FRP Sheets Research Articles

저속 충격하중에서의 FRP Sheet 및 강섬유 보강 콘크리트의 거동 해석

충격 하중 재하 실험의 경우 빠른 하중 재하 속도로 인해 실험 데이터를 측정하는 방법에 있어 많은 어려움이 있다. 또한 부재의 국부적 손상을 측정하지 못함으로써 부재의 동적 거동을 왜곡하는 문제점이 존재한다. 따라서, 본 연구에서는 충격 실험에서의 한계를 극복하기 위하여 명시적(explicit) 유한요소 해석 프로그램인 LS-DYNA를 이용하여 FRP Sheet 및 강섬유로 보강된 콘크리트의 저속 충격 하중 하에서의 동적 거동을 분석하였다. 해석 모델은 1방향 및 2방향 부재이며 충격 하중 재하 시부재의 국부적 파괴를 고려하고 있다. 해석결과 강섬유에 의해 보강된 SFRC와 UHPC 부재의 경우 충격 저항성능이 크게 향상되었다. FRP Sheet로 보강한 경우 GFRP가 CFRP보다 우수한 충격 저항 성능을 보였으며 FRP Sheet의 방향성에 의한 영향은 크게 나타나지 않았다. 본 연구에서 수행된 해석은 충격 실험 결과와의 비교를 통해 그 신뢰성이 검증되었다. In the case of impact loading test, measurement of the test data has difficulties due to fast loading velocity. In addition, the dynamic behaviors of specimens are distorted by ignoring local fracture. In this study, therefore, finite element analysis which considers local fracture and strain rate effect on impact load was performed by using LS-DYNA, an explicit analysis program. The one-way and two-way specimens strengthened with FRP Sheets and steel fibers were considered as analysis models. The results showed that the impact resistance of steel fiber reinforced concrete (SFRC) and ultra high performance concrete (UHPC) was enhanced. In the case of specimens strengthened with FRP Sheets, GFRP was superior to CFRP in the performance of impact resistance, and there was little effect of the FRP Sheet orientation. The reliability of this analysis model was verified by comparing with previous experimental results.

Bridge Girder Heavily Damaged by Impact Vehicle Load: A Case Strategy for Structure Assessment and FRP Strengthening

Episodes like impact of traffic vehicles on existing bridges occur very frequently manly due to the increase of traffic volume and vehicles dimensions respect of actual roads and overpass geometry. An interesting case of FRP strengthening on existing deck bridge has been applied to one overpass of state road SS36 (Lecco, Italy). Heavy damage at both intrados and webs of I girder prestress beams was caused by impact of a vehicle higher then the vertical clear dimension between main road and crossing overpass. Extensive damage to concrete, subject to spalling and cracking, and complete failure of ordinary reinforcement and prestress tendons was recognized by detailed inspection. Main concern of strengthening strategy was driven by absence of original detail design project, because lack of project documents as drawings or calculation reports from road owner and maintenance office. For this reason it was necessary to apply “back analysis” strategy of strengthening, assessing original strength contribution (both in flexure and shear) of each bridge girder portion which showed evident damage or failure. After repairing of damaged concrete by specific mortar, to recover original element geometry and inertia, carbon FRP sheets was designed and applied at free surface to increase both flexure and shear prestress concrete girder capacity. Main advantages of this strengthening application were shown by minimize time and cost of repairing without closing of main road.

Parallel computation for debonding process of externally FRP plated concrete

In this paper, the three dimensional Parallel Realistic Failure Process Analysis (<TEX>$RFPA^{3D}$</TEX>-Parallel) code based on micromechanical model is employed to investigate the bonding behavior in FRP sheet bonded to concrete in single shear test. In the model, the heterogeneity of brittle disordered material at a meso-scale was taken into consideration in order to realistically demonstrate the mechanical characteristics of FRP-to-concrete. Modified Mohr-coulomb strength criterion with tension cut-off, where a stressed element can damage in shear or in tension, was adopted and a stiffness degradation approach was used to simulate the initiation, propagation and growth of microcracks in the model. In addition, a Master-Slave parallel operation control technique was adopted to implement the parallel computation of a large numerical model. Parallel computational results of debonding of FRP-concrete visually reproduce the spatial and temporal debonding failure progression of microcracks in FRP sheet bonded to concrete, which agrees well with the existing testing results in laboratory. The numerical approach in this study provides a useful tool for enhancing our understanding of cracking and debonding failure process and mechanism of FRP-concrete and our ability to predict mechanical performance and reliability of these FRP sheet bonded to concrete structures.

