Tension Face Research Articles

Simulation of concrete cover separation failure in FRP plated RC beams

The flexural strength of a reinforced concrete beam can be increased by bonding a FRP sheet to the tension face. Nevertheless, this type of reinforcement may cause a premature debonding failure. This paper is concerned with the failure by concrete cover separation; in other words by peeling-off. Four series of FRP plated RC beams and one group of control beams were tested in a four-point bending setup where each group consisted of three beams. All strengthened beams failed by peeling-off. As a part of this study numerical analyses using the commercial program; Abaqus have been carried out in order to predict this type of failure. Comparisons between the predictions of the numerical model and test results show a very good agreement indicating that the ultimate load, the beam behavior and the failure mode can be closely predicted by using the proposed model.

Plate end debonding in the constant bending moment zone of plated beams

Reinforced concrete (RC) beams strengthened in flexure by externally bonding fibre reinforced polymer (FRP) or steel plate on their tension face are susceptible to premature plate end debonding failures. Safe design of such a strengthened RC beam demands a reliable and predictive debonding strength model. There are two special cases of plate end debonding failures: flexural debonding for cases when the plate terminates within a constant bending moment region (CMR), and shear debonding for the case when the plate terminates where the shear force is large but the bending moment is minimal. A general plate end debonding case is usually considered as an interaction between these two special cases. This paper is concerned with flexural debonding. A brief review of existing models is presented before the plate end interfacial stresses are examined. Three new models with different levels of accuracy are then developed: a closed-form theoretical model based on a simplified interfacial fracture mechanics analysis; a semi-empirical model; and a wholly empirical model. These three models together with two existing models are assessed against a carefully constructed test database containing 67 test data from an extensive literature survey.

THEORETICAL INVESTIGATION ON FLEXURAL PERFORMANCE OF RC BEAMS STRENGTHENED EXTERNALLY BY CFRP

In this paper an analytical model is presented for reinforced concrete beams externally reinforced with carbon fiber reinforced polymer (CFRP) laminates using finite element method adopted by ANSYS. The finite element models are developed using a smeared cracking approach for concrete and three dimensional layered elements for the CFRP composites. In particular, attaching unidirectional CFRP to the tension face of RC beams has provided an increase in the stiffness and load capacity of the structure. However, due to the brittle nature of unidirectional CFRP, the ductility of the beam decreases. Consequently, the safety of the structure is compromised, due to the reduction in ductility. The purpose of this research is to investigate the behavior of normal and high strength reinforced concrete beams strengthened with CFRP sheets. The major test parameters included the grade of concrete, the different dimensions of the beam and the tensile reinforcement ratio. Furthermore, change in the strength, ductility and toughness of the beams, as the grade of concrete, the dimensions of the beam and tensile reinforcement bar ratios are altered, are investigated. Fifty-four reinforced concrete beams were modeled and analyzed up to failure.

Flexural behavior of plain concrete beams strengthened with ultra high toughness cementitious composites layer

Ultra high toughness cementitious composites (UHTCC), which has metal-like deformation and crack width restricting ability, is expected to be utilized as retrofit materials. For this application, much attention needs to be paid to the working performance of structure members composed of UHTCC and existing concrete. This paper presents an investigation on the flexural behavior of plain concrete beams strengthened with UHTCC layer in tension face. The effect of UHTCC layer thicknesses on first crack load, ultimate flexural load, crack width, and load–deflection relationship is examined. The experimental results indicate that the use of UHTCC layer significantly increases the first crack load and ultimate flexural load. The first crack load and ultimate flexural load of composites beams increased with the increase of the UHTCC layer thickness. Considerable reduction in crack width was observed for composite specimens, as UHTCC layer restricted the cracks in upper concrete and dispersed them into multiple fine cracks effectively. Moreover, in comparison to plain concrete beam, composite beams could sustain the loading at a larger deflection without failure. Based on the plane section assumption, etc., a calculation method to predict the flexural capacity of composite beam was proposed. Good agreement between predictions and experiments had been obtained.

