When did people start using concrete? It appears that it was over 3,000 years ago, between 1400 and 1200 BCE. Archaeologists have discovered concrete floors in the palace of Tiryns in Greece, which predate the Bronze Age. Because it was a bad formulation, it cracked easily. The city of Pompeii was built mainly with Roman concrete. In today's world, concrete is an incredibly important material. Civil engineers use concrete to build roads, bridges, dams, skyscrapers, runways, canals, and foundations on a massive scale. If you take it by weight, concrete is the number-one building material in the world by far. It's easy to understand why concrete is so popular: it gives engineers the ability to pour a liquid into a mold and create something similar to solid rock that can last for centuries. By adding steel rebar or pretensioned steel, the strength of concrete improves dramatically and makes it possible to create beams 100 feet (30 meters) long or more. Add to that the fact that most of concrete's weight comes in the form of sand and gravel, and you have a material that is inexpensive compared to alternatives. At today's prices, concrete costs less than three cents per pound (0.45 kg). Concrete has four ingredients: one part Portland cement, two parts sand, and three parts gravel with enough water to make a paste-like mix. The Portland cement, when mixed with water, acts like a glue that binds the sand and gravel into a dense solid. This is not like a glue that dries, however. It is more like a calcium-silicon-water epoxy that hardens through a chemical reaction. This reaction gives off heat and it is slow. It takes concrete several weeks of curing to reach reasonable strength. This is why you will often see workers pour a concrete foundation and then disappear for a month. They are waiting for the concrete to cure to the point where they can put weight on it. Although concrete is simple to make, it is important to do it right. When a road or foundation is poured, engineers will often take a cylindrical sample and do a crush test to confirm its compressive strength.