Close communication between the designers and the Chicago ready mix industry has been an important factor in the development of new products, among them high-strength concrete. In the Chicago area, high-strength concrete refers to normal weight concretes between 6000 and 11,000 psi, and lightweight concretes from 5000 to 8000 psi compressive strengths, both at 56 days. The information provided in this article refers only to 9000 psi normal weight concrete which has been used in columns of high-rise buildings since 1972, and is a product familiar to and accepted by the Chicago area construction industry.
Because of varying chemical compositions of cements, their ability to perform may differ when they are used with various chemical admixtures and fly ashes. The selection of a cement should not be based solely on mortar cubes but more so on its performance in concrete at 28, 56, and 90 days. The use of a normal-set water reducer, retarding water reducer, high-range water reducer, or a combination of these becomes necessary to efficiently use all cementitious materials and to maintain the lowest practical water-cement ratio. Air entraining agents are not generally used because of the accompanying strength loss and because the type of application normally does not require them.
After a set of materials is chosen, uniformity must be maintained throughout the production, placement and testing. If variations become excessive, the required average strength may be unattainable. In the production of high-strength concrete, many factors normally accepted and not specified must be considered. For example, cardboard cylinder molds cause a loss in compressive strength and an increase in strength variations. A significant improvement can be noted when metal or plastic molds are used.