The results of research and development efforts of organizations such as Material Service Corporation in Chicago have increased many designers' confidence in the performance of concrete for tall building. The report of the firm's research on the subject defines high strength concrete as concrete having compressive strengths of 6000 psi or more. The high strength mix designs produced by Material Service Corporation and used in Water Tower Place were based on this research. One of the conclusions of the report is that the production of high strength concrete places the primary responsibility for concrete performance on the producer of ready mixed concrete. In developing the mix designs for Water Tower Place the concrete producer investigated, through trial mixes, the properties of concrete made from different materials used in various proportions. The result was that each of the many mixes used in Water Tower Place made optimum use of materials at the lowest cost. According to the report cited, the brand and type of cement is probably the most important factor in the selection of materials for high strength concrete. Variation in cement will cause the concrete strength to fluctuate more than will variations of any other single material. Some cements may produce very high strengths at early ages but exhibit little strength gain at extended ages. Initial tests with mixes of the same cement content indicated that the use of locally available gravel coarse aggregate produced concrete with lower compressive strength and modulus of elasticity than did crushed stone of the same size. Tests also indicated that smaller size aggregate produced higher compressive strength. Based on strength and workability, Size Number 7 stone was selected for 9000 psi concrete. Size Number 67 stone was found to be optimum for 7500 psi concrete. However, the testing program showed considerable differences in the strengths of concretes produced with different aggregates from different sources when mixed in comparable batches of identical proportions. These differences make a testing program necessary when choosing a specific aggregate from several available.