Improvement of Serviceability and Strength of Textile Reinforced Concrete by using Short Fibres

Nowadays, thin-walled load bearing structures can be realised using textile reinforced concrete (BRAMESHUBER and RILEM TC 201-TRC [1]). The required tensile strength is achieved by embedding several layers of textile. By means of the laminating technique the number of textile layers that can be included into the concrete could be increased. To further increase the first crack stress and the ductility as well as to optimize the crack development, fine grained concrete mixes with short fibres can be used. By a schematic stress-strain curve the demands on short fibres are defined. Within the scope of this study, short fibres made of glass, carbon, aramid and polyvinyl alcohol are investigated in terms of their ability to fit these requirements. On the basis of these results, the development of hybrid fibre mixes to achieve the best mechanical properties is described. Additionally, a conventional FRC with one fibre type is introduced. Finally, the fresh and hardened concrete properties as well as the influence of short fibres on the load bearing behaviour of textile reinforced concrete are discussed

Tragverhalten von Textilbeton mit Kurzfasern

Das Tragverhalten von Textilbeton kann durch den Einsatz von Kurzfasern erheblich verbessert werden. Untersuchungen zeigen eine Anhebung der Erstrisslast, ein dehnungsverfestigendes Verhalten, eine Verfeinerung des Rissbildes sowie eine Anhebung der maximalen Tragfähigkeit. Unklar sind bisher noch die genauen Mechanismen, die eine gezielte Einstellung des Tragverhaltens ermöglichen. Der Beitrag fasst die Untersuchungen zu den einzelnen Bereichen der Spannungs- ehnungslinie von Textilbeton mit Kurzfasern zusammen und versucht möglichst allgemeingültige Zusammenhänge darzustellen. Dabei werden die Erhöhung der Erstrisslast, die Rissüberbrückung und die Auswirkungen auf das Gesamttragverhalten betrachtet. Abschließend wird das zyklische Biegetragverhalten von Textilbeton mit Kurzfasern im gerissenen Zustand dargestellt.The load bearing behaviour of textile reinforced concrete can be significantly improved by the addition of short fibres. Investigations show an increase of the first crack strength, a strain-hardening behaviour, a better crack pattern distribution as well as an increase of the load carrying capacity. However, the exact mechanisms leading to this behaviour are not yet known such that a prediction of the load bearing behaviour is not possible. This paper summarises the investigations on the different parts of the stress-strain curve of textile reinforced concrete with short fibres and tries to describe general relationships. In this context the increase of the first crack stress, the crack bridging behaviour and the influence on the load-bearing behaviour in general are considered. Finally, the dynamic flexural behaviour of textile reinforced concrete with short fibres in state II is presented

Problemas de contorno de la geodesia física

Capítulo publicado en el "IV Curso de Geodesia superior", págs. 37-96, y como trabajo monográfico en la publicación del Instituto de Astronomía y Geodesia.1. Introducción 2. Planteamiento clásico 2.1. Problema de Molodensky o problema gravimétrico vectorial 2.2. Linealización 2.3. Aproximación esférica 2.4. Reducciones al elipsoide y el problema del Geoide 3. Solución de los problemas de contorno sobre la esfera 3.1. Introducción a la teoria de armónicos esféricos. 3.2. Problema de Dirichlet 3.3. Problema de Hotine 4.3. Problema de Stokes 4. Teoria del truncamiento con optimización para la fórmula de Stokes 5. Problema gravimétrico escalar y problema gravimétrico con contorno fijo 6. Problemas de Altimetría-GravimetríaPeer reviewe

Einfluss von Kurzfasern auf die Frisch- und Festbetoneigenschaften sowie das Tragverhalten von Textilbeton

This thesis deals with the combined use of textile reinforcement and short fibres in textile reinforced concrete. The consideration of the interaction of both kinds of reinforcement takes place against the background of higher first crack stresses, a smoother crack development as well as a better global load bearing behaviour. In different chapters the influences of short fibres on the workability of fresh concrete, on the fine grained concrete properties and on the composite material is being described. The main influence on the workability of fine grained concretes with short micro fibres that was found during the investigations is the fineness of the grading curve. Also a round particle size is advantageous while the binder content has no direct influence apart from a refinement of the grading curve. Well known rheological aspects like packing density and viscosity are less important at the investigated fine grained fibre concretes. A fundamental realisation during the investigations was the dimension of the possible drying sensitivity of fine grained concretes which makes the evaluation of crack strengths more difficult. On the first hand, the investigations show the necessity of a direct water supply at early ages when using fine grained concretes with high binder contents. On the other hand, dried fine grained concretes with uncoated textile reinforcement can have considerably reduced first crack stresses even if they were properly cured within the first weeks. This effect was not found at permanently water stored concretes as well as with coated textile reinforcements and is therefore ascribed to the increased and crack initiating drying process of the concrete in presence of uncoated yarns. Based on a maximum tensile strength of consequently water stored fine grained concretes a further significant increase in tensile strength is not observed with textiles or short fibres. However, drying induced strength losses can be completely compensated by short fibres if the hydration process is not interrupted at an early stage. In this context also synergistic effects between textiles and short fibres were found. A new approach taking into account not only fibre content and geometry but also fibre stiffness allows an estimation of the increased tensile strength. In contrast to existing approaches also very different short fibre types can be evaluated after a calibration to the used concrete. In the same manner, the crack strength of textile reinforced concrete with short fibres can be estimated. Short fibres play an important role at the transition of the tensile loads from the concrete to the textile reinforcement. High cracking strengths of the concrete matrix in combination with textile reinforcements with poor bond properties lead to a lack of reinforcement at the crack opening. This leads to a significantly reduced stiffness of the stress-strain curve during crack formation. By adding short fibres this lack of reinforcement and the resulting crack widths can be reduced. For a prediction of minimum required fibre contents a procedure based on the superposition of the crack bridging portions of each component (concrete, textile and short fibres) is suggested. The procedure allows a significant increase in crack strength without a loss of local stiffness during crack development. A comparison between measured crack spacings and first crack stresses of textile reinforced concretes with short fibres reveals that the crack spacings decrease disproportionately with increasing fibre content despite of the accompanying higher crack stresses of the matrix. This leads to the conclusion that the load introducing capabilities of the short fibres are more pronounced than the ability to increase the crack stresses of the concrete. Investigations on the crack development during tensile loading confirm a continuous cracking until textile rupture. The crack development is also supported by an additional crack mechanism which can be described as successive crack division by the short fibres. For the local stiffness of the stress-strain curve after first cracking it is decisive if the short fibres mainly reduce the crack widths or form new cracks in a stronger manner. Also the range of crack stresses governed by the scattering of the concrete plays an important role. The ultimate strain of textile reinforced concrete with short fibres can be increased by short fibres if the fibres are evenly distributed and long enough. Using short fibres with a larger diameter leading to a more uneven fibre distribution the existence of one weak cross-section can cause a premature failure compared to plain textile reinforced concrete without short fibres