Flexural strengthening of RC slabs with prestressed CFRP strips: serviceability and ultimate load state behavior

O presente trabalho pretende contribuir para o conhecimento da técnica EBR (Externally Bonded Reinforcement) usando laminados de CFRP (Carbon Fiber-Reinforced Polymer) pré-esforçados no reforço à flexão de estruturas de betão, sob condições de serviço (SLS) e estado limite último (ULS). Para tal, foi desenvolvido um programa experimental composto por 8 lajes ensaiadas até à rotura. Duas lajes foram usadas como de referência; 5 reforçadas com laminados de EBR/CFRP pré-esforçados; finalmente, 1 laje reforçada com um laminado EBR/CFRP. Dois sistemas distintos de ancoragem foram estudados: MA (Mechanical Anchorage) e GA (Gradient Anchorage). O efeito da largura e espessura do laminado foi também estudado. A performance dos protótipos é criticamente analisada ao nível dos SLS e ULS.The present work intends to contribute to the knowledge of Externally Bonded Reinforcement (EBR) technique using prestressed Carbon Fiber-Reinforced Polymer laminates (CFRP) to strengthen concrete structures in flexure, at both serviceability (SLS) and ultimate load levels (ULS). For that purpose, an experimental program was carried out with 8 slabs monotonically tested under displacement control up to failure. Two slabs were used as reference specimens (unstrengthened); 5 slabs were strengthened with prestressed EBR CFRP laminates; finally, 1 slab was strengthened EBR CFRP laminates without being prestressed. To fix the ends of the prestressed CFRP laminates two different anchorage systems were used: Mechanical Anchorage (MA) and Gradient Anchorage (GA). The effect of the width and thickness of the CFRP laminate were also studied. The observed performance of the tested slabs is critically analyzed in terms of SLS and ULS aspects

Flexural strengthening of RC slabs with prestressed CFRP strips using different anchorage systems

In the field of flexural strengthening of concrete structures the Externally Bonded Reinforcement (EBR) technique using carbon fiber-reinforced polymers (CFRP) is commonly used. This technique offers several structural advantages when the CFRP material is prestressed. One of the critical aspects of a prestressing technique is the type of end anchorage. Two systems are investigated in the present paper: the metallic anchorage elements fixed to the ends of the FRP reinforcement and the gradient anchorage. In order to assess the performance of these two systems, an experimental program was carried out. In addition to the anchorage systems, the cross-section geometry of the CFRP strip was also included in this study. Eight slabs were monotonically tested under displacement control up to failure by using a four-point bending test configuration. The observed performance of the tested slabs is critically analyzed.Fundação para a Ciência e a Tecnologia (FCT

Criteria for fast and selective α precipitation at β grain boundaries in Ti-alloys Consequence for in-service microstructures

International audienceThe potential for β grain boundaries (GBs) to give rise to large Widmanstätten colonies was determined through the observation of a large amount of precipitates at β GBs in a β-metastable titanium alloy using electron backscatter imaging and diffraction. The most critical boundaries are those which transform early and where a type of Variant Selection (VS) called double Burgers VS occurs. This mechanisms take place at 'special' β boundaries misoriented so that an α precipitate can be related to both grains through the Burgers relation. It was shown that the most critical GBs have a disorientation at an angle of less than 10° from 49.5°/ or 60°/. A simulation study allowed those boundaries to be quantifed in crystallographic textures typical of industrial products. Those texture have then been discussed as a function of their potential to form large Widmanstätten colonies

Criteria for fast and selective α precipitation at β grain boundaries in Ti-alloys Consequence for in-service microstructures

