Flexural Strengthening Effect of Reinforced Concrete Slabs Bonded with Carbon FRP Grid
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This paper presents the strengthening effect and the behavior of reinforced concrete slab strengthened with Carbon Fiber Reinforced Polymer (CFRP) grid. Parameters involved in this experimental study are FRP grid reinforcement quantity, repair mortar thickness, presence of anchor, and the location of strengthening. Generally, flexural members strengthened with reinforcement material (sheet, plate) made of Fiber Reinforced Polymer (FRP) have the failures, such as delamination failure debonding failure before reaching the maximum stress of FRP reinforcement. However, in this study there are various failure types with increasing the strengthening reinforcement. On the moderate strengthening level, there is a failure in the interface of carbon FRP grid because of the excessive shear deformation. On the other hand, there is a failure in shear on the high strengthening level. With the increasing of FRP grid reinforcement, the strengthening effect in flexure increased, but the ductility decreased. By limiting the strengthening level, it can be achieved to prevent shear failure, to increase the efficiency in strengthening, and to improve the ductility.Keywords:
Ductility (Earth science)
Delamination
Slab
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With passage of time, strengthening techniques have become more and more refined. Carbon fiber reinforced polymer (CFRP) and steel plates have been adopted since years as few of the popular materials for strengthening of structural elements such as beams and columns. A series of studies have been carried out in the past for shear strengthening of reinforced concrete beams using various mechanisms of strengthening, and response of such strengthened structural elements is found to be quite satisfactory as compared to non-strengthened structural elements. De-laminationDe-lamination /debonding is a major issue faced while strengthening any structural member using fiber reinforced polymer (FRP). Debonding occurs at regions of high stress concentration, which are often associated with material discontinuities and with presence of cracks. However, this can be avoided, if strengthening is done after proper understanding and analysis of the problem. Adaptation of proper guideline to overcome debonding plays a vital role in the resolution of debonding problem, however these guidelines are also limited. The increasing use of FRP in structural strengthening although has revoked a need for framing of guidelines in this segment, but eventually fails to address the debonding aspect to a larger extent. Also, the debonding load after strengthening remains unknown. The magnitude of this load, if known, can contribute to a much greater extent as long as FRP strengthening is concerned. This paper aims at highlighting the method for strengthening of reinforced concrete beam in flexure and shear using CFRP strip mechanism and thereby overcoming CFRP debonding problem in order to achieve enhanced performance in flexure and shear along with prevention of strengthened member failure against debonding.
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Remodeling buildings often implies a modification of the mechanical behavior of the structural system, which may include the application of concentrated loads to beams that before have only been subjected to distributed loads. In the case of reinforced concrete beams, the new condition causes the beam to support a concentrated load in the cracked condition determined by the distributed loads that had been acting in the past. If the concentrated load is applied at or near the midspan of the beam, consequently, the shear demand reaches its maximum where the shear capacity is low. At and near the midspan, in fact, the cracks in the tension zone are vertical, as the stirrups are. Therefore, the truss mechanism provides the shear capacity with a nil contribution, since both the struts and ties are vertical. Fiber-reinforced polymer composites allow these beams to increase their shear strength, up to guaranteeing adequate safety. This paper presents a method for analyzing the mechanical behavior of beams with vertical cracks, strengthened with FRP reinforcement, subjected to concentrated loads. The method defines the direction of the fibers that the reinforcement has to be composed of and computes the shear strength of the beam.
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Many studies performed on reinforced concrete (RC) members strengthened in flexure with externally bonded (EB) fiber-reinforced polymers (FRPs) have indicated quite low strengthening efficiency caused by debonding of the FRP from the concrete surface prior to the capacity of the FRP material being achieved. It should be emphasized that although flexural strengthening with FPR increases the load-bearing capacity of RC members, it has little effect on the serviceability limit state (i.e., cracking moment and deflections). Prestressing the EB FRP has been proposed as a method of increasing utilization of the FRP tensile strength and of improving the efficiency of strengthening in terms of serviceability limit states. An experimental research program consisting of three series of RC slabs with variations in the longitudinal steel reinforcement ratio, concrete strength, preloading level before strengthening, and adhesion between the CFRP laminates and the concrete is described. A practical and unique aspect of the program focuses on an analysis of the effect of preloading on the strengthening efficiency of RC slabs strengthened with prestressed carbon fiber-reinforced polymer (CFRP) laminates. Although the preloading is one of the most important parameters to be accounted for in the design of strengthening existing RC structures, this aspect has been investigated only rarely. Two levels of slabs preloading were considered: the slab self-weight acting alone and the self-weight plus an additional external load. The self-weight preloading level corresponded to 25 and 14% of the yield strength of nonstrengthened slabs in Series I and III, respectively. The higher preloading level, equal to 76% of the yield strength of the nonstrengthened slab, was chosen to approach the elastic limit of the slab behavior. Experimental tests yielded promising results for the ultimate and serviceability limit states of the strengthened slabs. The strengthening ratio, defined as the ratio of the difference between the ultimate load of the strengthened and nonstrengthened slabs to the ultimate load of the nonstrengthened slab, reached values in the range of 0.64–1.19. The influence of the tensile steel reinforcement ratio, adhesion between the prestressed CFRP laminate and concrete, and preloading level on the ultimate load carrying capacity following strengthening is discussed.
