Near-surface Mounted CFRP Strips Research Articles

Experimental study of flexural behaviour of RC beams strengthened using near surface mounted CFRP strips and ropes

In this article, an experimental program was carried out to investigate the flexural behaviour of RC beams strengthened using side and bottom near-surface mounted (NSM) CFRP strip and rope. Thirteen reinforced concrete beams (150x250x1600mm) with flexural deficiency were cast. Different configurations have been used for strengthening RC beams, including side NSM, bottom NSM and both of them. Test results have shown that specimens strengthened with NSM CFRP strips have higher load carrying capacity and higher ultimate deflection than that strengthened with NSM CFRP ropes for the same configuration. FRP debonding, concrete cracks and concrete cover separations are more depicted in specimens strengthened with NSM CFRP ropes than that strengthened with NSM CFRP strips. Using NSM CFRP ropes or strips has increased the elastic stiffness of the strengthened specimens. Flexural strengthening of RC beams using CFRP ropes and strips has increased the load-carrying capacity NSM in a range of (18%–51%) and (30%–80%), respectively, compared to the control specimen. It is also found that beams with bottom strengthening increased the flexural capacity by (25%–32%) compared to RC beams with side strengthening

Flexural Strengthening of Reinforced Concrete Beams with Longitudinal Circular Hole Using Near-Surface Mounted CFRP Strips

Flexural Strengthening of Reinforced Concrete Beams with Longitudinal Circular Hole Using Near-Surface Mounted CFRP Strips

NLFE modeling of bond between NSM CFRP strips and heat-damaged concrete

A nonlinear finite element (NLFE) model was developed to describe bond behavior between heat-damaged concrete and near-surface mounted (NSM) fiber-reinforced polymer (FRP) strips. The different components of near-end pullout specimens were represented using proper elements in ABAQUS software with the cohesive element adopted to consider bond behavior. The model is calibrated using data from two published experimental works, before its predictions were extrapolated to consider the effect of temperatures (23–550 °C), along with the effects of concrete strength (25–55 MPa), bond length (100–350 mm), spacing (50–200 mm) and fiber type of NSM CFRP strips. Elevated temperatures, in excess of 300 °C, caused a reduction in residual pullout force, yet an increase in bond ductility. The degradation in bond was more pronounced for specimens with higher-grade concrete and lower spacing between strips. Exposing concrete blocks to 450 °C and above undermined the contribution of NSM CFRP strips (100–350 mm) to bond strength. The type of composite had a minor (if any) impact upon bond behavior between heat-damaged concrete and FRP strips for temperatures higher than 300 °C. The NLFE cracking patterns indicated that bond failure mode in the pullout specimens is dictated by the spacing between strips with minimal impact of exposure temperature or bond length.

Behaviour of prestressed hollow core slabs strengthened with NSM CFRP strips around openings: A finite element investigation

Prestressed Hollow Core Slabs (PHCSs) have become widely used in the construction industry owing to their economic benefits. During their service life, changes that would require openings to be situated along their spans may arise. A strengthening technique to recover or improve the original serviceability and resistances becomes indispensable. Limited research interest has been given to develop appropriate strengthening methods for the PHCSs with openings. The Near Surface Mounted (NSM) strengthening technique is among the practical and feasible solutions to rehabilitate or enhance the performance of these PHCSs. This paper demonstrates a Finite Element (FE) analysis technique to predict the responses of the PHCSs with openings, either unstrengthened or strengthened with NSM strips. The Concrete Damage Plasticity (CDP) model was successfully implemented to model the non-linear behaviour of concrete. The prestressing transmission length and the bond behaviour between the epoxy adhesive and concrete interfaces were put into practice. Acceptable agreements were found, upon validating the FE analysis results against the experimental data available in the literature. Maximum differences, compared to the experimentally attained results in the ultimate loads and the corresponding deformations of nearly 4% and −8.4%, respectively, were detected. An extensive parametric study was executed to assess the effect of various parameters on the overall behaviour of the examined slabs. The PHCS cross-sectional shape, CFRP reinforcement percentage, average precompression, opening location and size and the concrete compressive strength were taken as parameters. Above all, a gap of knowledge exits regarding the presence of design guidelines that evaluate reductions in the ultimate capacities of the PHCSs associated with the presence of openings at multiple locations along their spans and improvements experienced by employing the NSM strengthening technique. This could be bridged by utilizing the suggested modelling approach and the parametric study results in this research.

