Determination of the compressive strength of concrete in existing reinforced concrete structures is important, particularly in some cases. Destructive and non-destructive test methods are used to determine the compressive strength in such structures. Amongst these, coring is the most widely used method as a destructive method in determining the compressive strength in existing reinforced concrete structures. However, even though numerous studies have been carried out on the variation of the determined compressive strength depending on the coring direction, the discussions continue. In this study, concrete blocks containing fly ash (FA) and silica fume (SF) and aggregates of different maximum sizes were produced. In the mixtures, cement was replaced by fly ash at ratios of 20%, 40%, and 60% and silica fume at ratios of 5%, 10%, and 15%, respectively. Two aggregates with the maximum aggregate sizes of 16 mm and 31.5 mm were used in the production of the concrete blocks. The concrete blocks were kept in a laboratory by covering them with burlaps moistened intermittently for 28 days and then cores of 10 cm diameter were taken and cut to have 20 cm height then capped and tested to determine compressive strength. Core drilling was carried out parallel and perpendicular to the casting direction and then the compressive strengths were determined. Cubes of 15 cm were also prepared and tested to determine the compressive strength level of concrete and to make comparisons accordingly. The compressive strength of mineral-added core samples taken parallel to the casting direction is higher than those of taken perpendicular to the casting direction, in all mixtures. The ANOVA test applied on the results obtained, it was found that the maximum aggregate size (Dmax) and the core drilling direction with respect to casting direction is statistically significant in terms of the compressive strength of concrete produced using fly ash and silica fume at certain substitution ratios.