The mitigation of blast hazards on critical reinforced concrete structures has become a major concern in regards to the safety of people and the integrity of buildings. Recent terrorist incidents and accidental explosions have demonstrated the need to study the effects of such threats on structures in order to develop effective methods of reducing the overall impact of blast loads. With the arrival of innovative materials such as steel fibre reinforced concrete (SFRC), ultra-high performance fibre reinforced concrete (UHPFRC) and high strength steel reinforcement, research is required in order to successfully adapt these new materials in blast-resistant structures. Hence, the objective of this thesis to conduct an experimental parametric study with the purpose of investigating the implementation of these innovative materials in reinforced concrete slabs and panels. As part of the study, a total of fourteen one-way slab specimens with different combinations of concrete, steel fibres and steel reinforcement are tested under simulated blast loads using the University of Ottawa Shock-Tube Facility. The test program includes three slabs constructed with normal-strength concrete, five slabs constructed with SFRC and six slabs constructed with UHPFRC. Among these specimens, four are reinforced with high-performance steel reinforcement. The specimens are subjected to repeated blast loading with gradually increasing reflected pressure and reflected impulse until failure. The performance of the slabs is studied using various criteria such as failure load and mode, maximum and residual deflections, as well as tensile cracking, spalling and secondary fragmentation control. The behaviour of all specimens is compared in different categories to determine the effects of concrete type, steel reinforcement type, steel fibre content and steel fibre type on blast performance. As part of the analytical study the response of the slab specimens is predicted using dynamic inelastic single-degree-of-freedom (SDOF) analysis. The dynamic analysis is conducted by generating load-deformation resistance functions for the slabs incorporating dynamic material properties.

Recent attacks and accidental explosions have demonstrated the necessity of ensuring the blast resistance of critical buildings and infrastructure in Canada such as federal and provincial offices, military buildings and embassies. Of particular importance is the blast resistance of ground-story columns in buildings which must be properly detailed to provide the necessary strength and ductility to prevent progressive collapse. There exists a need to explore the use of innovative materials that can simultaneously improve the performance of such columns, while also allowing for a relaxation of required detailing to ease construction. Advancements in concrete material science have led to the development of ultra-high performance fiber reinforced concretes (UHPFRC) which show superior mechanical properties when compared to conventional concrete, such as increased compressive strength, tensile resistance and toughness. These enhanced properties make UHPFRC an attractive material for use in the blast design of reinforced concrete columns. This thesis presents the results of a research program examining the performance of UHPFRC columns under simulated blast loads. As part of the experimental program twelve half-scale UHPFRC specimens, six built with regular grade steel reinforcement and six built with steel high-strength steel reinforcement, are tested under blast loading using the University of Ottawa shock tube. The specimens were designed according to CSA A23.3 standard requirements for both seismic and non-seismic regions, using various fibre types, fibre amounts and longitudinal reinforcement ratios, allowing for an investigation of various design parameters on blast behaviour. The results demonstrate that the use of UHPFRC improves the blast performance of columns by reducing displacements, increasing resistance and enhancing damage tolerance. The results also indicate that fiber content, fiber properties, seismic detailing, longitudinal reinforcement ratio and longitudinal reinforcement strength are factors which can affect the behaviour and failure mode of UHPFRC columns. As part of the analytical study the response of the UHPFRC columns is predicted using dynamic inelastic analysis. The dynamic responses of the columns are predicted by generating dynamic load-deformation resistance functions for UHPFRC and conducting single-degree-of-freedom (SDOF) analysis using software RC-Blast.

