Locking Front Model for pull-out behaviour of PVA microfibre embedded in cementitious matrix

High-performance polymeric microfibres are widely used along with cementitious matrices to produce Strain-Hardening, Cement-based Composites (SHCCs); a specific class of Fibre-Reinforced Cementitious Composites (FRCCs), which show high ductility in the post-cracking stage. As in all composites, the performance of SHCC is determined by the properties of its constituents: fibre, matrix and the interface between them. Above all, the interface plays an important role in the crack-bridging behaviour of fibres. Interface properties are commonly investigated by means of single fibre pull-out tests. The article at hand proposes a physical explanation of the pull-out behaviour of PVA microfibre in the post-debonding stage. It is shown that a particular type of damage with characteristic geometry creates a “locking front” somewhere on the fibre's embedded length, which leads to a rise in force in the post-debonding regime. The evidence supporting the given explanation is provided by microscopic analysis of the embedded parts of the fibres. First, single-sided fibre pull-out tests are performed followed by the spalling of the matrix by a special technique and by micro-mechanical, numerical simulations of the pull-out experiment. The Locking Front Model suggests that the interaction between fibre and matrix is not uniform over the embedded length, but it is rather local. Thus, the locking front of fibre-matrix interaction could be much shorter than the embedded length, indicating that the pull-out behaviour might be not proportionally dependent on embedded length for this type of microfibre.