Thermo-mechanical loading of GFRP reinforced thin concrete panels

The experimental investigation is focused on the thermo-mechanical behaviour of thin concrete panels reinforced with GFRP rebars. The considered thin panels (thickness of 4 cm) were exposed to increasing temperature and bending loading. These concrete elements are typical for low bearing function concrete layers in façade claddings. The influence of two aspects was studied: the concrete cover and the external surface of rebars. The heating condition was such that the temperature of the internal GFRP rebars reached about the transition temperature of the resins. This allowed to verify the variation of the deformability and the load carrying capacity of the panels with post-heating bending tests. As main outcome, the imposed temperature did not generate evident degradation of the GFRP reinforcement and of its adhesion to the concrete, while a reduction of the initial global stiffness was measured.     

Modelling the response of reinforced concrete panels under blast loading

A finite element model is developed for the simulation of the structural response of steel-reinforced concrete panels to blast loading using LS-DYNA. The effect of element size on the dynamic material model of concrete is investigated and strain-rate effects on concrete in tension and compression are accounted for separately in the model. The model is validated by comparing the computed results with experimental data from the literature. In addition, a parametric study is carried out to investigate the effects of charge weight, standoff distance, panel thickness and reinforcement ratio on the blast resistance of reinforced concrete panels.     

Strengthening of a reinforced concrete bridge with externally bonded steel reinforced polymer (SRP)

This paper introduces steel reinforced polymer (SRP) as a way to strengthen civil structures. This technique consists of cords formed by interwoven high-strength steel wires embedded within a polymeric resin. A comprehensive study addressing analysis, design, installation, load rating and monitoring of a bridge strengthened with this technology is reported. The SRP system was easily installed, demonstrating its similarity to more traditional fiber reinforced polymer (FRP) strengthening techniques. Load tests were performed to evaluate the bridge structural behavior prior and after the strengthening in accordance with AASHTO specifications, showing that the rehabilitation performed as predicted and therefore created the possibility to remove the bridge load posting.     

Flexural analysis and design of reinforced concrete beams with externally bonded FRP reinforcement

This work addresses the flexural analysis of reinforced concrete beams with externally bonded FRP (Fibre Reinforced Polymer) reinforcement. A numerical method has been developed for the computation of the bending moment capacity of FRP-plated reinforced concrete beams and prediction of the flexural failure modes. The expressions for the upper and lower values of the characteristic plate reinforcement ratios are derived for rectangular and T-sections using the Eurocode 2 model for concrete. A flow-chart of the numerical procedure, suitable for computer implementation, is included and its accuracy is validated with available experimental results. Some of the novel features of the numerical analysis are demonstrated through a brief investigation of the effects of loads acting at the stage of strengthening on the ultimate flexural capacity and deformation behaviour of FRP-plated R.C. beams.

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Résumé Ce travail analyse le comportement en flexion de poutres en béton armé avec renforcement extérieur par PRF (polymères renforcés de fibres). Une méthode numérique a été développée pour le calcul de la capacité du moment de flexion de poutres en béton renforcé de plaques en PRF, et pour la prévision des modes de défaillance en flexion. Des expressions pour les valeurs supérieures et inférieures des rapports caractéristiques de ce renforcement ont été dérivées pour une section rectangulaire et des sections en T en utilisant le modèle d'Eurocode 2 pour le béton. Un organigramme du procédé numérique, approprié à l'exécution numérique, est inclus et son exactitude est validée par les résultats expérimentaux disponibles. Certaines des caractéristiques originales de l'analyse numérique ont été démontrées par la brève recherche sur les effets du chargement précédemment appliqué sur la capacité en flexion finale et les déformations des poutres en béton armé renforcés de plaques en PRF.

     

Experimental analysis of flexural behaviour of externally bonded CFRP reinforced concrete structures

This pper focuses on the flexural behaviour of RC beams strengthened using carbon fibre pultruded plates. Several parameters (number of layers, effect of a precracking) are experimentally investigated on full-scale beams.     