Ultra-high performance fiber-reinforced concrete (UHPFRC) is an advanced cement-based composite with enhanced compressive strength, tensile resistance and toughness when compared to conventional concrete. Interest in the application of UHPFRC as a retrofit material has been rapidly increasing, and a few existing studies have examined the ability of UHPFRC to retrofit and strengthen existing reinforced concrete (RC) structures under static loading; however, very limited studies have examined the effectiveness of UHPFRC to improve the response of RC members under blast loading. This thesis aims at filing this research gap and investigates the behavior of UHPFRC retrofitted RC flexural members under both static and blast loads. A total of twenty-one (21) specimens, in two different series are tested. Series 1 includes nine (9) singly-reinforced beams built with high-strength concrete (HSC) and strengthened by UHPFRC to improve shear and flexural behaviour. Series 2 includes a further twelve (12) doubly-reinforced beams/columns built with normal-strength concrete (NSC), and strengthened by UHPFRC to improve response under blast, or combined blast-axial loading. Various test parameters are examined including the effects of varying retrofit types (full jacket, U-jacket or T-sided), surface roughening methods, longitudinal steel reinforcement ratio, single vs. repeated blasts, and the effects of axial loading. The results from this thesis are presented in six journal articles. Papers 1 and 2 study the effects of UHPFRC jacketing on the static and blast behaviour of the singly-reinforced HSC beams in Series 1, while Paper 3 discusses the effects of additional parameters such as: the effect of retrofit type, roughening method and steel detailing on blast behaviour. Under static loading (Paper 1), the UHPFRC jacketing was found to be effective in increasing shear resistance (by preventing shear failure), and improving flexural behaviour (by increasing strength, stiffness, ductility and overall toughness) when compared to control beams built without UHPFRC. Similarly, under blast loads (Paper 2) the use of UHPFRC jacketing prevented shear failure, and improved flexural behaviour by reducing displacements at equivalent blasts, increasing overall blast capacity, and improving damage tolerance. On the other hand, the results show that UHPFRC-retrofitted beams with low longitudinal steel ratios may be vulnerable to brittle bar fracture failures. As part of the numerical research, finite element (FE) modelling is used to predict the static and blast behaviour of the test beams using software LS-DYNA (Papers 1 and 2). The results from Paper 3, provide further insights into the effects of retrofit type (FJ, UJ and T) and roughening method on blast performance; both the UJ and FJ retrofits were found to be effective in increasing shear resistance, reducing blast-displacements and increasing blast capacity, while the benefit of the T-sided retrofit was limited by the crushing capacity of HSC concrete. The effect of roughening method was found to be negligible, except at the very late stages of blast loading. Papers 4, 5 and 6 present the experimental results from the doubly-reinforced NSC beams tested in Series 2, with a focus on the effect of UHPFRC jacketing, UHPFRC retrofitting type and Axial loading, respectively. Paper 4 shows that the UHPFRC jacketing increased the stiffness and strength of the beams under both static and blast loading, however the high bond capacity of the UHPFRC and relatively low tension steel ratio increased the vulnerability of bar rupture failure. The numerical parametric study investigates the effects of steel ratio and blast load scenario, jacket thickness and interface location on blast performance and failure model. Paper 5 confirms that the blast performance of the beams is influenced by the retrofit type, with optimal performance obtained when using full- or U-jacketing. The efficient use of localized "hinge" retrofits was also found to be effective, and reduced the vulnerability to bar rupture. The numerical parametric study investigates the effects of steel ratio and blast load scenario (single vs. repeated) on the blast performance of the beams. Paper 6, studies the effect of UHPFRC jacketing in columns tested under combined axial and blast loading. The retrofit is shown to increase blast capacity and reduce blast-induced displacements and damage, though the final failure of the columns was governed by bar rupture. As part of the numerical parametric study the effects of axial load ratio, boundary conditions, steel ratio, jacket thickness and jacket design are studied numerically and found to have significant effects on blast behaviour and failure mode.

