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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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