NSM-CFRP Strips Research Articles

Regaining Structural Performance of Thermally Damaged Beams Using NSM CFRP Strips: A NLFE Study

Regaining Structural Performance of Thermally Damaged Beams Using NSM CFRP Strips: A NLFE Study

Effects of the embedding depth on the interfacial bond performance of horizontally NSM CFRP strips to concrete structures

Horizontally NSM CFRP strip reinforcing concrete structures are a new type of reinforcing method, as they require a small groove depth compared with that of the traditional NSM strengthening method. In order to explore the effect and mechanism of the embedding depth of horizontally NSM CFRP strip on interfacial bond performance, the single shear pull-out test was used to compare and analyze the bond properties of concrete specimens reinforced with horizontally NSM CFRP strips, EB CFRP strips, and NSM CFRP strips, and explore the effect of horizontally NSM reinforcement methods. Then, the influence of the embedding depth on interface bonding was studied. Finally, the finite element software was used to establish a finite element model for numerical simulation to verify the test results of horizontally NSM CFRP strips. The results show that the specimens reinforced with horizontally NSM CFRP strips have better bonding capacity and interface bonding stiffness. The embedding depth of the CFRP strip affects the bond efficiency between the CFRP strip and concrete, and the interfacial bonding performance is the best when the embedding depth is 12 mm. The maximum bond capacity, strain distribution, and load-end slip curve of CFRP strip under finite element numerical simulation are in good agreement with the experimental results. This indicates that the bonding performance between the horizontally NSM CFRP strip and concrete can be analyzed by the finite element method. This provides a theoretical basis for the application of horizontally NSM CFRP strip reinforcement technology in practical engineering.

Effect of maximum coarse aggregate size upon shear strengthening of RC beams using NSM-CFRP strips

Reinforced concrete (RC) is widely used because of its many desirable characteristics, such as high strength, fire resistance, and durability. However, structures that are made of concrete may need strengthening or repairing due to concrete deterioration, the harshness of the environment, steel corrosion, errors in design, and damage caused by earthquake load. The use of composite materials has recently become more popular for strengthening and repairing damaged concrete structures. It is also known that properties of the aggregates, such as shape, size, and surface texture, have important influences on the behavior of concrete. In fact, many studies have shown that changes in coarse aggregate will affect the concrete's strength and fracture properties. Towards this end, very little research has been carried out on the effect of maximum aggregate size (MAS) on the shear behavior of reinforced concrete beams. The impact of maximum coarse aggregate size on shear strengthening using the methods proposed in this study was not studied. This research is driven by both environmental and economic aspects where the use of large-size coarse aggregates provides a large volume and less void space compared to small-size aggregates. From an environmental perspective, such an approach will reduce the need for cement in concrete unit volume, hence reducing the pollution. The main purpose of this study is to examine the behavior of reinforced concrete beams under general loading and how this is affected by using different coarse aggregate sizes. This work’s main objective is to investigate the effect of maximum coarse aggregate size on the shear behavior of reinforced concrete beams and study the effectiveness of using the Near Surface Mounted (NSM) – Carbon Fiber Reinforced Polymer (CFRP) strips technique on the shear strengthening of the RC beams. The experimental variables of the study consist of different maximum coarse aggregate sizes (9.5 mm, 12.5 mm, 19 mm, and 25 mm), Carbon Fiber Reinforced Polymer (CFRP) strips spacing (75 mm and 150 mm), and concrete compressive strength (25 MPa and 40 MPa). Fourteen RC beams (150 mm × 200 mm × 1150 mm) were cast using six mixes of concrete to be prepared and tested. The experimental results showed that using maximum coarse aggregate size has a positive effect on ultimate shear strength, maximum deflections, toughness, and stiffness of strengthened RC beams. Using NSM-CFRP strips change the failure mode from pure shear for the control beams to shear-flexural failure for the strengthened beams, which is an ideal enhancement.

