Fire behaviour of reinforced concrete beams strengthened with CFRP laminates: Protection systems with insulation of the anchorage zones

This paper presents experimental and numerical investigations about the fire behaviour of reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) laminates. The main objective was to assess the efficacy of different fire protection systems and to evaluate the viability of their use in floors of buildings. Fire resistance tests were conducted on an intermediate scale oven to investigate the behaviour under fire (ISO 834) of loaded CFRP-strengthened RC beams. The fire protection systems comprised calcium silicate boards and layers of vermiculite/perlite cement based mortar, with thicknesses of 25 mm and 40 mm, applied along the bottom soffit of the beams that was directly exposed to fire. In addition, the anchorage zones of the CFRP laminates were highly thermally insulated in order to evaluate the benefits of this particular constructive detail. Member deflection and temperatures throughout the midspan section were measured and recorded during the tests. When the strengthening system was left unprotected in the exposed length of the beam, the CFRP laminate anchorage debonded after about 23 min. When the above mentioned fire protection materials were applied in the exposed length of the beams, the strengthening system debonded after between 60–89 min (25 mm thickness) and 137-167 min (40 mm). Two-dimensional finite element thermal models of all beams tested were also developed in order to predict the evolution of temperatures in the materials. The calculated temperatures compared reasonably well with those measured in the tests.     

CFRP加固混凝土梁各受力阶段的剥离机理

粘贴碳纤维片加固混凝土梁的试验数据和破坏模式表明,在锚固措施可靠的情况下,界面粘贴失效或基面混凝土剥离是加固混凝土梁的主要早期破坏形态。为研究混凝土梁不同受力阶段对界面粘结失效或混凝土剥离的影响程度,针对实际加固工程中常见的混凝土梁损伤状况并结合室内试验结果,分别研究了粘贴碳纤维片加固完整梁及不同开裂程度梁在不同受力阶段中的界面应力分布与剥离机理,指出了加固梁的开裂或裂缝扩展是导致界面或粘贴基面混凝土剥离的主要原因。最后,结合实际混凝土梁的损伤特点,提出了加固设计施工过程中的注意事项及应采取的技术措施。     

剪跨比对CFRP加固无腹筋混凝土梁剪切破坏及尺寸效应的影响研究

剪跨比对FRP抗剪加固梁的裂缝开展和破坏模式有重要影响,但对FRP加固梁抗剪强度及尺寸效应的影响研究较少。采用三维细观数值模拟方法,考虑混凝土细观组成的非均质性及碳纤维布(CFRP)与混凝土之间的相互作用,建立了CFRP加固无腹筋钢筋混凝土梁剪切破坏力学分析模型。在验证细观模拟方法合理性的基础上,拓展模拟与分析了剪跨比对CFRP加固钢筋混凝土梁剪切破坏及尺寸效应的影响机制与规律。研究结果表明:剪跨比对CFRP抗剪加固梁剪切破坏模式影响较大,剪跨比越大,加固梁愈趋近于延性较好的斜拉破坏;剪跨比对CFRP加固梁抗剪承载力有较大影响,对抗剪强度尺寸效应影响较小;剪跨比对加固梁中的CFRP剪切贡献影响较大,剪跨比越大,CFRP对加固梁的抗剪效果越好,其中对中型剪跨比(λ=2.5)的梁加固效果最有效。     

Near surface mounted CFRP laminates for shear strengthening of concrete beams

A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminates into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets, applied according to the externally bonded reinforcement (EBR) technique; and two beams strengthened with NSM precured laminates of CFRP, one of them with laminates positioned at 90° and the other with laminates positioned at 45° in relation to the beam axis. Influences of the laminate inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminates at 45° was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. The NSM was also faster and easier to apply than the EBR technique. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.     

