Cementitious composites with high tensile strength and ductility through hybrid fibres: Fid-Bau Portal

Cementitious composites with high tensile strength and ductility through hybrid fibres

Mier, Jan G.M. van

Increasing the tensile strength and ductility is the aim of hybrid fibre concretes (HFC), defined as a concrete containing different types of fibres (geometry), or fibres made of different materials, or combinations thereof. In principle any combination is possible. Actively interfering with the microcrack process and with macrocrack growth is required to achieve these goals. The addition of fibres resembles the use of external active confinement. The fibre cocktails must be selected such that the microcrack length is minimized as long as possible, and the bridging of the cracks must be such that no sudden large deformations develop during (macro) crack growth. The fibres must effectively withstand loss of load carrying capacity from the concrete itself. Finally the mixtures must be self-compacting to ensure that proper fibre distributions are obtained. Nevertheless since structural shape and flow properties of the mixtures determine the ultimate fibre distribution, standard test methods must be designed so that the material is tested as it appears in the structure. HFC seems particularly suitable for the pre-cast concrete industry, where the higher demands for quality control can be met. Examples of applications are long-span lightweight girders, sheet-piling, and so on. Reducing conventional (and expensive) steel reinforcement can be achieved. Moreover, since HFC displays a clear hardening behaviour, the widths of cracks are very small, which positively affects durability. The high cement content is a favourable factor in this respect as well, since self-healing of microcracks can help to ensure durability for extended periods. In order not to obstruct the fibre distribution, the material should contain small aggregates only (dmax < 1 mm), which implies that the cement content has to increase as well. The mixtures studied to date contained up to 6 % (vol) of steel or PVA fibres, and the best mechanical properties were obtained with the high modulus steel fibres. The amount of large fibres (l > 30 mm) is limited by the demand for self-compactibility. Shorter fibres have a smaller influence on flow properties, and balling does not occur. In bending a considerable effect of the fibres in the boundary layers of the specimens was observed, which actually indicates that the material should be tested in the same situation as it is applied in a structure. This again complicates the development of a standard test method. Recently it was found that the permeability of high performance fibre concrete was much decreased, likely as a result of the fact that these materials contain large amounts of un-hydrated cement. Continued hydration, and even self-healing of cracks is possible over extended periods of time. The great disadvantage will be considered the costs of a mixture with 4-5 % (vol) steel fibres, and with three times as much cement as normal. Considering that many properties are improved, that durability problems are probably smaller, and that smaller amounts of the material are needed than in applications with conventional concrete, the sum of factors might actually be in favour of HFC. Advantages and disadvantages are summarized for structures made with conventional concrete, or with hybrid fibre concrete.