Concreto

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Exterior do Panteão Romano , concluído em 128 DC, a maior cúpula de concreto não armado do mundo. [1]
Interior da cúpula do Panteão, visto de baixo. O concreto da cúpula em caixotão foi colocado em moldes, montados em andaimes provisórios.
Opus caementicium exposta em um arco romano característico. Em contraste com as estruturas de concreto modernas, o concreto usado nos edifícios romanos era geralmente coberto com tijolo ou pedra.

O concreto é um material compósito composto por agregados finos e grossos unidos por um cimento fluido (pasta de cimento) que endurece (cura) com o tempo. No passado, ligantes de cimento à base de cal , como massa de cal, eram frequentemente usados, mas às vezes com outros cimentos hidráulicos (resistentes à água), como cimento de aluminato de cálcio ou com cimento Portland para formar concreto de cimento Portland (nomeado por sua semelhança visual com Pedra de Portland ). [2] [3] Muitos outros tipos de concreto sem cimento existem com outros métodos de ligação de agregados, incluindoconcreto asfáltico com um ligante de betume , que é freqüentemente usado para superfícies de estradas , e concretos de polímero que usam polímeros como um ligante. O concreto é diferente da argamassa . Enquanto o concreto é um material de construção, a argamassa é um agente de ligação que normalmente mantém tijolos , telhas e outras unidades de alvenaria juntas. [4]

Quando o agregado é misturado com cimento Portland seco e água , a mistura forma uma pasta fluida que é facilmente derramada e moldada em forma. O cimento reage com a água e outros ingredientes para formar uma matriz dura que une os materiais em um material durável semelhante a uma pedra que tem muitos usos. [5] Freqüentemente, aditivos (como pozolanas ou superplastificantes ) são incluídos na mistura para melhorar as propriedades físicas da mistura úmida ou do material acabado. A maioria do concreto é derramado com materiais de reforço (como vergalhões ) embutidos para fornecer resistência à tração , produzindo concreto armado .

O concreto é um dos materiais de construção mais usados. Seu uso em todo o mundo, tonelada por tonelada, é o dobro do aço, madeira, plástico e alumínio combinados. [6] Globalmente, a indústria de concreto usinado, o maior segmento do mercado de concreto, está projetada para exceder $ 600 bilhões em receitas até 2025. [7] Este uso generalizado resulta em uma série de impactos ambientais . Mais notavelmente, o processo de produção de cimento produz grandes volumes de emissões de gases de efeito estufa , levando a 8% líquido das emissões globais. [8] [9] Pesquisa e desenvolvimento significativos estão sendo feitos para tentar reduzir as emissões ou tornar concreta uma fonte de sequestro de carbono. Outras preocupações ambientais incluem a mineração ilegal de areia generalizada , impactos no meio ambiente, como aumento do escoamento superficial ou efeito de ilha de calor urbana e potenciais implicações para a saúde pública de ingredientes tóxicos. O concreto também é usado para mitigar a poluição de outras indústrias, capturando resíduos como cinzas volantes de carvão ou rejeitos e resíduos de bauxita .

Etimologia

A palavra concreto vem da palavra latina " concretus " (que significa compacto ou condensado), [10] o particípio passivo perfeito de " concrescere ", de " con -" (juntos) e " crescere " (crescer).

História

Tempos antigos

O concreto maia nas ruínas de Uxmal é citado em Incidents of Travel in the Yucatán, de John L. Stephens . “O telhado é plano e foi forrado com cimento”. "Os pisos eram de cimento, em alguns lugares duros, mas, devido à longa exposição, estavam quebrados e agora desmoronando sob os pés." "Mas toda a parede era sólida e consistia em grandes pedras embutidas em argamassa, quase tão duras quanto rocha."

A produção em pequena escala de materiais semelhantes ao concreto foi iniciada pelos comerciantes nabateus que ocuparam e controlaram uma série de oásis e desenvolveram um pequeno império nas regiões do sul da Síria e norte da Jordânia a partir do século 4 aC. Eles descobriram as vantagens da cal hidráulica , com algumas propriedades autocimentantes, por volta de 700 aC. Eles construíram fornos para fornecer argamassa para a construção de casas de alvenaria de entulho , pisos de concreto e cisternas subterrâneas impermeáveis . Eles mantiveram as cisternas em segredo, pois permitiram que os nabateus prosperassem no deserto. [11] Algumas dessas estruturas sobrevivem até hoje. [11]

Era clássica

Nas eras egípcias antigas e romanas posteriores , os construtores descobriram que a adição de cinzas vulcânicas à mistura permitia que ela se fixasse na água.