Experimental Behaviour of RC Beams Shear Strengthened with NSM CFRP Laminates

Abstract: The near‐surface mounted (NSM) is one of the most recent techniques applied for the increase of the shear resistance of reinforced concrete (RC) beams. This technique involves the installation of carbon fibre reinforcement polymers (CFRP) laminates into thin slits open on the concrete cover of the elements to strengthen. The effectiveness of this technique for the shear strengthening of T cross‐section RC beams was assessed by experimental research. For this purpose, three inclinations of laminates were tested (45º, 60º and 90º) and, for each inclination, three percentages of CFRP were applied in RC beams with a percentage of steel stirrups of 0.10% (ρ_sw). The highest percentage of laminates was designed to provide a maximum load similar to the reference RC beam, which was reinforced with a reinforcement ratio of steel stirrups of 0.28% (ρ_sw = 0.28%). For each percentage of laminates, a homologous RC beam strengthened with unidirectional U‐shaped CFRP wet lay‐up sheets (discrete strips) applied according to the externally bonded reinforcement technique was also tested, with the purpose of comparing the effectiveness of these two CFRP‐strengthening techniques. To evaluate the influence of the percentage of steel stirrups in the effectiveness of the NSM technique, some of the abovementioned CFRP configurations were also applied in beams with ρ_sw = 0.17%.     

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 (T_g). The comparison with the EBR-strengthened beams confirmed the much better performance of the NSM strengthening.     

NSM shear strengthening technique with CFRP laminates applied in high-strength concrete beams with or without pre-cracking

The effectiveness of the near surface mounted (NSM) shear strengthening technique with carbon fiber reinforced polymer (CFRP) laminates applied in high-strength concrete beams with a certain percentage of existing steel stirrups is assessed by experimental research. In this context, the influence of the following main parameters are investigated: (i) the percentage and the inclination of the CFRP laminates; (ii) the percentage of existing steel stirrups; (iii) the existence of cracks when the reinforced concrete (RC) beams are shear strengthened with NSM CFRP laminates. The results showed that the NSM shear strengthening technique with CFRP laminates is more effective when applied to RC beams of high-strength concrete, not only in terms of increasing the load carrying capacity of the beams, but also in assuring higher mobilization of the tensile properties of the CFRP. Inclined laminates were more effective than vertical laminates and the shear resistance of the beams has increased with the percentage of laminates. Pre-cracked RC beams strengthened with NSM CFRP laminates have presented a load carrying capacity similar to that of the homologous uncracked strengthened beams.     

Long-term behavior of existing low-strength reinforced concrete beams strengthened with carbon fiber composite sheets

This paper presents the results of an experimental study of the short- and long-term behavior of low-strength reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP). A numerical analysis model was developed and verified for the calculation of the long-term deflection and maximum allowable long-term load of such RC beams. A parametric study was also conducted and it was found that the maximum allowable long-term load of a CFRP-strengthened beam was dominated by the deflection of RC beam when the cubic compressive strength of concrete was less than a certain value. For concrete of higher strength, the maximum allowable long-term load was dominated by the stress levels of the steel bars. It was also found that the yielding load of the strengthened beams increased significantly with areas strengthened by CFRP sheets and steel bars, while the maximum allowable long-term load only increased slightly.     

Interfacial shear stress concentration in FRP-strengthened beams

This paper reports the results of an experimental programme designed to study the interfacial shear stress concentration at the plate curtailment of reinforced concrete (RC) beams strengthened in flexure with externally bonded carbon fibre-reinforced polymer (CFRP). Specifically, the study looks at the relationship between the CFRP plate thickness and the interfacial shear stress concentration at the plate curtailment, the failure modes of the CFRP-strengthened beams as well as the efficiency of the CFRP external reinforcing system. Comparing the experimental results with existing models' predictions is another objective of this study. The experimental programme included five RC beams 115 mm×150 mm in cross-section and 1500 mm in length. Four of the RC beams were reinforced externally with CFRP plates of different thicknesses. Tests in this study showed that the thickness of CFRP plate affects not only the load-carrying and deflection capacities of the strengthened beam, but also the shear stress concentration at the CFRP/concrete interface and the beam failure mode.     

