1、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

2、 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 NSMspec

3、imens 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 spe

4、cimen 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 de

5、veloped 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 reinforcement

6、s 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 re

7、cently. 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 rein

8、forcement offered the opportunity to implement NSM system (Hassan and Rizkalla 2003; Täljsten and Carolin 2001). 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 econo

9、mically viable alternative to traditional repair systems and materials(Mirmiran et al. 2004). Rizkalla and Hassan (2002) 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

10、 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 teste

11、d 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 c

12、oncrete 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 t

13、he 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 syste

14、ms. 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

15、 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 proce

16、eded 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 grind

17、er, 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 o

18、f 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

19、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 rein

20、forcement 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

21、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 th

22、e 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

23、-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 a

24、nd 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 NS

25、M 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 mu

26、ch 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

27、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 th

28、e ultimate strain, and 80 to 100% for NSM specimens. This proves that the NSM system is utilizing CFRP reinforcement efficiently2S 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

29、 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 wi

30、th 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

31、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 necessar

32、y 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. (2003), Investig