Viscoelastic analysis of FRP strengthened reinforced concrete beams

External bonding of FRP plates or sheets has become a popular method for strengthening reinforced concrete structures. Stresses along the FRP-concrete interface are critical to the effectiveness of this technique because high stress concentration along the FRP-concrete interface can lead to the FRP debonding from the concrete beam. Although the short-term stress distribution along the FRP-concrete interface has been studied extensively, very few studies have been conducted on the long-term stress distribution, which closely simulates the behavior of the structure during the service-life. In this study, we develop a viscoelastic solution for the long-term interface stress distribution in a FRP plate strengthened reinforced concrete beam. In this solution, the RC beam and the FRP plate are modeled as elastic materials; while the adhesive layer is modeled as a viscoelastic material using the Standard Linear Solid model. Closed-form expressions of the interface stresses and deflection of the beam are obtained using Laplace transform and calculated using the Zakian’s numerical method. The validation of this viscoelastic solution is verified by finite element analysis using a subroutine UMAT based on the Standard Linear Solid model.

Piezoresistive Effect of Multi-Wall Carbon Nanotube Reinforced Epoxy Resin Coating

Some conductive MWNT with different loading was filled in epoxy resin through the dispersion technique of surfactant-decoration, high-intensive shear mixing, and bath sonication process. This kind of conductive functional resin coating was then simply brushed on a FRP sheet surface. The electrical conductivity of the cured nanocomposite coating was acquired by four-electrode method to explore the percolation threshold and piezoresistive effect. The change in its longitudinal strain of the coating and FRP sheet was simultaneously sampled under three-point bending. Results reveal that, at the onset percolation threshold (2.0%), the nanocomposite possesses superior piezoresistive effect with high strain-sensitivity (up to 27), which favors to health monitoring to FRP structure used as a novel self-sensing coating.

Elimination of Debonging of FRP Strips in Shear Strengthened Beams Using Grooving Method

One of the methods of retrofitting reinforced concrete structures is use of FRP sheets to increase the flexural and shear strength of concrete elements. Use of FRP materials is in rapid progress because of their high strength, light weight and easy installation of FRP composites. Poisson’s ratio mismatch between FRP sheets and concrete is resulted premature debonding of FRP strips. Hence, the use of ultimate strength of FRP sheets is difficult if EB (External bonded) is applied in installation of FRP. Therefore, debonding has been repeatedly reported in most experimental studies. Recent studies in Isfahan University of Technology (IUT) show that use of grooving method (GM) to attach the FRP sheet to concrete surface for flexural strengthening is much more effective compared to conventional methods. In present experimental study, small scale reinforced concrete beams were strengthened with FRP strips for shear. Comparison between the ultimate load and deflection of the test beams demonstrates that grooving method was effective and led to elimination of debonding of FRP strips used for shear strengthening.

Experimental Study on Mechanical Properties of Timber Columns Strengthened with Carbon-Aramid Hybrid FRP Sheets under Axial Compression

Based on the experiment of timber circular columns made of pine and fir confined by carbon-aramid hybrid FRP (HFRP) sheets under axial compression, the failure modes, axial bearing capacities, peak compression strains were analyzed. The results showed that: Compared to the unconfined specimens, the axial bearing capacities and peak compression strains of the specimens confined by HFRP sheets were all improved. Axial bearing capacities of pine and fir specimens improved 6.6％～16.8％ and 5.0％～16.9％,respectively. Peak compression strains of pine and fir specimens improved 8.9％～60.2％ and 11.5％～56.8％, respectively.

Experimental Study on Bending Behavior of Timber Beams Reinforced with CFRP/AFRP Hybrid FRP Sheets

Based on the experiment of timber rectangle beams made of pine and fir reinforced with CFRP/AFRP hybrid FRP (HFRP) sheets. The failure modes and bending bearing capacity and load-deflection curves and strain distribution at mid-span section were analyzed. The results showed that: Comparing with the specimens without being reinforced by HFRP, there is certain improvement in bending bearing capacity and stiffness of the specimens reinforced with HFRP respectively. Bending bearing capacity of the pine specimens improve 18.1%~62.0% and bending bearing capacity of the fir specimens improve 7.7%~29.7%. Stiffness of the pine specimens improve 13%~21%, and stiffness of the fir specimens improve 6%~10%. Based on the experimental data, the computing formulas of bending bearing capacity of timber rectangle beams made of pine and fir reinforced with HFRP were presented.

