Concrete is a
compositeComposite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
construction material, composed of
cementIn the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
(commonly
Portland cementPortland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
) and other cementitious materials such as
fly ashFly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
and slag cement,
aggregateConstruction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
(generally a coarse aggregate made of gravel or crushed rocks such as
limestoneLimestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
, or
graniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
, plus a fine aggregate such as
sandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
),
waterWater is the most abundant compound on Earth's surface, covering about 70%. In nature, it exists in liquid, solid, and gaseous states. It is in dynamic equilibrium between the liquid and gas states at standard temperature and pressure. At room temperature, it is a tasteless and odorless liquid,...
and
chemicalChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
admixtures.
The word concrete comes from the Latin word "concretus" (meaning compact or condensed), the perfect passive participle of "concrescere", from "con-" (together) and "crescere" (to grow).
Concrete solidifies and hardens after mixing with water and placement due to a
chemical processA chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
known as
hydrationMineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....
. The water reacts with the cement, which bonds the other components together, eventually creating a robust stone-like material. Concrete is used to make
pavementsA sidewalk, or pavement, footpath, footway, and sometimes platform, is a path along the side of a road. A sidewalk may accommodate moderate changes in grade and is normally separated from the vehicular section by a curb...
, pipe,
architectural structureAn architectural structure is a free-standing, immobile outdoor constructed element. The structure may be temporary or permanent.Structures include buildings and nonbuilding structures . Examples of building structures include houses, town halls, libraries, and skyscrapers...
s, foundations, motorways/roads, bridges/
overpassAn overpass is a bridge, road, railway or similar structure that crosses over another road or railway...
es,
parkingParking is the act of stopping a vehicle and leaving it unoccupied for more than a brief time. Parking on one or both sides of a road is commonly permitted, though often with restrictions...
structures, brick/block walls, footings for gates,
fenceA fence is a freestanding structure designed to restrict or prevent movement across a boundary. It is generally distinguished from a wall by the lightness of its construction: a wall is usually restricted to such barriers made from solid brick or concrete, blocking vision as well as passage .Fences...
s and
polesA utility pole is a pole used to support overhead power lines and various other public utilities, such as cable, fibre optic cable, and related equipment such as transformers and street lights. It can be referred to as a telephone pole, power pole, hydro pole, telegraph pole, or telegraph post,...
and even
boatA boat is a watercraft of any size designed to float or plane, to provide passage across water. Usually this water will be inland or in protected coastal areas. However, boats such as the whaleboat were designed to be operated from a ship in an offshore environment. In naval terms, a boat is a...
s.
Concrete is used more than any other man-made material in the world. As of 2006, about 7.5 billion cubic meters of concrete are made each year—more than one cubic meter for every person on Earth.
Concrete powers a industry, employing more than two million workers in the United States alone. More than 55000 miles (88,513.7 km) of highways in the United States are paved with this material.
Reinforced concreteReinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
,
prestressed concretePrestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete...
and
precast concreteBy producing precast concrete in a controlled environment , the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. Utilizing a Precast Concrete system offers many potential advantages over site casting of concrete...
are the most widely used types of concrete functional extensions in modern days.
History
Concrete was used for construction in many ancient structures.
During the
Roman EmpireThe Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....
,
Roman concreteRoman concrete was a material used in construction during the late Roman Republic through the whole history of the Roman Empire. Roman concrete was based on a hydraulic-setting cement with many material qualities similar to modern Portland cement...
(or
opus caementicium) was made from quicklime,
pozzolanaPozzolana, also known as pozzolanic ash , is a fine, sandy volcanic ash. Pozzolanic ash was first discovered and dug in Italy, at Pozzuoli. It was later discovered at a number of other sites as well...
and an aggregate of
pumicePumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
. Its widespread use in many Roman structures, a key event in the
history of architectureThe history of architecture traces the changes in architecture through various traditions, regions, overarching stylistic trends, and dates.-Neolithic architecture:Neolithic architecture is the architecture of the Neolithic period...
termed the
Roman Architectural RevolutionThe Roman Architectural Revolution, also known as the concrete Revolution, is a term used to describe the widespread use in Roman architecture of the previously little-used architectural forms of the arch, vault and dome...
, freed
Roman constructionRomans are famous for their advanced engineering accomplishments, although some of their own inventions were improvements on older ideas, concepts and inventions. Technology for bringing running water into cities was developed in the east, but transformed by the Romans into a technology...
from the restrictions of stone and brick material and allowed for revolutionary new designs in terms of both structural complexity and dimension.
Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of archAn arch is a structure that spans a space and supports a load. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.-Technical aspects:The...
es, vaultsA Vault is an architectural term for an arched form used to provide a space with a ceiling or roof. The parts of a vault exert lateral thrust that require a counter resistance. When vaults are built underground, the ground gives all the resistance required...
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 as much compressive strength as modern Portland-cement concrete (ca. 200 kg/cm
2). However, due to the absence of
steel reinforcementReinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
, 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 rubbleRubble is broken stone, of irregular size, shape and texture. This word is closely connected in derivation with "rubbish", which was formerly also applied to what we now call "rubble". Rubble naturally found in the soil is known also as brash...
