*5.1. Performance at Elevated Temperatures*

Mobasher et al. [99] examined the cracking process in cement composites, with different polypropylene fibers content, using an acoustic emission technique and quantitative image analysis. The morphology of the crack system was investigated for samples loaded to specific strain magnitudes. For this purpose, the crack density, length and spacing were determined. An average crack density of 1.27 mm−<sup>1</sup> was obtained. It was observed that as the strain level increases, the formation of new cracks is accompanied by decrease in the crack spacing. Thus, the presence of polypropylene fibers in the cement composite resulted, first of all, in the necessity to supply more energy to the system so that the existing cracks could further propagate; secondly, an increase in the degree of cracks and microcracks dispersion in the material, which undoubtedly improved the mechanical properties of the composite.

Xu et al. [9] analyzed the morphology of the cracking patterns of concretes subjected to a high temperature. The subject of the study were concretes differing in the *w*/*b* ratio (water/binder) and fly ash content. A visual imaging technique was used to study, among other things, the crack density. It was noted that the crack density increased almost linearly up to 650 ◦C. There was a rapid increase in this magnitude between 650–800 ◦C. The degree of cracks dispersion increased as the fly ash content increased. Smaller *w*/*b* values were accompanied by a smaller number of cracks, however, the resulting cracks were characterized by a larger opening width and length, compared to concretes with a larger *w*/*b* ratio. It was found that a decrease in tensile strength with an increase in thermal load is identical with an increase in the crack density.

The studies carried out by Kim et al. [120] concerned the influence of thermal load (in the range 25–1000 ◦C) on the properties of cement paste. Among others, the 3D X-ray Computer Tomography was used to determine the change of pore structure and to evaluate the process of initiation and spatial configuration of thermal cracks, as a function of the temperature acting. It was observed that at temperatures higher than 900 ◦C a massive cracking network is formed in the edge zones of the material. This effect was considered to be the main reason for the occurrence of explosive spalling, which is one of the most dangerous phenomena accompanying cement composites exposed to the fire.

Magalhaes et al. [100] examined the mechanical response of a cement composite with PVA fibers. The samples were preheated at 90–250 ◦C. Next, uniaxial tensile, bending and compressive strength tests were carried out, during which the crack width and the crack density were measured. The studies showed that the crack density takes less and less values at the same level of deformation as the preheating temperature increases. In the case of the crack width, the inverse relationship was observed. The maximum crack widths were in the range 100–220 μm. The conducted research confirmed that in terms of mechanical performance, the more favorable situation is when there are more cracks in the analyzed area, but they are of smaller widths.

Szel ˛ag evaluated the influence of microsilica [14,98,102], metacaolinite [16,101], polypropylene fibres [12] and multi-wall carbon nanotubes [13] on the process of formation of the cracking patterns in cement pastes, under thermal load, in the range 200–250 ◦C. The influence of the shape and size of the sample on the geometric characteristics of the cracking patterns was also investigated [15]. To describe the morphology of the cracking patterns, the following parameters were used: the area and perimeter of the cluster, the width of the crack opening and the crack density. It was found that modification of the cement matrix with the addition of microsilica causes densification of the crack network. There are more cracks, but they have a smaller opening widths, which has a positive effect on mechanical properties. In case of modification with metacaolinite, a decrease in the number of cracks formed with comparable widths of their opening in comparison to unmodified cement matrix was observed. The use of polypropylene fibers resulted in the reduction of area and perimeter of the clusters, with a negligible effect on the opening widths of the cracks. The polypropylene fibers melted due to thermal load and the cracks forming the cracking pattern were largely contained in cavities left by the fibers. In case of modification of the cement matrix with carbon nanotubes a drastic reduction of cracks system density was observed, with a simultaneous high increase in crack widths. It had a negative effect on mechanical performance. However, the effect observed was more related to the phenomenon of cement matrix foaming caused by the way carbon nanotubes were introduced than to their presence itself.