Strengthening of concrete slab-column connections using CFRP strips

This paper presents experimental data and results on the effect of externally bonded carbon fiber-reinforced polymer (CFRP) strips on the punching shear of interior slab-column connections. A total of six square slabs with a concentric column were constructed with overall dimensions of 1220 mm by 1220 mm and 100 mm thick slab and 150 × 150 mm column. The test variables were the configuration and amount of CFRP strips externally bonded to the tension face of the slab. The specimens were simply supported along their edges and tested in punching with vertical load applied through the central column. The test results clearly showed that CFRP strengthening leads to significant improvements in the structural behaviour of slab-column connections. The increase in punching capacity of strengthened slabs was up to 29%, while the increase in stiffness was up to 80% compared to the unstrengthened slab. Punching capacities of the test specimens were evaluated using a recently developed model that accounts for both configuration and amount of CFRP strips. The predicted punching capacities using the analytical model were compared with the experimental results, and good agreement was found.

Monitoring crack development in fiber concrete beam by using electrical resistivity imaging

Accurate detection of damaged concrete zones plays an important role in selecting the proper remedial technique. This study presents results from an application of the electrical imaging method to monitor the development of cracks in fiber concrete beams. The study showed that resistivity measurements on the concrete specimens were able to detect the increase of concrete resistivity with the curing time that reached about 65 Ωm after 28 days of curing. A similar development trend of concrete compressive strength was also found. Two types of cracks were investigated, i.e., artificial cracks made of plastic sheets inserted in concrete and cracks developed during a four-step loading test. A mini-electric imaging survey with Wenner array was conducted on the tension face of the beams. To deal with the effect of the beam size new procedures to correct resistivity measurements before inversion were proposed and successfully applied in this study. The results indicated that both crack direction and depth could be accurately determined in the inverted resistivity sections. ► The imaging technique is applied to inspect the damage in concrete structures. ► New type of non-penetrating electrode was designed and applied. ► The change in resistivity was correlated with the increase in concrete strength. ► The 3D electric forward modeling can correct the resistivity measurement.

Composite Steel Stud Blast Panel Design and Experimental Testing

Based on Federal Aviation Authority (FAA) requirements, project specific blast loads are determined for the design of a new airport traffic control tower. These blast loads must be resisted by exterior wall panels on the control tower, protecting building occupants from intentional explosives attack scenarios. Such blast resistant walls are typically constructed of thick reinforced concrete panels or composite steel plate and rolled sections, as conventional building cladding systems have relatively low blast resistance. While these more robust design approaches are valid, the additional cladding mass they represent will significantly increase the base shear and overturning demand in seismic zones. This paper investigates the use of a light structural system comprised of a steel stud wall assembly partially embedded in a thin layer of concrete to obtain composite action. Fiber reinforced polymer (FRP) composites are also included to increase the blast resistance and aid in keeping the panel weight to a minimum. Two full-scale composite steel stud walls are designed, constructed, and tested dynamically in the BakerRisk shock tube. The stud walls consist of back-to-back 150 mm deep, 14 gauge (1.8 mm thick), cold-formed steel studs spaced at 610 mm on center. Both specimens have a 50 mm thick normal weight concrete layer, reinforced with welded wire mesh that is welded to the stud compression flanges to achieve composite action. Two layers of Tyfo® SEH-51A fiber reinforced composites are used on the tension flange of the steel studs. A single layer of Tyfo® SEH-51A composites is used on the tension face of the concrete layer between the studs for one of the specimens. Web stiffeners are used at the bearing support to prevent premature web crippling shear failure of the specimens. The stud walls are analyzed using single-degree-of-freedom (SDOF) models. A non-linear moment-curvature relationship, accounting for actual material constitutive properties, is used for determining the resistance function of the walls. Blast pressure and impulse data from the shock tube tests is used to compare analytical predictions to the measured displacement-time response. Analytical predictions of panel response for both tests are within ten percent of the observed response based on displacement.

Deflection Prediction of FRP-Strengthened Reinforced Concrete Beams

Bonding fiber reinforced polymer (FRP) laminates to the tension face of RC members has been proven to be an effective method to improve the flexural strength. However, structural members are not only needed to have adequate strength, but also to have adequate performance of deformation at service load levels. To evaluate the deflection of externally FRP-strengthened RC beams, a total of 18 RC beams, including 16 beams strengthened with CFRP laminate under different preload levels and 2 control beams, were tested. Based on the assumption that the section of the beam behaves a tri-linear moment-curvature response characterized by pre-crack stage, post-crack stage and failure stage and the test results, this paper presents a modified model to evaluate the deflection of CFRP-strengthened RC beams. The present modified model was verified by the similar test results, and shows a good agreement with the test results.