. 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 widespread use of concrete in many Roman structures has ensured that many survive to the present day. The
Baths of CaracallaThe Baths of Caracalla in Rome, Italy were Roman public baths, or thermae, built in Rome between AD 212 and 216, during the reign of the Emperor Caracalla.- History :...
in Rome are just one example. Many
Roman aqueductThe Romans constructed numerous aqueducts to serve any large city in their empire, as well as many small towns and industrial sites. The city of Rome had the largest concentration of aqueducts, with water being supplied by eleven aqueducts constructed over a period of about 500 years...
s and bridges have masonry cladding on a concrete core, as does the dome of the
PantheonThe Pantheon ,Rarely Pantheum. This appears in Pliny's Natural History in describing this edifice: Agrippae Pantheum decoravit Diogenes Atheniensis; in columnis templi eius Caryatides probantur inter pauca operum, sicut in fastigio posita signa, sed propter altitudinem loci minus celebrata.from ,...
.
Some have stated that the secret of concrete was lost for 13 centuries until 1756, when the British engineer
John SmeatonJohn Smeaton, FRS, was an English civil engineer responsible for the design of bridges, canals, harbours and lighthouses. He was also a capable mechanical engineer and an eminent physicist...
pioneered the use of
hydraulic limeHydraulic lime is a variety of lime, a slaked lime used to make lime mortar. Hydraulicity is the ability of lime to set under water. Hydraulic lime is produced by heating calcining limestone that contains clay and other impurities. Calcium reacts in the kiln with the clay minerals to produce...
in concrete, using pebbles and powdered brick as aggregate. However, the
Canal du MidiThe is a long canal in Southern France . The canal connects the Garonne River to the on the Mediterranean and along with the Canal de Garonne forms the Canal des Deux Mers joining the Atlantic to the Mediterranean. The canal runs from the city of Toulouse down to the Étang de Thau...
was built using concrete in 1670. Likewise there are concrete structures in Finland that date back to the 16th century.
Portland cementPortland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
was first used in concrete in the early 1840s.
Additives
Concrete
additiveAdditive may refer to:* Additive function, a function that preserves the addition operation* Additive inverse, an arithmetic concept* Additive category, a preadditive category with finite biproducts...
s 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 hairHorsehair is the long, coarse hair growing on the manes and tails of horses. It is used for various purposes, including upholstery, brushes, the bows of musical instruments, a hard-wearing fabric called haircloth, and for horsehair plaster, a wallcovering material formerly used in the construction...
made concrete less liable to crack while it hardened and adding blood made it more frost-resistant.
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 ashFly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
, a by-product of coal-fired power plants. This use reduces the amount of quarrying and landfill space required as the ash acts as a cement replacement thus reducing 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.
Composition
There are many
types of concreteThere are many types of concrete, variations of installation, composition, finish and performance characteristics.-Mix design:Modern concrete mix designs can be complex...
available, created by varying the proportions of the main ingredients below. In this way or by substitution for the cemetitious and aggregate phases, the finished product can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.
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,
mortarMortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder...
and
plasterPlaster is a building material used for coating walls and ceilings. Plaster starts as a dry powder similar to mortar or cement and like those materials it is mixed with water to form a paste which liberates heat and then hardens. Unlike mortar and cement, plaster remains quite soft after setting,...
. English masonry worker
Joseph AspdinJoseph Aspdin was a British cement manufacturer who obtained the patent for Portland cement on 21 October 1824....
patented Portland cement in 1824; it was named because of its similarity in color to
Portland limestonePortland stone is a limestone from the Tithonian stage of the Jurassic period quarried on the Isle of Portland, Dorset. The quarries consist of beds of white-grey limestone separated by chert beds. It has been used extensively as a building stone throughout the British Isles, notably in major...
, quarried from the English
Isle of PortlandThe Isle of Portland is a limestone tied island, long by wide, in the English Channel. Portland is south of the resort of Weymouth, forming the southernmost point of the county of Dorset, England. A tombolo over which runs the A354 road connects it to Chesil Beach and the mainland. Portland and...
and used extensively in London architecture. It consists of a mixture of oxides of
calciumCalcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
,
siliconThe chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
and
aluminiumAluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
. Portland cement and similar materials are made by heating
limestoneLimestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
(a source of calcium) with clay and grinding this product (called
clinkerthumb|200px|right|Typical clinker nodulesthumb|200px|right|Hot clinkerIn the manufacture of Portland cement, clinker is lumps or nodules, usually 3-25 mm in diameter, produced by sintering limestone and alumino-silicate during the cement kiln stage.-Uses:...
) with a source of
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
(most commonly
gypsumGypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
).
Water
Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it and allows it to flow more freely.
Less water in the cement paste will yield a stronger, more durable concrete; more water will give a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure.
Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete, to form a solid mass.
Reaction:
- Cement chemist notation
Cement chemist notation was developed to simplify the formulas cement chemists use on a daily basis. It is a "short hand" way of writing the chemical formula of oxides of calcium, silicon, and various metals.-Abbreviations of oxides:...
: 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
Aggregates
Fine and coarse aggregates make up the bulk of a concrete mixture.
SandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, natural gravel and
crushed stoneCrushed stone or angular rock is a form of construction aggregate, typically produced by mining a suitable rock deposit and breaking the removed rock down to the desired size using crushers...
are used mainly 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 furnaceA blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...
slag and
bottom ashBottom ash refers to part of the non-combustible residues of combustion. In an industrial context, it usually refers to coal combustion and comprises traces of combustibles embedded in forming clinkers and sticking to hot side walls of a coal-burning furnace during its operation. The portion of...
are also permitted.
Decorative stones such as
quartziteQuartzite is a hard metamorphic rock which was originally sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to gray, though quartzites often occur in various shades of pink...
, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.
The presence of aggregate greatly increases the robustness of concrete above that of cement, which otherwise is a brittle material and thus concrete is a true composite material.
Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients.
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 crack, allowing the structure to fail.