탄소섬유쉬트로 보강된 원형CFT기둥의 압축거동과 ACI 440 code를 응용한 압축내력예측식 제안

This study investigates the axial behavior of CFRP strengthened circular CFT columns and proposes the design formula of CFRP strengthened circular CFT columns. 10 specimens were prepared and axial loading test were conducted to investigate the retrofitting effects of CFRP composites on CFT columns. The main parameters are the number of FRP sheets and D/t ratio. Test results showed that the CFRP retrofitting enhanced the load bearing capacity of the circular CFT columns. Finally, A ACI 440 code applied ultimate axial strength formula is proposed to predict the ultimate strength of CFRP strengthened circular CFT columns. The proposed formula are good agreement with the test results.

Intrados strengthening of brick masonry arches with composite materials

The objective of this study is to investigate the effectiveness of an innovative technique for strengthening masonry arches at their intrados, based on the use of carbon plates. Although FRP sheets or strips are successfully used as strengthening elements for this kind of application, they present several critical issues that can compromise these upgrading works, leading to the local collapse mechanism, which corresponds to the debonding of reinforcement from the masonry substrate. As an alternative to FRP sheets, the use of FRP plates presents instead several interesting aspects which make them very attractive for intrados strengthening. More precisely, FRP plates have an inherent bending and axial stiffness that may overcome problems concerning premature peeling. Fifteen prototypes of brickwork arches strengthened at their intrados with GFRP sheets or CFRP plates were tested under a monotonic vertical load applied at the keystone. The influence of the types of reinforcement (glass fibers and carbon plates), properties of the bonding system and masonry substrate and the presence of anchor spikes has been investigated.

FRP Sheet와 비좌굴 가새를 적용한 보-기둥 접합부의 횡방향 보강효과에 관한 실험적 연구

FRP Sheet와 비좌굴가새를 적용한 보-기둥 접합부의 보강효과를 평가하기 위하여 보-기둥 접합부 실험체에 축력 및 반복 횡가력을 가하여 실험을 수행하였다. 동일한 크기의 6개의 실험체를 제작하였으며 FRP Sheet의 종류 및 비좌굴 가새의 유무를 변수로 하였다. 실험체의 파괴양상 및 최대하중, 연성지수, 에너지소산능력의 측면에서 실험결과를 분석하였다. 실험결과 CFRP Sheet와 비좌굴가새를 혼용한 보강방법이 가장 우수한 성능을 나타냈다. The strengthening effect of CFRP sheet and AFRP sheet with buckling-restrained brace for Beam-Column joints under constant axial and cyclic lateral loading is evaluated experimentally in this paper. Six test specimens were constructed. The main test parameters included the FRP Sheet and Buckling-restrained braces. The results of the tests were analyzed by focusing on their mode of failure, maximum load, ductility indexes, and energy dissipation capacity. Test results indicated that CFRP Sheet with the buckling-restrained brace system significantly increased the strength and stiffness of the specimen and that it was the most adequate retrofitting method.

A New Approach for Adhesive Stress Analysis of a Beam Bonded with Composite Plate Having Variable Fiber Spacing

External bonding of FRP plates or sheets has emerged as a popular method for strengthening reinforced concrete structures. Debonding along the FRP–concrete interface can lead to premature failure of the structures. Shear and normal stresses (in the thickness direction) at interfaces (referred to as interfacial shear and transverse normal stresses hereafter) have played a significant role in understanding this premature debonding failure of such hybrid structures. Extensive experimental and analytical analyses have been undertaken to investigate this problem. Large discrepancies have been found from various studies. This paper reports a closed-form formulation for interfacial stresses in plated beams. The analysis is based on the deformation compatibility approach developed by Tounsi and Benyoucef [8] where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. The composite plate with variable fiber spacing bonded to reinforced concrete beam is considered in the present study. In addition, an unrealistic restriction of the same curvatures in the RC beam and FRP panel commonly used in most of the existing studies is released in the present theoretical formulation. Compared with previously published analytical results, this one improves the accuracy of predicting the transverse normal stresses and the solution remains a closed form.

