity increased with the increase of the NSM reinforcement ratio. The CFRP wraps in the NSM technique can provide hoop confinement to avoid the buckling failure of NSM AA bars, and improve the seismic behavior by mitigating spalling of concrete. When increasing the axial compression ratio from 0.1 to 0.4, the lateral load-bearing capacity and cumulative energy dissipation capacity increased significantly, while the stiffness degradation became obvious. In addition, three-dimensional (3D) finite element (FE) models were developed and verified by comparisons in the deformation and lateral loads at characteristic points, as well as the strain distribution along NSM AA bars. The verified FE models can be used to analyze the damage of concrete and the stress distribution at the interface between epoxy and concrete.

Using NSM pre-stressed rebars for flexural strengthening of RC beams subjected to constant loading

The present study introduces an innovative technique for flexural strengthening of reinforced concrete (RC) beams subjected to constant loading and explores its efficacy through experimental evaluation. The technique involves the application of near-surface mounted (NSM) pre-stressed rebars, which exploits the advantages of both the NSM technique, as an effective one commonly used in strengthening RC beams, and pre-stressing, as an effective technique to eliminate excessive cracking and deformation. The experimental program consists of eight identically reinforced RC beams. Before strengthening, longitudinal grooves are cut out along the tensile face of the specimens in which steel bars are placed and anchored by end plates and an epoxy adhesive. Some of the bars are pre-stressed to a predetermined level using an end bolt and a torque-meter wrench. The test results indicate that the NSM pre-stressed steel rebars lead not only to almost identical amounts of significant increases in the ultimate flexural capacity of RC beams but also to decreased initial cracking and deflection.

A review on performance of near‐surface mounted‐carbon fiber‐reinforced polymer laminates bonded into slits

AbstractThe near‐surface mounted (NSM) technique over the years has become the most popular retrofitting technique for obtaining enhanced strength and durability in reinforced concrete structural elements. It is mostly preferred for its easy application of composites in grooves cut on specimen concrete cover. The specimen surface is prepared with slits where polymer composite laminates are placed along with adhesives as filler to attain good bonding between laminates and the surface. The paper reviews the NSM carbon fiber reinforced polymer technique and its ascendancies over externally bonded composites retrofitting approach. Members strengthened with the NSM technique show enhanced shear strength, flexural capacity, and fatigue resistance. Motivated by the urge of gaining knowledge on various advantages of this emerging method, the paper reviews the load‐carrying capacity, failure modes, durability, and bonding behavior along with the enhancement ascertained in shear, flexure, and fatigue resistance by the NSM technique. In addition to offering guidance on the experimental and computational investigations on the overall behavior of the technique, the present review also intends to encourage further development in the near‐surface mounted carbon fiber polymer strengthening system of retrofit.

Flexural response of FRP-strengthened lightweight RC beams: hybrid bond efficiency of L‐shape ribbed bars and NSM technique

This paper presents the results of an experimental study on employing near surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement technique, and L‐shape ribbed bars, for flexural strengthening of lightweight reinforced concrete (RC) beams. 18 RC beams including 14 lightweight RC beams and four normal-weight concrete beams were designed. The beams were strengthened with glass fiber-reinforced polymer (GFRP) bars and carbon fiber-reinforced polymer (CFRP) laminate in bending tests. Test parameters included: (1) different FRP materials (glass bars and carbon sheets), (2) longitudinal steel reinforcement ratio, and (3) type of strengthening technique used (NSM reinforcement or hybrid). The ultimate tensile strength, deflection, compressive and tensile strain of concrete, and failure mode of the beams were examined under four-point flexural test. Results showed that the ultimate strength of all RC beams increased between 33 and 105% compared to the control beam. The ultimate strength of beams reinforced with CFRP in the mid-span region was 10% higher than that of beams strengthened at both ends, although the former exhibited 28% lower ultimate deflection. The ultimate strength and deflection of RC beams strengthened with combined steel reinforcing bars and GFRP bars were 10% and 108% higher, respectively, compared to those of RC beams strengthened with GFRP bars only. Hybrid L‐shape ribbed bars beams showed a considerably higher ductility (up to 170% increase in the ultimate deflection) compared to other beams. The comparison of the experimental results of the ultimate strength of the beams with ACI440-2R guidelines indicated a reasonable and conservative prediction of the code expression.