With our experience building precast concrete fences and walls from our main office in Los Angeles, we've become pretty comfortable with a good understanding of the concrete and cement business. Generally "concrete" and "cement" are not really the same thing. Sidewalks are made from concrete, not cement, although cement is a vital ingredient of concrete. There are other ingredients which may include gravel or crushed stone (also known as aggregate), sand, water and, other optional performance-enhancing additives. The trucks you see that people refer to as cement mixers are really concrete mixers.

The cement in concrete is called Portland cement, because Joseph Aspdin, an English bricklayer who invented the earliest version, felt that its color was almost the same as limestone quarried on the Isle of Portland. Aspdin got a patent for cement as far back as 1824. He used to heat limestone and clay in a kiln until parts of the mixture fused, then he ground the mixture into a fine powder. Adding water to the powder produced a workable paste and started a complex chemical process, called hydration, in which the water bonded with compounds of calcium, silicon, aluminum and iron, and caused the whole thing to combine into a rigid mass. Wet Portland cement doesn't merely "dry," the way mud does; hydration transforms it into a chemically distinct material, which continues to strengthen over time.

Concrete is actually pretty easy to pull apart. A way to compensate for this tensile weakness (that means it's easy to break apart) is to add steel reinforcing rods, known as rebar, which hold the concrete in place overall when it cracks. Concrete reinforced with rebar must crack, Meyer explained. "That may sound funny to the layperson," he said, "but the reason is that if it doesn't crack, you wouldn't need the steel. It is the challenge of the engineer and the understanding of the architect to keep cracks small, so that rather than having a few big cracks, we have many little cracks."

Concrete is essentially fireproof, but it can fall apart in very high temperatures as free water trapped inside turns to steam, expands, and blows it apart from within. So if you want to reinforce cement even more, you can add lengths of threadlike fibers made of steel, polypropylene, polyolefin, and other materials-samples. Such concrete can provide extra protection in structures that may be exposed to any of a variety of increasingly ordinary-seeming perils of modern existence, among them fires, explosions, and bomb blasts. Polypropylene is a good idea for another reason - it may provide extra fire protection. By adding polypropylene fibers to the mix it can reduce the risk of such failures, because in high heat the fibers melt, leaving voids that act like relief valves for steam.