Investigation into the time-dependent behaviour of reinforced concrete specimens strengthened with externally bonded CFRP-plates

This paper deals with the influence of an external CFRP-reinforcement on the time dependent behaviour of flexural concrete beams. For this purpose, four reinforced concrete beams are tested in relaxation under three-point bending. Three of the four are strengthened with a composite plate bonded to their tensile soffit. The latter is not strengthened. The phenomenon of relaxation leads to a decrease in the loading force in the course of time. The test results are analysed in three ways: the first is a comparison between the global behaviour of the four beams. The second is a detailed analysis of the internal equilibrium of the beams in the course of time. The third is a global approach based on an incremental method suited to time analysis of composite structures. It can be concluded that composite reinforcement has a positive influence on the long term behaviour of strengthened beams. Their time dependent behaviours are different depending on whether the beam is pre-cracked before being strengthened. This difference may be taken into account using a coefficient of influence applied to the composite reinforcement. The existence of a period of loading before strengthening has a positive influence on the long-term behaviour of the beam.     

In-plane shear behavior of insulated precast concrete sandwich panels reinforced with corrugated GFRP shear connectors

Precast concrete sandwich panels often are used for the exterior cladding of residential and commercial buildings due to their thermal efficiency. Precast concrete sandwich panel systems consist of two precast reinforced concrete walls that are separated by a layer of insulation and joined by connectors that penetrate the insulation layer and are anchored to two precast concrete wythes. This paper presents push-out test results of concrete sandwich panels with and without corrugated shear connectors to investigate in-plane shear performance. The variables in this study are two types of insulation materials and the width, pitch, and embedment length of shear connectors. The test results indicate that the type of insulation material that is used in the system considerably affects the bond strength between the concrete walls and the insulation layer. A design equation adopted in ICC-ES is revised to determine the shear design capacity of precast concrete sandwich panels with various configurations of shear connectors.     

Shear strengthening of reinforced concrete beams using different configurations of externally bonded carbon fibre reinforced plates

This paper deals with research undertaken at Oxford Brookes University into shear strengthening of reinforced concrete beams using externally bonded carbon fibre reinforced plastic plates (CFRP). Thirty-eight reinforced concrete beams of 1.8 m length were constructed and tested and are described in this paper. The relative performance of a group of sixteen beams with the same steel reinforcement but with different amounts of shear strengthening is discussed. All the beams were designed to fail in shear using a spreadsheet program. The spreadsheets were designed to ensure that the beams' flexural capacity exceeded the shear capacity after strengthening. The variables were: main reinforcement ratio, spacing between links in the shear span and different configurations of CFRP plates on shear spans. The concrete had an average compressive strength of 61.76 N/mm². The majority of the beams tested showed a significant improvement in shear strength by the addition of CFRP plates, with increases of between 19–122% over the control beams.     

Punching shear strength of strengthened deck panels with externally bonded plates

This paper proposes an analytical method to predict the punching shear strength of bridge decks that are strengthened using external bonding techniques. Since the steel reinforcement affects the punching strength of a bridge deck, the deck will have a mechanical effect on the strengthening material. The strengthening procedure will increase the effective punching depth over which the critical crack inclinations can be divided into two inclination angles, depending on whether they are located above or below the main steel reinforcement. To verify the analytical method that is proposed in this paper, comparisons were made with existing experimental results in the literature, which showed that the proposed method accurately predicts the punching strength of bridge decks strengthened using external bonding techniques.     

