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Concrete and aggregate
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In construction, concrete is a composite building material made from
the combination of aggregate and cement binder. The most common form of concrete consists of Portland cement, mineral aggregates (generally gravel and sand) and water. Contrary to common belief, concrete does not solidify from drying after mixing and placement. Instead, the cement hydrates, gluing the other components together and eventually creating a stone-like material. When used in the generic sense, this is the material referred to by the term concrete. Concrete is used to make pavements, building structures, foundations, motorways/roads, overpasses, parking structures, brick/block walls and bases for gates, fences and poles. Concrete is used more than any other man-made material on the planet. An old name for concrete is liquid stone. It has been suggested that instead of naming our era "The nuclear age" it should be named "The Concrete Age" as almost all of our modern lifestyle and constructions depend on this material. It has also been called the "Rodney Dangerfield" of modern materials, for the apparent lack of recognition to its importance. |
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As of 2005 over six billion tons of concrete are made each year,
amounting to the equivalent of one ton for every person on Earth,
and powers a US$35 billion industry which employs over two million
workers in the United States alone. Over 55,000 miles of freeways
and highways in America are made of this material. China currently
consumes 40% of world cement production. However, asphalt concrete is strictly speaking a form of concrete as well. The Assyrians and Babylonians used clay as cement in their concretes. 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. In 1756, British engineer John Smeaton pioneered the use of portland cement in concrete, using pebbles and powdered brick as aggregate. In the modern day, the use of recycled/reused materials as concrete ingredients is gaining popularity due to increasingly stringent environmental legislation. The most conspicuous of these is pulverized fuel ash, recycled from the ash by-products of coal power plants. This has a significant impact in reducing the amount of quarrying and the ever-attenuating landfill space. The composition of concrete is determined initially during mixing and finally during placing of fresh concrete. The type of structure being constructed as well as the method of construction determine how the concrete is placed and therefore also the composition of the concrete mix or mix design. Water suitable for human or animal consumption can be used for the manufacture of concrete. The water cement ratio is the key factor that determines the strength of concrete. The water and cement paste hardens and develops strength over time. In order to ensure an economical and practical solution fine and coarse aggregates are utilised to make up the bulk of the concrete mixture. Sand and crushed stone are used for this purpose. Decorative stones such as quartzite or small river stones are sometimes added to the surface for a decorative "exposed aggregate" finish, which is popular among landscape designers. AdmixturesAdmixtures are organic or non-organic materials in form of solids or fluids that are added to the concrete to give it certain characteristics. In normal use the admixtures make up less than 5% of the cement weight and are added to the concrete at the time of batching/mixing. The most used types of admixtures are: AcceleratorsAccelerators: Speed up the hydration (strengthening) of the concrete.Retarders: Slow the hydration of concrete. Air-entrainers: Add and distributes tiny air bubbles to the concrete, which reduces damage due to freeze-thaw cycles. Plasticizers: Can be used to increase the workability of concrete, allowing it be placed more easily with less compactive effort. Superplasticisers allow a properly designed concrete to flow around congested reinforcing bars. Alternatively, they can be used to reduce the water content of a concrete (termed water reducers) yet maintain the original workability. This improves its strength and durability characteristics Pigments: Change the colour of concrete for aesthetics. AdditionsFly ash: A by-product of coal-fire electric generating plants, it is used to partially replace Portland cement by up to 40% by weight.Ground granulated blastfurnace slag (ggbs): A by-product of steel making, it is used to partially replace Portland cement by up to 80% by weight. Silica fume: A byproduct of the production of silicon and ferrosilicon alloys. Silica fume is a very reactive pozzolan that is used to increase strength and durability of concrete. CharacteristicsDuring hydration and hardening, concrete needs to develop certain physical and chemical properties, among others, mechanical strength, low permeability to ingress of moisture, and chemical and volume stability. 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 these facts is that concrete elements that are subjected to tensile stresses must be reinforced. To illustrate this difference in compressive and tensile strength for unreinforced concrete one only has to imagine a 10' x 10' section of concrete 4 inches thick suspended on its edges. This section of concrete would be unable to support its own weight and would crack in two. Concrete is most often constructed with the addition of steel bar or fiber reinforcement. The reinforcement can be by bars (rebars), mesh, or fibres to produce reinforced concrete. Concrete can also be prestressed (reducing tensile stress) using steel cables, allowing for beams or slabs with a longer span than is practical with reinforced concrete only.The ultimate strength of concrete is related to water-cement ratio (w/c), the proportion and type of cement to fillers, and the size, shape, and strength of the aggregate used. Concrete with lower water-cement ratio (down to 0.35) makes a stronger concrete than a higher ratio. Concrete made with smooth pebbles is weaker than that made with rough-surfaced broken rock pieces. For example, pebbles require more bonding material ("cement") per area than larger rock, which has less surface area to bond than the smaller "pea gravel". A much higher compressive strength though can be achieved with a "pea gravel" or even better with crushed 3/8" aggregate, even with a lower cement content. Limestone has much better bonding characteristics than conventional "gravel" or igneous type aggregates. Experimentation with various mix designs is generally done by specifying desired workability as defined by a given slump and a required 28 day compressive strength. The characteristics of the course and fine aggregates determine the water demand of the mix in order to achieve the workability. The 28 day compressive strength is obtained by determination of the correct amount of cement to achieve the required water cement ratio. Only with very high strength concrete does the strength and shape of the course aggregate become very critical in determination of 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. A structural member such as a bridge beam may have a bending moment induced in it by tensioning pre-stress tendons (wire or cable), placed at the correct eccentricity along the beam, which ensures that the concrete remains in compression when bending moments are created by loads passing along the beam. Workability is the ability of a fresh (plastic) concrete mix to fill the form/mould properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, additives, aggregate (shape and size distribution) and age (level of hydration). Raising the water content or adding plasticizer will increase the workability. Too much water will lead to bleeding (loss of water) and/or segregation (concrete starts to get heterogeneous) and the resulting concrete will have reduced quality. Workability is normally 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 the Abrams cone with a sample from a fresh batch of concrete, inverting the cone and setting it on a level surface. When the cone is carefully lifted off, the enclosed material will slump a certain amount due to its water content. A relatively dry sample will slump very little, and be given a slump value of one or two inches (25 or 50 mm), while a relatively wet concrete sample may slump as much as six or seven inches (150 to 175 mm). To increase the slump, the rule of thumb is: US units Add 1 US gallon of water per cubic yard of concrete in the mixer truck to increase slump by 1 inch. Adding 27 US gallons to 9 cubic yards of batched concrete will therefore increase the slump by about 3 inches. Metric units (converted from US rule of thumb) Add 2 litres of water per cubic metre of concrete in the mixer truck to increase slump by 1 cm. Adding 60 litres to 10 cubic metres of batched concrete will therefore increase the slump by about 3 cm. Slump can also be increased by adding a plasticizer, without changing the water/cement ratio. High flow concrete, like self compacting concrete, are normally tested by other flow-measuring methods. |
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