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in Toronto , Ontario , Canada .]] in a floor slab during a concrete pour]] Concrete is a construction material that consists of Cement (commonly Portland Cement ), Aggregate (generally Gravel and Sand ), Water and admixtures. Concrete solidifies and hardens after mixing 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. It is used to make Pavements , Architectural Structure s, Foundation s, Motorway s/ Road s, Overpass es, Parking structures, Brick / Block walls and Footing s for gates, Fence s and Pole s. Concrete is used more than any other manmade material on the planet.The Skeptical Environmentalist: Measuring the Real State of the World, by Bjorn Lomborg , p 138. As of 2005 about six Billion Cubic Meters of concrete are made each year, which equals one cubic meter for every person on Earth. Concrete powers a US $ 35 billion industry which employs more than two million workers in the United States alone. More than 55,000 Mile s of Freeway s and Highway s in America are made of this material. The People's Republic Of China currently consumes 40% of the world's cement {Link without Title} production. HISTORY In Serbia , remains of a hut dating from 5600 BC have been found, with a floor made of Red Lime , sand, and gravel. The Pyramid s of Shaanxi in China , built thousands of years ago, contain a mixture of lime and Volcanic Ash or clay. The Assyrians and Babylonians used Clay as cement in their concrete. The Egyptians used Lime and Gypsum cement. In the Roman Empire , concrete made from Quicklime , Pozzolanic Ash / Pozzolana and an aggregate made from Pumice was very similar to modern Portland cement concrete. The secret of concrete was lost for 13 centuries until in 1756 , 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. In modern times 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 byproduct of Coal fired power plants. This has a significant impact by reducing the amount of quarrying and landfill space required. The properties of concrete have been altered 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. In modern times, researchers have added other materials to create concrete that is extremely strong, and even concrete that can conduct electricity. COMPOSITION paved with concrete.]] The composition of concrete is determined initially during mixing and finally during placing of fresh concrete. The type of structure being built as well as the method of construction determine how the concrete is placed and therefore the composition of the concrete mix (the ''mix design''). 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 ). When mixed with water, the resulting powder will become a Hydrate d solid over time. High temperature applications, such as Masonry Oven s and the like, generally require the use of a Refractory Cement ; concretes based on Portland cement can be damaged or destroyed by elevated temperatures, but refractory concretes are better able to withstand such conditions. Water , the amount of cement paste in the overall mix and the physical characteristics (maximum size, shape, and grading) of the aggregates. Aggregates The water and cement paste hardens and develops strength over time. In order to ensure an economical and practical solution, both fine and coarse aggregates are utilised to make up the bulk of the concrete mixture. Sand , natural gravel and Crushed Stone are mainly used for this purpose. However, it is increasingly common for recycled aggregates (from construction, demolition and excavation waste) to be used as partial replacements of natural aggregates, whilst 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. 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.2 The most common types of admixtures are:
Mineral admixtures and blended cements There are inorganic materials that also have Pozzolan ic or latent hydraulic properties. These very Fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cements).3
Fibers Short fibers of steel, glass, synthetic or natural materials can be incorporated in the concrete during mixing. See Fiber Reinforced Concrete . 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 and any remaining batch water, and final mixing is completed in conventional concrete mixing equipment. Measuring, Mixing, Transporting, and Placing Concrete 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 Mixer . This paste can be used itself or foamed (expanded) for lightweight concrete. - Method for producing construction mixture for concrete 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 concrete roof and siding tiles, paving stones and lightweight concrete block production. CHARACTERISTICS During hydration and hardening, concrete needs to develop certain physical and Chemical Properties . Among other qualities, Mechanical Strength , low moisture permeability, and chemical and volumetric stability are necessary. Workability ''Workability'' (or ''consistence'', as it is known in Europe) 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 "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 Agent s (super-plasticizers) without changing the Water/cement Ratio . It is bad practice to add extra water at the concrete mixer. 