The standard configuration of the US Interstate Highway System is a layer of about 28 centimeters of non-reinforced concrete on top of a twice-as-thick layer of natural aggregates (stones, gravel, sand)—and the entire Interstate system contains about 50 million tons of cement, 1.5 billion metric tons of aggregates, and only about 6 million tons of steel (for structural support and culvert pipes).[94]
