"vibrocompressed concrete" has a density of 2000 – 2200 kg/mc, which can be reduced, using expanded clay, up to 1500 kg/mc, and even Less.
It is a very different product from the "cast concrete" used in Constructions. The vibrations and compressions applied in production cannot easily be reproduced on a "cube", so there is no mention of "rck of concrete" but of "resistance of the artifact".