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. Curing Because the cement requires time to fully hydrate before it acquires strength and hardness, concrete must be ''cured'' once it has been placed and achieved initial setting. Curing is the process of keeping concrete under a specific environmental condition until hydration is relatively complete. Good curing is typically considered to provide a moist environment and control temperature. A moist environment promotes hydration, since increased hydration lowers permeability and increases strength resulting in a higher quality material. Allowing the concrete surface to dry out excessively can result in tensile stresses, which the still-hydrating interior cannot withstand, causing the concrete to crack. Also, the amount of heat generated by the Exothermic chemical process of hydration can be problematic for very large placements. Allowing the concrete to freeze in cold climates before the curing is complete will interrupt the hydration process, reducing the concrete strength and leading to scaling and other damage or failure. The effects of curing are primarily a function of geometry (the relation between exposed surface area and volume), the Permeability of the concrete, curing time, and curing history. Improper curing can lead to several serviceability problems including cracking, increased scaling, and reduced abrasion resistance. Strength Concrete has relatively high Compressive Strength , but significantly lower Tensile Strength (about 10% of the compressive strength). As a result, concrete always fails from tensile stresses — Even When loaded in compression. The practical implication of this is that concrete elements subjected to tensile stresses must be reinforced. Concrete is most often constructed with the addition of steel or Fiber reinforcement. The reinforcement can be by bars ( Rebar ), mesh, or fibres, producing Reinforced Concrete . 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. The ultimate strength of concrete is influenced by the Water-cement Ratio ''(w/c)'' materials 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 a higher ratio. The total quantity of cementitious materials (Portland cement, slag cement, pozzolans) can affect strength, water demand, shrinkage, abrasion resistance and density. As concrete is a liquid which hydrates to a solid, plastic shrinkage cracks can occur soon after placement; but if the evaporation rate is high, they often can occur during finishing operations (for example in hot weather or a breezy day). Aggregate interlock and steel reinforcement in structural members often negates the effects of plastic shrinkage cracks, rendering them aesthetic in nature. Properly tooled control joints or saw cuts in slabs provide a plane of weakness so that cracks occur unseen inside the joint, making a nice aesthetic presentation. In very high strength concrete mixtures (greater than 10,000 psi), the strength of the aggregate can be a Limiting Factor to the ultimate compressive strength. In lean concretes (with a high water-cement ratio) the use of coarse aggregate with a round shape may reduce aggregate interlock. Experimentation with various mix designs begins by specifying desired "workability" as defined by a given slump, durability requirements, and the required 28 day compressive strength. The characteristics of the cementitious content, coarse and fine aggregates, and chemical admixtures determine the water demand of the mix in order to achieve the desired workability. The 28 day compressive strength is obtained by determination of the correct amount of cementitious to achieve the required water-cement ratio. Only with very high strength concrete does the strength and shape of the coarse aggregate become critical in determining ultimate compressive strength. The internal forces in certain shapes of structure, such as Arch es and Vaults , are predominantly compressive forces, and therefore concrete is the preferred construction material for such structures. Wired.com reported on April 13th, 2007, that a team from the 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). 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 linear at low stress levels but becomes increasing Non-linear 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. 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 any further after 30 years). The relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface. Cracking Concrete cracks due to tensile stress induced by shrinkage or by applied loading. Engineers are familiar with the tendency of concrete to crack, and where appropriate, special design precautions are taken to ensure crack control. This entails the incorporation of secondary reinforcing, for example deformed steel bars, placed at the desired spacing to limit the crack width to an acceptable level. Water retaining structures and concrete highways are examples of structures where crack control is exercised. The objective is to encourage a large number of very small cracks, rather than a small number of large, randomly-occurring cracks. 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 very simple, using a large volume of concrete, and Maillart noticed that large areas of the structure were very cracked. He then realised that if the concrete was very cracked, it must not be contributing to the strength of the structure - but yet the structure clearly worked. Therefore, his later designs simply removed the cracked areas, leading to slender, beautiful concrete arches. The Salginatobel Bridge is an example of this. Cracking is also a primary indicator of structural distress in reinforced concrete elements. For example, a properly designed reinforced concrete beam failing as a result of overloading will exhibit a pronounced increase in the number and width of cracks. This can allow remediation, repair, or if necessary, evacuation of an unsafe area. 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. Concrete is placed while in a wet (or plastic) state, and therefore can be manipulated and moulded as needed. 