Design Formulas of Concrete Filled Circular Steel Tubes Reinforced by Carbon Fiber Reinforced Plastic Sheets

Concrete-filled steel tube structure has advantages such as excellent deformation capacity, energy absorption and fireresistance. However, CFT column has the drawback of local buckling at steel tube due to the deterioration of confinement effect after steel tube yield. To solve the problem, CFT column system reinforced by CFRP(Carbon Fiber Sheets) will be introduced and the structural behaviors of CFRP reinforced columns will be compared with those of the current CFT columns. The main experimental parameters are diameter-thickness (D/t) and number of carbon FRP sheet layers. 10 specimens were prepared with the main experimental parameters. Axial compression test were conducted and axial behavior maximum strength capacity, and ductility capacity were analyzed between CFRP reinforced columns and the current CFT columns. To propose of design formula for FRP reinforced CFT columns, confinement effectiveness coefficient of confined concrete were proposed based on the experimental results. Finally, base on the coefficient, the axial design formula for FRP reinforced CFT columns were proposed and compared the design strength values with the experimental strength values.

Uniaxial compression test of steel plate bonded FRP sheet for rational repair and reinforcement of web in steel girder bridge

Uniaxial compression test of steel plate bonded FRP sheet for rational repair and reinforcement of web in steel girder bridge

Genetic algorithm model for shear capacity of RC beams reinforced with externally bonded FRP

A variety of on-site construction applications using FRP materials have been realized worldwide. However, this technology is currently at a stage where its future widespread implementation and competitiveness will depend on the development of reliable design guidelines based on sound engineering principles. This paper presents simple, yet improved, equations to calculate the shear capacity of FRP bonded-reinforced concrete beams based on the genetic algorithms (GAs) approach applied to 212 experimental data points available in the open literature. The performance of the proposed equations was compared to that of commonly used shear design methods, namely the ACI 440, Eurocode (EC2), the Matthys Model, Colotti model and the ISIS Canada guidelines. Results demonstrate that the proposed equations better agree with the available experimental data than the existing models investigated. Moreover, a sensitivity analysis was carried out to investigate the effect of the shear span-to-depth ratio on the shear capacity contributed by concrete, the ultimate effective strain in FRP sheets, and the ultimate effective stress in transverse rebars. Results indicate that the shear span-to-depth ratio has a significant effect on the shear behaviour of FRP bonded-reinforced concrete beams.

Durability of BFRP and Hybrid FRP Sheets under Freeze-Thaw Cycling

Coupon tests were conducted to investigate the mechanical characteristics of basalt FRP (BFRP) sheet, basalt-carbon hybrid FRP sheets and the corresponding epoxy rein under the effect of freeze-thaw cycling. FRP sheets and epoxy rein coupons were subjected to up to 200 and 250 freeze-thaw cycles respectively. Test parameters included the number of freeze-thaw cycles and the types of FRP composites. Test results show that (1) BFRP sheet perform better than CFRP or GFRP sheets under high freeze-thaw cycles; (2) exposed hybrid FRP sheets not only show very little loss in mechanical properties, but also contribute to the stability of test data; (3) mechanical properties of rein epoxy decrease significantly with increasing freeze-thaw cycles.

Retrofitting Effect of Unreinforced Masonry Walls Using FRPs

Unreinforced masonry buildings have significant portion of existing and historical buildings around the world. Recent earthquakes have shown the needs of seismic retrofitting for these types of buildings. There are many types of retrofitting materials for URM(unreinforced masonry buildings) such as shotcrete, ECC and FRPs. Many engineers use many types of fiber reinforced polymers because these types of material enhance the shear strength of wall without expansion of wall sectional area and additional weight of total structure. However, the complexity of mechanical behavior of masonry shear wall and the lack of experimental data of masonry wall which was retrofitted by FRPs may cause the problem that engineers hard to determine the retrofitting level. Determining and providing the information for retrofitting effect of FRPs for masonry shear wall, this paper investigate in-plane shear behavior of URM and retrofitted masonry shear walls using two types of different FRP materials. Specimens were designed to idealize the wall of low rise apartment which was built in 1970s Korea with no seismic reinforcements and have 1 aspect ratio. Retrofitting materials were carbon FRP and Hybrid sheet which have different elastic modulus and ultimate strain. Consequently, this study will evaluate the structural capacity of masonry shear wall and retrofitting effect of FRP sheet for in plane shear behavior comparing with evaluation method for reinforced concrete beam which was retrofitted by FRPs.

