Concrete is a construction material composed of cement (commonly Portland cement) as well as other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate such as gravel, limestone, or granite, plus a fine aggregate such as sand), water, and chemical admixtures. The word concrete comes from the Latin word "concretus" (meaning compact or condensed), the past participle of "concresco", from "com-" (together) and "cresco" (to grow).

Concrete solidifies and hardens after mixing with water and placement due to a chemical process known as hydration. The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material. Concrete is used to make pavements, pipe, architectural structures, foundations, motorways/roads, bridges/overpasses, parking structures, brick/block walls and footings for gates, fences and poles.

Concrete is used more than any other man-made material in the world.^[1] As of 2006, about 7.5 cubic kilometres of concrete are made each year—more than one cubic metre for every person on Earth.^[2] Concrete powers a US $35-billion industry which employs more than two million workers in the United States alone.^{[citation needed]} More than 55,000 miles (89,000 km) of highways in the United States are paved with this material. Reinforced concrete and prestressed concrete are the most widely used modern kinds of concrete functional extensions.

[edit] History

During the Roman Empire, Roman concrete (or Opus caementicium) was made from quicklime, pozzolanic ash/pozzolana, and an aggregate of pumice. Its widespread use in many Roman structures, a key event in the history of architecture termed the Concrete Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionarily new designs both in terms of structural complexity and dimension.^[4]

Concrete, as the Romans knew it, was in effect 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 which trouble the builders of similar structures in stone or brick.^[5]

Modern tests show Opus caementicium similarly strong as modern Portland cement concrete in its compressive strength (ca. 200 kg/cm²).^[6] However, due to the absence of reinforced steel, 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 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.^[7]

The widespread use of concrete in many Roman structures has ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example of the longevity of concrete, which allowed the Romans to build this and similar structures across the Roman Empire. Many Roman aqueducts and Roman bridges have masonry cladding to a concrete core, a technique they used in structures such as the Pantheon, the dome of which is concrete.

The secret of concrete was lost for 13 centuries until 1756, when the British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. Portland cement was first used in concrete in the early 1840s. This version of history has been challenged however, as the Canal du Midi was constructed using concrete in 1670.^[8]

Recently, the use of recycled materials as concrete ingredients is gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This has a significant impact by reducing the amount of quarrying and landfill space required, and, as it acts as a cement replacement, reduces the amount of cement required to produce a solid concrete.^{[citation needed]}

Concrete additives 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 hair made concrete less liable to crack while it hardened, and adding blood made it more frost-resistant.^[9]

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.

[edit] Composition

There are many types of concrete available, created by varying the proportions of the main ingredients below.

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.

[edit] Cement

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar, and plaster. English engineer Joseph Aspdin patented Portland cement in 1824; it was named because of its similarity in colour to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay, and grinding this product (called clinker) with a source of sulfate (most commonly gypsum). The manufacturing of Portland cement creates about 5 percent of human CO₂ emissions.^[10]

[edit] 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 easily.

Less water in the cement paste will yield a stronger, more durable concrete; more water will give an easier-flowing concrete with a higher slump.^[11]

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: Ca₃Si + H₂O → CaSiH(gel) + CaOH

Standard notation: Ca₃SiO₅ + H₂O → (CaO)•(SiO₂)•(H₂O)(gel) + Ca(OH)₂

Balanced: 2Ca₃SiO₅ + 7H₂O → 3(CaO)•2(SiO₂)•4(H₂O)(gel) + 3Ca(OH)₂

[edit] Aggregates

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are mainly used 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 furnace slag and bottom ash are also permitted.

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.

[edit] 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 concrete solves these problems by adding either metal reinforcing bars, steel fibers, glass fiber, or plastic fiber to carry tensile loads.

