It has been generally accepted that the tensile Strength and elastic modulus of concrete is proportional to the square-root of its compressive Strength. This relationship, however, may not be applicable for high-performance concrete. The study presents data on Strength and stiffness of concretes containing a laboratory produced metakaolin and commercial silica fume as cement replacement materials, with water-to-cementitious materials ratio of 0.27 to 0.33. Approximately 750 specimens were tested and compressive Strength of up to 110 MPa at 90 days was reported. Analysis of the best-fit relationships for tensile-compressive Strength and stiffness-compressive Strength found that the square-root function recommended by most codes of practice is inadequate when applied to concretes of higher Strength, particularly in the case for tensile Strength prediction.