Because of the wide availability and dependability of ready mixed concrete, the modern builder may have little occasion to involve himself directly in the selection of aggregates. But a basic understanding of these important materials is an inseparable part of understanding concrete itself; aggregates profoundly affect almost all of the important characteristics of concrete, including economy, weight, strength, workability, durability, texture, finishing characteristics, and even color. There are a number of aggregate properties that affect the performance of concrete. Hardness of aggregates refers to their abrasion resistance. Aggregates such as trap rock, granite and quartz are noted for their hardness and are more suited for industrial floors carrying heavy steel wheel traffic than aggregates that are more soft and friable. Toughness, an aggregates's resistance to impact, is another quality which is of paramount importance in floor construction. Soundness of an aggregate is its ability to withstand freezing and thawing and other types of weathering. This characteristic must be carefully evaluated when exposed concrete is to be placed. So called normal weight aggregates produce concrete that weighs around 150 pounds per cubic foot, or 2 tons per cubic yard. Contrast this to lightweight aggregates which produce concretes of 80 to 115 pounds per cubic foot. The advantages of lightweight aggregate concrete are many. It often greatly reduces foundation costs, especially in areas with poor subsoils. Its insulating value is appreciably higher than normal weight concrete. It also sometimes makes possible reduction by an appreciable amount of the concrete and reinforcement in the overall design. Concrete weighing as much as 385 pounds per cubic foot has been specified in recent years, mainly for protection from radiation. A number of materials are used as heavyweight aggregates. These include the following (1) barytes, (2) limonite, (3) magntite, (4) ilmenite, (5) hemotite, (6) steel punchings, (7) crushed cast iron shot, and (8) ferrophosphourous.