Why Use Concrete?
Stonehenge in England, the Great Pyramid of Giza, the Peruvian citadel at Machu Picchu—three wondrous examples of how stone structures can last hundreds or even thousands of years. But though stone is one of the oldest and most durable building materials, it isn’t exactly easy to work with. It’s heavy, hard to transport, and usually comes in giant chunks, which have to be laboriously cut to shape. Wouldn’t it be great if there were a recipe for stone—a kind of gooey cake mixture we could throw together wherever it was needed, simply pressing it into molds to make buildings and structures of any shape or size? Well that kind of “liquid stone” really does exist: we call it concrete. Although it sometimes gets a bad press, because many people associate it with brutal urban architecture from the mid-20th century, this dynamic material is the great, unsung hero of the modern, material world.
From the Hoover Dam to the Sydney Opera House, you’ll find it in the world’s tallest skyscrapers, biggest bridges, longest highways, deepest tunnels, and quite probably even under the floor in your own humble little home. Concrete is pretty amazing stuff, but what is it and how exactly does it work? Let’s take a closer look!
What is concrete?
The word “concrete” comes from a Latin word, concretus, meaning to grow together—and that’s exactly what it does when you combine its three ingredients, which are:
- A mixture of coarse and fine aggregates (sand, gravel, stones, larger bits of crushed rock, recycled glass, bits of old recycled concrete, and pretty much anything equivalent)—typically 60–75 percent.
- Cement (the everyday name for calcium silicates and aluminates)—typically 10–15 percent.
- Water—typically 15–20 percent.
Thrown together and mixed well, these simple ingredients make a composite, which is the name we give to a hybrid material that’s better in some important way than the materials from which it’s made. In the case of concrete, the “important” thing is that it’s strong, hard, and durable. Thinking of concrete as a composite material, the cement hydrate is the background, binding material (technically called the “matrix”) to which the sand and gravel add extra strength (the “reinforcement”).
How does concrete form from ingredients that are nothing like the final product? When you add water to cement, crystals of cement hydrate (technically, calcium-silica-hydrate) start to grow, which lock the sand and gravel tightly together. It’s this gradual crystal formation that gives concrete its strength, rather than the simple fact that it’s drying out. Indeed, the reason you have to keep wetting concrete for several days, as it sets, is to “power” the chemical reactions that hydrate the cement. The mushy slushy mixture that tumbles from your concrete mixer gradually turns much harder than the materials from which it’s formed. “Liquid stone” becomes stone for real—well, artificial stone, at least. And by “gradually,” I really do mean gradually: concrete hardens in hours, gets properly hard after about a month, but continues to harden and strengthen for at least five years after that.
An interesting fact, from recent scientific studies of concrete, is that the “crystals” inside it aren’t really crystals at all: they’re not well-ordered and perfectly regular, as crystals are supposed to be, but actually have some of the random structure you find in materials like glass (scientifically known as amorphous solids). Concrete contains quite a bit of trapped air (as much as 5–10 percent) because there is some space around the open, three-dimensional structure of cement hydrate crystals and the sand and gravel trapped between them. And that, in turn, explains why concrete can bend and flex, stretch and compress (just a little bit, anyway).
Just like any recipe, you can vary the mixture for concrete somewhat (more water, perhaps, more aggregates, or even chemicals of different kinds) to produce concrete that flows faster, sets harder or more quickly, weathers better, or has a particular colour or appearance. Adding a pigment called titanium dioxide, for example, is a simple way of making concrete bright and white—a million miles from the drab grey stuff that gives concrete car parks a bad name. Another variant is aerated concrete, which looks a bit like a very hard sponge with masses of tiny air pockets inside. These allow the concrete to expand and contract in hot and cold weather without fatally cracking and also make it an excellent heat insulating material.
McDaid’s have been involved in the quarrying and construction industry for over 50 years. We can provide a huge range of concrete to meet your individual needs www.mcdaid.ie/concrete.