O presente trabalho avaliou a influência dos concretos de ultra-alto desempenho com fibras (UHPFRC) e sem fibras (UHPC) no reforço de pilares curtos de concreto armado de seção transversal circular e quadrada. Avaliou-se também a adição de armaduras adicionais de reforço e de polímeros reforçados com fibras de carbono (PRFC) em alguns pilares reforçados. Para a avaliação deste novo sistema de reforço optou-se pela realização de um programa experimental e simulações numéricas. É importante ressaltar que no programa experimental, nenhum pilar reforçado possuía seção transversal maior que a seção do pilar de referência. Foi verificado por meio do programa experimental, que as camisas de UHPC apresentaram ruína de natureza frágil e não se recomenda a sua utilização a menos que acompanhada de mecanismos que garantam adequado confinamento do pilar reforçado. Nos pilares circulares e quadrados reforçados com UHPFRC foram verificados, respectivamente, incrementos de resistência de 106,4% e 83,6% onde o concreto do cobrimento foi substituído por UHPFRC, 154,3% e 111,7% onde além da substituição do cobrimento foram inseridas armaduras adicionais e 160% e 85,6% onde houve a colocação de PRFC após a substituição do cobrimento. Todos os pilares reforçados com UHPFRC não apresentaram destacamento da camisa de reforço. Foram realizadas simulações numéricas variando a espessura da camisa de UHPFRC e do número de camadas de PRFC tanto nos pilares de seção circular como nos pilares de seção quadrada. Por meio destas simulações, notou-se que a adição de pequenos incrementos de espessura da camisa de UHPFRC, proporciona elevados incrementos de resistência ao pilar reforçado, ao passo que o aumento do número de camadas de PRFC não influenciaria significantemente no incremento de resistência e sim na ductilidade do conjunto. / The present work evaluated the influence of ultra-high performance concrete with fibers (UHPFRC) and without fibers (UHPC) on the strengthening of short columns of reinforced concrete of circular and square cross section. It was also evaluated the addition of additional reinforcement and carbon fiber reinforced polymers (PRFC) on some strengthened columns. For the evaluation of this new system of strengthening we opted for the realization of an experimental program and numerical simulations. It is important to note that in the experimental program, no strengthened columns had a larger cross section than the reference column section. It was verified through the experimental program that the UHPC shirts presented ruin of a fragile nature and their use is not recommended unless accompanied by mechanisms that guarantee adequate confinement to the strengthened columns. In the circular and square columns strengthened with UHPFRC, respectively, resistance increments of 106.4% and 83.6% were verified, where the cover concrete was replaced by UHPFRC, 154.3% and 111.7%, in addition to the substitution of additional reinforcement were inserted and 160% and 85.6% where PRFC placement was performed after the replacement of the cover. All strengthened columns with UHPFRC did not present detachment of the strengthening jacket. Numerical simulations were performed by varying the thickness of the UHPFRC jacket and the number of PRFC layers on both the circular section columns and the square section columns. Through these simulations, it was noted that the addition of small thickness increments of the UHPFRC jacket would provide high increments of strength to the strengthened columns, while increasing the number of PRFC layers would not significantly influence the increase in strength but rather ductility of the assembly.

O presente trabalho avaliou a influência dos concretos de ultra-alto desempenho com fibras (UHPFRC) e sem fibras (UHPC) no reforço de pilares curtos de concreto armado de seção transversal circular e quadrada. Avaliou-se também a adição de armaduras adicionais de reforço e de polímeros reforçados com fibras de carbono (PRFC) em alguns pilares reforçados. Para a avaliação deste novo sistema de reforço optou-se pela realização de um programa experimental e simulações numéricas. É importante ressaltar que no programa experimental, nenhum pilar reforçado possuía seção transversal maior que a seção do pilar de referência. Foi verificado por meio do programa experimental, que as