Repair of thermally shocked reinforced concrete beams using near-surface mounted – Carbon fiber reinforced polymers ropes and strips

The use of near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) ropes and strips is a recent and promising repairing/strengthening technique for improving the mechanical properties and structural performance of damaged reinforced concrete (RC) members. This study aimed to evaluate the behavior of RC beams after being damaged by thermal shock and the efficiency of using NSM-CFRP strips and ropes as repairing techniques. Therefore, eleven RC beams with dimensions of (150 mm in width × 200 mm in depth × 1150 mm in length) were cast. Nine of the specimens were heated to a temperature of 600 °C for three hours, cooled rapidly by being immersed in water for 15 min, and repaired with different configurations of NSM-CFRP strips and ropes and CFRP sheets. All specimens were tested under a four-point loading test. Three main parameters were studied, namely: NSM-CFRP type, configuration, and the number of U-wrapping ropes used. Experimental results showed a significant reduction in the concrete’s compressive strength with a percent of about 49% after being damaged by thermal shock. The thermally shocked RC beam exhibited a significant degradation in the mechanical and structural performance represented by a reduction in the ultimate capacity, stiffness, and toughness of the beam with a percentage of about 14.4%, 39%, and 11.8% if compared with the control beam, respectively. Also, using an efficient repairing technique can improve the properties of these damaged beams, especially when using NSM-CFRP ropes with U-wrapping CFRP sheets which contributed to the increase in the ultimate load capacity, stiffness, and toughness by a percent of about 54.7%, 23.8%, and 32.8% if compared with the thermally shocked beam, respectively.

Durability of Bond between NSM CFRP Strips and Concrete under Real-Time Field and Laboratory Accelerated Conditioning

Durability of Bond between NSM CFRP Strips and Concrete under Real-Time Field and Laboratory Accelerated Conditioning

Development of interaction diagrams of reinforced concrete short columns strengthened by NSM-CFRP strips using finite element method

PurposeThe main aim of this study is to examine the behavior of reinforced concrete short columns strengthened using longitudinal near surface mounted (NSM)-carbon fiber reinforced polymer (CFRP) strips.Design/methodology/approachA full 3D-finite element (FE) model was developed using ABAQUS in order to conduct the analysis. The model is first validated based on experimental data available in the literature, and then the effect of concrete compressive strength, number of CFRP strips that are used and the spacing between them were taken in consideration for both concentric and eccentric loading cases. The parametric study specimens were divided into three groups. The first group consisted of unstrengthened columns and served as control specimens. The second group consisted of columns strengthened by longitudinal CFRP strips at two opposite column faces.FindingsThe results of this study are used to develop interaction diagrams for CFRP-strengthened short columns and to develop best-fit equations to estimate the nominal axial load and moment capacities for these strengthened columns. The results showed that the specimens that were strengthened using more longitudinal CFRP strips showed a significant increase in axial load capacity and a significant improvement in the interaction diagram, especially at large load eccentricity values. This result can be justified by the fact that longitudinal strips effectively resist the bending moment that is generated due to eccentric loading. Generally, the process of strengthening using longitudinal strips only has a reasonable effect and it can be typically considered an excellent choice considering the economic aspect when the budget of strengthening is limited.Originality/valueThis research aims at studying the performance of strengthened rectangular reinforced concrete short columns with CFRP strips using FE method, developing interaction diagrams of strengthened columns in order to investigate the effect of different parameters such as compressive strength (20, 30 and 40 MPa), number of CFRP strips (1, 2, 3 and 4) and the spacing between CFRP strips in terms of the ratio of CFRP center point distance to column outside dimension ratio (0.60, 0.70 and 0.80) on the behavior of strengthened RC columns and improving empirical formulas to predict the nominal axial load and moment capacities of strengthened RC columns. These parameters that directly affect short column load carrying capacity are presented in ACI-318 (2014).

