Concrete

Concrete

Concrete, from the Latin word concretus meaning having grown together, is an artificially engineered composite material generally consisting of various proportions of Portland cement, water, relatively unreactive fillers called fine and coarse aggregates, and a small amount of air. The filler is usually a conglomerate of gravel, sand, and blast-furnace stony matter known as slag. Fine aggregate particles are generally considered smaller than 0.25 in (6.4 mm) in size, while coarse aggregates are usually over 0.25 (6.4 mm) in size.

Concrete is used in such construction projects as highways and streets, buildings, bridges, dams, aqueducts, airport runways, irrigation structures, piers,

sidewalks, and farm buildings and structures (such as silos). Concrete is also used as radiation shielding.

Concrete is considered the most widely used material used for construction around the world. In fact, it is used twice as much as all the other building materials, which includes wood, steel, plastic, and aluminum. According to the Cement Association of Canada, in 2004 the annual world production of concrete was about 5.0 billion cubic yards (3.8 billion cubic meters). One year later, world concrete production rose to about 7.8 billion cubic yards (6.0 billion cubic meters). China is the largest producer and user of concrete, producing about one-third of it each year in the first half of the 2000s and consuming about 40% of it annually. The concrete industry in the United States is worth about $35 billion. It employs over two million workers.

Portland cement consists of finely pulverized matter produced by burning mixtures of lime, silica, alumina, and iron oxide at about 2, 642°F (1, 450°C). Chemically, Portland cement is a mixture of calcium aluminum silicates, typically including tricalcium silicate (3CaO SiO₂ ), dicalcium silicate (2CaO SiO₂ ), and tricalcium aluminate (3CaO Al₂ O₃ ). It may also contain tetracalcium aluminoferrate (4CaO Al₂ O₃ Fe₂ O₃ ). Small amounts of sulfur, potassium, sodium, and magnesia may also be present. The properties of Portland cement may be varied by changing the relative proportions of the ingredients and by grinding the cement to different degrees of fineness.

When Portland cement is mixed with water, the various ingredients begin to react chemically with the water. For a short time, the resultant mix can be poured or formed, but as the chemical reactions continue, the mix begins to stiffen, or set. Even after the mix has finished setting, it continues to combine chemically with the water, acquiring rigidity and strength. This process is called hardening.

Ordinarily, an excess of water is added to the concrete mix to decrease its viscosity so that it can be poured and shaped. When the chemical reactions have more or less finished taking place, the excess water, which is only held by secondary chemical bonds, either escapes, leaving behind voids, or remains trapped in tiny capillaries.

It is important to recognize that setting and hardening result from chemical reactions between the Portland cement and the water. They do not occur as the result of the mixture drying out. In the absence of water, the reactions stop. Likewise, hardening does not require air to take place and will take place even under water.

The strength of concrete is determined by the extent to which the chemical reactions have taken place by the filler size and distribution, the void volume, and the amount of water used. Depending on the materials used, concrete can support, in compression, over 10, 000 lb/sq in (700 kg/sq cm). Concrete is usually much stronger in compression than in tension. However, by properly designing steel reinforcing, concrete members can be as strong in tension as in compression. Reinforced concrete is made by placing steel mesh or bars into the concrete form or mold and then pouring concrete around it. The steel adds strength.

Other qualities of concrete are its economy of use and durability. Concrete is so durable, for example, that structures built by the ancient Egyptians over 3, 500 years ago have been discovered still standing. Concrete may also be modified with plastic (polymeric) materials to improve its properties of strength and durability.

See also Bond energy.