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WHAT IS CONCRETE?
Contrary to popular usage, "concrete" and "cement" are not synonyms. Sidewalks and foundations are made of concrete, not of cement, although cement is concrete’s most important ingredient. The other ingredients are gravel or crushed stone (also known as aggregate), sand, water and, optionally, various performance-enhancing additives. The vehicles that most people call cement mixers are actually concrete mixers; cement, like talcum powder, is transported mainly in tank trucks.
The cement in concrete is called Portland cement, because Joseph Aspdin, an English bricklayer who is credited with the invention of its earliest version, felt that its color was similar to that of limestone quarried on the Isle of Portland, a peninsula on England’s southern coast. Aspdin received his cement patent in 1824. His process involved heating limestone and clay in a kiln until parts of the mixture fused, then grinding the burned and desiccated result into a fine powder. Adding water to the powder yielded 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 lock together in 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 gain strength indefinitely.
Though concrete is very hard to crush, it’s relatively easy to pull apart. The usual way to compensate for this tensile weakness is to add steel reinforcing rods, known as rebar, which take over when the concrete 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 didn’t need the steel. It is the challenge of the engineer to keep cracks small, so that rather than having a few big cracks, we have many little cracks."
One area of continuing interest, Meyer told me, involves adding short lengths of threadlike fibers made of steel, polypropylene, polyolefin, and other materials-samples. Polypropylene can also be used to provide extra fire protection. Concrete is essentially fireproof, but it can fail in very high temperatures as free water trapped inside turns to steam, expands, and blows it apart from within; adding polypropylene fibers can reduce the risk of such failures, because in high heat the fibers melt, leaving voids that act like relief valves for steam. 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.
As excerpted from New Yorker, Nov 2003
www.newyorker.com
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