Thomas Klenck, Popular Mechanics

Today, it seems as if we define civilization in terms of the small things, like decoded strands of DNA, splitting atoms and microscopic bumps on plastic compact discs.

With a focus so fine, it's easy to forget the bigger, simpler things that got us here and continue to define our lives. Back up a bit and look at what the real world is made of. In there with wood, steel and glass you'll find another material quietly holding it all up. You'll find concrete -- and, a lot of it.

Concrete is everywhere. It's underfoot as you walk down the street and it keeps your home up off the ground. It's the stuff cities are made of -- and a good deal of the roads and bridges as well. You'll find concrete bricks and birdbaths, swimming pools and sailboats. And in years to come, we may even build with it on the moon.

Concrete is a unique building material. It doesn't rot or burn like wood. It won't rust and it's heavy -- when you build something out of concrete, you know it will be around for some time.

Most of all, though, concrete is special because it starts out as a thick liquid that can be molded in various shapes.

Like Baking A Cake

To understand the nature of concrete, it might help to think of a cake -- a fruitcake. To make one, you mix up some flour, baking powder, butter and eggs, add dried fruit and nuts and pour the whole thing into a cake pan. Apply heat for a while, and presto -- that unappetizing paste is suddenly something new. The original batter has changed, while the fruit and nuts are pretty much the same -- just suspended and held together by the cake.

Well, concrete isn't exactly fruitcake, but there are similarities. In place of the flour mixture, concrete requires portland cement. Instead of dried fruit and nuts, you have sand and gravel. And a concrete form takes the place of the cake pan. Finally, instead of the heat that bakes the cake, concrete uses water. While heat is the prime mover in getting the cake batter to change its characteristics, in the concrete mix, it's water that gets things going.

Perhaps the most important thing to understand about concrete is the role of water. First, it provides plasticity so the concrete can be poured in a form. Its real importance, however, is in the hardening process. Wet concrete doesn't harden by drying. Instead, the water is a chemical component in a curing process. The compounds that react with the water are in the portland cement.

Isle Of Portland

While cement in one form or another has been around for centuries, the type we use was invented in 1824 in Britain. It was named portland cement because it looked like the stone quarried on the Isle of Portland.

Portland cement is produced by mixing ground limestone, clay or shale, sand and iron ore. This mixture is heated in a rotary kiln to temperatures as high as 1600 degrees Celsius. The heating process causes the materials to break down and recombine into new compounds that can react with water in a crystallization process called hydration.

Concrete cures in several stages -- a factor that allows it to be trucked to the job site and still be ready to pour. With the concrete in the form, the cement begins a slow cure and the mix hardens. After about 36 hours, most of the hydration process is complete, but the cement will continue to cure as long as water and unhydrated compounds are present. While the process can take years, strength tests are typically done after 28 days.

It's important to use the right amount of water. Too much makes for weaker concrete. However, too little makes the mix hard to pour. The best mix is a compromise between strength and workability.

From Cement To Concrete

While cement and water are the active components, it's not economical to use them alone.

Instead, aggregates are added to increase the volume and tailor the concrete to its final use. Typically, 60 to 80 percent of the concrete is aggregate. In most cases, the aggregates are sand and gravel. When sand is used alone, the result is mortar. When both are present, the result is concrete.

However, other aggregates might be used depending on the required characteristics of the cured mix. For example, vermiculite or perlite aggregates produce a lightweight concrete that has good insulating properties and can be easily sawn.

Improving Performance

Concrete suppliers often use additives, called admixtures, to alter or improve the qualities of the mix for a specific application. When it's important to have a workable concrete that pours easily without adding extra water, a mineral additive such as fly ash is added. Alternatively, superplasticizers are used to improve workability while increasing strength because less water is required. Retarding and accelerating admixtures are used to alter curing time as necessitated by climatic conditions.

One problem with concrete is a tendency for freeze/thaw cycles to cause cracks. To help remedy this, air-entraining agents are added. These admixtures create a dispersion of very fine air bubbles that cushion the concrete against the effects of freezing water.