FRP-strengthened RC slabs anchored with FRP anchors

An abundance of tests over the last two decades has shown the bending capacity of flexural members such as reinforced concrete (RC) beams and slabs to be enhanced by the bonding of fibre-reinforced polymer (FRP) composites to their tension face. The propensity of the FRP to debond, however, limits its effectiveness. Different types of anchorages have therefore been investigated in order to delay or even prevent debonding. The so-called FRP anchor, which is made from rolled fibre sheets or bundles of lose fibres, is particularly suitable for anchoring FRP composites to a variety of structural element shapes. Studies that assess the effectiveness of FRP anchors in anchoring FRP strengthening in flexural members is, however, limited. This paper in turn reports a series of tests on one-way spanning simply supported RC slabs which have been strengthened in flexure with tension face bonded FRP composites and anchored with different arrangements of FRP anchors. The load–deflection responses of all slab tests are plotted, in addition to selected strain results. The behaviours of the specimens including the failure modes are also discussed. The greatest enhancement in load and deflection experienced by the six slabs strengthened with FRP plates and anchored with FRP anchors was 30% and 110%, respectively, over the unanchored FRP-strengthened control slab. The paper also discusses the strategic placement of FRP anchors for optimal strength and deflection enhancement in FRP-strengthened RC slabs.

GFRP 보강근으로 겹이음된 콘크리트 보의 보강비에 따른 거동특성

기존의 철근 콘크리트 구조물에서 나타나는, 극한 환경하에서의 철근의 부식 문제 때문에 GFRP 보강근으로 철근을 대체하고 있다. 최근 들어 GFRP를 보강근으로 사용한 보의 성능에 대한 해석적, 실험적 연구가 지속적으로 행해지고 있지만 아직 철근 콘크리트 보의 연구에 대한 수에 비하여 이에 대한 연구 결과는 매우 적어 신뢰성을 얻기 힘든 상황이다. 이에 본 연구에서는 겹침이음된 GFRP 보강근을 보에 적용하여 모멘트-처짐 관계에 대한 실험적 연구를 수행하였다. 실험 변수는 GFRP의 보강비와 피복 두께에 대한 것으로 총 6개의 GFRP 보강 콘크리트 보의 실험체가 제작되었다. 모든 실험체는 4000mm의 스팬을 가지고 있으며 12.7mm의 지름을 가지는 GFRP 보강근을 사용하였다. 보강근이 겹침이음된 부분에 일정한 모멘트가 작용하게 하기 위해 2점 가력 방식을 사용하였다. 실험 결과 보강근비의 증가에 따라 극한 하중의 크기가 증가하였다. 파괴 모드는 보강근비에 따라 매우 민감하게 변화하였으며 피복 두께는 인장측의 콘크리트의 탈락에 의해 최대 강도와 처짐량을 결정하는 요인이 되는 것으로 나타났다. The use of glass-fiber-reinforced polymer (GFRP) bars in reinforced concrete (RC) structures has emerged as an alternative to traditional RC due to the corrosion of steel in aggressive environments. Although the number of analytical and experimental studies on RC beams with GFRP reinforcement has increased in recent decades, it is still lower than the number of such studies related to steel RC structures. This paper presents the experimental moment deflection relations of GFRP reinforced beam which are spliced. Test variables were different reinforcement ratio and cover thickness of GFRP rebars. Seven concrete beams reinforced with steel GFRP re-Bars were tested. All the specimens had a span of 4000mm, provided with 12.7mm nominal diameter steel and GFRP rebars. All test specimens were tested under 2-point loads so that the spliced region be subject to constant moment. The experimental results show that the ultimate moment capacity of beam increasing of the reinforcement ratio. Failure mode of these specimens was sensitively vary according to the reinforcement ratio. The change of beam effective depth, which was caused by cover thickness variation, controlled the maximum strength and deflection because of cover spalling in tension face.

