Near-surface Mounted Carbon Fibre Reinforced Polymer Strips Research Articles

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.

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.

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.

Influence of Compressive Strength of Concrete on Shear Strengthening of Reinforced Concrete Beams with Near Surface Mounted Carbon Fiber-Reinforced Polymer

This paper investigates the effect of using near-surface mounted carbon fiber-reinforced polymer (NSM-CFRP) on the shear strengthening of rectangle beams with low strength concrete (f′c = 17 MPa), medium strength concrete (f′c = 32 MPa), and high strength concrete (f′c = 47 MPa). The experimental program was performed by installing NSM-CFRP strips vertically in three different configurations: aligned with the internal stirrups, one vertical NSM-CFRP strip between every two internal stirrups, and two vertical NSM-CFRP strips between every two internal stirrups. All tested beams were simply supported beams and tested under a three-point loading test. The experimental results were compared with the theoretical capacities that were calculated according to the ACI 440.2R-17 and finite element analysis (FEA) that was conducted using ABAQUS software to simulate the behavior of all beams. The experimental results indicated that using NSM-CFRP limited the failure mode of all beams to pure shear failure with no debonding or rapture of the carbon strips. Moreover, the use of NSM-CFRP proved its efficiency by increasing the shear capacity of all beams by a range of 4% to 66%, in which the best enhancement was recorded for the case of using two unaligned NSM-CFRP strips. In general, the experimental shear capacities increased with the increase in the compressive strength of all beams. On the other hand, the ACI 440.2R-17 was conservative in predicting the theoretical shear capacities, and the FEA results agreed well with the experimental results.

Bond Behaviour of Near-Surface Mounted Strips in RC Beams-Experimental Investigation and Numerical Simulations.

This paper presents an investigation of the bond mechanism between carbon fibre reinforced polymer (CFRP) laminates, concrete and steel in the near-surface mounted (NSM) CFRP-strengthened reinforced concrete (RC) beam-bond tests. The experimental program consisting of thirty modified concrete beams flexurally strengthened with NSM CFRP strips was published in. The effects of five parameters and their interactions on the ultimate load carrying capacities and the associated bond mechanisms of the beams are investigated in this paper with consideration of the following investigated parameters: beam span, beam depth, longitudinal tensile steel reinforcement ratio, the bond length of the CFRP strips and compressive concrete strength. The longitudinal steel reinforcement was cut at the beam mid-span in four beams to investigate a better assessment of the influence of the steel reinforcement ratio on the bond behaviour of CFRP to concrete bond behaviour. The numerical analysis implemented in this paper is based on a nonlinear micromechanical finite element model (FEM) that was used for investigation of the flexural behaviour of NSM CFRP-strengthened members. The 3D model based on advanced CFRP to concrete bond responses was introduced to modelling of tested specimens. The FEM procedure presents the orthotropic behaviour of the CFRP strips and the bond response between the CFRP and concrete. Comparison of the experimental and numerical results revealed an excellent agreement that confirms the suitability of the proposed FE model.

Behavior of NSM CFRP reinforced concrete columns: Experimental and analytical work

This paper presents the results of experimental testing on fifteen reinforced concrete (RC) columns repaired or strengthened using Near Surface Mounted-Carbon Fiber Reinforced Polymer (NSM-CFRP) under concentric load, eccentric load with 125 mm eccentricity (balanced state), and pure bending moment load, while considering the effect of several configurations on the interaction diagram. Furthermore, this paper investigates the efficiency of these new configurations in the capacity of the RC columns by varying the direction of CFRP strips to be used as reinforcement. The specimens of RC columns, having cross-sectional dimensions 1200 mm × 200 mm × 200 mm (length × width × height) with 150 mm corbel at both ends (in the plane of bending moment) and 1.13 % steel reinforcement ratio, were used. The specimens were divided into five groups: control group, preloaded until 80 % transversely strengthened group, preloaded until 80 % longitudinally strengthened group, preloaded until 80 % transversely and longitudinally strengthened group, and un-loaded transversely and longitudinally strengthened group. Each group included three specimens with a different type of loading as described above. The overall response of the specimens was investigated in terms of axial load carrying capacity, axial displacement, toughness, and failure modes. Furthermore, the interaction diagram for each group was developed and investigated. The strengthened and retrofitted specimens showed a significant increase in axial capacity and significant enhancement in the interaction diagram for all groups. The study showed that the most improvement in the interaction diagram, for both tension and compression control region, occurred when specimens were strengthened in both directions (transversely and longitudinally). In addition, the interaction diagrams were modified to provide a direct solution for the NSM-CFRP ratio. The modified design model was found to be capable of predicting the axial load capacity of NSM-CFRP strips columns.

