Concrete

Concrete is a composite material composed of fine and coarse

aggregate bonded together with a fluid cement (cement paste) that
hardens (cures) over time. Concrete is said to be the second
substance most used in the world after water, and is one of the
most frequently used building materials. Its usage worldwide, ton
for ton, is twice that of steel, wood, plastics, and aluminum
combined.[2] Globally, the ready-mix concrete industry, the largest
segment of the concrete market, is projected to exceed $600 billion
in revenue by 2025.[3] This widespread use results in a number of
environmental impacts. Most notably, the production process for Exterior of the Roman Pantheon,
cement produces large volumes of greenhouse gas emissions, finished 128 AD, the largest
leading to net 8% of global emissions.[4][5] Other environmental unreinforced concrete dome in the
concerns include widespread illegal sand mining, impacts on the world.[1]
surrounding environment such as increased surface runoff or urban
heat island effect, and potential public health implications from
toxic ingredients. Significant research and development is being
done to try to reduce the emissions or make concrete a source of
carbon sequestration, and increase recycled and secondary raw
materials content into the mix to achieve a circular economy.
Concrete is expected to be a key material for structures resilient to
climate disasters,[6] as well as a solution to mitigate the pollution of
other industries, capturing wastes such as coal fly ash or bauxite
tailings and residue.

When aggregate is mixed with dry Portland cement and water, the
mixture forms a fluid slurry that is easily poured and molded into
shape. The cement reacts with the water and other ingredients to
form a hard matrix that binds the materials together into a durable Interior of the Pantheon dome, seen
stone-like material that has many uses.[7] Often, additives (such as from beneath. The concrete for the
pozzolans or superplasticizers) are included in the mixture to coffered dome was laid on moulds,
improve the physical properties of the wet mix or the finished mounted on temporary scaffolding.
material. Most concrete is poured with reinforcing materials (such
as rebar) embedded to provide tensile strength, yielding reinforced
concrete.

In the past, lime based cement binders, such as lime putty, were often used but sometimes with other
hydraulic cements, (water resistant) such as a calcium aluminate cement or with Portland cement to form
Portland cement concrete (named for its visual resemblance to Portland stone).[8][9] Many other non-
cementitious types of concrete exist with other methods of binding aggregate together, including asphalt
concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use
polymers as a binder. Concrete is distinct from mortar. Whereas concrete is itself a building material, mortar
is a bonding agent that typically holds bricks, tiles and other masonry units together.[10]

Contents
Etymology
History
Ancient times
Classical era
Middle Ages
Industrial era
Composition
Cement
Water
Aggregates
Admixtures
Mineral admixtures and blended cements
Opus caementicium
Production
exposed in a characteristic
Design mix
Roman arch. In contrast to
Mixing modern concrete structures,
Sample analysis - Workability the concrete used in Roman
Curing buildings was usually
covered with brick or stone.
Alternative types
Asphalt
Concretene
Microbial
Nanoconcrete
Pervious
Polymer
Volcanic
Waste light
Properties
Energy efficiency
Fire safety
Earthquake safety
Construction with concrete
Reinforced concrete
Precast concrete
Mass structures
Surface finishes
Prestressed structures
Cold weather placement
Underwater placement
Roads
Environment, health and safety
Concrete, cement and the environment
Concrete and climate change mitigation
Concrete and climate change adaptation
Concrete – health and safety
Circular economy
End-of-life: concrete degradation and waste
Reuse of concrete
Recycling of concrete
Cradle-to cradle challenges
World records
See also
References
External links

Etymology
The word concrete comes from the Latin word "concretus" (meaning compact or condensed),[11] the
perfect passive participle of "concrescere", from "con-" (together) and "crescere" (to grow).

History

Ancient times

Mayan concrete at the ruins of Uxmal is referenced in Incidents of Travel in the Yucatán by John L.
Stephens. "The roof is flat and had been covered with cement". "The floors were cement, in some places
hard, but, by long exposure, broken, and now crumbling under the feet." "But throughout the wall was
solid, and consisting of large stones imbedded in mortar, almost as hard as rock."

Small-scale production of concrete-like materials was pioneered by the Nabatean traders who occupied and
controlled a series of oases and developed a small empire in the regions of southern Syria and northern
Jordan from the 4th century BC. They discovered the advantages of hydraulic lime, with some self-
cementing properties, by 700 BC. They built kilns to supply mortar for the construction of rubble masonry
houses, concrete floors, and underground waterproof cisterns. They kept the cisterns secret as these enabled
the Nabataeans to thrive in the desert.[12] Some of these structures survive to this day.[12]

Classical era

In the Ancient Egyptian and later Roman eras, builders discovered that adding volcanic ash to the mix
allowed it to set underwater.

Concrete floors were found in the royal palace of Tiryns, Greece, which dates roughly to 1400–1200
BC.[13][14] Lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan
Aqueduct (688 BC) made use of waterproof concrete.[15] Concrete was used for construction in many
ancient structures.[16]

The Romans used concrete extensively from 300 BC to 476 AD.[17] During the Roman Empire, Roman
concrete (or opus caementicium) was made from quicklime, pozzolana and an aggregate of pumice. Its
widespread use in many Roman structures, a key event in the history of architecture termed the Roman
architectural revolution, freed Roman construction from the restrictions of stone and brick materials. It
enabled revolutionary new designs in terms of both structural complexity and dimension.[18] The
Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's
largest unreinforced concrete dome.[19]

Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of
arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal
thrusts and strains that troubled the builders of similar structures in stone or brick.[20]

Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement
concrete (ca. 200 kg/cm2 [20 MPa; 2,800 psi]).[21] However, due to the absence of reinforcement, its
tensile strength was far lower than modern reinforced concrete, and its mode of application also
differed:[22]

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.[23]

The long-term durability of Roman concrete structures has been found to be due to its use of pyroclastic
(volcanic) rock and ash, whereby the crystallization of strätlingite (a specific and complex calcium
aluminosilicate hydrate)[24] and the coalescence of this and similar calcium–aluminum-silicate–hydrate
cementing binders helped give the concrete a greater degree of fracture resistance even in seismically active
environments.[25] Roman concrete is significantly more resistant to erosion by seawater than modern
concrete; it used pyroclastic materials which react with seawater to form Al-tobermorite crystals over
time.[26][27]

The widespread use of concrete in many Roman structures ensured that many survive to the present day.
The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges, such as the
magnificent Pont du Gard in southern France, have masonry cladding on a concrete core, as does the dome
of the Pantheon.

