Near Surface Mounted Technique Research Articles

NSM shear strengthening technique with CFRP laminates applied in high T cross section RC beams

Abstract The effectiveness of the Near Surface Mounted (NSM) technique with Carbon Fiber Reinforced Polymer (CFRP) laminates for the shear strengthening of relatively high T cross section (h = 600 mm) Reinforced Concrete (RC) beams was assessed by experimental research. Two inclinations of the laminates were tested (52° and 90°) and, for each one, two percentages of CFRP were adopted. The RC beams shear strengthened with NSM laminates had a percentage of steel stirrups of 0.09%. The experimental results showed that NSM shear strengthening technique with CFRP laminates is very effective in RC beams of relatively high cross section height, not only in terms of increasing the load carrying capacity of the beams, but also in assuring higher mobilization of the tensile capacity of the CFRP. Furthermore, inclined laminates were more effective than vertical laminates and the shear capacity of the beams increased with the percentage of laminates. Taking into account available experimental results obtained with the same test setup, but using RC beams of lower depth of T cross section (h = 400 mm), it can be concluded that the effectiveness of the NSM technique with CFRP laminates increase with the increase of the depth of the cross section. The formulations provided by Nanni et al. (2004) and Dias and Barros (2013) for the NSM shear strengthening technique predicted a CFRP contribution around 51% and 67%, respectively, of the experimentally registered values.

Torsional strengthening of reinforced concrete beams using different configurations of NSM FRP with epoxy resins and cement-based adhesives

The strength of reinforced concrete (RC) members has been improved with a good deal of success using carbon fibre-reinforced polymers (CFRP) with two application techniques i.e. externally-bonded reinforcement (EBR) and the near-surface mounted (NSM) technique. However, to date no studies have been reported on torsional strengthening using NSM methods, as the research on torsional strengthening has been conducted using EBR techniques only. Thus far, NSM strengthening has been successfully employed with epoxy in improving the flexure and shear strength of RC beams. As toxic fumes and poor performance in high-temperature environments are major problems associated with using epoxy, some studies have used a new cement-based adhesive as an alternative to epoxy. This paper presents the results of experimental work on torsional strengthening using the NSM technique in a specialized torsional rig system. Eight RC members were tested to evaluate the strength enhancement provided by two different NSM FRP configurations. Six beams were strengthened with CFRP laminates embedded into pre-cut grooves. The laminates were applied to all four faces of two beams (full wrapping) and to only three faces of the other four beams (u-wrapping). An epoxy and a new cement-based adhesive were examined in this study. For each type of adhesive, one beam with full wrapping and two beams with u-wrapping were considered. Two control beams were initially tested and a comparison was made with the results of the strengthened beams in terms of the total performance and the ultimate strength of the beams. The results show that the u-wrapping configuration provides a reasonable improvement to the torsional behaviour of the beams when using epoxy resin as the bonding agent. On the other hand, cement-based adhesive is less efficient in increasing the torsional strength than epoxy. A theoretical design capacity method was adopted in this study to compare its prediction to the experimental data.

Efficient near surface mounting CFRP strengthening of pretensioned hollowcore slabs with opening – An experimental study

Carbon fiber reinforced polymers (CFRP) are commonly used in strengthening of civil infrastructure due to their superior mechanical properties. Near surface mounting (NSM) of CFRP laminates is an efficient strengthening scheme to restore the capacity of concrete elements. Hollowcore slabs are pretensioned precast concrete elements reinforced only with prestressing strands. Due to various service requirements, openings/cut-outs are provided in the slabs. Reinstating the capacity of hollowcore slabs lost due to the presence of openings is very important as openings can severely reduce the capacity of these slabs and alter the load distribution in the floor system. This paper investigates the efficiency of NSM technique using CFRP laminates in restoring the capacity of hollowcore slabs with openings. Ten full scale hollowcore slabs were tested at two shear span to depth (a/d) ratios and with two different locations of opening. Presence of opening reduced the ultimate capacity of slab by 44% and NSM CFRP strengthening fully restored the lost capacity based on a/d ratio. The adopted strengthening was more effective in shear dominated than in flexure dominated specimens. However, in both a/d ratios, good improvement in initial stiffness was observed due to NSM CFRP strengthening.

