Extrusion of engineered cement-based composite material: Fid-Bau Portal

Extrusion of engineered cement-based composite material

Koker, Don de / Zijl, G.P.A.G.

The primary function of fibers in Engineered Cement-based Composites (ECC) is to bridge, arrest and divert micro-cracks developing in concrete and increase its tensile strength In the post-peak region of the tensile stress-strain behavior, the number of fibers per unit area of the cracked section plays a governing role. This number of fibers bridging a crack and the primary failure mode of either these fibers, or the matrix-fiber interfacial, bond governs the ductile performance of the ECC. At the onset of a comprehensive research program to characterize the influence on ECC mechanical behavior when subjected to extrusion, this paper has reported the potential of beneficial increased alignment of fibers, accompanied by matrix-fiber interface strengthening through densification by the high pressure formation process. Thereby, fiber mode of failure may be altered to cause rupture, in turn causing strong, but brittle composite tensile response. The inability of such brittle tensile response to exploit the post first cracking plateau, or even strain hardening for stress redistribution to its full potential in loading cases causing stress gradients, was illustrated by simplified analysis. Subsequently, a mixture of experience gained through preliminary experimentation on crude extrusion facilities of both piston and auger action types, as well as research results from the literature was presented towards identifying the major mechanisms at play in extrusion processing of ECC. Fiber breakage generates higher composite peak strength, whereas fiber pull-out leads to increased ductility. The type of manufacturing process of ECC has a significant influence on the fiber orientation. Orientation of fibers is mainly in the load bearing direction of the process driving the mechanism. While piston extrusion aligns the fibers in the direction of extrusion, auger extrusion aligns the fibers diagonally. This leads to enhanced mechanical properties in the direction ofmain fiber orientation. Interfacial bonding between fibers and the matrix is coupled with the primary mode of failure. Depending on the interfacial bond, an optimal fiber length must be chosen. Since extrusion produces high compaction of ECC, reducing the porosity, the optimal fiber length for extruded composites is shorter than that for cast composites, in terms of improved mechanical performance. Extrusion is sensitive to different fiber lengths for the same fiber volume ratio. The optimal fiber length not only relates to the die opening geometry, but is also bound by the preferred mode of failure. Extrusion provides an increase in shear resistance between the fibers and the matrix. If no fly ash is used in the mix, the shearing resistance of the fiber-matrix interface is increased to such an extent that fiber breakage may become the dominating failure mechanism. If fly ash is introduced in the extrusion mix, the lower matrix strength counteracts this mechanism, hence allowing fiber pull-out as the primary mode of failure, enabling favorable composite strain-hardening behavior.