Q:Where can we get magnetite for use in making shielding concrete of 235 pounds per cubic foot? Does such heavyweight concrete have other properties similar to regular concrete? We are particularly interested in coefficient of expansion and also thermal conductivity.

A: TMF Mineral Resources Ud. of Mountain, Ontario, KOE 1S0 has announced availability of Canadian magnetite for this purpose. The same product is marketed in the United States through de Rohan Associates, 3500 Vista Terrace, Cincinnati, Ohio 45208. The aggregate comes in coarse, medium and fine sizes with the following characteristics:

Size, inches

Specific gravity


3/4 to 3




1/4 to 1 1/2




100 mesh to 1/4



A mix containing 470 pounds of cement, 1755 pounds of coarse aggregate, 2025 pounds of medium aggregate, and 2070 pounds of fine aggregate and with a water-cement ratio of 0.53 produces concrete with a unit weight of 235 to 245 pounds per cubic foot. Other typical properties are:

Ultimate compressive strength 3600 psi
Ultimate tensile strength 430 psi
Allowable bond strength 300 psi
Allowable shear strength 300 psi
Allowable bearing strength 750 psi
Modulus of elasticity 400,000 psi
Poisson's ratio 0.15
Coefficient of linear expansion 0.000510% per degree F
Thermal conductivity 1.2 Btu per hour per square foot per degree F per foot