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More About Concrete Services from Wikipedia


is a composite material composed of fine and coarse construction aggregate bonded together with a fluid cement (cement paste) that hardens over time—most frequently a Lime (material)-based cement binder, such as Portland cement, but sometimes with other hydraulic cements, such as a calcium aluminate cements. It is distinguished from other, non-cementitious types of concrete all binding some form of aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder.


When aggregate is mixed together with dry Portland cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix that binds the materials together into a durable stone-like material that has many uses.Zongjin Li; ''Advanced concrete technology''; 2011 Often, additives (such as pozzolans or superplasticizers) are included in the mixture to improve the physical properties of the wet mix or the finished material. Most concrete is poured with reinforcing materials (such as rebar) embedded to provide tensile strength, yielding reinforced concrete.


Famous concrete structures include the Hoover Dam, the Panama Canal and the Roman Pantheon, Rome. The earliest large-scale users of concrete technology were the ancient Romans, and concrete was widely used in the Roman Empire. The Colosseum in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome. Today, large concrete structures (for example, dams and multi-storey car parks) are usually made with reinforced concrete.


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.


Etymology


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


History


Prehistory

Small-scale production of concrete-like materials dates to 6500 BC, pioneered by the Nabataea traders or Bedouins, who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan. 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-wall houses, concrete floors, and underground waterproof cisterns. They kept the cisterns secret as these enabled the Nabataea to thrive in the desert. Some of these structures survive to this day.From The History of Concrete - InterNACHI http://www.nachi.org/history-of-concrete.htm#ixzz31V47Zuuj Amelia Carolina Sparavigna, [https://arxiv.org/ftp/arxiv/papers/1110/1110.5230.pdf "Ancient concrete works"] Lime mortars were used in Greece, Crete, and Cyprus in 800 BC. The Assyrian Jerwan Aqueduct (688 BC) made use of waterproof concrete.Jacobsen T and Lloyd S, (1935) "Sennacherib's Aqueduct at Jerwan," ''Oriental Institute Publications'' 24, Chicago University Press Concrete was used for construction in many ancient structures. During the Roman Empire, Roman concrete (or ''opus caementicium'') was made from quicklime, pozzolana and an aggregate of pumice. Its widespread use in many Architecture of ancient Rome, a key event in the history of architecture termed the Roman Architectural Revolution, freed Roman engineering from the restrictions of stone and brick materials. It enabled revolutionary new designs in terms of both structural complexity and dimension.


Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, Vault (architecture) and List of Roman domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick.D.S. Robertson: ''Greek and Roman Architecture'', Cambridge, 1969, p. 233

Modern tests show that ''opus caementicium'' had as much compressive strength as modern Portland-cement concrete (ca. ).Henry Cowan: ''The Masterbuilders,'' New York 1977, p. 56,


Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.Robert Mark, Paul Hutchinson: "On the Structure of the Roman Pantheon", ''Art Bulletin'', Vol. 68, No. 1 (1986), p. 26, fn. 5

The long-term durability of Roman concrete structures has been found to be due to its use of Pyroclastic rock (volcanic) rock and ash, whereby crystallization of strätlingite and the coalescence of calcium–aluminum-silicate–hydrate cementing binder helped give the concrete a greater degree of fracture resistance even in seismically active environments.. allacademic.com


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.


A method for producing Portland cement was developed in England and patented by Joseph Aspdin in 1824.


Reinforced concrete was invented in 1849 by Joseph Monier.[http://inventors.about.com/library/inventors/blconcrete.htm The History of Concrete and Cement]. Inventors.about.com (2012-04-09). Retrieved on 2013-02-19. and the first house was built by François Coignet in 1853.
The first concrete reinforced bridge was designed and built by Joseph Monier in 1875 « Château de Chazelet » [archive], notice no PA00097319, base Mérimée, ministère français de la Culture..