Pisos de concreto foram encontrados no palácio real de Tiryns , na Grécia, que data de aproximadamente 1400–1200 aC. [12] [13] Argamassas de cal foram usadas na Grécia, Creta e Chipre em 800 AC. O aqueduto assírio Jerwan (688 aC) utilizava concreto impermeável . [14] O concreto foi usado para construção em muitas estruturas antigas. [15]

Os romanos usaram o concreto extensivamente de 300 aC a 476 dC. [16] Durante o Império Romano, o concreto romano (ou opus caementicium ) era feito de cal viva , pozolana e um agregado de pedra-pomes . Seu uso difundido em muitas estruturas romanas , um evento chave na história da arquitetura chamada de revolução arquitetônica romana , libertou a construção romana das restrições de materiais de pedra e tijolo. Permitiu novos designs revolucionários em termos de complexidade estrutural e dimensão. [17] O Coliseu in Rome was built largely of concrete, and the concrete dome of the Pantheon is the world's largest unreinforced concrete dome.[18]

Concrete, as the Romans knew it, was a new and revolutionary material. Laid in the shape of arches, vaults and domes, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains that troubled the builders of similar structures in stone or brick.[19]

Testes modernos mostram que o opus caementicium tinha tanta resistência à compressão quanto o concreto moderno de cimento Portland (ca. 200 kg / cm 2  [20 MPa; 2.800 psi]). [20] No entanto, devido à ausência de reforço, sua resistência à tração era muito menor do que o concreto armado moderno , e seu modo de aplicação também diferia: [21]

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.[22]

A durabilidade de longo prazo das estruturas de concreto romano foi encontrada devido ao uso de rocha piroclástica (vulcânica) e cinzas, por meio da qual a cristalização de estratlingita (um hidrato de aluminossilicato de cálcio complexo e específico) [23] e a coalescência deste e ligantes semelhantes de cimentação de hidrato de silicato de cálcio-alumínio ajudaram a dar ao concreto um maior grau de resistência à fratura, mesmo em ambientes sismicamente ativos. [24] O concreto romano é significativamente mais resistente à erosão pela água do mar do que o concreto moderno; ela usava materiais piroclásticos que reagem com a água do mar para formar cristais de Al- tobermorite ao longo do tempo. [25] [26]

O uso generalizado de concreto em muitas estruturas romanas garantiu que muitas sobrevivessem até os dias atuais. As Termas de Caracalla, em Roma, são apenas um exemplo. Muitos aquedutos e pontes romanas , como a magnífica Pont du Gard no sul da França, têm revestimento de alvenaria em um núcleo de concreto, assim como a cúpula do Panteão .

Após o colapso do Império Romano, o uso de concreto tornou-se raro até que a tecnologia foi redesenhada em meados do século XVIII. Em todo o mundo, o concreto superou o aço na tonelagem de material usado. [27]

Idade Média

Após o Império Romano, o uso de cal queimada e pozolana foi bastante reduzido. Baixas temperaturas do forno na queima de cal, falta de pozolana e má mistura contribuíram para um declínio na qualidade do concreto e da argamassa. A partir do século 11, o aumento do uso de pedra na construção de igrejas e castelos levou a um aumento da demanda por argamassa. A qualidade começou a melhorar no século 12 por meio de uma melhor moagem e peneiramento. As argamassas de cal e os concretos medievais não eram hidráulicos e eram usados ​​para a ligação de alvenaria, "coração" (ligação de núcleos de entulho de alvenaria ) e fundações. Bartholomaeus Anglicus em seu De proprietatibus rerum(1240) descreve a fabricação de argamassa. Em uma tradução inglesa de 1397, lê-se "lyme ... é um brent de pedra; por meio dele com sonda e sement de água é feito". A partir do século XIV, a qualidade da argamassa foi novamente excelente, mas apenas a partir do século XVII a pozolana foi comumente adicionada. [28]

O Canal du Midi foi construído com concreto em 1670. [29]

Era industrial

Torre Smeaton

Talvez o maior avanço no uso moderno de concreto tenha sido a Torre Smeaton , construída pelo engenheiro britânico John Smeaton em Devon, Inglaterra, entre 1756 e 1759. Este terceiro Farol de Eddystone foi pioneiro no uso de cal hidráulica em concreto, usando seixos e tijolos em pó como agregar. [30]

A method for producing Portland cement was developed in England and patented by Joseph Aspdin in 1824.[31] Aspdin chose the name for its similarity to Portland stone, which was quarried on the Isle of Portland in Dorset, England. His son William continued developments into the 1840s, earning him recognition for the development of "modern" Portland cement.[32]

O concreto armado foi inventado em 1849 por Joseph Monier . [33] e a primeira casa de concreto armado foi construída por François Coignet [34] em 1853. A primeira ponte de concreto armado foi projetada e construída por Joseph Monier em 1875. [35]

Composição

O concreto é um material compósito, que compreende uma matriz de agregado (normalmente um material rochoso) e um ligante (tipicamente cimento Portland ou asfalto ), que mantém a matriz unida. Muitos tipos de concreto estão disponíveis, determinados pelas formulações dos ligantes e pelos tipos de agregados usados ​​para se adequar à aplicação do material. Essas variáveis ​​determinam a resistência e a densidade, bem como a resistência química e térmica do produto acabado.

O agregado consiste em grandes pedaços de material em uma mistura de concreto, geralmente um cascalho grosso ou rochas trituradas, como calcário ou granito , junto com materiais mais finos, como areia .

