CONCRETE

CONCRETE

Introduction

Concrete is a composite construction material which composing primarily of a hard, chemically inert particulate substance, known as an aggregate that is bonded together by cement and water. The aggregate is generally coarse gravel or crushed rocks such as limestone or granite, along with a fine aggregate such as sand. Portland cement (the common used cement) and other cementitious materials such as fly ash and slag cement, serve as a binder for the aggregate. Various chemical admixtures are also added to achieve varied properties. Water is then mixed with this dry composite which enables it to be shaped and then solidified and hardened into rock-hard strength through a chemical process known as hydration.

The water reacts with the cement which bonds the other components together, eventually creating a robust stone-like material. Concrete has relatively high compressive strength, but much lower tensile strength. For this reason is usually reinforced with materials (usually steel) that are strong in tension.

Concrete is widely used for making architectural structures, foundations, brick or block walls, pavements, bridges or overpasses, motorways or roads, runways, parking structures, dams, pools or reservoirs, pipes, footings for gates, fences and poles and even boats. Famous concrete structures include the Panama Canal and the Roman Pantheon.

The environmental impact of concrete is a complex mixture of not entirely negative effects; while concrete is a major contributor to greenhouse gas emissions, recycling of concrete is increasingly common in structures that have reached the end of their life. Structures made of concrete can have a long service life. As concrete has a high thermal mass and very low permeability, it can make for energy efficient housing.

History

The Assyrians and Babylonians used clay as the bonding substance or cement. The Egyptians used lime and gypsum cement.  During the Roman Empire, Roman concrete (or opus caementicium) was made from quicklime, pozzolana and an aggregate of pumice. 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 material and allowed for revolutionary new designs in terms of both structural complexity and dimension.

The Romans knew concrete as a new and revolutionary material. Laid in the shape of arches, vaults and domes, it is 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. Modern tests show that opus caementicium had as much compressive strength as modern Portland-cement concrete (ca. 200 kg/cm²). However, due to the absence of steel reinforcement, its tensile strength was far lower and its mode of application was also different.

Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.

The widespread use of concrete in many Roman structures has ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example. Many Roman aqueducts and bridges have masonry cladding on a concrete core, as does the dome of the Pantheon.

Some have stated that the secret of concrete was lost for 13 centuries until 1756, when the British engineer, John Smeaton made the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powered brick into the cement. In 1824, English inventor, Joseph Aspdin invented Portland Cement, which is the first true artificial cement in the early 1840s by burning ground limestone and clay together. The burning process changed the chemical properties of the materials and he created stronger cement than what using plain crushed limestone would produce. Portland cement was first used in concrete in the early 1840s. However, the Canal du Midi was built using concrete in 1670. Likewise there are concrete structures in Finland that date back to the 16th century.

Concrete additives have been used since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened and adding blood made it more frost-resistant.

Composition

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone with clay and grinding clinker (the product) with a source of sulfate (most commonly gypsum).

Portland cement concrete shall consist of an intimate mixture of Portland cement, other approved cementitious material (when used), fine aggregate, coarse aggregate, water, and admixtures, if ordered or permitted by the Engineer, proportioned in accordance with the following requirements:

TYPE	28-day Minimum Compressive Strength	Water / Cement; or Water / Cement plus other approved Cementitious Material (by mass) Maximum	Minimum Cementitious Material Required (kg/m 3)
Pavement	25 MPa	0.49	365
Class  "A"	21 MPa	0.53	365
Class  "C"	21 MPa	0.53	390
Class "F"	28 MPa	0.44	390
Slope Paving	14 MPa	0.69	270

The water shall be reasonably clean, shall not be salty or brackish, and shall be free from oil, acid and injurious alkali or vegetable matter. Water shall not be taken from shallow or muddy sources. In cases where sources of supply are relatively shallow, they shall be so enclosed as to exclude silt, mud, grass and etc. and the water in the enclosure shall be maintained at a depth of not less than 610 mm under the intake of the suction pipe.

Combining water with a cementitious material forms a cement paste by the process of hydration. The cement paste glues the aggregate together, fills voids within it, and allows it to flow more freely. Less water in the cement paste will yield a stronger, more durable concrete; more water will give a freer-flowing concrete with a higher slump. Impure water used to make concrete can cause problems when setting or in causing premature failure of the structure. Hydration involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.

Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel and crushed stone are used mainly for this purpose. Fine aggregate shall be sand consisting of clean, hard, durable, uncoated particles of quartz or other rock, free from lumps of clay, soft or flaky material, loam, organic or other injurious material. In no case shall sand containing lumps of frozen material be used.