camisas de UHPC apresentaram ruína de natureza frágil e não se recomenda a sua utilização a menos que acompanhada de mecanismos que garantam adequado confinamento do pilar reforçado. Nos pilares circulares e quadrados reforçados com UHPFRC foram verificados, respectivamente, incrementos de resistência de 106,4% e 83,6% onde o concreto do cobrimento foi substituído por UHPFRC, 154,3% e 111,7% onde além da substituição do cobrimento foram inseridas armaduras adicionais e 160% e 85,6% onde houve a colocação de PRFC após a substituição do cobrimento. Todos os pilares reforçados com UHPFRC não apresentaram destacamento da camisa de reforço. Foram realizadas simulações numéricas variando a espessura da camisa de UHPFRC e do número de camadas de PRFC tanto nos pilares de seção circular como nos pilares de seção quadrada. Por meio destas simulações, notou-se que a adição de pequenos incrementos de espessura da camisa de UHPFRC, proporciona elevados incrementos de resistência ao pilar reforçado, ao passo que o aumento do número de camadas de PRFC não influenciaria significantemente no incremento de resistência e sim na ductilidade do conjunto. / The present work evaluated the influence of ultra-high performance concrete with fibers (UHPFRC) and without fibers (UHPC) on the strengthening of short columns of reinforced concrete of circular and square cross section. It was also evaluated the addition of additional reinforcement and carbon fiber reinforced polymers (PRFC) on some strengthened columns. For the evaluation of this new system of strengthening we opted for the realization of an experimental program and numerical simulations. It is important to note that in the experimental program, no strengthened columns had a larger cross section than the reference column section. It was verified through the experimental program that the UHPC shirts presented ruin of a fragile nature and their use is not recommended unless accompanied by mechanisms that guarantee adequate confinement to the strengthened columns. In the circular and square columns strengthened with UHPFRC, respectively, resistance increments of 106.4% and 83.6% were verified, where the cover concrete was replaced by UHPFRC, 154.3% and 111.7%, in addition to the substitution of additional reinforcement were inserted and 160% and 85.6% where PRFC placement was performed after the replacement of the cover. All strengthened columns with UHPFRC did not present detachment of the strengthening jacket. Numerical simulations were performed by varying the thickness of the UHPFRC jacket and the number of PRFC layers on both the circular section columns and the square section columns. Through these simulations, it was noted that the addition of small thickness increments of the UHPFRC jacket would provide high increments of strength to the strengthened columns, while increasing the number of PRFC layers would not significantly influence the increase in strength but rather ductility of the assembly.

The use of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) in the construction industry started in the 1990s and has since then been used for bridges all over the world. The mechanical properties and the dense matrix result in lower material usage and superior durability compared to conventional concrete, but the implementation of UHPFRC in the Swedish industry has been delayed. The most evident explanation, based on interview with industry representatives, as to why UHPFRC is not commonly used in Sweden are due to the lack of standards and knowledge. UHPFRC also has a high cement content and the cement industry contributes with high carbon dioxide (CO2) emissions to the total CO2 emissions in the world. This MSc Thesis looks into if a UHPFRC bridge is a feasible alternative to a conventional reinforced concrete structure bridge from design and material usage perspectives, regarding reduction of CO2 emissions. The project’s overall goal is to increase the knowledge in Sweden about the material, regarding the production, mechanical properties and behaviour of UHPFRC, and the design, regarding the difference in design between UHPFRC and conventional concrete bridges. To examine the material, a UHPFRC mixture with short straight steel fibres was developed. Specimens were tested to see how the different fibre contents affect the mechanical