NSM-CFRP Strips Research Articles

Effective strain of CFRP in RC beams strengthened in shear with NSM reinforcements

This paper investigates the structural behavior of reinforced concrete (RC) beams strengthened in shear by near surface mounted (NSM) method through experimental and numerical studies. Furthermore, the effect of applying different concrete strengths and steel shear bars is studied through the case studies. Four reinforced concrete beam specimens with dimensions of 2000 × 300 × 200 mm were cast in two series (with and without stirrups). The beams were tested under four-point flexural loading and their load-deflection curves as well as their failure mechanisms were compared. Experimental results showed that bonding carbon-fiber-reinforced polymer (CFRP) reinforcements into the grooves enhanced the shear capacity up to 42% and 68% in the beams with and without stirrups, respectively; and the reinforcing strips failed due to CFRP rupture. Furthermore, in NSM strengthened beams, ductility index decreased for both strengthened beams with and without stirrups compared to the reference beams. In addition, the overall stiffness of the strengthened specimen with stirrup was slightly more than that of the strengthened beam without stirrup. Finite element parametric studies through modeling bond behavior for the CFRP strips showed that as the concrete strength decreases, the debonding possibility may decrease, and as the shear reinforcement amount increases, the ultimate strain in CFRP strips decreases. Based on the results of the parametric study, a new relationship was proposed for determining the effective strain in NSM-CFRP strips.

Performance of RC Beam Strengthened with NSM-CFRP Strip Under Pure Torsion: Experimental and Numerical Study

This paper presents experimental and finite element model (FEM) on reinforced concrete (RC) beam behavior strengthened by near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips subjected to pure torsional loading. Seven rectangular reinforced concrete RC beams of 250 mm × 250 mm × 1600 mm were constructed and tested considering the effect of length, inclination, arrangement of longitudinal and scheme of NSM-CFRP strips. The outcomes of the tests indicated remarkable enhancement in the behavior of torsional strengthened beams using NSM-CFRP strips. In general, the beams strengthening with inclined CFRP-NSM strips exhibited an increase in torsion moment strength and angle of twist more than the beams strengthening with vertical CFRP-NSM strips. The experimental measured results are validated with a 3D numerical simulation carried out using nonlinear finite element (FE) modeling. Finally, it can be seen that the calculated numerical torsional moment–angle of twist behavior is in agreement with the experimental results for all RC beams.

Corroded RC Beams at Service Load before and after Patch Repair and Strengthening with NSM CFRP Strips

This paper presents the experimental results of the structural behavior of four reinforced concrete beams with corroded steel reinforcement at service loads. One beam was non-corroded, one beam was corroded under an accelerated electrochemical technique to a small corrosion level (for one corrosion cycle), while two beams were corroded under the same conditions of an accelerated electrochemical technique and then subjected to vertical service loads that corresponded to 60% and 75% of the yield load of the non-corroded beam respectively for three corrosion cycles (with maximum mass loss around 25% for the first and 31% for the latter). Longitudinal cracks due to corrosion and flexural cracks due to loading were thoroughly recorded at the end of each cycle. The beam under the 75% service load had higher deflection increase for heavier corrosion. After the three successive serviceability load tests, the cracked concrete cover was removed and the steel rebars were treated. The cement-based repair mortar and two NSM FRP laminates were applied to both beams and were tested to failure. Despite the heavy corrosion, the patch repair and NSM strengthening enhanced the load-bearing capacity of the beams when compared with the non-corroded beam. All 10 tests are thoroughly discussed.

