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AN EXPERIMENTAL STUDY ON FLEXURAL BEHAVIOR OF RC BEAMSSTRENGTHENED WITH NSM REINFORCEMENTWoo-Tai JUNG1, Young-Hwan PARK2, Jong-SupABSTRACT: This study presents the results of experiments performed on RC (Reinforced Concrete) beams strengthened with NSM(Near Surface Mounted) reinforcement. A total of 6 specimens have been tested. The specimens can be classified into EBR(Externally Bonded Reinforcement) specimen and NSM reinforcements specimens. Two NSM specimens with space variables were strengthened with 2 CFRP(Carbon Fiber Reinforced Polymer) strips. Experimental results revealed that NSMspecimens used CFRP reinforcements moreefficiently than the EBR specimens. Even if CFRP crosssection areas of NSM specimens have 30%,50% of EBR Specimen, the strengthening effect of NSMspecimens is superior to EBR specimen. NSM specimens with space variables showed that thstrengthening effect of the specimen with narrow space is slightly increased as compared to thespecimen with wide spaceuKEYWORDS: carbon fiber reinforced polymer, externally bonded CFRP reinforcements, nearsurface mounted CFRP reinforcements, strengthening1. INTRODUCTIONAmong the various strengthening techniques that have been developed and applied to strengthendeteriorated RC structures, a number of applications using FRP reinforcements have significantly increased recently. FRP reinforcements are bonded to concrete surfaces by adhesives but frequently experience debonding failure at the interface between FRP reinforcements and concrete. Most research, to date, has focused on investigating the strengthening effects and failure modes of EBR systemThe problem of premature failure of EBR system may be solved by increasing the interface between FRP and concrete. Using this principle, the NSM system has been introduced recently. The NSM system for concrete structure using steel reinforcement already began in 1940s. However, the corrosion of the steel reinforcement and the poor bonding performance of the grouting material largely impaired its application. The development of improved epoxy and the adoption of FRP reinforcement offered the opportunity to implement NSM system (Hassan and Rizkalla ; Tljsten and Carolin ). Because of their light weight, ease of installation, minimal labor costs and site constraints, high strength-to-weight ratios, and durability, FRP repair systems can provide an economically viable alternative to traditional repair systems and materials(Mirmiran et al. ). Rizkalla and Hassan () have compared EBR and NSM system in terms of cost, including costs of materials and labor,and strengthening effect. They concluded that the NSM system was more cost-effective than the EBR system using CFRP strips.This experimental study investigates the applicability and strengthening performances of NSM using CFRP strips. For comparison, flexural tests on RC