Reinforced concreteReinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
solves these problems by adding either
steel reinforcing barsA rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
, steel fibers, glass fiber, or plastic fiber to carry tensile loads. Thereafter the concrete is reinforced to withstand the tensile loads upon it.
Chemical admixtures
ChemicalChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing. The common types of admixtures are as follows.
- Accelerators
Accelerants play a major role in chemistry. Most chemical reactions can be hastened with an accelerant. Accelerants alter a chemical bond, speed up a chemical process, or bring organisms back to homeostasis. Accelerants are not necessarily catalysts as they may be consumed by the process...
speed up the hydration (hardening) of the concrete. Typical materials used are Calcium chloride, CaCl2, is a salt of calcium and chlorine. It behaves as a typical ionic halide, and is solid at room temperature. Common applications include brine for refrigeration plants, ice and dust control on roads, and desiccation...
, Ca(NO3)2Calcium nitrate, also called Norgessalpeter , is the inorganic compound with the formula Ca2. This colourless salt absorbs moisture from the air and is commonly found as a tetrahydrate. It is mainly used as a component in fertilizers but is found other applications...
and NaNO3Sodium nitrate is the chemical compound with the formula NaNO3. This salt, also known as Chile saltpeter or Peru saltpeter to distinguish it from ordinary saltpeter, potassium nitrate, is a white solid which is very soluble in water...
. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored.
- Retarders
A retarder is a chemical agent that slows down a chemical reaction. For example, retarders are used to slow the chemical hardening of plastic materials such as wallboard, concrete, and adhesives.Sugar water acts as a retarder for the curing of concrete...
slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable. Typical polyolA polyol is an alcohol containing multiple hydroxyl groups. In two technological disciplines the term "polyol" has a special meaning: food science and polymer chemistry.- Polyols in food science :...
retarders are sugarSugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
, sucroseSucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
, sodium gluconateSodium gluconate is a compound with formula NaC6H11O7. It is the sodium salt of gluconic acid.It has E number E576....
, glucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
, citric acidCitric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...
, and tartaric acidTartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...
.
- Air entrainment
-Air entrainment in concrete:Air entrainment is the intentional creation of tiny air bubbles in concrete. The bubbles are introduced into the concrete by the addition to the mix of an air entraining agent, a surfactant...
s add and entrain tiny air bubbles in the concrete, which will reduce damage during freeze-thawWeathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
cycles, thereby increasing the concrete's durability. However, entrained air entails a trade off with strength, as each 1% of air may result in 5% decrease in compressive strength.
- Plasticizer
Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
s increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics. SuperplasticizerSuperplasticizers, also known as high range water reducers, are chemicals used as admixtures where well-dispersed particle suspension are required. These polymers are used as dispersants to avoid particle aggregation, and to improve the flow characteristics of suspensions such as in concrete...
s (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Compounds used as superplasticizers include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers.
- Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s can be used to change the color of concrete, for aesthetics.
- Corrosion inhibitor
A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy. The effectiveness of a corrosion inhibitor depends on fluid composition, quantity of water, and flow regime...
s are used to minimize the corrosion of steel and steel bars in concrete.
- Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) .
- Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.
Mineral admixtures and blended cements
There are inorganic materials that also have
pozzolanA pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete, and in some cases...
ic or latent hydraulic properties. These very
fine-grainedGranularity is the extent to which a system is broken down into small parts, either the system itself or its description or observation. It is the "extent to which a larger entity is subdivided...
materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cements).
- Fly ash
Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
: A by-product of coal-fired electric generating plantsA power station is an industrial facility for the generation of electric energy....
, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareousCalcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...
fly ash has latent hydraulic properties.
- Ground granulated blast furnace slag
Ground-granulated blast-furnace slag is obtained by quenching molten iron slag from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder.-Applications:GGBS is used to make durable concrete structures in combination with ordinary...
(GGBFS or GGBS): A by-product of steel production is used to partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties.
- Silica fume
Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.It is sometimes confused with fumed silica and colloidal silica...
: A by-product of the production of silicon and ferrosiliconFerrosilicon, or ferrosilicium, is a ferroalloy, an alloy of iron and silicon with between 15% and 90% silicon. It contains a high proportion of iron silicides. Its melting point is about 1200 °C to 1250 °C with a boiling point of 2355 °C...
alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.
- High reactivity Metakaolin
Metakaolin is a dehydroxylated form of the clay mineral kaolinite.Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.-Forming...
(HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high-reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.
Concrete production
The processes used vary dramatically, from hand tools to heavy industry, but result in the concrete being placed where it cures into a final form. Wide range of technological factors may occur during production of concrete elements and their influence to basic characteristics may vary.
When initially mixed together, Portland cement and water rapidly form a
gelA gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
, formed of tangled chains of interlocking crystals. These continue to react over time, with the initially fluid gel often aiding in placement by improving workability. As the concrete sets, the chains of crystals join and form a rigid structure, gluing the aggregate particles in place. During curing, more of the cement reacts with the residual water (
hydrationIn organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous...
).
This curing process develops physical and
chemical propertiesA chemical property is any of a material's properties that becomes evident during a chemical reaction; that is, any quality that can be established only by changing a substance's chemical identity...
. Among these qualities are
mechanical strengthIn materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...
, low moisture permeability and chemical and volumetric stability.
Mixing concrete
Thorough mixing is essential for the production of uniform, high quality concrete. For this reason equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce
uniform mixtures of the lowest slump practical for the work.
Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with
aggregatesConstruction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
can increase the
compressive strengthCompressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g...
of the resulting concrete. The paste is generally mixed in a
high-speed, shear-type mixer at a
w/cmThe water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix and has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place...
