concrete

 

structural masonry material made by mixing broken stone or gravel with sand, cement, and water and allowing the mixture to harden into a solid mass. The cement is the chemically active element, or matrix; the sand and stone are the inert elements, or aggregate. Concrete is adaptable to widely varied structural needs, is available practically anywhere, is fire resistant, and can be used by semiskilled workers.

The use of artificial masonry similar to modern concrete dates from a remote period but did not become a standard technique of construction until the Romans adopted it (after the 2d cent. B.C.) for roads, immense buildings, and engineering works. The concrete of the Romans, formed by combining pozzuolana (a volcanic earth) with lime, broken stones, bricks, and tuff, was easily produced and had great durability (the Pantheon of Rome and the Baths of Caracalla were built with it). Enormous spaces could be roofed without lateral thrusts by vaults cast in the rigid homogeneous material.

Scientifically proportioned concrete formed with cement is an invention of modern times; the name did not appear until c.1830. Modern portland cement has revolutionized the production and potentialities of concrete and has superseded the natural cements, to which it is vastly superior. The component materials of concrete are mixed in varying proportions, according to the strength required and the function to be fulfilled; the proportions were first worked out by Duff Abrams in 1918. The ideal mixture is that which solidifies with the minimum of voids, the mortar and small particles of aggregate filling all interstices. A typical proportioning is 1:2:5, i.e., one part of cement, two parts of sand, and five parts of broken stone or gravel, with the proper amount of water for a pouring consistency. A simple test called a “slump test” is used to confirm the proportions and consistency of the mixture, and it is then poured into wood or steel molds, called forms. Concrete usually takes about five days to cure, or reach acceptable hardness, but a technique called steam saturation can shorten that curing time to less than 18 hours. A wide variety of additives allow the concrete to harden faster or slower, resist scaling, or adopt the final shape more easily.

Concrete used without strengthening is termed mass, or plain, concrete and has the structural properties of stone—great strength under compressive forces and almost none under tensile ones. F. Joseph Monier, a French inventor, found that the tensile weakness could be overcome if steel rods were embedded in a concrete member. The new composite material was called reinforced concrete, or ferroconcrete. It was patented in 1857, and a private house in Port Chester, N.Y., first demonstrated (1857) its use in the United States. It is now rivaled in popularity as a structural material only by steel. Concrete reinforced with polypropylene fibers instead of steel yields equivalent strength with a fraction of the thickness. Reinforced concrete was improved by the development of prestressed concrete—that is, concrete containing cables that are placed under tension opposite to the expected compression load before or after the concrete hardens. Another improvement, thin-shell construction, takes advantage of the inherent structural strength of certain geometric shapes, such as hemispherical and elliptical domes; in thin-shell construction great distances are spanned with very little material. The perfecting of reinforced concrete has profoundly influenced structural building techniques and architectural forms.

Bibliography

See A. A. Raafat, Reinforced Concrete in Architecture (1958); J. J. Waddell, Concrete Construction Handbook (1968); D. F. Orchard, Concrete Technology (1976).

Concrete is a construction material that consists of cement (commonly Portland cement) as well as other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate such as gravel limestone or granite, plus a fine aggregate such as sand or manufactured sand and water) and chemical admixtures.

Concrete solidifies and hardens after mixing and placement due to a chemical process known as hydration.The water reacts with the cement, which bonds the other components together, eventually creating a stone-like material. It is used to make pavements, architectural structures, foundations, motorways/roads, overpasses, parking structures, brick/block walls and footings for gates, fences and poles.

Concrete is used more than any other man-made material on the planet.^[1] As of 2005 about six billion cubic meters of concrete are made each year, which equals one cubic meter for every person on Earth. Concrete powers a US $35 billion industry which employs more than two million workers in the United States alone. More than 55,000 miles of freeways and highways in America are made of this material. The People's Republic of China currently consumes 40% of the world's cement [concrete] production.