Coarse aggregate shall be broken stone, gravel, or reclaimed concrete aggregate defined as mortar-coated rock, consisting of clean durable fragments of uniform quality throughout. It shall be free from soft, disintegrated pieces, mud, dirt, organic or other injurious material and shall not contain more than one percent of dust by mass, as determined by the testing method used by the Laboratory. Reclaimed concrete aggregate shall not be used in prestressed concrete members. Coarse aggregate of a size retained on a 28.6 mm square opening sieve shall not contain more than 8 percent of flat or elongated pieces, whose longest dimension exceeds five times their maximum thickness.

The presence of aggregate greatly increases the robustness of concrete above that of cement, which otherwise is a brittle material and thus concrete is a true composite material. Redistribution of aggregates after compaction often creates inhomogeneity due to the influence of vibration. This can lead to strength gradients.

Concrete is strong in compression as the aggregates carried the compression load efficiently but weak in tension as the place it holds aggregates will crack which causes the building structure to fail. Steel reinforcing bars, steel fibers, glass fiber, or plastic fiber can be added to reinforce the concrete which enable it to carry tensile loads without crack. Steel is the most common material used as reinforcement. The reinforcement must be of the right kind, of the right amount, and in the right place in order for the concrete structure to meet its requirements for strength and serviceability.

Chemical admixtures are natural or manufactured chemicals which are added to the concrete before or during mixing. The most often used admixtures are air- entraining agents, water- reducing retarders and accelerators. The air- entraining agents are added and entrained tiny air bubbles in the concrete, which will reduce damage during freeze thereby increasing the durability of concrete. However, entrained air entails a trade off with strength, as each 1% of air may result in 5% decrease in compressive strength. The water – reducing 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. Then, the accelerators speed up hydration of the concrete. The materials that are mostly used are calcium chloride, calcium nitrate and sodium nitrate. But, the use of chlorides may cause corrosion in steel reinforcing  and so the nitrate is favoured to be used in reinforcement.

The blended cements are manufactured by adding pozzolanic or cementitious materials like fly ash or ground granulated blast furnace slag (GGBFS) or condensed silica fumes (CSF) to Portland cement clinker and gypsum. Alternatively, these pozzolanic and cementitious materials can be introduced into Portland cement concrete during concrete making operations.

Fly ash which is 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. 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.

Silica fume is a by-product of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.

A metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark grey or black in color, high-reactivity metakaolin (HRM) is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.

The production of concrete

When Portland cement and water initially mixed together, they rapidly form a gel, formed of tangled chains of interlocking crystals. These continue to react over time, with the initially fluid gel often aiding in placement by improving workability. As the concrete sets, the chains of crystals join and form a rigid structure, gluing the aggregate particles in place. During curing, more of the cement reacts with the residual water (hydration). This curing process develops physical and chemical properties. Among these qualities are mechanical strength, low moisture permeability and chemical and volumetric stability.

Concrete mixing is essential for the uniform and high quality production of concrete. For this reason, equipment and methods should be capable of mixing concrete materials efficiently and containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work.

Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete. 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.

High-energy mixed (HEM) concrete is produced by means of high-speed mixing of cement, water and sand with net specific energy consumption of at least 5 kilojoules 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 this process, sand provides dissipation of energy and creates high-shear conditions on the surface of cement particles. This results in the full volume of water interacting with cement. The liquid activated mixture can be used by itself or foamed (expanded) for lightweight concrete. HEM concrete hardens in low and subzero temperature conditions and possesses an increased volume of gel, which drastically reduces capillarity in solid and porous materials.

Workability

Workability is the property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed, consolidated, and finished. It is considered to increase or improve as the ease of placement, consolidation, and finishing of a concrete increase.

A list of factors that influence workability are the properties and the amount of the cement; grading, shape, angularity and surface texture of fine and coarse aggregates; proportion of aggregates; amount of air entrained; type and amount of mineral admixtures; mixing time . These factors interact so that changing the proportion of one component to produce a specific characteristic requires that other factors be adjusted to maintain workability.

In most mixture-proportioning procedures, the water content is assumed to be a factor directly related to the consistency of the concrete for a given maximum size of coarse aggregate. If the water content and the content of cementitious materials are fixed, workability is largely governed by the maximum coarse aggregate size, aggregate shape angularity, texture, and grading. The coarse-aggregate grading that produces the most workable concrete for one water-cement ratio (w/c) may not produce the most workable concrete for another w/c. In a general way, the higher the w/c, the finer the aggregate grading required to produce appropriate flow without segregation.

