FRP Sheets Research Articles

Effect of Preload Level on Flexural Load-carrying Capacity of RC Beams Strengthened by Externally Bonded FRP Sheets

Most of the laboratory tests investigated the flexural performance of un-preloaded or undamaged RC beams strengthened with CFRP composites. However, in engineering applications, the structural member must carry a certain load or damage. There is a lack of systematical investigations on the effects of preload or damage level on the flexural load-carrying capacity of CFRP-strengthened RC beams. This paper tested 22 RC beams to investigate the influence of preload level on flexural load-carrying capacity of CFRP-strengthened RC beams. The test variables are preload level, amount of CFRP sheets, tension rebar ratio, and concrete strength. The test results show that if the preload level is not more than 80% of the yielding strength of the original beam, the preload or damage level does not influence the flexural load-carrying capacity of CFRP-strengthened RC beams. However, the ultimate flexural load-carrying capacity is significantly poor than that of RC beam strengthened under a preload level not more than 80% of the yielding strength, if the RC beams are strengthened under a preload level more than 90% of the yielding strength.

Reinforced concrete beams strengthened with SRP/SRG systems: Experimental investigation

Steel reinforced polymer (SRP) and steel reinforced grout (SRG) have emerged as promising and cost-effective technologies for the external strengthening of RC structures.Although the first studies date back to 2004, so far the literature related to the flexural strengthening of RC slabs/beams with steel tapes is rather limited. As a result, the application of such materials on a real structural member may be discouraged and, conversely, the use of carbon/glass FRP alternative systems may be preferred.The study presented in this paper contributes to filling this knowledge gap by presenting the results of 10 four-point bending tests performed on RC slabs strengthened with SRG/SRP systems. Test results have provided valuable information in terms of maximum forces, deformability and failure modes by varying number of layers and density of the steel tape. In particular, it has been shown that, disregarding the nature of the matrix (inorganic or polymeric), the presence of the external strengthening significantly increased the flexural strength of slabs, with percentage increases over the control (unstrengthened) member ranging from a minimum value of 27%, when using a single layer of low density tape, to a maximum of 106% in the case of SRP system with one layer of high density sheet.Preliminary analytical studies were also performed in order to investigate the possibility of extending to the SRP/SRG systems the applicability of formulations currently reported in some national and international guidelines for the flexural strengthening of RC members with FRP sheets.

Modified force method for the nonlinear analysis of FRP reinforced concrete beams

Among the different techniques adopted to retrofit structures that had become inadequate, the use of fiber reinforced composite materials (FRP) has gained more and more popularity for their relevant properties such as high strength to weight ratio and excellent corrosion resistance. However, debonding of FRP lamina from the substrate, made of either concrete or steel, may prevent the achievement of the full load bearing capacity of the reinforced element. In this paper a numerical method, able to follow the complete equilibrium path of a reinforced concrete beam externally strengthened by means of FRP sheets up to the complete detachment of the external reinforcement, is proposed. According to this method, the FRP sheets are separated from the reinforced concrete substructure by displaying the interface stresses that are taken into account by means of the corresponding interface slips (which are the primary unknowns). Compatibility equations are then imposed at the interface in order to restore the behavior of the externally reinforced beam. The method is first validated by benchmarking experimental results taken from literature and then exploited to perform a parametric analysis in order to point out the effect of the main design parameters on the response of the reinforced beam.

Prestressed FRP and interface slip effect on interfacial stresses analysis: the new FRP sheets rigidity model

This paper presents an improved bi-material beam theory with adhesive interface and the new prestressed-fiber reinforced polymer (FRP) – model, which has been applied to the study the problem of interfacial stresses. This work explicitly considers the interfacial slip effect on the structural performance by including both the effect of adherend shear deformations and the fiber volume fraction of the prestressed laminates. This new method needs only one differential equation to determine both shear and normal interfacial stress, which is one aspect that has not been taken into account by the previous studies in the literature. A parametrical study is carried out to show the effects of some design variables, e.g., stiffness and thickness of adhesive layer and FRP plate.