History

In Serbia, remains of a hut dating from 5600 BC have been found, with a floor made of red lime, sand, and gravel. The pyramids of Shaanxi in China, built thousands of years ago, contain a mixture of lime and volcanic ash or clay.^[2]

The Assyrians and Babylonians used clay as cement in their concrete. The Egyptians used lime and gypsum cement. In the Roman Empire, concrete made from quicklime, pozzolanic ash/pozzolana and an aggregate made from pumice was very similar to modern Portland cement concrete. The secret of concrete was lost for 13 centuries until in 1756, the British engineer John Smeaton pioneered the use of hydraulic lime in concrete, using pebbles and powdered brick as aggregate. Portland cement was first used in concrete in the early 1840s. In modern times the use of recycled materials as concrete ingredients is gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a byproduct of coal fired power plants. This has a significant impact by reducing the amount of quarrying and landfill space required. The properties of concrete have been altered since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened, and adding blood made it more frost resistant. In modern times, researchers have added other materials to create concrete that is extremely strong, and even concrete that can conduct electricity.

Composition

The composition of concrete is determined initially during mixing and finally during placing of fresh concrete. The type of structure being built as well as the method of construction determine how the concrete is placed and therefore the composition of the concrete mix (the mix design).

Cement

Portland cement is the most common type of cement in general usage. It is a basic ingredient of concrete, mortar and plaster. English engineer Joseph Aspdin patented Portland cement in 1824; it was named because of its similarity in colour to Portland limestone, quarried from the English Isle of Portland and used extensively in London architecture. It consists of a mixture of oxides of calcium, silicon and aluminium. Portland cement and similar materials are made by heating limestone (a source of calcium) with clay, and grinding this product (called clinker) with a source of sulfate (most commonly gypsum). When mixed with water, the resulting powder will become a hydrated solid over time.

High temperature applications, such as masonry ovens and the like, generally require the use of a refractory cement; concretes based on Portland cement can be damaged or destroyed by elevated temperatures, but refractory concretes are better able to withstand such conditions.

Water

Potable water can be used for manufacturing concrete. The water/cement ratio (mass ratio of water to cement) is the key factor that determines the strength of concrete. A lower w/c ratio will yield a concrete which is stronger and more durable, while a higher w/c ratio yields a concrete with a larger slump, so it may be placed more easily.^[3] Cement paste is the material formed by combination of water and cementitious materials; that part of the concrete which is not aggregate or reinforcing. The workability or consistency is affected by the water content, the amount of cement paste in the overall mix and the physical characteristics (maximum size, shape, and grading) of the aggregates.

Specifically, for every 4 lbs (or kg) of cement, 1 lb (or kg) of water is needed to complete the hydration reaction. This results in a water/cement ratio of 1/4 or 25%. In reality, a batch of concrete made with a 25% ratio would be too dry to be workable, so ratios of 35% to 40% are used, with plasticizers added to increase workability if needed.

Aggregates

The water and cement paste hardens and develops strength over time. In order to ensure an economical and practical solution, both fine and coarse aggregates are utilised to make up the bulk of the concrete mixture. Sand, natural gravel and crushed stone are mainly used for this purpose. However, it is increasingly common for recycled aggregates (from construction, demolition and excavation waste) to be used as partial replacements of natural aggregates, whilst a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.

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

Typically, a batch of concrete can be made by using 1 part portland cement, 2 parts dry sand, 3 parts dry stone, 1/2 part water. The parts are in terms of weight - not volume. For example, 1 cubic foot of concrete would be made using 22 lbs cement, 10 lbs water, 41 lbs dry sand, 70 lbs dry stone (1/2" to 3/4" stone). This would make 1 cubic foot of cement and would weigh about 143 lbs. The sand should be mortar or brick sand (washed and filtered if possible) and the stone should be washed if possible. Organic materials (leaves, twigs, etc) should be removed from the sand and stone to insure the highest strength.