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. Plastic-shrinkage cracks are immediately apparent, visible within 0 to 2 days of placement, while drying-shrinkage cracks develop over time. Precautions such as mixture selection and joint spacing can be taken to encourage cracks to occur within an aesthetic joint instead of randomly. Tension cracking Concrete members may be put into tension by applied loads. This is most common in concrete Beam s, 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 in so doing provides a warning mechanism. 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 forces is prone to 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. Physical Properties The coefficient of thermal expansion of Portland cement concrete is 0.000008 to 0.000012 (per degree Celsius).http://www.fhwa.dot.gov/pavement/pccp/thermal.cfm The density varies, but is around 150 pounds per cubic foot (2400 kg/m³).http://hypertextbook.com/facts/1999/KatrinaJones.shtml DAMAGE MODES Fire Due to its low Thermal Conductivity , a layer of concrete is frequently used for Fireproofing of steel structures. However, concrete itself may be damaged by fire. Up to about 300 °C, the concrete undergoes normal Thermal Expansion . Above that temperature, shrinkage occurs due to water loss; however, the aggregate continues expanding, which causes internal stresses. Up to about 500 °C, the major structural changes are carbonation and coarsening of pores. At 573 °C, Quartz undergoes rapid expansion due to Phase Transition , and at 900 °C Calcite starts shrinking due to decomposition. At 450-550 °C the cement hydrate decomposes, yielding calcium oxide. Calcium Carbonate decomposes at about 600 °C. Rehydration of the calcium oxide on cooling of the structure causes expansion, which can cause damage to material which withstood fire without falling apart. Concrete in buildings that experienced a fire and were left standing for several years shows extensive degree of carbonation. Concrete exposed to up to 100 °C is normally considered as healthy. The parts of a concrete structure that is exposed to temperatures above approximately 300 °C (dependent of water/cement ratio) will most likely get a pink color. Over approximately 600 °C the concrete will turn light grey, and over approximately 1000 °C it turns yellow-brownNorwegian Building Research Institute, publication 24. Fire-damage to buildings. One rule of thumb is to consider all pink colored concrete as damaged, and to be removed. Fire will expose the concrete to gasses and liquids that can be harmful to the concrete, among other salts and acids that occur when fire-gasses get in contact with water. Aggregate expansion Various types of aggregate undergo chemical reactions in concrete, leading to damaging expansive phenomena. The most common are those containing reactive silica, that can react (in the presence of water) with the alkalis in concrete (K2O and Na2O, coming principally from cement). Among the more reactive mineral components of some aggregates are Opal , Chalcedony , Flint and strained Quartz . Following the reaction ( Alkali Silica Reaction or ASR), an expansive gel forms, that creates extensive cracks and damage on structural members. On the surface of concrete pavements the ASR can cause pop-outs, i.e. the expulsion of small cones (up to 3 cm about in diameter) in correspondence of aggregate particles. When some aggregates containing Dolomite are used, a dedolomitization reaction occurs where the Magnesium Carbonate compound reacts with hydroxyl ions and yields Magnesium Hydroxide and a Carbonate Ion . The resulting expansion may cause destruction of the material. Far less common are pop-outs caused by the presence of Pyrite , an iron sulfide that generates expansion by forming iron oxide and Ettringite . Other reactions and recrystallizations, e.g. hydration of Clay Minerals in some aggregates, may lead to destructive expansion as well. Sea water effects Concrete exposed to Sea Water is susceptible to its corrosive effects. The effects are more pronounced above the tidal zone than where the concrete is permanently submerged. In the submerged zone, magnesium and Hydrogen Carbonate ions precipitate about 30 micrometers thick layer of Brucite on which a slower deposition of calcium carbonate as Aragonite occurs. These layers somewhat protect the concrete from other processes, which include attack by magnesium, chloride and sulfate ions and carbonation. Above the water surface, mechanical damage may occur by Erosion by waves themselves or sand and gravel they carry, and by crystallization of salts from water soaking into the concrete pores and then drying up. Pozzolanic cements and cements using more than 60% of slag as aggregate are more resistant to sea water than pure Portland cement. Bacterial corrosion Bacteria themselves do not have noticeable effect on concrete. However, Anaerobic Bacteria in untreated sewage tend to produce Hydrogen Sulfide , which is then oxidized by Aerobic Bacteria present in Biofilm on the concrete surface above the water level to Sulfuric Acid which dissolves the carbonates in the cured cement and causes strength loss. Concrete floors lying on ground containing Pyrite are also at risk. Using Limestone as the aggregate makes the concrete more resistant to acids, and the sewage may be pretreated by ways increasing pH or oxidizing or precipitating the sulphides in order to inhibit the activity of sulphide utilizing bacteria. Chemical attacks Carbonation Chlorides |
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