•Specimens drilled from slabs are tested to qualitatively assess fibre orientations.•Plastic fibres tend to be oriented parallel to the formwork walls.•Plastic fibres tend to be oriented perpendicular to the flow direction for a free surface.•CT-scan can be used to identify the dispersion and orientation of plastic fibres. This paper focuses on the study of the influence of flowability and wall-effects of the formwork in the orientation pattern of macro-plastic fibres. In order to identify the preferential orientation of fibres caused by the geometry of slabs, pairs of specimens drilled from PFRC slabs with different width/length – ratio are tested using the multidirectional double punch test (MDPT). The results show that plastic fibres tend to be oriented parallel to the walls or surfaces of the formwork and perpendicular to the flow direction for a free surface flow. The side walls slightly redistribute the fibre orientation, as the transverse dimension of the slabs is reduced. Additionally, a computed tomography (CT-scans) was, for the first time, successfully applied to assess the amount of macro-plastic fibres as well as its distribution and orientation in a prismatic core.