The real mechanical properties of concrete play an important role in the structural analysis of the behaviour of precast, thin-walled elements, especially those without passive reinforcement. The experimental research of pre-tensioned hollow-core slabs has shown that the actual strength and elasticity of concrete might be about 30 to 50 % lower than the so-called ”standard” properties assigned to a specific class of concrete. Reasons for the strength properties reduction in the narrow ribs of slabs with very heterogeneous structure subjected to the influence of interrelated hydrothermal fields, during the process of casting, are given in the paper.