An Experimental Study on Flexural Behavior of Reinforced Concrete Beams Strengthened with High-Performance Ferrocement

Three reinforced concrete (RC) beams strengthened by high-performance ferrocement and two control specimens without strengthened are investigated when RC beams have low compressive strength. Flexural behaviors of strengthened RC beams with high-performance ferrocement are evaluated based on comparative analysis with RC beams. The strengthening results of steel meshes with U-shape (i.e. ferrocements are put onto the tension face and two profile faces) are analyzed. The flexural capacity, deflection and crack width of RC flexural beams are measured, and then comparative analysis is carried out for deformation performance and law of crack development. The test results show that ferrocement contributes greatly to increase the flexural capacity and raise crack-resisting capacity. The experimental results can provide a theoretical reference for actual engineering designs.

Comparison of experimental internal forces with full and partial interaction theories in steel-concrete-steel sandwich beams

Steel-concrete-steel sandwich beam (double skin composite beam - DSC) experimental results including axial forces in steel plates and shear forces between the layers are compared with full and partial interaction theories. The flexibility of shear stud connectors on both tension and compression faces are taken into account in the partial interaction analysis including the influence of frictional forces between the concrete and external steel plates at the supports and load points. Quasi-static test results on DSC beams are compared with the theoretical solutions based on partial interaction theory assuming realistic material and shear connector properties. The comparison of results indicates that the proposed theoretical method shows good correlation with real behaviour and may be reliably used for the analysis of simply supported DSC beams. Key words: Steel-concrete-steel sandwich beams, double skin composite construction, partial interaction, full interaction, shear connectors, frictional force, quasi-static loading.

앵커볼트 체결 Slit형 강판 보강 RC보의 전단거동에 관한 실험적 연구

기존 구조물에서 RC보는 여러 가지 이유로 불충분한 전단에 대한 문제에 직면하게 된다. 전단내력이 부족한 RC보의 전단 보강방법으로 강판이 널리 사용되고 있다. 본 연구에서는 앵커볼트가 체결된 경사, 수직 슬릿형 강판의 표면부착에 의해 전단보강된 RC보에 대한 실험을 하였으며, 여러 형태의 앵커볼트 체결 슬릿형 강판으로 보강된 RC보에 대한 전단보강효과, 파괴모드 및 전단내력을 평가하는 것을 연구의 목적으로 하였다. 실험의 변수는 앵커볼트가 부착된 슬릿의 폭, 간격, 경사각 및 수직 길이로 하였다. 연구 결과, 에폭시 부착과 볼트 체결로 보강된 슬릿형 강판 실험체의 파괴 유형은 최대하중 시 전단파괴 모드로 나타났다. 휨균열은 보의 인장측에서 최초로 발생하였으며, 경사 균열은 전단스팬에서 발생하였다. 최종적으로 에폭시 부착과 볼트 체결로 보강된 슬릿형 강판에서의 급격한 박리현상은 지연되었으며, RC보의 본체로부터 완전하게 분리 되지는 않음을 알 수 있었다.

Strengthening Two-Way Reinforced Concrete Floor Slabs Using Polypropylene Fiber Reinforcement

Two methods for strengthening two-way reinforced concrete floor slabs subjected to out-of-plane bending loads are compared through experiments on seven test specimens and subsequent analyses. The seven test specimens were two unstrengthened regular reinforced concrete slabs (control), two slabs strengthened using glass-fiber-reinforced polymer (GFRP) sheets, and three slabs strengthened with an innovative method of applying a layer of fiber-reinforced cement (FRC) in varying thicknesses to the tension face of the slab. All specimens were 1.5 m×1.5 m (5 ft×5 ft) and were designed to resist bending in both directions. The advantages and disadvantages of the two strengthening methods are discussed in terms of structural considerations, e.g., increase in load carrying capacity and ductility, and construction considerations, e.g., economy and ease of application. Experimental results show a significant increase in the ultimate load capacity of all five strengthened slabs over the two control slabs. The FRC-strengthened slabs exhibit superior ductility and larger measured displacements than the GFRP-strengthened slab. The two methods are comparable in terms of ease of application but FRC is more cost-effective. Theoretical values, which are calculated using existing analytical methods, such as strain compatibility, yield-line analysis, and, as appropriate, flexural shear stress analysis, are generally in good agreement with experimental data. Furthermore, by modifying similar analytical methods for fiber-reinforced polymers (FRPs) found in the literature an analytical method is derived for FRC. The methodologies utilized here provide a means for analysis and design of such rehabilitation schemes. As a result of this study, it is concluded that FRC has great potential as a strengthening method, and future work is recommended to further optimize the proposed strengthening technique.