(water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders,
superplasticizersPlasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
,
pigmentA pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s, or
silica fumeSilica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.It is sometimes confused with fumed silica and colloidal silica...
. The premixed paste is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment.
High-energy mixed (HEM) concrete is produced by means of high-speed mixing of cement, water and sand with net
specific energyEnergy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
consumption of at least 5 kilojoules per kilogram of the mix. A
plasticizerPlasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
or a
superplasticizerSuperplasticizers, also known as high range water reducers, are chemicals used as admixtures where well-dispersed particle suspension are required. These polymers are used as dispersants to avoid particle aggregation, and to improve the flow characteristics of suspensions such as in concrete...
is then added to the activated mixture, which can later be mixed with aggregates in a conventional
concrete mixerA concrete mixer is a device that homogeneously combines cement, aggregate such as sand or gravel, and water to form concrete. A typical concrete mixer uses a revolving drum to mix the components...
. In this process, sand provides dissipation of energy and creates high-shear conditions on the surface of cement particles. This results in the full volume of water interacting with cement. The liquid activated mixture can be used by itself or foamed (expanded) for lightweight concrete. HEM concrete hardens in low and subzero temperature conditions and possesses an increased volume of gel, which drastically reduces capillarity in solid and porous materials.
Workability
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
hydrationIn organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous...
) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (
surface waterSurface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....
) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot be readily made more workable by addition of reasonable amounts of water.
Workability can be measured by the
concrete slump testThe concrete slump test is used for the measurement of a property of fresh concrete.The test is an emprical test that measures the workability of fresh concrete. More specifically, it measures the consistency of the concrete in that specific batch. It is also used to determine consistency between...
, a simplistic 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 "
Abrams coneDuff A. Abrams was an American researcher in the field of composition and properties of concrete. He developed the basic methods for testing concrete characteristics still in use today. A professor with the Lewis Institute, he studied the component materials of concrete in the early 20th...
" 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 in order to consolidate the layer. When the cone is carefully lifted off, the enclosed material will slump a certain amount due to gravity. A relatively dry sample will slump very little, having a slump value of one or two inches (25 or 50 mm). A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by using the
flow table testThe flow table test or flow test is a method to determine the consistence of fresh concrete.ApplicationWhen fresh concrete is delivered to a site by a truck mixer it is sometimes necessary to check its consistence before pouring it into formwork....
.
Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the
water-cement ratioThe water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix and has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place...
. Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.
High-flow concrete, like
self-consolidating concreteSelf-consolidating concrete is characterized by a low yield, high deformability, and moderate viscosity necessary to ensure uniform suspension of solid particles during transportation, placement , and thereafter until the concrete sets.Such concrete can be used for casting heavily reinforced...
, 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.
Curing
In all but the least critical applications, care needs to be taken to properly
cure concrete, to achieve best strength and hardness. This happens after the concrete has been placed. Cement requires a moist, controlled environment to gain strength and harden fully. The cement paste hardens over time, initially setting and becoming rigid though very weak and gaining in strength in the weeks following. In around 3 weeks, typically over 90% of the final strength is reached, though strengthening may continue for decades. The conversion of
calcium hydroxideCalcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...
in the concrete into
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
from absorption of
CO2Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
over several decades further strengthen the concrete and making it more resilient to damage. However, this reaction, called
carbonationCarbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...
, lowers the pH of the cement pore solution and can cause the reinforcement bars to corrode.
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. Strength of concrete changes (increases) up to three years. It depends on cross-section dimension of elements and conditions of structure exploitation.
During this period concrete needs to be kept under controlled temperature and humid atmosphere. In practice, this is achieved by spraying or ponding the concrete surface with water, thereby protecting the concrete mass from ill effects of ambient conditions. The pictures to the right show two of many ways to achieve this, ponding – submerging setting concrete in water and wrapping in plastic to contain the water in the mix. Additional common curing methods include wet burlap and/or plastic sheeting covering the fresh concrete, or by spraying on a water-impermeable temporary curing membrane.
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
exothermicIn thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
setting of cement. Improper curing can cause scaling, reduced strength, poor
abrasionAbrasion is the process of scuffing, scratching, wearing down, marring, or rubbing away. It can be intentionally imposed in a controlled process using an abrasive...
resistance and
crackingA fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
.
Properties
Concrete has relatively high
compressive strengthCompressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g...
, but much lower
tensile strengthUltimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
. For this reason is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures will crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to
creepIn materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material....
.
Tests can be made to ensure the properties of concrete correspond to specifications for the application.
Environmental and health
- For the environmental impact of cement production see Cement
Carbon dioxide emissions and climate change
The cement industry is one of two primary producers of carbon dioxide (CO
2), creating up to 5% of worldwide man-made emissions of this gas, of which 50% is from the chemical process and 40% from burning fuel. The carbon dioxide (CO
2) produced for the manufacture of one tonne of structural concrete (using ~14% cement) is estimated at 410 kg/m
3 (~180 kg/tonne @ density of 2.3g/cm
3) (reduced to 290 kg/m
3 with 30% fly ash replacement of cement). The CO
2 emission from the concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO
2 are emitted for the fabrication of every ton of cement.
Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
is thermally decomposed, producing lime and
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, and also through the use of energy, particularly from the combustion of
fossil fuelFossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
s.
Surface runoff
Surface runoffSurface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...
, when water runs off
impervious surfaceImpervious surfaces are mainly artificial structures--such as pavements that are covered by impenetrable materials such as asphalt, concrete, brick, and stone--and rooftops...
s, such as non-porous concrete, can cause heavy soil erosion and flooding.