International audienceThe potential for β grain boundaries (GBs) to give rise to large Widmanstätten colonies was determined through the observation of a large amount of precipitates at β GBs in a β-metastable titanium alloy using electron backscatter imaging and diffraction. The most critical boundaries are those which transform early and where a type of Variant Selection (VS) called double Burgers VS occurs. This mechanisms take place at 'special' β boundaries misoriented so that an α precipitate can be related to both grains through the Burgers relation. It was shown that the most critical GBs have a disorientation at an angle of less than 10° from 49.5°/<110> or 60°/<110>. A simulation study allowed those boundaries to be quantifed in crystallographic textures typical of industrial products. Those texture have then been discussed as a function of their potential to form large Widmanstätten colonies

Reforço à flexão de lajes de betão armado com laminados de CFRP pré-esforçados : comportamento em estado limite último e de utilização

No contexto do reforço estrutural à flexão com recurso aos laminados de CFRP (Carbon Fiber-Reinforced Polymers), a técnica EBR (Externally Bonded Reinforcement) é frequentemente utilizada. A aplicação de CFRP pré-esforçados acumula as vantagens da técnica EBR com as do pré-esforço externo. O sistema de ancoragem desempenha um papel fundamental nos sistemas de reforço com laminados de CFRP pré-esforçados. Por esta razão, neste trabalho desenvolveu-se um programa experimental, composto por 8 lajes de betão armado, com o principal objetivo de analisar dois sistemas de ancoragem: (i) ancoragem metálica e (ii) ancoragem do gradiente de força. Além dos sistemas de ancoragem, foi também analisada a influência da geometria do laminado de CFRP. Através dos resultados obtidos verificou-se uma melhoria do comportamento das lajes pré-esforçadas ao nível do estado limite último e de utilização

Comportamento de lajes de betão armado reforçadas com laminados de CFRP pré-esforçados

O presente artigo resume o trabalho experimental relativo a ensaios à flexão até à rotura em 12 lajes de betão armado reforçadas com laminados de CFRP pré-esforçados. Foram estudados os seguintes parâmetros: dois sistemas de ancoragem (ancoragem metálica e ancoragem do gradiente de força), sistema passivo versus passivo, largura (50 mm e 80 mm) e espessura (1.2 mm e 1.4 mm) do laminado de CFRP e a influência da geometria da laje (2600 mm e 2200 mm de comprimento). No presente artigo o programa experimental é descrito e os principais resultados são apresentados e analisados

Transformation kinetics and microstructures of Ti17 titanium alloy during continuous cooling

International audienceWe have investigated the microstructure evolutions in the Ti17 near Click to view the MathML source titanium alloy during heat treatments. The phase transformation has first been studied experimentally by combining X-ray diffraction analysis, electrical resistivity and microscopy observations. From a series of isothermal treatments, a IT diagram has been determined, which takes into account the different morphologies. Then, a Johnson–Mehl–Avrami–Kolmogorov (JMAK) model has been successfully used to describe the phase transformation kinetics during either isothermal or cooling treatments. Finally, the coupling of the JMAK model to the finite element software ZeBuLoN allowed us to investigate the evolution of the spatial distribution of the different morphologies during the cooling of an aircraft engine shaft disk after forging

Flexural behaviour of RC slabs strengthened with prestressed CFRP strips using different anchorage systems

The Externally Bonded Reinforcement (EBR) technique using Carbon Fiber-Reinforced Polymers (CFRP) has been commonly used to strengthen concrete structures in flexure. The use of prestressed CFRP material offers several advantages well-reported in the literature. Regardless of such as benefits, several studies on different topics are missing. The present work intends to contribute to the knowledge of two commercially available systems that differ on the type of anchorage: (i) the Mechanical Anchorage (MA), and (ii) the Gradient Anchorage (GA). For that purpose, an experimental program was carried out with twelve slabs monotonically tested under displacement control up to failure by using a four-point bending test configuration. The effect of type of anchorage system (MA and GA), prestrain level (0 and 0.4%), width (50 mm and 80 mm) and thickness (1.2 mm and 1.4 mm) of the CFRP laminate, and the surface preparation (grinded and sandblasted) on the flexural response were the main studied parameters. Better performance was observed for the slabs: (i) with prestressed laminates, (ii) for the MA system, and (iii) with sandblasted surface preparation.This work is supported by FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project FRPreDur - PTDC/ECM-EST/2424/2012. The authors also like to thank all the companies that have been involved supporting and contributing for the development of this study, mainly: S&P Clever Reinforcement Iberica Lda, S&P Clever Reinforcement Company (Switzerland), Tecnipor - Gomes & Taveira Lda., Via lam - Industrias Metalurgicas e Metalomecanicas, Lda., Hilti Portugal-Produtos e Servicos, Lda. The first and second authors wish also to acknowledge the grants SFRH/BD/98309/2013 and SFRH/BD/94850/2013, respectively, provided by FCT