A General Analytical Method for the Analysis of Interfacial Stresses in Plated Beams under Arbitrary Loading

Recent developments in structural engineering have demonstrated effective enhancement in strength and performance of reinforced concrete (RC) and metallic beams bonded with a fibre reinforced polymer (FRP) composite or steel plate on their tension face. This technique is now popularly adopted for the retrofitting and strengthening of existing structures. Under applied loading after strengthening, interfacial shear and normal stresses are developed between the adherends in such plated beams due to the transfer of stresses between the bonded plate and the original beam. The combination of these stresses may be responsible for premature plate end debonding failure of the plate from the original beam in a brittle manner. Consequently, many analytical solutions have been developed to quantify these interfacial stresses. However, almost all of these solutions are applicable only to thin plates bonded to the beam and are specific to pre-defined simple loading arrangements, so each solution is commonly only applicable to a specific loading. This paper presents a new analytical solution for the interfacial stresses in a simply supported beam bonded with a thin or thick plate to the tension face. The solution is generic and applicable to beams and plates made of any structural materials within the linear elastic range, in common with almost all previous studies. The novelty of this work lies in the application of the superposition principle so that the simple solution is applicable to any arbitrary loading arrangement. Numerical comparison of the new solution with one of the existing solutions and finite element predictions for three loading cases illustrate the accuracy and applicability of the new analytical solution.

Comparing experimental deformations of steel-concrete-steel sandwich beams with full and partial interaction theories

In this study, experimental deformation results (that is beam deflection and slip between layers) of steel-concrete-steel sandwich beams (double skin composite beam - DSC) are compared with full and partial interaction theories. The flexibility of shear stud connectors on both tension and compression faces is taken into account in the partial interaction analysis including the influence of frictional forces between the concrete and external steel plates at the supports and load points. Quasi-static test results on DSC beams are compared with the theoretical solutions based on partial interaction theory assuming realistic material and shear connector properties. The comparison of results indicates that the proposed theoretical method shows good correlation with real behaviour and may be reliably used for the analysis of simply supported DSC beams. Key words: Steel-concrete-steel sandwich beams, double skin composite construction, partial interaction, full interaction, shear connectors, deflection, slip, frictional force, quasi-static loading.

Acoustic emission activities and damage evaluation of reinforced concrete beams strengthened with CFRP sheets

In this study, the applicability of acoustic emission (AE) techniques to monitor damage evolution in reinforced concrete (RC) beams strengthened in flexure with carbon fiber reinforced polymer (CFRP) sheets is investigated. The objective is to initiate the creation of a user-friendly health monitoring system for RC structures strengthened with CFRP sheets using AE techniques. Five beams, 200mm×300mm in cross-section, were tested under three-point bending over a span of 1700mm. One of the beams was tested in its virgin condition to serve as reference; the remaining four beams were tested after being strengthened with CFRP sheets bonded on the tension face. The parameters investigated in this study include both the amount of CFRP sheets and construction imperfections (the CFRP sheets were intentionally bonded without adhesive in the centermost 10% and 20% bonding area). The AE signals were collected and analyzed for all specimens. The AE parameters were analyzed for four levels of damage based on initial crack, propagation, yielding of main bars, and fracture or rip-off of the CFRP sheets. The frequency-peak magnitude distribution of the AE parameters was used to determine the b-value, defined by the Gutenberg–Richter relationship, for evaluating the damage evolution and fracture process of RC beams strengthened in flexure with CFRP sheets. From the results of this study, the signal characteristics – event, amplitude versus frequency, and amplitude versus duration – show clear differences in the different loading stages, depending upon the active damage mechanism. The b-value is correlated to the fracture process of the RC beams bonded with CFRP sheets and the degree of localization of damage. The AE technique is a useful nondestructive technique for monitoring the behavior of RC beams that are externally reinforced in flexure with CFRP sheets.

