On the long–term strength losses of silica–fume high–strength concretes

This Paper presents studies and tests performed at the Laboratoire Central des Ponts et Chaussées with a view to a better understanding of the long–term mechanical properties of silica–fume high–strength concretes (HSC) and very–high–strength (VHSC) concretes. After noting the discrepancies among the various references in the literature, we propose to explain the drop in strength observed by some authors after three months by the hypothesis of dryig–related effects. To check the soundness of this hypothesis, mechanical tests were carried out on three different concretes, their drying was investigated by gammadensime try, and accelerated tests were performed on mortars. The homogeneous drying to which the specimens are subjected has no negative effect on their mechanical strengths. The losses of strength observed on some concretes are therefore probably not caused by a local effect of the drying. Examination of the water content fields of VHSC specimens four years old shows that the drying occurs with a high gradient and affects only a small thickness. Mechanical tests in this case show a non–negligible drop in compressive strength. Calculation of the self–stress field due to drying shrinkage indicates that the compression at the core of the specimen is of the same order as the drop in strength observed. To conclude, it is shown that, given some hypotheses concerning the water content field, the drop in strength linked to the structural effects of drying can be not more than twice the tensile strength.