• Journal of The Korean Society of Agricultural Engineers
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• v.46 no.5
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• pp.69-77
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• 2004

FRP rebar has low bond performance than steel rebar. Usually, FRP rebar has about 60% of bond strength of steel rebar. Without adequate bond to concrete, the full composite action between reinforcement and concrete matrix can not be achieved. Therefore, FRP rebars must also have surface deformations that provide good bond to concrete. The purpose of this research was decided an optimum surface deformation patterns through bond test of FRP rebar. Eighteen surface deformation patterns of FRP rebar with widely different geometries were investigated. Based on the test results, we established optimum surfale deformation pattern. Bond tests were performed for three types of surface deformation patterns of FRP rebar including sand coated rebar, ribbed rebar, and wrapped and sand coated rebar that commercially available, and two types of FRP rebar including CFRP, GFRP rebars that optimum surface deformation pattern is applied. According to bond test results, FRP rebars that optimum surface deformation pattern is applied were found to have better bond strength with concrete than currently using FRP rebar.

• Journal of the Korea Concrete Institute
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• v.25 no.2
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• pp.187-194
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• 2013

Fiber reinforced polymer (FRP) has been gathering interest from designers and engineers for its possible usage as a replacement reinforcement of a steel reinforcing bar due to its advantageous characteristics such as high tensile strength, non-corrosive material, etc. Since it is manufactured with various contents ratios, fiber types, and shapes without any general specification, test results for concrete members reinforced with these FRP reinforcing bars could not be systematically used. Moreover, since investigations for FRP reinforced members have mainly focused on short-term behavior, the purpose of this study is to evaluate long-term behaviors of glass FRP (GFRP) reinforcing bar and concrete beams reinforced with GFRP. In this paper, test results of tensile and bond performance of GFRP reinforcing bar and creep behavior are presented. In the creep tests, results showed that 100 years of service time can be secured when sustained load level is below 55% of tensile strength of GFRP reinforcing bar. A modification factor of 0.73 used to calculate long-term deflection of GFRP reinforced beams was acquired from the creep tests for GFRP reinforced concrete beams. It is expected that these test results would give more useful information for design of FRP reinforced members.

• Journal of the Korea Concrete Institute
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• v.19 no.3
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• pp.323-331
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• 2007

The development length equations of the GFRP rebars are suggested based on the pullout tests performed in this study. A total of 48 pullout and modified pullout tests were completed. Test variables included embedment length (L=10, 15, 20, and $30d_b$), vertical and horizontal installation of the rebars, height of the rebars (H=100 and 300 mm), and cover thickness $(C=2{\sim}5d_b)$. D13 GFRP rebars domestically developed were used in the experimental program. The average of the bond strength of all vertically installed GFRP rebars was 6.39 MPa with a 5% fractile of 4.63 MPa. A basic development length equation was derived that resulted in an equation equivalent to the one proposed in the ACI 440.1R-03. Careful reevaluation of the bond strength using the modified pullout test indicated that a modification of the design equation was necessary so that the basic development length increases by 11%. The top bar effect of the horizontally installed rebars as well as the effect of the cover thickness were determined and included in the set of suggested equations. Since the current equations were derived from testing rebars embedded in relatively low strength concrete $(f_{ck}=20{\sim}24MPa)$, they result in conservative development lengths when applied to bars embedded in higher strength concretes.

• Journal of the Korea Concrete Institute
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• v.26 no.4
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• pp.421-428
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• 2014

This paper describes the experimental results of insulated concrete sandwich wall panels (CSWP) with corrugated glass fiber-reinforced polymer (GFRP) shear connectors under in-plane shear loading. Corrugated GFRP shear connectors were used to improve the thermal property of insulated CSWP and to achieve composite action between the interior and exterior concrete wall panels. Test specimens were consist of three concrete panels with two insulation layers between concrete panels and middle concrete panels was loaded in the direction of gravity. To evaluate the effects of insulation types (extruded polystyrene, XPSS and expanded polystyrene, EPS), shear connector pitch (300 and 400 mm) and width (10 and 15 mm) on in-plane shear behavior of insulted CSWP, failure mode and shear flow-average relative slip relationship of specimens were investigated. Test results indicate that the bond stress between concrete panel and insulation is considerable initially. Especially in case of insulated CSWP without shear connector, initial stiffness of CSWP with XPSS is superior to that of CSWP with EPS. The shear connector's contribution to in-plane shear performance of insulated CSWP depends on the type of insulation.

• Journal of the Korea institute for structural maintenance and inspection
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• v.15 no.1
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• pp.281-287
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• 2011

GFRP rebars could be deteriorated by concrete alkalinity. This paper focuses on the investigation of durability of GFRP rebars with ribs exposed to alkaline environment of concrete. It has been reported that the milled E-glass fibers in the ribs of GFRP rebar can increase bond strength between GFRP rebars and the concrete. In this study, the effect of milled alkaline resistant glass fibers (milled AR glass) and milled E-glass in the ribs on the durability of GFRP rebar is investigated through ISS tests and moist absorption tests of the bare rebar. To accelerate the effect of the alkalinity, high temperature($40^{\circ}C$) was applied. According to the test results, mix ratio of milled glass fibers in the ribs by weight had significant effect on durability of GFRP rebars with ribs. It is because that the high mix ratio may leads more voids in the ribs due to lower workability and formability. On the other hand, changing fiber type in the ribs from E-glass to AR-glass had no improvements on ISS strength of the GFRP rebar. Therefore, it is found that determination of proper mix ratio of milled glass fiber in the mixture for the formation of the ribs of the GFRP rebar is important.