[edit] Chemical admixtures

Chemical 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.^[12] The most common types of admixtures^[13] are:

• Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are CaCl₂ and NaCl.
• Acrylic retarders slow the hydration of concrete, and are used in large or difficult pours where partial setting before the pour is complete is undesirable. A typical retarder is table sugar, or sucrose (C₁₂H₂₂O₁₁).
• Air entrainments add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles thereby increasing the concrete's durability. However, entrained air is a trade-off with strength, as each 1% of air may result in 5% decrease in compressive strength.
• Plasticizers (water-reducing admixtures) increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. Superplasticizers (high-range water-reducing admixtures) are a class of plasticizers which have fewer deleterious effects when used to significantly increase workability. Alternatively, plasticizers can be used to reduce the water content of a concrete (and have been called water reducers due to this application) while maintaining workability. This improves its strength and durability characteristics.
• Pigments can be used to change the color of concrete, for aesthetics.
• Corrosion inhibitors 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.
• Pumping aids improve pumpability, thicken the paste, and reduce dewatering – the tendency for the water to separate out of the paste.

[edit] Mineral admixtures and blended cements

There are inorganic materials that also 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),^[12] or as a replacement for Portland cement (blended cements).^[14]

• 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, silicious fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.^[15]
• 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.^[16]
• 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.^[17]
• High Reactivity Metakaolin (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.

[edit] 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.

When initially mixed together, Portland cement and water rapidly form a gel, 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 up, and form a rigid structure, gluing the aggregate particles in place. During curing, more of the cement reacts with the residual water (Hydration).

This curing process develops physical and chemical properties. Among other qualities, mechanical strength, low moisture permeability, and chemical and volumetric stability.

[edit] Mixing concrete

Thorough mixing is essential for the production of uniform, high quality concrete. Therefore, 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 aggregates can increase the compressive strength of the resulting concrete.^[18] 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, e.g. accelerators or retarders, plasticizers, pigments, or fumed silica. The latter is added to fill the gaps between the cement particles. This reduces the particle distance and leads to a higher final compressive strength and a higher water impermeability.^[19] The premixed paste is then blended with aggregates and any remaining batch water, and final mixing is completed in conventional concrete mixing equipment.^[20]

High-Energy Mixed Concrete (HEM concrete) is produced by means of high-speed mixing of cement, water and sand with net specific energy consumption at least 5 kilojoules per kilogram of the mix. It is then added to a plasticizer admixture and mixed after that with aggregates in conventional concrete mixer. This paste can be used itself or foamed (expanded) for lightweight concrete.^[21] Sand effectively dissipates energy in this mixing process. HEM concrete fast hardens in ordinary and low temperature conditions, and possesses increased volume of gel, drastically reducing capillarity in solid and porous materials. It is recommended for precast concrete in order to reduce quantity of cement, as well as for concrete roof and siding tiles, paving stones and lightweight concrete block production.

[edit] 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 hydration), and can be modified by adding chemical admixtures. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (surface 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 Test, 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 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 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 six or seven inches (150 to 175 mm).

Slump can be increased by adding chemical admixtures such as mid-range or high-range water reducing agents (super-plasticizers) without changing the water-cement ratio. It is bad practice to add excessive water upon delivery to the jobsite, however in a properly designed mixture it is important to reasonably achieve the specified slump prior to placement as design factors such as air content, internal water for hydration/strength gain, etc. are dependent on placement at design slump values.

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.

[edit] Curing

In all but the least critical applications, care needs to be taken to properly cure concrete, and 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 days and weeks following. In around 3 weeks, over 90% of the final strength is typically reached though it may continue to strengthen for decades.^[22]

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 significant strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased by keeping it damp for a longer period 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 which increases shrinkage and cracking.

During this period concrete needs to be in conditions with a controlled temperature and humid atmosphere. In practice, this is achieved by spraying or ponding the concrete surface with water, thereby protecting 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.

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 (the Hoover Dam used pipes carrying coolant during setting to avoid damaging overheating). Improper curing can cause scaling, reduced strength, poor abrasion resistance and cracking.

[edit] Properties

[edit] Strength

Concrete has relatively high compressive strength, but significantly lower tensile strength. It is fair to assume that a concrete sample's tensile strength is about 10%-15% of its compressive strength.^[23] As a result, without compensating, concrete would almost always fail from tensile stresses – even when loaded in compression. The practical implication of this is that concrete elements subjected to tensile stresses must be reinforced with materials that are strong in tension.

Reinforced concrete is the most common form of concrete. The reinforcement is often steel, rebar (mesh, spiral, bars and other forms). Structural fibers of various materials are available.

