The use of high-strength concrete is increasing. 6000 psi is considered the lower limit for "high-strength concrete," but mixes ranging from 7500 to 12,000 psi are routinely produced across the country. Selecting the correct and economical combination of materials is important in production of high-strength concrete. Successful production of high-stress concrete demands strict requirements for material selection. Choose the portland cement carefully. Use a Type I portland cement unless high initial strength is required. Both fine and coarse aggregates should meet ASTM C 33 requirements. Fine aggregates should have a rounded particle shape to reduce mix water and raise compressive strength. High-strength concrete may also require stronger aggregates. Fly ash and silica fume can be used to increase compressive strength by reacting with water and products of portland cement hydration. A high-range water-reducing agent, also called a superplasticizer, can be used to reduce the amount of mix water while retaining workablility. Slump requirements should be based on batching and placing techniques. Use a superplasticizer to achieve proper slump. Use air entrainment only when necessary--air content reduces strength. Consider construction loads for setting strength requirements at a time other than 28 days. Test labs should meet the requirements of ASTM C 1077, at a minimum. Cylinder storage must meet ASTM C 31 requirements.