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Externally bonded FRP reinforcement for strengthening RC structures
Abstract
The issue of upgrading the existing civil engineering infrastructure has been one of great importance over a decade. Deterioration of bridge decks, beams, girders and columns, buildings, parking structures and others may be attributed to ageing, environmentally induced degradation, poor initial design and/or construction, lack of maintenance, and to accidental events such as earthquakes. The infrastructure's increasing decay is frequently combined with the need of upgrading so that the structures can meet more stringent design requirements (for example, increased traffic volumes on bridges exceeding the initial design loads), and hence the aspect of civil engineering infrastructure renewal has received considerable attention over the past few years throughout the world. At the same time, seismic retrofit has become important, especially in the areas of high seismic risk.
... Reinforced concrete structures during the past decades have reached the increasingly wide application in numerous engineering projects. However, there are the number of reasons, which cause challenges for their exploitation: environmentally induced degradation, not sufficient design strength, lack of maintenance, accidental events, poor level of construction, etc (DebMallik P., 2004;Blikharskyy et al, 2021(a,b,c)). All the listed above factors lead to deterioration of existing RC structures and buildings and enormous economic and technical consequences. ...
... In engineering practice strengthening of RC structures could be performed in the various ways: epoxy bonded steel plates, additional rebar, concrete cages, etc (DebMallik P., 2004). However, as the most of conventional strengthening methods may be problematic, the innovative methods with the use of composite strengthening materials were developed. ...
... Reasons to use composite materials include high immunity to corrosion, high strength/weight ratio, comparatively easy application, reduction in labor costs and time. Also, such materials are characterized with increased tensile strength, consistent stiffness, large deformation capacity and wide range of available forms, dimension and sizes (DebMallik P., 2004). However, the effectiveness of such strengthening mechanisms is strongly dependent on specifics of their stress-strain state and load distribution. ...
Wide spread of RC structures in construction projects indicates the necessity of their service life prolongation. Perspective possibility to increase strength of RC structures is use of external composite tapes. The article presents theoretical research on the effectiveness of strengthening of RC bended elements with the use of composite tapes. Work also includes comparative analysis on the basis of calculations, according to normative regulations. With the use of developed algorithm parameters of stress-strain state and deformability of RC beams, strengthened with composite tapes were obtained. Study identified, that the most critical parameter was the steel bars` strain and yield point. Interesting finding of the study is the no-linearity of the strength growth of the studied elements with the increase of additional reinforcement amount. Consistent literature review was conducted, which identified the necessity to take into account different external factors and failure mode.
... The wide acceptance of externally bonded fiber reinforced polymer (FRP) laminates (including wet layup FRP sheets and pultruded FRP plates) in strengthening existing reinforced concrete (RC) structures has been a major structural engineering development over the past two decades [1, 2]. Design approaches for FRP-strengthened RC structures at room/ambient temperature are now widely available, but a rational design method for the fire resistance evaluation of FRP-strengthened RC structures is not yet available in design guidelines (e.g., [3, 4]). ...
... This is because during fire exposure the FRP strengthening system transforms into an external ''cable'' fixed in its two anchorage zones, where the FRP-to-concrete interface retains most of its bond strength. However, there is a lack of reliable design approach for this ''partial'' fire insulation scheme, and a simplistic approach that requires a thick insulation layer is still employed by the current design guidelines [3, 4]. Such a thick insulation layer may greatly compromise the advantages of the FRP strengthening technique in terms of ease of installation and minimal alterations of structural dimensions. ...
In the strengthening of reinforced concrete (RC) structures with fiber-reinforced polymer (FRP) composites, fire resistance design is an important issue. If fire insulation is not provided, the fire resistance of an FRP-strengthened RC beam can be approximated as that of an equivalent bare RC beam with a much higher load ratio as the mechanical resistance of the unprotected FRP strengthening system disappears within minutes of fire initiation. Due to this difference in load ratio, existing fire resistance design methods for conventional RC beams cannot be used in the fire resistance design of these equivalent bare RC beams. This paper aims to address this deficiency in existing research by presenting a reliable design method for the fire resistance of bare RC beams that provides close predictions for a wide range of load ratios, particularly those of high load ratios. Numerical results obtained using an advanced finite element approach developed by the authors are presented to examine the effects of various parameters on the fire resistance of bare RC beams, including the load ratio, the concrete cover depth, the reinforcement ratio of tension steel rebars, the distribution ratio of tension steel rebars, the cross-sectional dimensions, and the aggregate type of concrete. A design equation for predicting the fire resistance period of bare RC beams under standard fire exposure, formulated on the basis of the numerical results, is proposed. It is demonstrated that the proposed equation provides reasonably close fire resistance predictions for bare RC beams with a wide range of load ratios.