Response of Reinforced Concrete Tapered Beams Strengthened Using NSM-CFRP Laminates

The present study is about an experimental investigation of the strength and response of reinforced concrete tapered beams strengthened by near surface mounting (NSM)-CFRP technique. The influence of several parameters on the behavior of beams is investigated. And those parameters are: angle of inclination and strengthening patterns by NSM-CFRP technique. The experimental program involved testing nine specimens of reinforced concrete tapered beams and one prismatic beam. There were two groups in the experimental program; the first group included four unstrengthening reference beams to investigate the effect the angle of inclination, and the second group included six specimens were strengthened by NSM-CFRP strips with different strengthening patterns (U-strips) and (UL-strips) were used to strengthen the tapered beams against shear and flexural forces. The results of the experiments indicated that as the angle of inclination increased, from (0˚) to (7°), (14°) and (21°) decreases the ultimate load capacity (Pu) by about (4)%, (14)% and (41)% respectively for unstrengthen tapered beams, as well as the deflection at the ultimate load. Also, when compared unstrengthen with strengthened beams, the ultimate load capacity will increase with the angle of inclination increased, from (0˚) to (7°), (14°) and (21°), for (U-strips) patterns the percentages of increase were (7)%, (9)% and (33) % respectively, while (UL-strips) increased the ultimate load capacity by about (19)%, (19)% and (44)% respectively.

Seismic performance of reinforced concrete beam-column joints strengthened with NSM steel bars and NSM CFRP strips

Three strengthening methods for RC beam-column joints using NSM steel bars and NSM CFRP strips were experimentally evaluated by cyclic loading. These methods increase joints capacity and prevent the plastic hinge formation near the column while try to minimize destructive task on floor slab. Strengthened specimens and a control specimen of RC beam-column joint from an intermediate moment-resisting frame were constructed on one-half scale and subjected to cyclic loading according to ACI 374.1-05 simultaneously with a constant column’s axial load to study their seismic behavior. Experimental results indicated that the proposed methods could increase joints capacity up to 30% compared to the control specimen and two of them improve joints ductility by removing the plastic hinge from the joint core. In addition, the energy dissipation of the retrofitted specimens was increased up to 100% and they experienced less pinching hysteresis behavior. The debonding of CFRP strips in one of specimens showed that the penetration depth of CFRP strips into the core should be increased to avoid this phenomenon. In addition, the anchorage method of the added steel rebar inside the joint core worked well and forces were transferred to the column properly.

Models for Predicting Strength of RC Columns Strengthened with NSM-CFRP Strips and CFRP-Fabric Wraps

A comprehensive experimental investigation was carried out on reinforced concrete (RC) columns strengthened with near surface mounted (NSM) Carbon fiber reinforced polymer (CFRP) strips with CFRP fabric wraps as confinement. The number of NSM-CFRP strips and the number of CFRP-Fabric wraps were varied. The RC columns were subjected to concentric and eccentric loads with different eccentricities that resulted in uniaxial bending. Significant improvements in strength and ductility were observed in the strengthened columns compared to that of the control specimen. In this investigation four empirical models, based on the experimental results, were developed to predict the un-strengthened and the NSM-CFRP strengthened RC columns using different parameters. The capacities of the columns predicted by the developed empirical models were validated using the obtained experimental results. It is observed that the developed empirical models have exponential decay and a near-perfect fit to the experimental results, with R2 averaging more than 99% and an average mean absolute percent error (MAPE) of 6.6%. It is concluded that the developed models can be used as useful tools for predicting capacity of columns with NSM-CFRP strips and CFRP wraps under axial loading and uniaxial bending.

Performance of Strengthened/Repaired Heat-Damaged One-Way Self-Compacted Concrete Slabs with Openings Using Nsm-Cfrp Strips

Performance of Strengthened/Repaired Heat-Damaged One-Way Self-Compacted Concrete Slabs with Openings Using Nsm-Cfrp Strips

Behavior of RC Beams Strengthened with NSM-CFRP Strips Subjected to Fire Exposure: A Numerical Study

