The formulation of high-performance concretes is deduced from that of ordinary concretes which are a mixture in the appropriate proportions of binder, aggregates, water and optionally, admixtures. These types of concrete composed of ultrafine materials, including metakaolin in the case of this study, are characterized by high mechanical resistance and good workability due to the addition of a water-reducing superplasticizer. However, for a successful formulation, materials must be carefully studied to ensure their use. This research project aims to characterize all the materials used in the composition of high-performance concrete to serve as a reference in other similar work in Chad. These are the physical, chemical and mechanical characteristics of materials including cement, gravel, sand, superplasticizer and metakaolin, coming from various locations in Chad. After analysis, the characteristics of these materials are satisfactory and comply with international standards and hence favorable for the composition of high-performance and sustainable concretes.

This paper focuses on the improvement of the nonlinear fatigue law of B. Tikri from 2011, which was born from the old laws of Chaboche according to the Basquin model. Several parameters can influence the life of structures in service. It is a question in this work to take into account the influence of the residual stress on the damage law proposed in the literature. Two values of stress relaxation coefficients of 0.5 and 1 have been considered to test the proposed model with two different overload ratios in order to evaluate the influence of overload on steels used in the automotive industry. The ratios were 1.4 and 2.3. Two findings were made, the first is that when the relaxation coefficient is 0.5 the residual stress improves the life but it is far from the experimental case contrary to the case of relaxation coefficient equal to the unit. For the second case, the model is much more satisfactory for overloads of ratio 1.4 than for overloads of ratio 2.3 for HE360D materials. For future designs, the consideration of its residual stresses over the life of parts or structures in service is paramount.

This study is interested in the connection between steel and concrete with recycled gravel. The work is carried out in the civil engineering laboratory of the Ecole Nationale Supérieure des Travaux Publics ENSTP and then completed in the mechanical engineering laboratory of the Institut National Supérieur des Sciences et Techniques d’Abeché INSTA. Two types of concrete are formulated, one with 100% ordinary crushed gravel and the other with 100% recycled gravel. In terms of compressive strength, the present study with 100% recycled gravel varies from 19 to 32 MPa at 28 days for the different dosages. And the concrete with ordinary crushed gravel offers a strength ranging from 22 to 37 MPa. Regarding steel concrete bonding, several types of reinforced concrete specimens are manufactured by anchoring a steel bar in the concrete. These specimens are subjected to the test of extracting the steel bar from the concrete. The values of the bond stress at rupture are calculated according to the formulas given by the Euro code, the BAEL, and then calculated experimentally. These values vary according to the compressive strength of the concrete.