Behavior of RC beams strengthened with NSM CFRP strips under flexural repeated loading

Strengthening with near surface mounted carbon fibre reinforced polymers (NSM-CFRP) is a strengthening technique that have been used for several decades to increase the load carrying capacity of reinforced concrete members. In Iraq, many concrete buildings and bridges were subjected to a wide range of damage as a result of the last war and many other events. Accordingly, there is a progressive increase in the strengthening of concrete structures, bridges in particular, by using CFRP strengthening techniques. Near-surface mounted carbon fibre polymer has been recently proved as a powerful strengthening technique in which the CFRP strips are sufficiently protected against external environmental conditions especially the high-temperature rates in Iraq. However, this technique has not been examined yet under repeated loading conditions such as traffic loads on bridge girders. The main objective of this research was to investigate the effectiveness of NSM-CFRP strips in reinforced concrete beams under repeated loads. Different parameters such as the number of strips, groove size, and two types of bonding materials (epoxy resin and cement-based adhesive) were considered. Fifteen NSM-CFRP strengthened beams were tested under concentrated monotonic and repeated loadings. Three beams were non-strengthened as reference specimens while the remaining were strengthened with NSM-CFRP strips and divided into three groups. Each group comprises two beams tested under monotonic loads and used as control for those tested under repeated loads in the same group. The experimental results are discussed in terms of load-deflection behavior up to failure, ductility factor, cumulative energy absorption, number of cycles to failure, and the mode of failure. The test results proved that strengthening with NSM-CFRP strips increased both the flexural strength and stiffness of the tested beams. An increase in load carrying capacity was obtained in a range of (1.47 to 4.49) times that for the non-strengthened specimens. Also, the increase in total area of CFRPs showed a slight increase in flexural capacity of (1.02) times the value of the control strengthened one tested under repeated loading. Increasing the total area of CFRP strips resulted in a reduction in ductility factor reached to (0.71) while the cumulative energy absorption increased by (1.22) times the values of the strengthened reference specimens tested under repeated loading. Moreover, the replacement of epoxy resin with cement-based adhesive as a bonding material exhibited higher ductility than specimen with epoxy resin tested under monotonic and repeated loading.

Flexural Behavior of RC Beams Strengthened by NSM-CFRP Laminates or Bars

The strengthening and enhancing the structures represents an important aspect in the construction industry due to the growing need to increase the tolerability of origin to a specific level and within the required rehabilitation and maintenance work. This paper assessed the performance and effectiveness of the Near Surface Mounted (NSM) strengthening technique for the reinforced concrete beams. Three (140x260x2700 mm) reinforced concrete beams were strengthened in flexure with NSM strengthening systems using Carbon Fiber Reinforced Polymer (CFRP) strips, bars, and cement-based adhesive as a binding materials. The flexural behaviour of the beams was evaluated by testing the specimens under three-point loading to failure. The structural performance, deflection, ductility, stiffness, and modes of failure of the tested beams are presented and discussed in this paper. The test results indicate that using NSM-CFRP strips and bars is practical and significantly improves the stiffness and increases the flexural capacity of reinforced concrete beams. The strength increments were 48, 42, and 15 percent recorded with CFRP bars, rough strips, and smooth strips respectively. The deflection of the strengthened beams was reduced by about 66, 48, and 58 percent for CFRP smooth strips, rough strips, and CFRP bars respectively, compared with the control beam due to the increased stiffness of the strengthened beams.

Performance of heat-damaged partially-insulated RC beams strengthened with NSM CFRP strips and epoxy adhesive

This paper describes an experimental and numerical study of the behaviour of RC beams damaged by heating and subsequently strengthened using NSM CFRP laminates. In the experiments, two beams were normal temperature controls, and the other beams were heated under the ISO-834 standard fire curve. The flexural behaviour was studied after exposure to 700 °C and 800 °C with and without insulation on the top surface of the concrete. The parameters studied in this investigation were unheated and heat-damaged beams, level of heat exposure and use of insulation. Three beams were un-strengthened fire-damaged control beams and the other beams were strengthened with CFRP laminates after being heated. Although the ultimate load and stiffness decreased, repair with NSM CFRP laminates using epoxy adhesive was found to increase the load capacity and stiffness of the beams. Finite element models (FEMs) were able to predict the experimental behaviour reasonably well.