beams strengthened by EBR and by NSM have been performed. In addition, specimens with space variables have been tested to compare the strengthening performance by cross section with wide and narrow space.2. EXPERIMENTAL PROGRAM2.1 MANUFACTURE OF SPECIMENSA total of 6 specimens of simply supported RC beams with span of 3m have been cast. The details andcross-section of the specimens are illustrated in Figure 1. A concrete with compressive strength of31.3 MPa at 28 days has been used. Steel reinforcements D10(9.53mm) of SD40 have been arrangedwith steel ratio of 0.0041 and a layer of three D13(12.7mm) has been arranged as compressionreinforcements. Shear reinforcements of D10 have been located every 10 cm in the shear zone to avoidshear failure. Table 1 summarizes the material properties used for the test beams.2.2 EXPERIMENTAL PARAMETERSTable 2 lists the experimental parameters. The control specimen, an unstrengthened specimen, has been cast to compare the strengthening performances of the various systems. CPL-50-BOND, EBR specimen, has been strengthened with CFRP strip. The remaining 4 specimens were strengthened with NSM CFRP strips. Among the specimens strengthened with NSM reinforcements, an embedding64 depth of NSM-PL-15 and NSM-PL-25 is 15mm and 25mm, respectively. A space between grooves of NSM-PL-25*2 and NSM-PL-2S is 60mm and 120mm, respectively. The strengthened length of all thespecimens has been fixed to 2,700 mm2.3 INSTALLATION OF THE FRP REINFORCEMENTSFigure 2 shows the details of cross-sections of the specimens. The strengthening process of EBR specimen (CPL-50-BOND) was proceeded by the surface treatment using a grinder, followed by the bonding of the CFRP strip. The strengthened beams were cured at ambient temperature for 7 days for the curing of epoxy adhesive. The process for NSM strengthening progressed by cutting the grooves at the bottom of the beams using a grinder, cleaning the debris, and embedding the CFRP strip after application of the adhesive. The strengthened beams were cured for 3 days so that the epoxy adhesive achieves its design strength.2.4 LOADING AND MEASUREMENT METHODSAll specimens were subjected to 4-point bending tests to failure by means of UTM (Universal Testing Machine) with capacity of 980 kN. The loading was applied under displacement control at a speed of 0.02 mm/sec until the first 15 mm and 0.05 mm/sec from 15 mm until failure. The measurement of alltest data was recorded by a static data logger and a computer at intervals of 1 second. Electrical resistance strain gauges were fixed at mid-span and L/4 to measure the strain of steel reinforcements.Strain gauges to measure the strain of concrete were located at the top, 5 cm and 10 cm away from the top on one side at mid-span. Strain gauges were also placed on the FRP reinforcement located at the bottom of the mid-span and loaded points to measure the strain according to