A cement, most commonly Portland cement, is the most prevalent kind of concrete binder. For cementitious binders, 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 process called hydration. The water reacts with the cement, which bonds the other components together, creating a robust, stone-like material. Other cementitious materials, such as fly ash and slag cement, are sometimes added—either pre-blended with the cement or directly as a concrete component—and become a part of the binder for the aggregate.[36]As cinzas volantes e a escória podem melhorar algumas propriedades do concreto, como propriedades frescas e durabilidade. [36]

Misturas são adicionadas para modificar a taxa de cura ou as propriedades do material. Os aditivos minerais usam materiais reciclados como ingredientes de concreto. Materiais conspícuos incluem cinzas volantes , um subproduto de usinas elétricas movidas a carvão ; escória de alto forno granulada moída , um subproduto da siderurgia ; e sílica ativa , um subproduto dos fornos elétricos industriais a arco .

As estruturas que utilizam concreto de cimento Portland costumam incluir reforço de aço, pois esse tipo de concreto pode ser formulado com alta resistência à compressão , mas sempre apresenta menor resistência à tração . Portanto, geralmente é reforçado com materiais que são resistentes à tração, geralmente vergalhões de aço .

Outros materiais também podem ser usados ​​como ligante de concreto: a alternativa mais prevalente é o asfalto , que é usado como ligante no concreto asfáltico .

O projeto da mistura depende do tipo de estrutura que está sendo construída, como o concreto é misturado e entregue e como é colocado para formar a estrutura.

Cimento

Several tons of bagged cement, about two minutes of output from a 10,000 ton per day cement kiln

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar, and many plasters.[37] British masonry worker Joseph Aspdin patented Portland cement in 1824. It was named because of the similarity of its color to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of calcium silicates (alite, belite), aluminates and ferrites—Compostos que combinam cálcio, silício, alumínio e ferro em formas que reagem com a água. O cimento Portland e materiais semelhantes são feitos aquecendo o calcário (uma fonte de cálcio) com argila ou xisto (uma fonte de silício, alumínio e ferro) e triturando este produto (chamado clínquer ) com uma fonte de sulfato (mais comumente gesso ).

Nos modernos fornos de cimento , muitos recursos avançados são usados ​​para reduzir o consumo de combustível por tonelada de clínquer produzida. Os fornos de cimento são instalações industriais extremamente grandes, complexas e inerentemente empoeiradas, e têm emissões que devem ser controladas. Dos vários ingredientes usados ​​para produzir uma determinada quantidade de concreto, o cimento é o mais caro energeticamente. Mesmo fornos complexos e eficientes requerem 3,3 a 3,6 gigajoules de energia para produzir uma tonelada de clínquer e depois transformá-la em cimento . Muitos fornos podem ser abastecidos com resíduos de difícil disposição, sendo os pneus usados ​​mais comuns. As temperaturas extremamente altas e longos períodos de tempo nessas temperaturas permitem que os fornos de cimento queimem de forma eficiente e completa até mesmo combustíveis difíceis de usar. [38]

Água

A combinação da água com um material cimentício forma uma pasta de cimento pelo processo de hidratação. A pasta de cimento cola o agregado, preenche os vazios dentro dele e faz com que flua mais livremente. [39]

Conforme declarado pela lei de Abrams , uma relação água-cimento mais baixa produz um concreto mais resistente e durável , enquanto mais água resulta em um concreto de fluxo mais livre com maior abatimento . [40] Água impura usada para fazer concreto pode causar problemas durante o assentamento ou causar falha prematura da estrutura. [41] A hidratação envolve muitas reações, geralmente ocorrendo ao mesmo tempo. À medida que as reações prosseguem, os produtos do processo de hidratação do cimento unem gradualmente as partículas individuais de areia e cascalho e outros componentes do concreto para formar uma massa sólida. [42]

Reação: [42]

Notação química do cimento : C 3 S + H → CSH + CH
Notação padrão: Ca 3 SiO 5 + H 2 O → (CaO) · (SiO 2 ) · (H 2 O) (gel) + Ca (OH) 2
Equilibrado: 2Ca 3 SiO 5 + 7H 2 O → 3 (CaO) · 2 (SiO 2 ) · 4 (H 2 O) (gel) + 3Ca (OH) 2 (aproximadamente; as razões exatas de CaO, SiO 2 e H 2 O em CSH pode variar)

Agregados

Agregado de pedra triturada

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.

A distribuição do tamanho do agregado determina quanto aglutinante é necessário. O agregado com uma distribuição de tamanho muito uniforme tem as maiores lacunas, enquanto a adição de agregados com partículas menores tende a preencher essas lacunas. O aglutinante deve preencher as lacunas entre o agregado, bem como colar as superfícies do agregado, e é normalmente o componente mais caro. Assim, a variação nos tamanhos do agregado reduz o custo do concreto. [43] O agregado é quase sempre mais resistente do que o aglutinante, portanto, seu uso não afeta negativamente a resistência do concreto.

A redistribuição de agregados após a compactação geralmente cria falta de homogeneidade devido à influência da vibração. Isso pode levar a gradientes de força. [44]

Pedras decorativas, como quartzito , pequenas pedras de rio ou vidro triturado, às vezes são adicionadas à superfície do concreto para um acabamento decorativo de "agregado exposto", popular entre os paisagistas.

