*5.5. Chemical Corrosion*

Xu et al. [9] apart from studying the development of the cracking patterns as a function of the influence of elevated temperature of concrete with fly-ash, also conducted the rapid chloride diffusion test. The results obtained indicated that deterioration of concrete durability is correlated with the crack density.

Zhuang et al. [116] performed a fractal analysis of the crack patterns on the surface of reinforced concrete piles bonded with carbon fiber reinforced polymer. The factor causing the cracks was the corrosion of the reinforcement caused by a harsh marine environment. Within the framework of the research the damage index was developed to assess the degree of degradation of the piles, the method of calculation of which was very similar to the one presented in [105]. The assessment was made at different stages of reinforcement corrosion development. Together with the progressing corrosion, an increase in the value of fractal dimension of the cracking patterns was observed. Moreover, a study of maximum load of piles was carried out, which showed a very high correlation with the fractal dimension. The observed strength loss-fractal dimension dependence assumed a linear dependence. As a result of the conducted tests it was found that supplementing the measurements with the analysis

of cracks width, a high quality information could be obtained regarding the degree of structure degradation in the situation when it is impossible to perform mechanical tests.

### **6. Summary**

The paper provides a literature review of the development process, analysis and impact of the cracking patterns on the properties of cement composites. The review focuses on four main aspects related to the analysis of the cracking patterns:


Each of these four aspects requires a different scientific approach, making comprehensive analysis of the cracking patterns a difficult and multidisciplinary issue. The three main scientific areas that underpin this research area are the fracture mechanics, image analysis and cement composites technology.

The factors that determine the cracking process, i.e., the number and type of structure defects and the composition of the material, are discussed. It was found that critical stress intensity factor (*KIC*) being the product of crack work and Young's modulus is the most commonly studied parameter which determines the ability of the material to propagate and develop cracks in the branched cracks system. The methodology of analysis of the cracking patterns requires the use of advanced tools for the detection and extraction of cracks. Research carried out so far in this field indicates that by far the most effective and popular group of tools is the computer image analysis. The simplest of them are threshold operations, e.g., the global thresholding, locally adaptive thresholding or the Otsu thresholding. However, such operations, despite the fact that they are simple to implement, are sometimes characterized by a very large detection error. More advanced techniques, such as the genetic algorithms, artificial neural networks or machine learning algorithms, are characterized by very good detection accuracy. Their disadvantage is the need for much more computational resources, which slows down the whole process and makes it sometimes impossible to analyze a large set of data. The literature review shows that the most accurate methods of cracks detection on cement composite surfaces are characterized by an accuracy of more than 95%.

Previous research indicates that the simplest parameter to measure, in terms of the complexity and morphology of the cracking patterns, is the width of the crack opening. However, this indicator does not provide information on the layout and organization of the cracks structure in the material. The parameter that provides this information is the crack density and the resultant the crack spacing. Recent studies postulate the use of fractal geometry to quantify the cracking patterns. In these studies, the concept of fractal dimension is used, which by definition determines the degree of complexity of the analyzed structure. Analyses carried out with the use of this parameter indicate a strong relationship with the mechanical features of a degraded cement composite. In the research results published so far, the greatest attention is paid to determining the influence of the cracking patterns on the performance of cement composites at elevated temperatures. The system of cracks formed due to the shrinkage and drying of the material as well as those formed due to chemical corrosion is also analyzed. The influence of the cracking patterns on water permeability and mechanical strength of cement composites is also evaluated. The presented results indicate that with the progressive development of the cracking patterns, practically every property of the hardened cement composite deteriorates.

The authors of many research point to the key importance of the degree of development of the cracking patterns for the functional properties and durability of cement composites. However, there are still few works in which the dependence of the cracking patterns on the physical and mechanical properties of the material is determined directly, in a numerical way. Knowledge in this area seems to be crucial in the aspect of designing durable cement composites, which would be resistant to cracking and crack development process in specific environments. From the methodological point of view, further development of works aimed at developing more and more accurate, automatic crack detection systems is indicated. Such systems could be used very successfully by building managers to effectively detect areas in need of remedial action. The search for new methods and the development of the relationship between the cracking patterns and material properties is crucial in the development of non-destructive testing methods in cement composite technology.

**Funding:** This research was funded by the Polish Ministry of Science and Higher Education with the statutory research number FN14/ILT/2019, the subsidy for the development of young scientists— FNM14.1/ILT/2019.

**Acknowledgments:** The author is supported by the Foundation for Polish Science (FNP).

**Conflicts of Interest:** The author declares no conflict of interest.
