Hybrid fibre concrete: is there a synergetic effect?

Abstract

Concrete can be characterized as a brittle material with a low tensile strength and strain capacity. A more ductile material can be achieved by the use of fibres in the concrete. Indeed fibre concrete shows a softening response after cracking. Fibres of various shapes and sizes and produced from different materials can be used. Hybrid fibre concrete is defined as concrete containing combinations of different types of fibres.

Synergy is the phenomenon where acting of two or more subjects together leads to a better result than the action of the same subjects independently of each other. That means that applied to hybrid fibre concrete one could suppose that the synergy of short and long fibres leads to an improved tensile response of the hybrid fibre concrete, compared to the arithmetic sum of tensile responses of two concretes, one of which contains only long and the other only short fibres in the same dosage as the hybrid fibre concrete.

This contribution presents the mechanical properties (compressive strength, tensile behaviour according to RILEM TC162-TDF) of a number of steel fibre concretes. Part of them are hybrid steel fibre concretes. The investigated parameters are: steel fibre type and fibre dosage. The program includes 15 different steel fibre concrete mixes. The obtained results of CMOD (crack mouth opening displacement) controlled bending tests show that the short fibres (L=13mm) are very effective in the region of the small crack widths while the longer fibres (L=35mm) with the hooked end provide a good ductility at large crack openings. The efficiency of the very short fibres (L=6 mm) in concrete with normally used aggregates is rather small. For most hybrid mixes, no synergetic effect has been found.