After the Roman Empire collapsed, use of concrete became rare until the technology was redeveloped in
the mid-18th century. Worldwide, concrete has overtaken steel in tonnage of material used.[28]

Middle Ages

After the Roman Empire, the use of burned lime and pozzolana was greatly reduced. Low kiln
temperatures in the burning of lime, lack of pozzolana, and poor mixing all contributed to a decline in the
quality of concrete and mortar. From the 11th century, the increased use of stone in church and castle
construction led to an increased demand for mortar. Quality began to improve in the 12th century through
better grinding and sieving. Medieval lime mortars and concretes were non-hydraulic and were used for
binding masonry, "hearting" (binding rubble masonry cores) and foundations. Bartholomaeus Anglicus in
his De proprietatibus rerum (1240) describes the making of mortar. In an English translation from 1397, it
reads "lyme ... is a stone brent; by medlynge thereof with sonde and water sement is made". From the 14th
century, the quality of mortar was again excellent, but only from the 17th century was pozzolana commonly
added.[29]
The Canal du Midi was built using concrete in 1670.[30]

Industrial era

Perhaps the greatest step forward in the modern use of concrete was
Smeaton's Tower, built by British engineer John Smeaton in Devon,
England, between 1756 and 1759. This third Eddystone Lighthouse
pioneered the use of hydraulic lime in concrete, using pebbles and
powdered brick as aggregate.[31]

A method for producing Portland cement was developed in England and

patented by Joseph Aspdin in 1824.[32] Aspdin chose the name for its
similarity to Portland stone, which was quarried on the Isle of Portland in
Dorset, England. His son William continued developments into the 1840s,
earning him recognition for the development of "modern" Portland
cement.[33]

Reinforced concrete was invented in 1849 by Joseph Monier.[34] and the

first reinforced concrete house was built by François Coignet[35] in 1853. Smeaton's Tower
The first concrete reinforced bridge was designed and built by Joseph
Monier in 1875.[36]

Composition
Concrete is an artificial composite material, comprising a matrix of cementitious binder (typically Portland
cement paste or asphalt) and a dispersed phase or "filler" of aggregate (typically a rocky material, loose
stones, and sand). The binder "glues" the filler together to form a synthetic conglomerate.[37] Many types
of concrete are available, determined by the formulations of binders and the types of aggregate used to suit
the application of the engineered material. These variables determine strength and density, as well as
chemical and thermal resistance of the finished product.

Aggregates consist of large chunks of material in a concrete mix, generally a coarse gravel or crushed rocks
such as limestone, or granite, along with finer materials such as sand.

Cement paste, most commonly made of Portland cement, is the most prevalent kind of concrete binder. For
cementitious binders, water is mixed with the dry cement powder and aggregate, which produces a semi-
liquid slurry (paste) that can be shaped, typically by pouring it into a form. The concrete solidifies and
hardens through a chemical process called hydration. The water reacts with the cement, which bonds the
other components together, creating a robust, stone-like material. Other cementitious materials, such as fly
ash and slag cement, are sometimes added—either pre-blended with the cement or directly as a concrete
component—and become a part of the binder for the aggregate.[38] Fly ash and slag can enhance some
properties of concrete such as fresh properties and durability.[38] Alternatively, other materials can also be
used as a concrete binder: the most prevalent substitute is asphalt, which is used as the binder in asphalt
concrete.

Admixtures are added to modify the cure rate or properties of the material. Mineral admixtures use recycled
materials as concrete ingredients. Conspicuous materials include fly ash, a by-product of coal-fired power
plants; ground granulated blast furnace slag, a by-product of steelmaking; and silica fume, a by-product of
industrial electric arc furnaces.
Structures employing Portland cement concrete usually include steel reinforcement because this type of
concrete can be formulated with high compressive strength, but always has lower tensile strength.
Therefore, it is usually reinforced with materials that are strong in tension, typically steel rebar.

The mix design depends on the type of structure being built, how the concrete is mixed and delivered, and
how it is placed to form the structure.

Cement

Portland cement is the most common type of cement in general

usage. It is a basic ingredient of concrete, mortar, and many
plasters.[39] British masonry worker Joseph Aspdin patented
Portland cement in 1824. It was named because of the similarity of
its color to Portland limestone, quarried from the English Isle of
Portland and used extensively in London architecture. It consists of
a mixture of calcium silicates (alite, belite), aluminates and ferrites
—compounds which combine calcium, silicon, aluminum and iron
in forms which will react with water. Portland cement and similar
Several tons of bagged cement,
materials are made by heating limestone (a source of calcium) with
about two minutes of output from a
clay or shale (a source of silicon, aluminum and iron) and grinding 10,000 ton per day cement kiln
this product (called clinker) 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 grind it into cement. 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.[40]

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 makes it flow more freely.[41]

As stated by Abrams' law, a lower water-to-cement ratio yields a stronger, more durable concrete, whereas
more water gives a freer-flowing concrete with a higher slump.[42] Impure water used to make concrete can
cause problems when setting or in causing premature failure of the structure.[43]

Portland cement consists of five major compounds of calcium silicates and alumninates ranging from 5 to
50% in weight, which all undergo hydration to contribute to final material's strength. Thus, the hydration of
cement involves many 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.[44]

Hydration of tricalcium silicate

Cement chemist notation: C3S + H → C-S-H + CH + heat

 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)[44]

Due to the nature of the chemical bonds created in these reactions and the final characteristics of the
particles formed, the process of cement hydration is considered irreversible, which makes methods of
cement recycling prohibitive.[45]

Aggregates

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 Crushed stone aggregate
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 paste 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.[46] The aggregate is
nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.

Redistribution of aggregates after compaction often creates non-homogeneity due to the influence of
vibration. This can lead to strength gradients.[47]

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.