Reforço

Construir uma gaiola de vergalhão que será permanentemente embutida em uma estrutura acabada de concreto armado

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 adds either steel reinforcing bars, steel fibers, aramid fibers, carbon fibers, glass fibers, or plastic fibers to carry tensile loads.

Admixtures

As misturas são materiais na forma de pó ou fluidos que são adicionados ao concreto para dar a ele certas características não obtidas com as misturas de concreto simples. As misturas são definidas como adições "feitas à medida que a mistura de concreto está sendo preparada". [45] As misturas mais comuns são retardadores e aceleradores. Em uso normal, as dosagens de mistura são inferiores a 5% em massa de cimento e são adicionadas ao concreto no momento da dosagem / mistura. [46] (Ver § Produção abaixo.) Os tipos comuns de aditivos [47] são os seguintes:

  • Os aceleradores aceleram a hidratação (endurecimento) do concreto. Os materiais típicos usados ​​são cloreto de cálcio , nitrato de cálcio e nitrato de sódio . No entanto, o uso de cloretos pode causar corrosão nas armaduras de aço e é proibido em alguns países, de modo que os nitratos podem ser favorecidos, embora sejam menos eficazes do que o sal de cloreto. As misturas de aceleração são especialmente úteis para modificar as propriedades do concreto em climas frios.
  • Air entraining agents add and entrain tiny air bubbles in the concrete, which reduces damage during freeze-thaw cycles, increasing durability. However, entrained air entails a tradeoff with strength, as each 1% of air may decrease compressive strength by 5%.[48] If too much air becomes trapped in the concrete as a result of the mixing process, defoamers can be used to encourage the air bubble to agglomerate, rise to the surface of the wet concrete and then disperse.
  • Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) with wide temperature tolerance and corrosion resistance.
  • Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
  • Crystalline admixtures are typically added during batching of the concrete to lower permeability. The reaction takes place when exposed to water and un-hydrated cement particles to form insoluble needle-shaped crystals, which fill capillary pores and micro-cracks in the concrete to block pathways for water and waterborne contaminates. Concrete with crystalline admixture can expect to self-seal as constant exposure to water will continuously initiate crystallization to ensure permanent waterproof protection.
  • Pigments can be used to change the color of concrete, for aesthetics.
  • Os plastificantes aumentam a trabalhabilidade do plástico, ou concreto "fresco", permitindo que ele seja colocado com mais facilidade e menor esforço de consolidação. Um plastificante típico é o lignossulfonato. Plastificantes podem ser usados ​​para reduzir o teor de água de um concreto, mantendo a trabalhabilidade e, às vezes, são chamados de redutores de água devido a esse uso. Esse tratamento melhora suas características de resistência e durabilidade.
  • Superplasticizers (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Superplasticizers are used to increase compressive strength. It increases the workability of the concrete and lowers the need for water content by 15–30%. Superplasticizers lead to retarding effects.
  • Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.
  • Os retardadores retardam a hidratação do concreto e são usados ​​em derramamentos grandes ou difíceis, onde o endurecimento parcial é indesejável antes da conclusão do derramamento. Os retardadores de poliol típicos são açúcar , sacarose , gluconato de sódio , glicose , ácido cítrico e ácido tartárico .

Aditivos minerais e cimentos compostos

Componentes do cimento:
comparação das características químicas e físicas [a] [49] [50] [51]
Propriedade
Cimento Portland
Cinza volante siliciosa [b]
Cinza volante calcária [c]
Escória de
cimento
Sílica
ativa
Proporção em massa (%)
SiO 2 21,9 52 35 35 85-97
Al 2 O 3 6,9 23 18 12 -
Fe 2 O 3 3 11 6 1 -
CaO 63 5 21 40 <1
MgO 2,5 - - - -
SO 3 1,7 - - - -
Superficial específica (m 2 / kg) [d] 370 420 420 400 15.000
- 30.000
Gravidade Específica 3,15 2,38 2,65 2,94 2,22
Propósito geral Fichário primário Substituição de cimento Substituição de cimento Substituição de cimento Intensificador de propriedade
  1. ^ Os valores mostrados são aproximados: os de um material específico podem variar.
  2. ^ ASTM C618 Classe F
  3. ^ ASTM C618 Classe C
  4. ^ Medições de superfície específicas para sílica ativa pelo método de adsorção de nitrogênio (BET), outras pelométodo de permeabilidade ao ar (Blaine).

Inorganic materials that have pozzolanic or latent hydraulic properties, these very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures),[46] or as a replacement for Portland cement (blended cements).[52] 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,[53] as well as lowering costs, improving concrete properties, and recycling wastes.