Shear strengthening of RC beams using side near surface mounted CFRP ropes and strips

In this study the effectiveness of near surface mounted (NSM) rope and strip in shear strength of reinforced concrete beams was experimentally investigated. Fifteen reinforced concrete (RC) beams (150 mm × 250 mm × 1200 mm) (width × thickness × length) designed to fail in shear were constructed and tested. The following parameters were considered in each configuration: CFRP orientation angle, the spacing between CFRP and the strengthening scheme to study the load displacement behavior of RC beams strengthened by NSM Rope and Strip FRP. The results showed that NSM CFRP rope is an effective technique in strengthening of reinforced concrete beams. Using inclined CFRP ropes or strips have the largest load carrying capacity compared to all other configurations where the ultimate load capacity increased by (150–170%) of the control specimen depending on the spacing between CFRP. The inclined NSM CFRP ropes or strips have changed the type of failure in the strengthened specimens from brittle shear to a flexural shear failure.

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.

Flexural behaviour of FRP reinforced concrete beams strengthened with NSM CFRP strips

This paper presents the results on an experimental programme studying the flexural behaviour of internally reinforced Glass-Fibre Reinforced Polymer (GFRP) Reinforced Concrete (RC) beams strengthened with Carbon-FRP strips using the Near-Surface Mounted (NSM) technique. Their theoretical load-carrying capacity is assessed by a cracked sectional analysis. It is seen that NSM CFRP results an effective technique to enhance the flexural capacity of RC beams internally reinforced with GFRP bars, despite the high degrees of deformability of GFRP RC. Additionally, an a analytical study to evaluate the influence of different parameters on the flexural capacity of NSM CFRP strengthened concrete beams internally reinforced with either steel or GFRP bars is performed. In general, increasing the reinforcement ratio and mechanical properties, either internal or NSM, increases the flexural capacity. However, the change in the parameters may affect in a different way the ratios of increase and cause different modes of failure.

A nonlinear finite element model for shear deficient heat-damaged concrete beams repaired using NSM CFRP strips

The mechanical performance of heat-damaged and shear-deficient beams, retrofitted using near surface mounted (NSM) CFRP strips, was investigated using nonlinear finite element (NLFE) modeling. This was carried out using the software ABAQUS with the cohesive element and the virtual crack closure technique (VCCT) implemented to model bond interface between concrete and NSM CFRP strips and concrete cover separation, respectively. The NLFE model was validated using experimental data from tests on shear-deficient and heat-damaged beams before predictions expanded to consider wide spectrum of exposure temperatures in the range of 23–700 °C and varying spacing between vertical NSM CFRP strips (75–200 mm). NLFE predictions of mechanical response and cracking and failure modes for intact, heat-damaged, retrofitted and strengthened beams were in an excellent agreement with experimentally obtained results. The study revealed that the retrofitting efficiency hence benefit was higher for beams, exposed to temperatures below 600 °C and repaired using NSM CFRP strips at spacing greater than 75 mm. In general, concrete beams, exposed to 400 °C and greater then repaired for shear, tended to fail by concrete cover separation; especially when NSM CFRP strips were inserted at spacing values of 100 mm and less; others experienced dominant typical shear failure. Finally, the present NLFE model explained failure mechanism of NSM CFRP shear-strengthened beams through concrete cover separation.