properties and which fibre content that is most favourable. Three different fibre contents were tested: 1.5%, 2.0% and 2.5% of the total volume of the mixture. The tested and evaluated mechanical properties were workability, flexural strength, tensile strength, fracture energy, compressive strength and modulus of elasticity. This study does not contain tests of durability of UHPFRC, however trough the literature review it was investigated to what extent the fibres affect the durability. It was concluded that an increase in fibre content results in improved mechanical properties, except for workability and in some cases when using a fibre content of 2.5%. The increase in the mechanical properties is due to the increased cracking resistance and the bond strength between the fibres and the matrix. The decrease in the mechanical properties, e.g. characteristic tensile strength and compressive strength of cylinders, for 2.5% in fibre content can be due to uneven fibre distribution and higher amount of air in the specimens which result in less strength. It was concluded that 2.0% in fibre content is most favourable. It was possible to conclude that the degradation of the fibres takes a long time, however not to what extent the fibres will affect the durability. To evaluate if UHPFRC is a viable economical and environmental alternative to regular concrete bridges, three cases of bridge design are considered. Two cases with UHPFRC (different thickness) and one case with conventional concrete. Up to 2017 only technical guidelines and recommendations for design with UHPFRC existed, but in 2017 the first approved standards in the world were published. The French national standards cover material (NF P18-470, 2016) and design (NF P18-710, 2016) and were used for the design process. The material usage regarding the amount of reinforced UHPFRC/concrete and steel reinforcement as well as the amount of CO2 emissions from the production of cement and steel (fibre and steel reinforcement) used for the bridges in the mid-span and at the support were investigated. The design process was also evaluated. It was concluded that the UHPFRC bridge with an optimized thickness was 47% lighter than the conventional concrete bridge, but the amount of CO2 emissions was still higher (e.g. 23% from the support). To be able to determine if a UHPFRC bridge is a feasible alternative to a conventional concrete bridge, with regards to the reduction of CO2 emissions, the CO2 emissions have to be observed in a wider perspective than only from the production of cement and steel, e.g. fewer transports and longer lifetime. / Användningen av ultrahögpresterande fiberbetong (UHPFRC) i anläggningsindustrin började på 1990-talet och har sedan dess använts till broar i hela världen. De mekaniska egenskaperna och den täta UHPFRC matrisen resulterar i lägre materialanvändning och bättre beständighet i jämförelse med konventionell betong, men användningen av UHPFRC har inte slagit igenom i den svenska industrin. De största förklaringarna till varför UHPFRC sällan används i Sverige är för att det inte har funnits kunskap och standarder. UHPFRC har också en hög cementhalt och cementindustrin bidrar med höga koldioxid (CO2) utsläpp till de totala CO2 utsläppen i världen. Den här masteruppsatsen skrevs för att undersöka om en UHPFRC bro är ett möjligt alternativ till en konventionell betongbro ur dimensionering- och materialanvändningssynpunkt med avseende på reduktion av CO2 utsläpp. Projektets övergripande mål är att öka kunskapen om materialet, med avseende på tillverkningen, de mekaniska egenskaperna och beteendet av UHPFRC, och dimensionering, med avseende på skillnaden i dimensionering mellan UHPFRC broar och konventionella betongbroar. I materialdelen utvecklades ett UHPFRC recept med korta raka stålfibrer. Provkroppar testades för att se hur olika fiberinnehåll påverkade de mekaniska egenskaperna och vilket fiberinnehåll som var mest gynnsamt. Tre olika fiberinnehåll testades: 1.5%, 2.0% och 2.5% av total volym av blandningen. De mekaniska egenskaperna som testades och utvärderades var bearbetbarheten, böjhållfasthet, draghållfasthet, fraktur energi, tryckhållfasthet och elasticitetsmodul. Beständigheten av UHPFRC testades aldrig men i vilken omfattning fibrerna påverkar beständigheten undersöktes i den litteraturstudie som