the loading process.3. EXPERIMENTAL RESULTS3.1 FAILURE MODESBefore cracking, all the strengthened specimens exhibited bending behavior similar to theunstrengthened specimen. This shows that the CFRP reinforcement is unable to contribute to the increase of the stiffness and strength in the elastic domain. However, after cracking, the bending stiffness and strength of the strengthened specimens were seen to increase significantly until failure compared to the unstrengthened specimens.Examining the final failure, the unstrengthened control specimen presented typical bending failure mode which proceeds by the yielding of steel reinforcement followed by compression failure of concrete. The failure of CPL-50-BOND, EBR specimen, began with the separation of CFRP reinforcement and concrete at mid-span to exhibit finally brittle debonding failure (Figure 3). Failure of NSM-PL-15, NSM specimen, occurred with the rupture of the FRP reinforcement. Failure of the remaining NSM specimens(NSM-PL-25, NSM-PL25*2, and NSM-PL-2S) occurred through the simultaneous separation of the CFRP reinforcement and epoxy from concrete (Figure 4, 5, and 6).Table 3 summarizes the failure modes.3.2 STRENGTHENING EFFECTFigure 7 ploted the load-deflection curves of EBR and NSM specimens. The specimens with EBR,CPL-50-BOND, presented ultimate load increased by 30% compared to the unstrengthened specimen, while NSM specimens (NSM-PL-15, NSM-PL-25) increased the ultimate load by 40 to 53%.Observation of Figure 7 reveals that even if CPL-50-BOND with relatively large cross-sectional areaof CFRP reinforcement developed larger initial stiffness, premature debonding failure occurred because its bonding area is much smaller than NSM-PL-15, NSM-PL-25. EBR specimen behaved similarly to the unstrengthened control specimen after debonding failure. In Figure 7, the stiffness of NSM specimens before yielding of steel reinforcement was smaller than the stiffness developed by EBR specimen because NSM specimens have the smaller cross-sectional area of CFRP reinforcement than EBR specimen. The ultimate load and yield load are seen to increasewith the cross-sectional area of NSM reinforcement.Examining the ultimate strain of FRP summarized in Table 3, the maximum strain for EBR specimenappears to attain 30% of the ultimate strain, and 80 to 100% for NSM specimens. This proves that the NSM system is utilizing CFRP reinforcement efficiently（2S with the same cross-sectional area as CPL-50-Bond resented ultimate load increased by 95%, 90% compared to the unstrengthened specimen,respectively. Considering the same cross-sectional area, the strengthening effect of NSM specimens issuperior to the EBR specimen. In Figure 8,NSM-PL-25*2 and NSM-PL-2S, NSM specimens with space variables,showed that the strengthening effect of the specimen with narrow spaceis slightly increased by 2.5%as compared to the specimen with wide