Modelling of NSM CFRP strips embedded in concrete after exposure to elevated temperature using epoxy adhesives

The repair and strengthening of concrete structures with composite material has become common during the last decade. Based on experimental results of specimens damaged by heating and subsequently repaired using CFRP laminate and epoxy adhesive, finite element analysis (FEA) was conducted to simulate the bond behaviour between NSM CFRP strips and heat-damaged concrete. The theoretical parameters included unheated and heat-damaged concrete and level of heat exposure. The finite element models (FEMs) showed reasonable agreement with the experimental results, and can therefore be used to predict the effects of high temperature on pull-out values after exposure to elevated temperatures.

The performance of near-surface mounted CFRP strengthened RC beam in fire

Carbon fiber reinforced polymers (CFRP) have been widely used in the retrofitting and strengthening of concrete structures. However, the layouts of CFRP in externally bonded (EB) strengthening system and near surface mounted (NSM) system result in the vulnerability of fire resistance. To explore the fire resistance potential of NSM-CFRP strengthening system, 15 RC beams strengthened with CFRP and 2 reference beams were loaded to failure either under the ISO834 standard fire or at ambient temperature. In test, the following factors were considered: the strengthening method (near surface mounted technique/externally bonded technique), adhesive (epoxy and magnesium-oxychloride cement), fire protection materials (thin intumescent fire retardant coating and thick fire retardant coating) and location of fire protection (local patch protection/single-sided protection/U-shaped protection). With appropriate fire protection, the RC beams strengthened with NSM-CFRP resisted the standard fire for more than 3h at high load level. Besides the effects of the aforementioned factors, the friction stress at the CFRP/matrix interface when CFRP is in globally slipping is found playing a crucial role for the fire resistance of NSM-CFRP strengthening system. The experimental observation sheds some light on how the NSM-CFRP strips can retain the contribution at elevated temperatures and why NSM-CFRP strengthening outperforms EB-CFRP strengthening when exposed to fire.

Numerical Modeling of Reinforced Concrete Beams Strengthened by NSM-CFRP Technique

The NSM method has been used in recent years for the strengthening of RC beams with CFRP materials. A numerical study has been carried out on rectangular RC beams, strengthened with NSM-CFRP by using FE code ATENA. The numerical FE simulations were compared with experimental measurement tested by other investigators on specimen strengthened with NSM CFRP strips. On overall, the predicted FE peak loads and mid-span deflection responses agreed quite well with the corresponding measured experimental tested data at all stages of flexural loading. Therefore, the developed FE model is suitable as a practical and economical tool especially in parametric studies for accurate modeling and analysis of flexural strengthening of RC members with NSM-FRP reinforcement.

Recovering shear capacity of heat-damaged beams using NSM-CFRP strips

The potential of recovering shear capacity of heat-damaged beams using near surface mounted (NSM)-carbon fiber reinforced polymer (CFRP) strips was investigated in this work. Twenty-two under-reinforced concrete beams with shear reinforcement deficiency (150×250×1400) were cast and heat-treated for two hours in an electric furnace; under different temperatures (300, 500, and 600°C). Later, control beams were strengthened, whereas post-heated ones repaired with NSM-CFRP strips, directed vertically across the depth of the beams and at different spacing values ranging from 100 to 200mm. The mechanical behavior of the beams was evaluated under three-point loading test setup; with data collected and analyzed for the characteristics of the relationships between load and deflection or slippage (between concrete and reinforcing steel). Cracking and failure modes were monitored and characterized, as well. The experimental results indicated that the efficiency of repairing heat-damaged reinforced concrete beams using NSM-CFRP strips depended mainly upon damage level and spacing between NSM-CFRP strips. The result showed that using NSM-CFRP allowed restoring load capacity of the beams, damaged at 300 and 500°C; but not so for those, post-heated to 600°C. The latter achieved an utmost load capacity recovery of about 91% of original value. Lowering spacing between the CFRP strips contributed to increasing shear load capacity of repaired beams as long as peeling-off failure of the concrete cover across the depth of beam’s soffits was avoided. The flexural stiffness was not recovered for heat-damaged and repaired beams whereas toughness and ductility were noticeably improved.

