Concrete: It’s Everywhere, You Probably Don’t Understand It, And It’s Changing

06.21.2011 |

Concrete has been around for a long time, but researchers are still fine-tuning it.

We are always surrounded by things we don’t really understand, but there are few man-made substances as common, but little understood, as concrete. We walk on it, drive on it and live in buildings built on it (or even from it). And, while most people hardly ever think about it, there are researchers who think about it all the time. And they want to make it better.

So, let’s tip our hat to this seemingly omnipresent material by understanding how it works, and how it is changing.

Concrete is said to be the most plentiful manmade material on Earth. Romans mixed lime-based cement with various other materials (primarily sand) to make mortar. In a sense, that was the first concrete. But concrete as we know it wasn’t possible until about 200 years ago. That’s because modern concrete is basically a mixture of water, aggregate (i.e., small rocks), sand and Portland cement. And Portland cement wasn’t invented until the early 19th century.

Portland cement is the dry, grey powder that most people think of (wrongly) as concrete. But without cement, you can’t have concrete. Cement is the part of concrete that hardens when it reacts with water. When cement hardens, it isn’t “drying” – which is to say, the water isn’t evaporating. Instead, the water is chemically combining with the powdery cement to create a new material – a calcium silicate hydrate. That’s why cement can “set” even if it is underwater.

But concrete has attributes that cement alone does not. First, it’s more economical. Rock and sand are cheaper than cement alone, so mixing them in makes concrete cheaper than pure cement. But that rock and sand isn’t just filler. Those components make concrete significantly more durable than pure cement. For example, concrete  can usually handle temperature fluctuations (e.g., freezing and thawing) much better than cement can.

And concrete is strong stuff. Everyday concrete can handle at least 3,000 pounds per square inch (psi) of pressure before breaking. It’s also cheap, selling in the U.S. for around $110 per cubic yard (for reference, one cubic yard weighs about 2 tons).

Because concrete is so strong, so cheap, and can be cast into almost any conceivable shape, it is almost ideal for all sorts of construction projects. Almost.

Here’s the problem. Concrete is brittle, and subject to something called brittle fracture. Imagine that you are holding a long piece of chalk in both hands, and you try to bend it. No matter how much pressure you exert, the chalk will not bend – but eventually it will snap in two. That’s brittle fracture. Now, imagine that the piece of chalk is a bridge. Well, you can see the potential problem with brittle fracture there.

To address this problem in large-scale construction projects, concrete is reinforced by an internal skeleton or framework – which is usually made of steel. Steel is very strong when pulled (i.e., under tension), but can buckle when pushed (i.e., under compression). Concrete is exactly the opposite – strong under compression, weak under tension. Reinforcing concrete with steel capitalizes on the strengths of each, making the final product less brittle and more stable.

One area of ongoing concrete research focuses on the search for new reinforcing materials, because steel is not perfect. For example, it’s virtually impossible to prevent microscopic cracks from forming in concrete over time. If those cracks allow water or salt to reach the reinforcing steel, the metal will corrode. That, obviously, is bad.

So, researchers are seeing what else might work. For example, a lot of research is being done to determine whether carbon composites might be able to serve as more durable replacements for steel. The composites are stronger than steel, but can be too brittle if improperly designed (that’s one thing researchers are working on). From a practical standpoint, another concern is whether the composites could be made cheaply enough to replace steel.

Another major area of concrete research has to do with making the concrete itself stronger. Normal concrete, as we said, can handle 3,000 psi before failing. High performance concrete, which is also commercially available, can handle up to about 18,000 psi before it breaks. But researchers are now trying to create ultra-high performance concrete (UHPC), that could take up to 30,000 psi before breaking.

Why? Economy again. If the material is stronger, you can use less of it. But another reason is that it would allow designers to dream up structures that we simply can’t build today – taller buildings, longer bridges, you name it.

UHPC research focuses primarily on the components that go into concrete. For example, to make UHPC, you want to use extremely hard sands and aggregates. And you want those sands and aggregates to be of particular shapes and sizes that will settle tightly together. The less space there is between the components, the less room there is for cracks to develop. UHPC researchers are also exploring the addition of tiny glass or steel fibers into the mixture, to help prevent microcracks from forming.

Concrete researchers are also experimenting with new chemical admixtures, hoping to tweak the chemical reaction in the cement to create a stronger and more durable finished product.

All in all, concrete will continue to be a common building material for projects large and small. But while it may look the same, our children will likely be walking and driving on a substance that is different chemically and physically from the concrete you and I grew up playing hopscotch on. (Yes, I played hopscotch).

Note: Many thanks to Greg Lucier, lab manager of NC State’s Constructed Facilities Laboratory, for taking the time to talk to me about concrete. Any errors in the above post are mine, and mine alone.



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