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European Network for Durable Reinforcement and Rehabilitation Solutions

Contract Number: MC-ITN-2013-607851

Shear Strengthening of RC Beams

Jaime Gonzalez-Libreros

Università degli Studi di Padova

 

Overview

The need for strengthening existing reinforced concrete (RC) structures may arise from their deterioration with age, change in applied loads due to modification of their original use, or upgrading to current design codes. The intervention of these structures requires the use of satisfactory rehabilitation and/or strengthening techniques that result in adequate behavior of the structure after the retrofitting process is carried out. Traditional techniques such as the increase of concrete section using concrete jackets or the use of externally bonded steel elements, which are common especially in developing countries, can often be considered as structurally acceptable but may not comply with modern requirements in which time- and cost-efficient interventions are usually required. In addition, they have some disadvantages such as increase in self-weight of the structure, undesirable change in stiffness, and the need for handling of heavy steel parts.

For this reason, there is growing interest to develop strengthening techniques that overcome the aforementioned drawbacks. Among these techniques, externally bonded Fiber Reinforced Polymer (FRP) composites have proven to be an effective solution. FRP composites are comprised of a reinforcing fiber and an organic matrix, usually epoxy based, that are bonded to the concrete surface to provide additional strength for flexural, shear, torsional, and axial loads. FRP composites have benefits including low invasiveness, high strength-to-weight ratio, and ease of application. However, some drawbacks of the use of FRP composites have been reported, such as poor behavior at temperatures, poor compatibility with the substrate, inability to apply onto wet surfaces or at low temperatures, and difficulty in carrying out post-earthquake assessment of damaged structures, which are linked mainly to the use of organic resins used as matrix. This suggests that the use of FRP might not be suitable for all applications, and new techniques that overcome some of these limitations are needed.

For this reason, composites in which an inorganic matrix (mortar) replaces the organic resin matrix have recently raised interest as they allow to overcome some of the limitations associated to the use of FRP composites. The term fiber reinforced cementitious matrix (FRCM) has been used to designate these types of composite, although some other names can be found in the available literature such as textile reinforced concrete (TRC), textile reinforced mortar (TRM), and mineral based composites (MBC). Although research conducted on the topic is still scarce, the effectiveness of this technique for flexural, shear, and torsional strengthening, and for confinement of axially or eccentrically loaded RC elements is confirmed by the available experimental evidence.

The case of shear is of interest due to the undesirable brittle failure mode associated with shear failure of RC members. In addition, the shear behavior of RC beams is quite complex due to the interaction of different mechanisms. In general, the factors that contribute to the strength of a RC beam without stirrups, are the area of uncracked concrete in compression, aggregate interlock, dowel action, and arch action. For the case of RC beams with stirrups, the contribution of the reinforcement must be considered. For RC beams strengthened in shear with externally bonded composites, either with FRP or FRCM, the effect of the composite on the strength of the beam and its interaction with the aforementioned mechanisms should be taken into account.

Taking into account the points described above, this research project is developed with the aim of studying the behavior of RC beams strengthened in shear with FRCM composites, comparing their performance with elements strengthened with FRP composites. As first step, a summary of the state of research on the topic of shear strengthening of RC beams using externally bonded FRCM composites is performed, including a detailed bibliographical review of the literature on the topic. As next step, the behavior of the strengthened elements is evaluated by means of experimental tests carried out in the laboratory. Variables investigated included the type of composite (FRP or FRCM), type of fiber, stirrup spacing, and the use of anchors.

 

Failure mode of tested the beams

 

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