Influence of temperature on the curing of an epoxy adhesive and its influence on bond behaviour of NSM-CFRP systems

In NSM-CFRP installations, the mechanical behaviour of the strengthening system is strongly influenced by the epoxy adhesive, particularly at early ages. In the present work, the influence of temperature on the curing process of the epoxy was investigated. Three distinct temperatures were studied: 20, 30 and 40 °C. The elastic modulus of the adhesive was monitored through EMM-ARM (Elasticity Modulus Monitoring through Ambient Response Method). Direct pull-out tests with concrete specimens strengthened with NSM CFRP strips were carried out at the same three distinct temperatures to compare the evolution of bond performance with the E-modulus of epoxy since early ages. The results showed that increasing the curing temperature significantly accelerated both the curing process of the epoxy adhesive and the evolution of bond performance. The EMM-ARM technique has revealed its ability in clearly identifying the hardening kinetics of epoxy adhesives, allowing also thermal activation analysis. Finally, existing models for predicting temperature-dependent mechanical properties were extended to also describe the bond behaviour of NSM-CFRP applications.

Fire behaviour of thermally insulated RC beams strengthened with NSM-CFRP strips: Experimental study

This paper presents the results of fire resistance tests on reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) strips installed according to the near surface mounted (NSM) technique using two different adhesives. The beams were simultaneously subjected to a service load and the ISO 834 standard fire. Different fire protection schemes were studied, comprising a thinner insulation layer along the bottom soffit of the beams and a thicker one at the CFRP anchorage zones. The main objectives of this paper were (i) to understand in further depth the fire behaviour of NSM-strengthened RC beams, in particular the structural effectiveness of the strengthening system during fire, (ii) to evaluate the efficiency of the above-mentioned fire protection strategy in extending the CFRP mechanical contribution during fire, and (iii) to compare the fire performance of the NSM-strengthening system with that of the alternative externally bonded reinforcement (EBR) technique, recently investigated under similar test conditions. The results obtained showed that using the adopted insulation schemes (i.e., thicker insulation at the anchorage zone and thinner insulation in the current zone), even after the CFRP-concrete bond is highly damaged in the central zone of the beams, the strengthening system is able to retain its structural effectiveness through a cable mechanism: for insulation thicknesses of 25 mm (current zone) and 50 mm (anchorage zones), the fire resistance of the strengthening system was extended up to 114 min. The loss of effectiveness of the CFRP system occurred when the average temperature in the adhesive at the CFRP anchorage zones attained values ranging from 2.2 to 5.6 times its glass transition temperature (Tg). The comparison with the EBR-strengthened beams confirmed the much better performance of the NSM strengthening.

Effects of Sustained Load and Freeze-Thaw Exposure on RC Beams Strengthened with Prestressed NSM-CFRP Strips

An experimental program was conducted to investigate the combined effects of sustained load and freeze-thaw cycling exposure on the flexural behaviour of reinforced concrete (RC) beams strengthened in flexure using prestressed Near-Surface Mounted (NSM) Carbon Fibre Reinforced Polymer (CFRP) strips. Two sets of five large-scale simply supported rectangular RC beams (sets BS-F and BS-FS) were tested. Each set consisted of one un-strengthened control beam and four beams strengthened using NSM-CFRP strips prestressed to 0, 20, 40, and 60% of the CFRP ultimate tensile strength. After strengthening, the beams from set BS-F were exposed to 500 freeze-thaw cycles. The beams from set BS-FS were exposed to 500 freeze-thaw cycles; simultaneously, each beam in this set was subjected to a sustained load representing 47% of the theoretical ultimate capacity of the non-prestressed NSM-CFRP strengthened beam. Thereafter, the beams were tested at room temperature under static monotonic loading in four-point bending configuration until failure. The damage done to the beams due to environmental exposure and long-term loading was evaluated, and furthermore, the effects of sustained load and freeze-thaw exposure were discussed on the load-deflection response, type of failure, ductility, energy absorption, and strain in CFRP strips. Analysis of the results revealed the significant effects of the sustained load and freeze-thaw cycling exposure on the flexural performance of RC beams strengthened with prestressed and non-prestressed NSM-CFRP strips.