Chemical admixtures

Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement, and are added to the concrete at the time of batching/mixing.^[4] The most common types of admixtures are:

• Accelerators speed up the hydration (hardening) of the concrete. Without accelerants, concrete may take centuries to cure. Craig Taylor at Los Alamos says "The cement in the Great Wall of China has not yet reached a chemically neutral state. But the supercritical carbon dioxide treatment achieves the chemically stable condition in minutes or hours." [1].
• Retarders slow the hydration of concrete, and are used in large or difficult pours where partial setting before the pour is complete is undesirable.
• Air-entrainers add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles thereby increasing the concrete's durability. However, entrained air is a trade-off with strength, as each 1% of air may result in 5% decrease in compressive strength.
• Plasticizers (water-reducing admixtures) increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort.
• Superplasticizers (high-range water-reducing admixtures) are a class of plasticizers which have fewer deleterious effects when used to significantly increase workability. Alternatively, plasticizers can be used to reduce the water content of a concrete (and have been called water reducers due to this application) while maintaining workability. This improves its strength and durability characteristics.
• 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.
• Pumping aids improve pumpability, thicken the paste, and reduce dewatering of the paste.

Mineral admixtures and blended cements

There are inorganic materials that also 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),^[4] or as a replacement for Portland cement (blended cements).^[5]

• 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, silicious fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.^[6]
• 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.^[7]
• Silica fume: A byproduct 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.^[8]
• High Reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.

Fibers

Short fibers of steel, glass, synthetic or natural materials can be incorporated in the concrete during mixing. See Fiber reinforced concrete.

Mixing concrete

Thorough mixing is essential for the production of uniform, high quality concrete. Therefore, equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work. Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete.^[9] The paste is generally mixed in a high-speed, shear-type mixer at a w/cm of 0.30 to 0.45 by mass. The premixed paste is then blended with aggregates and any remaining batch water, and final mixing is completed in conventional concrete mixing equipment.^[10]

High-Energy Mixed Concrete (HEM concrete) is produced by means of high-speed mixing of cement, water and sand with net specific energy consumption at least 5 kilojoules per kilogram of the mix. It is then added to a plasticizer admixture and mixed after that with aggregates in conventional mixer. This paste can be used itself or foamed (expanded) for lightweight concrete.^[11] Sand effectively dissipates energy in this mixing process. HEM concrete fast hardens in ordinary and low temperature conditions, and possesses increased volume of gel, drastically reducing capillarity in solid and porous materials. It is recommended for precast concrete in order to reduce quantity of cement, as well as concrete roof and siding tiles, paving stones and lightweight concrete block production.

Characteristics

During hydration and hardening, concrete needs to develop certain physical and chemical properties. Among other qualities, mechanical strength, low moisture permeability, and chemical and volumetric stability are necessary.

Workability

Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration), and can be modified by adding chemical admixtures. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (surface water) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot be readily made more workable by addition of reasonable amounts of water.

Workability can be measured by the "slump test," a simplistic measure of the plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2 test standards. Slump is normally measured by filling an "Abrams cone" with a sample from a fresh batch of concrete. The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod in order to consolidate the layer. When the cone is carefully lifted off, the enclosed material will slump a certain amount due to gravity. A relatively dry sample will slump very little, having a slump value of one or two inches (25 or 50 mm). A relatively wet concrete sample may slump as much as six or seven inches (150 to 175 mm).

Slump can be increased by adding chemical admixtures such as mid-range or high-range water reducing agents (super-plasticizers) without changing the water/cement ratio. It is bad practice to add extra water at the concrete mixer.

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

Curing

Because the cement requires time to fully hydrate before it acquires strength and hardness, concrete must be cured once it has been placed and achieved initial setting. Curing is the process of keeping concrete under a specific environmental condition until hydration is relatively complete. Good curing is typically considered to provide a moist environment and control temperature. A moist environment promotes hydration, since increased hydration lowers permeability and increases strength resulting in a higher quality material. Allowing the concrete surface to dry out excessively can result in tensile stresses, which the still-hydrating interior cannot withstand, causing the concrete to crack.