  • Fly ash: A by-product of coal-fired electric generating plants, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.[54]
  • 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.[55]
  • Sílica ativa : Subproduto da produção de silício e ligas de ferrossilício . A sílica ativa é semelhante à cinza volante, mas tem um tamanho de partícula 100 vezes menor. Isso resulta em uma relação superfície-volume mais alta e uma reação pozolânica muito mais rápida. A sílica ativa é usada para aumentar a resistência e a durabilidade do concreto, mas geralmente requer o uso de superplastificantes para a trabalhabilidade. [56]
  • Metacaulim de alta reatividade (HRM): O metacaulim produz concreto com resistência e durabilidade semelhantes ao concreto feito com sílica ativa. Enquanto a sílica ativa é geralmente cinza escuro ou preto, o metacaulim de alta reatividade é geralmente branco brilhante, tornando-o a escolha preferida para concreto arquitetônico onde a aparência é importante.
  • Carbon nanofibers 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.[57]
  • Carbon products have been added to make concrete electrically conductive, for deicing purposes.[58]

Production

Concrete plant showing a concrete mixer being filled from ingredient silos
Concrete mixing plant in Birmingham, Alabama in 1936

A produção de concreto é o processo de mistura de vários ingredientes - água, agregado, cimento e quaisquer aditivos - para produzir concreto. A produção de concreto é sensível ao tempo. Uma vez que os ingredientes são misturados, os trabalhadores devem colocar o concreto no lugar antes que ele endureça. No uso moderno, a maior parte da produção de concreto ocorre em um grande tipo de instalação industrial chamada planta de concreto , ou geralmente uma planta de lote.

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

A concrete plant consists of large storage hoppers for various reactive ingredients like cement, storage for bulk ingredients like aggregate and water, mechanisms for the addition of various additives and amendments, machinery to accurately weigh, move, and mix some or all of those ingredients, and facilities to dispense the mixed concrete, often to a concrete mixer truck.

O concreto moderno é geralmente preparado como um fluido viscoso, de modo que pode ser derramado em formas, que são recipientes erguidos no campo para dar ao concreto a forma desejada. A cofragem de concreto pode ser preparada de várias maneiras, como a conformação deslizante e a construção em chapa de aço . Alternativamente, o concreto pode ser misturado em formas mais secas e não fluidas e usado em configurações de fábrica para fabricar produtos de concreto pré-moldado .

Uma ampla variedade de equipamentos é usada para o processamento de concreto, desde ferramentas manuais até máquinas industriais pesadas. Qualquer que seja o equipamento usado pelos construtores, o objetivo é produzir o material de construção desejado; os ingredientes devem ser devidamente misturados, colocados, moldados e retidos dentro dos limites de tempo. Qualquer interrupção no lançamento do concreto pode fazer com que o material inicialmente colocado comece a endurecer antes que o próximo lote seja adicionado ao topo. Isso cria um plano horizontal de fraqueza denominado junta fria entre os dois lotes. [59] Uma vez que a mistura esteja onde deveria estar, o processo de cura deve ser controlado para garantir que o concreto atinja os atributos desejados. Durante a preparação do concreto, vários detalhes técnicos podem afetar a qualidade e a natureza do produto.

Mistura

A mistura completa é essencial para produzir concreto uniforme e de alta qualidade.

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.[60] The paste is generally mixed in a high-speed, shear-type mixer at a w/cm (water to cement ratio) of 0.30 to 0.45 by mass. The cement paste premix may include admixtures such as accelerators or retarders, superplasticizers, pigments, or silica fume. The premixed paste is then blended with aggregates and any remaining batch water and final mixing is completed in conventional concrete mixing equipment.[61]

Proporções de mistura

As misturas de concreto são divididas principalmente em dois tipos, mistura nominal e mistura de design :

As relações nominais de mistura são dadas em volume de. As misturas nominais são uma maneira simples e rápida de obter uma ideia básica das propriedades do concreto acabado, sem ter que realizar testes com antecedência.

Various governing bodies (such as British Standards) define nominal mix ratios into a number of grades, usually ranging from lower compressive strength to higher compressive strength. The grades usually indicate the 28-day cube strength.[62] For example, in Indian standards, the mixes of grades M10, M15, M20 and M25 correspond approximately to the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively.[citation needed]

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

Workability

Piso de concreto de garagem sendo colocado
Derramando e alisando concreto no Palisades Park em Washington, DC

Trabalhabilidade é a capacidade de uma mistura de concreto fresco (plástico) preencher a forma / molde adequadamente com o trabalho desejado (vazamento, bombeamento, espalhamento, compactação, vibração) e sem reduzir a qualidade do concreto. A trabalhabilidade depende do teor de água, agregado (forma e distribuição granulométrica), teor de cimento e idade (nível de hidratação ) e pode ser modificada pela adição de aditivos químicos, como superplastificantes. Aumentar o teor de água ou adicionar aditivos químicos aumenta a trabalhabilidade do concreto. O excesso de água leva ao aumento do sangramento ou segregação dos agregados (quando o cimento e os agregados começam a se separar), com o concreto resultante tendo qualidade reduzida. O uso de uma mistura agregada com uma gradação indesejável [ carece de fontes? ] can result in a very harsh mix design with a very low slump, which cannot readily be made more workable by addition of reasonable amounts of water. An undesirable gradation can mean using a large aggregate that is too large for the size of the formwork, or which has too few smaller aggregate grades to serve to fill the gaps between the larger grades, or using too little or too much sand for the same reason, or using too little water, or too much cement, or even using jagged crushed stone instead of smoother round aggregate such as pebbles. Any combination of these factors and others may result in a mix which is too harsh, i.e., which does not flow or spread out smoothly, is difficult to get into the formwork, and which is difficult to surface finish.[63]

