HYBRID FRP ROD FOR REINFORCEMENT AND SMART-MONITORING IN CONCRETE STRUCTURE

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1 Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 5) Chen and Teng (eds) 5 International Institute for FRP in Construction HYBRID FRP ROD FOR REINFORCEMENT AND SMART-MONITORING IN CONCRETE STRUCTURE Jongsung Sim 1, Doyoung Moon, Hongseob OH 3, Cheolwoo Park, and Sungjae Park 5 1 Professor, Department of Civil & Environmental Engineering, Hanyang University Research fellow, Doctor, Korea Infrastructure Safety & Technology Corporation 3 Full-time lecturer, Department of Civil Engineering, Jinju National University Research Professor, Department of Civil & Environmental Engineering, Hanyang University 5 Graduate student, Department of Civil & Environmental Engineering, Hanyang University ABSTRACT This study focused on the development of a hybrid fiber-reinforced polymer (FRP) rod for the purpose of reinforcements and smart monitoring in concrete structures. The hybrid FRP rod consisted of glass FRP and a FBG fiber-optic sensor (FOS), which provided reinforcement and smart monitoring for the concrete structure respectively. Tensile strength and pull-out bonding characteristics of hybrid FRP rod were carried. And strain acting on the hybrid FRP was measured. From the results, it is concluded that hybrid FRP rod can be successfully used for the purpose of reinforcement and smart monitoring in concrete structures. KEYWORDS Bonding strength, Fiber Optic Sensor, Hybrid FRP Rod, Smart-monitoring INTRODUCTION Generally, external loading and severe environmental conditions such as corrosion of steel rebar, alkali reaction, de-icing salt, and freeze-thaw are the major cause of deterioration in the reinforced concrete structure. These deterioration can be caused to serious structural damage, structural failure. Specially, the corrosion of steel reinforcing rebar is a dominant cause of concrete structure degradation. So, most effective way to prevent corrosion of steel rebar is to use a reinforcing material that does not corrode [1, ]. It is FRP rebar that is developed to solve problem of corrosion, and such as deteriorations. Recently, the interest in the safety of concrete structures has increased. It is necessary to monitor and maintain the safety of concrete structures, which requires smart monitoring systems that can make long-term monitoring. A smart monitoring system that can be applied to concrete structures that use FRP would also be useful, since it provides a way to inspect and assess any structural damage [3]. This study focused on the development of Hybrid FRP rods for reinforcement replaced steel rebar and for smart monitoring of concrete structures. The developed Hybrid FRP rod consisted of glass FRP and a FBG fiber-optic sensor (FOS), which provided reinforcement and smart monitoring for the concrete structure respectively. The developed Hybrid FRP rod has two advantages: first, it creates an anti-corrosive concrete structure replaced steel reinforcements with FRP, and it creates self-monitoring system used FOS embedded in FRP rods. Figure 1 shows a conceptual smart concrete member used a Hybrid FRP rod system. The Hybrid FRP rod embedded in concrete beam bends when an external force is carried. Then, the data acquisition system, which supplies light to the FOS in the Hybrid FRP rod, detects a change in wavelength of returned light. From the analyzing the changed wavelength, we can assess both the strain of Hybrid FRP rod and deflection of the concrete beam. Therefore, it is necessary to be verified mechanical properties of Hybrid FRP rod and to be tested deflection of FOS embedded in Hybrid FRP rod for the application of the developed Hybrid FRP rod system on concrete structure. 393