Also, the amount of heat generated by the exothermic chemical process of hydration can be problematic for very large placements. Allowing the concrete to freeze in cold climates before the curing is complete will interrupt the hydration process, reducing the concrete strength and leading to scaling and other damage or failure.

The effects of curing are primarily a function of geometry (the relation between exposed surface area and volume), the permeability of the concrete, curing time, and curing history.

Improper curing can lead to several serviceability problems including cracking, increased scaling, and reduced abrasion resistance.

Strength

Concrete has relatively high compressive strength, but significantly lower tensile strength (about 10% of the compressive strength). As a result, concrete always fails from tensile stresses — even when loaded in compression. The practical implication of this is that concrete elements subjected to tensile stresses must be reinforced. Concrete is most often constructed with the addition of steel or fiber reinforcement. The reinforcement can be by bars (rebar), mesh, or fibres, producing reinforced concrete. Concrete can also be prestressed (reducing tensile stress) using internal steel cables (tendons), allowing for beams or slabs with a longer span than is practical with reinforced concrete alone. Inspection of concrete structures must be non-destructive, so equipment such as a Schmidt hammer is used to estimate concrete strength.

The ultimate strength of concrete is influenced by the water-cement ratio (w/c) [water-cementitious materials ratio (w/cm)], the design constituents, and the mixing, placement and curing methods employed. All things being equal, concrete with a lower water-cement (cementitious) ratio makes a stronger concrete than a higher ratio. The total quantity of cementitious materials (Portland cement, slag cement, pozzolans) can affect strength, water demand, shrinkage, abrasion resistance and density. As concrete is a liquid which hydrates to a solid, plastic shrinkage cracks can occur soon after placement; but if the evaporation rate is high, they often can occur during finishing operations (for example in hot weather or a breezy day). If no restraints existed the concrete would simply shrink, aggregate interlock and steel reinforcement cause tensile stresses to develop within the concrete and due to its low tensile strength, has the effect of plastic shrinkage cracking of various depths at the surface. Properly tooled control joints or saw cuts in slabs provide a plane of weakness so that cracks occur unseen inside the joint, making a nice aesthetic presentation. In very high strength concrete mixtures (greater than 10,000 psi), the strength of the aggregate can be a limiting factor to the ultimate compressive strength. In lean concretes (with a high water-cement ratio) the use of coarse aggregate with a round shape may reduce aggregate interlock.

Experimentation with various mix designs begins by specifying desired "workability" as defined by a given slump, durability requirements, and the required 28 day compressive strength. The characteristics of the cementitious content, coarse and fine aggregates, and chemical admixtures determine the water demand of the mix in order to achieve the desired workability. The 28 day compressive strength is obtained by determination of the correct amount of cementitious to achieve the required water-cement ratio. Only with very high strength concrete does the strength and shape of the coarse aggregate become critical in determining ultimate compressive strength.

The internal forces in certain shapes of structure, such as arches and vaults, are predominantly compressive forces, and therefore concrete is the preferred construction material for such structures.

Wired.com reported on April 13th, 2007, that a team from the University of Tehran, competing in a contest sponsored by the American Concrete Institute, demonstrated several blocks of concretes with abnormally high compressive strengths between 50,000 and 60,000 PSI at 28 days[2]. The blocks appeared to use an aggregate of steel fibres and quartz -- a mineral with a compressive strength of 160,000 PSI, much higher than typical high-strength aggregates such as granite (15,000-20,000 PSI).

C20 is a standard abbreviation for a mix of concrete that has a compressive strength of 20 N/mm²(2900psi).

Elasticity

The modulus of elasticity of concrete is a function of the modulus of elasticity of the aggregates and the cement matrix and their relative proportions. The modulus of elasticity of concrete is relatively linear at low stress levels but becomes increasing non-linear as matrix cracking develops. The elastic modulus of the hardened paste may be in the order of 10-30 GPa and aggregates about 45 to 85 GPa. The concrete composite is then in the range of 30 to 50 GPa.