A trabalhabilidade pode ser medida pelo teste de abatimento do concreto , uma medida simples da plasticidade de um lote fresco de concreto seguindo os padrões de teste ASTM C 143 ou EN 12350-2. A queda é normalmente medida enchendo um " cone de Abrams"com uma amostra de um novo lote de concreto. O cone é colocado com a extremidade larga voltada para baixo em uma superfície nivelada e não absorvente. Em seguida, é preenchido em três camadas de igual volume, com cada camada sendo compactada com uma barra de aço para consolidar a camada. Quando o cone é cuidadosamente levantado, o material fechado desce um pouco, devido à gravidade. Uma amostra relativamente seca desce muito pouco, tendo um valor de queda de uma ou duas polegadas (25 ou 50 mm) em um pé (305 mm). Uma amostra de concreto relativamente úmido pode afundar até oito polegadas. A trabalhabilidade também pode ser medida pelo teste da mesa de fluxo .

Slump can be increased by addition of chemical admixtures such as plasticizer or superplasticizer without changing the water-cement ratio.[64] 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

A concrete slab being kept hydrated during water curing by submersion (ponding)

Concrete must be kept moist during curing in order to achieve optimal strength and durability.[65] During curing hydration occurs, allowing calcium-silicate hydrate (C-S-H) to form. Over 90% of a mix's final strength is typically reached within four weeks, with the remaining 10% achieved over years or even decades.[66] The conversion of calcium hydroxide in the concrete into calcium carbonate from absorption of CO2 over several decades further strengthens the concrete and makes it more resistant to damage. This carbonation reaction, however, lowers 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.[67] Addition of short-cut polymer fibers can improve (reduce) shrinkage-induced stresses during curing and increase early and ultimate compression strength.[68]

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

Techniques

During the curing period, concrete is ideally maintained at controlled temperature and humidity. To ensure full hydration during curing, concrete slabs are often sprayed with "curing compounds" that create a water-retaining film over the concrete. Typical films are made of wax or related hydrophobic compounds. After the concrete is sufficiently cured, the film is allowed to abrade from the concrete through normal use.[69]

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

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

Alternative types

Asphalt

Asphalt concrete (commonly called asphalt,[70] blacktop, or pavement in North America, and tarmac, bitumen macadam, or rolled asphalt in the United Kingdom and the Republic of Ireland) is a composite material commonly used to surface roads, parking lots, airports, as well as the core of embankment dams.[71] Asphalt mixtures have been used in pavement construction since the beginning of the twentieth century.[72] It consists of mineral aggregate bound together with asphalt, laid in layers, and compacted. The process was refined and enhanced by Belgian inventor and U.S. immigrant Edward De Smedt.[73]

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

Concretene

Concretene is graphene-enriched concrete. Graphene acts as mechanical support, increasing strength by around 30%, and offers an extra catalyst surface for the chemical reactions that produce concrete.

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.[citation needed] Bacillus sp. CT-5. can reduce corrosion of reinforcement in reinforced concrete by up to four times. Sporosarcina pasteurii reduces water and chloride permeability. B. pasteurii increases resistance to acid.[citation needed] Bacillus pasteurii and B. sphaericuscan induce calcium carbonate precipitation in the surface of cracks, adding compression strength.[74]

Nanoconcrete

Decorative plate made of Nano concrete with High-Energy Mixing (HEM)

Nanoconcrete (also spelled "nano concrete"' or "nano-concrete") is a class of materials that contains Portland cement particles that are no greater than 100 μm[75] and particles of silica no greater than 500 μm, which fill voids that would otherwise occur in normal concrete, thereby substantially increasing the material's strength.[76] It is widely used in foot and highway bridges where high flexural and compressive strength are indicated.[77]

Pervious

Pervious concrete is a mix of specially graded coarse aggregate, cement, water, and little-to-no fine aggregates. This concrete is also known as "no-fines" or porous concrete. Mixing the ingredients in a carefully controlled process creates a paste that coats and bonds the aggregate particles. The hardened concrete contains interconnected air voids totaling approximately 15 to 25 percent. Water runs through the voids in the pavement to the soil underneath. Air entrainment admixtures are often used in freeze-thaw climates to minimize the possibility of frost damage. Pervious concrete also permits rainwater to filter through roads and parking lots, to recharge aquifers, instead of contributing to runoff and flooding.[78]

Polymer

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

Volcanic

Volcanic concrete substitutes volcanic rock for the limestone that is burned to form clinker. It consumes a similar amount of energy, but does not directly emit carbon as a byproduct.[79]

Waste-light

A form of polymer modified concrete. The specific polymer admixture allows the replacement of all the traditional aggregates (gravel, sand, stone) by any mixture of solid waste materials in the grain size of 3-10mm to form a low compressive strength (3-20 N/mm2) product[80] for road and building construction. 1 m3 of waste light concrete contains 1.1-1.3 m3 of shredded waste and no other aggregates.