space.4. CONCLUSIONSPerformance tests have been carried out on RC beams strengthened with NSM systems. The followingconclusions were derived from the experimental results.It has been seen that NSM specimens utilized the CFRP reinforcement more efficiently than the EBR specimen. According to the static loading test results, the strengthening performances were improvedin NSM specimens compared with EBR specimen. However, the specimens NSM-PL-25, NSM-PL-25*2 and NSM-PL-2S failed by the separation of the CFRP reinforcements and epoxy adhesive from the concrete. Consequently, it is necessary to take somecountermeasures to prevent debonding failure for NSM specimens.Considering the same cross-sectional area, the strengthening effect of NSM specimens is superior to EBR specimen. NSM-PL-25*2 and NSM-PL-2S, NSM specimens with space variables, showed that the strengthening effect of the specimen with narrow space is slightly increased as compared to the specimen with wide space.5. REFERENCES1. Hassan, T. and Rizkalla, S. (), Investigation of Bond in Concrete Structures Strengthenedwith Near Surface Mounted Carbon Fiber Reinforced Polymer Strips”, Journal of Composites for Construction, Vol 7, No. 3, pp. 248-2572. Tljsten, B. and Carolin, A. (), “Concrete Beams Strengthened with Near Surface MountedCFRP Laminates”, Proceeding of the fifth international conference of ibre-reinforced plastics forreinforced concrete structures (FRPRCS-5), Cambridge, UK, 16-18 July , pp. 107-1163. Mirmiran, A., Shahawy, M., Nanni, A., and Karbhari, V. (), “Bonded Repair and Retrofit ofConcrete Structures Using FRP Composites”, Recommended Construction Specifications andProcess Control Manual, NCHRP Report 514, Transportation Research Board4. Rizkalla, S., and Hassan, T. (), “Effectiveness of FRP for Strengthening Concrete Bridges”,Structural Engineering International, Vol. 12, No. 2, pp. 89-95字典近表面埋置加固的钢筋混凝土梁抗弯性能实验研究Woo-Tai JUNG1, Young-Hwan PARK2, Jong-Sup PARK3摘要：本研究简介了近表面贴埋置加固钢筋混凝土（RC）实验成果。共有6个实验试件。试件可分为EBR的（外部粘结）和NSM（近表埋置）两种。埋置两片CFRP（碳纤维增强聚合物）的试件埋置间距不同。实验成果表白，用碳纤维增强材料NSM试件比EBR试件更有效。虽然NSM 试件断面面积是EBR的30和50，其效果也优于EBR试件。间距不同的试件成果表白，小间距略比大间距试件效果好。核心词：碳纤维增强复合材料，碳纤维增强材料外贴，近碳纤维增强材料表面安装，加固1简介在多种不同的技术的开发和应用，加强钢筋混凝土构造的技术越来越多，用玻璃钢增强混凝土的申请项目已显着多起来。玻璃钢加固混凝土表面的粘合胶，但常常在玻璃钢和混凝土界面脱粘导致失败。大多数研究，到今天为止，始终专注于加强调查EBR的影响和破坏模式系统。EBR的系统的过早失效的问题如果得到解决，就会增长玻璃钢和混凝土之间的接口的粘合度。运用这一原则，NSM系统被引进用于钢筋混凝土构造体系是在20世纪40年代开始的。然而，容易受腐蚀的钢筋和灌浆材料的胶合性能较差损害了其应用。提高环氧树脂的发展和采用玻璃钢加固所提供的机会实行NSM（Hassan和Rizkalla; Tljsten和Carolin ）。由于其重量轻，安装以便，最小的劳动力成本和场地条件，高强度与重量比，和耐用性，玻璃钢修复系统可以提供一种经济上可行替代老式的修复系统和材料（Mirmiran等。）。 Rizkalla和Hassan（）EBR和NSM在成本方面的比较，涉及原材料和劳动力成本的制度，尚有强化作用等。她们的结论是，NSM系统在成本效益上更符合规定。为了便于比较，有关钢筋混凝土简支梁的抗弯实验EBR的和NSM开始做了。此外，间距变量试件进行测试，以比较通过加强断面宽和窄的间距影响。2实验项目2.1试样的制造 有6组钢筋混凝土简支梁的试件共300次已经实验。具有代表性的试件如图1所示。一种与混凝土抗压强度在28天31.3兆帕斯卡已被使用。钢筋D10中的SD40（9.53mm）已安排钢比0.0041和三个D13号层（12.7mm）已安排了压缩增援。增援的D10的剪切已经找到剪切带中的每一种10厘米，以避免剪切破坏。表1总结了材料实验梁的属性。