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Available online at www.rilem.net Materials and Structures Vol (Month Year) pp-

Available online at www.rilem.net Materials and Structures Vol (Month Year) pp-pp ABSTRACT Using test results it is demonstrated how thin layers of concrete with textile reinforcement can be used for strengthening for reinforced concrete (RC) members. The enhancement of bending capacity is illustrated with flexural strengthened RC-slabs. It is also established that the shear capacity may be increased through strengthening of RC-beams, and that properties of serviceability are improved, in particular the reduction of deflections and crack widths. The detailed problem of force transfer from the textile reinforced strengthening layer to the existing concrete of RC-members is then explained and subsequently the relation between the transferable bond force and the associated bond length is shown. A simple model for dimensioning the flexural strengthening of RC-slabs is presented and necessary model extensions are additionally pointed out. 1359-5997 © 2005 RILEM. All rights reserved. RÉSUMÉ Il est montré avec les résultats d’essai, que les minces couches avec une armature textile peuvent être utilisée pour un renforcement des éléments de construction en béton armé. Par les plaques, il est prouvé l’agrandissement de la capacité à la flexion. Il est montré pareillement, que la capacité au cisaillement peut être agrandie avec un renforcement des poutres en béton armé. Il se montre, que la qualité d’état de service peut être bonifiée. Ce sont surtout les flèches réduites et les larges de fissure plus petits. Le détail de la transmission de la force du renforcement armé textile au vieux béton est expliqué, et le rapport entre la force d’adhérence transférable et la longueur d’ancrage adéquat est montré. Un simple modèle de dimensionnement de la section transversale pour les plaques renforcées avec béton armé textile est présenté, et c’est indiquer les élargissements nécessaires. 1. INTRODUCTION Textile reinforced concrete is a relatively new and sophisticated composite material. It generally comprises a cementitious matrix and alkali-resistant (AR) glass fibre reinforcement, although the reinforcement may also consist of any other fibre material, for example carbon fibres. In contrast to steel reinforcement, the single AR glass or carbon fibres in the textile can be positioned in almost any direction and subsequently nearly perfectly adopted to the orientation of the applied load. It is thus possible to create an extremely effective reinforcement. The textile reinforcement is built in as a multiaxial fabric. The fibres can be arranged on top of each other, in different directions, with a maximum of four layers. High strength fine grained concrete with a maximum aggregate size of 1 mm is used as a matrix. Due to the very small diameter of the reinforcement, it is possible to get new, very thin concrete elements as applications for the new material. The diameter of the fibres in the textile reinforcement is usually one or two dimensions lower than the necessary diameter of steel reinforcement. In addition no minimum thickness of concrete cover is now needed to prevent corrosion of the reinforcement, because the fibres cannot rust like steel. Both of these advantages allow the development of concrete elements with a thickness of only 10 to 20 mm. Lower thickness may not only be helpful for new concrete elements, but also makes the lightweight strengthening of already existing concrete structures possible. Strengthening with textile reinforced concrete noticeably increases both the ultimate load carrying behaviour, as well as the serviceability. This will be shown in the paper using experimental results from flexural strengthened slabs and shear strengthened beams. 1359-5997 © 2005 RILEM. All rights reserved. doi:10.1617/***** Ref. MS1783 A. Brückner1, R. Ortlepp1 and M. Curbach1 (1) Institute of Concrete Structures, Dresden University of Technology, Germany Received: Day Month Year; accepted: Day Month Year Textile reinforced concrete for strengthening in bending and shear Available online at www.rilem.net Materials and Structures Vol (Month Year) pp-pp 2. GENERAL REMARKS ABOUT STRENGTHENING During the day-to-day use of buildings, many issues can arise which may lead to the requirement of a strengthening of RC-members. Examples are structural damage or an increase of live loads. Enhancement of load carrying capacity can be achieved by different methods. A relatively new possibility is by strengthening with textile reinforced concrete. This strengthening process is suitable for nearly every acting force. It is possible to strengthen for bending, shear, torsion or axial forces. In every case of textile reinforcement, the following criteria must be kept in mind:  the load