Compressive behaviours of square timber columns reinforced by partial wrapping of FRP sheets

Based on the experimental tests of timber square columns strengthened by fibre-reinforced polymer sheets under axial compression, the failure modes, load carrying capacity, loading–strain curves, ductile and stiffness factors were examined. The results showed that in comparison with specimens without FRP reinforcement, the axial load carrying capacity, the peak of compression strain and stiffness of specimens with fibre-reinforced polymer reinforcement were all improved. The ultimate load carrying capacity of specimens with fibre-reinforced polymer reinforcement was improved by 68·9–100·2%. However, the ultimate displacement was decreased by 46·1–56·9%, and the lowest displacement was found in the specimens strengthened by two layers of basalt fibre-reinforced polymer. Meanwhile, specimens reinforced by two layer of carbon fibre-reinforced polymer and aramid fibre-reinforced polymer sheets showed a higher level of ultimate displacement and stiffness than specimens with basalt fibre-reinforced polymer reinforcement. Additionally, the failure mode of tested specimens was influenced not only by the type of fibre-reinforced polymer sheets but also by the fractured wood texture.

Analysis on Fatigue Failure Modes of Reinforced Concrete Beams Strengthened with BFRP

The fatigue failure modes of reinforced concrete beams externally strengthened with basalt fiber reinforced polymer(BFRP) sheets was examined in an experimental and analytical study. One strengthened beam failed due to the debonding and fracture of U-wraps FRP sheet at the anchors,and the others failed the fatigue fracture of the longitudinal tensile reinforcements. Based on Code for Design of strengthening concrete structure, the fatigue failure of RC bending members strengthened with FRP was analysed,and the indicate that with reinforcement ratio of 0.612% and fatigue stress ratio of 0.3, only the fatigue fracture of steel bars occured rather than the crushing of concrete in the compression zone.

Influence of overlap configuration on compressive behavior of CFRP-confined normal- and high-strength concrete

This paper presents the findings of an experimental investigation on the effect of overlap configuration on carbon fiber-reinforced polymer (CFRP)-confined normal- and high-strength concrete. A total of 33 specimens were prepared and tested under monotonic axial compression. All specimens were cylinders with 152 mm diameter and 305 mm height and confined by CFRP tubes. Two different concrete mixes were examined, with average compressive strengths of 52.0 and 84.7 MPa. The effect of overlap configuration was examined by manufacturing the specimens with different properties at the overlap region including overlap length, continuity and distribution. Axial and lateral behavior was recorded to observe the axial stress–strain relationship and hoop strain behavior for concentric compression. Ultimate axial and lateral conditions are tabulated and stress–strain curves have been provided. Detailed plots of hoop strain development and lateral confinement pressure at ultimate are presented. The results indicate that FRP overlap length has no significant influence on strain enhancement ratio (e cu /e co), but an increase in overlap length leads to a slight increase in strength enhancement ratio (f′ cc /f′ co), with these observations equally applicable to both continuously and discontinuously wrapped specimens. The results also indicate that continuity of the FRP sheet in the overlap region has some influence on the effectiveness of FRP confinement. Furthermore, it was observed that the distribution of FRP overlap regions for discontinuously wrapped specimens can influence the axial compressive behavior of these specimens in certain overlap configurations. Finally, it is found that the distribution of lateral confining pressure around specimen perimeter becomes less uniform for specimens with higher concrete strengths and those manufactured with overlap regions that are not evenly distributed.

The numerical simulations of the strengthened RC slabs with CFRPs using standard CDP material model of Abaqus code

In this research, the effect of the lateral and corner openings as well as the impact of using a kind of FRP sheets known as Carbon Fibre-Reinforced Polymer sheets (CFRPs) as a retrofitting approach on the RC two-way slabs' bearing capacity was investigated. To this end, at first, an enhanced finite-element (FE) model of an actual laboratory sample of a reinforced concrete slab with central opening was simulated using well-known Abaqus software and verified using available experimental results. For concrete behaviour simulation, concrete-damaged plasticity (CDP) material model of Abaqus standard code which can predict well the nonlinear behaviour of concrete from the initial steps of loading scenario up to the failure state was used and its parameters were identified for this particular application. For verifying the identification, two concrete beam test set-up were simulated and the obtained results were compared with the laboratory test results. The reason of implementing such a constitutive model is that when the concrete is retrofitted with CFRPs, its brittle fracture mode under bending actions transformed to a ductile–brittle fracture one. Therefore, a need for a model which can simulate the plastic deformations and ductile damages of concrete in compressive stress regions and brittle damages in tensile stress regions up to near failure state is generated. After that, two rectangular slabs with the lateral and corner openings were simulated and strengthened with some geometrical patterns of CFRPs. Obtained results indicated that CDP material model for concrete can perform well in the structural behaviour simulating of the slabs. Also, it has been distinguished that the CFRPs can recover the load-bearing capacity loss of such slabs due to mentioned openings considerably and even in some cases can improve it in comparison to the homogeneous slab (slab with no opening). It was revealed, moreover, that the effect of the lateral openings on reduction of load capacity of these slabs is greater than the central and corner openings. Furthermore, it was declared that the combined 45° and 90° arrangement of CFRPs can affect more efficiently in strengthening weakened slabs with openings than the other CFRPs arrangements.