Composition


Many types of concrete are available, distinguished by the proportions of the main ingredients below. In this way or by substitution for the cementitious and aggregate phases, the finished product can be tailored to its application. Strength, density, as well as chemical and thermal resistance are variables.


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


#Cement, most commonly Portland cement, is associated with the general term "concrete." A range of other materials can be used as the cement in concrete too. One of the most familiar of these alternative cements is asphalt concrete. Other cementitious materials, such as fly ash and slag cement, are sometimes added as mineral admixtures (see below) – either pre-blended with the cement or directly as a concrete component – and become a part of the binder for the aggregate.


To produce concrete from most cements (excluding asphalt), water is mixed with the dry powder and aggregate, which produces a semi-liquid slurry that can be shaped, typically by pouring it into a form. The concrete solidifies and hardens through a Chemical reaction called mineral hydration. The water reacts with the cement, which bonds the other components together, creating a robust stone-like material.


#Chemical admixtures are added to achieve varied properties. These ingredients may accelerate or slow down the rate at which the concrete hardens, and impart many other useful properties including increased tensile strength, entrainment of air and water resistance.


#Reinforcement is often included in concrete. Concrete can be formulated with high compressive strength, but always has lower tensile strength. For this reason it is usually reinforced with materials that are strong in tension, typically steel rebar.


#Mineral admixtures and blended cements have become more popular over recent decades. The use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation, and the discovery that such materials often have complementary and valuable properties. The most conspicuous of these are fly ash, a by-product of Fossil fuel power plant; ground granulated blast furnace slag, a byproduct of steelmaking; and silica fume, a byproduct of industrial electric arc furnaces. The use of these materials in concrete reduces the amount of resources required, as the mineral admixtures act as a partial cement replacement. This displaces some cement production, an energetically expensive and environmentally problematic process, while reducing the amount of industrial waste that must be disposed of. Mineral admixtures can be pre-blended with the cement during its production for sale and use as a blended cement, or mixed directly with other components when the concrete is produced.


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


Cement


Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar (masonry) and many plasters. British masonry worker Joseph Aspdin patented Portland cement in 1824. It was named because of the similarity of its colour to Portland stone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of calcium silicates (alite, belite), tricalcium aluminate and calcium aluminoferrite – compounds which combine calcium, silicon, aluminium and iron in forms which will react with water. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay or shale (a source of silicon, aluminium and iron) and grinding this product (called ''clinker (cement)'') with a source of sulfate (most commonly gypsum).


In modern cement kilns many advanced features are used to lower the fuel consumption per ton of clinker produced. Cement kilns are extremely large, complex, and inherently dusty industrial installations, and have emissions which must be controlled. Of the various ingredients used to produce a given quantity of concrete, the cement is the most energetically expensive. Even complex and efficient kilns require 3.3 to 3.6 gigajoules of energy to produce a ton of clinker and then Cement mill. Many kilns can be fueled with difficult-to-dispose-of wastes, the most common being used tires. The extremely high temperatures and long periods of time at those temperatures allows cement kilns to efficiently and completely burn even difficult-to-use fuels.


Reaction:
:Cement chemist notation: C3S + H → C-S-H + CH
:Standard notation: Ca3SiO5 + H2O → (CaO)·(SiO2)·(H2O)(gel) + Ca(OH)2
:Balanced: 2Ca3SiO5 + 7H2O → 3(CaO)·2(SiO2)·4(H2O)(gel) + 3Ca(OH)2 (approximately; the exact ratios of the CaO, SiO2 and H2O in C-S-H can vary)


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 biggest gaps whereas adding aggregate with smaller particles tends to fill these gaps. The binder must fill the gaps between the aggregate as well as pasting the surfaces of the aggregate together, and is typically the most expensive component. Thus variation in sizes of the aggregate reduces the cost of concrete.[http://www.engr.psu.edu/ce/courses/ce584/concrete/library/materials/Aggregate/Aggregatesmain.htm The Effect of Aggregate Properties on Concrete] . Engr.psu.edu. Retrieved on 2013-02-19. The aggregate is nearly always stronger than the binder, so its use does not negatively affect the strength of the concrete.


Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients.


Decorative stones such as quartzite, small river stones or crushed glass are sometimes added to the surface of concrete for a decorative "exposed aggregate" finish, popular among landscape designers.


In addition to being decorative, exposed aggregate may add robustness to a concrete.[http://www.uniquepaving.com.au/exposed_aggregate.htm Exposed Aggregate]


Concrete is strong in compression (physical), as the aggregate efficiently carries the compression load. However, it is weak in Tension (physics) as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete adds either rebar, Fiber-reinforced concrete, glass fibers, or plastic fibers to carry tension (physics).


Chemical admixtures

''Chemistry 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. (See the section on Concrete Production, below.)The common types of admixtures are as follows:



  • Accelerant 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. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.

  • Retarder (chemistry) slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable. Typical polyol retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.

  • Air entrainment add and entrain tiny air bubbles in the concrete, which reduces damage during Weathering cycles, increasing durability. However, entrained air entails a trade off with strength, as each 1% of air may decrease compressive strength by 5%. 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.

  • Plasticizers increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called ''water-reducers'' due to this use. Such treatment improves its strength and durability characteristics. 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. Compounds used as superplasticizers include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers.

  • 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 (typically a type of polymer) with wide temperature tolerance and corrosion resistance.

  • Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.


Mineral admixtures and blended cements

Inorganic materials that have pozzolanic or latent hydraulic properties, these very Granularity materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cements). Products which incorporate limestone, fly ash, blast furnace slag, and other useful materials with pozzolanic properties into the mix, are being tested and used. This development is due to cement production being one of the largest producers (at about 5 to 10%) of global greenhouse gas emissions,[http://web.mit.edu/newsoffice/2011/concrete-pavements-0829.html Paving the way to greenhouse gas reductions] . Web.mit.edu (2011-08-28). Retrieved on 2013-02-19. as well as lowering costs, improving concrete properties, and recycling wastes.



  • Fly ash: A by-product of coal-fired power station, 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.

  • 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.

  • 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.

  • Carbon nanofibres can be added to concrete to enhance compressive strength and gain a higher Young’s modulus, and also to improve the 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.

  • Carbon products have been added to make concrete electrically conductive, for deicing purposes.. Concreteinternational.com (1989-11-01). Retrieved on 2013-02-19. The paste is generally mixed in a ''high-speed'', shear-type mixer at a Water-cement ratio (water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders, Plasticizer, 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.


    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 reaction) and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability. Excessive water leads to increased bleeding or Segregation in concrete (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate blend with an undesirable gradation


    Workability can be measured by the concrete slump test, a simple measure of the plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Duff Abrams" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod to consolidate the layer. When the cone is carefully lifted off, the enclosed material slumps a certain amount, owing to gravity. A relatively dry sample slumps very little, having a slump value of one or two inches (25 or 50 mm) out of one foot (305 mm). A relatively wet concrete sample may slump as much as eight inches. Workability can also be measured by the flow table test.


    Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the water-cement ratio. Some other admixtures, especially air-entraining admixture, can increase the slump of a mix.


    High-flow concrete, like self-consolidating concrete, is tested by other flow-measuring methods. One of these methods includes placing the cone on the narrow end and observing how the mix flows through the cone while it is gradually lifted.


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


    Curing


    Concrete must be kept moist during curing in order to achieve optimal strength and durability."Curing Concrete" Peter C. Taylor CRC Press 2013. During curing hydrate occurs, allowing calcium-silicate hydrate (C-S-H) to form. Over 90% of a mix's final strength is typically reached within four weeks, with the remaining 10% achieved over years or even decades. The conversion of calcium hydroxide in the concrete into calcium carbonate from absorption of carbon dioxide over several decades further strengthens the concrete and makes it more resistant to damage. This carbonation reaction, however, lowers the pH of the cement pore solution and can corrode the reinforcement bars.


    Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained sufficient strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased if it is kept damp during the curing process. Minimizing stress prior to curing minimizes cracking. High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. The strength of concrete changes (increases) for up to three years. It depends on cross-section dimension of elements and conditions of structure exploitation.Resulting strength distribution in vertical elements researched and presented at the article [http://www.concreteresearch.org/PDFsandsoon/Inhomog%20Denver.pdf "Concrete inhomogeneity of vertical cast-in-place elements in skeleton-type buildings".] Addition of short-cut polymer fibers can improve (reduce) shrinkage-induced stresses during curing and increase early and ultimate compression strength.[http://www.minifibers.com/documents/ADMIXUS-Admixtures-for-Cementitious-Applications.pdf "Admixtures for Cementitious Applications."]


    Properly curing concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing or overheating due to the exothermic setting of cement. Improper curing can cause Spalling#Spalling in mechanical weathering, reduced strength, poor abrasion (mechanical) resistance and fracture.


    =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.
    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 totalling 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.


    Nanoconcrete


    Nanoconcrete is created by high-energy mixing (HEM) of cement, sand and water. To ensure the mixing is thorough enough to create nano-concrete, the mixer must apply a total mixing power to the mixture of 30–600 watts per kilogram of the mix. This mixing must continue long enough to yield a net Energy density expended upon the mix of at least 5000 joules per kilogram of the mix. A plasticizer or a superplasticizer is then added to the activated mixture which can later be mixed with aggregates in a conventional concrete mixer. In the HEM process, the intense mixing of cement and water with sand provides dissipation of energy and increases shear stresses on the surface of cement particles. This intense mixing serves to divide the cement particles into extremely fine nanometer scale sizes, which provides for extremely thorough mixing. This results in the increased volume of water interacting with cement and acceleration of Calcium Silicate Hydrate (C-S-H) colloid creation.


    The initial natural process of cement hydration with formation of colloidal globules about 5 nm in diameter spreads into the entire volume of cement – water matrix as the energy expended upon the mix approaches and exceeds 5000 joules per kilogram.


    The liquid activated high-energy mixture can be used by itself for casting small architectural details and decorative items, or foamed (Expanded polystyrene concrete) for lightweight concrete. HEM Nanoconcrete hardens in low and subzero temperature conditions and possesses an increased volume of gel, which reduces capillarity in solid and porous materials.


    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.
    Polymer concretes are mixtures of aggregate and any of various polymers and may be reinforced. The cement is more costly 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. They are frequently used for repair and construction of other applications such as drains.


    Safety


    Grinding of concrete can produce hazardous dust. Exposure to cement dust General Chipping. [https://generalchipping.com/protect-team-around-cement-dust/ “How Do I Protect My Team and Those Around Them from Cement Dust?”], General Chipping. Retrieved on November 5, 2018. can lead to issues such as silicosis, kidney disease, skin irritation and similar effects. The 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, OSHA Fact Sheet. [https://www.osha.gov/Publications/OSHA3682.pdf “OSHA’s Respirable Crystalline Silica Standard for General Industry and Maritime”], Occupational Safety and Health Administration. Retrieved on November 5, 2018. which OSHA put into effect Sept. 23, 2017 for construction companies, restricted the amount of respirable crystalline silica workers could legally come into contact with to 50 micrograms per cubic meter of air per 8-hour workday. That same rule went into effect June 23, 2018 for general industry, hydraulic fracturing and maritime. It should be noted, however, that the deadline was extended to June 23, 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.


