PSI - Issue 13

René Čechmánek et al. / Procedia Structural Integrity 13 (2018) 1780 – 1785 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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• Increases tensile strength and bending strength of concrete • Reduces deformation of concrete due to shrinkage, increases the modulus of elasticity • Reduces brittleness and increases toughness of concrete • Increases temperature resistance and reduces water permeability

Within testing of properties of such designed composites there is usually a realized fact, that variability of these properties is distinctive mainly for the reason of uneven dispersion of commonly used fibres in a whole matrix volume. Elimination of this phenomenon could be achieved by means of a design of the optimal homogenization process of dry mixture components with fibre reinforcement. Properties of fibre reinforced concrete (FRC) are mostly influenced by the fibre type and the amount of fibres. The bond of fibres mainly depends on material, shape and surface of fibres and mechanical properties of a matrix, the amount of fibres as well as the loading rate, as described by Kovacs and Balazs (2003). Consequently, almost all properties of FRC changes with the changing of the surface and the shape of fibres. Only little information exists on the influence of the mixing procedure on the properties of fibres. The research conducted in Budapest University of Technology and Economics was directed to the possible influences of mixing time on the tensile properties of fibres and the flexural properties of FRC. The research team realized that some of the fibres may suffer from too long mixing time of concrete mixture, as observed by Czoboly and Balazs (2015). Several research works have been aimed to monitoring of fibre dispersion and orientation in cementitious composites, e.g. a magnetic method could be employed for this purpose, but only for composites reinforced with steel fibres, as conducted by Ferrara et al. (2012). Other techniques could be used for cracking assessment of concrete structures by means of electric resistivity measurement, as tested by Lataste et al. (2003), including the research conducted by the authors´ team from the Research Institute for Building Materials using electrically conductive elements with modified cement matrix, by Čechmánek et al. (2016) and Junek et al. (2014). The Research Institute for Building Materials has designed new experimental devices for the Discovery Hybrid Rheometer (DHR-1) to measure rheological properties of fresh cement-based mixtures. They are three-blade and three ball agitators with a complementary role for absolute measurements using a system of coaxial cylinders. For slurries with low viscosity blade agitators are used and for higher viscosity suspensions ball agitators or a board-to-board system with roughened surface may be used, as described by Kuder et al. (2007). Results of the measurements of cement mixtures with non-metallic fibres are depicted in Fig. 1. 2. Rheological properties of mixtures

Fig. 1. Plastic viscosity of fibre-cement fresh mixtures.

The size of the ball depends on the viscosity of the material. A suitable approach how to obtain rheological parameters is to use a ball agitator and take measurements of one revolution only. As to cement mortar, the structure is broken and does not retrieve its original state even after material relaxation. Watery mortars close up, however, water is separated into a groove formed by the passing ball and thick mortars create a permanent groove. It means that repeated passage of the ball shows misleading values.