Apply for RC Beams and Slabs Strengthened with New Inorganic Adhesives Sticked FRP Sheets

A new inorganic nonmetallic cementing material rather than epoxy resin was adopted in carbon fiber sheet (CFS) strengthened concrete beams and slabs in this paper. Firstly, the types of inorganic adhesive were introduced and double-shear tests and the compressive strength of specimens as a function of temperature were studied to prove the effectiveness of inorganic adhesive in CFS strengthened concrete beams and slabs was studied. The practice shows that MPB has good performance when applying at high temperature. RC beams and slabs strengthened with new inorganic adhesives sticked FRP sheets are feasible. The appearance of MPB renders an extensive prospect for carbon fiber sheet (CFS) application in building structure reinforcement.

Experimenting with a superconducting levitation train

The construction and operation of a prototype high- T c superconducting train model is presented. The train is levitated by a melt-processed GdBa 2 Cu 3 O x (Gd-123) superconducting material over a magnetic rail (track). The oval shaped track is constructed in S-N-S or PM3N configuration arranged on an iron plate. The train bodies are constructed with FRP sheets forming a vessel to maintain the temperature of liquid nitrogen. The superconductors are field-cooled on the magnetic track, which provides a large stability of the levitation due to strong flux pinning of the melt-processed superconductors. The setup enables to test parameters like stability, speed, and safety, of the superconducting train for various gaps (ranging between 1 mm to 15 mm) between the train and the magnetic track. The experimental results indicate that trains with 1 to 2 mm gaps cannot run properly due to the friction applied to the track. The trains with 10 or 15 mm gaps do not run stable on the track. Our results confirm that a gap of 5 mm is the optimum distance to run the train showing also stability to run fast on the track. The present results clearly demonstrate that the stability of the superconducting trains depends on the gap between the rail and train, which is an important parameter also for the real Maglev trains.

Two new composite plate elements with bond–slip effect for nonlinear finite element analyses of FRP-strengthened concrete slabs

Two 8-node 48-DOF rectangular composite layered plate elements, CPEB-1 and CPEB-2, which account for the bond–slip effect between FRP sheets and concrete, are developed for nonlinear finite element analyses of FRP-strengthened concrete slabs in this paper. A layered approach is used to form an integrated plate element for modelling of concrete, FRP, adhesive layer and steel reinforcement and also for modelling of debonding. In element CPEB-1, a zero thickness interfacial element is used to connect concrete and FRP layer and to model the debonding between concrete and FRP layer. In element CPEB-2, the bond–slip is assumed to occur within a smeared layer with real thickness, which connects the concrete tensile zone with the adhesive layer. Shear deformation effects are included in these models and the shear locking problem is avoided by employing the Timoshenko’s composite beam functions to describe the deflection and rotation of the layered plate elements. Both geometric and material nonlinearity are included. The accuracy and efficiency of the proposed elements are validated by comparing the computed results with experimental results obtained from literatures. The two elements and their computation accuracy and computation effectiveness are also compared.

Effect of tapered-end shape of FRP sheets on stress concentration in strengthened beams under thermal load

Repairing and strengthening structural members by bonding composite materials have received a considerable attention in recent years. The major problem when using bonded FRP or steel plates to strengthen existing structures is the high interfacial stresses that may be built up near the plate ends which lead to premature failure of the structure. As a result, many researchers have developed several analytical methods to predict the interface performance of bonded repairs under various types of loading. In this paper, a numerical solution using finite - difference method (FDM) is used to calculate the interfacial stress distribution in beams strengthened with FRP plate having a tapered ends under thermal loading. Different thinning profiles are investigated since the later can significantly reduce the stress concentration. In the present theoretical analysis, the adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both beam and bonded plate. The shear correction factor for I-section beams is also included in the solution. Numerical results from the present analysis are presented to demonstrate the advantages of use the tapers in design of strengthened beams.