Admixtures

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. Admixtures are defined as additions "made as the
concrete mix is being prepared".[48] The most common admixtures are retarders and accelerators. 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.[49] (See § Production below.) The common types of admixtures[50] are as follows:

Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are
calcium chloride, calcium nitrate and sodium nitrate. However, use of chlorides may cause
corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be
favored, even though they are less effective than the chloride salt. Accelerating admixtures
are especially useful for modifying the properties of concrete in cold weather.
Air entraining agents add and entrain tiny air bubbles in the concrete, which reduces
damage during freeze-thaw cycles, increasing durability. However, entrained air entails a
tradeoff with strength, as each 1% of air may decrease compressive strength by 5%.[51] If too
much air becomes trapped in the concrete as a result of the mixing process, defoamers can
be used to encourage the air bubble to agglomerate, rise to the surface of the wet concrete
and then disperse.
 Bonding agents are used to create a bond between old and new concrete (typically a type of
polymer) with wide temperature tolerance and corrosion resistance.
Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
Crystalline admixtures are typically added during batching of the concrete to lower
permeability. The reaction takes place when exposed to water and un-hydrated cement
particles to form insoluble needle-shaped crystals, which fill capillary pores and micro-
cracks in the concrete to block pathways for water and waterborne contaminates. Concrete
with crystalline admixture can expect to self-seal as constant exposure to water will
continuously initiate crystallization to ensure permanent waterproof protection.
Pigments can be used to change the color of concrete, for aesthetics.
Plasticizers increase the workability of plastic, or "fresh", concrete, allowing it to 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.
Superplasticizers (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. Superplasticizers are used to increase compressive strength. It
increases the workability of the concrete and lowers the need for water content by 15–30%.
Superplasticizers lead to retarding effects.
Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.
Retarders slow the hydration of concrete and are used in large or difficult pours where partial
setting is undesirable before completion of the pour. Typical polyol retarders are sugar,
sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.

Mineral admixtures and blended cements

Inorganic materials that have pozzolanic or latent hydraulic properties, these very fine-grained materials are
added to the concrete mix to improve the properties of concrete (mineral admixtures),[49] or as a
replacement for Portland cement (blended cements).[55] Products which incorporate limestone, fly ash, blast
furnace slag, and other useful materials with pozzolanic properties into the mix, are being tested and used.
These developments are ever growing in relevance to minimize the impacts caused by cement use,
notorious for being one of the largest producers (at about 5 to 10%) of global greenhouse gas emissions.[56]
The use of alternative materials also is capable of lowering costs, improving concrete properties, and
recycling wastes, the latest being relevant for Circular Economy aspects of the construction industry, whose
demand is ever growing with greater impacts on raw material extraction, waste generation and landfill
practices.

Fly ash: A by-product of coal-fired electric generating plants, 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 calcareous fly ash has latent
hydraulic properties.[57]
Ground granulated blast furnace slag (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.[58]
Silica fume: A by-product of the production of silicon and ferrosilicon 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.[59]
 High reactivity Components of cement:
Metakaolin (HRM): comparison of chemical and physical characteristics[a][52][53][54]
Metakaolin
Portland Siliceous[b] Calcareous[c] Slag Silica
produces concrete Property
cement fly ash fly ash cement fume
with strength and
durability similar to SiO2 21.9 52 35 35 85–97

Proportion by mass (%)

concrete made with Al2O3 6.9 23 18 12 —
silica fume. While
silica fume is Fe2O3 3 11 6 1 —
usually dark gray or CaO 63 5 21 40 <1
black in color, high-
reactivity metakaolin MgO 2.5 — — — —
is usually bright SO3 1.7 — — — —
white in color,
making it the Specific
15,000
surface 370 420 420 400
preferred choice for – 30,000
(m2/kg)[d]
architectural
concrete where Specific
3.15 2.38 2.65 2.94 2.22
appearance is gravity
important. General Primary Cement Cement Cement Property
Carbon nanofibers purpose binder replacement replacement replacement enhancer
can be added to
concrete to enhance a. Values shown are approximate: those of a specific material may vary.
compressive b. ASTM C618 Class F
strength and gain a c. ASTM C618 Class C
higher Young’s
modulus, and also d. Specific surface measurements for silica fume by nitrogen adsorption (BET)
to improve the method, others by air permeability method (Blaine).
electrical properties
required for strain monitoring, damage evaluation and self-health monitoring of concrete.
Carbon fiber has many advantages in terms of mechanical and electrical properties (e.g.,
higher strength) and self-monitoring behavior due to the high tensile strength and high
conductivity.[60]
Carbon products have been added to make concrete electrically conductive, for deicing
purposes.[61]

Production
Concrete production is the process of mixing together the various ingredients—water, aggregate, cement,
and any additives—to produce concrete. Concrete production is time-sensitive. Once the ingredients are
mixed, workers must put the concrete in place before it hardens. In modern usage, most concrete
production takes place in a large type of industrial facility called a concrete plant, or often a batch plant.

In general usage, concrete plants come in two main types, ready mix plants and central mix plants. A ready-
mix plant mixes all the ingredients except water, while a central mix plant mixes all the ingredients
including water. A central-mix plant offers more accurate control of the concrete quality through better
measurements of the amount of water added, but must be placed closer to the work site where the concrete
will be used, since hydration begins at the plant.

A concrete plant consists of large storage hoppers for various reactive ingredients like cement, storage for
bulk ingredients like aggregate and water, mechanisms for the addition of various additives and
amendments, machinery to accurately weigh, move, and mix some or all of those ingredients, and facilities
to dispense the mixed concrete, often to a concrete mixer truck.
Modern concrete is usually prepared as a viscous fluid, so that it may be
poured into forms, which are containers erected in the field to give the
concrete its desired shape. Concrete formwork can be prepared in several
ways, such as slip forming and steel plate construction. Alternatively,
concrete can be mixed into dryer, non-fluid forms and used in factory
settings to manufacture precast concrete products.

A wide variety of equipment is used for processing concrete, from hand

tools to heavy industrial machinery. Whichever equipment builders use,
however, the objective is to produce the desired building material;
ingredients must be properly mixed, placed, shaped, and retained within
time constraints. Any interruption in pouring the concrete can cause the
initially placed material to begin to set before the next batch is added on
top. This creates a horizontal plane of weakness called a cold joint
between the two batches.[62] Once the mix is where it should be, the Concrete plant showing a
curing process must be controlled to ensure that the concrete attains the concrete mixer being filled
desired attributes. During concrete preparation, various technical details from ingredient silos
may affect the quality and nature of the product.

Design mix

Design mix ratios are decided by an engineer after analyzing the properties
of the specific ingredients being used. Instead of using a 'nominal mix' of 1
part cement, 2 parts sand, and 4 parts aggregate (the second example from
above), a civil engineer will custom-design a concrete mix to exactly meet
the requirements of the site and conditions, setting material ratios and often
designing an admixture package to fine-tune the properties or increase the
performance envelope of the mix. Design-mix concrete can have very
broad specifications that cannot be met with more basic nominal mixes, but
the involvement of the engineer often increases the cost of the concrete
mix. Concrete mixing plant in
Birmingham, Alabama in
Concrete Mixes are primarily divided into nominal mix, standard mix and 1936
design mix.

Nominal mix ratios are given in volume of . Nominal mixes are a simple,
fast way of getting a basic idea of the properties of the finished concrete without having to perform testing
in advance.