... Com a finalidade de analisar a eficiência do reforço nas vigas, denominadas de V1, V2 e V3, o autor considerou que ocorreu perdas de área da seção transversal da armadura tracionada da ordem de 10%, 20% e 30%. Portanto, o reforço à flexão das vigas de Paliga[6] foram projetados seguindo as recomendações da fib bulletin 14[9], em lâminas de PRFC colados na face tracionada das vigas, a fim de recuperar a capacidade de carga original da peça. ...
... Since the beginning of the present century, researchers have paid much attention to the torsional behavior of reinforced concrete (RC) beams strengthened with fiber-reinforced polymer (FRP) sheets by experimental investigation [1][2][3]; however, very few models exist to predict the ultimate strength of strengthened RC beams. The fib Bulletin 14 [4] proposed equations to calculate the ultimate torsional moment of strengthened beams involving two typical failure patterns, namely, FRP fracture and debonding. Ghobarah [2] proposed a model of the torsional moment contributed by externally bonded FRP and assumed that the mean ultimate strain of fiber was approximately 0.003, as indicated by experimental records. ...
The ultimate torque of reinforced concrete (RC) members strengthened with fiber reinforced polymer (FRP) sheets does not only depend on the torque of RC members, but also on the FRP contribution to the torque. For structural design, predicting the accurate torsional capacity of the strengthened beams is considerably important. Three existing models for calculating the ultimate torsional moment of RC beams and two existing models for computing the FRP contribution to the ultimate torque are described and combined. Based on an experimental database collected from existing literature, six combinations were discussed and evaluated from the calculative values compared with the experimental results. The comparison shows that the combination of ACI 318 and fib Bulletin 14 models (Group 2), as well as Chinese and Ghobarah models (Group 6), can reasonably and accurately predict the ultimate torque of beams strengthened with FRP sheet. Furthermore, the ultimate torque of six boxsection beams strengthened with fully wrapping or U-wrap calculated by the Group 6 shows closely to the experimental results.
In the last decades, the use of Fiber Reinforced Polymer (FRP) in the strengthening of reinforced concrete structures has been gaining popularity due to its outstanding properties, such as high strength, low specific weight and durability. However, its use is still limited by its behavior in fire, seen the severe degradation that the system suffers when exposed to elevated temperatures. One of the main consequences of the thermal action is the loss of bond between composite and concrete associated, mainly due to the degradation of the resin used in the for bonding the material. The present study evaluated the effect of elevated temperatures on the residual bond of concrete strengthened with Carbon Fiber Reinforced Polymer (CFRP) laminate installed according to the Near Surface Mounted (NSM) technique. Bond was evaluated by means of beam pullout test, based on the test method proposed by RILEM RC 05 (1982) document. Initially, six tests were conducted at room temperature, in order to determine the influence of the bond length. After, ten samples were heated at different target temperatures (50 °C to 250 °C). Lastly, bond after heating was evaluated, in order to determine the reduction properties in function of temperature. In relation to room temperature, higher bond strength was obtained with an anchorage length of 120mm - value assumed for the other experimental parts. About the thermal program, it was verified that glass transition temperature (Tg) was reached in the resin at the target temperature of 100 °C. Color changes in the resin were observed at the target temperatures of 200 °C and 250 °C. In relation to bond after heating, the bond strength according to the reached temperature. At 50 °C, before reaching the Tg, the system presented an increase of 7,3 % in the bond strength, mostly due to the post-curing promoted by cooling. After, the bond strength reduced significantly, reaching 45,1 % of initial value at 150 °C. This present work highlights the effects of thermal action in CFRP, since, even in lower temperatures, the system lost most of its bond properties
The use of fiber reinforced polymer (FRP) for torsional strengthening of reinforced concrete (RC) single cell box beams has been analyzed considerably by researchers worldwide. However, little attention has been paid to torsional strengthening of multicell box girders in terms of both experimental and numerical research. This paper reports the experimental work in an overall investigation for torsional strengthening of multicell box section RC girders with externally-bonded Carbon Fiber Reinforced Polymer CFRP strips. Numerical work was carried out using non-linear finite element modeling (FEM). Good agreement in terms of torque-twist behavior, steel and CFRP reinforcement responses, and crack patterns was achieved. The unique failure modes of all the specimens were modeled correctly as well.