폭발 손상을 입은 강섬유 및 FRP 시트 보강 철근콘크리트 보의 국부손상 및 잔류성능 평가

본 연구에서는 폭발에 의한 국부손상이 철근콘크리트 보의 잔류 휨강도 및 연성에 미치는 영향을 알아보기 위하여 160×290×2200㎜의 철근콘크리트 보에 대하여 근접 폭발 시험과 정하중 휨시험을 수행하였다. 또한 이 시험을 통해 강섬유 및 FRP 시트로 보강한 철근콘크리트 보의 폭발저항성능을 평가하였다. 장약량 1㎏, 이격거리 0.1m의 폭발 시험을 수행하였으며, 시험 후 시험체의 crater, spall 지름과 무게손실(weight loss)을 측정하여 시험체의 국부손상을 평가하였다. 또한 폭발하중을 받지 않은 시험체와 폭발하중을 받아 국부손상이 발생한 시험체의 정하중 휨시험을 수행하여 휨강도 및 연성지수를 측정하였다. 시험결과, 보통콘크리트 시험체(NC)는 비교적 큰 crater와 spall이 발생하였으며 국부손상에 의한 무게 손실이 23.5㎏ 발생하였다. 반면 강섬유보강콘크리트 시험체(SFRC)와 FRP 시트 보강 시험체(NC-F, NC-FS)는 NC에 비해 crater 크기와 무게손실이 감소하며 향상된 폭발저항성능을 나타내었다. 또한 폭발하중에 의해 국부손상이 발생한 시험체들은 휨강도와 연성지수가 감소하였다. 특히 NC는 잔류 휨강도가 기존 강도에 비해 크게 감소하였고 압축철근의 좌굴현상이 나타나며 취성적으로 파괴되었다. 강섬유와 FRP 시트로 보강한 시험체의 경우 잔류 휨강도 및 연성지수가 NC에 비해 증가하였다. 이를 통해 폭발이 발생할 경우, 충분한 이격거리를 확보하지 못하면 장약량이 적더라도 구조부재에 심각한 국부손상을 발생 시킬 수 있다는 것을 알 수 있었으며, 강섬유와 FRP 시트에 의한 보강 방법이 폭발저항성능을 향상시킨다는 것을 확인할 수 있었다.

Bond behavior of roughing FRP sheet bonded to concrete substrate

With the thickness of 1–2mm made of FRP, a thin concrete scaleboard which can be used accompanying with concrete pouring was presented with the aim to improve the durability of concrete, in which the use of porous lightweight aggregate as roughing coating during FRP sheet producing was innovated to strengthen the bond performance between FRP sheet and concrete. Results indicated that lightweight aggregate with a particle size distribution from 3.15 to 4.75mm showed nice bond performance, the average bond strength was 2.17MPa. Bond strength was determined by mechanical interlocking and interfacial structure, and a bond equation was established.

Experimental Study on Masonry Panels Strengthened by GFRP: The Role of Inclination between Mortar Joints and GFRP Sheets

For the shear strengthening of masonry walls, different configurations for FRP sheets are currently used in real application, such as vertical, horizontal or diagonal strips. In the last configuration the FRP sheet is inclined with respect to the direction of mortar joints. In the experimental campaign presented in this paper, it is investigated whether the FRP-masonry bond could be affected by this inclination. In order to analyze this issue, three different typologies of masonry panels (with different textures) retrofitted by FRP sheets, inclined of 45 degrees with respect to the vertical axis of the specimen, are subjected to single-lap shear tests. Results of shear tests are presented in terms of maximum debonding forces, force-elongation curves, failure modes and strain profiles along the specimens. The use of Digital Image Correlation (DIC) technique allowed to obtain complete strain maps on the surface of the specimens tested, with the purpose of investigating possible variations in the strain field within the bonded area.