Flexural Response of Inorganic Hybrid Composites With E-Glass and Carbon Fibers

By far, carbon and glass fibers are the most popular fiber reinforcements for composites. Traditional carbon composites are relatively expensive since the manufacturing process requires significant heat and pressure, while the carbon fibers themselves are inherently expensive to produce. In addition, they are often flammable and their use is restricted when fire is a critical design parameter. Glass fabrics are approximately one order of magnitude less expensive than similar carbon fabrics. However, they lack the stiffness and the durability needed for many high performance applications. By combining these two types of fibers, hybrid composites can be fabricated that are strong, yet relatively inexpensive to produce. The primary objective of this study was to experimentally investigate the effects of bonding high strength carbon fibers to E-glass composite cores using a high temperature, inorganic matrix known as geopolymer. Carbon fibers were bonded to E-glass cores (i) on only the tension face, (ii) on both the tension and compression faces, or (iii) dispersed throughout the core in alternating layers to obtain a strong, yet economical, hybrid composite laminate. For each response measured (flexural capacity, stiffness, and ductility), at least one hybrid configuration displayed mechanical properties comparable to all carbon composite laminates. The results indicate that hybrid composite plates manufactured using 3k unidirectional carbon tape exhibit increases in flexural capacity of approximately 700% over those manufactured using E-glass fibers alone. In general, as the relative amount of carbon fibers increased, the likelihood of precipitating a compression failure also increased. For 92% of the specimens tested, the threshold for obtaining a compression failure was utilizing 30% carbon fibers. The results presented herein can dictate future studies to optimize hybrid performance and to achieve economical configurations for a given set of design requirements.

Simple General Solution for Interfacial Stresses in Plated Beams

The flexural strength of a reinforced concrete, metallic or timber beam can be increased by bonding a thin plate, made of steel or fiber-reinforced polymer, to its tension face. A main failure mode of such plated beams involves debonding of the plate end from the beam and such plate-end debonding depends strongly on the interfacial stresses between the beam and the plate. Consequently, many analytical solutions have been developed for the interfacial stresses of specific plated beam problems, with almost all of them being for simply supported plated straight beams of constant section subjected to simple loadings. The existing analytical solutions are therefore neither general enough nor simple enough for direct exploitation in assessing the risk of plate-end debonding failure. This paper corrects this deficiency by presenting a simple, accurate yet general solution for interfacial stresses. The solution is applicable to plated beams of all geometric (e.g., curved beams), sectional (e.g., tapered beams), loading (e.g., a linearly varying distributed load), and boundary conditions (e.g., continuous beams). The accuracy of the solution is demonstrated through comparisons with finite element results. The paper also presents simple and accurate approximations for the peak values of interfacial shear and normal stresses at the plate end. In these approximate expressions, only the sectional forces and properties of the plate end section are involved, which greatly facilitates their direct exploitation in predicting debonding failure.

Flexural Behavior Of R/C Beams With Externally Bonded Gfrp Sheets

During the latest decades there has been a significant increase in using FRP (Fiber Reinforced Polymers) as a main material for external reinforcement in the construction industry.Externally bonded FRP sheets have been used to increase moment capacity of flexural members and to improve confinement in compression members. This paper summarizes the result of experimental and analytical studies on the flexural strengthening of reinforced concrete beams by external bonding of glass fiber reinforced polymer sheets to the tension face of the beam. Four beams, three with different thickness of GFRP sheets and one without GFRP sheets were tested using third-point loading over a span of 900mm.The tests were carried out under load control. The results indicate that the flexural strength of the beams increased significantly as the thickness of the sheet increased. Analytical study using a computer program based on strain compatibility is presented to predict the ultimate strength and load-deflection behavior of the beams. When the experimental results were compared with theoretical ones, good acceptable agreement was obtained which make it possible to consider and recommended this model in the design. Keyword: Analytical Study, Flexural Behavior, GFRPSheets, Strengthening.