Concrete can also be prestressed (reducing tensile stress) using internal steel cables (tendons), allowing for beams or slabs with a longer span than is practical with reinforced concrete alone. Inspection of concrete structures can be non-destructive if carried out with equipment such as a Schmidt hammer, which is used to estimate concrete strength.

The ultimate strength of concrete is influenced by the water-cementitious ratio (w/cm), the design constituents, and the mixing, placement and curing methods employed. All things being equal, concrete with a lower water-cement (cementitious) ratio makes a stronger concrete than that with a higher ratio. The total quantity of cementitious materials (Portland cement, slag cement, pozzolans) can affect strength, water demand, shrinkage, abrasion resistance and density. All concrete will crack independent of whether or not it has sufficient compressive strength. In fact, high Portland cement content mixtures can actually crack more readily due to increased hydration rate. As concrete transforms from its plastic state, hydrating to a solid, the material undergoes shrinkage. Plastic shrinkage cracks can occur soon after placement but if the evaporation rate is high they often can actually occur during finishing operations, for example in hot weather or a breezy day. In very high-strength concrete mixtures (greater than 10,000 psi) the crushing strength of the aggregate can be a limiting factor to the ultimate compressive strength. In lean concretes (with a high water-cement ratio) the crushing strength of the aggregates is not so significant.

The internal forces in common shapes of structure, such as arches, vaults, columns and walls are predominantly compressive forces, with floors and pavements subjected to tensile forces. Compressive strength is widely used for specification requirement and quality control of concrete. The engineer knows his target tensile (flexural) requirements and will express these in terms of compressive strength.

Wired.com reported on April 13, 2007 that a team from the University of Tehran, competing in a contest sponsored by the American Concrete Institute, demonstrated several blocks of concretes with abnormally high compressive strengths between 50,000 and 60,000 PSI at 28 days.^[24] The blocks appeared to use an aggregate of steel fibres and quartz – a mineral with a compressive strength of 160,000 PSI, much higher than typical high-strength aggregates such as granite (15,000-20,000 PSI).

Reactive Powder Concrete, also known as Ultra-High Performance Concrete, can be even stronger, with strengths of up to 116,000 PSI (800 MPa).^[25] These are made by eliminating large aggregate completely, carefully controlling the size of the fine aggregates to ensure the best possible packing, and incorporating steel fibers (sometimes produced by grinding steel wool) into the matrix. Reactive Powder Concretes may also make use of silica fume as a fine aggregate. Commercial Reactive Powder Concretes are available in the 25,000-30,000 PSI strength range.

[edit] Elasticity

The modulus of elasticity of concrete is a function of the modulus of elasticity of the aggregates and the cement matrix and their relative proportions. The modulus of elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. The elastic modulus of the hardened paste may be in the order of 10-30 GPa and aggregates about 45 to 85 GPa. The concrete composite is then in the range of 30 to 50 GPa.

The American Concrete Institute allows the modulus of elasticity to be calculated using the following equation:^[23]

(psi)

where

w_c = weight of concrete (pounds per cubic foot) and where

f'_c = compressive strength of concrete at 28 days (psi)

This equation is completely empirical and is not based on theory. Note that the value of E_c found is in units of psi. For normalweight concrete (defined as concrete with a w_c of 150 pcf and subtracting 5pcf for steel) E_c is permitted to be taken as .

[edit] Expansion and shrinkage

Concrete has a very low coefficient of thermal expansion. However, if no provision is made for expansion, very large forces can be created, causing cracks in parts of the structure not capable of withstanding the force or the repeated cycles of expansion and contraction.

As concrete matures it continues to shrink, due to the ongoing reaction taking place in the material, although the rate of shrinkage falls relatively quickly and keeps reducing over time (for all practical purposes concrete is usually considered to not shrink due to hydration any further after 30 years). The relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface.

Because concrete is continuously shrinking for years after it is initially placed, it is generally accepted that under thermal loading it will never expand to its originally placed volume.

[edit] Cracking

All concrete structures will crack to some extent. One of the early designers of reinforced concrete, Robert Maillart, employed reinforced concrete in a number of arched bridges. His first bridge was simple, using a large volume of concrete. He then realized that much of the concrete was very cracked, and could not be a part of the structure under compressive loads, yet the structure clearly worked. His later designs simply removed the cracked areas, leaving slender, beautiful concrete arches. The Salginatobel Bridge is an example of this.

