The bond between fibres and cementitious matrix is one of the most important factors influencing the material properties of fibre composites. Several different techniques have been proposed in recent years to determine and measure the critical pull-out load P_(krit) using pull-out tests and so deriving the bond strength assuming a certain shear distribution over the embedded length of the fibre. One of the most important factors regarding the relation between the pull-out load and the bond strength τ_(max) is the perimeter of the fibre used in contact with the matrix. This fibre perimeter is easy to determine e.g. for steel fibres. However, it is not quite as easy to define this perimeter for a glass fibre multi-filament yarn which is in contact with the matrix. In this paper a method is presented how this contact area can be determined by means of the light transmission properties of the fibres and a computer aided image analysing system. The determined parameters of the area and the perimeter of the yarns in contact with the matrix are cross-checked using of electron microscopy.
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