Simultaneous flexural and punching strengthening of RC slabs according to a new hybrid technique using U-shape CFRP laminates

One of the main concerns related to flat reinforced-concrete (RC) slabs is the slab’s punching capacity. Punching can occur not only due to a deficient transverse reinforcement, but also when the flexural capacity of the slab needs to be increased. To increase the flexural capacity, carbon-fiber-reinforced-polymer (CFRP) composites have been applied according to near-surface-mounted (NSM) or external-bonded-reinforcement (EBR) techniques, while for the punching strengthening CFRP reinforcements have been applied according to embedded-through-section (ETS) technique. To take advantage of strengthening benefits of the NSM and ETS techniques, in the present paper a new type of CFRP laminate of U-shape is used by adopting a novel hybrid technique for the simultaneous flexural and punching strengthening of existing RC slabs. Besides, this hybrid technique aims to provide a better bond performance for the ETS and NSM CFRPs by improving the anchorage conditions. Moreover, a higher resistance to the susceptibility of occurrence of other premature failure modes, like concrete cover delamination, is offered by using this hybrid technique. A 3D nonlinear finite-element (FE) model is developed to simulate the experimental tests by considering the nonlinear behavior of the constituent materials. The experimental program and numerical model are described, and the relevant results are analyzed.

End concrete cover separation in RC structures strengthened in flexure with NSM FRP: Analytical design approach

Fiber-reinforced-polymer (FRP) composite materials applied according to the near-surface-mounted (NSM) technique are very effective for the flexural strengthening of reinforced-concrete (RC) structures. However, the flexural strengthening effectiveness of this NSM technique is sometimes compromised by end concrete cover separation (CCS) failure, which is a premature failure before occurring the conventional flexural failure modes. Due to the complexity of this failure mode, no analytical approach, with a design framework for its accurate prediction, was published despite the available experimental results on this premature failure. In the present study, a novel simplified analytical approach is developed based on a closed form solution for an almost accurate prediction of CCS failure in RC structures strengthened in flexure with NSM FRP reinforcement. After demonstrating the good predictive performance of the proposed model, it was used for executing parametric studies in order to evaluate the influence of the material properties and FRP strengthening configuration on the susceptibility of occurring the CCS failure. At the end, regarding to the FRP strengthening configuration, some design recommendations were proposed to maximize the resistance of NSM FRP strengthened structures to the susceptibility of occurring the CCS failure.

Experimental investigation into the use of NSM FRP to increase the torsional resistance of RC beams using epoxy resins and cement-based adhesives

Carbon fibre-reinforced polymers (CFRPs) have been used to increase the strength of reinforced concrete (RC) members using both externally bonded reinforcement (EBR) and near surface mounted (NSM) techniques with a great deal of success. However, currently no studies have been conducted on torsional strengthening using NSM methods as current research on torsional strengthening has been conducted using EBR techniques only. It is well documented that flexure and shear strength of RC beams has been improved using NSM strengthening technology applied with epoxy adhesives. However, due to epoxy adhesive problems associated with toxic fumes and poor performance in high-temperature environments, a new cement-based adhesive has been developed and used as an alternative to epoxy in several studies. This paper reports on experimental results of ten RC beams tested in torsion using a specialized torsional rig system. Two control beams were considered and eight beams strengthened with CFRP laminates applied to all four faces of the beam and embedded into pre-cut grooves using the NSM technique were prepared. An epoxy adhesive was used in four beams and a new cement-based adhesive was used as a substitute for epoxy in the remaining four beams. For each type of adhesive, two different spacings between the grooves were examined. The performances and the ultimate strengths of the beams are presented and the results expressed in terms of the maximum torsional capacity with respect to the peak angle of twist. According to the results, the torsional behaviour of the beams was improved in the strengthened beams to varying degrees when using epoxy resin as the bonding agent. On the other hand, the NSM technique with the cement-based adhesive was also an effective method in increasing the torsional strength.

Modified cement-based adhesive for near-surface mounted CFRP strengthening system

The strengthening system with near-surface mounted (NSM) has emerged in last decades as a promising technology to improve the flexural and shear strength of concrete structures. Large number of concrete structures have been strengthened worldwide by this technique using epoxy adhesives. However, the rapid deterioration of the mechanical properties of epoxy-based polymer matrix at elevated temperature, and the hazardous effects of toxic fumes during the application made the need of replacing this polymer with a new cementitious bonding agent to enhance the performance of concrete structures in high-temperature environments and reduce the environmental and health hazards. The application of cement-based adhesive has been studied for externally-bonded techniques using CFRP textile. Although, the good bond properties has been achieved, debonding was the critical failure between fibre and concrete substrate. To exploit the advantages of NSM technique, a new modification of cement-based adhesive has been achieved to be compatible with requirements of NSM application system. To assess the efficiency of this adhesive, an experimental investigation of pull-out testing using a single-lap shear test set-up was conducted to study the bond characteristics between CFRP textile and concrete. Four different mixes were used in this investigation. The test results show the efficiency of the NSM technique using modified cement adhesive and its superior bond properties compared to externally-bonded CFRP.