Finite Element Modeling of Reinforced Concrete Slabs Strengthened in Flexural with NSM-CFRP

This study is devoted to investigate the behavior and performance of R.C. slabs strengthened with near surface mounted (NSM-CFRP). A system of computer program (ANSYS V.12.1) is used for analysis at this study. The ordinary reinforced concrete is modeled by 8-node isoparametric brick elements, while the steel reinforcing bars is modeled as axial members (bar elements) connecting opposite nodes in the brick elements with a full interaction assumption. The NSM CFRP strips were modeled by shell element. The effect of some selected parameters (thickness and shape) is studied. It is found that the ultimate load increased efficiently with increasing the thickness of NSM (87% & 123% for slabs S1 & S2 respectively). Concrete strength of R.C. slabs tested is (fc' 18.85). The maximum increment in terms of the slab load carrying capacity is limited by the concrete crushing.

Bond–slip model for CFRP strips near-surface mounted to concrete

A relatively recent method for the strengthening of concrete structures involves the embedding of fiber reinforced polymer (FRP) bars/strips into pre-cut, adhesive-filled grooves in the cover concrete. This paper presents a finite element (FE) study into the bond behavior of NSM CFRP strip-to-concrete bonded joints using a three-dimensional (3-D) meso-scale FE model developed by the authors. The effects of various parameters on the bond behavior of NSM CFRP strips are first clarified with the assistance of FE results, leading to the identification of the groove height-to-width ratio and the concrete strength as the two important parameters for the bond behavior. Based on the results of a FE parametric study on the effects of these two parameters, an analytical bond–slip model is proposed.

탄소섬유판 (CFRP) 표면매립 (NSM) 공법을 이용한 콘크리트 구조물 휨 보강에 관한 실험 연구

본 연구에서는 탄소섬유판을 콘크리트 표면에 매립하는 표면매립(NSM) 공법을 적용하여 콘크리트 구조물 보강을 수행하고, 콘크리트 내구성 증진 뿐 아니라 구조물 수명을 향상 시킬 수 있는 노후구조물 보수 보강 체계를 구축하고자 하였다. 이를 위하여, 표면매립 탄소섬유판 및 고성능 단면복구 모르터로 보강된 철근 콘크리트 부재를 제작하여 실험을 수행하였다. 실험결과, 에폭시로 탄소섬유판을 매립 부착한 철근콘크리트 부재는 보강되지 않은 부재보다 초기 강성 및 휨강도가 현저하게 증가하는 것으로 나타났다. 탄소섬유판으로 보강된 철근콘크리트 부재는 탄소섬유판의 인장 파괴로 인하여 시작되며, 탄소섬유판의 저항능력은 매우 우수하며, 보강 효율이 우수한 것으로 나타났다. 탄소섬유판을 매립하고 표면부에 고성능 모르터로 단면을 복구하는 보강 방식은 보의 강성 및 내구성을 향상시킬 뿐 아니라 미관이 우수하여 보수와 보강이 혼합된 적절한 보강 방식으로 판단된다. The purpose of this study is to establish the flexural strengthening method of the concrete members. To accomplish this objective, a total of seven concrete beams were tested. From this study, it is found that the initial flexural stiffness and strength of the beams reinforced with NSM CFRP strips were significantly improved compared to the beam without CFRP strip. Failure of the beam reinforced with NSM strips is initiated by failure of NSM strips, eventually sudden explosive compressive failure in the loaded region. This strengthening method combined with NSM CFRP strips and high performance mortar for concrete cover recovery is evaluated by a good strengthening method for the strength, durability and good appearance of concrete structures.