Various governing bodies (such as British Standards) define nominal mix ratios into a number of grades,
usually ranging from lower compressive strength to higher compressive strength. The grades usually
indicate the 28-day cube strength.[63]

Mixing

Thorough mixing is essential to produce uniform, high-quality concrete.

Separate paste mixing has shown that the mixing of cement and water into a paste before combining these
materials with aggregates can increase the compressive strength of the resulting concrete.[64] The paste is
generally mixed in a high-speed, shear-type mixer at a w/cm (water to cement ratio) of 0.30 to 0.45 by
mass. The cement paste premix may include admixtures such as accelerators or retarders, superplasticizers,
pigments, or silica fume. The premixed paste is then blended with aggregates and any remaining batch
water and final mixing is completed in conventional concrete mixing equipment.[65]

Sample analysis - Workability

Workability is the ability of a fresh (plastic) concrete mix to fill the

form/mold properly with the desired work (pouring, pumping,
spreading, tamping, 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) 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 of aggregates (when the
cement and aggregates start to separate), with the resulting Concrete floor of a parking garage
concrete having reduced quality. The use of an aggregate blend being placed
with an undesirable gradation can result in a very harsh mix design
with a very low slump, which cannot readily be made more
workable by addition of reasonable amounts of water. An
undesirable gradation can mean using a large aggregate that is too
large for the size of the formwork, or which has too few smaller
aggregate grades to serve to fill the gaps between the larger grades,
or using too little or too much sand for the same reason, or using
too little water, or too much cement, or even using jagged crushed
stone instead of smoother round aggregate such as pebbles. Any
combination of these factors and others may result in a mix which
is too harsh, i.e., which does not flow or spread out smoothly, is
difficult to get into the formwork, and which is difficult to surface Pouring and smoothing out concrete
finish.[66] at Palisades Park in Washington, DC

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 "Abrams cone" 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 (300 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.[67] 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.
Curing

Concrete must be kept moist during curing in order to achieve

optimal strength and durability.[68] During curing hydration
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.[69] The conversion of calcium hydroxide in the concrete
into calcium carbonate from absorption of CO2 over several
decades further strengthens the concrete and makes it more
resistant to damage. This carbonation reaction, however, lowers A concrete slab being kept hydrated
the pH of the cement pore solution and can corrode the during water curing by submersion
reinforcement bars. (ponding)

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.[70] Addition of short-cut polymer fibers can
improve (reduce) shrinkage-induced stresses during curing and increase early and ultimate compression
strength.[71]

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 scaling, reduced strength, poor abrasion resistance
and cracking.

Techniques

During the curing period, concrete is ideally maintained at controlled temperature and humidity. To ensure
full hydration during curing, concrete slabs are often sprayed with "curing compounds" that create a water-
retaining film over the concrete. Typical films are made of wax or related hydrophobic compounds. After
the concrete is sufficiently cured, the film is allowed to abrade from the concrete through normal use.[72]

Traditional conditions for curing involve spraying or ponding the concrete surface with water. The adjacent
picture shows one of many ways to achieve this, ponding—submerging setting concrete in water and
wrapping in plastic to prevent dehydration. Additional common curing methods include wet burlap and
plastic sheeting covering the fresh concrete.

For higher-strength applications, accelerated curing techniques may be applied to the concrete. A common
technique involves heating the poured concrete with steam, which serves to both keep it damp and raise the
temperature so that the hydration process proceeds more quickly and more thoroughly.

Alternative types

Asphalt
Asphalt concrete (commonly called asphalt,[73] blacktop, or pavement in North America, and tarmac,
bitumen macadam, or rolled asphalt in the United Kingdom and the Republic of Ireland) is a composite
material commonly used to surface roads, parking lots, airports, as well as the core of embankment
dams.[74] Asphalt mixtures have been used in pavement construction since the beginning of the twentieth
century.[75] It consists of mineral aggregate bound together with asphalt, laid in layers, and compacted. The
process was refined and enhanced by Belgian inventor and U.S. immigrant Edward De Smedt.[76]

The terms asphalt (or asphaltic) concrete, bituminous asphalt concrete, and bituminous mixture are
typically used only in engineering and construction documents, which define concrete as any composite
material composed of mineral aggregate adhered with a binder. The abbreviation, AC, is sometimes used
for asphalt concrete but can also denote asphalt content or asphalt cement, referring to the liquid asphalt
portion of the composite material.

Concretene

Concretene is very similar to concrete except that during the cement-mixing process, a small amount of
graphene (< 0.5% by weight) is added.[77]

Microbial

Bacteria such as Bacillus pasteurii, Bacillus pseudofirmus, Bacillus cohnii, Sporosarcina pasteuri, and
Arthrobacter crystallopoietes increase the compression strength of concrete through their biomass. Not all
bacteria increase the strength of concrete significantly with their biomass. Bacillus sp. CT-5. can reduce
corrosion of reinforcement in reinforced concrete by up to four times. Sporosarcina pasteurii reduces water
and chloride permeability. B. pasteurii increases resistance to acid. Bacillus pasteurii and B. sphaericuscan
induce calcium carbonate precipitation in the surface of cracks, adding compression strength.[78]

Nanoconcrete

Nanoconcrete (also spelled "nano concrete"' or "nano-concrete") is

a class of materials that contains Portland cement particles that are
no greater than 100 μm[79] and particles of silica no greater than
500 μm, which fill voids that would otherwise occur in normal
concrete, thereby substantially increasing the material's
strength.[80] It is widely used in foot and highway bridges where
high flexural and compressive strength are indicated.[81]

Decorative plate made of Nano

Pervious
concrete with High-Energy Mixing
(HEM)
Pervious concrete is a mix of specially graded coarse aggregate,
cement, water, and little-to-no fine aggregates. This concrete is also
known as "no-fines" or porous concrete. Mixing the ingredients in
a carefully controlled process creates a paste that coats and bonds the aggregate particles. The hardened
concrete contains interconnected air voids totaling approximately 15 to 25 percent. Water runs through the
voids in the pavement to the soil underneath. Air entrainment admixtures are often used in freeze-thaw
climates to minimize the possibility of frost damage. Pervious concrete also permits rainwater to filter
through roads and parking lots, to recharge aquifers, instead of contributing to runoff and flooding.[82]
Polymer

Polymer concretes are mixtures of aggregate and any of various polymers and may be reinforced. The
cement is costlier than lime-based cements, but polymer concretes nevertheless have advantages; they have
significant tensile strength even without reinforcement, and they are largely impervious to water. Polymer
concretes are frequently used for the repair and construction of other applications, such as drains.

