Résumé Cet article rapporte les résultats d'une recherche expérimentale entreprise en vue de déterminer si des fibres métalliques incorporées au béton peuvent contribuer à réduire les flèches en service d'éléments structurels en béton armé. L'alternative pour la réduction de ces flèches réside dans l'emploi de béton à hautes performances. Trois séries de trois poutres ont été essayées. Les séries diffèrent par le pourcentage d'armature ordinaire et chaque série comprend une poutre de référence en béton normal, une poutre en béton de fibres (dosées à 0,4% en volume) et une poutre en béton à hautes performances (60 MPa). A pourcentage d'armature ordinaire égal, il apparaît que l'emploi de béton à hautes performances est plus efficace que celui de béton de fibres pour réduire les déformations du béton armé sous charge de service. A la ruine, on observe également un meilleur comportement des poutres en béton à hautes performances qui se sont avérées plus ductiles que les poutres en béton armé normal; la ductilité des poutres en béton de fibres était par contre paradoxalement beaucoup plus basse que la ductilité des poutres en béton armé normal.

Summary This paper reports the results of experimental research focusing on two possible methods to enhance the service load behaviour of ordinary reinforced concrete structural elements, i.e. the use of steel-fibre concrete and the use of high-strength concrete. The programme involved the testing of nine reinforced concrete beams with rectangular cross-section (b=25 cm, h=15 cm) and span L=140 cm. The reinforcement ratios considered (ρ=0.33, 0.52 and 0.75%) are usual for slabs, for which the deflection limit state may often become a design factor. For each reinforcement ratio, we tested a beam in ordinary concrete (BN; 32 MPa), a beam in high-strength concrete (BHP; 60 MPa) and a beam in fibre-reinforced concrete (BF; 38 MPa; fibre content 0.4 vol%). We also tested a fibre-reinforced concrete beam with no bar reinforcement (BRF; ρ=0) to determine the material properties of fibre concrete that we needed to compute the carrying capacity of the BF beams. Test results show that the use of high-strength concrete seems more effective than the use of fibre concrete to reduce the deflections at service load level. BF beams are slightly stiffer than the BHP beams for higher loading levels (stabilized cracking phase of BN and BHP beams) for the two lowest reinforcement ratios. The carrying capacity of the BF beams is greater than the carrying capacity of the BN beams; the difference in carrying capacity may be estimated on the safe side with a simple prediction model, and is significant (experimentally 32%) for the lowest reinforcement ratio. However, the ultimate load behaviour of the BF beams is far less satisfactory than the ultimate load behaviour of the BN and BHP beams. For the BF beams, we observed the concentration of all inelastic deformations at failure in a single cracked section and the ductility factor varied between 1 and 2.8 compared with 4.5–6.3 for the BN beams and 8.7–10.6 for the BHP beams. The ultimate load behaviour of the BHP beams was quite satisfactory, with the largest number of cracks equally spaced and equally open and the largest deformation capacity. We conclude from these tests that the tensile properties of cracked fibre-reinforced concrete are too limited to reach the level of deformation that is required in safe reinforced concrete design and that the use of steel fibres as complementary reinforcement of rebars should be avoided.