*4.3. The Crack Density*

In the early 1990s, Mobasher et al. [99] defined a new parameter to describe the morphology of the cracking patterns, which is commonly used today, namely the crack density. This parameter was defined as the ratio of the average length of the crack to the area of the surface under analysis, according to the equation:

$$L\_A = \frac{2\overline{L}}{a^2} \tag{3}$$

where:

*LA*—the crack density [mm<sup>−</sup>1], *L*—the average crack length [mm], *a*2—the area of the test section [mm2].

A slightly modified version of the crack density, i.e., the parameter expressed as number of cracks per meter [13,96,100], is also encountered in the research. In this version, the crack density is more convenient to use, because in order to get the result it is necessary to count the number of cracks crossing the test line, and relate the result to the situation when the test line would be 1 m long. In this case, with the crack density it can be very easily calculated the crack spacing, which is a size that symbolizes the average distances between the cracks.