Safety

Grinding of concrete can produce hazardous dust. Exposure to cement dust can lead to issues such as silicosis, kidney disease, skin irritation and similar effects. The U.S. National Institute for Occupational Safety and Health in the United States recommends attaching local exhaust ventilation shrouds to electric concrete grinders to control the spread of this dust.[81] 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,[82] which OSHA put into effect 23 September 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 23 June 2018 for general industry, hydraulic fracturing and maritime. That the deadline was extended to 23 June 2021 for engineering controls in the hydraulic fracturing industry. Companies which fail to meet the tightened safety regulations can face financial charges and extensive penalties.

Properties

Concrete has relatively high compressive strength, but much lower tensile strength.[83] Therefore, it is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep.

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

Compression testing of a concrete cylinder

The ingredients affect the strengths of the material. Concrete strength values are usually specified as the lower-bound compressive strength of either a cylindrical or cubic specimen as determined by standard test procedures.

The strengths of concrete is dictated by its function. Very low-strength—14 MPa (2,000 psi) or less—concrete may be used when the concrete must be lightweight.[84] Lightweight concrete is often achieved by adding air, foams, or lightweight aggregates, with the side effect that the strength is reduced. For most routine uses, 20 MPa (2,900 psi) to 32 MPa (4,600 psi) concrete is often used. 40 MPa (5,800 psi) concrete is readily commercially available as a more durable, although more expensive, option. Higher-strength concrete is often used for larger civil projects.[85] Strengths above 40 MPa (5,800 psi) are often used for specific building elements. For example, the lower floor columns of high-rise concrete buildings may use concrete of 80 MPa (11,600 psi) or more, to keep the size of the columns small. Bridges may use long beams of high-strength concrete to lower the number of spans required.[86][87] Occasionally, other structural needs may require high-strength concrete. If a structure must be very rigid, concrete of very high strength may be specified, even much stronger than is required to bear the service loads. Strengths as high as 130 MPa (18,900 psi) have been used commercially for these reasons.[86]

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.[88] Concrete is used more than any other artificial material in the world.[89] As of 2006, about 7.5 billion cubic meters of concrete are made each year, more than one cubic meter for every person on Earth.[90]

Mass structures

Aerial photo of reconstruction at Taum Sauk (Missouri) pumped storage facility in late November 2009. After the original reservoir failed, the new reservoir was made of roller-compacted concrete.

Due to cement's exothermic chemical reaction while setting up, large concrete structures such as dams, navigation locks, large mat foundations, and large breakwaters generate excessive heat during hydration and associated expansion. To mitigate these effects, post-cooling[91] 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.[91]

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

Surface finishes

Advantage and Disadvantage of Concrete

Black basalt polished concrete floor

Raw concrete surfaces tend to be porous and have a relatively uninteresting appearance. Many finishes can be applied to improve the appearance and preserve the surface against staining, water penetration, and freezing.

Examples of improved appearance include stamped concrete where the wet concrete has a pattern impressed on the surface, to give a paved, cobbled or brick-like effect, and may be accompanied with coloration. Another popular effect for flooring and table tops is polished concrete where the concrete is polished optically flat with diamond abrasives and sealed with polymers or other sealants.

Other finishes can be achieved with chiseling, or more conventional techniques such as painting or covering it with other materials.

The proper treatment of the surface of concrete, and therefore its characteristics, is an important stage in the construction and renovation of architectural structures.[92]

Prestressed structures

Stylized cacti decorate a sound/retaining wall in Scottsdale, Arizona

Prestressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose tensile stresses experienced in use. This can greatly reduce the weight of beams or slabs, by better distributing the stresses in the structure to make optimal use of the reinforcement. For example, a horizontal beam tends to sag. Prestressed reinforcement along the bottom of the beam counteracts this. In pre-tensioned concrete, the prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting, or for post-tensioned concrete, after casting.

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

Cold weather placement

Extreme weather conditions (extreme heat or cold; windy conditions, and humidity variations) can significantly alter the quality of concrete. Many precautions are observed in cold weather placement.[93] Low temperatures significantly slow the chemical reactions involved in hydration of cement, thus affecting the strength development. Preventing freezing is the most important precaution, as formation of ice crystals can cause damage to the crystalline structure of the hydrated cement paste. If the surface of the concrete pour is insulated from the outside temperatures, the heat of hydration will prevent freezing.

The American Concrete Institute (ACI) definition of cold weather placement, ACI 306,[94] 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 CSA A23.1:

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

The minimum strength before exposing concrete to extreme cold is 500 psi (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing.

