*4.2. Length and Orientation of the Cracks*

The length of the cracks or the entire cracks system, as well as the orientation of the cracks, are one of the basic parameters for determining the properties of a cracks. It is relatively easy to measure when implementing computer image analysis into a research program. The combination of these two properties, which characterize the cracking pattern, can provide a lot of information, e.g., regarding the direction of shrinkage or swelling of the cement composite, the direction of the aggressive agent action, etc. The orientation of cracks on a degraded composite is very important for the prediction of its mechanical properties, as well as for predicting the direction of further development of the cracks network.

In his research, Szel ˛ag [12–16,98] introduced a modification of the crack length measurement, by measuring the length of the perimeter line of the area which is limited by the cracks. This approach applies to the analysis of cracks that form closed areas called as the clusters, on the surface of the material. The clusters are formed only when there is a strong volumetric cracks propagation, e.g., in case of a thermal load.