高延性混凝土加固RC梁抗剪性能试验研究

剪跨比对FRP抗剪加固梁的裂缝开展和破坏模式有重要影响,但对FRP加固梁抗剪强度及尺寸效应的影响研究较少。采用三维细观数值模拟方法,考虑混凝土细观组成的非均质性及碳纤维布(CFRP)与混凝土之间的相互作用,建立了CFRP加固无腹筋钢筋混凝土梁剪切破坏力学分析模型。在验证细观模拟方法合理性的基础上,拓展模拟与分析了剪跨比对CFRP加固钢筋混凝土梁剪切破坏及尺寸效应的影响机制与规律。研究结果表明：剪跨比对CFRP抗剪加固梁剪切破坏模式影响较大,剪跨比越大,加固梁愈趋近于延性较好的斜拉破坏;剪跨比对CFRP加固梁抗剪承载力有较大影响,对抗剪强度尺寸效应影响较小;剪跨比对加固梁中的CFRP剪切贡献影响较大,剪跨比越大,CFRP对加固梁的抗剪效果越好,其中对中型剪跨比(λ=2.5)的梁加固效果最有效。     

Fatigue behavior of externally bonded steel fiber reinforced polymer (SFRP) for retrofit of reinforced concrete

Steel fiber reinforced polymer (SFRP) strips comprised of multiple high-strength wires have been introduced into the repertoire of the structural engineer in recent years. The deleterious effects of fatigue loading on FRP-to-concrete bond have been identified in previous studies by the author; therefore the effect of fatigue loading on the bond behavior of SFRP is investigated. Four large-scale beam specimens (4.9 m long) having externally bonded SFRP retrofits are tested. These specimens are paired with unretrofit and CFRP-retrofit companion specimens allowing a number of direct comparisons to be made. Of the SFRP specimens, one is tested in monotonic loading to failure while the remaining three are tested at various fatigue load levels ranging from service load level to an extreme load level. Service load fatigue is cycled for 2 million cycles and the specimen is then tested monotonically to failure to assess the effects of fatigue conditioning on the ultimate performance of the beam. Extreme loading is selected to result in fatigue-induced failure of the internal reinforcing steel.     

Fatigue of CFRPs externally bonded to concrete

The external bonding of fibre reinforced polymers (FRPs) to concrete beams is a particularly attractive rehabilitation technique for increasing or restoring the strength of existing reinforced concrete flexural members. The durability and the performance of FRP-strengthened structures subjected to cyclic loading depend directly on the fatigue behaviour of the FRP itself, as well as on that of the FRP-to-concrete interface. This paper aims at increasing the database on the fatigue behaviour of these types of reinforcement using two test set-ups developed in two distinct research laboratories. The primary purpose of this paper is to propose a methodology for the determination of the value of the allowable shear bond strength (f_adh,v,u) and the tensile FRP strength (f_f,u) as a function of the numbers of cycles of loading. Results from an experimental investigation on the cyclic behaviour of single-lap and double-lap CFRP-concrete joints are reported. For both types of test set-up, the cyclic bond stress-slip relations are presented and discussed. Analytical S-N relations (cyclic stress range vs. the number of cycles to failure) are proposed.

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Résumé Les renforts externes mis en œuvre par collage constituent une méthode efficace pour le renforcement en partie tendue des éléments de type poutres. La durabilité et la performance des structures renforcées par matériaux composites sont directement liées à l'endommagement du composite et de son interface avec le béton. Cet article vise à définir la durabilité vis-à-vis de sollicitations de fatigue grace à deux essais développés dans deux laboratoires distincts. L'objectif principal de cet article est de définir à la fois les niveaux de contraintes admissibles en cisaillement d'un joint de colle (f_adh,v,u) et les résistances en traction d'un composite (f_f,u) en fonction du nombre de cycles de chargement. Les résultats expérimentaux obtenus à partir d'un essai sur joint de colle à simple recouvrement et sur un joint de colle à double recouvrement sont présentés. Pour chaque type d'essais, la relation contrainte d'adhérence-glissement sous charges cycliques est analysée. Des courbes d'endurance de type S-N (contrainte à la rupture en fonction du nombre de cycles) sont proposées.

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Editorial note Prof. Patrice Hamelin is a RILEM Senior Member.     

