Quality control processes require accurate knowledge of in situ compressive strength of concrete. The most widely accepted method of determining the compressive strength of concrete in existing structures is to test core specimens drilled from hardened concrete. The strength level is an appropriate criterion because it is a decisive factor in determining the quality of concrete, and in addition, other most significant properties of the concrete can be related to its compressive strength. According to the widely accepted international standards and recommendations, the minimum core diameter is generally fixed at 100 mm. However, drilling such rather large size cores may prove hazardous in many cases. In this regard, estimation of concrete strength in structures may be gained from compression tests conducted on cores having a diameter considerably less than the recommended one of 100 mm. As it is not always safe to drill cores of 100 mm, utilizing smaller diameters is more practical. Microcores can be drilled in situ with very light equipment, resulting in greatly reduced costs and less damage to the structure. The statistical characteristics of microcores on the other hand are not understood completely.