Bond performance of NSM CFRP strips embedded in concrete using direct pull-out testing with cementitious adhesive made with graphene oxide

Concrete structures are normally strengthened using fibre reinforced polymer (FRP) with epoxy adhesives and polymer cementitious mortars. Epoxy adhesives have significant issues, such as the release of toxic fumes throughout curing, loss of strength and stiffness when exposed to hot temperatures, and low permeability and weakness to UV radiation. In the case of polymer cementitious adhesives, their properties are adversely affected by hydrothermal conditions. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) has recently been developed by the authors which uses graphene oxide and cementitious materials. This paper presents the bond response of carbon FRP strips bonded to concrete substrate using near-surface mounted (NSM) technique with IHSSC-CA, epoxy and polymer cement-based adhesives using direct pull-out tests. The behaviour of each adhesive is presented and compared and the local bond-slip relationship is calculated. Finally, an analytical model is proposed to predict the ultimate pull-out force (bond strength). The results of this study confirm the effectiveness of using IHSSC-CA to improve the bond strength, stiffness, CFRP strip utilisation, ductility and residual strength of NSM CFRP system. Moreover, the proposed analytical model can simulate experimental conditions reasonably well.

Reinforced concrete beams strengthened in flexure with near-surface mounted (NSM) CFRP strips: Current status and research needs

Abstract The near-surface mounted (NSM) FRP strengthening technique has attracted worldwide attention as an effective alternative to the externally bonded (EB) FRP strengthening technique. In the NSM FRP strengthening method, grooves are first cut in the concrete cover of a concrete member for the FRP reinforcement to be inserted and embedded using an adhesive. The NSM FRP method has many advantages over the EB FRP method, including a higher bonding efficiency and a better protection of the FRP reinforcement. Existing experimental studies have shown that FRP strips owned a better bond efficiency compared with other section shapes (e.g. round bars and square bars), due to the fact that they had a larger perimeter-to-cross-sectional area ratio. This paper presents a state-of-the-art review, particularly on the flexural strengthening of RC beams with NSM CFRP strips. The observed failure modes in laboratory experiments of such FRP-strengthened RC beams are classified and the existing strength models are examined along with the failure mechanisms behind. The main knowledge gaps to be bridged in future studies are also identified. This review partially formed the basis of the development of design provisions on the NSM strengthening technique in the relevant Hong Kong design guideline.

Bond Behavior between NSM CFRP Strips and Concrete Exposed to Elevated Temperature Using Cement-Based and Epoxy Adhesives

AbstractThe repair and strengthening of existing concrete structures with composite material has become more common during the last decade. The near-surface-mounted (NSM) technique was used in this...

Fatigue performance of NSM CFRP strips embedded in concrete using innovative high-strength self-compacting cementitious adhesive (IHSSC-CA) made with graphene oxide

The efficiency of strengthening reinforced concrete (RC) structures using the near-surface mounted (NSM) fibre reinforced polymer (FRP) technique depends on the bond between the NSM FRP and the concrete substrate. This paper presents the results of an experimental study of NSM carbon FRP strips bonded to concrete substrate with innovative high-strength self-compacting cementitious adhesive (IHSSC-CA) under fatigue loading using single-lap shear tests. Graphene oxide and cementitious materials were used to synthesise the IHSSC-CA. The test variables were: CFRP strip dimensions (1.4×10mm, 1.4×20mm) and CFRP strip surface conditions (smooth, rough). 61 specimens were tested under different fatigue load ranges to develop the load range (LR)-fatigue life (N) relationships. Finally, equations were developed to predict fatigue lives based on the experimental results. It was found that the rough surface CFRP strips 1.4×20mm in dimensions showed the best fatigue life under different fatigue load levels. Moreover, the test results confirm the effectiveness of using IHSSC-CA to bond NSM CFRP strips to the concrete substrate. Furthermore, the proposed equations can predict fatigue life reasonably well and can be used in the design of NSM CFRP-strengthened RC members.

Bond behaviour between NSM CFRP strips and concrete at high temperature using innovative high-strength self-compacting cementitious adhesive (IHSSC-CA) made with graphene oxide

Strengthening of reinforced concrete (RC) structures using the near-surface mounted (NSM) fibre reinforced polymer (FRP) method is becoming an attractive technique for upgrading the existing structural elements. The exposure of the NSM FRP strengthening system to high temperatures greatly affects the bond between FRP, adhesive material and the concrete substrate. Organic adhesives are widely used in the NSM strengthening technique. However, their low fire resistance is a major drawback. This study investigates the performance of NSM carbon FRP strengthening technique using single-lap shear tests (pull-out tests) with innovative high-strength self-compacting cementitious adhesive (IHSSC-CA) under high temperatures. Graphene oxide and cementitious materials were used to synthesise the IHSSC-CA. Single-lab shear tests were conducted on NSM strengthened specimens which were exposed to high temperatures of 800°C. The results obtained from the pull-out tests signifies that the samples made with IHSSC-CA were less affected by high temperature, and maintain considerable residual bond strength. Theoretical equations are proposed to simulate the effect of high temperature on bond strength.

