Concrete
Concrete is a composite material composed of [|aggregate] bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance, the most widely used building material, and the most-manufactured material in the world.
When aggregate is mixed with dry Portland cement and water, the mixture forms a fluid slurry that can be poured and molded into shape. The cement reacts with the water through a process called hydration, which hardens it after several hours to form a solid matrix that binds the materials together into a durable stone-like material with various uses. This time allows concrete to not only be cast in forms, but also to have a variety of tooled processes performed. The hydration process is exothermic, which means that ambient temperature plays a significant role in how long it takes concrete to set. Often, additives are included in the mixture to improve the physical properties of the wet mix, delay or accelerate the curing time, or otherwise modify the finished material. Most structural concrete is poured with reinforcing materials embedded to provide tensile strength, yielding reinforced concrete.
Before the invention of Portland cement in the early 1800s, lime-based cement binders, such as lime putty, were often used. The overwhelming majority of concretes are produced using Portland cement, but sometimes with other hydraulic cements, such as calcium aluminate cement. Many other non-cementitious types of concrete exist with other methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder.
Concrete is distinct from mortar. Whereas concrete is itself a building material, and contains both coarse and fine aggregate particles, mortar contains only fine aggregates and is mainly used as a bonding agent to hold bricks, tiles and other masonry units together. Grout is another material associated with concrete and cement. It also does not contain coarse aggregates and is usually either pourable or thixotropic, and is used to fill gaps between masonry components or coarse aggregate which has already been put in place. Some methods of concrete manufacture and repair involve pumping grout into the gaps to make up a solid mass in situ.
Etymology
The word concrete comes from the Latin word "concretus", the perfect passive participle of "concrescere", from "con-" and "crescere".History
Ancient times
Concrete floors were found in the royal palace of Tiryns, Greece, which dates roughly to 1400 to 1200 BC. Lime mortars were used in Greece, such as in Crete and Cyprus, in 800 BC. The Assyrian Jerwan Aqueduct made use of waterproof concrete. Concrete was used for construction in many ancient structures.Mayan concrete at the ruins of Uxmal is referenced in Incidents of Travel in the Yucatán by John L. Stephens. "The roof is flat and had been covered with cement". "The floors were cement, in some places hard, but, by long exposure, broken, and now crumbling under the feet." "But throughout the wall was solid, and consisting of large stones imbedded in mortar, almost as hard as rock."
Small-scale production of concrete-like materials was pioneered by the Nabatean traders who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan from the 4th century BC. They discovered the advantages of hydraulic lime, with some self-cementing properties, by 700 BC. They built kilns to supply mortar for the construction of rubble masonry houses, concrete floors, and underground waterproof cisterns. They kept the cisterns secret as these enabled the Nabataeans to thrive in the desert. Some of these structures survive to this day.
In the Roman era, builders discovered that adding volcanic ash to lime allowed the mix to set underwater. They discovered the pozzolanic reaction.
Classical era
The Romans used concrete extensively from 300 BC to AD 476. During the Roman Empire, Roman concrete 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 materials. It enabled revolutionary new designs in terms of both structural complexity and dimension. The Colosseum in Rome was built largely of concrete, and the Pantheon has the world's largest unreinforced concrete dome.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 troubled the builders of similar structures in stone or brick.
Modern tests show that opus caementicium had a similar compressive strength to modern Portland-cement concrete. However, due to the absence of reinforcement, its tensile strength was far lower than modern reinforced concrete, and its mode of application also differed:
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.
The long-term durability of Roman concrete structures was found to be due to the presence of pyroclastic rock and ash in the concrete mix. The crystallization of strätlingite during the formation of the concrete and its merging with similar calcium–aluminium-silicate–hydrate structures helped give the Roman concrete a greater degree of fracture resistance compared to modern concrete. In addition, Roman concrete is significantly more resistant to erosion by seawater than modern concrete; the aforementioned pyroclastic materials react with seawater to form Al-tobermorite crystals over time. The use of hot mixing in preparation of concrete, leading to the formation of lime clasts in the final product, has been proposed to give the Roman concrete a self-healing ability.
