Miguel Angel Vicente

This study characterizes the sequential flexural fatigue behavior of fiber-reinforced concrete (FRC), in which two damage mechanisms occur in sequence: (i) matrix fatigue up to cracking, followed by (ii) fiber-matrix bond fatigue. A simple criterion is proposed that, based on the relationship between static strengths (fLOP and fR) and the maximum cyclic stress level (σfat,max), allows anticipation of this sequential behavior. When σfat,max is below both fLOP and fR, the cyclic creep curves (defined as the maximum CMOD per cycle versus the number of cycles) display two concatenated inverted-S segments, reflecting the two damage processes.The method is validated through an experimental program on three FRCs with fiber contents of 0.3%, 0.6%, and 1.0%, subjected to flexural fatigue under constant stress levels. The results confirm that the proposed criterion correctly discriminates between the general case and particular cases in which only one of the two mechanisms operates.In addition, the Sparks–Menzies relation between the slope of the central segment of the cyclic creep curve (dCMOD/dn) and the fatigue life (N) is extended to derive closed-form expressions that estimate, with high accuracy, the number of cycles to matrix fracture (Nm) and the remaining cycles to final failure (Nf–m). This tool provides a practical basis for assessing accumulated damage and optimizing the design and maintenance of FRC structural elements subjected to cyclic loading.

This paper proposes a reliable and comprehensive procedure for the assessment of steel fibre distribution in fibre reinforced concrete. Two techniques have been used: computed tomography (CT) scanning and inductive testing. Both techniques have been applied to study the distribution of steel fibres in specimens extracted from structural elements, carrying out a comparative analysis of the results. Firstly, the data from the CT-scanning and the inductive method have been processed to obtain the density of steel fibres and their orientation in all the specimens, before comparing the results. Then, a complementary study was carried out to validate the inductive method, using hand-made cylinders with a previously known fibre dosage and orientation. Finally, the dual energy CT technique was applied and some conclusions about its potential were discussed. Both methods exhibited comparable accuracy and precision in measuring fibre density and orientation. Considering that CT-scanning is the reference technique, this mutual agreement reinforces the reliability of the inductive method. Additionally, the authors employed the dual energy CT aiming to explore the potential of this novel technique within the domain of fibre reinforced concrete research.

Este estudio explora cómo el nanosílice afecta las propiedades mecánicas y el comportamiento de fractura del hormigón de ultra alta prestaciones (UHPC). Profundizamos en el papel que el nanosílice juega en la mejora de las reacciones puzolánicas y sus efectos sobre la estructura porosa del UHPC. Para ello, se utilizaron técnicas avanzadas como la microscopía electrónica de barrido y la porosimetría por intrusión de mercurio. Se prepararon muestras de UHPC con diferentes concentraciones de nanosílice, sustituyendo el cemento entre un 0 y un 7,5% en peso, para comprender a fondo su influencia. Nuestros resultados revelan que, aunque un mayor contenido de nanosílice es beneficioso en algunos aspectos, compromete la capacidad autocompactante del UHPC. Esto hizo necesario realizar ensayos de escurrimiento para evaluar su trabajabilidad. Sorprendentemente, la adición de nanosílice llevó a una reducción tanto de los poros capilares como de los macroporos, mejorando la densidad y resistencia del hormigón. Concretamente, una adición máxima del 5% en peso de nanosílice resultó en un aumento del 13% en la resistencia a compresión. Sin embargo, esta mejora en la resistencia conlleva una contrapartida. La energía de fractura del UHPC disminuyó en un 23,2%, lo que indica un aumento en la fragilidad debido al nanosílice. Curiosamente, la resistencia a tracción aumentó en un 10,5%, principalmente.

The accurate determination of the tensile strength of concrete is a topic of growing interest, especially driven by the new ultra-high tensile strength fiber-reinforced concretes. However, this is a complex task, as traditional methods provide inconsistent results (both splitting tensile test and 3-point bending test). The most robust method is the direct tensile test. However, in concrete, it poses a technical challenge that has not yet been satisfactorily solved. The aim of this study is the design of a test specimen that maximizes the percentage of successful failure and also allows a viable anchorage solution to the load application equipment, especially for high and ultra-high tensile strength concretes. An optimized geometry of a bone-shaped specimen that maximizes the probability of collapse occurring at the central neck is presented. First, a mathematical function of the generatrix of the bone-shape specimen is presented. The behavior of the solution is also analyzed numerically and statistically, comparing it with other commonly used solutions for direct tensile tests. Additionally, the design, fabrication, and calibration of a tailored cardan joint for load centering, thus minimizing its eccentricity and the dispersion of the results. Finally, the results of a static test campaign carried out on six carbon-fiber reinforced high tensile strength concrete specimens. The proposed bone-shaped specimen shows a much higher percentage of successful failures than the other specimen geometries commonly used for tensile testing, which improves the quality of tensile characterization test campaigns for high and ultra-high tensile strength concretes.

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.