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SFRC - shear load bearing capacity and tunnel linings

Falkner, Horst / Henke, Volker

Recently, the use of structural elements made of steel fibre reinforced concrete is becoming more common. Steel fibres are used instead of ordinary steel reinforcement or in addition with reinforcing bars. Both, under service and ultimate loading conditions, the fibre reinforcement is subjected to carry tensile loads. The addition of steel fibres to plain concrete changes its mechanical properties. Depending on the type and amount of fibres an increase in ductility and a better cracking behaviour can be achieved. Especially by the use of steel wire fibres remarkable stresses can be transferred across cracks. The fibre itself can be seen as a kind of reinforcement. Verification concepts for SFRC structural members can be derived from this principle. Based on these considerations, research work carried out with respect to the shear design of SFRC and SFsRC beams and tunnel linings will be presented in this paper. The investigations concerning the shear load-carrying behaviour of steel fibre reinforced beams carried out within the scope of a Brite/Euram programme showed that the empirical design approach according to Rilem TC 162 TDF 727 results in a safe estimation of the shear load-carrying capacity. Based on further tests, the steel fibre action on the different components influencing the shear load-carrying capacity of rectangular beams became further investigated. It could be established that the main effect of the steel fibres can be seen in the tensile force component acting in the inclined crack. The steel fibre orientation plays an important role for the effectiveness degree of the steel fibres, as it could be shown that the post cracking tensile strength depends to a high degree on the concreting direction and loading direction, respectively. The shear design concept presented here would make it possible to replace the minimum re-bar shear reinforcement which has to be provided according to the present standards by steel fibres. It considers the specific failure mechanism observed for steel fibre reinforced beams and/or beams with a mixed reinforcement. As far as tunnel linings are concerned, the concept presented here provides the user with improved, technically as well as economically, design rules. The combined reinforcement concept leads to reduced crack width, compared to conventionally reinforced tunnel linings, as well as to a reduction of the wall thickness. This, in combination with nonlinear design and computation methods, leads to a for more robust and economical design.