The widespread use of concrete in many Roman structures ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges, such as the magnificent Pont du Gard in southern France, have masonry cladding on a concrete core, as does the dome of the Pantheon.
Middle Ages
After the Roman Empire, the use of burned lime and pozzolana was greatly reduced. Low kiln temperatures in the burning of lime, lack of pozzolana, and poor mixing all contributed to a decline in the quality of concrete and mortar. From the 11th century, the increased use of stone in church and castle construction led to an increased demand for mortar. Quality began to improve in the 12th century through better grinding and sieving. Medieval lime mortars and concretes were non-hydraulic and were used for binding masonry, "hearting" and foundations. Bartholomaeus Anglicus in his De proprietatibus rerum describes the making of mortar. In an English translation from 1397, it reads "lyme... is a stone brent; by medlynge thereof with sonde and water sement is made". From the 14th century, the quality of mortar was again excellent, but only from the 17th century was pozzolana commonly added.The Canal du Midi was built using concrete in 1670.File:Smeaton's Lighthouse00.jpg|thumb|upright=1.1|Smeaton's Tower in Devon, England
Industrial era
Perhaps the greatest step forward in the modern use of concrete was Smeaton's Tower, built by British engineer John Smeaton in Devon, England, between 1756 and 1759. This third Eddystone Lighthouse pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate.A method for producing Portland cement was developed in England and patented by Joseph Aspdin in 1824. Aspdin chose the name for its similarity to Portland stone, which was quarried on the Isle of Portland in Dorset, England. His son William continued developments into the 1840s, earning him recognition for the development of "modern" Portland cement.
Reinforced concrete was invented in 1849 by Joseph Monier. and the first reinforced concrete house was built by François Coignet in 1853.
The first concrete reinforced bridge was designed and built by Joseph Monier in 1875.
Prestressed concrete and post-tensioned concrete were pioneered by Eugène Freyssinet, a French structural and civil engineer. Concrete components or structures are compressed by tendon cables during, or after, their fabrication in order to strengthen them against tensile forces developing when put in service. Freyssinet patented the technique on 2 October 1928.
Composition
Concrete is an artificial composite material, comprising a matrix of cementitious binder and a dispersed phase or "filler" of aggregate. The binder "glues" the filler together to form a synthetic conglomerate. Many types of concrete are available, determined by the formulations of binders and the types of aggregate used to suit the application of the engineered material. These variables determine strength and density, as well as chemical and thermal resistance of the finished product.File:DB Museum rail and concrete sleeper cross section 2.jpg|thumb|Cross section of a concrete railway sleeper below a rail
Construction aggregates consist of large chunks of material in a concrete mix, generally a coarse gravel or crushed rocks such as limestone, or granite, along with finer materials such as sand.
Cement paste, most commonly made of Portland cement, is the most prevalent kind of concrete binder. For cementitious binders, water is mixed with the dry cement powder and aggregate, which produces a semi-liquid slurry that can be shaped, typically by pouring it into a form. The concrete solidifies and hardens through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, creating a robust, stone-like material. Other cementitious materials, such as fly ash and slag cement, are sometimes added—either pre-blended with the cement or directly as a concrete component—and become a part of the binder for the aggregate. Fly ash and slag can enhance some properties of concrete such as fresh properties and durability. Alternatively, other materials can also be used as a concrete binder: the most prevalent substitute is asphalt, which is used as the binder in asphalt concrete.
Admixtures are added to modify the cure rate or properties of the material. [|Mineral admixtures] use recycled materials as concrete ingredients. Conspicuous materials include fly ash, a by-product of coal-fired power plants; ground granulated blast furnace slag, a by-product of steelmaking; and silica fume, a by-product of industrial electric arc furnaces.
Structures employing Portland cement concrete usually include [|steel reinforcement] because this type of concrete can be formulated with high compressive strength, but always has lower tensile strength. Therefore, it is usually reinforced with materials that are strong in tension, typically steel rebar.
The mix design depends on the type of structure being built, how the concrete is mixed and delivered, and how it is placed to form the structure.