is a composite material composed of fine and coarse construction aggregate bonded together with a fluid cement (cement paste) that hardens over time—most frequently a Lime (material)-based cement binder, such as Portland cement, but sometimes with other hydraulic cements, such as a calcium aluminate cements. It is distinguished from other, non-cementitious types of concrete all binding some form of 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.
When aggregate is mixed together with dry Portland cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix that binds the materials together into a durable stone-like material that has many uses.Zongjin Li; ''Advanced concrete technology''; 2011 Often, additives (such as pozzolans or superplasticizers) are included in the mixture to improve the physical properties of the wet mix or the finished material. Most concrete is poured with reinforcing materials (such as rebar) embedded to provide tensile strength, yielding reinforced concrete.
Famous concrete structures include the Hoover Dam, the Panama Canal and the Roman Pantheon, Rome. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome. Today, large concrete structures (for example, dams and multi-storey car parks) are usually made with reinforced concrete.
Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, Vault (architecture) and List of Roman 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: ''Greek and Roman Architecture'', Cambridge, 1969, p. 233
Modern tests show that ''opus caementicium'' had as much compressive strength as modern Portland-cement concrete (ca. ).Henry Cowan: ''The Masterbuilders,'' New York 1977, p. 56,
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.Robert Mark, Paul Hutchinson: "On the Structure of the Roman Pantheon", ''Art Bulletin'', Vol. 68, No. 1 (1986), p. 26, fn. 5
The long-term durability of Roman concrete structures has been found to be due to its use of Pyroclastic rock (volcanic) rock and ash, whereby crystallization of strätlingite and the coalescence of calcium–aluminum-silicate–hydrate cementing binder helped give the concrete a greater degree of fracture resistance even in seismically active environments.. allacademic.com
Reinforced concrete was invented in 1849 by Joseph Monier.[http://inventors.about.com/library/inventors/blconcrete.htm The History of Concrete and Cement]. Inventors.about.com (2012-04-09). Retrieved on 2013-02-19.
In 1889 the first concrete reinforced bridge was built, and the first large concrete dams. Auburn.edu. Retrieved on 2013-02-19.
#Aggregates consists 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, most commonly Portland cement, is associated with the general term "concrete." A range of other materials can be used as the cement in concrete too. One of the most familiar of these alternative cements is asphalt concrete. Other cementitious materials such as fly ash and slag cement, are sometimes added as mineral admixtures (see below) – either pre-blended with the cement or directly as a concrete component – and become a part of the binder for the aggregate.
To produce concrete from most cements (excluding asphalt), water is mixed with the dry 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 reaction called mineral hydration. The water reacts with the cement, which bonds the other components together, creating a robust stone-like material.
#Chemical admixtures are added to achieve varied properties. These ingredients may accelerate or slow down the rate at which the concrete hardens, and impart many other useful properties including increased tensile strength, entrainment of air and water resistance.
#Reinforcement is often included in concrete. Concrete can be formulated with high compressive strength, but always has lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension, typically steel rebar.
#Mineral admixtures and blended cements have become more popular over recent decades. The use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation, and the discovery that such materials often have complementary and valuable properties. The most conspicuous of these are fly ash, a by-product of Fossil fuel power plant, ground granulated blast furnace slag, a byproduct of steelmaking, and silica fume, a byproduct of industrial electric arc furnaces. The use of these materials in concrete reduces the amount of resources required, as the mineral admixtures act as a partial cement replacement. This displaces some cement production, an energetically expensive and environmentally problematic process, while reducing the amount of industrial waste that must be disposed of. Mineral admixtures can be pre-blended with the cement during its production for sale and use as a blended cement, or mixed directly with other components when the concrete is produced.
The ''types of concrete#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.
Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar (masonry) and many plasters. British masonry worker Joseph Aspdin patented Portland cement in 1824. It was named because of the similarity of its colour to Portland stone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of calcium silicates (alite, belite), tricalcium aluminate and calcium aluminoferrite – compounds which combine calcium, silicon, aluminium and iron in forms which will react with water. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay or shale (a source of silicon, aluminium and iron) and grinding this product (called ''clinker (cement)'') with a source of sulfate (most commonly gypsum).
In modern cement kilns many advanced features are used to lower the fuel consumption per ton of clinker produced. Cement kilns are extremely large, complex, and inherently dusty industrial installations, and have emissions which must be controlled. Of the various ingredients used to produce a given quantity of concrete, the cement is the most energetically expensive. Even complex and efficient kilns require 3.3 to 3.6 gigajoules of energy to produce a ton of clinker and then Cement mill. Many kilns can be fueled with difficult-to-dispose-of wastes, the most common being used tires. The extremely high temperatures and long periods of time at those temperatures allows cement kilns to efficiently and completely burn even difficult-to-use fuels.
:Cement chemist notation: C3S + H → C-S-H + CH
:Standard notation: Ca3SiO5 + H2O → (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2
:Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)·2(SiO2)·4(H2O)(gel) + 3Ca(OH)2 (approximately; the exact ratios of the CaO, SiO2 and H2O in C-S-H can vary)
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.