Concrete cracks due to tensile stress induced by shrinkage or stresses occurring during setting or use. Various means are used to overcome this. Fiber reinforced concrete uses fine fibers distributed throughout the mix or larger metal or other reinforcement elements to limit the size and extent of cracks. In many large structures joints or concealed saw-cuts are placed in the concrete as it sets to make the inevitable cracks occur where they can be managed and out of sight. Water tanks and highways are examples of structures requiring crack control.

[edit] Shrinkage cracking

Shrinkage cracks occur when concrete members undergo restrained volumetric changes (shrinkage) as a result of either drying, autogenous shrinkage or thermal effects. Restraint is provided either externally (i.e. supports, walls, and other boundary conditions) or internally (differential drying shrinkage, reinforcement). Once the tensile strength of the concrete is exceeded, a crack will develop. The number and width of shrinkage cracks that develop are influenced by the amount of shrinkage that occurs, the amount of restraint present and the amount and spacing of reinforcement provided.

Plastic-shrinkage cracks are immediately apparent, visible within 0 to 2 days of placement, while drying-shrinkage cracks develop over time. Autogenous shrinkage also occurs when the concrete is quite young and results from the volume reduction resulting from the chemical reaction of the Portland cement.

[edit] Tension cracking

Concrete members may be put into tension by applied loads. This is most common in concrete beams where a transversely applied load will put one surface into compression and the opposite surface into tension due to induced bending. The portion of the beam that is in tension may crack. The size and length of cracks is dependent on the magnitude of the bending moment and the design of the reinforcing in the beam at the point under consideration. Reinforced concrete beams are designed to crack in tension rather than in compression. This is achieved by providing reinforcing steel which yields before failure of the concrete in compression occurs and allowing remediation, repair, or if necessary, evacuation of an unsafe area.

[edit] Creep

Creep is the term used to describe the permanent movement or deformation of a material in order to relieve stresses within the material. Concrete which is subjected to long-duration forces is prone to creep. Short-duration forces (such as wind or earthquakes) do not cause creep. Creep can sometimes reduce the amount of cracking that occurs in a concrete structure or element, but it also must be controlled. The amount of primary and secondary reinforcing in concrete structures contributes to a reduction in the amount of shrinkage, creep and cracking.

[edit] Liquid concrete

After mixing, concrete is a fluid and can be pumped to where it is needed.

[edit] Physical properties

The coefficient of thermal expansion of Portland cement concrete is 0.000008 to 0.000012 (per degree Celsius) (8-12 1/MK).^[26] The density varies, but is around 150 pounds per cubic foot (2400 kg/m³).^[27]

[edit] Environmental concerns

For the environmental impact of cement production see Cement

[edit] Worldwilde CO₂ emissions and global change

The cement industry is one of two primary producers of carbon dioxide (CO2), creating up to 5 percent of worldwide emissions of this gas. The embodied carbon dioxide (ECO₂) of a tonne of concrete varies with mix design and is in the range of: 75-176 kg CO₂/tonne 0.075 - 0.176 tonne CO₂/tonne^[28] Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is heated, producing lime and carbon dioxide^[29], and also indirectly through the use of energy, particularly if the energy is sourced from fossil fuels. The cement industry produces 5% of global man-made CO₂ emissions, of which 50 % is from the chemical process, and 40 % from burning fuel.^[30]

[edit] CO₂ uptake by concrete in the Biosphere 2 project building

A deficit of CO₂ was observed in the mass balance of the gases in the closed atmosphere environments of the Biosphere 2 project. It was found that the respiration rate was faster than the photosynthesis resulting in a slow decrease of oxygen. An unresolved question accompanied the oxygen decline: the corresponding increase in carbon dioxide did not appear in the mass balance calculations. This concealed the underlying process until an investigation by Severinghaus et al. (1994) of Columbia University’s Lamont-Doherty Earth Observatory using isotopic analysis showed that carbon dioxide was reacting with exposed concrete inside Biosphere 2 to form calcium carbonate, thereby sequestering the carbon dioxide. ^[31][32] After being poured, concrete can absorb CO₂ for up to 5 years until fully cured.