Fatigue performance of NSM CFRP strips embedded in concrete using epoxy adhesive

Strengthening of reinforced concrete (RC) structures using the near-surface mounted (NSM) fibre reinforced polymer (FRP) technique is becoming an attractive system for upgrading these structures. The efficiency of this method depends on the bond between the concrete and the NSM FRP. Although different experimental and analytical studies on the bond of NSM FRP to concrete substrate under monotonic loading have been carried out, very few studies have been conducted under fatigue loading conditions. This paper presents the results of an experimental study of NSM carbon FRP bonded to concrete with epoxy adhesive under fatigue loading using single-lap shear tests. The test variables were: CFRP strip dimensions (1.4×10mm, 1.4×20mm), and CFRP strip surface conditions (smooth, rough). 52 specimens were tested under different fatigue load ranges to develop the load range (LR)-fatigue life (N) relationships. Finally, equations were developed to predict fatigue lives based on the experimental results. It was found that the rough surface CFRP strips 1.4×20mm in dimensions showed the best fatigue life under different fatigue load levels. Moreover, the proposed equations can predict fatigue life reasonably well and can be used in the design of NSM CFRP-strengthened RC members.

Pullout strength of NSM CFRP strips bonded to vintage clay brick masonry

The frequently observed inadequate seismic performance of unreinforced masonry (URM) buildings necessitates the development of cost effective minimally-invasive seismic improvement techniques for this type of construction. One promising solution is use of the near surface mounted (NSM) technique to incorporate fibre reinforced polymers (FRP) strips as longitudinal reinforcement. In particular, the NSM technique provides several advantages over externally bonded (EB) FRP as a seismic improvement technique including significantly higher axial strain at debonding, minimal negative impact upon the aesthetics of the structure, reduced installation time, and superior protection from fire and the environment, thus providing a cost effective and minimally-invasive option for seismically strengthening URM buildings. An experimental program consisting of 39 pull tests was conducted using NSM carbon (C)FRP strips bonded to vintage solid clay brick masonry, to provide data with which to validate the accuracy of existing predictive FRP-to-masonry bond models. Based on experimental findings, a variation of an existing analytical FRP-to-masonry bond model is proposed and the effects of geometric variation of the NSM groove and the reinforcing CFRP strip are discussed.

스터럽 절단 탄소섬유판 표면매립공법의 휨 보강 성능 평가

Recently, the near surface mounted (NSM) FRP strengthening technique has been actively applied to deteriorated concrete structures for rehabilitation purposes. However, the use of this conventional NSM technique could be restricted due to the insufficient height or strength of the concrete cover. In this study, the stirrup-Cutting Near Surface Mounted(CNSM) technique was considered as an alternative, whereby NSM strips are placed at a deeper level, namely at the level of the main steel reinforcement. A flexural test of a concrete beam strengthened with CNSM technique was performed and the results were then compared to those for a concrete beam strengthened by the conventional NSM technique. The embedment length of the CFRP strips was varied in order to increase the effect of the anchoring depth of the NSM and CNSM CFRP strips in the beam specimens. From the results of the test, the beam with the CNSM CFRP strip showed typical structural behavior similar to that of the beam with the NSM CFRP strip. Moreover, there was no apparent structural degradation resulting from the stirrup partial-cutting. Consequently, the CNSM strengthening technique can be suitably utilized for extensively damaged concrete structures where it is difficult to apply the conventional NSM technique.