The size distribution of the aggregate determines how much binder is required. Aggregate with a very even size distribution has the biggest gaps whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate as well as pasting the surfaces of the aggregate together, and is typically the most expensive component. Thus variation in sizes of the aggregate reduces the cost of concrete.[http://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Aggregate/Aggregatesmain.htm The Effect of Aggregate Properties on Concrete] . Engr.psu.edu. Retrieved on 2013-02-19. The aggregate is nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.
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.
In addition to being decorative, exposed aggregate may add robustness to a concrete.[http://www.uniquepaving.com.au/exposed_aggregate.htm Exposed Aggregate]
Concrete is strong in compression (physical), as the aggregate efficiently carries the compression load. However, it is weak in Tension (physics) as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete adds either rebar, Fiber-reinforced concrete, glass fibers, or plastic fibers to carry tension (physics).
''Workability'' is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration reaction) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability. Excessive water leads to increased bleeding or Segregation in concrete (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate blend with an undesirable gradation
Workability can be measured by the concrete slump test, a simple measure of the plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Duff Abrams" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod to consolidate the layer. When the cone is carefully lifted off, the enclosed material slumps a certain amount, owing to gravity. A relatively dry sample slumps very little, having a slump value of one or two inches (25 or 50 mm) out of one foot (305 mm). A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by the flow table test.
Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the water-cement ratio. Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.
High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.
After mixing, concrete is a fluid and can be pumped to the location where needed.
Concrete must be kept moist during curing in order to achieve optimal strength and durability."Curing Concrete" Peter C. Taylor CRC Press 2013. During curing hydrate occurs, allowing calcium-silicate hydrate (C-S-H) to form. Over 90% of a mix's final strength is typically reached within four weeks, with the remaining 10% achieved over years or even decades. The conversion of calcium hydroxide in the concrete into calcium carbonate from absorption of carbon dioxide over several decades further strengthens the concrete and makes it more resistant to damage. This carbonation reaction, however, lowers the pH of the cement pore solution and can corrode the reinforcement bars.
Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained sufficient strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. The strength of concrete changes (increases) for up to three years. It depends on cross-section dimension of elements and conditions of structure exploitation.Resulting strength distribution in vertical elements researched and presented at the article [http://www.concreteresearch.org/PDFsandsoon/Inhomog%20Denver.pdf "Concrete inhomogeneity of vertical cast-in-place elements in skeleton-type buildings".] Addition of short-cut polymer fibers can improve (reduce) shrinkage-induced stresses during curing and increase early and ultimate compression strength.[http://www.minifibers.com/documents/ADMIXUS-Admixtures-for-Cementitious-Applications.pdf "Admixtures for Cementitious Applications."]
Properly curing concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing or overheating due to the exothermic setting of cement. Improper curing can cause Spalling#Spalling in mechanical weathering, reduced strength, poor abrasion (mechanical) resistance and fracture.
Nanoconcrete is created by high-energy mixing (HEM) of cement, sand and water. To ensure the mixing is thorough enough to create nano-concrete, the mixer must apply a total mixing power to the mixture of 30–600 watts per kilogram of the mix. This mixing must continue long enough to yield a net Energy density expended upon the mix of at least 5000 joules per kilogram of the mix. A plasticizer or a superplasticizer is then added to the activated mixture which can later be mixed with aggregates in a conventional concrete mixer. In the HEM process, the intense mixing of cement and water with sand provides dissipation of energy and increases shear stresses on the surface of cement particles. This intense mixing serves to divide the cement particles into extremely fine nanometer scale sizes, which provides for extremely thorough mixing. This results in the increased volume of water interacting with cement and acceleration of Calcium Silicate Hydrate (C-S-H) colloid creation.
The initial natural process of cement hydration with formation of colloidal globules about 5 nm in diameter spreads into the entire volume of cement – water matrix as the energy expended upon the mix approaches and exceeds 5000 joules per kilogram.
The liquid activated high-energy mixture can be used by itself for casting small architectural details and decorative items, or foamed (Expanded polystyrene concrete) for lightweight concrete. HEM Nanoconcrete hardens in low and subzero temperature conditions and possesses an increased volume of gel, which reduces capillarity in solid and porous materials.
Tests can be performed to ensure that the properties of concrete correspond to specifications for the application.
Different mixes of concrete ingredients produce different strengths. Concrete strength values are usually specified as the lower-bound compressive strength of either a cylindrical or cubic specimen as determined by standard test procedures.
Different strengths of concrete are used for different purposes. Very low-strength –
Concrete is one of the most durable building materials. It provides superior fire resistance compared with wooden construction and gains strength over time. Structures made of concrete can have a long service life. Concrete is used more than any other artificial material in the world.
