The strength of any building mainly depends on the quality of the materials used. Concrete is a macro content. Even though we have tested its micro contents such as sand, cement, blue metal, and water. The mixing method and proportioning of the micro-ingredients may affect the strength of the concrete.
Overall Strength of a structure such as flexural resistance and abrasion directly depends upon the compressive strength of concrete. According to Wikipedia, Compressive Strength of concrete is defined as the Characteristic strength of 150mm size concrete cubes tested at 28 days.