Volcanic

Volcanic concrete substitutes volcanic rock for the limestone that is burned to form clinker. It consumes a
similar amount of energy, but does not directly emit carbon as a byproduct.[83] Volcanic rock/ash are used
as supplementary cementitious materials in concrete to improve the resistance to sulfate, chloride and alkali
silica reaction due to pore refinement.[84] Also, they are generally cost effective in comparison to other
aggregates,[85] good for semi and light weight concretes,[85] and good for thermal and acoustic
insulation.[85]

Pyroclastic materials, such as pumice, scoria, and ashes are formed from cooling magma during explosive
volcanic eruptions. They are used as supplementary cementitious materials (SCM) or as aggregates for
cements and concretes.[86] They have been extensively used since ancient times to produce materials for
building applications. For example, pumice and other volcanic glasses were added as a natural pozzolanic
material for mortars and plasters during the construction of the Villa San Marco in the Roman period (89
B.C. – 79 A.D.), which remain one of the best-preserved otium villae of the Bay of Naples in Italy.[87]

Waste light

Waste light is form of polymer modified concrete. The specific polymer admixture allows the replacement
of all the traditional aggregates (gravel, sand, stone) by any mixture of solid waste materials in the grain size
of 3-10 mm to form a low compressive strength (3-20 N/mm2 ) product [88] for road and building
construction. One cubic meter of waste light concrete contains 1.1-1.3 m3 of shredded waste and no other
aggregates.

Properties
Concrete has relatively high compressive strength, but much lower tensile strength.[89] Therefore, it 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 crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration
forces is prone to creep.

Tests can be performed to ensure that the properties of concrete correspond to specifications for the
application.

The ingredients affect the strengths of the material. 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.

The strengths of concrete is dictated by its function. Very low-strength—14 MPa (2,000 psi) or less—
concrete may be used when the concrete must be lightweight.[90] Lightweight concrete is often achieved by
adding air, foams, or lightweight aggregates, with the side effect that the strength is reduced. For most
routine uses, 20 to 32 MPa (2,900 to 4,600 psi) concrete is often used.
40 MPa (5,800 psi) concrete is readily commercially available as a more
durable, although more expensive, option. Higher-strength concrete is
often used for larger civil projects.[91] Strengths above 40 MPa (5,800 psi)
are often used for specific building elements. For example, the lower floor
columns of high-rise concrete buildings may use concrete of 80 MPa
(11,600 psi) or more, to keep the size of the columns small. Bridges may
use long beams of high-strength concrete to lower the number of spans
required.[92][93] Occasionally, other structural needs may require high-
strength concrete. If a structure must be very rigid, concrete of very high
strength may be specified, even much stronger than is required to bear the
service loads. Strengths as high as 130 MPa (18,900 psi) have been used
Compression testing of a
commercially for these reasons.[92] concrete cylinder

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. Similarly, relatively little energy is
used in producing and combining the raw materials (although large amounts of CO2 are produced by the
chemical reactions in cement manufacture).[94] The overall embodied energy of concrete at roughly 1 to 1.5
megajoules per kilogram is therefore lower than for most structural and construction materials.[95]

Once in place, concrete offers great energy efficiency over the lifetime of a building.[96] Concrete walls
leak air far less than those made of wood frames.[97] 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.[98] 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.

Fire safety

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.
Boston City Hall (1968) is a Brutalist
Options for non-combustible construction include floors, ceilings
design constructed largely of precast
and roofs made of cast-in-place and hollow-core precast concrete. and poured in place 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.

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 tensile and
compressional loads. Concrete structures without reinforcement, like other unreinforced masonry structures,
can fail during severe earthquake shaking. Unreinforced masonry structures constitute one of the largest
earthquake risks globally.[99] These risks can be reduced through seismic retrofitting of at-risk buildings,
(e.g. school buildings in Istanbul, Turkey[100]).

Construction with concrete

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.[101]
Concrete is used more than any other artificial material in the world.[102]
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.[103]

Reinforced concrete

The use of reinforcement, in the form of iron was introduced in the 1850s
by French industrialist François Coignet, and it was not until the 1880s that
German civil engineer G. A. Wayss used steel as reinforcement. Concrete
is a relatively brittle material that is strong under compression but less in
tension. Plain, unreinforced concrete is unsuitable for many structures as it The City Court Building in
Buffalo, New York
is relatively poor at withstanding stresses induced by vibrations, wind
loading, and so on. Hence, to increase its overall strength, steel rods, wires,
mesh or cables can be embedded in concrete before it is set. This
reinforcement, often known as rebar, resists tensile forces.[104]

Reinforced concrete (RC) is a versatile composite and one of the most widely used materials in modern
construction. It is made up of different constituent materials with very different properties that complement
each other. In the case of reinforced concrete, the component materials are almost always concrete and
steel. These two materials form a strong bond together and are able to resist a variety of applied forces,
effectively acting as a single structural element.[105]

Reinforced concrete can be precast or cast-in-place (in situ) concrete, and is used in a wide range of
applications such as; slab, wall, beam, column, foundation, and frame construction. Reinforcement is
generally placed in areas of the concrete that are likely to be subject to tension, such as the lower portion of
beams. Usually, there is a minimum of 50 mm cover, both above and below the steel reinforcement, to resist
spalling and corrosion which can lead to structural instability.[104] Other types of non-steel reinforcement,
such as Fibre-reinforced concretes are used for specialized applications, predominately as a means of
controlling cracking.[105]
Precast concrete

Precast concrete is concrete which is cast in one place for use elsewhere and is a mobile material. The
largest part of precast production is carried out in the works of specialist suppliers, although in some
instances, due to economic and geographical factors, scale of product or difficulty of access, the elements
are cast on or adjacent to the construction site. [106] Precasting offers considerable advantages because it is
carried out in a controlled environment, protected from the elements, but the downside of this is the
contribution to greenhouse gas emission from transportation to the construction site.[105]

Advantages to be achieved by employing precast concrete: [106]

Preferred dimension schemes exist, with elements of tried and tested designs available from
a catalogue.
Major savings in time result from manufacture of structural elements apart from the series of
events which determine overall duration of the construction, known by planning engineers
as the 'critical path'.
Availability of Laboratory facilities capable of the required control tests, many being certified
for specific testing in accordance with National Standards.
Equipment with capability suited to specific types of production such as stressing beds with
appropriate capacity, moulds and machinery dedicated to particular products.
High-quality finishes achieved direct from the mould eliminate the need for interior
decoration and ensure low maintenance costs.