Near Surface Mounted Composites for Flexural Strengthening of Reinforced Concrete Beams

Existing structural components require strengthening after a certain period of time due to increases in service loads, errors in design, mechanical damage, and the need to extend the service period. Externally-bonded reinforcement (EBR) and near-surface mounted (NSM) reinforcement are two preferred strengthening approach. This paper presents a NSM technique incorporating NSM composites, namely steel and carbon fiber-reinforced polymer (CFRP) bars, as reinforcement. Experimental and analytical studies carried out to explore the performance of reinforced concrete (RC) members strengthened with the NSM composites. Analytical models were developed in predicting the maximum crack spacing and width, concrete cover separation failure loads, and deflection. A four-point bending test was applied on beams strengthened with different types and ratios of NSM reinforcement. The failure characteristics, yield, and ultimate capacities, deflection, strain, and cracking behavior of the beams were evaluated based on the experimental output. The test results indicate an increase in the cracking load of 69% and an increase in the ultimate load of 92% compared with the control beam. The predicted result from the analytical model shows good agreement with the experimental result, which ensures the competent implementation of the present NSM-steel and CFRP technique.

Flexural performance of RC beams strengthened with near surface mounted CFRP strips

The near surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement is emerging as a promising alternative strengthening technique to externally bonded reinforcement (EBR) for increasing the load carrying capacity of reinforced concrete (RC) members. NSM FRP technique has several advantages, in comparison with the EBR method, such as reducing the risk of debonding, and a better protection from the external sources of damage. In this research, the performance and effectiveness of the NSM and EBR techniques for the flexural strengthening of RC beams are compared. In order to achieve this objective, six full-scale, RC beams were strengthened with different carbon FRP (CFRP) schemes and tested. Such beams were designed to fail in a flexural mode. Test results indicated that if the same amount of CFRP is used, beams strengthened with NSM strips achieved higher ultimate load than those strengthened with EBR. Such increase in the ultimate load ratio ranged between 12% and 18%. Furthermore, a design approach for computing the moment capacity of RC flexural members strengthened with NSM CFRP strips is developed and presented in this paper.

Exploring new NSM reinforcements for the flexural strengthening of RC beams: Experimental and numerical research

Carbon-fiber-reinforced-polymer (CFRP) composite materials applied according to near-surface-mounted (NSM) technique constitute an effective technique for the flexural and shear strengthening of reinforced-concrete (RC) structures. However, the NSM CFRP reinforcement ratio is limited by the thickness of concrete cover of the longitudinal tensile steel bars, and the minimum distance between consecutive CFRPs, below which premature fracture of surrounding concrete occurs due to group effect. Hence, the current study aims to experimentally and numerically evaluate the strengthening potentialities of a novel NSM system (with high CFRP ratio capability) for the flexural strengthening of RC beams. This new system combines externally-bonded-reinforcement (EBR) and NSM techniques in the same application using T-shaped CFRP profiles. The obtained experimental results of the RC beams strengthened with CFRP profiles are presented and discussed with the aim of evaluating the influence of CFRP profile reinforcement ratio on the strengthening efficiency of this technique. A developed 3D finite-element (FE) approach is used to simulate the experimental tests. After demonstrating its good predictive performance, a series of parametric studies is performed to assess the influence of the main material properties, and ratio of bond area to cross sectional area of the CFRP profiles on the efficiency of the proposed system.

Numerical modelling of the bond between concrete and CFRP laminates at elevated temperatures

This paper presents numerical investigations on the bond between concrete and CFRP strengthening systems at elevated temperatures. The numerical study was developed using the commercial package Abaqus and comprised the simulation of double-lap shear tests previously performed by the authors, in which the influence of the strengthening technique – externally bonding reinforcement (EBR) or near surface mounted (NSM) – on the bond behaviour was evaluated along with the temperature increase. In those tests, the two types of specimens (EBR and NSM) were first heated up to elevated temperatures (20°C, 55°C, 90°C, 120°C and 150°C), and then loaded up to failure. Three-dimensional finite element (FE) models of the specimens were developed, in which the global behaviour of the CFRP-concrete interaction was simulated by means of bi-linear bond–slip laws. These laws, which can be used for design purposes, were derived for each of the tested temperatures based on an inverse analysis, in order to maximize the agreement between experimental and numerical results in terms of (i) failure loads, (ii) load vs. slip responses, and (iii) strain distributions. Compared to the EBR specimens, the bond–slip relationships derived for the NSM specimens at ambient temperature presented (i) similar stiffness and (ii) higher values of maximum shear stress. With increasing temperatures, although both strengthening techniques exhibited comparable stiffness decrease, the maximum shear stresses for the NSM technique were higher than the EBR counterparts, attesting the better performance of the former technique.

