A stone floor must bear the weight of humans and large things. As a result, compressive strength is the maximum load per unit area that the stone can withstand without giving way. A compressive strength between 1,800 psi (12.45 MPa) to 19,000 psi (131 MPa) indicates that the stone can withstand a larger crushing load. Compressive strength varies depending on how the stone is made up and the type of material it is composed of. Concrete is a mixture of water, cement, sand, and gravel used to make buildings, bridges, and roads. Concrete has greater compressive strength when it's fresh out of the mixer; however, as it cures over time, its compressive strength increases further. For example, if you pour a concrete slab today, it will be at its lowest strength until it cures completely in one year. After that, it can be carved, drilled, or cut into any shape you like—as long as you don't put any weight on it. The average person weighs about 140 lb (63 kg). This means that the maximum load that a 4-foot by 8-foot (1.22 m by 2.44 m) section of concrete would need to support is about 56,000 lb (25,500 kg). However, for most people to be able to walk across it, the surface would need to be at least 6 inches (15 cm) deep.