*4.1. Opening Width of the Cracks*

The crack opening width is one of the simplest parameters to measure, as measurements can be taken locally using portable optical microscopes. However, such a measurement is burdened by the error of the human eye. Greater measurement accuracy is achieved by using a digital measurement, on a previously prepared image of the tested surface. This parameter is also of great practical value, since practically every aspect of durability of cement composites depends on the opening width of cracks. Due to the simplicity of the measurement, it is very often used to measure the width of cracks opening as a supplementary test to the main research thread concerning the cracking patterns. It is also valuable to give the maximum crack width, because in the case of a cracked cement composite, its mechanical strength is largely dependent on this parameter.

Wagner et al. [96,97] proposed a classification of the measured crack width distributions as the crack width polygons. Cracks of particular widths are accumulated in percentage. The graph of the crack width polygons allows very quickly to determine the percentage of cracks with a width, which is below a certain limit. This parameter allows to characterize the whole of the cracking pattern, because it is physically very similar to the construction of the particle size distribution curves—in case of analyzing aggregates for cement composites.
