Browsing by Author "Silva, Pedro"
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- A durabilidade do betão auto-compactável (BAC)Publication . Silva, Pedro; Brito, JorgeNas últimas décadas, têm surgido novos tipos de betões associados a novas tecnologias com o objectivo de dar resposta aos problemas de durabilidade do betão bem como procurando melhorar o seu comporta-mento em ambientes agressivos. É o caso do betão auto-compactável (BAC), tema de grande interesse actual pois, apesar de o seu desenvolvimento inicial aparecer originalmente associado à procura da reso-lução de problemas de índoles várias em estruturas de betão armado associados à falta de mão-de-obra especializada, apresenta igualmente evidentes efeitos positivos, por exemplo na melhoria das condições de trabalho e do meio envolvente à obra através da redução do ruído associado à vibração. As propriedades do BAC no estado fresco são a grande diferença quando comparado com um betão con-vencional (BC). No entanto, e devido às diferenças apresentadas entre ambos quanto às quantidades dos materiais constituintes e ao processo de colocação, a durabilidade do BAC é diferente da apresentada por um BC. Desse modo, pretende-se, neste artigo, apresentar um resumo sobre o estado actual do conheci-mento relativo ao comportamento do BAC em termos de durabilidade. In the last decades, new types of concrete associated with new technologies have been developed in order to provide solutions to concrete durability problems as well as to improve its behaviour in aggressive environments. It is the case of self-compacting concrete (SCC), which is of great interest nowadays since it produces, for example, positive effects in the improvement of working conditions and in the environ-ment surrounding the worksite through noise reduction associated with vibration, even though its initial creation seems to be associated with the search for a solution to various problems of reinforced concrete structures linked to the lack of specialised labour. The properties of SCC in the fresh state are the major difference when comparing SCC to conventional concrete (CC). On the other hand, and due to their dif-ferences in what regards the quantities of constituent materials and the casting, the durability of SCC is different from that of a CC. Within this context, the aim of this article is to present a summary on the state-of-the-art related to the behaviour of SCC in terms of durability.
- Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt, Southern Morocco)Publication . Neres, Marta; Silva, Pedro; Ikenne, Moha; Martins, Sofia; Hafid, Ahmid; Mata, João; Almeida, Francisco; Youbi, Nasrrddine; Boumehdi, Moulay AhmedNo paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/ chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma.
- Exploring offshore sediment evidence of the 1755 CE tsunami (Faro, Portugal): implications for the study of outer shelf tsunami depositsPublication . Kümmerer, Vincent; Drago, Teresa; Veiga-Pires, C.; Silva, Pedro; Magalhaes, Vitor; Mena, Anxo; Lopes, Ana; Rodrigues, Ana Isabel; Schmidt, Sabine; Terrinha, Pedro; Baptista, Maria AnaOuter shelf sedimentary records are promising for determining the recurrence intervals of tsunamis. However, compared to onshore deposits, offshore deposits are more difficult to access, and so far, studies of outer shelf tsunami deposits are scarce. Here, an example of studying these deposits is presented to infer implications for tsunami-related signatures in similar environments and potentially contribute to pre-historic tsunami event detections. A multidisciplinary approach was performed to detect the sedimentary imprints left by the 1755 CE tsunami in two cores, located in the southern Portuguese continental shelf at water depths of 58 and 91 m. Age models based on C-14 and Pb-210(xs) allowed a probable correspondence with the 1755 CE tsunami event. A multi-proxy approach, including sand composition, grain-size, inorganic geochemistry, magnetic susceptibility, and microtextural features on quartz grain surfaces, yielded evidence for a tsunami depositional signature, although only a subtle terrestrial signal is present. A low contribution of terrestrial material to outer shelf tsunami deposits calls for methodologies that reveal sedimentary structures linked to tsunami event hydrodynamics. Finally, a change in general sedimentation after the tsunami event might have influenced the signature of the 1755 CE tsunami in the outer shelf environment.