skrevs inför testerna och tillverkningen av UHPFRC. Det konstaterades att en ökning i fiberinnehåll resulterade i en ökning av de mekaniska egenskaperna, förutom för bearbetbarheten och i vissa fall när ett fiberinnehåll av 2.5% användes. Ökningen av de mekaniska egenskaperna berodde på det ökande sprickmotståndet och bindningsstyrka mellan fibrerna och matrisen. Minskningen av de mekaniska egenskaperna, till exempel den karakteristiska drag- och tryckhållfastheten, när ett fiberinnehåll på 2.5% i cylindrar användes kan bero på ojämn fiberfördelning och större mängd luft i provkropparna vilket resulterar i lägre hållfasthet. Det konstaterades att ett fiberinnehåll på 2.0% var det mest gynnsamma. Det kunde inte konstateras i vilken omfattning fibrerna påverkar beständigheten men det kunde konstateras att nedbrytningen av fibrerna tar lång tid. I dimensioneringsdelen utformades tre slakarmerade balkbroöverbyggnader, i två fall var överbyggnaden med UHPFRC (olika tjocklekar) och i ett fall var den med konventionell betong. Fram till 2017 fanns det bara tekniska riktlinjer och rekommendationer för UHPFRC men 2017 publicerades de första godkända standarderna i världen. De franska nationella standarderna täcker material (NF P18-470, 2016) och dimensionering (NF P18-710, 2016) och användes vid dimensioneringen. Materialanvändningen med avseende på mängd armerad UHPFRC/betong och slakarmering och mängd CO2 utsläpp från produktionen av cement och stål (fibrer och slakarmering) som användes till broarna i mittenspannet och vid stöden undersöktes. Även dimensioneringsprocessen utvärderades. Det konstaterades att UHPFRC bron med optimerad tjocklek var 47% lättare än betongbron men mängden CO2 utsläpp var fortfarande högre (till exempel 23% högre från stödet). Det konstaterades att om det ska vara möjligt att fastställa att en UHPFRC bro är ett möjligt alternativ till en konventionell betongbro, med avseende på reduktion av CO2 utsläpp, måste CO2 utsläppen ses från ett bredare perspektiv än från bara produktion av cement och stål, till exempel mindre transporter och längre livslängd.

Ausgangspunkt für dieses Anschlussprojekt am Institut für Tragwerksplanung der TU Braunschweig war der Wunsch, die Effektivität des Faseranteils derjenigen Betonbauteile zu erhöhen, die zuvor im SPP-Projekt Entwicklung neuartiger Verbindungen für komplexe Stab-, Flächen- und Raumtragelemente aus UHPFRC (S. 50 ff . in diesem Buch) hergestellt und untersucht wurden. Voruntersuchungen und Versuche zum Thema der magnetische Faserausrichtung in UHPFRC werden am ITE seit 2014 kontinuierlich durchgeführt [1]–[4]. Diese Voruntersuchungen berührten bereits zentrale Aspekte dieses Forschungsvorhabens und lieferten konkrete Hinweise auf die zu erwartenden Ergebnisse zur robotergestützten, magnetischen Ausrichtung und Verteilung der Mikrostahlfasern (MSF). Im Fokus der Forschung standen zum einen die Möglichkeiten der digitalen und robotergestützten Bauteilfertigung und zum anderen das Potenzial der Faserausrichtung zur Steigerung der Materialeffizienz von UHPFRC. In der Entwicklung des Verfahrens der magnetischen Faserausrichtung (MFA) wurden diese beiden Ansätze zusammengeführt. / The starting point for this follow-up project, which was carried out at the Institute of Structural Design at the Technical University of Braunschweig, was the desire to increase the effectiveness of the fibre content of the type of concrete components that were previously manufactured and investigated in the SPP project Development of novel jointing systems for complex beam surface and spatial elements made of UHPFRC (p. 50 et seq. in this book). Preliminary investigations and tests on the topic of magnetic fiber alignment in UHPFRC have been carried out continuously at ITE since 2014 [1]–[4]. These preliminary investigations already touched upon central aspects of this research project and provided concrete indications of the expected fi ndings on robot-assisted magnetic alignment and distribution of the micro steel fi bres (MSF). The research focused on the possibilities of digital and robot based component production on the one hand and the potential of fibre orientation to increase the material efficiency of UHPFRC on the other. In the development of the magnetic fibre alignment (MFA) process, these two approaches were brought together.

