During recent years, a number of materials known as lightweight aggregates have played an increasingly important role in the concrete industry. Lightweight aggregates, used to produce lightweight concretes and plasters, have for some time been valuable in the manufacture of precast concrete. Among lightweight aggregates are expanded slags, expanded clays, shales, and slates, perlite, and vermiculite. Slag, clay, shale, and slate aggregates are used in structural concrete and for fill and insulating purposes. Lightweight concrete might be compared with conventional sand and gravel aggregate concrete on the following points: (1) Lightweight concrete ranges in weight from 20 pounds per cubic foot for the lightest insulating and fill concrete to 115 pounds per cubic foot for structural grade concrete. (2) Lightweight structural compressive strength varies from 500 psi to 1000 psi for insulating material to 7000 psi and better for structural concrete. Normal weight concrete ranges to 8000 plus psi. (3) The modulus of elasticity varies from around 65,000 psi for the ultra lightweights to 3 million psi for the medium lightweight concretes used for structural work. Generally the modulus of elasticity for lightweight concretes is about one-half to two-thirds that of gravel, stone and air-cooled slag concretes of equal strength. (4) Lightweight concrete shrinkage also varies according to the aggregate, being about .1 to .2 percent for perlite, from .1 to .7 for vermiculite, from .04 to .06 percent for expanded slag, and from .02 to .08 percent for shale, slate and clay. And (5) the thermal conductivity of lightweight concrete, in BTU per hour per square foot per degree F. per inch, ranges from .6 to .97 for vermiculite, .5 to .9 for perlite.