2 HYBRID FRP ROD SYSTEM Manufacture of the Hybrid FRP rod Figure shows a schematic of a deformed Hybrid FRP rod. Fiber Bragg Grating (FBG) sensor was used for the FOS system in this study. The shape of rod is similar to that of a steel reinforcing rebar. The bearing angle and rib spacing were designed to obtain optimal bonding performance from the various analyze. The height, pitch, and thickness of the rib were expressed as functions of nominal diameter of the FRP rod. The bearing angle was selected 8. It is possible to use the bonding behavior and structural model of steel rebar on Hybrid FRP rod, because the typical shape of the deformed FRP rod developed is similar to that of steel rebar. Section of Hybrid FRP rod is made up of 3 parts: FRP matrix as the main structural member, rib as element for the getting bonding characteristics, fiber optic as sensing elements. C o n n e c to r fo r F O S Analysis of structurald a ta FO S system in FRP rod D ata acquisition system Figure 1 Conceptual monitoring system of Hybrid FRP rod.d Lug α P=.63d t =.15d d' d w=.3d P = Lug Pitc h α= 8 Loguitu d inal indented.d h=.d Figure Shape and cross-section of Hybrid FRP rod The rib used in the Hybrid FRP rod was a mixture of epoxy resin and milled glass fibers. The main FRP matrix of glass fibers and epoxy resin, which resisted and the tensile stress and protected the FOS. Table 1 shows material properties of the developed FRP rod. Table 1 Material properties of the developed FRP rod Glass Fiber Epoxy Resin Hybrid FRP rod Tensile strength (MPa) Elongation (%) Specific gravity Table Volume fraction of Hybrid FRP Rod (%, per unit length) FRP matrix Rib section Glass fiber (%) Epoxy resin (%) Epoxy resin (%) Milled Glass fiber (%) Hybrid FRP rod Fabrication of deformed Hybrid FRP rod The deformed Hybrid FRP rod as glass FRP rod has nominal diameter of 9mm and total diameter included rip of 39

3 11.mm. Nominal pitch is about 5.7mm, height and thickness of rip is.mm and 1.mm, respectively. Table shows volume fraction of Hybrid FRP matrix and rib section mixed. Manufacturing procedures of Hybrid FRP rod system Figure 3 shows the process of the deformed Hybrid FRP rod. The procedures make up four steps: (a) arranging the glass fibers, (b) impregnating the epoxy resin and the pultrusion process, (c) pressing and curing the rod/rib section, and (d) calibrating the FOS system. The Hybrid FRP rod was fabricated in the laboratory because of test work required to develop the product. A fiber volume fraction of 58% was used in this study. Figure 3(a) and (b) show the arranging glass fibers and FOS. The impregnation was performed after arranging process. As depicted in Figure 3(c), the FRP matrix is generally distributed over all of the fibers and FOS by passing the rod through an opened epoxy thank. After impregnating, a pultrusion process was preformed (see Figure 3(e)). The rib sections were fabricated after the pultrusion procedure in order to improve the bonding characteristics of the Hybrid FRP rod. Figure 3(f) shows the pressing and molding equipment used to fabricate the rib sections. The pressing equipment slid the mold up and down, and then cured the materials at 15 for minutes. The Fiber Bragg Grating used for the smart sensing system could be damaged if curing temperature was maintained at 15 for longer than 3 minutes. Therefore, maximum temperature and curing time were carefully controlled. In order to demonstrate the workability of the FOS system embedded in FRP rod, the FOS was measured using optic sensor measuring equipment. The FOS was connected to a data logger using fiber-optic splicing equipment (see Figure 3(h)) and the data logger analyzed the measured FOS signal. (a) Fabrication bed (b) Arrangement of glass fiber and FOS (c) Impregnation (d) Impregnation of glass fiber and FOS (e) Pultrusion of undeformed Hybrid FRP (f) Pressing process and curing of FRP rod (g) Measurement of curing temperature (h) FOS splicing equipment (i) Calibration of FOS Fig. 3 Fabrication procedure BONDING CHARACTERISTIC OF DEVELOPED HYBRID FRP ROD Table 3 shows the mix proportion of concrete. To compare with the bonding characteristic of FRP rod, the used steel reinforcement [DS] has elastic modulus of. 1 5 MPa and yield strength of MPa. Four different types 395

