BEFIB2012 – Fibre reinforced concrete Joaquim Barros et al. (Eds) © UM, Guimarães, 2012 DESIGN OF UHPFRC WITH ENHANCED DUCTILITY AND FLOWABILITY E. Ghafari * , H. Costa ** , E. Júlio *** * ICIST & Dep. Civil Eng., Faculty of Sciences and Technology, University of Coimbra Rua Luis Reis Santos (Polo II), 3030-788 Coimbra, Portugal e-mail: ghafari@dec.uc.pt ** ICIST & Polytechnic Institute of Coimbra – ISEC, Coimbra, Portugal Rua Pedro Nunes, Quinta da Nora 3030-100 Coimbra, Portugal e-mail: hcosta@isec.pt *** ICIST& DECivil IST, Technical University of Lisbon, Portugal ejulio@civil.ist.utl.pt Keywords: UHPFRC, steel fibers, aspect ratio, flowability. Summary: The experimental study herein described was conducted aiming to design UHPFRC with enhanced ductility and flowability. The influence of different type and dosage of steel fibres on the behaviour of UHPFRC was analysed. Steel fibers with different aspect ratio (l/d) were added, individually and blended, to the mixtures. The flexural strengths of all UHPFRC mixtures were measured. The concrete flowability was assessed using both the slump flow test and the V-funnel test. Conclusions are drawn in relation to the optimal addition of steel fibres, i.e., the one leading to the highest flexural strength and simultaneously ensuring self-compacting properties of the mixtures. 1 INTRODUCTION Ultra high performance concrete (UHPC) presents both ultra-high compressive strength and durability. The major drawback is the reduction in ductility, since this parameter decreases with the increase of the compressive strength. The addition of steel micro fibers to UHPC mixtures improves its ductility, as well as other mechanical properties, due to the crack bridging effect of the fibers. In fact, interlocking effect between the fibers and the binding matrix is responsible for transferring stresses from the matrix to the fibers. Thus, after the matrix cracking, all stresses are transferred to the fibers [1]. For this reason, quality enhancement of the matrix-fiber interface is a key issue. In addition, other factors affect the mechanical properties of steel fibers reinforced concrete as well, such as: the fiber type; the volume fraction; and the aspect ratio (length to diameter ratio). Among all the variables mentioned above, the latter has a significant role to improve the fiber-matrix bond, since a higher aspect ratio results in higher pull-out strength of the fiber [1], which finally enhances the mechanical properties of hardened concrete. Nevertheless, higher aspect ratio raised the problem of insufficient workability in the fresh state. On the other hand, the orientation and the uniform distribution of steel fibers in the mixtures, the two most influential parameters on the mechanical properties, are mainly governed by fresh state properties. In fact, orientation and alignment of the fibers is highly dependent on the flowability [2], whereas uniformly dispersed steel fibers can be achieved through the high degree of the mixture’s stability, in which the risk of segregation drops significantly. P.Stähli et al. [3] showed that the alignment of the fibers increases with the increase of the flowability of the mixtures regardless of fiber’s aspect ratio. Self-compacting concrete with high degree of flowability and stability during the placement would be the ideal situation to govern both fiber orientation and dispersion. Markovic [4] expressed that the self-compacting hybrid fiber concrete achieved a much better performance in the mechanical properties as well as pull-out strength, when compared to those of ordinary fiber reinforced concrete.