1、 1 AN EXPERIMENTAL STUDY ON FLEXURAL BEHAVIOR OF RC BEAMS STRENGTHENED WITH NSM REINFORCEMENT Woo-Tai JUNG1, Young-Hwan PARK2, Jong-Sup ABSTRACT: This study presents the results of experiments performed on RC (Reinforced Concrete) beams strengthened with NSM(Near Surface Mounted) reinforcement. A to
2、tal 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 N
3、SMspecimens 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 t
4、he specimen with narrow space is slightly increased as compared to thespecimen with wide spaceu KEYWORDS: carbon fiber reinforced polymer, externally bonded CFRP reinforcements, nearsurface mounted CFRP reinforcements, strengthening 1. INTRODUCTION Among the various strengthening techniques that hav
5、e 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 rein
6、forcements and concrete. Most research, to date, has focused on investigating the strengthening effects and failure modes of EBR system The 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 int
7、roduced 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 o
8、f FRP reinforcement offered the opportunity to implement NSM system (Hassan and Rizkalla 2003； Tljsten 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 e
9、conomically 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
10、 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
11、 tested to compare the strengthening performance by cross section with wide and narrow space. 2. EXPERIMENTAL PROGRAM 2.1 MANUFACTURE OF SPECIMENS A total of 6 specimens of simply supported RC beams with span of 3m have been cast. 2 The details andcross-section of the specimens are illustrated in Fi
12、gure 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
13、 10 cm in the shear zone to avoidshear failure. Table 1 summarizes the material properties used for the test beams. 2.2 EXPERIMENTAL PARAMETERS Table 2 lists the experimental parameters. The control specimen, an unstrengthened specimen, has been cast to compare the strengthening performances of the
14、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 betwe
15、en 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 mm 3 2.3 INSTALLATION OF THE FRP REINFORCEMENTS Figure 2 shows the details of cross-sections of the specimens. The strengthening process of EBR specimen (CPL-5
16、0-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 be
17、ams 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 METHODS All specimens were subjected to 4-point bending tests to
18、 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 comput
19、er 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 pla
20、ced 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 RESULTS 3.1 FAILURE MODES Before cracking, all the strengthened specimens exhibited bending behavior similar to the unstrengthened 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