Serviceability (structure)
Limit state design
Slab
Ultimate load
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Slab
Ductility (Earth science)
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The technique of FRP(fibre reinforced polymer) fast strengthening steel members is presented in concept and operation.The effectiveness of the technique is proven through the preliminary experiments of three specimens.Then,another 14 specimens have been tested under axial compressive loading,through which the compressive behaviour and strengthening effects are investigated with different configuration parameters,including two slenderness ratios,two confinement configurations(pultruded FRP profile and hand lay-up fibber sheets),two filled materials(mortar and bamboo splits) and the end connections(confined end-plate connection and single sideband pinned end conditions).The strengthening effects are analyzed by the comparison of both theoretical and test results,which shows that the overall buckling failure of steel members can be prevented by FRP strengthening,and the ultimate loading capacity and deformation capacity of steel members are enhanced obviously.The highest loading capacity of a strengthening member is 2.86 times of a non-strengthened one.Ductile failure tends to happen for the strengthened member.All these concluded that the technique of FRP fast strengthening steel members is feasible and effective.
Pultrusion
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Bonding carbon fibre-reinforced polymers (FRP) to reinforced concrete (RC) flexural members has become a popular means to enhance load carrying capacity and prolong service life. Considering the type of FRP strengthening system, the near-surface mounting (NSM) technique of embedding FRP strips in saw-cut grooves within the concrete cover of RC members to flexurally strengthen beams and slabs is known to have desirable bond behaviour benefits over external bonded FRP plates. Further the NSM technique has attracted growing attention in strengthening of statically indeterminate RC members. However, little is known about the durability of the critical bond between the embedded FRP and the concrete when installed using this technique. In this paper, experimental results of continuous flexural members strengthened with NSM FRP is collated and reviewed to investigate failure modes, ductility and moment redistribution capability. Ductility and moment redistribution characteristics of strengthened RC members relies on the bond performance of reinforcement to concrete and hence this paper also reviews durability testing of NSM joints. Regression analysis shows that concrete compressive strength and steel reinforcement ratios are paramount to maintaining ductility under all strengthening schemes.
Ductility (Earth science)
Concrete cover
Service life
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Many factors can affect the shear capacity of fiber-reinforced polymer in reinforced concrete beams shear-strengthened with externally bonded fiber-reinforced polymer composites. Undoubtedly, the interaction of concrete-stirrup-fiber-reinforced polymer system is one of the key factors. However, most of the existing fiber-reinforced polymer design guidelines do not take account of this important factor on predicting fiber-reinforced polymer shear capacity. This study provides an advanced strengthening model that comprehensively considers the interaction among concrete, stirrup, and fiber-reinforced polymer for calculating the fiber-reinforced polymer effective strain. The advanced strengthening model provides a more accurate prediction for the fiber-reinforced polymer shear contribution compared with existing design guidelines.
Stirrup
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This paper presents the failure mode and strengthening design of reinforced concrete slab strengthened with Carbon Fiber Reinforced Polymer(CFRP) grid. Parameters involved in this experimental study are FRP grid reinforcement quantity, repair mortar thickness, the presence of anchor, and strengthening in compression. In this study, there are different failure types with increasing the CFRP grid strengthening reinforcement. On the low strengthening level, CFRP grid in repair mortar cover ruptures. On the moderate strengthening level, there is a debonding shear failure in the interface of carbon FRP grid because of the excessive shear deformation. On the high strengthening level, diagonal shear failure occurs. With the increasing of FRP grid reinforcement, the strengthening effect increased, but the ductility decreased. By limiting the strengthening level, it can be achieved to prevent shear failure which result in sudden loss in the resisting load capacity. CFRP rupture failure is desirable, because CFRP ruptured concrete slab keeps the same load capacity and ductility haying before strengthening even after failure. Finally, design guideline and procedure are given for strengthening of concrete slab with CFRP grid.
Slab
Ductility (Earth science)
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