BEHAVIOR OF REINFORCED CONCRETE CONTINUOUS BEAMS STRENGHENED WITH NEAR SURFACE MOUNTED REINFORCEMENT

Using Near Surface Mounted (NSM) technique for flexural strengthening of reinforced concrete continuous beams became particularly attractive for researchers and applicators. Due to the existence of carbon fiber reinforced polymer (CFRP)laminates as external strengthening it is subjected to severe damage resulting from mechanical and environmental conditions. There is limited experimental work investigating the behavior of reinforced concrete continuous beams strengthened using NSM technique.This paper presents an experimental investigation of the behavior of reinforced concrete continuous beams strengthened with near surface mounted (NSM) technique in the hogging and sagging moment region. The main variable was changing the lengths of the NSM strips within the negative and positive moment region. The effect of the change in lengths of the NSM strips on the failure load, mid-span deflection, strain on reinforcement, slippage of CFRP strips and crack widths were investigated. The study revealed that the NSM technique can enhance crack and failure loads and controls crack widths.

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

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

A FEM-based model to study the behaviour of corroded RC beams shear-repaired by NSM CFRP rods technique

This paper presents the main features of a finite element FE numerical model developed using the computer code FEMIX to predict the contribution of near-surface mounted (NSM) carbon-fibre-reinforced polymer (CFRP) rods to the shear repair of corroded reinforced concrete RC beams. In the RC beams repaired in shear with the NSM technique, the CFRP rods are placed inside grooves pre-cut into the concrete cover of the RC beam’s lateral faces and are bonded to the concrete with high performance epoxy adhesive. Experimental and 3D numerical modelling results are presented here in terms of load–deflection curves and failure modes for 4 short corroded beams: two corroded beams (A1CL3-B and A1CL3-SB) and two control beams (A1T-B and A1T-SB), the beams noted B were repaired in bending only with NSM CFRP rods while those noted SB were repaired in both bending and shear with the NSM technique. The corrosion of the tensile steel bars and its effect on the shear capacity of the RC beams is discussed. Results show that the FE model is able to capture the main aspects of the experimental load–deflection curves of the RC beams. They also present the experimental failure modes and FE numerical modelling crack patterns, both of which gave similar results for non-shear repaired beams, which showed a diagonal tension mode of failure, and for shear-repaired beams, which failed due to a large flexural crack in the middle of the beams, along with the concrete crushing. Three-dimensional crack patterns were produced for shear-repaired beams in order to investigate the splitting cracks occurring at the middle of the beams and near the support.

Effect of wet-dry cycles on the bond behaviour of concrete elements strengthened with NSM CFRP laminate strips

The near-surface mounted (NSM) strengthening technique is capable of effectively increase the bearing capacity of structural concrete elements. This technique which basically consists of placing FRP reinforcements inside small grooves cut in the concrete cover, has been widely investigated in terms of structural performance and ability to improve the flexural and shear behaviour of reinforced concrete beams and columns. However, little research has been carried out concerning to the NSM long-term performance and durability. Motivated by the need of increasing the knowledge on the expected durability of the NSM technique using CFRP laminates, this paper presents an experimental program in which direct pull-out tests are carried out for evaluating the bond behaviour of specimens aged through wet-dry cycles. A total of 30 specimens are tested, analysing the effect of the bond length, the groove width, the groove depth and the aging effect on the bond behaviour. Digital image correlation method is also used to identify the bond resistant mechanism developed in an element strengthened using NSM technique. Finally, using the experimental results, an analytical–numerical strategy is applied to establish the local bond stress–slip relationship.

Transfer zone of prestressed CFRP reinforcement applied according to NSM technique for strengthening of RC structures

This study presents an experimental program to assess the tensile strain distribution along prestressed carbon fiber reinforced polymer (CFRP) reinforcement flexurally applied on the tensile surface of RC beams according to near surface mounted (NSM) technique. Moreover, the current study aims to propose an analytical formulation, with a design framework, for the prediction of distribution of CFRP tensile strain and bond shear stress and, additionally, the prestress transfer length. After demonstration the good predictive performance of the proposed analytical approach, parametric studies were carried out to analytically evaluate the influence of the main material properties, and CFRP and groove cross section on the distribution of the CFRP tensile strain and bond shear stress, and on the prestress transfer length. The proposed analytical approach can also predict the evolution of the prestress transfer length during the curing time of the adhesive by considering the variation of its elasticity modulus during this period.