[edit] Surface runoff

Surface runoff, when water runs off impervious surfaces, such as non-porous concrete, can cause heavy soil erosion. Urban runoff tends to pick up gasoline, motor oil, heavy metals, trash and other pollutants from sidewalks, roadways and parking lots.^[33][34] The impervious cover in a typical city sewer system prevents groundwater percolation five times than that of a typical woodland of the same size.^[35] A 2008 report by the United States National Research Council identified urban runoff as a leading source of water quality problems.^[36]

[edit] Urban heat

Both concrete and asphalt are the primary contributors to what is known as the Urban heat island effect.

Using light-colored concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature.^[37] A low albedo 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.^[38]

Many U.S. cities show that pavement comprise approximately 30-40% of their surface area.^[37] This directly impacts the temperature of the city, as demonstrated by the urban heat island effect. In addition to decreasing the overall temperature of parking lots and large paved areas by paving with light-colored concrete, there are supplemental benefits. One example is 10-30% improved nighttime visibility.^[37] The potential of energy saving within an area is also high. With lower temperatures, the demand for air conditioning decreases, saving vast amounts of 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.^[39] New York City offers another example. The Design Trust for Public Space in New York City found that by slightly raising the albedo value in their 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.^[38]

[edit] 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 earthquake.^[40]

[edit] Health concerns

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.^[41] 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.

[edit] Damage modes

Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, leaching, physical damage and chemical damage (from carbonation, chlorides, sulfates and distillate water).

[edit] Types of concrete

There are many different types of concrete including Mix design, Regular concrete, High-strength concrete, Stamped concrete, High-performance concrete, Self-consolidating concretes, Vacuum concretes, Shotcrete, Pervious concrete, Cellular concrete, Cork-cement composites, Roller-compacted concrete, Glass concrete, Asphalt concrete, Rapid strength concrete, Rubberized concrete, Polymer concrete, Geopolymer or green concrete, Limecrete, Refractory Cement, Concrete cloth, Innovative mixtures and Gypsum concrete.

[edit] Concrete handling / Safety precautions

Handling of wet concrete must always be done with proper protective equipment. Contact with wet concrete can cause skin burns due to the caustic nature of the mix with cement and water.

[edit] Concrete testing

Engineers usually specify the required compressive strength of concrete, which is normally given as the 28 day compressive strength in megapascals (MPa) or pounds per square inch (psi). Twenty eight days is a long wait to determine if desired strengths are going to be obtained, so three-day and seven-day strengths can be useful to predict the ultimate 28-day compressive strength of the concrete. A 25% strength gain between 7 and 28 days is often observed with 100% OPC (ordinary Portland cement) mixtures, and up to 40% strength gain can be realized with the inclusion of pozzolans and supplementary cementitious materials (SCMs) such as fly ash and/or slag cement. Strength gain depends on the type of mixture, its constituents, the use of standard curing, proper testing and care of cylinders in transport, etc. It is imperative to accurately test the fundamental properties of concrete in its fresh, plastic state.

Concrete is typically sampled while being placed, with testing protocols requiring that test samples be cured under laboratory conditions (standard cured). Additional samples may be field cured (non-standard) for the purpose of early 'stripping' strengths, that is, form removal, evaluation of curing, etc. but the standard cured cylinders comprise acceptance criteria. Concrete tests can measure the "plastic" (unhydrated) properties of concrete prior to, and during placement. As these properties affect the hardened compressive strength and durability of concrete (resistance to freeze-thaw), the properties of workability (slump/flow), temperature, density and age are monitored to ensure the production and placement of 'quality' concrete. Tests are performed per ASTM International, European Committee for Standardization or Canadian Standards Association. As measurement of quality must represent the potential of concrete material delivered, placed and properly cured, it is imperative that concrete technicians performing concrete tests are certified to do so according to these standards. Structural design, material design and properties are often specified in accordance with national/regional design codes such as American Concrete Institute.