Structural response of hyperstatic concrete beams reinforced with GFRP bars: Effect of increasing concrete confinement

The design of concrete structures reinforced with glass fibre reinforced polymer (GFRP) bars is influenced by their reduced stiffness and brittleness. In hyperstatic structures, the methodology used in force analysis depends on the ductility of the structural systems, which in this case, being essentially provided by the concrete, can be potentially increased by confining concrete in critical zones. This paper presents experimental and numerical investigations about the flexural behaviour of continuous beams reinforced with GFRP bars, namely of their service and failure responses, and the effect of increasing concrete confinement in critical cross-sections. A calculation procedure to quantify the confinement effect in beams due to the reduction of the spacing between shear stirrups is first presented. The experimental investigations comprised a comparative study in which two-span concrete beams reinforced with either GFRP or steel bars were tested in bending. In the former, the effect of reducing the shear stirrups spacing was analyzed together with the under- and over-reinforcement at the central support and midspan cross-sections, respectively. The development of a crack hinge in the continuity support zone highlighted the better performance of beams under-reinforced on the top layer with GFRP bars compared to “equivalent” beams reinforced with steel, namely at the resistance level. In addition, the confinement at critical zones increased significantly the strength and ductility. The numerical investigations included the development of non-linear finite element models for all beams tested - numerical results are in good agreement with test data and seem to confirm the confinement effect observed in the experiments.     

Crushing energy absorption of GFRP sandwich panels and corresponding monolithic laminates

Steady quasi-static compression of GFRP monolithic laminates and sandwich panels made of a randomly oriented continuous filament mat/polyester were undertaken. The effects of facing/laminate thickness, trigger collapse system and aspect ratio on their failure mechanisms, hence their energy absorption capability were examined. A numerical model, using a non-linear finite element explicit code, LS-DYNA, was used for pre-analysis of the effect of aspect ratio. A collapse trigger configuration was also studied numerically. The experimental data showed that high values of energy absorbed per unit mass were a predominant feature of the thickest monolithic laminates and sandwich panels with the thickest facings. The monolithic laminates showed higher specific energy than their sandwich panel counterparts. It seems that this difference was due to instability of the sandwich specimens.     

Holistic design of RC beams and slabs strengthened with externally bonded FRP laminates

Structural strengthening with externally bonded reinforcement is now recognized as a cost-effective, structurally sound and practically efficient method for rehabilitating deteriorated and damaged reinforced concrete structures. Although a variety of worldwide on-site applications using composite materials have been realized for the rehabilitation and reinforcement of structural elements, the technology is now at a stage where its future development and competitiveness with conventional methods will depend on the definition of valid design guidelines based on sound engineering principles rather than on the availability of new materials or production processes.The main objective of this paper is to present a general design philosophy for externally plated reinforced concrete beams and slabs, based on a holistic approach, in which appropriate strategies for achieving durable and safe strengthened structures are described.Essential to the design for safety, durability and ductility is the availability of structural models which are: (i) based on sound engineering principles; (ii) capable of reflecting the physical behaviour of strengthened members; (iii) of general applicability, irrespective of the type of external reinforcement material (steel or fiber-reinforced polymer), and the reinforcement configuration (web or tension plate); (iv) capable of describing all possible failure modes, in order to predict the weakest link chain of resistance of a structural member.It will be shown, with a series of numerical/experimental comparisons, that such requirements can be conveniently obtained with a unified approach in which materials and structures, calculation and experimental verification, modelling and analysis are integrated.     

Flexural strengthening of concrete beams with CFRP laminates bonded into slits

Near surface mounted (NSM) strengthening technique using carbon fibre reinforced polymer (CFRP) laminate strips was applied for doubling the load carrying capacity of concrete beams failing in bending. This objective was attained and the deformational capacity of the strengthened beams was similar to the corresponding reference beams. The NSM technique has provided a significant increment of the load at serviceability limit state, as well as, the stiffness after concrete cracking. The maximum strain in the CFRP laminates has attained values between 62% and 91% of its ultimate strain. A numerical strategy was developed to simulate the deformational behaviour of RC beams strengthened by NSM technique. Not only the load carrying capacity of the tested beams was well predicted, but also the corresponding deflection.     