Experimental and analytical investigation of reinforced high strength concrete continuous beams strengthened with fiber reinforced polymer

Carbon and glass fiber reinforced polymer (CFRP and GFRP) are two materials suitable for strengthening the reinforced concrete (RC) beams. Although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminate. In addition, most design guidelines were developed for simply supported beams with external FRP laminates. This paper presents an experimental program conducted to study the flexural behavior and redistribution in moment of reinforced high strength concrete (RHSC) continuous beams strengthened with CFRP and GFRP sheets. Test results showed that with increasing the number of CFRP sheet layers, the ultimate strength increases, while the ductility, moment redistribution, and ultimate strain of CFRP sheet decrease. Also, by using the GFRP sheet in strengthening the continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase ultimate strength of beam. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. An analytical model for moment–curvature and load capacity are developed and used for the tested continuous beams in current and other similar studies. The stress–strain curves of concrete, steel and FRP were considered as integrity model. Stress–strain model of concrete is extended from Oztekin et al.’s model by modifying the ultimate strain. Also, new parameters of equivalent stress block are obtained for flexural calculation of RHSC beams. Good agreement between experiment and prediction values is achieved.     

Flexural strengthening of concrete beams with CFRP laminates bonded into slits

Near surface mounted (NSM) strengthening technique using carbon fibre reinforced polymer (CFRP) laminate strips was applied for doubling the load carrying capacity of concrete beams failing in bending. This objective was attained and the deformational capacity of the strengthened beams was similar to the corresponding reference beams. The NSM technique has provided a significant increment of the load at serviceability limit state, as well as, the stiffness after concrete cracking. The maximum strain in the CFRP laminates has attained values between 62% and 91% of its ultimate strain. A numerical strategy was developed to simulate the deformational behaviour of RC beams strengthened by NSM technique. Not only the load carrying capacity of the tested beams was well predicted, but also the corresponding deflection.     

Nonlinear finite element modeling of concrete deep beams with openings strengthened with externally-bonded composites

This paper aims to develop 3D nonlinear finite element (FE) models for reinforced concrete (RC) deep beams containing web openings and strengthened in shear with carbon fiber reinforced polymer (CFRP) composite sheets. The web openings interrupted the natural load path either fully or partially. The FE models adopted realistic materials constitutive laws that account for the nonlinear behavior of materials. In the FE models, solid elements for concrete, multi-layer shell elements for CFRP and link elements for steel reinforcement were used to simulate the physical models. Special interface elements were implemented in the FE models to simulate the interfacial bond behavior between the concrete and CFRP composites. A comparison between the FE results and experimental data published in the literature demonstrated the validity of the computational models in capturing the structural response for both unstrengthened and CFRP-strengthened deep beams with openings. The developed FE models can serve as a numerical platform for performance prediction of RC deep beams with openings strengthened in shear with CFRP composites.     

Flexural strengthening of RC beams using Hybrid Composite Plate (HCP): Experimental and analytical study

Hybrid Composite Plate (HCP) is a reliable recently proposed retrofitting solution for concrete structures, which is composed of a strain hardening cementitious composite (SHCC) plate reinforced with Carbon Fibre Reinforced Polymer (CFRP). This system benefits from the synergetic advantages of these two composites, namely the high ductility of SHCC and the high tensile strength of CFRPs. In the material-structural of HCP, the ultra-ductile SHCC plate acts as a suitable medium for stress transfer between CFRP laminates (bonded into the pre-sawn grooves executed on the SHCC plate) and the concrete substrate by means of a connection system made by either chemical anchors, adhesive, or a combination thereof. In comparison with traditional applications of FRP systems, HCP is a retrofitting solution that (i) is less susceptible to the detrimental effect of the lack of strength and soundness of the concrete cover in the strengthening effectiveness; (ii) assures higher durability for the strengthened elements and higher protection to the FRP component in terms of high temperatures and vandalism; and (iii) delays, or even, prevents detachment of concrete substrate. This paper describes the experimental program carried out, and presents and discusses the relevant results obtained on the assessment of the performance of HCP strengthened reinforced concrete (RC) beams subjected to flexural loading. Moreover, an analytical approach to estimate the ultimate flexural capacity of these beams is presented, which was complemented with a numerical strategy for predicting their load-deflection behaviour. By attaching HCP to the beams' soffit, a significant increase in the flexural capacity at service, at yield initiation of the tension steel bars and at failure of the beams can be achieved, while satisfactory deflection ductility is assured and a high tensile capacity of the CFRP laminates is mobilized. Both analytical and numerical approaches have predicted with satisfactory agreement, the load-deflection response of the reference beam and the strengthened ones tested experimentally.     