Mass structures

Due to cement's exothermic chemical reaction while setting up,

large concrete structures such as dams, navigation locks, large mat
foundations, and large breakwaters generate excessive heat during
hydration and associated expansion. To mitigate these effects, post-
cooling[107] is commonly applied during construction. An early
example at Hoover Dam used 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 Aerial photo of reconstruction at
ambient air temperature, the cooling process may last for many Taum Sauk (Missouri) pumped
months after the concrete is placed. Various methods also are used storage facility in late November
to pre-cool the concrete mix in mass concrete structures.[107] 2009. After the original reservoir
failed, the new reservoir was made of
Another approach to mass concrete structures that minimizes roller-compacted concrete.
cement's thermal by-product 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 compacted into a dense, strong mass.

Surface finishes

Raw concrete surfaces tend to be porous and have a relatively uninteresting appearance. Many finishes can
be applied to improve the appearance and preserve the surface against staining, water penetration, and
freezing.
Examples of improved appearance include stamped concrete
where the wet concrete has a pattern impressed on the surface, to
give a paved, cobbled or brick-like effect, and may be
accompanied with coloration. Another popular effect for flooring
and table tops is polished concrete where the concrete is polished
optically flat with diamond abrasives and sealed with polymers or
other sealants.

Other finishes can be achieved with chiseling, or more

conventional techniques such as painting or covering it with other
Black basalt polished concrete floor
materials.

The proper treatment of the surface of concrete, and therefore its

characteristics, is an important stage in the construction and renovation of architectural structures.[108]

Prestressed structures

Prestressed concrete is a form of reinforced concrete that builds in

compressive stresses during construction to oppose tensile stresses
experienced 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
tends to sag. Prestressed reinforcement along the bottom of the
beam counteracts this. In pre-tensioned concrete, the prestressing is
achieved by using steel or polymer tendons or bars that are
subjected to a tensile force prior to casting, or for post-tensioned
Stylized cacti decorate a
concrete, after casting.
sound/retaining wall in Scottsdale,
Arizona
There are two different systems being used: [105]

Pretensioned concrete is almost always precast, and

contains steel wires (tendons) that are held in tension while the concrete is placed and sets
around them.
Post-tensioned concrete has ducts through it. After the concrete has gained strength,
tendons are pulled through the ducts and stressed. The ducts are then filled with grout.
Bridges built in this way have experienced considerable corrosion of the tendons, so
external post-tensioning may now be used in which the tendons run along the outer surface
of the concrete. In pre-tensioned concrete, the prestressing 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.

More than 55,000 miles (89,000 km) of highways in the United States are paved with this material.
Reinforced concrete, prestressed concrete and precast concrete are the most widely used types of concrete
functional extensions in modern days. For more information see Brutalist architecture.

Cold weather placement

Extreme weather conditions (extreme heat or cold; windy conditions, and humidity variations) can
significantly alter the quality of concrete. Many precautions are observed in cold weather placement.[109]
Low temperatures significantly slow the chemical reactions involved in hydration of cement, thus affecting
the strength development. Preventing freezing is the most important precaution, as formation of ice crystals
can cause damage to the crystalline structure of the hydrated
cement paste. If the surface of the concrete pour is insulated from
the outside temperatures, the heat of hydration will prevent
freezing.

The American Concrete Institute (ACI) definition of cold weather

placement, ACI 306,[110] is:

A period when for more than three successive days the

average daily air temperature drops below 40 °F (~
4.5 °C), and Pohjolatalo, an office building made
of concrete in the city center of
Temperature stays below 50 °F (10 °C) for more than
Kouvola in Kymenlaakso, Finland
one-half of any 24-hour period.

In Canada, where temperatures tend to be much lower during the

cold season, the following criteria are used by CSA A23.1:

When the air temperature is ≤ 5 °C, and

When there is a probability that the temperature may fall below 5 °C within 24 hours of
placing the concrete.

The minimum strength before exposing concrete to extreme cold is 500 psi (3.4 MPa). CSA A 23.1
specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.

Underwater placement

Concrete may be placed and cured underwater. Care must be taken in the
placement method to prevent washing out the cement. Underwater
placement methods include the tremie, pumping, skip placement, manual
placement using toggle bags, and bagwork.[111]

Grouted aggregate is an alternative method of forming a concrete mass

underwater, where the forms are filled with coarse aggregate and the voids
then completely filled with pumped grout.[111]

Roads

Concrete roads are more fuel efficient to drive on,[112] more reflective and
last significantly longer than other paving surfaces, yet have a much
smaller market share than other paving solutions. Modern-paving methods
and design practices have changed the economics of concrete paving, so
that a well-designed and placed concrete pavement will be less expensive
on initial costs and significantly less expensive over the life cycle. Another
major benefit is that pervious concrete can be used, which eliminates the Assembled tremie placing
need to place storm drains near the road, and reducing the need for slightly concrete underwater
sloped roadway to help rainwater to run off. No longer requiring
discarding rainwater through use of drains also means that less electricity is
needed (more pumping is otherwise needed in the water-distribution system), and no rainwater gets
polluted as it no longer mixes with polluted water. Rather, it is immediately absorbed by the ground.
Environment, health and safety
The manufacture and use of concrete produce a wide range of environmental, economic and social impacts.

Concrete, cement and the environment

A major component of concrete is cement, a fine, soft, powdery-type substance, used mainly to bind fine
sand and coarse aggregates together in concrete. Although a variety of cement types exist, the most
common is “Portland cement”, which is produced by mixing clinker with smaller quantities of other
additives such as gypsum and ground limestone. The production of clinker, the main constituent of cement,
is responsible for the bulk of the sector’s greenhouse gas emissions, including both energy intensity and
process emissions.[113]

The cement industry is one of the three primary producers of carbon dioxide, a major greenhouse gas – the
other two being energy production and transportation industries. On average, every tonne of cement
produced releases one tonne of CO2 into the atmosphere. Pioneer cement manufacturers have claimed to
reach lower carbon intensities, with 590 kg of CO2 eq per tonne of cement produced.[114] The emissions
are due to combustion and calcination processes,[115] which roughly account for 40% and 60% of the
greenhouse gases, respectively. Considering that cement is only a fraction of the constituents of concrete, it
is estimated that a tonne of concrete is responsible for emitting about 100-200 kg of CO2 .[116][6] Every
year more than 10 bilion tonnes of concrete are used worldwide.[6] In the coming years, large quantities of
concrete will continue to be used, and the mitigation of CO2 emissions from the sector will be even more
critical.

Concrete is used to create hard surfaces that contribute to surface runoff, which can cause heavy soil
erosion, water pollution, and flooding, but conversely can be used to divert, dam, and control flooding.
Concrete dust released by building demolition and natural disasters can be a major source of dangerous air
pollution. Concrete is a contributor to the urban heat island effect, though less so than asphalt.

