Saint Louis’ most famous structure is an arch made of steel and concrete. Begun in 1959, the Gateway Arch is the centerpiece of the Jefferson National Expansion Memorial, and at 630 feet, it’s the tallest man-made monument in the United States. Saint Louis, Missouri concrete contractors and workers completed the structure in 1967, and it currently hosts over four million visitors annually. According to Saint Louis concrete companies and engineers, the iconic structure consists of a stainless steel skin which covers a sandwich of two carbon-steel walls filled with 25,980 short tons of reinforced concrete. What’s more, the arch is hollow and houses a unique tram system that takes visitors to the top.