    Cement dust isn’t the only concern crews face when working with concrete. Improper ergonomics can lead to muscle pains and strains. General Chipping. [https://generalchipping.com/safety-forefront-ergonomics-concrete-work/ “Safety at the Forefront: Ergonomics in Concrete Work”], General Chipping. Retrieved on November 5, 2018. As such, it is important for crews to practice proper stretching General Chipping. [https://generalchipping.com/why-and-how-you-should-stretch-before-strenuous-physical-work/ “Why (and How) You Should Stretch Before Strenuous Physical Work”], General Chipping. Retrieved on November 5, 2018. before embarking on a busy workday. Working in close quarters can also place team members in danger. General Chipping. [https://generalchipping.com/confined-spaces-understanding-hazards-concrete-chipping/ “Confined Spaces: Understanding the Hazards of Concrete Chipping”], General Chipping. Retrieved on November 5, 2018. Not only can crowded workspaces make it difficult for crews to safely exit, but certain work environments — such as jobs carried out inside cement mixing drums — make ventilation an obstacle. Furthermore, crews who carry out much of their work outdoors face weather-related concerns. It is important for crews to take special precautions when faced with extremely hot conditions, General Chipping. [https://generalchipping.com/can-protect-team-hot-summer-months/ “How Can I Protect My Team During Hot Summer Months?”], General Chipping. Retrieved on November 5, 2018. as well as cold conditions. General Chipping. [https://generalchipping.com/can-encourage-cold-weather-safety-crew/ “How Can I Encourage Cold Weather Safety for My Crew?”], General Chipping. Retrieved on November 5, 2018. Such is the case for all outdoor crews, whether they work with concrete or not.


    Properties


    For this reason 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 (deformation).


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


    Different mixes of concrete ingredients produce different strengths. Concrete strength values are usually specified as the lower-bound compressive strength of either a cylindrical or cubic specimen as determined by standard test procedures.


    Different strengths of concrete are used for different purposes. Very low-strength –


    In construction



    Concrete is one of the most durable building materials. It provides superior fire resistance compared with wooden construction and gains strength over time. Structures made of concrete can have a long service life. Concrete is used more than any other artificial material in the world.


    Due to cement's exothermic chemical reaction while setting up, large concrete structures such as dams, navigation locks, large mat foundations, and large breakwater (structure) generate excessive heat during hydration and associated expansion. To mitigate these effects ''post-cooling''[http://www.ce.berkeley.edu/~paulmont/165/Mass_concrete2.pdf Mass Concrete] . (PDF) . Retrieved on 2013-02-19. is commonly applied during construction. An early example at Hoover Dam 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 ambient air temperature, the cooling process may last for many months after the concrete is placed. Various methods also are used to pre-cool the concrete mix in mass concrete structures.


    Another approach to mass concrete structures that minimizes cement's thermal byproduct is the use of roller-compacted concrete, which uses a dry mix which has a much lower cooling requirement than conventional wet placement. It is deposited in thick layers as a semi-dry material then roller compactor into a dense, strong mass.


    Surface finishes

    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. See Brutalism.


    Cold weather placement

    Extreme weather conditions (extreme heat or cold; windy condition, and humidity variations) can significantly alter the quality of concrete. Many precautions are observed in cold weather placement. 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 placmement, ACI 306, is:



    • A period when for more than three successive days the average daily air temperature drops below 40 ˚F (~ 4.5 °C), and

    • Temperature stays below 50 ˚F (10 °C) for more than 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 Canadian Standards Agency 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 (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.


    Roads

    Road surface#Concrete are more fuel efficient to drive on, 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 need to place storm drains near the road, and reducing the need for slightly 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.


    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#CO2 emissions).


    Once in place, concrete offers great energy efficiency over the lifetime of a building.John Gajda (2001) [https://web.archive.org/web/20110916080530/http://www.arxx.com/Assets/CMS/WYSIWYGBase/file/5825%20PCA%20CD026.pdf Energy Use of Single Family Houses with Various Exterior Walls], Construction Technology Laboratories Inc. Concrete walls leak air far less than those made of wood frames. While insulation reduces energy loss through the building envelope, thermal mass uses walls to store and release energy. Modern concrete wall systems use both external insulation and thermal mass to create an energy-efficient building. Insulating concrete forms (ICFs) are hollow blocks or panels made of either insulating foam or rastra that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.


