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Environmentally friendly FRP composites, made of natural fibres and bio-based polymer matrices, may be used as externally bonded reinforcement for civil structures or buildings subjected to moderate outdoor conditions, in replacement of traditional carbon/epoxy systems. However, a major drawback of natural fibers is their sensitivity to moisture, which can affect both the mechanical properties of FRP composites and their adhesive bond with concrete. This research, funded by the French National Research Agency (ANR Project MICRO), aims at studying the influence of hygrothermal ageing on the performances of “green composites” manufactured by hand lay-up process using unidirectional flax fabrics and a bio-based epoxy matrix. The test program consists in subjecting FRP laminates and FRP strengthened concrete slabs to accelerated ageing conditions under various combinations of temperature and humidity. Aged laminates are then periodically characterized by tensile tests and interlaminar shear tests, while the bond properties of concrete/composite assemblies are assessed by pull-off tests. This paper presents the first results of this ongoing program which is scheduled over a period of 2 years. Results are discussed in the light of complementary investigations (water sorption behaviour, microscopic observations and evaluation of the glass transition temperature by differential scanning calorimetry – DSC) in order to relate observed performance evolutions to actual microstructural changes or damage processes taking place in the material.

In a previous study [6], we have presented an original experimental procedure for results expansion alcali aggregate reaction. Our main goal is to identify the influence of the confinement by composite materials on alkali-aggregate expansion.In this paper, we are going to present a serie of compressive tests that were carried out in order to investigate the mechanical behaviour during alkali aggregate attack at different ageing periods. We also evaluate the influence of composite material confinement on the mechanical characteristics of these concretes. This will be demonstrated in this second part.The specimens used for expansion were loaded in compression until they fractured at four successive stages in the ageing process.Our results show a significant increasein the mechanical characteristics of the confined alkali reactive specimens in comparison withthe non confined specimens: the confined specimens not only withstand compressive stresses four to five times as high as nonconfined ones, but their stress/strain curves follow a 'linear evolution' whereas non-confined specimens seem to behave like powdery materials. Our experiments show that the increase in strength is much more I. Mohamed · S. Ronel · L. Curtil Laboratoire Mécanique Matériaux Structure (L2MS) -Université Claude Bernard Lyon I, 82, bd Niels Bohr Domaine Scientifique de la Doua, 69622 Villeurbanne Cedex, France important with this kind of alkali aggregate concrete confined by composite materials. Résumé Dans une première partie [6], nous avons exposé nos travaux sur un protocole expérimental original qui nous a permis d'obtenir des résultats d'expansion d'un béton alcali réactif, tout en identifiant l'influence d'un confinement par matériaux composites sur le gonflement dûà l'alcali-réaction.En parallèle, nous avons mené une campagne d'essais de compression afin d'étudier le comportement mécanique d'un béton alcali-réactifà différents stades de vieillissement et surtout d'évaluer l'influence des matériaux composites sur les caractéristiques mécaniques de tels bétons. Ceci fait l'objet de cette deuxième partie. Leséprouvettes ayant servi pour l'expansion ontété chargées en compression jusqu'à ruptureà quatreétapes temporelles successives. Nos résultats montrent un gain notable sur les caractéristiques mécaniques deséprouvettes alcaliréactives confinées par rapportà celles non confinées : non seulement, les confinées supportent des contraintes de compression 4à 5 fois supérieuresà celles ne l'étant pas, mais leurs courbes contraintesdéformations suivent une "évolution linéaire" alors que les non confinées semblent se comporter comme un matériau pulvérulent. Le gain en résistance est important et l'apport du confinement par matériaux composites est net.