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An original concept is presented for the durable rehabilitation and strengthening of concrete structures. The main idea is to use Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) complemented with steel reinforcing bars to "harden" and strengthen those zones of the structure that are exposed to severe environmental influences and high mechanical loading. This concept combines efficiently protection and resistance properties of UHPFRC and significantly improves the structural performance of the rehabilitated concrete structure in terms of durability. The concept has been validated by means of field applications demonstrating that the technology of UHPFRC is mature for cast in-situ and prefabrication using standard equipment for concrete manufacturing. This novel technology is a step forward towards more sustainable structures.This keynote paper presents an original concept for the rehabilitation and strengthening of concrete structures. The concept is described and some scientific background regarding the structural behaviour of RC elements strengthened with UHPFRC is given. Finally, this novel technology is validated by means of applications. This paper summarizes more than 12 years of intensive research and development of a novel technology to improve concrete structures.

Structural elements combining Ultra-High Performance Fiber Reinforced Concretes (UHPFRC) and concrete offer a high potential in view of rehabilitation and modification of existing structures. The investigation of the time-dependent behavior of composite "UHPFRC-concrete" elements is a fundamental step in the determination of durability and serviceability. For this, an experimental program was conducted on large composite "UHPFRC-concrete" beams and a numerical model was validated with the test results. The experimental results and a parametric study performed with the numerical model showed that UHPFRC and normal strength reinforced concrete are compatible in the long-term and that the critical period of composite "UHPFRC-concrete" elements are the first 90 days after the casting of the UHPFRC layer. Thus, the high potential of such composite elements can be exploited also in the long term. Résumé Leséléments structuraux composés de Béton Fibré Ultra Performant (BFUP) et de béton armé offrent un grand potentiel dans la réhabilitation etla modification des structures existantes. L'étude du comportement différé de telséléments composés est indispensable pour déterminer leur durabilité et leur aptitude au service. Une campagne expérimentale aété réalisée sur des poutres mixtes BFUP -béton armé, et un modèle numérique aété validé avec les résultats d'essais. Ces résultats et uneétude numérique avec le modèle numérique montrent que le BFUP et le béton armé sont compatibles sur le bilan déformationnelà long terme et que la période critique deséléments mixtes est les premiers 90 jours après le bétonnage de la couche de BFUP. Ainsi, le potentielélevé de telś eléments mixtes peutêtre exploitéà long terme.

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a b s t r a c tUltra-High Performance Fibre Reinforced Concrete (UHPFRC) is charact erized by a unique combination of extremely low permeability, high strength and deformability. Extensive R&D works and applications over the last 10 years have demonstrated that cast on site UHPFRC is a fast, efficient and price competitive method for the repair/re habilitation of existing structures. More recently, an original concept of ECO-UHPFRC with a high dosage of mineral addition, a low clinker content, and a majority of local components has been applied successfully for the reh abilitation of a bridge in Slovenia. The objective of the present study is to evaluate the global warming impact of bridge rehabilitations with different types of UHPFRC and to compare them to more standard solutions, both on the basis of the bridge rehabilitation performed in Slovenia. Life Cycle Assessment (LCA) methodology is used. The analysis shows that rehabilitations with UHPFRC, and even more ECO-UHPFRC, have a lower impact than traditional methods over the life cycle.

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Ultra High Performance Fiber Reinforced Concretes (UHPFRC) present outstanding mechanical properties and a very low permeability. Those characteristics make them very attractive for the rehabilitation of existing structures and the conception of new structures. To define the range of admissible tensile deformation in those materials, the influence of imposed tensile deformation and subsequent cracking on permeability and absorption was studied. The transport properties of water and glycol were assessed in order to estimate the effect of the interaction of water with a specific UHPFRC. The experimental results demonstrate that permeability and absorption increase steadily until a residual tensile deformation of 0.13% is reached in the material, then water seeping rises distinctly. During experiments, the interaction of water with the UHPFRC decreases by 1 to 3 orders of magnitude the permeability and reduces absorption by approximately 50 to 85%. Test results reveal the high capability of the material to seal cracks and improve its water-tightness with time.