The evolution of nuclear power into peaceful application has given rise to an expanding use of heavy weight concrete in private industry. The type of material used as shielding in any specific application depends on available space. Where every possible amount of space must be conserved, a highly dense metal such as lead is frequently employed. When space is available, normal weight concrete is used. However, there are occasions where concrete considerably heavier than normal proves to be the most practical material. The determining factor in choice of materials for shielding is weight. Heavyweight concrete used for counter weights frequently entails the additional performance requirement of weatherability. To make heavyweight concrete, special consideration must be given to all parts of the concrete mix. Heavyweight concrete should be as dense as absolutely possible. Much cement that is sold as non-air-entraining nevertheless contains certain amounts of materials that will entrain air. In this work, it is absolutely necessary that the amount of air entrainment be kept to a bare minimum. Careful testing of the cement used for heavyweight concrete should be performed to accurately assess its air-entraining potential. The aggregates most commonly used for the manufacture of heavyweight concrete are limonite, barytes, magnetite, ilmenite, steel punchings and sand, ferro-phosphorus, boron additives, serpentine and geothite. These aggregates will vary considerably in the weight of concrete they will produce and in their physical properties, such as gradation and durability. An air-de-training agent is often helpful. However, in a few exposed applications, 2 or 3 percent of entrained air has been allowed to create a more nearly impermeable cement paste that will provide greater durability. Air entrainment will also reduce the amount of bleeding and promote a more homogeneous mix. In addition to mass, substances containing appreciable amounts of iron, hydrogen, and boron or cadmium are needed to contain neutrons. Under some conditions, the amount of hydrogen contained in the moisture, which is present in all concrete in their natural state, is sufficient to attenuate neutrons. Such moisture may occur as the water of hydration is chemically bound to the cement.