carrying capacity,  the criterion of the minimum reinforcement,  the necessary anchorage of the textile reinforced concrete layer. The orientation of the AR-glass fibres will correlate with the direction of the main load. For example, in the flexural strengthening of the test slabs a biaxial fabric was used. The main reinforcement of this fabric lies in the longitudinal direction of the slabs. With a fineness of 1100 tex (that means 1100 g/km of length) the cross section area of one of these fibres is approximately 0.39 mm². The percentage of reinforcement in the perpendicular direction is nearly 25 % of the longitudinal reinforcement. Before the strengthening layer can be applied, the old RC-member has to be prepared by sandblasting. Any fine grains on the surface have to be removed, so that a bond can be formed between the old concrete and the strengthening layer. Then the textile reinforced concrete strengthening is applied in layers; alternate layers of fine grained concrete, applied with a spatula, and the textile reinforcement are applied to the RC-members. 3. BONDING BEHAVIOUR OF THE STRENGTHENING LAYER Bond Problems and Failure Layers When an old RC-member is strengthened with a new layer, then the additional tension force in the strengthening layer has to be transferred back to the old concrete at the end of the RC-member. This may be problematic, when a subsequently applied flexural strengthening layer of slabs, which are installed in the structure, ends in front of the supports (Fig. 1, top). Also in the case of a shear strengthening of T-beams, the strengthening layer does not reach the compression zone (Fig. 1, right). The questions regarding force transfer mechanisms between the textile-reinforced strengthening layer and the old concrete are investigated using separate bond test specimens. The aim of these bond tests is to describe the relationship between shear loading and deformation, as well as the necessary bond length and the transferable bond forces. Bond models for adhesive bonded steel plates or CFRP-strips can be found e.g. [1,2]. Strengthening layers made from textile-reinforced concrete show, as opposed to steel plates and CFRP-strips, a pronounced non-linear behaviour of the material due to crack formation [1]. Furthermore, there is strong interlocking between the old concrete and the strengthening layer as a result of the sandblasted surface of the old concrete and the direct laminating. Thus, bond failure can only occur in two different layers: firstly in the old concrete and secondly in the textile layer. Fig. 1 – Anchoring Problem. Load Carrying Capacity of Bond In order to transfer the tensile force from the strengthening layer to the old concrete via the bond joint, a definite bond length is required, which depends on certain variables, for example the properties of the textile reinforcement. If the available bond length is shorter than the necessary bond length, only a part of the ultimate strength of the strengthening layer can be anchored to the old concrete of an RC-member. In this case, the specimen fails due to bond failure in the test. Fig. 2 shows the ultimate bond forces, related to a width of 1 m, and the associated bond length of strengthening layers. These are reinforced with the same textile as is used for the strengthening of slabs. If the bond stress that has to be transferred becomes higher than the load carrying capacity of the old concrete of the RC-member, the bond will fail in the old concrete. If the bond stress reaches the load carrying capacity of the fine grained concrete in the textile layer before it reaches the load carrying capacity of the old concrete, a bond failure will occur through a delamination of the textile layer. To use the strengthening to full capacity, the bond properties should be optimised in such a way that the weak point of the construction is not situated in the newly applied strengthening layer, but in the old construction. This can be achieved through the enlargement of the inner width of the textile fabric. The necessary bond length, which is able to anchor the ultimate tensile force of the strengthening layer and 1359-5997 © 2005 RILEM. All rights reserved. doi:10.1617/***** Ref. MS1783 Available online at www.rilem.net Materials and Structures Vol (Month Year) pp-pp corresponds to the maximum bond stress can be determined for each textile in bond tests, which are carried out at the University of Dresden. According to the test results, this means that if the bond length is longer than a definite bond length then the strengthening layer fails because it reaches its ultimate tensile load before bond failure can occur. The load carrying capacity of the strengthening layer can therefore be fully utilized, with the use of uploads/Ingenierie_Lourd/ brckneretal-trcforstrengtheninginbendingandshear-authorsfinalversion.pdf