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Mod-01 Lec-21 Self-compacting concrete
Etymology[ edit ] 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. Prehistory[ edit ] Small-scale production of concrete-like materials dates to 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 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. German archaeologist Heinrich Schliemann found concrete floors, which were made of lime and pebbles, in the royal palace of Tiryns , Greece, which dates roughly to — BC. Its widespread use in many Roman structures , a key event in the history of architecture termed the Roman Architectural Revolution , freed Roman construction from the restrictions of stone and brick materials. It enabled revolutionary new designs in terms of both structural complexity and dimension. 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. 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. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges, such as the magnificent Pont du Gard in southern France, have masonry cladding on a concrete core, as does the dome of the Pantheon. Middle Ages[ edit ] After the Roman Empire, the use of burned lime and pozzolana was greatly reduced until the technique was all but forgotten between and the 14th century. From the 14th century to the midth century, the use of cement gradually returned. The Canal du Midi was built using concrete in This third Eddystone Lighthouse pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. His son William continued developments into the s, earning him recognition for the development of "modern" Portland cement. 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. Aggregate 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. 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 process called 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 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 coal-fired power plants , 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 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 Several tons of bagged cement, about two minutes of output from a 10, 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. British masonry worker Joseph Aspdin patented Portland cement in It was named because of the similarity of its colour to Portland limestone , quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of calcium silicates alite , belite , aluminates and ferrites — compounds which combine calcium, silicon, 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 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. 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. The cement paste glues the aggregate together, fills voids within it, and makes it flow more freely. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles and other components of the concrete to form a solid mass. Construction aggregate Crushed stone aggregate 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. Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients. In addition to being decorative, exposed aggregate may add robustness to a concrete. Reinforced concrete Constructing a rebar cage that will be permanently embedded in a finished reinforced concrete structure 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 , glass fibers, or plastic fibers to carry tensile loads. Chemical admixtures[ edit ] Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. The common types of admixtures  are as follows: Accelerators speed up the hydration hardening of the concrete. 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. Retarders 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 entraining agents add and entrain tiny air bubbles in the concrete, which reduces damage during freeze-thaw cycles, increasing durability. 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[ edit ] Components of Cement Comparison of Chemical and Physical Characteristicsa    Property.
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