An outgrowth of the development of nuclear energy, high-density concrete is gaining wide spread use as a shielding material to provide protection against radiation hazards. In contrast with conventional concrete weighing around 150 pounds per cubic foot, high-density concretes normally weigh from 200 to 250 pounds per cubic foot. Among the natural aggregates most commonly used are barite, magnetite, limonite, geothite, and ilmenite. These can be used for the lower and medium weight ranges. For densities over 250 pounds per cubic foot, expensive aggregates such as ferrophosphorus and iron punchings can be combined with magnetite and ilmenite. For highly specialized applications, densities as high as 425 pounds per cubic foot can be obtained using steel punchings and iron shot as the primary aggregate. In addition to high-density aggregates, additives containing boron are sometimes used to improve shielding properties. These usually consist of colemanite, boron frits, or borocalcite. Before utilizing any of these boron compounds, it is essential to make a trial mixes under field conditions to be sure that strength and setting time are not adversely affected. Certain impurities often associated with boron compounds will completely inhibit the setting of concrete. Other boron compounds such as borocalcite and boron frits appear to be less erratic in their performance, but they are usually considerably more expensive. When exceptionally heavy aggregates are used, the Prepakt method offers several advantages. In this method, coarse aggregates are first placed in the forms and the interstices are filled later with a special grout. Segregation is minimized and concrete of uniform density and composition can be placed in restricted areas and around embedded objects. Also, the Prepakt method consistently gives greater density and homogeneity than other methods.