The use of Near-Surface Mounted (NSM) Carbon-Fiber-Reinforced Polymer (CFRP) strips is an efficient technology for increasing flexural and shear strength or for repairing damaged Reinforced Concrete (RC) members. This strengthening method is a promising technology. However, the thin layer of concrete covering the NSM-CFRP strips is not adequate to resist heat effect when directly exposed to a fire or at a high temperature. There is clear evidence that the strength and stiffness of CFRPs severely deteriorate at high temperatures. Therefore, in terms of fire resistance, the NSM technique has a significant defect. Thus, it is very important to develop a set of efficient fire protection systems to overcome these disadvantages. This paper presents a numerical study that investigates the fire behavior of thermally insulated RC beams flexurally strengthened with NSM-CFRP strips and subjected to fire exposure according to the ISO 834 standard. The numerical study considered three-dimensional finite element models in the ABAQUS software that have been developed to simulate and predict the performance (thermal and structural response) of fire endurance tests on strengthened, uninsulated strengthened, and thermally insulated beams strengthened with NSM-CFRP strips, which were exposed to fire and had different fire insulation schemes. The insulation used was plaster from local material with a thickness range of 25 to 50mm. The variation of the thermal and mechanical properties with the temperature of the constituent materials was considered. All beams' mechanical and thermal responses were adequately simulated using numerical models. The results of the numerical simulations were in good agreement with the experimental data. The fire behavior of the NSM-CFRP strengthened RC beams was examined and particularly the efficiency of the NSM strengthening system during the fire. The behavior in the fire of the NSM-CFRP strengthening system on the RC beams thermally protected with different fire insulation schemes was assessed. Finally, the effectiveness of fire insulation was studied.

Experimental investigation for shear strengthening of reinforced self-compacting concrete beams using different strengthening schemes

This paper investigates the performance of four different strengthening techniques used to improve the shear capacity of RC beams. Three of the four strengthening techniques used were near-surface mounted (NSM) using carbon fiber-reinforced polymer (CFRP) strips, welded wire mesh (WWM) and textile reinforced epoxy mortar (TREM). The fourth strengthening technique consisted of a u-shape jacket made of ultra-high performance fiber-reinforced concrete (UHPFRC). Seven RC beams made of self-compacted concrete (SCC) were tested using a four-point bending setup. One beam was designed with sufficient shear reinforcement and served as the control in this study while the remaining six beams were cast with sufficient shear reinforcement in one half and no shear reinforcement in the other half. One beam was tested as a shear deficient control without any strengthening. The remaining five beams were strengthened with NSM-CFRP strips at 90°, NSM-CFRP strips at 45°, NSM-TREM strips, NSM-WWM strips and UHPFRC jacketing. The test results of all proposed strengthening techniques showed an increase in the shear strength of the RC beams by 47%–56% compared to using steel web reinforcement. The increase in the shear capacity of RC beams using 15 mm UHPFRC jacket was comparable to the other strengthening techniques. The UHPFRC jacket is considered a promising technique that could be utilized in the repair and rehabilitation of shear deficient RC beams. In addition to increasing shear capacity, the strengthened RC beams' mode of failure altered from brittle shear failure to ductile flexural failure with increased stiffness. Analytical modeling was conducted to predict the shear capacity of tested beams, with results that closely matched experimental observations.

Behavior of RC Beams Strengthened with NSM-CFRP Strips Exposure to Fire

The behavior of reinforced concrete beams strengthened with Near-Surface-Mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) strips using epoxy adhesive and insulated protection schemes after being exposed to fire was investigated in this paper. The following four beams will be subjected to an experimental test: As a reference specimen, a reinforced concrete beam flexural strengthened with NSM-CFRP strips using epoxy adhesive will be tested to failure at ambient temperature. Three RC beams were flexural reinforced with NSM-CFRP strips using epoxy adhesive, one of which was left uninsulated, while the other two were insulated with plastering, with the protection schemes including a thinner insulation layer along the bottom face of the beams (25 mm) and a thicker one at the CFRP anchorage areas (25 to 50 mm). These specimens will be exposed to fire in a gas furnace according to the ISO 834 standard fire curve, and adjustments will be observed after the specimens have been heated. In general, they were all heated without any load and then allowed to cool to room temperature. They were then subjected to four-point bending (simply supported and two-point load) until their failed. The fire behavior of NSM-CFRP reinforced RC beams will be investigated, with an emphasis on the NSM strengthening system's effectiveness during a fire. The conduct of the NSM-CFRP strengthening system on RC beams thermally insulated with fire protection schemes, on the other hand, will be evaluated in the event of a fire. Finally, the fire insulation's efficiency will be investigated.

