With our experience building precast concrete fences and walls in California, we've become pretty comfortable with a good understanding of the concrete and cement business. Generally "concrete" and "cement" are not really the same product. Sidewalks are constructed from concrete, not cement, although cement is a vital and significant ingredient of concrete. Other ingredients include gravel or crushed stone , sand, water and, other performance-enhancing additives. The trucks you see that most people call cement mixers are actually concrete mixers; cement, like talcum powder, is transported mainly in tank trucks.

The cement that you find in concrete is known as Portland cement, because Joseph Aspdin, an English bricklayer who is credited with the invention of its, felt that its color was similar to that of limestone quarried on the Isle of Portland, a peninsula on England's southern coast. Aspdin got a patent for cement in 1824. He used to heat limestone and clay in a kiln until parts of the mixture fused, then he ground the burned and desiccated result into a fine powder. Adding water to the powder produced a workable paste and initiated 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," hydration transforms it into a chemically distinct material, which continues to gain strength 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."

Another way to reinforce the cement is by adding threadlike fibers made of steel, polypropylene, polyolefin, and other materials-samples. Polypropylene is a good idea for an additional reason - it can provide extra fire protection. 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 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.