Fatigue is the process of mechanical degradation of a material, which leads to its collapse. Repeated load applications with a maximum value lower than the one that provokes the static failure of the material, causes internal damage in the material that, progressively, reduces its mechanical capacity until it finally collapses. The increasingly widespread use of high-strength concretes permits the construction of more lightweight structures. This implies that the variable loads (which are the causes of fatigue) represent an ever larger percentage of the total load. In consequence, fatigue is an increasingly important factor in concrete structures. In some cases, it even begins to be the dimensioning load of the structure. In addition, the presence of fibers within the concrete modifies the fatigue response of the concrete. In this chapter, the classic theory of fatigue is presented in detail and the most recent developments in the study of concrete fatigue are discussed.
Part of the book: High Performance Concrete Technology and Applications
Computed tomography (CT) is a nondestructive technique, based on absorbing X-rays, that permits the visualisation of the internal microstructure of material. The field of application is very wide. This is a well-known technology in medicine, because of its enormous advantages, but it is also very useful in other fields. Computed tomography is used in palaeontology to study the internal structure of the bones from ancient hominids. In addition, this technology is being used by engineers to analyse the microstructure of materials. Materials engineers use this technology to analyse or develop new materials. Mechanical engineers use CT scans to study the internal defects of materials. Geotechnical engineers use CT scans to study several aspects of the rocks and minerals (cracks, voids, etc). This technology is also very useful to study de microstructure of concrete, especially in case of the new concretes (ultra-high performance concrete, fiber reinforced concrete, etc). In this chapter, an extended state-of-the-art of the most relevant research, related to the use of computed tomography to explore the microstructure of materials in civil and mechanical engineering, is exposed. The main objective of this chapter is that the reader can discover new applications of the computed tomography, different from conventional ones.
Part of the book: Computed Tomography
Steel fiber-reinforced concrete (SFRC) is a composite material, consisting of conventional concrete with the addition of short, randomly distributed steel fibers. Fibers modify the mechanical behavior of concrete, improving some of its properties: they increase its ductility, enhance its residual tensile strength, and under certain conditions, increase its ultimate flexural strength. All these advantages make this material competitive with conventional reinforced concrete. However, the psychological barriers of the construction sector and the technical challenges that remain to be solved are slowing down the consolidation of this building material. One of these challenges is the improvement of the understanding of fatigue, which not only affects SFRC, but concrete in general. In this regard, work to date suggests that fibers, given certain circumstances, increase the fatigue life of concrete. This would result in SFRC being very effective in structures where fatigue is a critical action, such as wind turbine concrete towers.
Part of the book:[Working title]