Une étude sur 27 essais de tube en acier remplis de béton a été effectuée. Les paramè-tres, résistance du béton, rapport diamètre sur épaisseur du tube et nuance d'acier sont traités. La relation charge déformation et la variation du volume dV/V ont montré qu'à partir de 70% de la charge ultime, la déformation du composite se fait à volume croissant ce qui indique un changement de la structure du béton par la propagation de la microfissuration. Son coefficient de Poisson devient supérieur à 0,5 créant ainsi un état de confinement du noyau de béton. A la limite de ruine la déformation transversale dans le tube d'acier devient égale à celle longitudinale. Selon le critère de VonMises ceci ne peut être négligé dans le calcul de la résistance du composite ; l'EC4 donne une meilleure approche que l'ACI-318 qui néglige cet effet.A set of 27 tests was carried out on a concrete-filled steel tube column. The strength parameters of concrete, diameter-thickness ratio and steel grade were treated. The load strain relationship as well as the variation of volume dV/V showed that at 70% of failure load, the strain in the composite is continuous with increase in volume which indicates change of structure of concrete by propagation of micro-cracking. Poisson's coefficient of concrete becomes higher than 0,5 which produces confinement of the concrete core. At failure load, transversal strain in the steel tube becomes equal to longitudinal strain. According to the von Mises yield criterion this effect cannot be neglected in the strength calculation of composite; EC4 gives a better approach than ACI-318 which neglected this effect.

The reuse of concrete waste as a secondary aggregate could be an efficient solution for sustainable development and long-term environmental protection. However, the variable quality of waste concrete, especially with various compressive strengths, can have a negative effect on the final compressive strength of recycled concrete. In this approach, the major goal of this research is to study the effect of parent concrete qualities on the performance of recycled concrete. To accomplish this task, three grades of different compressive strengths (10 to 15) MPa, (20 to 25) MPa, and (30 to 40) MPa have been analyzed in an experimental test program, in which an unknown compressive strength is introduced as well. The experimental mix use 40% of secondary aggregates (both course and fine) and 60% of natural aggregates. This led to the decreasing of the compressive strength of the test concrete between 14% and 23.7% compared to the normal concrete. This loss was improved by adding an amount of cement equivalent to 4% of the weight of the recycled aggregate used. The achieved results prove that the strength properties of the parent concrete have a limited effect on the compressive strength of the recycled concrete. Additionally, low compressive strength parent concrete, when crushed, generates a high amount of fine aggregate and large percentage of recycled coarse aggregates with less attached mortar, and presents the same compressive strength as an excellent parent concrete.

For more than two decades, the construction technique using concrete filled steel tube (CFST) has been widespread throughout the world. Indeed, it has been demonstrated that the use of normal or high strength concrete, confined in a steel tube of circular shape can considerably improve its ductility as well as its load capacity, owing to the combination of the qualities of the two constituent materials; these tubes have an effortless execution, indeed, the concrete used in the CFST does not require formwork nor reinforcement, a durability of the two materials as well as a good behavior to fire, which was the effect desired at the origin of their elaboration. In this paper, we study the axial compression behavior of short circular steel tubes filled with concrete; their modelling will be performed using the ABAQUS/Standard calculation program. In order to accurately determine their behavior, we have created different models. Indeed, these tubes will be modeled in order to simulate different plastic state behaviours, namely a perfect elasto-plastic state, an elasto-plastic state with multilinear strain hardening and a third elasto-plastic behavior with strain hardening proposed by Tao et al. The tested columns consist of circular hollow sections which are designated in the literature as Concrete Filled Steel Tube (CFST), for which we vary the diameters, heights as well as the wall thicknesses, and which we fill with concrete of different qualities. The compressive behavior, including ultimate loads, confinement, load-deflection relationship and failure modes, was obtained from numerical models and compared with experimental and theoretical results based on Eurocode 4. All these results showed a good agreement and a satisfactory correlation, allowing us to assume that a correct modelling can be sufficient to simulate the behavior of CFST.