If the aggregate-cement ratio is reduced, the water content must increase for the w/c to remain constant. The water required to maintain a constant consistency will increase as the w/c is increased or decreased. The increase in fine aggregate or coarse aggregate ratio generally increases the water content required to produce a given workability. If finer aggregate is substituted in a mixture, the water content typically must be increased to maintain the same workability. Similarly, water content must be increased to maintain workability if angular aggregate is substituted for rounded aggregate. Crushed aggregates having numerous flat or elongated particles will produce less workable concrete that requires a higher mortar content and possibly a higher paste content. Aggregates with high absorption present a special case because, if they are batched with a large unsatisfied absorption, they can remove water from the final concrete mixture and, hence, reduce workability. The size and shape of particles in the fine aggregate affect the workability. For example, the use of very fine sand requires that more water be added to achieve the workability that coarser sand would provide. Angular fine aggregate particles interlock and reduce the freedom of movement of particles in the fresh concrete. Using angular fine aggregate increases the amount of fine aggregate that must be used for a given amount of coarse aggregate and generally requires that more water be added to achieve the workability obtained with rounded sand.

The workability of concrete mixtures commonly is improved by using air-entraining and water-reducing admixtures. Air entrainment typically increases paste volume and improves the consistency of the concrete while reducing bleeding and segregation. Water-reducing admixtures disperse cement particles and improve workability, increasing the consistency and reducing segregation. Small changes in the amounts of chemical admixtures used in concrete can profoundly affect workability. Some chemical admixtures interact in adverse ways with some Portland cements, resulting in accelerated hydration of the Portland cement.

Mineral admixtures or pozzolans are used to improve strength, durability, and workability in concrete. Freshly mixed concretes are generally more workable when a portion of the cementitious material is fly ash, in part because of the spherical shape of fly ash particles. Smoother mixtures are typically produced if the mineral admixture is substituted for sand rather than cement, but highly reactive or cementitious pozzolans can cause loss of workability through early hydration. Very finely divided mineral admixtures, such as silica fume, can have a very strong negative effect on water demand and hence workability, unless high-range water-reducing admixtures are used.

Freshly mixed concrete loses workability with time. The reduction in workability is generally attributed to loss of water absorbed into aggregate or by evaporation, or from chemical reaction with the cementitious materials in early hydration reactions. Elevated temperatures increase the rate of water loss in all of the modes mentioned above. The workability of air-entrained concretes is reported to be more easily reduced by elevated temperature than workability in similarly proportioned non air-entrained concretes.

Properties

Concrete has relatively high compressive strength, but lower tensile strength. To overcome this problem, it is usually reinforced with materials that are strong in tension such as steel. The elasticity of concrete is relatively constant at low stress levels but it starts to decrease at a higher stress level as matrix cracking develops. Concrete has a very low coefficient of thermal expansion and shrinks as it matures. All concrete structures will crack to some extent, due to shrinkage and tension. Concrete that is subjected to long-duration forces is prone to creep.

Environmental, health and safety

 A major component of concrete is cement, which has its own environmental and social impacts.The cement industry is one of two primary producers of carbon dioxide, a major greenhouse gas. Concrete is used to create hard surfaces which contribute to surface runoff, which can cause heavy soil erosion, water pollution and flooding. Concrete is a primary contributor to the urban heat island effect, but is less so than asphalt. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution. The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicidity and radioactivity. Wet concrete is highly alkaline and must be handled with proper protective equipment.

Concrete recycling is an increasingly common method of disposing of concrete structures. It is increasing due to improved environmental awareness, governmental laws and economic benefits. Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through a crushing machine, often along with asphalt, bricks and rocks.

Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Aggregate base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt placed over it. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions.

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). The overall embodied energy of concrete is therefore lower than for most structural materials other than wood.

Concrete offers significant energy efficiency over the lifetime of a building. Concrete walls leak air far less than those made of wood-frames. 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. 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.

Concrete buildings are more resistant to fire than those constructed using wood or steel frames since concrete does not burn. Concrete reduces the risk of structural collapse and is an effective fire shield, providing safe means of escape for occupants and protection for fire fighters. 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 fire-proof 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 the best resistance of any building material to high winds, hurricanes, tornadoes due to its lateral stiffness that results in minimal horizontal movement.

As what has 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 tensional and compressional loads. Concrete structures without reinforcing, like other unreinforced masonry structures, can fail during severe earthquake shaking. Unreinforced masonry structures constitute one of the largest earthquake risks globally. These risks can be reduced through seismic retrofitting of at-risk buildings.