The research project concerning the testing method of the Compressive Strength on the Stack has been finished in 2006. But there is no accepted reference material for calibrating the apparatus for testing of mechanical and  physical properties of aggregates. This could lead to a lack of precision or even misinterpretations due to petrographic or geometrical inhomogeneity’s of the laboratory samples, of the test portions or of the test specimens for determining the bias of the procedure.

It is therefore necessary to have recourse to reference material with well-defined characteristics for making precise and reliable determinations of the repeatability and reproductibilty of any testing method for determing the physical and the mechanical properties of aggregates.