Experimental Study on Flexural Behavior of Damaged Concrete Beams Strengthened with NSM-CFRP Strips

In order to investigate the strengthen effect of different embedment lengths of the NSM strip on different damage levels. A series of tests were conducted on damaged reinforced concrete (RC) beams in flexure strengthened with near surface mounted (NSM) carbon- fiber-reinforced polymer (CFRP) strips, and initial cracking load, ultimate capacity, loading-deflection curves, and failure modes are examined and analyzed in the paper. The results showed that not only the initial cracking loads and ultimate capacities of the beams are significantly increased，but also the flexural stiffness of the beams in the yield and ultimate behavior stages are improved by using NSM-CFRP strips. The strengthen effect on lower damage level RC beams has no obvious difference with that on non-damaged RC beams. Anchoring of the strip end can increase the ultimate load capacities and decrease the ductility of RC beams. Debonding was found to be the primary failure mode in all cases.

Nonlinear 3D finite element modeling of RC beams strengthened with prestressed NSM-CFRP strips

This paper demonstrates a comprehensive 3D nonlinear Finite Element (FE) analysis of Reinforced Concrete (RC) beams strengthened with prestressed Near-Surface Mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) strips. Debonding effect at the epoxy-concrete interface was considered in the model by identification of fracture energies of the interfaces and appropriate bilinear shear stress-slip and tension stress-gap models. Prestressing was applied to the CFRP strips by adopting the equivalent temperature method. A constitutive confined concrete model for flexural member was generated from unconfined concrete curve and assigned to the concrete materials. The results from the FE model were validated with experimental data available in the literature. Comparison between FE and test results confirms excellent accuracy of the proposed model. An optimum prestressing level in the NSM-CFRP strips was determined that enhances the beam performance under service and ultimate loads by maintaining the amount of energy absorption in the strengthened beam equal to the un-strengthened control beam.

Fatigue behavior of RC beams strengthened with prestressed NSM CFRP rods

The fatigue performance of Reinforced Concrete (RC) beams strengthened using NSM CFRP rods was examined in this study. The testing matrix consisted of one un-strengthened beam and four beams strengthened using NSM CFRP rods prestressed to effective strain values of 0, 3260, 6899, and 9177 μ representing 0%, 20.4%, 43.1%, and 57.4% of the CFRP rod ultimate tensile strain, 1.6%. All beams were tested in four-point bending under fatigue conditions representing in-service loading for 3 million cycles at a frequency of 2 Hz. Upper and lower load limits were chosen to induce a stress range of 125 MPa in the tension steel during the first cycle. The fatigue results were compared with experimental test results of identical beams strengthened using prestressed NSM CFRP strips tested under identical fatigue conditions found elsewhere in literature. Test results showed that all strengthened beams experienced deflection increase lower than that of the un-strengthened beam which indicates the efficiency of the strengthening process in reducing the damage accumulation. Also, the percentage deflection increase as well as the stiffness degradation after 3 million cycles are almost the same for all the strengthened beams which indicates that damage accumulation is independent from the prestress level. The groove dimensionality, rather than the CFRP geometry, has a detrimental effect on the bond behavior.

Performance of RC Beams Strengthened Using Prestressed NSM-CFRP Strips Subjected to Fatigue Loading

AbstractThe fatigue performance of Reinforced Concrete (RC) beams strengthened using prestressed Near Surface Mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) strips was evaluated in this paper. Five full-scale RC beams were tested under fatigue loading simulating in-service loading conditions: one control unstrengthened beam and four beams strengthened using NSM-CFRP strips prestressed to 0, 20, 40, and 60% of the CFRP ultimate tensile strength. All beams were subjected to fatigue loading for 3 million cycles at a frequency of 2.0 Hz such that the stress range induced in the tension steel, during the initial cycle, is 125 MPa as specified by the Canadian Highway Bridge Design Code. Experimental test results show that the percentage deflection increase at the end of fatigue loading was almost the same for all beams, which implies that damage accumulation is independent from the prestress level. CFRP strain variation at midspan indicated the occurrence of deboning during the initial cycling of beams pr...