Expansion and shrinkage

Concrete has a very low coefficient of thermal expansion. However if no provision is made for expansion very large forces can be created, causing cracks in parts of the structure not capable of withstanding the force or the repeated cycles of expansion and contraction.

As concrete matures it continues to shrink, due to the ongoing reaction taking place in the material, although the rate of shrinkage falls relatively quickly and keeps reducing over time (for all practical purposes concrete is usually considered to not shrink any further after 30 years). The relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface.

Because concrete is continuously shrinking for years after it is initially placed, it is generally accepted that under thermal loading it will never expand to it's originally-placed volume.

Cracking

Concrete cracks due to tensile stress induced by shrinkage or by applied loading. Engineers are familiar with the tendency of concrete to crack, and where appropriate, special design precautions are taken to ensure crack control. This entails the incorporation of secondary reinforcing, for example deformed steel bars, placed at the desired spacing to limit the crack width to an acceptable level. Water retaining structures and concrete highways are examples of structures where crack control is exercised. The objective is to encourage a large number of very small cracks, rather than a small number of large, randomly-occurring cracks.

All concrete structures will crack to some extent. One of the early designers of reinforced concrete, Robert Maillart, employed reinforced concrete in a number of arched bridges. His first bridge was very simple, using a large volume of concrete, and Maillart noticed that large areas of the structure were very cracked. He then realised that if the concrete was very cracked, it must not be contributing to the strength of the structure - but yet the structure clearly worked. Therefore, his later designs simply removed the cracked areas, leading to slender, beautiful concrete arches. The Salginatobel Bridge is an example of this.

Cracking is also a primary indicator of structural distress in reinforced concrete elements. For example, a properly designed reinforced concrete beam failing as a result of overloading will exhibit a pronounced increase in the number and width of cracks. This can allow remediation, repair, or if necessary, evacuation of an unsafe area.

Shrinkage cracking

Shrinkage cracks occur when concrete members undergo restrained volumetric changes (shrinkage) as a result of either drying, autogenous shrinkage, or thermal effects. Restraint is provided either externally (i.e. supports, walls, and other boundary conditions) or internally (differential drying shrinkage, reinforcement). Once the tensile strength of the concrete is exceeded, a crack will develop. the number and width of shrinkage cracks that develop are influenced by the amount of shrinkage that occurs, the amount of restraint present, and the amount and spacing of reinforcement provided.

Concrete is placed while in a wet (or plastic) state, and therefore can be manipulated and moulded as needed. Hydration and hardening of concrete during the first three days is critical. Abnormally fast drying and shrinkage due to factors such as evaporation from wind during placement may lead to increased tensile stresses at a time when it has not yet gained significant strength, resulting in greater shrinkage cracking. The early strength of the concrete can be increased by keeping it damp for a longer period during the curing process. Minimizing stress prior to curing minimizes cracking. High early-strength concrete is designed to hydrate faster, often by increased use of cement, which increases shrinkage and cracking.

Plastic-shrinkage cracks are immediately apparent, visible within 0 to 2 days of placement, while drying-shrinkage cracks develop over time. Precautions such as mixture selection and joint spacing can be taken to encourage cracks to occur within an aesthetic joint instead of randomly.

Tension cracking

Concrete members may be put into tension by applied loads. This is most common in concrete beams, where a transversely applied load will put one surface into compression and the opposite surface into tension (due to induced bending). The portion of the beam that is in tension may crack - the size and length of cracks is dependent on the magnitude of the bending moment and the design of the reinforcing in the beam at the point under consideration. Reinforced concrete beams are designed to crack in tension rather than in compression. This is achieved by providing reinforcing steel which yields before failure of the concrete in compression occurs and in so doing provides a warning mechanism.

Creep

Creep is the term used to describe the permanent movement or deformation of a material in order to relieve stresses within the material. Concrete which is subjected to forces is prone to creep. Creep can sometimes reduce the amount of cracking that occurs in a concrete structure or element, but it also must be controlled. The amount of primary and secondary reinforcing in concrete structures contributes to a reduction in the amount of shrinkage, creep and cracking.