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April 22, 2009 > TechKnow Talk: The concrete facts

TechKnow Talk: The concrete facts

To many, concrete may be dull. "As dull as cement," some would say. But construction industry workers understand that concrete is the basic building block of our civilization. Just look around. Concrete is everywhere: in roads, bridges, sidewalks, office buildings, and factories. Your home rests on a concrete foundation. It is by far the most ubiquitous man-made material in the world.

Materials similar to modern concrete may have been used by the ancient Egyptians and Chinese thousands of years B.C. Concrete was used extensively by the Romans, in building their famous aqueducts, baths, and other structures. It is so enduring that some of those structures still survive today. Following the fall of the Roman Empire, concrete was little used until it was "rediscovered" in the 18th century.

What is it, how is it made, and what makes it so useful? In very simple terms, concrete is composed of stones held together with a binder. The stones, called aggregate, may range from fine sand to coarse river rocks. The binder, called cement, fills the gaps between stones and binds them together. Chemical reactions with water cause the cement to harden over time, producing a very strong and durable material.

Let's begin with cement, as it is the key ingredient. Most modern concrete uses portland cement, first produced in 1824 by British stone mason Joseph Aspdin. It was named after its resemblance in color to limestone from the Isle of Portland and the Dorsetshire coast, a common construction material in southern England at the time.

Portland cement is a mixture of limestone and clay or shale. These materials are crushed together then baked at about 2600 degrees Fahrenheit in a rotary kiln. The kiln is a huge, descending column, with a screw-shaped interior. The limestone and clay mixture slowly travels down the rotating kiln over a period of an hour or more.

This process burns off moisture and causes the material to chemically react, forming calcium silicates. The product exiting the kiln is called clinker, and consists of marble-sized balls or pellets of cement. Large amounts of carbon dioxide are released in the production of cement, making it environmentally unfriendly.

Clinker contains four ingredients: tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. Ultimately, the clinker is crushed to an extremely fine powder and mixed with gypsum, which contributes sulfur to the cement to help regulate the curing process.

The aggregate is typically some combination of crushed rock, gravel, and sand, though the use of recycled, crushed concrete is increasing. This material is washed before mixing to ensure a strong bond with the cement. The specific type of aggregate used determines in part the physical properties, including density and wear resistance, of the concrete. Most concrete recipes are predominately aggregate. A typical composition is roughly 70% aggregate, 10% cement, 15% water, and 5% entrapped air.

Small amounts of liquid or powder additives, called admixtures, may also be used. These include agents to delay or accelerate early cure time (used in hot or cold temperatures, respectively), metal oxides to provide pigmentation, and polymers to improve workability, the ease with which the concrete can be poured, formed, and finished.

Shortly before the cement is to be poured, it is mixed with water, and the aggregate and admixtures are added to the cement paste. The proportion of water to cement is critical. If excess water is used, voids will form, and the concrete will be weakened. If insufficient water is used, the cement will not harden fully and fail to achieve its full strength. Too little water also limits the workability of the concrete.

The addition of water begins a chemical reaction called hydration that creates strong chemical bonds within the constituents of the cement and forms the aggregate into a solid mass. Each of the four cement components cures and hardens at a different rate. These reactions begin immediately, but continue as long as water is present within the material. The water must be relatively pure, as impurities may trigger undesirable chemical reactions.

Early curing is aided by keeping the fresh concrete wet for a few days. This is often accomplished by repeated spraying or flowing of water onto the concrete or wrapping it in plastic sheets to trap moisture. The industry standard for testing concrete strength is 28 days of cure time. By then, it has achieved about 90% of its ultimate strength but will continue to cure and strengthen for many years.

Fully cured concrete is very strong in compression (resistance to crushing). This is why it can support massive buildings. Its tensile strength (resistance to pulling apart) is poor in comparison, only about 10% of its strength in compression. This is why concrete cracks. This weakness is overcome by adding reinforcement to concrete structures, such as steel rods (rebar) and wire mesh. Some concrete is also "prestressed" with built-in compression to compensate for its poor tensile strength.

But its advantages far outweigh this weakness. Concrete is relatively inexpensive and requires very little maintenance. It will not rot or corrode as wooden or metal structures do and is uninteresting to termites and other destructive pests. It can be poured and molded into almost any shape imaginable. It is resistant to damage from storms, water, wind, and fire. And concrete, properly proportioned and cured, will last for hundreds of years.

The U.S. consumes more than 100 million tons of cement annually, a number that has been decreasing in recent years, mostly due to a reduction in residential construction. Because cement is expensive to ship long distances, the U.S. cement industry is widely dispersed, with cement plants in approximately 35 states. About 80% of the U.S. cement industry is owned by foreign companies.

Worldwide, enough concrete is produced each year to fill a cube more than a mile on each side. Another way to think of this is an annual concrete production sufficient to fill a three foot cube for every human on Earth. China's explosive industrialization has spawned an enormous appetite for concrete. As a result, China is by far the world's number one consumer of cement, utilizing about 40% of worldwide production.

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