Compressive strength tests are conducted using an instrumented hydraulic ram to compress a cylindrical or cubic sample to failure. Tensile strength tests are conducted either by three-point bending of a prismatic beam specimen or by compression along the sides of a cylindrical specimen. The same test can be done on site by a non-destructive test method using a rebound hammer.

[edit] Concrete recycling

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

Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through a crushing machine, often along with asphalt, bricks, and rocks.

Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Aggregate base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt placed over it. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions. On March 3, 1983, a government funded research team (the VIRL research.codep) approximated that almost 17% of worldwide landfill was by-products of concrete based waste.

Recycling concrete provides environmental benefits, conserving landfill space and use as aggregate reduces the need for gravel mining.

[edit] 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 21 million cubic yards over 17 years. Surpassing the previous record of 3.2 million cubic meters which was held by Itaipu hydropower station in Brazil. ^{[42] [43]}

[edit] Continuous pours

The world record for largest continuously poured concrete raft was achieved March 23rd, 2007 in Al Durrah, Dubai by contracting firm, Dubai Contracting Company. The pour was close to 10,500 cubic meters of concrete poured within a two day period. Surpassing the previous record which was also held by Dubai Contracting Company.^[44][45] The pour was part of the foundation for the Dubai's Sama Tower.

The world record for largest continuously poured concrete floor was completed November 8th, 1997 in Louisville, Kentucky by Design-build firm, EXXCEL Project Management. The pour consisted of 225,000 sq. ft of concrete within a 30 hour period with a flatness of FF 54.60 and levelness of FL 43.83. surpassing the previous record by 50% in total volume and 7.5% in total area.^[46][47]

[edit] Use of concrete in infrastructure

[edit] Mass concrete structures

These include gravity dams such as the Itaipu, Hoover Dam and the Three Gorges Dam and large breakwaters. Concrete that is poured all at once in one block (so that there are no weak points where the concrete is "welded" together) is used for tornado shelters.

[edit] Concrete textures

When one thinks of concrete, oftentimes the image of a dull, gray concrete wall comes to mind. With the use of form liner, concrete can be cast and molded into different textures and used for decorative concrete 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-foot lizard and 40-foot cacti along the 8-mile stretch. The project, titled "The Path Most Traveled," is one example of how concrete can be shaped using elastomeric form liner.

Three textures of concrete featured in Scottsdale, AZ, created with [formliner]. (Scott System)

A 67-foot concrete lizard basks in the sun, featured on a sound/retaining wall in Scottsdale, AZ. (Scott System)

40-foot cacti decorate a sound/retaining wall in Scottsdale, AZ. (Scott System)

[edit] Reinforced concrete structures

Reinforced concrete contains steel reinforcing that is designed and placed in structural members at specific positions to cater for all the stress conditions that the member is required to accommodate.

[edit] Prestressed concrete structures

Prestressed concrete is a form of reinforced concrete which 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 prestressed, it can counteract 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 concrete, after casting.

[edit] See also

Anthropic rock Biorock Construction Bunding Brutalist architecture, encouraging visible concrete surfaces Cement Geopolymers, a class of synthetic aluminosilicate materials Hempcrete, a mixture with hemp hurds Mudcrete, a soil-cement mixture Papercrete, a paper-cement mixture Portland cement, the classical concrete cement Cement accelerator Concrete moisture meter Concrete canoe Concrete curing Concrete leveling Concrete mixer Concrete masonry unit Concrete recycling Concrete step barrier Efflorescence Fireproofing Foam Index Form liner Formwork Controlled permeability formwork

KU Mix LiTraCon High performance fiber reinforced cementitious composites High Reactivity Metakaolin Mortar Plasticizer Prefabrication Pykrete, a composite material of ice and cellulose Silica fume Shallow foundation Types of concrete Asphalt concrete Aerated autoclaved concrete Decorative concrete Fiber reinforced concrete Lunarcrete Prestressed concrete Precast concrete Ready-mix concrete Reinforced concrete Salt-concrete Seacrete Terrazzo concrete Whitetopping World of Concrete

[edit] References

• Matthias Dupke: Textilbewehrter Beton als Korrosionsschutz. Examicus, Frankfurt am Main 2009, ISBN 978-3-86943-336-3.

[edit] External links

Wikimedia Commons has media related to: Concrete

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