    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.


    Options for non-combustible construction include floors, ceilings and roofs made of cast-in-place and hollow-core precast concrete. For walls, concrete masonry technology and Insulating concrete forms (ICFs) are additional options. ICFs are hollow blocks or panels made of fireproof insulating foam that are stacked to form the shape of the walls of a building and then filled with reinforced concrete to create the structure.


    Concrete also provides good resistance against externally applied forces such as high winds, hurricanes, and tornadoes owing to its lateral stiffness, which results in minimal horizontal movement. However this stiffness can work against certain types of concrete structures, particularly where a relatively higher flexing structure is required to resist more extreme forces.


    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.[http://www.fema.gov/library/viewRecord.do?id=4067 Unreinforced Masonry Buildings and Earthquakes: Developing Successful Risk Reduction Programs] , C.C. Simsir, A. Jain, G.C. Hart, and M.P. Levy, The 14th World Conference on Earthquake Engineering, 12–17 October 2008, Beijing, China).


    Degradation



    Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel rebar, freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distillate water).


    Concrete is widely used for making architectural structures, foundation (engineering), brick/Concrete masonry unit walls, Sidewalk, bridges/overpasses, highways, runways, parking structures, dams, pools/reservoirs, pipes, foundation (engineering) for gates, fences and Utility pole and even boats. Concrete is used in large quantities almost everywhere there is a need for infrastructure. Concrete is one of the most frequently used building materials in animal houses and for manure and silage storage structures in agriculture.
    The cement industry is one of the three primary producers of carbon dioxide, a major greenhouse gas (the other two being the energy production and transportation industries). As of 2001, the production of Portland cement contributed 7% to global anthropogenic CO2 emissions, largely due to the sintering of limestone and clay at
    This includes crew members who work in concrete chipping. Companies are charged with protecting their team members from the dangers of cement dust, General Chipping. [https://generalchipping.com/protect-team-around-cement-dust/ “How Do I Protect My Team and Those Around Them from Cement Dust?”], General Chipping. Retrieved on November 5, 2018. as exposure can result in a range of health issues The Center for Construction Research and Training. [https://www.silica-safe.org/ “Work Safely with Silica”], The Center for Construction Research and Training. Retrieved on November 5, 2018. including lung and kidney disease, chemical burns and skin irritation. In fact, in recent months, the Occupational Safety and Health Administration (OSHA) has placed more stringent regulations on companies that work around silica dust. The new silica rule Occupational Safety and Health Administration. [https://www.osha.gov/Publications/OSHA3682.pdf “OSHA’s Respirable Crystalline Silica Standard for General Industry and Maritime”], Occupational Safety and Health Administration. Retrieved on November 5, 2018. limits the amount of respirable crystalline silica workers can come in contact with to 50 micrograms per cubic meter of air per 8-hour workday. Companies found to be exposing workers to higher amounts could face financial payouts and other 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.


    Recycling


    Concrete recycling is an increasingly common method for disposing of concrete structures. Concrete debris was once routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws and economic benefits. General Chipping. [https://generalchipping.com/concrete-recycling-work/ “How Does Concrete Recycling Work?”], General Chipping. Retrieved on November 5, 2018.


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


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


    It is important to note that, as with concrete chipping, polishing and similar work, the process of recycling concrete can expose workers to dangerous silica dust. A recent silica rule update Occupational Safety and Health Administration. [https://www.osha.gov/Publications/OSHA3682.pdf “OSHA’s Respirable Crystalline Silica Standard for General Industry and Maritime”], Occupational Safety and Health Administration. Retrieved on November 5, 2018. set forth by the Occupational Safety and Health Administration (OSHA) limits the legal amount of respirable crystalline silica workers can come in contact with to 50 micrograms per cubic meter of air per 8-hour workday.