Concrete and climate change mitigation

Reducing the cement clinker content might have positive effects on the environmental life-cycle assessment
of concrete. Some research work on reducing the cement clinker content in concrete has already been
carried out. However, there exist different research strategies. Often replacement of some clinker for large
amounts of slag or fly ash was investigated based on conventional concrete technology. This could lead to a
waste of scarce raw materials such as slag and fly ash. The aim of other research activities is the efficient
use of cement and reactive materials like slag and fly ash in concrete based on a modified mix design
approach.[117]

An environmental investigation found that the embodied carbon of a precast concrete facade can be
reduced by 50% when using the presented fiber reinforced high performance concrete in place of typical
reinforced concrete cladding.[118]

Studies have been conducted with the expectation of being utilized as data for the commercialization of
low-carbon concretes. Life cycle assessment (LCA) of low-carbon concrete was investigated according to
the ground granulated blast-furnace slag (GGBS) and fly ash (FA) replacement ratios. Global warming
potential (GWP) of GGBS decreased by 1.1 kg CO2 eq/m3 , while FA decreased by 17.3 kg CO2 eq/m3
when the mineral admixture replacement ratio was increased by 10%. This study also compared the
compressive strength properties of binary blended low-carbon concrete according to the replacement ratios,
and the applicable range of mixing proportions was derived.[119]
Concrete and climate change adaptation

High-performance building materials will be particularly important for enhancing resilience, including for
flood defenses and critical-infrastructure protection. Risks to infrastructure and cities posed by extreme
weather events are especially serious for those places exposed to flood and hurricane damage, but also
where residents need protection from extreme summer temperatures. Traditional concrete can come under
strain when exposed to humidity and higher concentrations of atmospheric CO2 . While concrete is likely to
remain important in applications where the environment is challenging, novel, smarter and more adaptable
materials are also needed.[6]

Concrete – health and safety

Grinding of concrete can produce hazardous dust. Exposure to

cement dust can lead to issues such as silicosis, kidney disease,
skin irritation and similar effects. The U.S. National Institute for
Occupational Safety and Health in the United States recommends
attaching local exhaust ventilation shrouds to electric concrete
grinders to control the spread of this dust. In addition, the
Occupational Safety and Health Administration (OSHA) has
placed more stringent regulations on companies whose workers
regularly come into contact with silica dust. An updated silica rule,
which OSHA put into effect 23 September 2017 for construction Recycled crushed concrete, to be
companies, restricted the amount of breathable crystalline silica reused as granular fill, is loaded into
workers could legally come into contact with to 50 micro grams a semi-dump truck
per cubic meter of air per 8-hour workday. That same rule went
into effect 23 June 2018 for general industry, hydraulic fracturing
and maritime. That the deadline was extended to 23 June 2021 for engineering controls in the hydraulic
fracturing industry. Companies which fail to meet the tightened safety regulations can face financial charges
and extensive penalties. 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.

Circular economy
Concrete is an excellent material with which to make long-lasting and energy-effi cient buildings. However,
even with good design, human needs change and potential waste will be generated.[120]

End-of-life: concrete degradation and waste

Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel
reinforcement bars, 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).[121] The micro fungi Aspergillus alternaria and
Cladosporium were able to grow on samples of concrete used as a radioactive waste barrier in the
Chernobyl reactor; leaching aluminum, iron, calcium, and silicon.[122]

Concrete may be considered waste according to the European Commission decision of 2014/955/EU for
the List of Waste under the codes: 17 (construction and demolition wastes, including excavated soil from
contaminated sites) 01 (concrete, bricks, tiles and ceramics), 01 (concrete), and 17.01.06* (mixtures of,
separate fractions of concrete, bricks, tiles and ceramics containing hazardous substances), and 17.01.07
(mixtures of, separate fractions of concrete, bricks, tiles and
ceramics other than those mentioned in 17.01.06).[123] It is
estimated that in 2018 the European Union generated 371,910
thousand tons of mineral waste from construction and demolition,
and close to 4% of this quantity is considered hazardous. Germany,
France and the United Kingdom were the top three polluters with
86,412 thousand tons, 68,976 and 68,732 thousand tons of
construction waste generation, respectively.[124]

Currently, there is not a End-of-Waste criteria for concrete

materials in the EU. However, different sectors have been
proposing alternatives for concrete waste and re purposing it as a
secondary raw material in various applications, including concrete The Tunkhannock Viaduct in
manufacturing itself.[125] northeastern Pennsylvania opened in
1915 and is still in regular use today

Reuse of concrete

Reuse of blocks in original form, or by cutting into smaller blocks, has even less environmental impact;
however, only a limited market currently exists. Improved building designs that allow for slab reuse and
building transformation without demolition could increase this use. Hollow core concrete slabs are easy to
dismantle and the span is normally constant, making them good for reuse.[120]

Other cases of re-use are possible with pre-cast concrete pieces: through selective demolition, such pieces
can be disassembled and collected for further use in other building sites. Studies show that back-building
and remounting plans for building units (ie, re-use of pre-fabricated concrete) is an alternative for a kind of
construction which protects resources and saves energy. Especially long-living, durable, energy-intensive
building materials, such as concrete, can be kept in the life-cycle longer through recycling. Prefabricated
constructions are the prerequisites for constructions necessarily capable of being taken apart. In the case of
optimal application in the building carcass, savings in costs are estimated in 26%, a lucrative complement to
new building methods. However, this depends on several courses to be set.[126] The viability of this
alternative has to be studied as the logistics associated with transporting heavy pieces of concrete can
impact the operation financially and also increase the carbon footprint of the project. Also, ever changing
regulations on new buildings worldwide may require higher quality standards for construction elements and
inhibit the use of old elements which may be classified as obsolete.

Recycling of concrete

Concrete recycling is an increasingly common method for disposing of concrete structures. Concrete debris
were once routinely shipped to landfills for disposal, but recycling is increasing due to improved
environmental awareness, governmental laws and economic benefits.