4 of FRP rods were classified according to the style of surface shape. PRP rod [PR] which FRP surface do not any surface treatment, Sand-coated FRP Rod [SCR], Deformed Hybrid FRP Rod [DR] developed in this study and Surface Braided FRP Rod [SBR] which FRP surface is braided by fiber, were used in bonding test. Table shows material properties of used FRP rods and the Figure shows typical shape of used FRP rods. Figure 5 shows the test specimens consisted of the fixing and bonding blocks. The bonding block measure mm, after PVC pipe was installed at the unbonded region, concrete was placed. The section of fixing block is mm, The cup-shaped anchoring bulb in the fixing block was attached with epoxy resin to prevent the reinforcement from pulling out of the concrete member at the fixing block. Table 3 Mix proportion of concrete Slump (cm) Air (%) W/C (%) S/a Unit (kg/m 3 ) AD (kg) (%) W C S G AEA * 1 HRWRA * *1: AE, *: super platicizer Table Material properties of steel reinforcing bar and FRP rods fiber Tensile strength (MPa) Fiber volume fraction (%) Deformed steel rebar [DS] PR Glass Fiber 1, 7 SCR Glass Fiber 1, 7 DR Glass Fiber SBR Glass Fiber 91 7 (a) SCR (b) DR (c) SBR (d) PR Fig. The shape of various FRP rod used for bonding test 15 unbonded region 15 5 Var. 35-Var. anchoring of FRP rod embedded region (Unit : mm ) Fig. 5 Details of the pull-out test specimen There are many effective factors to decide the bonding characteristics of FRP rod and concrete. The major factors are surface shape, bonding length, and nominal diameter of FRP rod [-7]. Therefore, in this study, 39 specimens were tested with 3 test variables; surface shape, diameter, and bonding length. The test result was compared with steel reinforcement and other FRP rod used in Korea. The Mechanical effect of the milled fiber in the developed Hybrid deformed FRP rod [DR] was also used as a variable. The bonding length was selected to 396

5 be 5, 1, 15 and times the nominal diameter of the reinforcement. The nominal diameter of the rod was 6, 9, and 1 mm. Table 5 shows the test variables and results. Table 5 Variables and results of the bonding test Variables Specimen Diameter (d n, mm) Embedment Max. bond strength (MPa) length 1 3 Ave. Failure pattern Diameter SCR d n =6 mm Pull-out SCR d n = 1 mm Pull-out SBR d n = 9 mm Pull-out Surface type DR d n = 9 mm Pull-out PR d n = 9 mm Pull-out SCR d n = 9 mm Pull-out DR d n = 5 mm Pull-out DR d n = 9 mm Pull-out DR d n = 135 mm Pull-out Embedment length DR d n = 18 mm Tensile failure of FRP rod DS d n = 5 mm Pull-out DS d n = 9 mm Pull-out DS d n = 135 mm Tensile failure of FRP rod DS d n = 18 mm Tensile failure of FRP rod * Bold is the test result of the specimens included milled glass fiber on Rib The bonding test (see Figure 6) was performed using an actuator, which had capacity of kn. The loading was controlled at a speed of 1 mm/min. Four LVDT were installed to measure the displacement of the specimens. Two LVDT were installed at the left and right side of the fixing block to measure the total slip of the specimen, one LVDT was installed 3mm away from the fixed part of the specimen to measure the FRP rod displacement. And one LVDT was installed at the end of the FRP rod in the bonding part of the specimen to measure the slip of FRP rods. Fig. 6 Experimental details Fig. 7 Tension test apparatus for Hybrid FRP rod MONITORING TEST OF HYBRID FRP ROD To ensure that the FOS provided a good indication of strain acting on the Hybrid FRP rod, tensile test for smart monitoring was carried. Figure 7 shows the specimen and setup used to assess the tensile characteristic of Hybrid FRP rod. Four specimens were manufactured for the considering on reliability and error of sensing data. The wedge-shaped end block of the rod was made of carbon fiber sheet and epoxy resin in order to avoid stress concentration occurred at grip. The tensile test was performed using an UTM (Universal Testing Machine), 397