Underwater placement

Assembled tremie placing concrete underwater

Concrete may be placed and cured underwater. Care must be taken in the placement method to prevent washing out the cement. Underwater placement methods include the tremie, pumping, skip placement, manual placement using toggle bags, and bagwork.[95]

Grouted aggregate is an alternative method of forming a concrete mass underwater, where the forms are filled with coarse aggregate and the voids then completely filled with pumped grout.[95]

Roads

Concrete roads are more fuel efficient to drive on,[96] 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.[citation needed]

Energy efficiency

Energy requirements for transportation of concrete are low because it is produced locally from local resources, typically manufactured within 100 kilometers of the job site. Similarly, relatively little energy is used in producing and combining the raw materials (although large amounts of CO2 are produced by the chemical reactions in cement manufacture).[97] The overall embodied energy of concrete at roughly 1 to 1.5 megajoules per kilogram is therefore lower than for most structural and construction materials.[98]

Once in place, concrete offers great energy efficiency over the lifetime of a building.[99] Concrete walls leak air far less than those made of wood frames.[100] Air leakage accounts for a large percentage of energy loss from a home. The thermal mass properties of concrete increase the efficiency of both residential and commercial buildings. By storing and releasing the energy needed for heating or cooling, concrete's thermal mass delivers year-round benefits by reducing temperature swings inside and minimizing heating and cooling costs.[101] 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

Boston City Hall (1968) is a Brutalist design constructed largely of precast and poured in place concrete.

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.[102] These risks can be reduced through seismic retrofitting of at-risk buildings, (e.g. school buildings in Istanbul, Turkey[103]).

Degradation

Concrete spalling caused by the corrosion of rebar

Concrete can be damaged by many processes, such as the expansion of corrosion products of the steel reinforcement bars, freezing of trapped water, fire or radiant heat, aggregate expansion, sea water effects, bacterial corrosion, leaching, erosion by fast-flowing water, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distillate water).[104] The micro fungi Aspergillus alternaria and Cladosporium were able to grow on samples of concrete used as a radioactive waste barrier in the Chernobyl reactor; leaching aluminum, iron, calcium, and silicon.[105]

The Tunkhannock Viaduct in northeastern Pennsylvania opened in 1915 and is still in regular use today

Environmental and health

The manufacture and use of concrete produce a wide range of environmental and social consequences. Some are harmful, some welcome, and some both, depending on circumstances.

A major component of concrete is cement, which similarly exerts environmental and social effects.[citation needed] 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). Every tonne of cement produced releases one tonne of CO2 into the atmosphere.[106] As of 2019, the production of Portland cement contributed eight percent to global anthropogenic CO2 emissions, largely due to the sintering of limestone and clay at 1,500 °C (2,730 °F).[106][107] Researchers have suggested a number of approaches to improving carbon sequestration relevant to concrete production.[108] In August 2019, a reduced CO2 cement was announced which "reduces the overall carbon footprint in precast concrete by 70%."[109]

Concrete is used to create hard surfaces that contribute to surface runoff, which can cause heavy soil erosion, water pollution, and flooding, but conversely can be used to divert, dam, and control flooding. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution.

Concrete is a contributor to the urban heat island effect, though less so than asphalt.[110]

Workers who cut, grind or polish concrete are at risk of inhaling airborne silica, which can lead to silicosis.[111] This includes crew members who work in concrete chipping. 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.

Recycled crushed concrete, to be reused as granular fill, is loaded into a semi-dump truck

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.

World records

The world record for the largest concrete pour in a single project is the Three Gorges Dam in Hubei Province, China by the Three Gorges Corporation. The amount of concrete used in the construction of the dam is estimated at 16 million cubic meters over 17 years. The previous record was 12.3 million cubic meters held by Itaipu hydropower station in Brazil.[112][113][114]

The world record for concrete pumping was set on 7 August 2009 during the construction of the Parbati Hydroelectric Project, near the village of Suind, Himachal Pradesh, India, when the concrete mix was pumped through a vertical height of 715 m (2,346 ft).[115][116]

The Polavaram dam works in Andhra Pradesh on 6 January 2019 entered the Guinness World Records by pouring 32,100 cubic metres of concrete in 24 hours.[117] 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.[118][119] The pour (a part of the foundation for the Abu Dhabi's Landmark Tower) was 16,000 cubic meters of concrete poured within a two-day period.[120] 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.[121]

The world record for largest continuously poured concrete floor was completed 8 November 1997, in Louisville, Kentucky by design-build firm EXXCEL Project Management. The monolithic placement consisted of 225,000 square feet (20,900 m2) of concrete placed in 30 hours, finished to a flatness tolerance of FF 54.60 and a levelness tolerance of FL 43.83. This surpassed the previous record by 50% in total volume and 7.5% in total area.[122][123]

The record for the largest continuously placed underwater concrete pour was completed 18 October 2010, in New Orleans, Louisiana by contractor C. J. Mahan Construction Company, LLC of Grove City, Ohio. The placement consisted of 10,251 cubic yards of concrete placed in 58.5 hours using two concrete pumps and two dedicated concrete batch plants. Upon curing, this placement allows the 50,180-square-foot (4,662 m2) cofferdam to be dewatered approximately 26 feet (7.9 m) below sea level to allow the construction of the Inner Harbor Navigation Canal Sill & Monolith Project to be completed in the dry.[124]

See also

References

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External links

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