For the seismic assessment of existing RC buildings an important question is the reliable appraisal of the in situ concrete strength. This parameter can be performed by resorting to destructive tests (concrete core drilling) properly combined with qualitative information provided by Non-Destructive tests (NDTs). However, there are many factors that influence the results of the experimental tests, and it is difficult to establish whether the value obtained is really representative of the material in situ and if there are alterations which have arisen in time. In the paper, a methodology aimed at the definition of a coefficient called “C_DD” is proposed. This coefficient takes into account the effects of the deterioration and alteration of the drilled core, and provides an ideal percentage decrease of the mechanical strength (ΔR) of the concrete core (with respect to the original one), as a function of the compaction degree g_c. The proposed methodology was applied to 2 set of samples, including drilled concrete cores and cubic specimens, respectively coming from an existing and a brand new building. The method allows to appraise and critically compare the difference between the strength obtained from drilling tests and the one measured on the specimens sampled during the casting.