- A influência da porosidade na durabilidade do betão auto-compactável (BAC)Publication . Silva, Pedro; Brito, Jorge dePretende-se, neste artigo, apresentar um resumo sobre o estado actual do conhecimento relativo ao comportamento do BAC em termos de durabilidade, mais especificamente sobre o efeito da sua porosidade. Apesar das exigências dos BAC em termos de propriedades no estado endurecido serem as mesmas de um betão convencional (BC), as quantidades de amassadura utilizadas bem como as proporções nas quais os diversos componentes se combinam são diferentes. As diferenças referidas entre os BAC e os BC assentam na diminuição da relação entre as quantidades de agregado grosso e de argamassa com o consequente aumento do volume de pasta através da utilização de maiores volumes de material ultra fino (cimento + adições) e adjuvantes (tipo superplastificantes e/ou moduladores de viscosidade) e no adequado controlo da máxima dimensão de agregado. Apesar das razões, tanto de carácter técnico e económico, como até ecológicas (utilização dos recursos de modo a garantir um crescimento sustentável) para a utilização de maiores quantidades de adições, se estas forem demasiadamente elevadas, tal pode afectar substancialmente a durabilidade do BAC. Tal como acontece com um BC, para obter uma maior durabilidade, o BAC terá de dificultar a penetração dos agentes agressivos para o seu interior. A referida penetração ocorre por difusão ou absorção capilar devido à maior ou menor permeabilidade do BAC à água e aos gases. Desse modo, um conjunto de fluidos poderá penetrar no BAC e deteriorá-lo, tais como a água pura ou com iões agressivos, o CO2 e o oxigénio (como elemento regulador de todas as reacções de corrosão do aço embebido no betão). Com recurso à análise da estrutura dos poros de um betão, é possível prever a sua durabilidade uma vez que esta análise possibilita o conhecimento do modo como os agentes agressivos se difundem ou penetram na massa do betão.
- Local finiteness for Green's relations in semigroup varietiesPublication . Volkov, Mikhail; Silva, Pedro; Soares, FilipaA semigroup variety is said to be locally 𝒦-finite, where 𝒦 stands for any of Green’s relations ℋ, ℛ, ℒ, 𝒟, or 𝒥, if every finitely generated semigroup in this variety has only finitely many 𝒦-classes. We characterize locally 𝒦-finite varieties of finite axiomatic rank in the language of “forbidden objects”.
- Localization effect on AMS fabric revealed by microstructural evidence across small-scale shear zone in marblePublication . Kusbach, Vladimir; Machek, Matej; Roxerová, Z.; Racek, Martin; Silva, PedroAnisotropy of magnetic susceptibility (AMS) is regularly applied as a tool to infer structural analysis of deformation and flow in rocks, particularly, with low anisotropy. AMS integrates the magnetic signature of crystallographic and shape preferred orientation of all mineral grains present in the rock microstructure. Those preferred orientations result from multiple processes affecting the rock during its evolution, therefore the desirable AMS-strain relationship is not straightforward. Here we show that due to localization of deformation, AMS is indirectly dependent on the magnitude and character of deformation. In order to decipher the AMS-strain relationship, AMS studies should be accompanied by microstructural analyses combined with numerical modelling of magnetic fabric. A small-scale shear zone produced by single deformation event was studied. The resultant AMS fabric is “inverse” due to the presence of Fe-dolomite and controlled by calcite and dolomite crystallographic preferred orientations. The localized deformation resulted in the angular deviation between macroscopic and magnetic fabric in the shear zone, systematically increasing with increasing strain. This is a result of the presence of microstructural subfabrics of coarse porphyroclasts and fine-grained recrystallized matrix produced by localization.The localization of deformation is a multiscale and widespread process that should be considered whenever interpreting AMS in deformed rocks and regions.
- Magma flow, exsolution processes and rock metasomatism in the Great Messejana-Plasencia dyke (Iberian Peninsula)Publication . Silva, Pedro; Henry, Bernard; Marques, Fernando O.; Font, Eric; Mateus, António; Vegas, Ramon; Miranda, Jorge Miguel; Palomino, Ricardo; Palencia-Ortas, AliciaMagma flow in dykes is still not well understood; some reported magnetic fabrics are contradictory and the potential effects of exsolution and metasomatism processes on the magnetic properties are issues open to debate. Therefore, a long dyke made of segments with different thickness, which record distinct degrees of metasomatism, the Messejana-Plasencia dyke (MPD), was studied. Oriented dolerite samples were collected along several cross-sections and characterized by means of microscopy and magnetic analyses. The results obtained show that the effects of metasomatism on rock mineralogy are important, and that the metasomatic processes can greatly influence anisotropy degree and mean susceptibility only when rocks are strongly affected by metasomatism. Petrography, scanning electron microscopy (SEM) and bulk magnetic analyses show a high-temperature oxidation-exsolution event, experienced by the very early Ti-spinels, during the early stages of magma cooling, which was mostly observed in central domains of the thick dyke segments. Exsolution reduced the grain size of the magnetic carrier (multidomain to single domain transformation), thus producing composite fabrics involving inverse fabrics. These are likely responsible for a significant number of the 'abnormal' fabrics, which make the interpretation of magma flow much more complex. By choosing to use only the 'normal' fabric for magma flow determination, we have reduced by 50 per cent the number of relevant sites. In these sites, the imbrication angle of the magnetic foliation relative to dyke wall strongly suggests flow with end-members indicating vertical-dominated flow (seven sites) and horizontal-dominated flow (three sites).