Experimental study on bond behavior between heat-damaged recycled asphalt pavement concrete and NSM-CFRP strips

In this work, the effect of high temperature on the bond behavior between Recycled Asphalt Pavement (RAP) concrete and Fiber Reinforced Polymer (FRP) was studied. The use of RAP in concrete construction has been developed and utilized recently due to its various economic and environmental features. Nevertheless, very few studies have examined the bonding performance and failure mode between RAP concrete and FRP. A total of 46 asphalt pavement concrete specimens with three different concrete compressive strength were cast and tested under pullout test. The specimens were prepared using different RAP aggregate content and were subjected to elevated temperatures within a range from 23 °C to 600 °C. The heat damaged specimens were repaired using two Near-Surface Mounted Carbon Fiber Reinforced Polymer (NSM-CFRP) strips spaced at 50 mm with three different bonded lengths. According to the experimental results of the pullout test, RAP concrete specimens showed lower bonding strength and slippage than Natural Aggregate (NA) concrete specimens. Furthermore, specimens exposed to elevated temperatures showed a significant decrease in bonding strength and a significant increase in slippage. Results also indicated that as the temperature increased, specimens with high RAP aggregate content failed at higher slippage compared to specimens with low RAP aggregate content with the same compressive strength. Moreover, the relationships between bond load and strain of (CFRP) strips were established and discussed in the results. The failure mode in all specimens was concrete separation.

Experimental evaluation of factors affecting the behaviour of reinforced concrete beams strengthened by NSM CFRP strips

The bond between carbon fibre reinforced polymer (CFRP) and concrete is a significant factor that affects the load capacity of structural members strengthened by near-surface mounted (NSM) technique. In this study, several factors that affect the bond were investigated to find the optimum conditions to attain maximum performance by using this technique of strengthening. The cracking behaviour of each beam specimen was also investigated. The results showed that the groove depth, fibre shape, groove surface, and wrapping distribution were the critical factors that affected the capacity and failure mode of the specimens.

Hybrid UHPC/NSM CFRP strips vs. traditional systems for flexural upgrading of RC beams – Experimental and FE study

In this study, the efficacy of traditional versus innovative systems for enhancing the flexural strength of RC (reinforced concrete) beams is investigated experimentally as well as numerically. Four-point bending tests were conducted on seven RC beams. Test matrix comprised of two control and five strengthened beams. Strengthening techniques included: bonded steel plate, externally attached CFRP (carbon fiber reinforced polymer) composite, NSM (near-surface mounted) steel rebars, externally attached CFRCM (carbon fiber reinforced cementitious matrix) composite, and innovative hybrid system comprising of ultra-high performance concrete (UHPC) layer combined with NSM CFRP strips. Different strengthening systems were designed to provide approximately the same flexural strength enhancement. The performance of strengthened specimens was compared in terms of load–deflection characteristics. The peak load of the tested specimens was analytically predicted using the equations of ACI 318-19 code and ACI 440.2R-17 guidelines. Nonlinear FE (finite element) modeling was also carried out, and a comparison was conducted between the experimental and FE results showing good agreement. The validated FE models were extended for some useful parametric studies of interest.