Urban runoffUrban runoff is surface runoff of rainwater created by urbanization. This runoff is a major source of water pollution in many parts of the United States and other urban communities worldwide.-Overview:...
tends to pick up gasoline,
motor oilMotor oil or engine oil is an oil used for lubrication of various internal combustion engines. The main function is to lubricate moving parts; it also cleans, inhibits corrosion, improves sealing, and cools the engine by carrying heat away from moving parts.Motor oils are derived from...
, heavy metals,
trashWaste is unwanted or useless materials. In biology, waste is any of the many unwanted substances or toxins that are expelled from living organisms, metabolic waste; such as urea, sweat or feces. Litter is waste which has been disposed of improperly...
and other pollutants from sidewalks, roadways and parking lots. Without
attenuationIn physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
, the impervious cover in a typical urban area limits groundwater percolation and causes five times the amount of runoff generated by a typical woodland of the same size. A 2008 report by the
United States National Research CouncilThe National Research Council of the USA is the working arm of the United States National Academies, carrying out most of the studies done in their names.The National Academies include:* National Academy of Sciences...
identified urban runoff as a leading source of
water qualityWater quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
problems.
Urban heat
Both concrete and
asphaltAsphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
are the primary contributors to what is known as the
urban heat islandAn urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...
effect.
Using light-colored concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature. A low
albedoAlbedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
value, characteristic of black asphalt, absorbs a large percentage of solar heat and contributes to the warming of cities. By paving with light colored concrete, in addition to replacing asphalt with light-colored concrete, communities can lower their average temperature.
In many U.S. cities, pavement covers about 30–40% of the surface area. This directly affects the temperature of the city and contributes to the
urban heat islandAn urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...
effect. Paving with light-colored concrete would lower temperatures of paved areas and improve night-time visibility. The potential of energy saving within an area is also high. With lower temperatures, the demand for air conditioning decreases, saving energy.
Atlanta has tried to mitigate the heat-island effect. City officials noted that when using heat-reflecting concrete, their average city temperature decreased by 6°F (3.3°C). The Design Trust for Public Space found that by slightly raising the albedo value in New York City, beneficial effects such as energy savings could be achieved. It was concluded that this could be accomplished by the replacement of black asphalt with light-colored concrete.
However, in winter this may be a disadvantage as ice will form more easily and remain longer on the light colored surfaces as they will be colder due to less energy absorbed from the reduced amount of sunlight in winter.
Concrete dust
Building demolition and natural disasters such as earthquakes often release a large amount of concrete dust into the local atmosphere. Concrete dust was concluded to be the major source of dangerous air pollution following the
Great Hanshin earthquakeThe Great Hanshin earthquake, or Kobe earthquake, was an earthquake that occurred on Tuesday, January 17, 1995, at 05:46 JST in the southern part of Hyōgo Prefecture, Japan. It measured 6.8 on the moment magnitude scale , and Mj7.3 on JMA magnitude scale. The tremors lasted for approximately 20...
.
Toxic and radioactive contamination
The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns. Natural radioactive elements (K, U and Th) can be present in various concentration in concrete dwellings, depending on the source of the raw materials used. Toxic substances may also be added to the mixture for making concrete by unscrupulous makers. Dust from rubble or broken concrete upon demolition or crumbling may cause serious health concerns depending also on what had been incorporated in the concrete.
Handling precautions
Handling of wet concrete must always be done with proper protective equipment. Contact with wet concrete can cause skin
chemical burnA chemical burn occurs when living tissue is exposed to a corrosive substance such as a strong acid or base. Chemical burns follow standard burn classification and may cause extensive tissue damage. The main types of irritant and/or corrosive products are: acids, bases, oxidizers, solvents,...
s due to the caustic nature of the mixture of cement and water. Indeed, the pH of fresh cement water is highly alkaline due to the presence of free
potassiumPotassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
and
sodium hydroxides in solution (pH ~ 13.5). Eyes, hands and feet must be correctly protected to avoid any direct contact with wet concrete and washed without delay if necessary.
Damage modes
Concrete can be damaged by many processes, such as the expansion of
corrosionCorrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
products of the steel
reinforcement barsA rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
, 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 carbonation, chlorides, sulfates and distillate water).
Concrete recycling
Concrete recycling is an increasingly common method of disposing of concrete structures. Concrete debris was once routinely shipped to
landfillA landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...
s 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
crushing machineA crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Crushers may be used to reduce the size, or change the form, of waste materials so they can be more easily disposed of or recycled, or to reduce the size of a solid mix of raw materials , so that pieces...
, often along with asphalt, bricks and rocks.
Reinforced concrete contains
rebarA rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
and other metallic reinforcements, which are removed with
magnetA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
s 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 baseAggregate base is typically composed of crushed rock capable of passing through a rock screen. The component particles will vary in size from 3/4 inch down to dust...
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) approximated that almost 17% of worldwide landfill was by-products of concrete based waste.
World records
The world record for the largest concrete pour in a single project is the
Three Gorges DamThe Three Gorges Dam is a hydroelectric dam that spans the Yangtze River by the town of Sandouping, located in the Yiling District of Yichang, in Hubei province, China...
in Hubei Province, China by the Three Gorges Corporation. The amount of concrete used in the construction of the dam is estimated at 16 million cubic meters over 17 years. The previous record was 3.2 million cubic meters held by Itaipu hydropower station in Brazil.
Concrete pumping
The world record was set at on 7 August 2009 during the construction of the
ParbatiParbati River is a river in Himachal Pradesh, India that flows into the Beas River at Bhuntar, some 10 km south of Kullu. It rises from the Man Talai Glacier below the Pin Parbati pass and flows in a gradual curve from north-northwest to west-southwest past the important temple town of...