Performance of steel beams strengthened with CFRP laminate – Part 2: FE analyses

A new method for repairing and strengthening steel is under development and consists of using CFRP (carbon-fibre-reinforced-polymer) laminates bonded to the steel substrate. Research on this method has been conducted by a few research groups in recent years. The idea is to let the CFRP laminate carry a large part of the stresses and thereby reduce the load on the steel, which may have had its capacity lowered due to deterioration or fatigue. The present paper presents the results of FE analyses of steel beams strengthened with bonded CFRP laminates. The interfacial shear and peeling stresses that appear in the bond line between the steel and CFRP laminate are studied in both the elastic and plastic phase of the steel beam. Comparisons with the results obtained from laboratory tests conducted on steel beams strengthened with bonded CFRP laminates show that the behaviour of the strengthened beams can be captured using FE analyses. The distribution of the shear and peeling stresses near the end of the bond line were obtained from the FE analyses, together with the interfacial stresses that develop near beam mid-span due to the yielding of the steel. These stresses may exceed the capacity of the adhesive and cause debonding in this region.     

Assessment of CFRP strengthened RC beams through dynamic tests

The use of Fiber Reinforced Polymers (FRPs) for strengthening damaged RC beams has become common practice over recent years. Two methods adopted for repairing or strengthening such beams are FRP plates/sheets glued onto their concrete surface and FRP rods or strips inserted into grooves.This paper investigates experimental vibration monitoring of strengthening according to the two aforementioned methods through dynamic tests on six RC beam models strengthened using carbon FRP. Three beams were strengthened applying CFRP sheets on the tensile cracked surface after loading and three beams were strengthened by near surface mounted (NSM) CFRP rods. The experimental results include both the static tests to create damage and the dynamic tests of strengthened, measuring natural vibration modes and frequency values for free end beams.Comparison between experimental dynamic response and static behavior established that vibration monitoring is a convenient, non-destructive method for assessing strengthened beams under service loads. Further studies and tests must be developed in order to solve the issues that emerged following the analysis of the experimental data obtained.     

Behavior and performance of RC T-section deep beams externally strengthened in shear with CFRP sheets

A series of experimental tests were carried out to investigate the behavior and performance of reinforced concrete (RC) T-section deep beams strengthened in shear with CFRP sheets. Key variables evaluated in this study were strengthening length, fiber direction combination of CFRP sheets, and an anchorage using U-wrapped CFRP sheets. A total of 14 RC T-section deep beams were designed to be deficient in shear with a shear span-to-effective depth ratio (a/d) of 1.22. Crack patterns and behavior of the tested deep beams were observed during four-point loading tests. Except the CS-FL-HP specimen, almost all strengthened deep beams showed a shear–compression failure due to partial delamination of the CFRP sheets. From the load–displacement (p–u) curves, the effects of key variables on the shear performance of the strengthened deep beams were addressed. It was concluded from the test results that the key variables of strengthening length, fiber direction combination, and anchorage have significant influence on the shear performance of strengthened deep beams. In addition, a series of comparative studies between the present experimental data and theoretical results in accordance with the commonly applied design codes were made to evaluate the shear strength of a control beam and deep beams strengthened with CFRP sheets.     

