Issue 31

A. Abrishambaf et alii, Frattura ed Integrità Strutturale, 31 (2015) 38-53; DOI: 10.3221/IGF-ESIS.31.04 43 i) number of fibres per unit area, f N : is the ratio between the total number of fibres counted in the image, f T N , and the total area of the image, A : f f T N N A  (1) ii) fibre orientation factor,   : determined as the average orientation towards a certain plane surface by Eq. 2: 1 1 . cos f T N i f i T N       (2) where, f T N is the total number of fibres that can be determined by counting all the visible ellipses and circles at the cross section, θ is the out-plane angle that is defined as the angle between the fibre’s longitudinal axis and a vector orthogonal to the plane. iii) fibre segregation parameter, seg  : to calculate the location of the steel fibres gravity centre, an average value of the coordinates in the Y axis of entire fibres should be determined in the analysed cross-section. 1 1 . . f T N seg f i T y h N     (3) In this equation, y is the coordinate in the Y axis of the fibre’s gravity centre, and h is the height (or depth) of the analysed cross-section, iv) number of effective fibres per unit area, f eff N : the summation of fibres with deformed hooked in a unit area. Since this parameter cannot obtain from the image analysis results, therefore it was executed by visual inspection of the fracture surface. (a) (b) (c) (d) Figure 7 : Image processing steps: (a) converting a colored image to greyscale image (b) adjusting a threshold, (c) defining mask, noise (remove small noises) and watershed (separated fibres that are stuck together) functions, (d) fitting the best ellipse to each fibre. Tab. 2 includes the results of the image analysis performed on a plane surface of the uniaxial tensile specimens, see Fig. 8. The fibre distribution parameters were assessed on two orthogonal planes for each core location regarding to panel centre point (Fig. 1). The θ = 0 ° series presented a considerably higher average f N and f eff N , about 80% and 254%, respectively, when compared to the θ =9 0 ° series. The differences between these series corroborate that the fibres were preferentially reoriented due to the concrete flow. This could be clearly noticed if the orientation parameter,   , for each series was