Modelling of the bond–slip behavior in FRP reinforced masonry

The strengthening of out-of-plane loaded unreinforced masonry (URM) walls strengthened with near-surface mounted (NSM) FRP strips has proven to be an innovative and cost effective strengthening technique. Since the performance of FRP-strengthened URM walls is often controlled by the behavior of the interface between the FRP and masonry, it is very important to model the bond–slip relationship accurately. In this paper, the bond–slip behavior of the FRP-to-masonry interface in a pull test was simulated by solid continuum elements where detailed information of the interface could be modelled directly. The masonry prism in the pull test was modelled using a previously derived homogenized model. For comparisons of computational efficiency, a continuum model, in which the mortar and masonry brick units are discritized individually, and the commonly used smeared-crack model were also employed to model the masonry prism in the pull test. A thin layer of interface elements was used to simulate the behavior of the FRP-to-masonry interfaces in each model. The accuracy and efficiency of the homogenized model together with the interface elements was validated by experimental data from (a) uniaxial pull tests of a NSM strip bonded to a masonry prism and (b) an out-of-plane loaded full-scale masonry wall strengthened with NSM CFRP strips running in the vertical direction. The simulated results using the homogenized model were found to agree well with test data from both sets of tests.

Out-of-plane strength of brick masonry retrofitted with horizontal NSM CFRP strips

The magnitude of damage possible in unreinforced brick masonry construction subjected to high levels of out-of-plane loading is well understood. Due to the large global building stock of masonry structures, it is essential that efficient retrofitting techniques be developed. The use of fibre-reinforced polymer (FRP) strips has been shown to improve the load-carrying and displacement capacities of masonry sections subjected to out-of-plane loading. This paper presents the results of an experimental and theoretical investigation into the behaviour of modern clay brick masonry retrofitted with horizontally oriented FRP strips subjected to horizontal bending. Experimental tests were conducted on four masonry wallettes retrofitted with horizontal near-surface mounted (NSM) carbon FRP strips, subjected to horizontal bending. The specimens were tested in a vertical orientation with variation in vertical compressive loading. The application of compressive load improved the FRP confinement efficiency, and thus increased the maximum strength of the specimens. The primary failure modes were displacement induced (DI) debonding and FRP buckling. Mathematical models were developed to predict the moment at cracking and moment capacity for the specimens. The models explicitly account for the flexural strengths of the mortar and brick units, the torsional resistance of the bed joints, the NSM FRP, as well as the contributions to bed joint shear strength from the vertical compressive stress and friction. The proposed mathematical models were validated against the results of the experimental tests.

표면매입 및 외부부착 탄소섬유판으로 보강된 철근콘크리트 부재의 전단 거동에 관한 실험적연구

본 연구에서는 탄소섬유판이 외부 부착, 표면 매입 그리고 외부부착 및 표면매입이 혼합된 보강 방식의 철근콘크리트 부재의 전단 거동 및 전단 보강 효과를 구명하고자 하였다. 본 연구 결과, 표면매입 탄소섬유판 및 외부부착과 표면매입으로 혼용된 탄소섬유판으로 보강된 철근콘크리트 보의 전단 강성 및 극한 전단강도는 섬유판으로 보강되지 않은 보에 비하여 크게 증진되는 것으로 나타났다. 표면매입 탄소섬유판 및 외부부착과 표면매입으로 혼용된 탄소섬유판으로 보강된 철근콘크리트 보의 파괴는 전단균열로 시작되었으며, 하중이 증가함에 따라 하중 재하점과 가장 인접한 매입 탄소섬유판 하단 모서리에서 발생한 휨 균열은 하중재하점 방향으로 급격하게 진행되었고, 하중 재하점과 휨 균열을 연결하는 압축파괴가 발생되었다. 이러한 사실은 매입 섬유판의 전단보강 효과가 매우 우수하여 보가 전단으로 파괴되지 않고 섬유판이 매입 보강되지 않은 휨 구역에서 압축파괴가 발생하고 있음을 설명하고 있다. 매입 탄소 섬유판 보강부재 및 탄소섬유판 부착 및 매입을 혼용한 부재에서는 매입 탄소섬유판의 변형률이 각각 0.45% 및 0.35%로 나타났으며, 외부 부착 탄소섬유판의 변형률이 약 0.3%로 나타났다.