Concrete

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Image:Concrete pouring 0020.jpg Image:Concrete rebar 0030.jpg

For other uses, see Concrete (disambiguation).

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, with water being the only substance on Earth we utilize more. An old name for concrete is liquid stone.

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.

However, asphalt concrete is strictly speaking a form of concrete as well.

1 History
2 Characteristics
3 Admixtures
4 Additions
5 Workability
6 Self compacting concretes
7 Shotcrete / sprayed concrete
8 Post-tensioned concrete structures
9 See also
10 External links

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History

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.

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Characteristics

During 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 detirmination of the correct amount of cement to achieve the required water cement ratio. Only with very high strength concrete does the stength 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.

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Curing

Curing is the process of keeping concrete under a specific environmental condition. Good curing is typically considered to be a moist environment which promotes hydration. Increased hydration lowers permeability and increases strength, resulting in a higher quality material. The effects of curing are primarily a function of specimen geometry, the permeability of the concrete, curing length and curing history.

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Expansion and shrinkage

Concrete has a very low thermal expansion coeficient, however if no provision is made for expansion very large forces can be created which lead to cracking of parts of the structure which are 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. Brickwork of clay origin tends to expand with time after manufacture of the bricks and the relative shrinkage and expansion of concrete and brickwork need to be accommodated in appropriate detailing of joints and other components of the structure.

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Cracking

Concrete is placed in a wet or plastic state, and therefore can be manipulated and molded as needed. Hydration and hardening of concrete may lead to tensile stresses at a time when it has not yet gained significant strength, resulting in shrinkage cracks. Extending the period concrete stays damp during curing increases its strength. Minimizing stress prior to curing minimizes cracking. High early strength concrete is designed to cure faster and thus can be stressed earlier than other concretes.

Cracking may start out as micro cracking, thus not readily apparent.

Freezing of concrete (such as 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.

Concrete can be sampled and tested off site for strength. Such tests may use hydraulic ram compression. Construction site testing, including concrete testing, is typically performed by an accredited independent testing laboratory such as AASHTO, U.S. Army Corps.

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 placed at the desired spacing so as to limit the crack width to an acceptable level. Water retaining structures and conrete 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 occuring cracks.

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Creep

Creep is the term used to describe the permanent movement or deformation of a material in order to relieve stresses in the material. Concrete which is subjected to forces is prone to creep. The amount of cracking that occurs in a concrete structure or element is sometimes less than it would have been had creep not occurred. The amount of primary and secondary reinforcing in concrete structures contributes to a reduction in the amount of shrinkage, creep and cracking.

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Admixtures

Admixtures 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 addmixtures are:

Accelerators: 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 bar. 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 Liquid or powder colours: Change the colour of concrete for aesthetics.

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Additions

Fly 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.

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Workability

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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Self compacting concretes

During the 1980s a number of countries including Japan, Sweden and France developed a range of concretes that were self-compacting. These 'SCC's are characterised by their extreme fluidity (using superplasticizers), behaving more like a viscous fluid that is self-leveling than the traditional concrete that needs consolidating, normally by vibration.

SCCs are characterized by

extreme fluidity measured by flow, typically measured between 700-750 mm, rather than slump.
no need for vibrators to compact the concrete, which can be noisy and may cause hand-arm syndrome (whitefinger)
placing becomes simpler
no bleed water (excess water migrating to the surface of the concrete)

SCC can offer benefits of up to 50% in labour costs, due to it being poured up to 80% faster and having reduced wear and tear on formwork.

As of 2005, self compacting concretes account for 10-15% of concrete sales in some European countries.

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Shotcrete / sprayed concrete

Main article: Shotcrete

Shotcrete uses compressed air to shoot (cast) concrete to a frame or structure. Shotcrete is mostly used for rock support, especially in tunnelling. Today there are two application methods for shotcrete: the dry-mix and the wet-mix procedure. In Dry-mix the dry mixture of cement and aggregates is filled into the machine and conveyed with compressed air through the hoses. The water needed for the hydration is added at the nozzle. In Wet-mix the mixes are prepared with all necessary water for hydration. The mixes are pumped through the hoses. At the nozzle compressed air is added for spraying. For both methods additives such as plasticizers and accelerators may be used. Shotcrete is normally reinforced by fibers. <p>

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Post-tensioned concrete structures

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Buildings with monostrand post-tensioned slabs are a widely used application of prestressed concrete. This method achieves performance and construction improvements over other construction methods. However, in order to reap the benefits of this method, proficiency is required in both structural design and construction.

Post-tensioned slabs is a preferred method for industrial, commercial and residential floor slab construction. The extensive use of this method is due to its advantages and its nature of easy applicability to a wide variety of structure geometry and design solutions.

Prestressed floor systems using monostrand cables may be designed as either one or two way slab systems, and may be flat plate, flat slab waffle slab, or other slab sections. The prestressing is achieved by individually tensioning tendons, placed within internally greased protected plastic sleeves, arranged in the slab prior to casting. Compressive stresses are applied to the concrete via tendon anchors. Prestressing is performed within three to seven days of casting.

Unlike the multi-strand system (which is primarily suited for beams) the monostrand method allows prestressing of slabs as thin as 15cm and less, while maintaining vertical curvatures optimal for the structure. The monostrand system is also simpler, requires less in site organization, and is more forgiving to construction variances.

Advantages afforded by unbonded slab prestressing as compared with alternative designs include:

Increased speed of construction as prestressing allows for faster stripping and reuse of formwork.

Thinner slabs resulting from post-tensioning by virtue of improved deflection behavior and improved section utilization.

Improved economy due to reduced slab thickness and associated concrete costs, reducing building weight with the corresponding foundation reductions, reduced building height with the corresponding decrease in building skin area, and a reduced amount of mild reinforcing rebar.

Large area slabs can be maintained with no control joints.

Simpler coordination between consultants due to a flat slab underside, the design and installation of systems is simpler (heating, air conditioning, sprinklers, etc.)

Increased design flexibility allows simple solutions even for structures with irregular geometry, without the need for transverse or longitudinal beams.

Longer spans can be achieved improving the architectural structure flexibility.

Long-term deformations due to creep, which are usually significant in concrete slabs, are almost nonexistent in unbonded prestressed slabs.

Longer building life cycle due to the uncracked nature of the prestressed concrete. This advantage also creates slabs more resistant to water penetration, and the structure behaves monolithically.

[1] Typical projects

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