This paper deals with the effect of loading on the microstructure of reinforced high performance and normal concrete. In the case of reinforced concrete structures, cracking is the only visible sign of mechanical degradation and is accompanied by microscopic damage to the concrete. To account for this damage, experiments on three different concretes with compressive strengths (f c28) of 45 MPa, 80 MPa and 100 MPa were carried out. The loadless initial state of concrete and the damage state after loading were described using the single replica technique observed on SEM. The way in which the microcracks extended was revealed, with predominantly aggregated bond failure and crack branching occurring with cracks spreading out around the coarse aggregate under stable progressive fracture conditions. An increase in the specific area of microcracks was seen to result from an increase in load. This phenomenon could have an influence on the durability properties of reinforced concrete structures.