Effect of FRP U-jackets on the behaviour of RC beams strengthened in flexure with NSM CFRP strips

The near-surface mounted (NSM) fibre-reinforced polymer (FRP) strengthening technique has become a viable strengthening method for reinforced concrete (RC) beams. Premature debonding failure of the NSM FRP strips is a common failure mode though that severely limits the effectiveness of the FRP intervention that results in reduced beam strength and deformability. The application of FRP U-jackets near the ends of the NSM strips offers a potential solution to the debonding problem. This paper presents the first systematic experimental investigation on the use of FRP U-jackets for delaying or preventing debonding failure in RC beams strengthened in flexure with tension face NSM FRP strips. A total of 11 large-scale RC beams were tested, with the investigated parameters being (i) arrangement of NSM carbon FRP (CFRP) strips, (ii) U-jacket material type, (iii) layout, width and inclination angle of FRP U-jackets. Failure modes, load-deflection and strain distribution responses of all tested beams were recorded. The test results bring clarity and understanding to the tested parameters and they also demonstrate the effectiveness of the U-jacket intervention.

Seismic retrofit of exterior RC beam-column joints with bonded CFRP reinforcement: An experimental study

This paper presents an experimental study on the strengthening of seismically deficient RC beam-column joints using carbon fiber reinforced polymer (CFRP). Six exterior RC beam-column joint specimens were tested to identify an effective method for improving the seismic performance of such joints in terms of their lateral strength and ductility. These six specimens included one non-seismically designed specimen, one seismically designed specimen, and four specimens retrofitted using different schemes. In these schemes, both externally bonded CFRP sheets and near-surface mounted (NSM) CFRP strips were explored as strengthening options. The test results showed that by adding CFRP reinforcement, the seismic performance of a seismically deficient beam-column joint can be significantly improved. In particular, the use of NSM CFRP strips in beams and joints was found to effectively relocate the plastic hinge away from the joint region, thereby leading to a ductile failure mode (beam flexural failure), which demonstrates the effectiveness of this seismic retrofit method. A good understanding of the hinge relocation mechanisms has been achieved through extensive and detailed strain measurement during the tests.

Flexural Performance and Failure Modes of NSM CFRP-Strengthened Concrete Beams: A Parametric Study

The flexural performance and failure modes of concrete beams, strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips were investigated. For this, reinforced concrete beams (150 × 250 × 1400 mm) were cast, and cured for 28 days, then strengthened with NSM CFRP strips at varying numbers (1–3), lateral spacing (50–100 mm), and embedment lengths (150–450 mm). The impact of staggering of NSM CFRP strips upon strengthening efficiency was tackled, as well. The flexural mechanical response, strain in strips at failure, and cracking and failure modes were evaluated for all concrete beams under a four-point loading test setup. The findings indicated that inserting NSM CFRP strips at a certain lateral distance from the main steel bars prevented end-cover peeling-off. Furthermore, staggering NSM CFRP strips contributed to increasing the residual flexural capacity and toughness of strengthened beams by as much as 30 and 51%, respectively, although it led to a slight reduction in their ductility. In general, the present study confirmed the findings of different literature works with regard to the effect of key repair parameters using NSM CFRP strips.

Bond response of NSM CFRP strips in concrete under sustained loading and different temperature and humidity conditions

Near Surface Mounted (NSM) reinforcement has been increasingly recognized as an efficient technique for strengthening Reinforced Concrete (RC) structures using Fiber Reinforced Polymer (FRP) reinforcement. Although a number of studies have investigated the short-term bond behavior between NSM FRP and concrete, little work has been done on its long-term behavior under sustained loading. This paper aims to experimentally investigate the bond response of NSM Carbon FRP (CFRP) strips in concrete under sustained loading and different temperature and humidity conditions. Thirty-three single shear pull-out specimens were subjected to monotonic and sustained loading. The long-term pull-out specimens were subjected to different environmental regimes including four combinations of temperature, humidity and sustained loading. For each environmental combination, three bonded lengths were tested and two levels of sustained loading were applied. Specimens were kept loaded and conditioned in a climatic chamber for 1000 h, while the slip evolution with time was monitored throughout the testing period. Results revealed that the changes in the parameters being studied did have a relevant effect on the bond response.