Wolbachia Variants Induce Differential Protection to Viruses in Drosophila melanogaster: A Phenotypic and Phylogenomic Analysis

Wolbachia are intracellular bacterial symbionts that are able to protect various insect hosts from viral infections. This tripartite interaction was initially described in Drosophila melanogaster carrying wMel, its natural Wolbachia strain. wMel has been shown to be genetically polymorphic and there has been a recent change in variant frequencies in natural populations. We have compared the antiviral protection conferred by different wMel variants, their titres and influence on host longevity, in a genetically identical D. melanogaster host. The phenotypes cluster the variants into two groups--wMelCS-like and wMel-like. wMelCS-like variants give stronger protection against Drosophila C virus and Flock House virus, reach higher titres and often shorten the host lifespan. We have sequenced and assembled the genomes of these Wolbachia, and shown that the two phenotypic groups are two monophyletic groups. We have also analysed a virulent and over-replicating variant, wMelPop, which protects D. melanogaster even better than the closely related wMelCS. We have found that a ~21 kb region of the genome, encoding eight genes, is amplified seven times in wMelPop and may be the cause of its phenotypes. Our results indicate that the more protective wMelCS-like variants, which sometimes have a cost, were replaced by the less protective but more benign wMel-like variants. This has resulted in a recent reduction in virus resistance in D. melanogaster in natural populations worldwide. Our work helps to understand the natural variation in wMel and its evolutionary dynamics, and inform the use of Wolbachia in arthropod-borne disease control.FCT PhD fellowship: (SFRH/BD/51625/2011), Royal Society University Research Fellowship

Symbionts commonly provide broad spectrum resistance to viruses in insects: a comparative analysis of Wolbachia strains.

In the last decade, bacterial symbionts have been shown to play an important role in protecting hosts against pathogens. Wolbachia, a widespread symbiont in arthropods, can protect Drosophila and mosquito species against viral infections. We have investigated antiviral protection in 19 Wolbachia strains originating from 16 Drosophila species after transfer into the same genotype of Drosophila simulans. We found that approximately half of the strains protected against two RNA viruses. Given that 40% of terrestrial arthropod species are estimated to harbour Wolbachia, as many as a fifth of all arthropods species may benefit from Wolbachia-mediated protection. The level of protection against two distantly related RNA viruses--DCV and FHV--was strongly genetically correlated, which suggests that there is a single mechanism of protection with broad specificity. Furthermore, Wolbachia is making flies resistant to viruses, as increases in survival can be largely explained by reductions in viral titer. Variation in the level of antiviral protection provided by different Wolbachia strains is strongly genetically correlated to the density of the bacteria strains in host tissues. We found no support for two previously proposed mechanisms of Wolbachia-mediated protection--activation of the immune system and upregulation of the methyltransferase Dnmt2. The large variation in Wolbachia's antiviral properties highlights the need to carefully select Wolbachia strains introduced into mosquito populations to prevent the transmission of arboviruses.This is the final version published by PLoS in PLoS Pathogens here: http://www.plospathogens.org/article/info%3Adoi%2F10.1371%2Fjournal.ppat.1004369