图-1试件的细节表达图表-1实验梁的属性材料性能混凝土抗压强度（MPa） 31.3张力钢加强（D10中）屈服强度（MPa） 426拉伸强度（MPa）562直径（毫米）9.53面积（cm2）0.7133压缩钢加固（D13号）屈服强度（MPa） 481拉伸强度（MPa）608直径（毫米）12.7面积（cm2）1.267碳纤维带（光滑表面）厚度（毫米） 1.4拉伸强度（MPa）2452.59（GPA）的弹性模量165.49极限应变（）1.482.2实验参数表2列出了实验参数。控制试件，一不加强的试件， 被拿来比较各系统加强的状态。CPL- 50 卷，EBR的试件，加强了与碳纤维带。其他4个试件，加强了与NSM碳纤维带。内嵌加强了试件，嵌入分别为64对NSM-特等- 15和NSM-特等- 25深度为15mm和25mm，。沟槽之间的一种空间NSM-的PL- 25* 2和NSM-的PL- 2为60mm和120mm，分别为。加强的所有的长度试件已得到修复了二七零零毫米试件碳纤维复合材料试样面积（平方毫米）碳纤维复合材料碳纤维（）加强措施控制-不加强CPL-50-BOND70条0.1296EBR1NSM-PL-1521条0.0389NSM2)NSM-PL-2535条0.0648NSM2)NSM-PL-25*270条0.1296NSM-N3)NSM-PL-2S70条0.1296NSM-W4)1）EBR的：外部粘结2）NSM：近表埋置3）NSM- N的：2 NSM增援;槽空间：60毫米4）NSM瓦：2 NSM增援;槽空间：1202.3安装玻璃钢筋图2显示了跨越部分的试件的细节。 EBR的强化过程的试件（cpl- 50 粘合剂）是由表面进行研磨解决使用的，另一方面接合地带的碳纤维。加固梁凝结的7天环境温度环氧树脂胶粘剂的固化。加强对NSM进程在获得进展的沟槽切割底部横梁用粉碎机，清洗碎片，嵌入后的碳纤维带应用的胶粘剂。加固梁的3天凝结，可使环氧胶粘剂达到其设计强度。 图-2纤维增强塑料的加强2.4加载和测量措施所有试件受到4点弯曲测试，以衰竭的UTM手段（万能实验机）与980千牛的能力。根据应用的负载量为位移控制在速度0.02毫米/秒，直到第15毫米和0.05毫米/秒，从15毫米到失败。对所有测量测试数据记录静态数据记录器和一种1秒的间隔计算机。电电阻应变计固定在跨中和1 / 4来衡量钢筋应变。应变计来测量混凝土应变均位于上方，5厘米和10厘米的距离在顶部的一跨中的一面。应变计，寄存在位于上玻璃钢加固中期的跨度和负载点底部测量应变根据加载过程。3实验成果3.1失效模式开裂前，所有试件展出弯曲性能加强类似的不加强试件。这表白，碳纤维加固是无法作出奉献增长的刚度和强度在弹性域。然而，在开裂，弯曲刚度和强度得到加强的试件被视为显着增长，直到失败相比unstrengthened试件。检查的最后失败，unstrengthened控制试件呈现典型的弯曲破坏模式，由钢筋屈服所得另一方面是压缩失败混凝土。对于cpl- 50 条，EBR的试件，故障开始与碳纤维分离加强和跨中混凝土脆性剥离终于体现出故障（图3）。失败对NSM-特等- 15，NSM试件，用玻璃钢加固破裂发生。发生故障其他NSM试件器（NSM-特等- 25，NSM- PL25* 2，和NSM-特等- 2）通过发生同步碳纤维加固的分离，从具体环氧树脂（图4，5和6）。表3总结了失败的模式表-3实验成果试件Py (kN)dy (mm)Pu (kN)du (mm)Increase inPu(%)FailuremodeuFRP 控制46.8912.7856.1971.68 -(a) -CPL-50-BOND61.0410,5273.2416.0030(c)4449NSM-PL-1557.4715.578.4959.9440(d)15417NSM-PL-2561.9916.0686.1853.9853(b)12350NSM-PL-25*271.9616.46109.6646.9295(b)10260NSM-PL-2S70.4914.16107.0344.4490(b)11241（一）：钢产量依次为混凝土压碎（二）：剥离的内嵌FRP加固和环氧（三）：剥离了外贴FRP加固（四）：有关NSM增强纤维材料的破坏3.2增强效应图7 ploted和NSM试件的EBR的负载挠度曲线。与EBR的试件，cpl- 50 粘合剂，提出最后比不加强试件负荷增长了30，而NSM试件器（NSM-特等- 15，NSM-特等- 25）增长了40极限荷载的53。图7中观测发现，即cpl-50- Bond的相对较大的横截面积碳纤维加固的初始刚度较大的发展，过早发生剥离破坏由于它的粘接面积比NSM-特等- 15，NSM-特等- 25小。 EBR的试件体现同样以失败unstrengthened控制剥离后的试件。在图7，NSM试件钢筋屈服前刚度不不小于通过EBR的试件发展，由于NSM试件的刚度较小的横截面EBR的面积比碳纤维加固试件。极限载荷和载荷看到产量增长随着NSM加固截面积。检查的FRP极限应变总结在表3中，最大应变为EBR的试件似乎达到30的极限应变，80到100 NSM的试件。这证明NSM系统是有效地运用碳纤维加固。图-7 EBR和NSM的作用曲线 图-8 NSM的有效作用曲线4结论性能测试已经进行了钢筋混凝土梁内嵌系统的加强。如下结论来自实验成果。它经看到，NSM碳纤维加固的试件运用效率比EBR的试件。根据静载实验成果，强化性能得到改善在NSM试件相比，EBR的试件。然而，试件NSM -特等- 25，NSM - pl的，25 * 2和NSM - pl的由碳纤维增强的环氧树脂胶粘剂分离和从失败28-2具体。因此，有必要采用某些对策，以避免剥离破坏为NSM试件。考虑到相似的横截面面积，NSM试样加固效果优于EBR的试件。 NSM -的PL - 25 * 2和NSM -的PL - 2，空间变量NSM试件，成果表白本次与狭窄的空间补强效果略有增长相比，试件与广阔的空间。5.参照文献1. Hassan, T和Rizkalla译（），债券加固混凝土构造的调查与近表面安装碳纤维增强聚合物条“，为复合材料学报建筑，第7卷第3期，第248-2572. Tljsten，B.和Carolin，（），“混凝土梁加固表层嵌碳纤维复合材料层压板“，出发的第五届纤维增强塑料的国际会议钢筋混凝土结（FRPRCS - 5），剑桥，英国，7月16-18日，第107-1163. Mirmiran， Shahawy，Nanni，A.和Karbhari，五（），“保税维修与改造混凝土构造使用玻璃钢复合材料“，推荐施工规范和过程控制手册，NCHRP报告514，运送研究委员会4. Rizkalla，和Hassan, T.（），“混凝土桥梁的加固玻璃钢研究”，国际构造工程，卷12，第2，第89-95 内部资料请勿外传
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