Structural lightweight concrete is defined as having a 28 day compressive strength in excess of 2,500 psi and a unit weight ranging from 85 or 90 pcf to 115 pcf. Generally speaking, concrete in this range of strength and weight is produced with expanded shales, clays, slates, and slags. A fairly satisfactory means of cataloging the weight and strength range of these concretes is by a concrete "spectrum" which spreads the weight range of concretes from a low of 15 pounds per cubic foot to a high of 120 pounds per cubic foot. At the lower end of the scale are the "low density" lightweight aggregate concretes generally produced with vermiculite or perlite. These concretes have a high insulative value but with compressive strengths ranging from 300 psi to perhaps a high of 1,000 psi. The unit weight of these concretes rarely exceeds 50 psf oven dry. At the higher end of the scale are the structural lightweight concretes ranging in weight roughly from 85 to 120 pcf. The expanded shales, clays and slates produced by both the rotary kiln and the sintering process and expanded slags make up the build of these concrete. They are capable of developing compressive strengths in excess of 2,500 psi and the majority can achieve strengths of 6,000 psi or more. Between these two extremes, with strengths ranging from 1,000 to 2,000 psi, and unit weights from 50 to 85 pcf, are the fill concretes. This type of concrete has a certain amount of insulator value and also some inherent strength.