AbstractCracks in concrete are measured in 3D by microfocus X‐ray computer tomography. The tomographic images are thresholded matching a characteristic of the measured crack attenuation profile. A methodology is proposed to convert the 3D measured voxel data into a network of parallel plates. The methodology is based on the determination of the aperture map and skeleton of the void space, and the segmentation of the void space in crack segments. The segmentation and network approach allows to study crack aperture and connectivity distributions of the crack. Static invasion percolation and moving front technique are used to analyse liquid flow in cracks. One‐dimensional simulations of transport in a crack with variable crack width exemplify the retardation effect of narrow passages. In 2D, the narrow passages can be by‐passed resulting in preferential flow patterns, where coarse crack zones remain unfilled. Mesh sensitivity in the network approach is studied showing a limited influence of the mesh size on the filling patterns, caused by a change of connectivity when refining the mesh. Comparison of 3D and 2D simulations indicates that flow in 2D crack sections can strongly underestimate possible fluid penetration depths. Finally the network model is validated analysing water uptake in a fractured brick sample. Copyright © 2004 John Wiley & Sons, Ltd.