Jim describes how concrete has been being used in construction for thousands of years and illustrates this with the example of the concrete dome at the Pantheon.
- Ever since civilizations first started to build, we've sought a material that would bind stones into a solid formed mass. The Babylonians used clay for this purpose. Later on, the Egyptians advanced to the discovery of lime and gypsum mortar as a binding agent for building structures like the Great Pyramids. The Greeks made further improvements, and finally the Romans developed a cement that produced structures of remarkable durability. Now, when I say remarkable durability, I'm talking about the fact that some of these first early concrete structures are still standing today.
Structures like the Pantheon in Rome, which was built almost 2,000 years ago. It's still standing, and it's still considered to be the world's largest unreinforced concrete dome. The secret of Roman success in making cement was traced back to the mixing of lime with a volcanic ash from Mount Vesuvius, and they called this ash, "pozzolana." The process they used produced a cement capable of hardening underwater, and we now refer to that as hydraulic cement.
Then, fast-forward a little bit. In the mid-1700s, a British engineer named John Smeaton conducted experiments that led to the discovery of a cement made from limestone that also contained a considerable amount of clay, and it performed better than previously used cements, and it still hardened underwater. Go forward a little bit more, 1824, Joseph Aspdin, a bricklayer and mason in Leeds, England took out a patent on hydraulic cement, and he called it Portland cement. And he called it that because the color resembled the stone that they quarried on the Isle of Portland off the British coast.
Now, his method involved the careful proportioning of limestone and clay. He pulverized them, and then burned the mixture into what he called "clinker." That was then ground into the finished cement product. Now, this process that dates back to the 1800s really most resembles the method that we use to make cement today, and today we use these terms, Portland cement and cement, interchangeably. In addition, virtually all cements we use today are hydraulic cements, meaning they'll all harden underwater.
Alright, now that we've got a little history behind us, and we've introduced a few industry terms, let's continue and take a closer look at the difference between cement and concrete.
- Concrete versus cement
- Ingredients and terminology
- The chemistry of concrete
- Pozzolans and industrial by-products
- Using concrete as a building material
- Strategies to manage cracking
- Pre-stressed concrete
- Active versus passive reinforcing
- Pre-tensioning versus post-tensioning