Concrete

Concrete is a construction material composed of cement (commonly Portland cement) and other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravels or crushed rocks such as limestone, or granite, plus a fine aggregate such as sand), water, and chemical admixtures.
The word concrete comes from the Latin word "concretus" (meaning compact or condensed), the past participle of "concresco", from "com-" (together) and "cresco" (to grow).
Concrete solidifies and hardens after mixing with water 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. Concrete is used to make pavements, pipe, architectural structures, foundations, motorways/roads, bridges/overpasses, parking structures, brick/block walls and footings for gates, fences and poles.
Concrete is used more than any other man-made material in the world.[2] As of 2006, about 7.5 cubic kilometres of concrete are made each year—more than one cubic metre for every person on Earth.[3]
Concrete powers a US$35 billion industry, employing more than two million workers in the United States alone.[citation needed] More than 55,000 miles (89,000 km) of highways in the United States are paved with this material. Reinforced concrete, prestressed concrete and precast concrete are the most widely used types of concrete functional extensions in modern days.
Contents[hide]
1 History
1.1 Additives
2 Composition
2.1 Cement
2.2 Water
2.3 Aggregates
2.4 Reinforcement
2.5 Chemical admixtures
2.6 Mineral admixtures and blended cements
3 Concrete production
3.1 Mixing concrete
3.2 Workability
3.3 Concrete mixture placement
3.4 Concrete compaction
3.5 Curing
4 Properties
5 Environmental concerns
5.1 Worldwide CO2 emissions and global change
5.2 Surface runoff
5.3 Urban heat
5.4 Concrete dust
6 Health concerns
6.1 Concrete handling/safety precautions
7 Damage modes
8 Concrete repair
9 Concrete recycling
10 World records
10.1 Concrete pumping
10.2 Continuous pours
11 Use of concrete in infrastructure
11.1 Mass concrete structures
11.2 Reinforced concrete structures
11.3 Prestressed concrete structures
11.4 Concrete textures
12 Building with concrete
12.1 Environmentally sustainable
12.2 Energy efficiency
12.3 Fire safety and quality of life
12.4 Recycling and recyclable
13 See also
14 References
14.1 Notes
14.2 Bibliography
15 External links
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History
Concrete has been used for construction in various ancient civilizations.[4] An analysis of ancient Egyptian pyramids has shown that concrete was employed in their construction.[5]
During the Roman Empire, Roman concrete (or opus caementicium) was made from quicklime, pozzolana, and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture termed the Roman Architectural Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionary new designs both in terms of structural complexity and dimension.[6]
Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that trouble the builders of similar structures in stone or brick.[7]
Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement concrete (ca. 200 kg/cm2).[8] However, due to the absence of steel reinforcement, its tensile strength was far lower and its mode of application was also different:
Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.[9]
The widespread use of concrete in many Roman structures has ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges have masonry cladding on a concrete core, as does the dome of the Pantheon.
Some have stated that the secret of concrete was lost for 13 centuries until 1756, when the British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. However, the Canal du Midi was constructed using concrete in 1670 suggesting a continuous unpublished use since Roman times.[10] Likewise there are concrete structures in Finland that date back to the 1500s.[citation needed] Portland cement was first used in concrete in the early 1840s.

Additives
Concrete additives have been used since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened, and adding blood made it more frost-resistant.[11]
Recently, the use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This significantly reduces the amount of quarrying and landfill space required, and, as it acts as a cement replacement, reduces the amount of cement required.
In modern times, researchers have experimented with the addition of other materials to create concrete with improved properties, such as higher strength or electrical conductivity. Marconite is one example