Effect of Different Strengthening Techniques on the Structural Behaviour of Continuous Two-Way Slabs with Partially Corroded Reinforcement at Internal Support

This research studies the combined effects of partial corrosion of the negative reinforcement in two-way continuous slabs (two spans) with a construction joint at the internal support and investigates the efficiency of the proposed strengthening methods to overcome these effects. The partial corrosion in reinforcement comes from exposure to the corrosive outdoor environment due to an unplanned stoppage in concrete pouring for a long-time which left part of the negative reinforcement without protection. The construction joint formed at the internal support after resume the concrete pouring later. Three types of strengthening are proposed to overcome those effects. The study adopted an accelerated corrosion technique which is running an electrical current in the steel bars partially immersed in a 5% salt solution (elect-chemical cell). The proposed strengthening was: NSM CFRP bars, CFRP strips, and steel plates. All applied to the top face in the tension zone at the internal support. Six two-way continuous (two spans) concrete slabs were cast (2200, 1000, and 100 mm). The strengthening was: four strips of CFRP had 50 mm width, four of 6 mm CFRP bars applied using the NSM technique, and four steel plate 50 mm width and 5 mm thickness. All the strengthening extended to the third of span length and was aligned with the slab continuity axis. The specimens were tested and the deflection, first positive and negative crack width, and failure mode were recorded. Numerical analysis and comparison with the experimental results were conducted to verify the numerical model. The study found that the construction joint and corrosion had a clear influence on the structural behavior of the slab. Those effects were on the ultimate load, deflection, and the negative cracks. The proposed treatments were highly effective and efficient in increasing the ultimate load, delay the appearance of cracks, and reduction of deflection under the load points.

RC Columns Strengthened with NSM-CFRP Strips and CFRP Wraps under Axial and Uniaxial Bending: Experimental Investigation and Capacity Models

Abstract The efficiency of the use of near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips confined with CFRP wraps to strengthen reinforced concrete (RC) columns will be experi...

Response of Damaged Reinforced Concrete Beams Strengthened with NSM CFRP Strips

This paper presents a study (experimentally) for strengthening reinforced concrete (RC) beams with Near-Surface-Mounted (NSM) technique. The use of this technique with CFRP strips or rebars is an efficient technology for increasing the strength for flexure and shear or for repairing damaged reinforced concrete (RC) members. The objective of this research is to study, experimentally, RC beams either repaired or strengthened with NSM CFRP strips and follow their flexural behavior and failure modes. NSM-CFRP strips were used to strengthen three RC beam specimens, one of them was initially strengthened and tested up to failure. Four beam specimens have been initially subjected to preloading to 50% and 80% of ultimate load. Two of the specimens were either repaired or strengthened with NSM-CFRP strips. All the repaired/strengthened pre-damaged beams have been tested up to failure by using compression-testing machine. An appropriate-scale model was adopted. All the specimens have a cross-sectional dimension of 150 mm with an effective span of 110 mm. Depends on the experimental results, a better performance of the strengthened concrete specimens was obtained in both strength and serviceability. As a comparison with the control beam specimen, all the repaired specimens show a very good increase of about 40% in the load-carrying capacity and a high improvement in resistance to cracking of about 120% in NSM. On the other hand, the test results of NSM CFRP-strengthened concrete specimens with a preloading of 50% and 80% of the ultimate load show an increase of about 9% to 20% in the load-carrying capacity, for 50% and 80% pre-loading, respectively an improvement in deflection of about 2% to 27% in NSM, for 80% and 50% pre-loading, respectively.

New anchorage technique for NSM-CFRP flexural strengthened RC beam using steel clamped end plate

Using appropriate techniques, or systems, is mandatory to strengthen those elements, such as the Near Surface Mounted (NSM)-Carbon Fiber Reinforced Polymer (CFRP). The goal of this paper is to find a new mechanical anchor system to clamp the NSM-CFRP strip at the end of the beam and prevent the end cover separation of the CFRP strip to increase the ultimate beam capacity load. This technique is based on using a steel plate at the end of the beam. A total of eleven reinforced concrete beams were constructed with concrete compressive strength of 35 MPa and dimensions of 250 mm × 200 mm × 1600 mm (height × width × length) and divided into three groups (control samples, beams with CFRP, and beams with CFRP strips and plate steel clamped). The beams were strengthened with one, two, and three CFRP strips. The main parameters studied were the type of NSM-CFRP strip fixation technique and the number of NSM-CFRP strips used. The test results show that the use of a steel plate at the end with horizontal CFRP strips in the beam is more efficient in increasing the ultimate load capacity, deflection behavior, stiffness, and toughness compared to NSM-CFRP strengthened beams.