- Mantle source heterogeneity, magma generation and magmatic evolution at Terceira Island (Azores archipelago): Constraints from elemental and isotopic (Sr, Nd, Hf, and Pb) dataPublication . Madureira, Pedro; Mata, João; Mattielli, Nadine; Queiroz, Gabriela; Silva, PedroThis work addresses the present-day (<100 ka) mantle heterogeneity in the Azores region through the study of two active volcanic systems from Terceira Island. Our study shows that mantle heterogeneities are detectable even when "coeval" volcanic systems (Santa Barbara and Fissural) erupted less than 10 km away. These volcanic systems, respectively, reflect the influence of the Terceira and D. Joao de Castro Bank end-members defined by Beier et at (2008) for the Terceira Rift Santa Barbara magmas are interpreted to be the result of mixing between a HIMU-type component, carried to the upper mantle by the Azores plume, and the regional depleted MORB magmas/source. Fissural lavas are characterized by higher Ba/Nb and Nb/U ratios and less radiogenic Pb-206/Pb-204, Nd-143/Nd-144 and Hf-176/Hf-177, requiring the small contribution of delaminated sub-continental lithospheric mantle residing in the upper mantle. Published noble gas data on lavas from both volcanic systems also indicate the presence of a relatively undegassed component, which is interpreted as inherited from a lower mantle reservoir sampled by the ascending Azores plume. As inferred from trace and major elements, melting began in the garnet stability field, while magma extraction occurred within the spinel zone. The intra-volcanic system's chemical heterogeneity is mainly explained by variable proportions of the above-mentioned local end-members and by crystal fractionation processes.
- Multi-stage evolution of a sub-aerial volcanic ridge over the last 1.3 Myr: S. Jorge Island, Azores triple junctionPublication . Hildenbrand, Anthony; Madureira, Pedro Miguel; Marques, Fernando Ornelas; Cruz, Inês; Henry, Bernard; Silva, PedroNew K/Ar dating and geochemical analyses have been carried out on the WNW-ESE elongated oceanic island of S. Jorge to reconstruct the volcanic evolution of a linear ridge developed close to the Azores triple junction. We show that S. Jorge sub-aerial construction encompasses the last 1.3 Myr, a time interval far much longer than previously reported. The early development of the ridge involved a sub-aerial building phase exposed in the southeast end of the island and now constrained between 1.32 +/- 0.02 and 1.21 +/- 0.02 Ma. Basic lavas from this older stage are alkaline and enriched in incompatible elements, reflecting partial melting of an enriched mantle source. At least three differentiation cycles from alkaline basalts to mugearites are documented within this stage. The successive episodes of magma rising, storage and evolution suggest an intermittent reopening of the magma feeding system, possibly due to recurrent tensional or trans-tensional tectonic events. Present data show a gap in sub-aerial volcanism before a second main ongoing building phase starting at about 750 ka. Sub-aerial construction of the S. Jorge ridge migrated progressively towards the west, but involved several overlapping volcanic episodes constrained along the main WNW-ESE structural axis of the island. Malic magmas erupted during the second phase have been also generated by partial melting of an enriched mantle source. Trace element data suggest, however, variable and lower degrees of partial melting of a shallower mantle domain, which is interpreted as an increasing control of lithospheric deformation on the genesis and extraction of primitive melts during the last 750 kyr. The multi-stage development of the S. Jorge volcanic ridge over the last 1.3 Myr has most likely been greatly influenced by regional tectonics, controlled by deformation along the diffuse boundary between the Nubian and the Eurasian plates, and the increasing effect of sea-floor spreading at the Mid-Atlantic Ridge.
- Multidisciplinary characterization of Quaternary mass movement deposits in the Portimão Bank (Gulf of Cadiz, SW Iberia)Publication . Silva, Pedro; Roque, C.; Drago, Teresa; Belen, A.; Ercilla, Gemma; Lopes, Ana; Lopez-Gonzalez, Nieves; CASAS, DAVID; Naughton, F.; Vazquez, Juan-Tomas; Henry, B.We present a multidisciplinary study of the seismic stratigraphy, sedimentology, geochemistry and magnetism to characterize the Quaternary mass movement deposits (MMD) and the associated deformation in the Portimão Bank (Gulf of Cadiz, offshore SW Iberia). Two scales of approach have been applied. At large-scale (m to km) approach, were recognized and characterized a series of important and subsequent slide scars (tens of meters high) and MMD (the larger one with 10 km length) related to landslides, more prominent at Portimão Bank's southern flank. At small-scale (cm) approach, we point out on a piston core collected within a scar affected by MMD, a replication of the sedimentary column as evidenced by geochronological results and corroborated by sedimentology, geochemistry and magnetic data. Magnetic fabric data enabled the identification and characterization of the internal structure and deformation of MMD along the sedimentary column. For last, geochronology and the morphology of Portimão seamount are discussed in order to constraint the factors controlling MMD triggering. The multidisciplinary approach is useful in a better characterization of the MMD at different scales and improved the understanding of its dynamics.
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