Hydroelectric Project, near the village of Suind,
Himachal PradeshHimachal Pradesh is a state in Northern India. It is spread over , and is bordered by the Indian states of Jammu and Kashmir on the north, Punjab on the west and south-west, Haryana and Uttar Pradesh on the south, Uttarakhand on the south-east and by the Tibet Autonomous Region on the east...
, India, when the concrete mix was pumped through a vertical height of 715 m (2,345.8 ft).
Continuous pours
The world record for largest continuously poured concrete raft was achieved in August 2007 in Abu Dhabi by contracting firm Al Habtoor-CCC Joint Venture.
The pour (a part of the foundation for the Abu Dhabi's
Landmark TowerThe Landmark is a postmodern supertall skyscraper under construction in Abu Dhabi, United Arab Emirates. The mixed-use project will stand tall with 72 floors above ground and five basement levels.. The skyscraper is expected for completion in 2011....
) was 16,000 cubic meters of concrete poured within a two day period. The previous record (close to 10,500 cubic meters) was held by Dubai Contracting Company and achieved 23 March 2007.
The world record for largest continuously poured concrete floor was completed 8 November 1997, in
LouisvilleLouisville is the largest city in the U.S. state of Kentucky, and the county seat of Jefferson County. Since 2003, the city's borders have been coterminous with those of the county because of a city-county merger. The city's population at the 2010 census was 741,096...
, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement consisted of 225000 square feet (20,903.2 m²) of concrete placed within a 30 hour period, finished to a flatness tolerance of F
F 54.60 and a levelness tolerance of F
L 43.83. This surpassed the previous record by 50% in total volume and 7.5% in total area.
The record for the largest continuously placed underwater concrete pour was completed 18 October 2010, in New Orleans, Louisiana by contractor C. J. Mahan Construction Company, LLC of Grove City, Ohio. The placement consisted of 10,224 cubic yards of concrete placed in a 58 hour period using two concrete pumps and two dedicated concrete batch plants. Upon curing, this placement will allow the 50180 square feet (4,661.9 m²) cofferdam to be dewatered approximately 26 feet (7.9 m) below sea level to allow the construction of the IHNC GIWW Sill & Monolith Project to be completed in the dry.
Mass concrete structures
These large structures typically include gravity dams, such as the
Hoover DamHoover Dam, once known as Boulder Dam, is a concrete arch-gravity dam in the Black Canyon of the Colorado River, on the border between the US states of Arizona and Nevada. It was constructed between 1931 and 1936 during the Great Depression and was dedicated on September 30, 1935, by President...
, the Itaipu Dam and the
Three Gorges DamThe Three Gorges Dam is a hydroelectric dam that spans the Yangtze River by the town of Sandouping, located in the Yiling District of Yichang, in Hubei province, China...
,
arch damAn arch dam is a type of dam that is curved and commonly built with concrete. The arch dam is a structure that is designed to curve upstream so that the force of the water against it, known as hydrostatic pressure, presses against the arch, compressing and strengthening the structure as it pushes...
s, navigation locks and large
breakwatersBreakwaters are structures constructed on coasts as part of coastal defence or to protect an anchorage from the effects of weather and longshore drift.-Purposes of breakwaters:...
. Such large structures, even though individually placed in formed horizontal blocks, generate excessive heat and associated expansion; to mitigate these effects post-cooling is commonly provided in the design. 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.
Concrete that is poured all at once in one form (so that there are no weak points where the concrete is "welded" together) is used for tornado shelters.
Reinforced concrete structures
Reinforced concrete contains steel reinforcing that is designed and placed in the structure at specific positions to cater for all the stress conditions that the structure is required to accommodate.
Pre-stressed concrete structures
Pre-stressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose those found when in use. This can greatly reduce the weight of beams or slabs, by better distributing the stresses in the structure to make optimal use of the reinforcement. For example a horizontal beam will tend to sag down. If the reinforcement along the bottom of the beam is pre-stressed, it can counteract this.
In pre-tensioned concrete, the pre-stressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting, or for post-tensioned concrete, after casting.
Concrete textures
When one thinks of concrete, the image of a dull, gray concrete wall often comes to mind. With the use of form liner, concrete can be cast and molded into different textures and used for
decorative concreteDecorative concrete is the use of concrete as not simply a utilitarian medium for construction but as an aesthetic enhancement to a structure, while still serving its function as an integral part of the building itself such as floors, walls, driveways and patios.The transformation of concrete into...
applications. Sound/retaining walls, bridges, office buildings and more serve as the optimal canvases for concrete art. For example, the Pima Freeway/Loop 101 retaining and sound walls in Scottsdale, Arizona, feature desert flora and fauna, a 67 feet (20.4 m) lizard and 40 feet (12.2 m) cacti along the 8 miles (12.9 km) stretch. The project, titled "The Path Most Traveled," is one example of how concrete can be shaped using elastomeric form liner.
Building with concrete
Concrete is one of the most durable building materials. It provides superior fire resistance, compared with wooden construction and can gain strength over time. Structures made of concrete can have a long service life. Concrete is the most widely used construction material in the world with annual consumption estimated at between 21 and 31 billion tonnes.
