Abstract: Sedimentary rock aggregates are commonly used in Nigeria for structural concrete production. Due to its variability in quality, strength and contaminations it is being suspected as one of causes of many structural failures in the country. This paper investigated the quality of 8 – 16mm size gravels from Oji-River piths by optimizing the strength of concrete made from it through Scheffe’s simplex method of optimization and produced a mathematical model which gave an optimum compressive strength of 11.78N/mm2 and mix ratio of 1:1.4:2.5 for a water/cement ratio of 0.53. Because of the small nature of this optimum strength and the general predictions from the model, it was recommended that such aggregate (8 – 16mm from Oji-River) be restricted to oversite concretes, columns and lintels of buildings of load bearing walls and a maximum storey of three; and it should never be used for slabs of suspended floors, retaining walls, bridges and culverts.