Analysis and design of RC structures strengthened with mechanically fastened FRP laminates: A review

The use of Mechanically Fastened Fiber Reinforced Polymer (MF-FRP) laminates is emerging as a viable alternative to adhesively bonded FRP laminates for the rehabilitation of reinforced concrete (RC) members such as beams and slabs. A recently published state-of-the-art review of the experimental research has demonstrated the viability and effectiveness of MF-FRP systems. This paper provides a state-of-the-art review of the analytical and numerical studies performed over the last decade with the aim of: (a) predicting the strength, the load-deformation response and the failure mode of rehabilitated RC members, and (b) accounting for the interfacial behavior between the concrete and the MF-FRP laminate. Ultimate strength models and constitutive models are critically reviewed based on their key assumptions and formulations and compares the analytical predictions with previously reported experimental results.     

Efficacy of CFRP-based techniques for the flexural and shear strengthening of concrete beams

Near surface mounted (NSM) and externally bonded reinforcement (EBR) strengthening techniques are based on the use of carbon fiber reinforced polymer (CFRP) materials and have been used for the structural rehabilitation of concrete structures. In the present work, the efficacies of the NSM and EBR techniques for the flexural and shear strengthening of reinforced concrete beams are compared carrying out two experimental groups of tests. For the flexural strengthening, the efficacy of applying CFRP laminates according to NSM is compared to those resulting from applying CFRP laminates and wet lay-up CFRP sheets according to EBR technique. The influences of the equivalent reinforcement ratio (steel and laminates) and spacing of the laminates on the efficiency of the NSM technique for the flexural strengthening is also investigated. A numerical strategy is implemented to analyze the applicability of the FRP effective strain concept, proposed by ACI and fib in the design of FRP systems for the flexural strengthening. To assess the efficacy of the NSM technique for the shear strengthening of concrete beams, four beam series of distinct depth and longitudinal tensile steel reinforcement ratio are tested. Each series is composed of one beam without any shear reinforcement and one beam using the following shear reinforcing systems: conventional steel stirrups; strips of wet lay-up CFRP sheet of U configuration applied according to EBR technique; and laminates of CFRP embedded into vertical or inclined (45°) pre-cut slits on the concrete cover of the beam lateral faces, according to the NSM technique. Using the obtained experimental results, the performance of the analytical formulations proposed by ACI, fib and Italian guidelines is appraised.     

模拟不中断交通状况下粘贴碳纤维布加固钢筋混凝土梁试验研究

为了调查动荷载作用下碳纤维布与钢筋混凝土梁的粘贴性能及加固效果,进行了5根模拟交通荷载(等幅正弦波形动荷载)作用下粘贴碳纤维布加固钢筋混凝土梁和1根保持荷载下粘贴碳纤维布加固钢筋混凝土梁以及2根对比梁的试验研究。试验中考虑了混凝土等级、配筋率、有无锚固条、粘贴长度、荷载幅值5个变化参数。试验结果表明,在模拟交通荷载的作用下,碳纤维布加固的钢筋混凝土梁粘贴性能满足要求,粘贴效果良好。在模拟交通荷载后的静载作用下,试验梁的抗弯承载力提高较多,加固效果明显,进一步验证了粘贴效果。     

Structural behaviour of RC beams with external flexural and flexural–shear strengthening by FRP sheets

This paper presents experimental research on reinforced concrete (RC) beams with external flexural and flexural–shear strengthening by fibre reinforced polymer (FRP) sheets consisting of carbon FRP (CFRP) and glass FRP (GFRP). The work carried out has examined both the flexural and flexural–shear strengthening capacities of retrofitted RC beams and has indicated how different strengthening arrangements of CFRP and GFRP sheets affect behaviour of the RC beams strengthened. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the RC beam. In addition theoretical calculations are developed to estimate the bending and shear capacities of the beams tested, which are compared with the corresponding experimental results.