Due to cement's exothermic chemical reaction while setting up, large concrete structures such as dams, navigation locks, large mat foundations, and large breakwater (structure) generate excessive heat during hydration and associated expansion. To mitigate these effects ''post-cooling''[http://www.ce.berkeley.edu/~paulmont/165/Mass_concrete2.pdf Mass Concrete] . (PDF) . Retrieved on 2013-02-19. is commonly applied during construction. An early example at Hoover Dam, installed a network of pipes between vertical concrete placements to circulate cooling water during the curing process to avoid damaging overheating. Similar systems are still used; depending on volume of the pour, the concrete mix used, and ambient air temperature, the cooling process may last for many months after the concrete is placed. Various methods also are used to pre-cool the concrete mix in mass concrete structures.
Another approach to mass concrete structures that minimizes cement's thermal byproduct is the use of roller-compacted concrete, which uses a dry mix which has a much lower cooling requirement than conventional wet placement. It is deposited in thick layers as a semi-dry material then roller compactor into a dense, strong mass.
The minimum strength before exposing concrete to extreme cold is 500 psi (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.
Once in place, concrete offers great energy efficiency over the lifetime of a building.John Gajda (2001) [https://web.archive.org/web/20110916080530/http://www.arxx.com/Assets/CMS/WYSIWYGBase/file/5825%20PCA%20CD026.pdf Energy Use of Single Family Houses with Various Exterior Walls], Construction Technology Laboratories Inc. Concrete walls leak air far less than those made of wood frames. While insulation reduces energy loss through the building envelope, thermal mass uses walls to store and release energy. Modern concrete wall systems use both external insulation and thermal mass to create an energy-efficient building. Insulating concrete forms (ICFs) are hollow blocks or panels made of either insulating foam or rastra that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.
Concrete buildings are more resistant to fire than those constructed using steel frames, since concrete has lower heat conductivity than steel and can thus last longer under the same fire conditions. Concrete is sometimes used as a fire protection for steel frames, for the same effect as above. Concrete as a fire shield, for example Fondu fyre, can also be used in extreme environments like a missile launch pad.
Options for non-combustible construction include floors, ceilings and roofs made of cast-in-place and hollow-core precast concrete. For walls, concrete masonry technology and Insulating concrete forms (ICFs) are additional options. ICFs are hollow blocks or panels made of fireproof insulating foam that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.
Concrete also provides good resistance against externally applied forces such as high winds, hurricanes, and tornadoes owing to its lateral stiffness, which results in minimal horizontal movement. However this stiffness can work against certain types of concrete structures, particularly where a relatively higher flexing structure is required to resist more extreme forces.
Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel rebar, freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distillate water).
Concrete is widely used for making architectural structures, foundation (engineering), brick/Concrete masonry unit walls, Sidewalk, bridges/overpasses, highways, runways, parking structures, dams, pools/reservoirs, pipes, foundation (engineering) for gates, fences and Utility pole and even boats. Concrete is used in large quantities almost everywhere there is a need for infrastructure. Concrete is one of the most frequently used building materials in animal houses and for manure and silage storage structures in agriculture.
The cement industry is one of the three primary producers of carbon dioxide, a major greenhouse gas (the other two being the energy production and transportation industries). As of 2001, the production of Portland cement contributed 7% to global anthropogenic CO2 emissions, largely due to the sintering of limestone and clay at Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution.
The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicity and radioactivity.
Fresh concrete (before curing is complete) is highly alkaline and must be handled with proper protective equipment.
Concrete recycling is an increasingly common method for disposing of concrete structures. Concrete debris was once routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws and economic benefits.
Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through a crusher, often along with asphalt, bricks and rocks.
Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Aggregate base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt placed over it. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions. On 3 March 1983, a government-funded research team (the VIRL research.codep) estimated that almost 17% of worldwide landfill was by-products of concrete based waste.
The world record for concrete pumping was set on 7 August 2009 during the construction of the Parbati River (Himachal Pradesh) Hydroelectric Project, near the village of Suind, Himachal Pradesh, India, when the concrete mix was pumped through a vertical height of
The world record for the largest continuously poured concrete raft was achieved in August 2007 in Abu Dhabi by contracting firm Al Habtoor-CCC Joint Venture and the concrete supplier is Unibeton Ready Mix. The pour (a part of the foundation for the Abu Dhabi's The Landmark (Abu Dhabi)) was 16,000 cubic meters of concrete poured within a two-day period.[http://www.leighton.com.au/verve/_resources/AlHabtoorIssue24.pdf Al Habtoor Engineering] – ''Abu Dhabi – Landmark Tower has a record-breaking pour'' – September/October 2007, Page 7. The previous record, 13,200 cubic meters poured in 54 hours despite a severe tropical storm requiring the site to be covered with tarpaulins to allow work to continue, was achieved in 1992 by joint Japanese and South Korean consortiums Hazama Corporation and the Samsung C&T Corporation for the construction of the Petronas Towers in Kuala Lumpur, Malaysia.National Geographic Channel International / Caroline Anstey (2005), Megastructures: Petronas Twin Towers
The world record for largest continuously poured concrete floor was completed 8 November 1997, in Louisville, Kentucky, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement consisted of 12 May 2011