The use of fiber reinforced polymer (FRP) for torsional strengthening of reinforced concrete (RC) single cell box beams has been analyzed considerably by researchers worldwide. However, little attention has been paid to torsional strengthening of multicell box girders in terms of both experimental and numerical research. This paper reports the experimental work in an overall investigation for torsional strengthening of multicell box section RC girders with externally-bonded Carbon Fiber Reinforced Polymer CFRP strips. Numerical work was carried out using non-linear finite element modeling (FEM). Good agreement in terms of torque-twist behavior, steel and CFRP reinforcement responses, and crack patterns was achieved. The unique failure modes of all the specimens were modeled correctly as well.
Fiber Reinforced Polymers (FRP) is a recent technique to strengthen timber structures and the studies available discussing the debonding between these materials are limited. Therefore, the bond assessment between FRP composites and timber substrates is a topic that needs clarification. The present work analyses the debonding process between Carbon (C) FRP laminates and timber with rupture modes consistent with Mode II interfacial fracture, i.e. with the sliding mode where the bond stresses act parallel to the plane of the bonding surface. Several single-lap shear tests were performed and the experiments showed a nonlinear local behaviour of the CFRP-to-timber interface. An interfacial bond-slip model and its calibration procedure were also presented. Furthermore, the calibrated nonlinear bond-slip model was implemented in a numerical approach where the FRP composite and the adhesive are simulated by linear and nonlinear springs and the substrate is assumed rigid. The following influences on the debonding process of the CFRP-to-timber interface were also analysed: (i) the bonding technique (Externally Bonded Reinforcement - EBR; and Near Surface Mounted - NSM); and (ii) the use of an additional device to mechanically anchor the CFRP laminate. Besides the determination of the effective bond length for each bonding technique, a new concept defining the length beyond which the force at the anchorage device does not decrease with the bonded length and a proposal to estimate its value for any bonded length was also presented and discussed. The experimental tests have shown that the NSM technique has a better performance compared to the EBR technique, independently of the installation of mechanical anchorage devices. In the case of the EBR technique, the strains in the CFRP laminate increased at its vicinities due to the clamping force applied to the anchors, which affected the final strength of the interface.
With reference to seismic upgrading of poorly detailed reinforced concrete frame members using FRP jackets, the effective design strain of the jacket material (and the commensurate increase of compression strain ductility) are explored for confining applications in compressed members. For strain demands exceeding the effective value in some cases rupture of the FRP has been reported, accompanied by reinforcing bar buckling and disintegration of the concrete core. To investigate the conditions that lead to this behavior, an analytical model was developed and subsequently correlated with the experimental database assembled from literature. A primary output of the model is the compressive load carried by longitudinal reinforcement before and after attainment of instability conditions. Through the model it is possible to estimate stress redistribution between reinforcement and the confined core (if the latter has strength reserves due to confinement, so as to support the steel through the plateau well into strain hardening).
The negative impact of elevated temperature on polymers is well known. For epoxy adhesives that are commonly used in strengthening systems of FRP EBR as dangerous temperature is considered to 50°C. At this temperature they can start process called glass transition meaning the rapid decrease of the modulus of elasticity. The paper gives an overview of selected studies on this subject. In the second part shows the results of temperature measurements of several samples subjected to direct exposure of the sun for summer conditions in the southern Poland. Finally, on the basis of the received results, recommendations for the protection of composites from the sun are formulated.
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