The confinement of concrete columns provided by carbon fibre reinforced plastic (CFRP) sheets can be an efficient technique for their structural strengthening. The principal advantages of this technique are the high strength-to-weight ratio, good fatigue properties, noncorroding characteristics of the CFRP, and the facility of its application.An experimental research program, that included tests on 54 short column specimens, was carried out to investigate the gain in strength and ductility of concrete columns externally confined by CFRP wrapping. The variables studied were the column cross section shape (circular, square and rectangular) and the amount of confinement expressed in the number of CFRP sheet layers applied to the models (one or two layers).On the basis of the obtained results, equations were proposed to calcuhte the confined concrete strength and the ultimate confined concrete strain as a function of the confining lateral stress for each of the cross section geometry used, circular, square and rectangular. The estimations given by these equations and by those from formulas encountered in the literature were compared with the experimental ones and general conclusions were, finally, drawn. RESUME

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A B S T R A C T R i~ S U M i~Although the great advances in concrete technology have led to the possibility of obtaining ready-mix concrete with compressive strength around 100 MPa, some national and international codes for concrete structures do not cover concrete strengths above 50 MPa. Many codes are under revision, but some of them (including the Brazilian Code) will still not include high strength concretes.Due to the different characteristics of higher strength concrete some design procedures traditionally used in normal strength concrete structures have to be changed.Different types of stress-strain relationships for concrete have been proposed for the non-linear analysis of member behavior and for the ultimate state analysis of high strength concrete elements under combined flexure and axial load.In this work comparisons are made between proposed stress-strain curves and between the axial load-moment interaction diagrams based on these curves. Comparisons of test results with these diagrams, for columns subjected to eccentric compression, give an idea of the different degrees of safety obtained using those curves. Malgr~ les grands progr~s de la

R SUMMinimum reinforcement is provided in concrete beams in order to improve their behaviour towards cracking and ductility at failure.Generally, codes of practice equations for the minimum steel ratios, longitudinal and transversal, are mainly empirical and do not include all the influential parameters in them. For this reason and due to the fact that they do lack of a theoretical background, different codes can give values for the minimum steel ratios that greatly differs from one another. Also the validity of these equations may be questioned particularly in the case of high strength concrete beams and prestressed concrete beams for which limited test data are available.In this work, a theoretical approach for the minimum steel ratios that are required for the ductile behaviour at failure in bending, shear and torsion, in concrete beams made of concrete with different strengths is presented. Comparisons are also made between the proposed expressions, the codes expressions and available test results. Un renforcement minimum des poutres en b~ton arm~ est pr&u pour am(liorer leur comportement en cas de fissuration et leur ductilit~ dans la rupture. En g&&al les formules des normes techniques pour tes pourcenrages minimum des armatures longitudinales et transversales sont simplement empiriques et ne contiennent pas tous les param~tres influents. Pour cette raison et comme elles ne prennent pas en consid&ation des fondements th&riques, les diff&ents r~glements donnent des valeurs trbs diff&entes pour les pourcentages minimum d'acier. La validit~ de ces formules peut &e raise en doute particuli}rement clans les cas de poutres en b~ton h haute r&istance et des poutres pr~-contraintes pour lesquelles seuIs des r&ultats d' essais tr}s

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