CFRP 시트 및 강섬유로 보강된 RC 보의 충격저항 성능 평가

최근 콘크리트 구조물에 충격하중, 폭발하중 등 극한의 외력이 작용하는 경우가 빈번하게 발생하고 있다. 이 연구에서는 강섬유보강 RC보와 carbon FRP 시트를 이용하여 보강한 RC보를 이용하여 충격실험을 진행하였다. 강섬유 보강 RC보의 경우 0.75%부피비로 강섬유를 혼입하였으며 carbon FRP 시트의 경우 에폭시 레진을 이용하여 보강을 한후 보 부재를 완성하였다. FRP 시트 보강은 부재의 하단을 휨 보강하였으며 충격하중이 부재에 작용할 때 발생하는 shear-plug 균열을 제어하기 위하여 충격하중이 가해지는 국부에 CFRP 전단보강을 실시하였다. 실험진행은 drop-weight test 방식으로 직접 기기를 만들어 실행하였다. 각각의 부재에 단계별로 충격하중을 가하여 실험을 진행하였으며 균열과 균열폭을 측정하였다. 실험결과 강섬유보강 RC보가 일반 RC보에 비하여 균열폭 및 shear-plug 균열제어 그리고 스폴링 파괴에 더 높은 성능을 나타내었다. FRP로 부재의 하단을 휨 보강한 부재의 경우 균열의 제어에 어느정도 효과를 나타내고 있으나 충격하중이 가해질 시 콘크리트와 FRP 시트의 부착면에서 박리파괴가 빠르게 진행되었다. FRP 시트로 부재의 하단과 측면을 CFRP로 휨, 전단보강한 부재의 경우 shear-plug 균열제어에 가장 높은 저항성능을 보이고 있음을 확인할 수 있었다. 하지만 충격하중이 보강이 이루어지지 않은 부분에 작용할 경우 오히려 보강이 되지 않은 RC보에 비하여 콘크리트 스폴링 파괴에 더 취약함을 알 수 있었다.

Seismic Retrofitting of Historic Masonry Buildings – Case Study

Masonry heritage building built during early part of 19th century have characteristic colonial architecture using masonry walls and jack arch roofing supported on steel beams. They are highly vulnerable to failure during earthquakes. This paper describes a methodology to quantify their vulnerability and then based on this a scheme of structural retrofitting is suggested. The aim of this presentation, through 4 case studies of buildings located in Delhi, India, is to exemplify various aspects of analysis, design and execution methodology of the retrofitting scheme for such important heritage structures. The assessment of vulnerability is based on its location, codes of practice with respect to materials and loading. The main challenge in choosing the appropriate retrofitting scheme lies in retaining the architecture and aesthetics. Also the retrofitting has to be completed in the least possible time causing minimum disturbance to the occupants. This has been achieved through a combination of Ferro-cement bands and FRP sheets. The execution of retrofitting was considered to make use of available local materials and expertise. The building is analysed in detail and the areas where stress concentration takes place is further strengthened.

Silicate계 콘크리트 함침제 도포에 따른 부착특성 및 공극변화

There are the impregnating layer formation by surface protective materials or impregnants and the adhesion method by polymer, FRP sheet or steel plate in the surface protective method of concrete structure. The surface impregnation method by impregnants improves the durability of concrete structure by modifying the structure of the concrete surface and also have a merit that can be shortly applied in place without the decrease of concrete surface appearance and is easily applied again. This study is interested in manufacturing the concrete surface impregnants including lithium and potassium silicate for the repair of the exposed concrete and the color concrete requiring the advanced function in view of the concrete appearance. The durability and porosity properties was tested for the review of application. The result of this study show that the effective content of silicate ranges 5 to 20% and the separate application of the first impregnant and the second impregnant is effective for the optimum performance. The adhesion in tension is slightly increased but the reinforcement of concrete substrate is slight. So, the concrete impregnant of this study is more desirable for the improvement of durability rather than the reinforcement.