Preloading Effect on Strengthening Efficiency of RC Beams Strengthened with Non- and Pretensioned NSM Strips

The near surface mounted (NSM) technique has been shown to be one of the most promising methods for upgrading reinforced concrete (RC) structures. Many tests carried out on RC members strengthened in flexure with NSM fiber-reinforced polymer (FRP) systems have demonstrated greater strengthening efficiency than the use of externally-bonded (EB) FRP laminates. Strengthening with simultaneous pretensioning of the FRP results in improvements in the serviceability limit state (SLS) conditions, including the increased cracking moment and decreased deflections. The objective of the reported experimental program, which consisted of two series of RC beams strengthened in flexure with NSM CFRP strips, was to investigate the influence of a number of parameters on the strengthening efficiency. The test program focused on an analysis of the effects of preloading on the strengthening efficiency which has been investigated very rarely despite being one of the most important parameters to be taken into account in strengthening design. Two preloading levels were considered: the beam self-weight only, which corresponded to stresses on the internal longitudinal reinforcement of 25% and 14% of the yield stress (depending on a steel reinforcement ratio), and the self-weight with the additional superimposed load, corresponding to 60% of the yield strength of the unstrengthened beam and a deflection equal to the allowable deflection at the SLS. The influence of the longitudinal steel reinforcement ratio was also considered in this study. To reflect the variability seen in existing structures, test specimens were varied by using different steel bar diameters. Finally, the impact of the composite reinforcement ratio and the number of pretensioned FRP strips was considered. Specimens were divided into two series based on their strengthening configuration: series “A” were strengthened with one pretensioned and two non-pretensioned carbon FRP (CFRP) strips, while series “B” were strengthened with two pretensioned strips. Experimental tests illustrated promising results at ultimate and serviceability limit state conditions. A significant gain of the load bearing capacity, in the range between 56% and 135% compared to the unstrengthened beams, was obtained. Tensile rupture of the NSM CFRP strips was achieved, confirming full utilization of the material.

Assessing the Contribution of the CFRP Strip of Bearing the Applied Load Using Near-Surface Mounted Strengthening Technique with Innovative High-Strength Self-Compacting Cementitious Adhesive (IHSSC-CA).

Efficient transfer of load between concrete substrate and fibre reinforced polymer (FRP) by the bonding agent is the key factor in any FRP strengthening system. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) was recently developed by the authors and has been used in a number of studies. Graphene oxide and cementitious materials are used to synthesise the new adhesive. The successful implementation of IHSSC-CA significantly increases carbon FRP (CFRP) strip utilization and the load-bearing capacity of the near-surface mounted (NSM) CFRP strengthening system. A number of tests were used to inspect the interfacial zone in the bonding area of NSM CFRP strips, including physical examination, pore structure analysis, and three-dimensional laser profilometery analysis. It was deduced from the physical inspection of NSM CFRP specimens made with IHSSC-CA that a smooth surface for load transfer was found in the CFRP strip without stress concentrations in some local regions. A smooth surface of the adhesive layer is very important for preventing localized brittle failure in the concrete. The pore structure analysis also confirmed that IHSSC-CA has better composite action between NSM CFRP strips and concrete substrate than other adhesives, resulting in the NSM CFRP specimens made with IHSSC-CA sustaining a greater load. Finally, the results of three-dimensional laser profilometery revealed a greater degree of roughness and less deformation on the surface of the CFRP strip when IHSSC-CA was used compared to other adhesives.

Experimental and numerical study of strengthening of heat-damaged RC beams using NSM CFRP strips

An experimental and numerical study of eight reinforced concrete beams is reported in this paper. In the experiments, two beams were normal temperature controls, and the other six beams were heated under the ISO-834 standard fire curve. All beams had the same geometrical dimensions, reinforcement and loading and support arrangements. The flexural behaviour was studied after exposure to 600 and 700°C for two hours. Two beams were un-strengthened fire-damaged control beams and the other four beams were strengthened with carbon fibre-reinforced polymer (CFRP) laminates after exposure to heating. The results showed that the mid-span ultimate load, stiffness, and ductility of the heat-damaged beams decreased compared with those of the normal-temperature control beam. Although the ultimate load and stiffness decreased, repair with near-surface mounted (NSM) systems to the CFRP laminate using an epoxy- or cement-based adhesive in grooves cut into the concrete surface was found to increase the load capacity and stiffness of the beams. The finite element models (FEMs) were able to predict the experimental behaviour reasonably well.