NSM technique: Bond of CFRP rods and static/dynamic response of strengthened RC beams

The use of the near surface mounted (NSM) fiber reinforced polymer (FRP) technique in existing RC beams is becoming a common strengthening method in civil engineering. Nevertheless, many aspects regarding the behavior of strengthened beams have not been completely analyzed such as bond mechanisms or effects of cracking concrete and section curvature.This paper investigates the response of RC beams with NSM circular CFRP rods through static and dynamic tests, analyzing the failure mechanisms of beams due to loss of strength and/or delamination. Four RC beams were built and subjected to bending loading until failure; three of which were strengthened using NSM CFRP rods. The experimental tests regarded beams with NSM strengthening in absence of execution defects and with local lacks of adherence. Theoretical and experimental results obtained from pull-out tests of concrete elements with NSM CFRP rods, allowed obtaining shear-slip laws and estimating fracture energy value for analysis of bond capacity of the NSM CFRP rod contained in the beams. Finally, the dynamic behavior was investigated considering free–free vibrations of beams and the changes of frequency values due to damages. The experimental results obtained by static and dynamic tests were discussed and analyzed to assess the NSM strengthening.

Efficiency of CFRP NSM strips and EBR plates for flexural strengthening of RC beams and loading pattern influence

The efficiency of Fibre Reinforced Polymer (FRP) materials for strengthening existing Reinforced Concrete (RC) structures according to the Near Surface Mounted (NSM) technique can be greater than the External Bonded Reinforcement (EBR) technique because debonding phenomena are less relevant and thus the tensile strength of the FRP materials is better exploited. Some experimental results and numerical analyses performed in the past showed that the debonding load in RC beams reinforced with EBR-FRP could be influenced by the effect of the loading pattern. Nevertheless, most bending tests available in the literature have been conducted with a 3 or 4 point loading scheme and very few tests have been performed under distributed loads. This issue has not yet been investigated for RC beams reinforced with NSM-FRP.Therefore, the results of flexural tests on RC beams strengthened with both NSM and EBR techniques are discussed in this paper to show that the debonding phenomena for NSM strip strengthened beams are less significant than for EBR plate beams. Moreover, the effect of the loading pattern is analysed to evaluate the sensitivity of failure modes and loads to different distributions of bending moment and shear along the beam. Finally, the experimental results are compared with the predictions of theoretical models and code provisions.

Reviewing the FRP Strengthening Systems

Several methods have been invented for flexural strengthening of RC beams using the FRP materials in recent years. These techniques are categorized into two main groups: Externally Bonded Reinforcement (EBR) techniques and Near Surface Mounted (NSM) techniques. The EBR family contains EBR (with conventional surface preparation), EBROG, EBRIG, MF-EBR, HOLING methods. The NSM family contains NSM-FRP Rods, NSM-FRP Sheets, NSM-MMFRP Rods methods. The EBR family techniques Despite the ease of implementation have weaknesses such as vulnerability against sever environmental conditions. Although the NSM techniques, have longer installation time than the EBR family techniques but in these methods the strengthening materials are greatly protected against the environmental effects. In this paper, the various techniques for flexural strengthening of RC members from both the EBR and NSM families have been fully described and advantages and disadvantages of each technique have been discussed.

Application of near surface mounted (NSM) strengthening technique to traditional timber frame walls

Timber frame buildings are well known as an efficient seismic resistant structure popular all over the world not only due to their seismic performance, but also to their low cost and the strength they offer. These constructions still exist today and it is important to be able to preserve them, so a better knowledge on their behaviour is sought. Furthermore, historic technologies could be used even in modern constructions to build seismic resistant buildings using more natural materials with lesser costs.A great rehabilitation effort is being carried out on this type of buildings, as their neglect has led to decay or their change in use and alterations to the structure has led to the need to retrofit such buildings; only recently studies on their behaviour have become available and only a few of them address the issue of possible strengthening techniques for this kind of walls.In this scope, an innovative retrofitting technique (near surface mounted steel flat bars) is proposed and validated on traditional timber frame walls based on an extensive experimental program. The results of the static cyclic tests on distinct wall typologies retrofitted with the NSM technique are herein presented and discussed in detail. The main features on deformation, lateral stiffness, lateral resistance and seismic performance indexes are analysed.