Contrary to general belief, concrete recovery is achievable – concrete can be crushed and reused as
aggregate in new projects.[120]

Recycling or recovering concrete reduces natural resource exploitation and associated transportation costs,
and reduces waste landfill. However, it has little impact on reducing greenhouse gas emissions as most
emissions occur when cement is made, and cement alone cannot be recycled. At present, most recovered
concrete is used for road sub-base and civil engineering projects. From a sustainability viewpoint, these
relatively low-grade uses currently provide the optimal outcome.[127]
The recycling process can be done in situ, with mobile plants, or in specific recycling units. The input
material can be returned concrete which is fresh (wet) from ready-mix trucks, production waste at a pre-cast
production facility, Waste from construction and demolition. The most significant source is demolition
waste, preferably pre-sorted from selective demolition processes.[120]

By far the most common method for recycling dry and hardened concrete involves crushing. Mobile sorters
and crushers are often installed on construction sites to allow on-site processing. In other situations, specific
processing sites are established, which are usually able to produce higher quality aggregate. Screens are
used to achieve desired particle size, and remove dirt, foreign particles and fine material from the coarse
aggregate.[128][120]

Chloride and sulfates are undesired contaminants originated from soil and weathering and can provoke
corrosion problems on aluminum and steel structures.[128] The final product, Recycled Concrete Aggregate
(RCA), presents interesting properties such as: angular shape, rougher surface , lower specific gravity
(20%), higher water absorption, and pH greater than 11 – this elevated pH increases the risk of alkali
reactions.[120]

The lower density of RCA usually Increases project efficiency and improve job cost - recycled concrete
aggregates yield more volume by weight (up to 15%).[129] The physical properties of coarse aggregates
made from crushed demolition concrete make it the preferred material for applications such as road base
and sub-base. This is because recycled aggregates often have better compaction properties and require less
cement for sub-base uses. Furthermore, it is generally cheaper to obtain than virgin material.[120]

Applications of recycled concrete aggregate

The main commercial applications of the final recycled concrete aggregate are:

Aggregate base course (road base), or the untreated aggregates used as foundation for
roadway pavement, is the underlying layer (under pavement surfacing) which forms a
structural foundation for paving. To this date this has been the most popular application for
RCA due to technical-economic aspects.[130]
Aggregate for ready-mix concrete, by simply replacing from 10 to 45% of the natural
aggregates in the concrete mix with a blend of cement, sand and water. Some concept
buildings are showing the progress of this field. Because the RCA contains cement it, the
ratios of the mix have to be adjusted to achieve desired structural requirements such as
workability, strength and water absorption.[120]
Soil Stabilization, with the incorporation of recycled aggregate, lime, or fly ash into marginal
quality subgrade material used to enhance the load bearing capacity of that subgrade.[130]
Pipe bedding: serving as a stable bed or firm foundation in which to lay underground
utilities. Some countries' regulations prohibit the use of RCA and other construction and
demolition wastes in filtration and drainage beds due to potential contamination with
chromium and pH-value impacts.[120][130]
Landscape Materials: to promote green architecture. To date, recycled concrete aggregate
has been used as boulder/stacked rock walls, underpass abutment structures, erosion
structures, water features, retaining walls, and more.[130]

Cradle-to cradle challenges

The applications developed for RCA so far are not exhaustive, and many more uses are to be developed as
regulations, institutions and norms find ways to accommodate construction and demolition waste as
secondary raw materials in a safe and economic way. However, considering the purpose of having a
circularity of resources in the concrete life cycle, the only
application of RCA that could be considered as recycling of
concrete is the replacement of natural aggregates on concrete
mixes. All the other applications would fall under the category of
downcycling. It is estimated that even near complete recovery of
concrete from construction and demolition waste will only supply
about 20% of total aggregate needs in the developed world.[120]

The path towards circularity goes beyond concrete technology

itself, depending on multilateral advances in the cement industry, Circularity of Concrete: Cradle-to-
research and development of alternative materials, building design Cradle design
and management, and demolition as well as conscious use of
spaces in urban areas to reduce consumption.

World records
The world record for the largest concrete pour in a single project is the Three Gorges Dam 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 12.3 million cubic
meters held by Itaipu hydropower station in Brazil.[131][132][133]

The world record for concrete pumping was set on 7 August 2009 during the construction of the Parbati
Hydroelectric Project, near the village of Suind, Himachal Pradesh, India, when the concrete mix was
pumped through a vertical height of 715 m (2,346 ft).[134][135]

The Polavaram dam works in Andhra Pradesh on 6 January 2019 entered the Guinness World Records by
pouring 32,100 cubic metres of concrete in 24 hours.[136] 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.[137][138] The pour (a part of the foundation
for the Abu Dhabi's Landmark Tower) was 16,000 cubic meters of concrete poured within a two-day
period.[139] 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.[140]

The world record for largest continuously poured concrete floor was completed 8 November 1997, in
Louisville, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement
consisted of 225,000 square feet (20,900 m2 ) of concrete placed in 30 hours, finished to a flatness tolerance
of FF 54.60 and a levelness tolerance of FL 43.83. This surpassed the previous record by 50% in total
volume and 7.5% in total area.[141][142]

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 10,251 cubic yards of concrete placed in 58.5 hours using two concrete pumps
and two dedicated concrete batch plants. Upon curing, this placement allows the 50,180-square-foot
(4,662 m2 ) cofferdam to be dewatered approximately 26 feet (7.9 m) below sea level to allow the
construction of the Inner Harbor Navigation Canal Sill & Monolith Project to be completed in the dry.[143]

See also
Anthropic rock – Rock that is made, modified and moved by humans.
Biorock
Brutalist architecture – 20th century style of architecture
Bunding
Cement accelerator
Cenocell – Concrete material using fly ash in place of cement
Compressometer
Concrete canoe
Concrete chipping
Concrete leveling
Concrete mixer – Device that combines cement, aggregate, and water to form concrete
Concrete masonry unit – Rectangular block used in construction
Concrete moisture meter
Concrete plant
Concrete recycling
Concrete step barrier – Safety barrier used on the central reservation of motorways
Concrete sealer
Construction – Process of the building or assembling of a building or infrastructure
Diamond grinding of pavement – Technique for correcting surface defects
Efflorescence
Fireproofing – Rendering something (structures, materials, etc.) resistant to fire, or
incombustible
Foam Index
Form liner
High-performance fiber-reinforced cementitious composites
Interfacial Transition Zone (ITZ)
International Grooving & Grinding Association – Trade association in the concrete and
asphalt surface industry
Lift slab construction
LiTraCon
Metakaolin
Mortar – Workable paste which hardens to bind building blocks
Plasticizer – substance that is added to a material to make it softer and more flexible
Prefabrication
Pykrete – Ice alloy containing sawdust or another form of wood pulp
Rammed earth – Technique for constructing foundations, floors, and walls by compacting a
damp mixture of sub soil
Reinforced concrete structures durability
Rusticated concrete block – Type of concrete block
Shallow foundation
Silica fume – Silicon dioxide nano particles
Studcast
Translucent concrete
Whitetopping
World of Concrete
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