• Journal of the Korea Concrete Institute
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• v.19 no.6
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• pp.677-684
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• 2007

This study investigates the failure mechanism of RC beams strengthened with GFRP (glass fiber reinforced polymer) sheets. After analyzing failure mechanisms, the various methods to prevent the debonding failures, such as increasing bonded length of GFRP sheets, U-shape wrappings and epoxy shear keys are examined. The bonded length of GFRP sheets are calculated based on the assumed bond strengths of epoxy resin. The U-shape wrappings are either adopted at the end or center of the CFRP sheets bonded to the beam soft. The epoxy shear keys are embedded to the beam soft to provide sufficient bond strength. The end U-wrappings and the center U-wrappings are conventional, while epoxy shear keys are new details developed in this study. A total six half-scale RC beams have been constructed and tested to investigate the effectiveness of each methods to prevent debonding failure of GFRP sheets. From the experimental results, it was found that increasing bonded length or end U-wrappings do not prevent debonding failure. On the other hand, the beams with center U-wrappings and shear keys reached an ultimate state with their sufficient performance. The center U-wrappings tended to control debonding of the longitudinal GFRP sheets because the growth of the longitudinal cracks along the edges of the composites was delayed. In the case of shear keys, it was sufficient to prevent debonding and the beam was failed by GFRP sheets rupture.

• Structural Engineering and Mechanics
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• v.38 no.1
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• pp.27-37
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• 2011

This paper presents the results of a study on the capability of nonlinear quasi-static finite element modelling in simulating the hysteretic behaviour of CFRP and GFRP-retrofitted RC exterior beam-column joints under cyclic loads. Four specimens including two plain and two CFRP/GFRP-strengthened beam-column joints tested by Mahini and Ronagh (2004) and other researchers are modelled using ANSYS. Concrete in compression is defined by the modified Hognestad model and anisotropic multi-linear model is employed for modelling the stress-strain relations in reinforcing bars while anisotropic plasticity is considered for the FRP composite. Both concrete and FRP are modelled using solid elements whereas space link elements are used for steel bars considering a perfect bond between materials. A step by step load increment procedure to simulate the cyclic loading regime employed in the testing. An automatically reforming stiffness matrix strategy is used in order to simulate the actual seismic performance of the RC concrete after cracking, steel yielding and concrete crushing during the push and pull loading cycles. The results show that the hysteretic simulation for all specimens is satisfactory and therefore suggest that the numerical model can be used as an inexpensive tool to design of FRP-strengthened RC beam-column joints under cyclic loads.

• Journal of the Korea Concrete Institute
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• v.24 no.1
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• pp.45-52
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• 2012

A variety of retrofit material such as CFRP, GFRP, and PolyUrea have been developed for strengthening RC structures and infrastructures. From previously reported research results, the capacity of strengthened concrete structures was dictated by the behavior of the interface between retrofit material and concrete. In this study, bond-shear test was carried out to estimate the bond behavior between retrofit material and concrete using a newly developed test grip. The test results of load and slip relation and energy absorption capacity of each retrofit material were obtained. These test results will provide basic information for retrofit material selection to achieve target retrofit performance.

• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.5
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• pp.144-152
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• 2010

In this study, cement-based filler is used as an adhesive instead of organic adhesive, epoxy because there were problems under wet condition. First, the bond strength of cement-based filler was measured and the result was satisfied with KS F 4716. However, in case of wet condition, bond strength of epoxy adhesive decreased $0.73N/mm^2$ in 7 days and $0.84N/mm^2$ in 14 days from pilot test. This implies that there would be a problem on reinforced concrete structure in wet condition, such as tunnel and sewage box. In the second experiment, the flexural strength of RC beams with GFRP using different thickness of cement-based filler was investigated, and the result was indicated 113%, 66%, 75% increase in 10mm, 20mm, 30mm thickness, respectively. From the result, it was known that 10mm filler thickness produces stable bond performance.

• Proceedings of the Korea Concrete Institute Conference
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• 2006.05b
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• pp.345-348
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• 2006

FRP composites which are used in strengthening existing structure are usually adhered to the concrete surface, their performance are directly affected by environmental condition such as freezing-thawing and moisture. Accordingly, it is required to evaluate bond durability between FRP composite and concrete as well as FRP materials itself. The durability characteristics of FRP composite for freezing-thawing are evaluated in this study with the variables of concrete strength, type of FRP composite, freezing-thawing conditions and freezing-thawing cycle. In addition, material durability of GFRP sheet for high temperature/high humidity condition are examined in this experimental study.