    Chipping

    Concrete chipping is the act of breaking away the dried concrete that forms along the walls of central mixers and ready-mix concrete trucks. Whenever possible, teams carry out the work by physically entering the spaces and breaking away the dried material with help from handheld jackhammers and chisels. Once removed, the broken concrete is either hauled away to landfills, recycled or reused in some other way.


    While specific concrete chipping needs vary by fleet size, concrete blends at play and the specific drums in use, it is generally accepted that a company should have its drums chipped every three months. General Chipping. [https://generalchipping.com/often-plan-concrete-removal-drums-central-mixers/ “How Often Should I Plan Concrete Removal for My Drums and Central Mixers?”], General Chipping. Retrieved on November 5, 2018. Such maintenance helps companies avoid slowdowns in production, lowered drum capacity and breakdowns which can affect their bottom line.


    As with all concrete work, concrete chipping carries its share of on-the-job dangers. Silica dust exposure, General Chipping. [https://generalchipping.com/protect-team-around-cement-dust/ “How Do I Protect My Team and Those Around Them from Cement Dust?”], General Chipping. Retrieved on November 5, 2018. cramped quarters, General Chipping. [https://generalchipping.com/confined-spaces-understanding-hazards-concrete-chipping/ “Confined Spaces: Understanding the Hazards of Concrete Chipping”], General Chipping. Retrieved on November 5, 2018. flying debris and other hazards are all concerns team members can face at work. Regular training, as well as adherence to regulations set forth by the Occupational Safety and Health Administration (OSHA) help keep team members safe.


    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 Dam in Brazil.


    The world record for concrete pumping was set on 7 August 2009 during the construction of the Parbati River (Himachal Pradesh) Hydroelectric Project, near the village of Suind, Himachal Pradesh, India, when the concrete mix was pumped through a vertical height of


    The world record for the largest continuously poured concrete raft was achieved in August 2007 in Abu Dhabi by contracting firm Al Habtoor-CCC Joint Venture and the concrete supplier is Unibeton Ready Mix. The pour (a part of the foundation for the Abu Dhabi's The Landmark (Abu Dhabi)) was 16,000 cubic meters of concrete poured within a two-day period.[http://www.leighton.com.au/verve/_resources/AlHabtoorIssue24.pdf Al Habtoor Engineering] – ''Abu Dhabi – Landmark Tower has a record-breaking pour'' – September/October 2007, Page 7. The previous record, 13,200 cubic meters poured in 54 hours despite a severe tropical storm requiring the site to be covered with tarpaulins to allow work to continue, was achieved in 1992 by joint Japanese and South Korean consortiums Hazama Corporation and the Samsung C&T Corporation for the construction of the Petronas Towers in Kuala Lumpur, Malaysia.National Geographic Channel International / Caroline Anstey (2005), Megastructures: Petronas Twin Towers


    The world record for largest continuously poured concrete floor was completed 8 November 1997, in Louisville, Kentucky, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement consisted of
    * Anthropic rock
    * Biorock
    * Brutalist architecture
    * Bunding
    * Cement accelerator
    * Concrete canoe
    * Concrete chipping
    * Concrete leveling
    * Concrete mixer
    * Concrete masonry unit
    * Concrete moisture meter
    * Concrete plant
    * Concrete recycling
    * Concrete step barrier
    * Concrete sealers
    * Construction
    * Diamond grinding of pavement
    * Efflorescence
    * Fireproofing
    * Foam Index
    * Form liner
    * High performance fiber reinforced cementitious composites
    * Metakaolin
    * International Grooving & Grinding Association
    * Lift slab construction
    * LiTraCon
    * Mortar (masonry)
    * Plasticizer
    * Prefabrication
    * Pykrete
    * Rammed earth
    * Rusticated concrete block
    * Shallow foundation
    * Silica fume
    * Translucent concrete
    * Whitetopping
    * World of Concrete


References


Learn more about Concrete Services:

https://en.wikipedia.org/wiki/Concrete