CONCRETE

Concrete is a construction material that consists of cement (commonly Portland cement) as well as other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate such as gravel limestone or granite, plus a fine aggregate such as sand and water) and chemical admixtures. Concrete solidifies and hardens after mixing and placement due to a chemical process known as hydration.The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material. It is used to make pavements, architectural structures, foundations, motorways/roads, overpasses, parking structures, brick/block walls and footings for gates, fences and poles. More concrete is used than any other man-made material on the planet.[1] As of 2006 about seven billion cubic meters of concrete are made each year – more than one cubic meter for every person on Earth.[2] Concrete powers a US$35 billion industry which employs more than two million workers in the United States alone. More than 55,000 miles of freeways and highways in America are made of this material. The People's Republic of China currently consumes 40% of the world's cement [concrete] produc

Composition

There are many types of concrete available by varying the proportions of the main ingredients below.

The mix design depends on the type of structure being built, how the concrete will be mixed and delivered, and how it will be placed to form this structure.

Cement

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. English engineer Joseph Aspdin patented Portland cement in 1824; it was named because of its similarity in colour to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay, and grinding this product (called clinker) with a source of sulfate (most commonly gypsum).

High temperature applications, such as masonry ovens and the like, generally require the use of a refractory cement; concretes based on Portland cement can be damaged or destroyed by elevated temperatures, but refractory concretes are better able to withstand such conditions.

Water

Combined with a cementitious material, this forms a cement paste. The cement paste glues the aggregate together, fills voids between it, and allows it to flow more easily.

Less water in the cement paste will yield a stronger more durable concrete, more water will give an easier flowing concrete with a higher slump.[4]

Impure water used to make concrete can cause problems, either when setting, or later on.

Aggregates

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose. Recycled aggregates (from construction, demolition and excavation waste) are increasingly used as partial replacements of natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.

Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.

Reinforcement

Concrete is strong in compression, as the aggregate efficiently carries the compression load. However, it is weak in tension as the cement holding the aggregate in place can snap, allowing the structure to fail. Reinforced concrete solves these problems by adding metal reinforcing bars, glassfiber, or plastic fiber to carry tensile loads.