Digital Image Correlation Analysis of CFRP to Brick Bonded Joints Fastened by Fiber Anchor

In the framework of the reinforcement of masonry structures by FRP sheets (Fiber Reinforced Polymer), the use of “Fiber Anchor” is an innovative performing solution to increase the peak load and the dissipation capability of the strengthened system. To analyze the performance of this new technology an experimental campaign composed by 36 tests has been conducted. Six series of CFRP to brick bonded joint fastened by different configurations of fiber anchor have been tested. To deeply describe the measure fields, the Digital Image Correlation (DIC) has been used. The strain fields obtained in the series not fastened by anchors have permitted to define the bond stress field and the advancement of the stress transfer zone. The DIC appears as a performing methodology to use in a lot of civil engineering experimental problems.

Bond behavior of steel deformed bars embedded in concrete confined by FRP wire jackets

This study provides experimental results of bond tests for concrete confined by FRP wire jackets and investigates bond behavior. The used FRP wire jackets are newly introduced and found to be more effective in wrapping around concrete members than conventional FRP sheet jackets. Bond–slip tests were conducted by push-out tests using concrete cylinders with an embedded rebar at the center of the cylinders. Two parameters of concrete strength and amount of confinement were chosen. Bond strength and radial strain of concrete are discussed with the stiffness ratio of confinement. A method to estimate the increment of bond strength due to confinement is suggested. The bond strength of confined concrete shows a logarithmic relationship to the stiffness ratio of confinement and, thus, there is a limit on how to increase the bond strength by external confinement. It is found that radial strain at bond strength or the corresponding lateral confining pressure is a critical factor to increase bond strength.

Research on New Type Quantitative Nondestructive Pre-Stress FRP Sheet Tension Fixture

Traditional pre-stress FRP sheet tension fixture must have the beam of the bridge slotted to reduce the cross-section of the beam. Besides, stress on the FRP sheet is irregular with local stress concentration. Its indirect load pre-stress, as well as quantitative criterion of pre-stress born by FRP sheet is not accurate. To solve the above problems, a newly-developed self-balanced type pre-stress FRP sheet tension fixture has been introduced. Such a tension fixture consists of welded steel frame, a mechanical jackscrew, a dynamometer and a strain gauge. It is able to give specified stress onto the FRP sheet accurately while not damaging the beam cross-section. At the same time, with the combination of ANASYS Workbench and UG8.0 Software, also states the simulation analysis of the new type tension fixture, including its stress under practical work and deformation state, which initially proves the scientific and economic usefulness of such tension fixtures.

Axial behavior of FRP-wrapped circular ultra-high performance concrete specimens

Ultra-High Performance Concrete (UHPC) is an innovative new material that, in comparison to conventional concretes, has high compressive strength and excellent ductility properties achieved through the addition of randomly dispersed short fibers to the concrete mix. This study presents the results of an experimental investigation on the behavior of axially loaded UHPC short circular columns wrapped with Carbon-FRP (CFRP), Glass-FRP (GFRP), and Aramid-FRP (AFRP) sheets. Six plain and 36 different types of FRP-wrapped UHPC columns with a diameter of 100 mm and a length of 200 mm were tested under monotonic axial compression. To predict the ultimate strength of the FRP-wrapped UHPC columns, a simple confinement model is presented and compared with four selected confinement models from the literature that have been developed for low and normal strength concrete columns. The results show that the FRP sheets can significantly enhance the ultimate strength and strain capacity of the UHPC columns. The average greatest increase in the ultimate strength and strain for the CFRP- and GFRP-wrapped UHPC columns was 48% and 128%, respectively, compared to that of their unconfined counterparts. All the selected confinement models overestimated the ultimate strength of the FRP-wrapped UHPC columns.

Shear failure of RC elements strengthened with steel profiles and CFRP wraps

The paper presents an experimental activity carried out on RC elements strengthened in shear with different configurations of CFRP laminates. Nine specimens were tested with the aim of validating the performance of a new proposed technology that combines L shaped steel profiles and CFRP wrapped sheets: the steel profiles are placed along the section corners to increase the axial load carrying capacity, and the FRP sheets are devoted to increase the shear strength. The results of the experimental tests are compared with values predictable on the basis of both the Italian CNR-DT200 and the ACI 440.2R-02 codes. This comparison evidenced a weakness that lies in the expression reported in the Italian code.