Resumen de Microstructural analysis of heated ultra-high-performance fibre-reinforced-concrete under cyclic loading.
Concrete is currently the structural material more used in civil engineering and building construction. Nowadays, the application of the technological advances of the chemical industry in the processes of concrete manufacture, as well as the addition of fibres as reinforcement of the cementitious matrix, has meant a significant improvement in the mechanical and fracture behaviour of these materials. High performance and especially, ultra‐high‐performance concretes are examples of such advances. One of the advantages of the use of concrete as structural material is its high resistance in fire situations compared to other types of materials, like steel. However, the maximum service temperature of structural concrete is essentially limited by the internal damage caused by them. This thermal damage is evidenced by the generation of cracks on the border of the pore matrix, as a consequence of the pressure inferred by the evaporation of the free water and dehydration of the compounds arisen from the cement hydration. One of the alternatives to mitigate the thermal damage of the cementitious matrix is the addition of polymer fibres, which are melted at relatively low temperatures (approximately 160 oC). The melting of the fibres generates a network of micro‐channels that allow the evacuation of the evaporated water more efficiently, so that the pressure inside the pores of the matrix is reduced and consequently the thermal damage. Another alternative is the use of steel fibres as reinforcement of the concrete matrix. The presence of steel fibres, well‐distributed and in the right amount, hinders the free crack propagation because they act as barriers. Nevertheless, the addition of fibres generates a greater heterogeneity of the matrix that can induce the alteration of its microstructure and affect the mechanical and fracture properties of concrete. This effect, together with the high packing density of high‐performance concrete and, more significantly in ultrahigh‐ performance concrete, leads to the thermal damage produced in the matrix of the material must be conveniently analysed using the appropriate techniques, like the computed tomography. In some applications, the concrete elements must bear thermal and mechanical loads simultaneously at moderately temperatures, but for long periods of time. In these cases, the microstructure of the matrix, as well as its degradation by thermal damage, is closely related to the mechanical strength and fracture behaviour of the material. In other applications, concretes are subjected to cyclic thermo‐mechanical loads that entail the generation and propagation of cracks as a result of thermo‐mechanical fatigue. The reinforcement of concretes with a high number of steel fibres, as in the case of ultrahigh‐performance concretes, allows to reach very high values of the tensile and flexural strength. Thus, it is possible to reduce or dispense the use of prestressed reinforcement in those applications where it is necessary that the material resists high tensile stress without cracking. In all these cases, the analysis of the macro‐mechanical behaviour (mechanical and fracture properties) and its time evolution is closely related to the microstructure of the material and the mechanisms produced in the micro‐scale by the addition of fibres, the thermal degradation and the crack growth by cyclic loading. This thesis begins with the study of the microstructural behaviour and its effect on the macroscopic properties of high‐strength concrete reinforced with polypropylene fibres of different length and subjected to high temperatures (Chapter 2). The discussion focuses on the connection between the microstructure and the mechanical and fracture properties of the different concretes. In the Chapter 3, it is analysed the effect that the addition of different steel fibres has in the microstructure of an ultra‐high‐performance concrete and to determine its influence on the microstructure and the mechanical and fracture properties. In the next chapter, it is analysed the effect of temperature on the internal structure of the ultra‐high‐performance concrete of Chapter 3, as well as the influence of thermal damage on its mechanical and fracture properties. Chapter 5 focuses on the validation of a fatigue failure probability model in compression developed by Saucedo et al., for its application in flexural fatigue tests on high and ultrahigh‐ performance fibre‐reinforced concrete and the evaluation of the effect of fibres on the model parameters. Finally, in Chapter 6, the fatigue behaviour of an ultra‐highperformance concrete subjected to different temperatures is analysed by determining the Wöhler curves through a fatigue failure probability model developed by Castillo and Fernández‐Canteli. All the analysis carried out have focused on relating the effect of thermal damage and the influence of fibres on the microstructure, with the obtained macro‐mechanical properties.