Bond-slip relations for PBO-FRCM materials externally bonded to concrete

Existing reinforced concrete (RC) structures often need to be repaired, strengthened and upgraded to satisfy current code requirements. In recent years many interventions have been done bonding composite materials to the surface of existing RC elements. The structural effectiveness of these interventions strongly depends on the bond between the strengthening material and the concrete and on the mechanical properties of the concrete cover. In this paper the bond between fiber reinforced cementitious matrix (FRCM) materials made out of a poliparafenilenbenzobisoxazole (PBO) net embedded in a cement based matrix and the concrete is analytically analyzed with reference to the approach generally adopted for the fiber reinforced polymers (FRP) materials, which is based on the local bond-slip relation between the strengthening fibers and the supporting concrete. A local bond-slip relation is calibrated on the base of the results of an experimental investigation previously performed by the authors. The bond-slip relation is essential in the modeling of the structural behavior of RC elements strengthened with PBO-FRCM, in that it allows to calculate the force that can be transferred to the concrete, the effective anchorage length, the concrete cracks distance and opening.     

Bond-slip relations for PBO-FRCM materials externally bonded to concrete

Existing reinforced concrete (RC) structures often need to be repaired, strengthened and upgraded to satisfy current code requirements. In recent years many interventions have been done bonding composite materials to the surface of existing RC elements. The structural effectiveness of these interventions strongly depends on the bond between the strengthening material and the concrete and on the mechanical properties of the concrete cover. In this paper the bond between fiber reinforced cementitious matrix (FRCM) materials made out of a poliparafenilenbenzobisoxazole (PBO) net embedded in a cement based matrix and the concrete is analytically analyzed with reference to the approach generally adopted for the fiber reinforced polymers (FRP) materials, which is based on the local bond-slip relation between the strengthening fibers and the supporting concrete. A local bond-slip relation is calibrated on the base of the results of an experimental investigation previously performed by the authors. The bond-slip relation is essential in the modeling of the structural behavior of RC elements strengthened with PBO-FRCM, in that it allows to calculate the force that can be transferred to the concrete, the effective anchorage length, the concrete cracks distance and opening.     

High temperature effects on concrete members reinforced with GFRP rebars

GFRP rebars are often used for the internal reinforcement of concrete structures, such as bridge deck slabs, to improve the corrosion resistance. Several studies were conducted to evaluate the static and fatigue behaviour of these elements but the fire resistance still needs further investigation. This paper presents an experimental investigation aimed at understanding the static behaviour of concrete beams reinforced with GFRP rebars exposed to localized elevated temperatures. Two parameters were varied: the maximum temperature imposed on the bottom side of the specimens (230 °C and 550 °C) and the lapping scheme of the rebars, including rebars with hooks and laps of different lengths. The mechanical response was investigated by quasi-static three-points bending tests at room temperature and after heating. The results show that the geometry of the reinforcement has a more relevant influence on the ultimate load than on the initial stiffness of the specimens. The localized heating temperature generates damage in concrete and partial evaporation of the matrix in the GFRP rebars without causing the collapse of the element. The reduction of the load carrying capacity mainly depends on the reinforcement geometry in the overlapping areas.     

IR thermography for interface analysis of FRP laminates externally bonded to RC beams

Non-destructive (ND) evaluation by thermographic analysis was applied to reinforced concrete (RC) beams strengthened with CFRP laminates, to assess the quality of the interface between reinforcement and substrate before and under loading, during laboratory bending tests. The proper algorithm for processing data acquired from the thermographic system was first selected by testing reduced-scale fiber reinforced polymer (FRP)-strengthened concrete slabs with artificial defects and anomalies, deliberately placed at the interface. Portable heating equipment was purposely set-up to allow continuous scanning along the loaded beams. Results showed that infrared thermography (IRT) can supply significant qualitative and quantitative information on bonding of FRP materials applied to structural substrates, for both preliminary and in-situ investigations, by means of a reliable low-time-consuming procedure. Cross-evaluation of crack patterns during bending tests and thermographic results are also presented.