6 which had capacity of 1 kn. The displacement was controlled at a speed of mm/min. test result was compared with data measured from UTM and that from FBG sensing equipment. TEST RESULTS AND DISCUSSION Bonding test of deformed Hybrid FRP rod In these tests, specimens failed in two modes: tensile failure and pull-out failure and Fig. 8 shows the typical failure modes of the tested FRP rods. The FRP rod failed mainly due to the shear stress acting at the rib section, which eventually resulted in a pull-out failure. Table 5 gives the maximum bond strength and the comparison of the test results are depicted in Figure 9. (a) Bar failure (b) Pull-out failure Fig. 8 Failure modes of FRP rod τ(mpa) 6 τ(mpa) 6 (w ith m illed fiber) (w ith o u t m illed fiber) D S-9-9 SBR-9-9 D S-9-9* SC R-9-9 PR-9-9 (a) Deformed FRP rod (DR-9-135) (b) Surface type of FRP rod(embedment length=9 mm) 1 1 DS series DR series τ(mpa) 6 τ(mpa) 6 5m m 9m m 135m m 18m m SC R-6-6 SC R-1-1 (c) Embedment length of FRP rod (d) Diameter of FRP rod Fig. 9 The effect of experimental variables To compare the bonding characteristics between the specimens, the specimens had a diameter of 9 mm and an embedment length of 9 mm were selected. These specimens exhibited pull-out failure after the maximum bonding strength was reached. Load-slip relationship of specimens of this category showed the linearity up to failure. The DR specimens with milled glass fiber on their ribs had better bonding characteristic than the DR specimens without milled fibers on their ribs. The bonding strengths of the DR rod with milled fibers and the normal DR rod were and 8.5MPa, respectively. Therefore, the presence of milled fibers on the ribs improved the bonding characteristics of the FRP rod. 398

7 The DS specimens with embedment lengths of 5 and 9 mm failed in pull-out mode, and those with embedment lengths of 135 and 18 mm failed due to tensile failure of steel reinforcing bar. The FRP rod with embedment lengths of 5 and 9 mm failed in pull-out mode, and the FRP rod with an embedment length of 135 mm failed in pull-out and tension modes simultaneously. Finally, the FRP rod specimen with an embedment length of 18mm failed due to tensile failure of DR rod. In summary, rod specimens with an embedment length 15 times their nominal diameter failed in pull-out mode. However, those with an embedment length times their nominal diameters fail in tensile failure of FRP rod. According to these results, the maximum bonding length was between 15 and d n. The DR specimen failed in both tension and pull-out modes; therefore, its maximum bonding length was 15 times it nominal diameter. Monitoring test of Hybrid FRP rod It is not measured tensile test until rupturing of FRP rod because of sliding at taper. Therefore, we cannot measure yielding strength of FRP rod. It is necessary to study on the prevention of sliding at taper. Figure 1 shows that the measured result was a good accord with the electric strain gauge measurements Electronic strain gage Fiber optic sensor.6 strain CONCLUSION Time(sec) Fig. 1 Tension test results for Hybrid FRP rod In this study, the bonding tests and FOS monitoring tests of deformed Hybrid FRP rod were carried out. The results presented here lead to the following conclusions. 1. In this study, the bonding capacity of Hybrid FRP rod was 18% that of a deformed steel reinforcement. Therefore, the developed Hybrid FRP rod performed better than other FRP rod products. The milled fibers were used on the ribs to improve the bonding characteristics. A bonding test demonstrated that milled fibers could improve the bonding capacity by more than %. 3. From the result of tensile test and FOS monitoring test, The FOS monitoring system provided approximately strain results. However, it is considered to need a study for the prevention of sliding at taper.. The developed Hybrid FRP rod has sufficient tension and bonding capacities to be used as a replacement for steel reinforcements at concrete structures. Moreover, the FOS embedded in the developed Hybrid FRP rod has good strain value. Therefore, it is possible to obtain both sufficient bonding capacities and selfmonitoring performance using FOS embedded in FRP rod, also it is possible to apply using the developed Hybrid FRP rod at concrete structures. 399

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