The research has been undertaken to determine the strength and quality of concrete material used to build various types of structures in various localities in the city of Istanbul, located within the first-degree disastrous area for a potential earthquake risk. Concrete core samples collected from a total of 244 structures (buildings, industrial, and governmental structures), for which the construction details, such as the production type (cast in place and/or ready mixed) and class of concrete material are known, have been tested for their compressive strength. Statistical analyses of the collected data show that the tested core samples have equivalent specified strength of between 1.61 and 70.05 N/mm(2). The mean value for a total of 1,892 characteristic strength values has been found to be 22.01 N/mm(2) with a standard deviation value of 9.99 N/mm(2). The statistical results obtained using the production type and class (C14, C16, C18, C20, C25, and C30) of concrete indicate that the strength quality of both cast-in-place and ready-mixed concrete is, in general, quite far away from meeting the required concrete strength. Although ready-mixed concrete gives better values, the results show that approximately 42% of all the analyzed structures are below the quality required to build the structures safely.