Recovering flexural performance of thermally damaged concrete beams using NSM CFRP strips

The potential of recovering the flexural performance of thermally damaged concrete beams using near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) strips was experimentally investigated. Twenty reinforced concrete beams (150×250×1400mm) were cast then cured for 28days in moist burlap. A set of ten beams were heated at 600°C for 2h using an electrical furnace whereas those of the second set were left in laboratory air. Four pairs of beams from each set were repaired/strengthened at their tension side using similar configurations of NSM CFRP strips. Duplicate beams of each set were tested as controls. The mechanical performance of different beams was evaluated under four-point loading test setup including measurement of strain in NSM CFRP strips and slippage between NSM CFRP strips and concrete. Moreover, cracking and failure modes were monitored and characterized. Intact/strengthened and heat-damaged/repaired beams showed improved load capacity and toughness, yet experienced reductions in ductility and toughness as compared to control ones. Different performance indicators revealed good potential of repairing heat-damaged beams using NSM CFRP strips. End-cover separation failure mode was observed for both strengthened as well as repaired beams. Analytical predictions of ultimate load capacity for different beams confirmed experimentally obtained results.

Assessment of bond strength of NSM CFRP strips embedded in concrete using cementitious adhesive made with graphene oxide

Efficient transfer of load between concrete substrate and fibre reinforced polymer (FRP) by the bonding agent is the key factor in any FRP strengthening system. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) was recently developed by the authors and has been used in a number of studies. Graphene oxide and cementitious materials are used to synthesise the new adhesive. The successful implementation of IHSSC-CA significantly increases carbon FRP (CFRP) strip utilization and the load-bearing capacity of the near-surface mounted (NSM) CFRP strengthening system. A number of tests were used to inspect the interfacial zone in the bonding area of NSM CFRP strips, including physical examination, pore structure analysis, and three-dimensional laser profilometery analysis. It was deduced from the physical inspection of NSM CFRP specimens made with IHSSC-CA that a smooth surface for load transfer was found in the CFRP strip without stress concentrations in some local regions. A smooth surface of the adhesive layer is very important for preventing localized brittle failure in the concrete. The pore structure analysis also confirmed that IHSSC-CA has better composite action between NSM CFRP strips and concrete substrate than the other adhesives, resulting in the NSM CFRP specimens made with IHSSC-CA sustaining a greater load. Finally, the results of three-dimensional laser profilometery revealed a greater degree of roughness and less deformation on the surface of the CFRP strip when IHSSC-CA was used, compared to other adhesives.

Three-dimensional finite element modelling of the fire behaviour of insulated RC beams strengthened with EBR and NSM CFRP strips

This paper presents numerical simulations of the fire behaviour of reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) strips installed according to the externally bonded reinforcement (EBR) and near surface mounted (NSM) techniques. Three dimensional (3D) finite element (FE) models of the beams were developed, in which the temperature-dependent thermophysical and mechanical properties of the constituent materials was considered. The CFRP-concrete interaction was modelled by means of bi-linear bond-slip laws previously calibrated by the authors for different temperatures. Comparisons between numerical and previous experimental results confirmed the accuracy of the models in predicting the thermo-mechanical response of insulated CFRP-strengthened RC beams exposed to fire. The results obtained highlight the possibility of exploring the CFRP mechanical contribution in fire through a “cable” behaviour by applying a thicker insulation in the CFRP anchorage zones. Moreover, the numerical results confirmed the better fire performance of the NSM strengthening technique when compared to EBR: with the latter technique CFRP debonding occurred after considerably shorter periods of fire exposure and for much lower average temperatures in the adhesive in the anchorage zones, from 1.2×Tg to 1.4×Tg, with such range being 2.4×Tg to 4.2×Tg for the NSM system.