`Harvard Stadium reinforced concrete (1910)`

Cited as the oldest reinforced concrete structure in the country, used to illustrate that concrete continues to harden over years and decades — "you have to have a hammer drill with a carbide tip" to drill it now.

Student: Were you saying that concrete doesn't set correctly unless you compress it as it's curing? No, I'm just saying it takes time to get to its strength. Just like two-part epoxy, concrete is two-part — you take the dry Portland cement, you mix it with water, and it forms a chemical reaction. It takes a month or more to harden up. In fact it can take years. The oldest reinforced concrete structure in the country is Harvard football stadium. If you go ask the workmen around here about drilling in these concrete walls — this stuff is really hard after a hundred years of setting up. You have to have a hammer drill with a carbide tip.