When compared to traditional concrete, steel fibre reinforced concrete (SFRC) shows several enhancements in performance, including improved tensile resistance, toughness and ductility. One potential application for SFRC is in columns where the provision of steel fibres can improve performance under axial and lateral loads. The use of SFRC can also allow for partial replacement of transverse reinforcement required by modern seismic codes. To improve workability, self-consolidating concrete (SCC) can be combined with steel fibres, leading to highly workable SFRC suitable for structural applications. Recent advances in material science have also led to the development of ultra-high performance fibre reinforced concretes (UHPFRC), a material which exhibits very high compressive strength, enhanced post-cracking resistance and high damage tolerance. In heavily loaded ground-story columns, the use of UHPFRC can allow for reduced column sections. This thesis presents the results from a comprehensive research program conducted to study the axial behaviour of columns constructed with highly workable SFRC and UHPFRC. As part of the experimental program, twenty-three full-scale columns were tested under pure axial compressive loading. In the case of the SFRC columns, columns having rectangular section and constructed with SCC and steel fibres were tested, with variables including fibre content and spacing of transverse reinforcement. The results confirm that use of fibres results in improved column behaviour due to enhancements in core confinement and cover behaviour. Furthermore, the results demonstrate that the provision of steel fibres in columns can allow for partial replacement of transverse reinforcement required by modern codes. The analytical investigation indicates that confinement models proposed by other researchers for traditional RC and SFRC can predict the response of columns constructed with SCC and highly workable SFRC. In the case of the UHPFRC columns, variables included configuration and spacing of transverse reinforcement. The results demonstrate that the use of appropriate detailing in UHPFRC columns can result in suitable ductility. Furthermore, the results demonstrate the improved damage tolerance of UHPFRC when compared to traditional high-strength concrete. The analytical investigation demonstrates the need for development of confinement models specific for UHPFRC.

L'étude du comportement sous incendie des bétons a particulièrement pris son essor depuis des incendies répétés de tunnels comme celui sous la Manche ou du Mont-Blanc. La problématique de résistance au feu est essentielle puisque les éclatements qui peuvent accompagner la diminution de résistance du matériau sont de nature à fragiliser fortement les structures. Depuis plusieurs décennies, le domaine de la résistance au feu des bétons ordinaires et à hautes performances a été exploré tant expérimentalement que théoriquement. Cependant il existe des matériaux plus récents, que sont les bétons fibrés à ultra performances (BFUP), pour lesquels le phénomène d'instabilité thermique est accentué et dont le comportement mécanique à chaud n'a pas été beaucoup exploré. L'étude menée ici a pour vocation de s'intéresser à un BFUP particulier, le BCV®, fourni par l'entreprise Vicat. Les résultats exposés permettent de mieux comprendre l'efficacité des fibres de polypropylène face aux autres fibres synthétiques, grâce à une approche expérimentale originale complétée par une étude à l'échelle microscopique. Cette dernière est constituée de nombreuses analyses au microscope électronique à balayage ainsi que d'études de porosimétrie au mercure. Cette première phase est suivie d'un vaste programme de caractérisations mécaniques à chaud sur le BCV® à des températures variant de 20°C à 1100°C, essais peu courants pour ce genre de matériau. Ce travail contribue à la compréhension du comportement des BFUP à haute température, et montre que l'évolution avec la température de leurs caractéristiques mécaniques principales (résistance en flexion, résistance en compression et module d'élasticité) est assez semblable à celle des bétons ordinaires. Les résultats de cette étude contribuent à une meilleures connaissance du comportement à haute température des bétons fibrés à ultra performances. / Study of concrete behaviour under fire has really increased since the occurence of several fires in tunnels such as the Chunnel or the Mont-Blanc tunnel. Fire resistance of concrete is a very important subject because the possible spalling coupled with a decrease of the strength of the material can deeply weaken the structures. For decades the fire resistance field of ordinary concrete and high performance concrete was studied experimentally and theorically. However more recent materials such as ultra-high performance fibre reinforced concrete (UHPFRC), for which spalling phenomenon is emphasized and whose high temperature mechanical behaviour has not been studied so much. The present study is aimed to study a particular UHPFRC, the BCV®, supplied by Vicat. The results enable to better understand the effenciency of polypropylene fibres compared to other synthetic fibres, thanks to an original experimental approach coupled with a study at the microscopic scale. The latter consists in various scanning electron microscope analysis and mercury intrusion porosimetry. This first part is followed by a wide number of experimental results which characterize the mechanical behaviour of BCV® for a range of temperatures from 20°C to 1100°C. Experiments in such testing conditions are not common for this material. The work achieved contributes to a better understanding of the high performance behaviour of UHPFRC. It reveals also that the evolution with temperature of main mechanical characteristics (bending strength, compression strength and Young Modulus) is really close to those of ordinary concretes. The results of this study contributes to a better knowledge of ultra-high performance fibre reinforced concrete under high temperature.

