Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid. It is the second-most-used substance (after water), the most widely used building material, and the most-manufactured material in the world. Cement-bound concrete differs from the less rigid, less durable asphaltic concrete, which has a bituminous binder.
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 (such as pozzolans or superplasticizers) 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 (such as steel rebar) 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 (large) and fine (small) 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 "" (meaning compact or condensed), the perfect passive participle of "", from "-" (together) and "" (to grow).
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 (688 BC) made use of waterproof concrete. Concrete was used for construction in many ancient structures.
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.
Classical era
The Romans used concrete extensively from 300 BC to AD 476. 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 materials. It enabled revolutionary new designs in terms of both structural complexity and dimension. The Colosseum in Rome was built largely of concrete. 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.D.S. Robertson (1969). Greek and Roman Architecture, Cambridge, p. 233
Modern tests show that opus caementicium had a similar compressive strength to modern Portland-cement concrete (c. ), 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 (volcanic) rock and ash in the concrete mix. The crystallization of strätlingite (a complex calcium aluminosilicate hydrate) 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, sometimes discussed as having being self-healing qualities. 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 as the technique giving the Roman concrete a self-healing ability.
The widespread use of their 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 In Britannia after the Roman Empire, there is evidence of the continued use of burned lime and remains of an 8th century mortar mill have been found in Northamptonshire. It is thought that low kiln temperatures in the burning of lime, lack of pozzolana, poor mixing, and an Anglo-Saxon culture of timber construction all contributed to a decline in the quality of concrete and mortar there, but it did not become a lost art. From the 11th century in England, 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" (binding rubble masonry cores) and foundations. Bartholomaeus Anglicus in his De proprietatibus rerum (1240) 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.Peter Hewlett and Martin Liska (eds.), Lea's Chemistry of Cement and Concrete, 5th ed. (Butterworth-Heinemann, 2019), pp. 3–4.
Mayan concrete at the ruins of Uxmal (AD 850–925) 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."
Medieval hydraulic mortars were made from the addition of pozzolans such as crushed ceramics, volcanic soils, or metamorphosed soils from specific deposits with known pozzolanic properties, like Trass or noncrystalline opal. Hydraulic mortar was used in medieval Crete. The use of specific sands containing volcanic ash in 10th-11th century south-Italian mortars may indicate a knowledge of its properties.
The Roman architectural treatise De architectura by Vitruvius contained a description of Roman cement and a copy at the scriptorium of Charlemagne produced many of the medieval copies that subsequently survived. A copy at Saint Gall Abbey was found by Poggio Bracciolini in 1414. The first printed edition was published in 1486 by Fra Giovanni Sulpitius and versions in Italian, German, French, Spanish, and English were published between 1520 and 1692.
Early modern period A 16th-century dockyard in Venice used hydraulic lime mortars in foundations. Hydraulic mortar was also used in Ottoman baths in Budapest from the same period.
The Canal du Midi was built using concrete in 1670.
Trass from the Rhineland was added to lime in England in the 17th century, including for the mole of English Tangier (1663-1683), which also used Italian pozzolana recommended and supplied by Genoese engineers. At the port of Toulon, concrete foundations were built using pozzolana or trass, quicklime, sand, pebbles, and slag or cinder. The 1752 reconstruction of the foundations of Essex bridge by George Semple included a mixture of small stones, sand, and powdered lime.
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.
Modern period 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.« Château de Chazelet » [archive], notice no PA00097319, base Mérimée, ministère français de la Culture.
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 (typically Portland cement paste or asphalt) and a dispersed phase or "filler" of aggregate (typically a rocky material, loose stones, and sand). 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.