Experimental and Numerical Study of Behaviour of Reinforced Masonry Walls with NSM CFRP Strips Subjected to Combined Loads

Near surface mounted (NSM) carbon fibers reinforced polymer (CFRP) reinforcement is one of the techniques for reinforcing masonry structures and is considered to provide significant advantages. This paper is composed of two parts. The first part presents the experimental study of brick masonry walls reinforced with NSM CFRP strips under combined shear-compression loads. Masonry walls have been tested under vertical compression, with different bed joint orientations 90° and 45° relative to the loading direction. Different reinforcement orientations were used including vertical, horizontal, and a combination of both sides of the wall. The second part of this paper comprises a numerical analysis of unreinforced brick masonry (URM) walls using the detailed micro-modelling approach (DMM) by means of ABAQUS software. In this analysis, the non-linearity behavior of brick and mortar was simulated using the concrete damaged plasticity (CDP) constitutive laws. The results proved that the application of the NSM-CFRP strips on the masonry wall influences significantly strength, ductility, and post-peak behavior, as well as changing the failure modes. The adopted DMM model provides a good interface to predict the post peak behavior and failure mode of unreinforced brick masonry walls.

Influence of elevated temperatures on the bond behaviour between concrete and NSM-CFRP strips

This paper presents an experimental and numerical study about the effects of elevated temperatures on the bond behaviour between concrete and carbon fibre reinforced polymer (CFRP) strips installed according to the near-surface mounted (NSM) technique. In the first part of this study, single-lap shear tests were performed on concrete prisms strengthened with CFRP strips inserted into slits pre-cut in the concrete cover and bonded with an epoxy-based adhesive. The specimens were first heated up to the target temperature (from 20 to 270 °C, measured in the bonding adhesive) and then loaded up to failure. The results obtained show that with increasing temperatures (i) the bond strength is significantly reduced, (ii) the strain distribution along the bonded length becomes closer to linear, and (iii) the failure mode changes from CFRP tensile failure at 20 °C and 50 °C to adhesive failure in the CFRP-adhesive interface at higher temperatures. In the second part of this study, based on the results obtained in the experiments, a numerical procedure was developed to determine the local bond-slip laws at different elevated temperatures. In general, the stiffness and maximum bond stress of the obtained laws decreased with increasing temperatures; in addition, the predicted load vs. slip curves fit well the experimental counterparts.

표면매입 및 외부부착탄소섬유판으로 보강된 철근콘크리트 기둥의 파괴 거동 실험 연구

본 연구는 탄소섬유판 및 탄소섬유시트로 보강된 콘크리트 기둥의 역학적 거동 및 파괴특성 등을 규명함에 목적이 있다. 이를 위하여, 본 연구에서는 탄소섬유시트, 탄소섬유판 표면부착 및 표면매입으로 보강된 콘크리트 기둥 부재를 18개 제작하였으며, 이에 대한 일축 압축 시험을 시행하였다. 실험 결과, 표면매입탄소섬유판으로 보강된 기둥은 극한강도가 현저히 증가되며, 보강하지 않은 기둥보다 128 %의 보강 효과가 있는 것으로 나타났다. 또한, 매입탄소섬유판 혹은 탄소섬유시트로 보강된 철근콘크리트 기둥은 보강되지 않은 철근콘크리트 부재보다 선형구간이 확대되며, 극한하중이 크게 증진되는 것으로 나타났다. 매입탄소섬유판으로 보강된 기둥은 연직 방향으로 발생하는 균열을 충분히 제어할 수 있지만, 표면에 부착한 탄소섬유판은 연직방향으로 발생하는 균열을 충분히 제어하지 못하며 탄소섬유판이 콘크리트 계면에서 분리되는 취성 파괴가 나타났다.