R SUMMinimum reinforcement is provided in concrete beams in order to improve their behaviour towards cracking and ductility at failure.Generally, codes of practice equations for the minimum steel ratios, longitudinal and transversal, are mainly empirical and do not include all the influential parameters in them. For this reason and due to the fact that they do lack of a theoretical background, different codes can give values for the minimum steel ratios that greatly differs from one another. Also the validity of these equations may be questioned particularly in the case of high strength concrete beams and prestressed concrete beams for which limited test data are available.In this work, a theoretical approach for the minimum steel ratios that are required for the ductile behaviour at failure in bending, shear and torsion, in concrete beams made of concrete with different strengths is presented. Comparisons are also made between the proposed expressions, the codes expressions and available test results.
Un renforcement minimum des poutres en b~ton arm~ est pr&u pour am(liorer leur comportement en cas de fissuration et leur ductilit~ dans la rupture. En g&&al les formules des normes techniques pour tes pourcenrages minimum des armatures longitudinales et transversales sont simplement empiriques et ne contiennent pas tous les param~tres influents. Pour cette raison et comme elles ne prennent pas en consid&ation des fondements th&riques, les diff&ents r~glements donnent des valeurs trbs diff&entes pour les pourcentages minimum d'acier. La validit~ de ces formules peut &e raise en doute particuli}rement clans les cas de poutres en b~ton h haute r&istance et des poutres pr~-contraintes pour lesquelles seuIs des r&ultats d' essais tr}s
Besides the strength enhancement and strain improvement (strain-hardening behavior), the use of natural fibers as reinforcement in cement-based matrices can also be highlighted as an economical and eco-friendly alternative for the future of the construction industry. In the present work, cement-based composites reinforced by natural sisal fibers were produced and tested under direct tensile loading. The Portland cement was partially replaced by pozzolans (metakaolin -MK and fly ash -FA), aiming to produce a calcium hydroxide-free matrix to ensure the durability of the fibers. The natural sisal fibers were used in a 5% volume fraction (in mass), divided into three layers. The mechanical properties of composite plates were compared to other literature results and demonstrated to be compatible with recent research. The crack pattern was analyzed by Digital Image Correlation (DIC) for a better understanding of their failure mechanisms. The material presented a tensile strength increase after the first crack formation, marked by multiple cracking partners from this point. Finally, a comparison between direct (LVDTs) and indirect (DIC) methods of strain measurement was done and demonstrated minor results for the DIC, approximately 84% of those obtained from the LVDTs.
Resumoc r a c leves com resi O oncreto com agregado leve tem sido cada vez mais empregado em azão da reduzida massa específica, do elevado isolamento térmico e cústico e da melhoria na resistência estrutural a carregamentos íclicos. Neste trabalho foram estudados concretos com agregados stência à compressão em torno de 20 MPa e seu desempenho estrutural em lajes treliçadas pré-moldadas. Utilizaram-se três diferentes concretos com valores de massa específica entre 1.200 e 1.600 kg/m 3 . Os concretos foram avaliados com relação à resistência à compressão (aos 3, 7 e 28 dias) e à absorção total de água (aos 28 dias). Com os três tipos de concreto foram produzidas e ensaiadas à flexão lajes pré-moldadas (duas lajes para cada tipo de concreto), com a avaliação de seu comportamento no estado limite de serviço (ELS) para cargas de curta duração, ou seja, sem considerar efeitos ao longo do tempo. Os resultados mostraram o potencial do uso de concretos com agregados leves nesse tipo de laje estrutural.
Anderson de Oliveira
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