To assess the influence of the alkali-silica reaction (ASR) on pavement concrete 3D-CT imaging has been applied to concrete
samples. Prior to imaging these samples have been drilled out of a concrete beam pre-damaged by fatigue loading. The resulting
high resolution 3D-CT images consist of several gigabytes of voxels. Current desktop computers can visualize such big datasets
without problems but a visual inspection or manual segmentation of features such as cracks by experts can only be carried out on
a few slices. A quantitative analysis of cracks requires a segmentation of the whole specimen which could only be done by an
automatic feature detection.
This arises the question of the reliability of an automatic crack detection algorithm, its certainty and limitations. Does the algorithm
find all cracks? Does it find too many cracks? Can parameters of that algorithm, once identified as good, be applied
to other samples as well? Can ensemble computing with many crack parameters overcome the difficulties with parameter finding?
By means of a crack detection algorithm based on shape recognition (template matching) these questions will be discussed. Since
the author has no access to reliable ground truth data of cracks the assessment of the certainty of the automatic crack is restricted
to visual inspection by experts. Therefore, an artificial dataset based on a combination of manually segmented cracks processed
together with simple image processing algorithms is used to quantify the accuracy of the crack detection algorithm.
Part of the evaluation of cracks in concrete samples is the knowledge of the surrounding material. The surrounding material can
be used to assess the detected cracks, e.g. micro-cracks within the aggregate-matrix interface may be starting points for cracks
on a macro scale. Furthermore, the knowledge of the surrounding material can help to find better parameter sets for the crack
detection itself because crack characteristics may vary depending on their surrounding material. Therefore, in addition to a crack
detection a complete segmentation of the sample into the components of concrete, such as aggregates, cement matrix and pores is
needed. Since such a segmentation task cannot be done manually due to the amount of data, an approach utilizing convolutional
neuronal networks stemming from a medical application has been applied. The learning phase requires a ground truth i.e. a
segmentation of the components. This has to be created manually in a time-consuming task. However, this segmentation can be
used for a quantitative evaluation of the automatic segmentation afterwards.
Even though that work has been performed as a short term subtask of a bigger project funded by the German Research Foundation
(DFG) this paper discusses problems which may arise in similar projects, too.