Energy efficiency
Energy requirements for transportation of concrete are low because it is produced locally from local resources, typically manufactured within 100 kilometers of the job site. Once in place, concrete offers significant energy efficiency over the lifetime of a building. Concrete walls leak air far less than those made of wood-frames. Air leakage accounts for a large percentage of energy loss from a home. The thermal mass properties of concrete increase the efficiency of both residential and commercial buildings. By storing and releasing the energy needed for heating or cooling, concrete's thermal mass delivers year-round benefits by reducing temperature swings inside and minimizing heating and cooling costs . While insulation reduces energy loss through the building envelope, thermal mass uses walls to store and release energy. Modern concrete wall systems use both 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
rastraRastra is an insulating concrete form used to make walls for buildings. It is composed of concrete and thastyron. Thastyron is a mixture of plastic foam and binder that is composed of eighty-five percent recycled post consumer polystyrene waste that are molded into blocks...
that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.
Fire safety
Concrete buildings are more resistant to fire than those constructed using wood or steel frames, since concrete does not burn. Concrete reduces the risk of structural collapse and is an effective fire shield, providing safe means of escape for occupants and protection for fire fighters.
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 fire-proof 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 the best resistance of any building material to high winds, hurricanes, tornadoes due to its lateral stiffness that results in minimal horizontal movement.
Earthquake safety
As discussed above, concrete is very strong in compression, but weak in tension. Larger earthquakes can generate very large shear loads on structures. These shear loads subject the structure to both tensional and compressional loads. Concrete structures without reinforcing, like other unreinforced masonry structures, can fail during severe earthquake shaking. Unreinforced masonry structures constitute one of the largest earthquake risks globally. These risks can be reduced through seismic retrofitting of at-risk buildings, (e.g. School buildings in Istanbul, Turkey).
See also
- Anthropic rock
Anthropic rock is rock that is made, modified and moved by humans. Concrete is the most widely-known example of this. The new category has been proposed to recognise that man-made rocks are likely to last for long periods of Earth's future geological time, and will be important in humanity's...
- Biorock
Biorock, also known as Seacrete, is a substance formed by electro-accumulation of minerals dissolved in seawater. The building process, popularly called accretion, is not to be confused with Biorock sewage treatment...
- Brutalist architecture
Brutalist architecture is a style of architecture which flourished from the 1950s to the mid 1970s, spawned from the modernist architectural movement.-The term "brutalism":...
, encouraging visible concrete surfaces
- Bunding
Bunding, also called a bund wall, is the area within a structure designed to prevent inundation or breaches of various types.-Liquid containment:...
- Cement
- Geopolymers
Geopolymer is a term covering a class of synthetic aluminosilicate materials with potential use in a number of areas, essentially as a replacement for Portland cement and for advanced high-tech composites, ceramic applications or as a form of cast stone...
, a class of synthetic aluminosilicate materials
- Hempcrete
Hempcrete is a mixture of hemp hurds and lime used as a material for construction and insulation.It is marketed under names like Hemcrete, Canobiote, Canosmose, and Isochanvre....
, a mixture with hemp hurds
- Mudcrete
Mudcrete is a structural material made of mixing mud with sand and concrete/cement. It is used as a cheaper and more sustainable alternative to rock fill. It is also used in such projects as land reclamation....
, a soil-cement mixture
- Papercrete
Papercrete is a recently developed construction material which consists of re-pulped paper fiber with Portland cement or clay and/or other soil added. First patented in 1928, it has been revived since the 1980s. Although perceived as an environmentally friendly material due to the significant...
, a paper-cement mixture
- Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
, the classical concrete cement
- Cement accelerator
Cement Accelerator is an admixture for the use in concrete, mortar, rendering or screeds. The addition of Cement Accelerator speeds the setting time and thus cure time starts earlier . This allows concrete to be placed in winter without the worry of frost damage .Typical materials used for...
- Concrete canoe
A concrete canoe is a canoe made of concrete, typically created for an engineering competition.In spirit, the event is similar to that of a cardboard boat race—make the seemingly unfloatable float...
- Concrete curing
- Concrete leveling
Concrete leveling is a procedure that attempts to correct an uneven concrete surface by altering the foundation that the surface sits upon. It is a cheaper alternative to having replacement concrete poured, and commonly performed at small businesses and private homes. In 1977, the term concrete...
- Concrete mixer
A concrete mixer is a device that homogeneously combines cement, aggregate such as sand or gravel, and water to form concrete. A typical concrete mixer uses a revolving drum to mix the components...
- Concrete masonry unit
- Concrete moisture meter
- Concrete recycling
When structures made of concrete are demolished or renovated, concrete recycling is an increasingly common method of utilizing the rubble. Concrete was once routinely trucked to landfills for disposal, but recycling has a number of benefits that have made it a more attractive option in this age of...
- Concrete step barrier
A concrete step barrier is a safety barrier used on the central reservation of motorways and dual carriageways as an alternative to the standard steel crash barrier.The barrier has contained all vehicles up to 13.5 tonnes.-United Kingdom:...
- Construction
- Diamond grinding of pavement
- Efflorescence
In chemistry, efflorescence is the loss of water of crystallization from a hydrated or solvated salt to the atmosphere on exposure to air.-Examples:...
- Fireproofing
Fireproofing, a passive fire protection measure, refers to the act of making materials or structures more resistant to fire, or to those materials themselves, or the act of applying such materials. Applying a certification listed fireproofing system to certain structures allows these to have a...
- Foam Index
Foam Index test is a rapid method to determine the relative levels of Air Entraining Agent needed during concrete mixing, with or without mineral additives like combustion fly ash, that control air void volumes within cured concrete.-Introduction:...
- Form liner
- Formwork
Formwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering moulds.-Formwork and concrete form types:...
- Controlled permeability formwork
Controlled permeability formwork is a system proven to significantly enhance the durability of surface concrete during the casting process.-The need for CPF:...