Dans le cadre de l’utilisation de plus en plus courante des BFUP dans des structures techniquement ou architecturalement complexes et de la recherche d’optimisation de la quantité de matériau mis en œuvre, cette thèse s’intéresse au fonctionnement des jonctions en BFUP. Son objectif principal est de mettre en évidence les phénomènes physiques qui interviennent dans la jonction et de mieux les retranscrire dans les méthodes de calcul afin de mieux en maîtriser la sécurité et d’optimiser la matière mise en oeuvre.Le travail de thèse s’articule en quatre étapes :- Une étude bibliographique a permis d’établir les bases de connaissances permettant de dimensionner les campagnes expérimentales et de proposer une structure de modélisation des jonctions âme-membrure.- Le développement d’un modèle analytique avancé permettant de prédire le comportement d’une poutre en Té soumise à une sollicitation combinée de flexion longitudinale (et donc de cisaillement longitudinal dans la table) et de flexion transversale.- L’étude expérimentale, à l’échelle du matériau, du comportement des BFUP sous sollicitation de cisaillement pur. Cette étude permet d’enrichir les connaissances sur le comportement des BFUP (données encore indisponibles dans la carte d’identité des différents matériaux) et constitue également une donnée d’entrée pour la compréhension quantitative des phénomènes qui interviennent dans la jonction, à l’échelle de la structure, lorsqu’elle est sollicitée en cisaillement longitudinal.- Enfin, l’essai à rupture de 6 poutres en Té à l’échelle 1 a permis d’étudier le comportement expérimental des jonctions soumises à des sollicitations de cisaillement longitudinal. Les résultats expérimentaux de cette étude ont été comparés aux différentes prédictions analytiques possibles, y compris le modèle proposé.Ces travaux de thèse ont permis d’apporter de nouveaux outils de dimensionnement (notamment dans le cadre de l’étude de la résistance des jonctions en BFUP sous sollicitation de cisaillement pur). Ils mériteraient d’être complétés par des essais de poutres soumis à une sollicitation concomitante de flexion longitudinale et de flexion transversale, pour mieux conforter les méhodes d’analyse proposées / Ultra-high performance fiber-reinforced concrete (UHPFRC) are increasingly used for technically or architecturally complex structures and research is going on to optimize design and save implemented material quantities. In this context, this thesis focuses on the mechanical behavior of junctions in UHPFRC structures. The main goal is to highlight physical phenomena occurring in the junctions in order to optimally transpose them into calculation methods. In this way, both safety and cost-efficience should be better controlled.This thesis is divided in four steps :- A literature review enabled to establish the background knowledge allowing to design the experimental campaigns and to suggest a modeling for web-flange junctions.- An advanced analytical model was developed in order to predict the behavior of a T-beam subjected to combined longitudinal bending (and therefore longitudinal shear in the table) and transverse bending.- The expérimental study, at the material scale, of UHPFRC behavior under pure shear stress, allowed enriching the knowledge in this field (data still unavailable in the identity card of current UHPFRC mixes). It also constituted an input for the quantitative understanding of the phenomena occuring in the junction, at the structure scale, when it is loaded in longitudinal shear.- Finally, the test of 6 full scale T-beams up to failure made it possible to study the experimental behavior of the junctions subjected to longitudinal shear stresses. The experimental results were compared with the various analytical predictions, including the proposed model.A new approch has thus been proposed to design junctions under longitudinal shear. It should be completed with experiments on T-beams under combined longitudinal and transverse bending to further validate the proposed analysis methods