- High performance fiber reinforced cementitious composites
High-performance fiber-reinforced cementitious composites are a group of fiber-reinforced cement-based composites which possess the unique ability to flex and self-strengthen before fracturing...
- High Reactivity Metakaolin
Metakaolin is a dehydroxylated form of the clay mineral kaolinite.Rocks that are rich in kaolinite are known as china clay or kaolin, traditionally used in the manufacture of porcelain. The particle size of metakaolin is smaller than cement particles, but not as fine as silica fume.-Forming...
- International Grooving & Grinding Association
The International Grooving & Grinding Association is a non-profit trade association founded in 1972 that represents the industry that performs grooving and grinding of both concrete and asphalt surfaces in addition to Concrete Pavement Restoration and Concrete Pavement Preservation methods...
- LiTraCon
LiTraCon is a trademark for a translucent concrete building material. The name is short for "light-transmitting concrete". The technical data sheet from the manufacturersays the material is made of 96% concrete and 4% by weight of optical fibers, it was developed in 2001 by Hungarian architect...
- Mortar
Mortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder...
- Plasticizer
Plasticizers or dispersants are additives that increase the plasticity or fluidity of the material to which they are added; these include plastics, cement, concrete, wallboard, and clay. Although the same compounds are often used for both plastics and concretes the desired effects and results are...
- Prefabrication
Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located...
- Pykrete
Pykrete is a composite material made of approximately 14 percent sawdust or some other form of wood pulp and 86 percent ice by weight. Its use was proposed during World War II by Geoffrey Pyke to the British Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier...
, a composite material of ice and cellulose
- Shallow foundation
A shallow foundation is a type of foundation which transfers building loads to the earth very near the surface, rather than to a subsurface layer or a range of depths as does a deep foundation...
- Silica fume
Silica fume, also known as microsilica, is a fine-grain, thin, and very high surface area silica.It is sometimes confused with fumed silica and colloidal silica...
- Translucent concrete
Translucent concrete is a concrete based building material with light-transmissive properties due to embedded light optical elements - usually fibers. Light is conducted though the stone from one end to the other. Therefore the fibers have to go through the whole object...
- Whitetopping
Whitetopping is the covering of an existing asphalt pavement with a layer of Portland cement concrete. Whitetopping is divided into types depending on the thickness of the concrete layer and whether the layer is bonded to the asphalt substrate. Unbonded whitetopping, also called conventional...
- World of Concrete
The World of Concrete is an annual trade show for the commercial construction industry. It is held each year either in the months of January or February for four days in Las Vegas, Nevada. This event is a show where products, resources, and information related to concrete construction are shared...
- Types of concrete
There are many types of concrete, variations of installation, composition, finish and performance characteristics.-Mix design:Modern concrete mix designs can be complex...
- Aerated autoclaved concrete
Autoclaved aerated concrete , also known as autoclaved cellular concrete or autoclaved lightweight concrete , was invented in the mid-1920s by the Swedish architect and inventor Johan Axel Eriksson. It is a lightweight, precast building material that simultaneously provides structure, insulation,...
- Asphalt concrete
Asphalt concrete is a composite material commonly used in construction projects such as road surfaces, airports and parking lots. It consists of asphalt and mineral aggregate mixed together, then laid down in layers and compacted...
- Seacrete
Biorock, also known as Seacrete, is a substance formed by electro-accumulation of minerals dissolved in seawater. The building process, popularly called accretion, is not to be confused with Biorock sewage treatment...
- Decorative concrete
Decorative concrete is the use of concrete as not simply a utilitarian medium for construction but as an aesthetic enhancement to a structure, while still serving its function as an integral part of the building itself such as floors, walls, driveways and patios.The transformation of concrete into...
- ferrocement
The term ferrocement is most commonly applied to a mixture of Portland cement and sand reinforced with layers of woven or expanded steel mesh and closely spaced small-diameter steel rods rebar. It can be used to form relatively thin, compound curved sheets to make hulls for boats, shell roofs,...
- Fiber reinforced concrete
Fiber-reinforced concrete is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers...
- Lunarcrete
Lunarcrete, also known as "Mooncrete", an idea first proposed by Larry A. Beyer of the University of Pittsburgh in 1985, is a hypothetical aggregate building material, similar to concrete, formed from lunar regolith, that would cut the construction costs of building on the Moon.- Ingredients :Only...
- Precast concrete
By producing precast concrete in a controlled environment , the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. Utilizing a Precast Concrete system offers many potential advantages over site casting of concrete...
- Prestressed concrete
Prestressed concrete is a method for overcoming concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete...
- Ready-mix concrete
Ready-mix concrete is a type of concrete that is manufactured in a factory or batching plant, according to a set recipe, and then delivered to a work site, by truck mounted transit mixers . This results in a precise mixture, allowing specialty concrete mixtures to be developed and implemented on...
- Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
- Roller-compacted concrete
Roller-compacted concrete or rolled concrete is a special blend of concrete that has essentially the same ingredients as conventional concrete but in different ratios, and increasingly with partial substitution of fly ash for Portland cement. RCC is a mix of cement/fly ash, water, sand, aggregate...
- Salt-concrete
Salt-concrete is a construction material that is used to reduce the water inflow in mining shafts in salt mines. It is composed of 16% cement, 39% halite, 16% limestone powder, 14% water and 15% sand.-History:...
- Terrazzo
Terrazzo is a composite material poured in place or precast, which is used for floor and wall treatments. It consists of marble, quartz, granite, glass or other suitable chips, sprinkled or unsprinkled, and poured with a binder that is cementitious, chemical or a combination of both...
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