Concrete

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Concrete_Blonde :: Concrete_and_Mortar :: Concrete :: Concrete_Units :: Concrete_and_Paving

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.

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 ^*]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.

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.

Concrete has been a high-tech material 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 shrink while it hardened, and adding blood made it more frost-resistant. In modern times, researchers have added other materials to create concrete that is capable of conducting electricity.

Composition

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.

Cement

Portland cement is the most common type of cement in general usage, as 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 (a source of calcium) with clay or sand (a source of silicon) and grinding the product (clinker), with a source of sulfate (most commonly gypsum). The resulting powder, when mixed with water, 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; Portland cement-based concretes can be damaged or destroyed by high heat, whereas refractory concretes can absorb the heat better with less degradation.

Water

Water suitable for human or animal consumption can be used for the manufacture of concrete. The water-to-cement ratio is the key factor that determines the strength of concrete. A lower water-to-cement ratio will lead to a concrete which is stronger but less workable, and thus more difficult to handle. A higher water-to-cement ratio yields a concrete with lower strength but a higher workability.http://www.olemiss.edu/courses/engr313/engr314/materials.html

Aggregates

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.

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 admixtures 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 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.

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. Experiments have determined that the use of ash up to 95% can produce structurally sound concrete, but it is only useful under limited load pressures.
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.
Crushed Glass: Recycled, crushed glass can also be added in the production of concrete for an aesthetic effect in the construction of walkways.

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 section of concrete 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) or water-cementitious materials ratio (w/cm), 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 coarse 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 coarse 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

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, chemical admixtures, aggregate (shape and size distribution), cementitious content and age (level of hydration). 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 (concrete starts to get heterogeneous) with the resulting concrete having reduced quality. Likewise, very insufficient water can reduce the hydration of the cementitious particles.

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. The cone is inverted and placed on a level, non-absorptive 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 chemical admixtures such as mid-range or high-range water reducing agents (super-plasticizer), without changing the water/cement ratio. High flow concrete, like self consolidating concrete, are tested by other flow-measuring methods.

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.

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 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.

Cracking

Concrete is placed in a wet or plastic state, and therefore can be manipulated and molded as needed. Hydration and hardening of concrete during the first three days is critical and 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 shrinkage cracks. 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.

By nature, concrete shrinks and therefore cracks. Plastic-shrinkage cracks are immediately apparent, visible within 0 to 2 days of placement with drying-shrinkage cracks developing over time. Precautions such as mixture selection, joint timing and spacing can be taken to encourage cracks to occur within the aesthetic joint, instead of randomly.

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.

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 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.

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.

Types of concrete

Various types of concrete have been developed for specialist application and have become known by these names.

Regular concrete

Regular concrete can be described as a lay term to describe concrete that is produced by following the mixing instuctions that are commonly published on packets of cement. This concrete can be produced to yield concrete varying in strength from about 10 Mpa to about 40 Mpa depending on the purpose, ranging from blinding to structural concrete respectively.

Self-consolidating concretes

During the 1980s a number of countries including Japan, Sweden and France developed a range of concretes that were self-consolidating. These SCC's are characterised by their extreme fluidity, 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 or aggregate segregation
no need for a Viscosity Modifying Agent (VMA)

SCC can offer benefits of up to 50% in labor 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. In the USA precast concrete industry SCC represents over 75% of concrete production. 38 departments of transportation in the US accept the use of SCC for road and bridge projects.

This emerging technology is made possible by the use of Polycarboxylates instead of older High Range Water Reducers. The world's foremost producer of Polycarboxylates is Sika-Corp.

Shotcrete

Main article: Shotcrete

Shotcrete uses compressed air to shoot (cast) concrete to a frame or structure. Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminates the need for formwork. It is sometimes 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 accelerators and fiber reinforcement may be used. http://www.shotcrete.org/

The term Gunite is occasionally used for shotcrete, but properly refers only to dry-mix shotcrete, and used to be a proprietary name.

No fines concrete

No fines concrete consists of concrete mix design which is carefully formulated and has almost no sand in the mix and just enough water to ensure that very little paste settles to the bottom during placing. The result is a porous mass which can be utilised to provide drainage much like a french drain. No fines concrete may be referred to as washed out concrete.

Cellular concrete

Aerated concrete produced by the addition of an air entraining agent to the concrete or a lightweight aggregate like vermiculite is sometimes called Cellular concrete.

Roller-compacted Concrete

Roller-compacted concrete, sometimes called rollcrete, is a low-cement-content stiff concrete placed using techniques borrowed from earthmoving and paving work. The concrete is placed on the surface to be covered, and is compacted in place using rollers typically used in earthwork. The concrete mix achieves a high density and cures over time into a strong monolithic block. http://www.cement.org/pavements/pv_rcc.asp Roller-compacted concrete is typically used for concrete pavement, but has been used to build concrete dams, as the low cement content causes less heat to be generated than typical for normally placed mass concrete pours.

Asphalt concrete

Strictly speaking, asphalt is a form of concrete as well, with bituminous materials replacing portland cement as the binder.

Concrete testing

Engineers usually specify the required compressive strength of concrete which is normally given as the 28 day compressive strength in megapascals (Mpa) or pounds per square inch (psi). Twenty eight days is however a long time to wait to determine if desired strengths are going to be obtained, so three-day and seven-day strengths can be useful to predict the ultimate 28-day compressive strength of the concrete. A 25% strength gain between 7 and 28 days is often observed with 100% OPC (ordinary portland cement) mixtures, and frequently a 40% strength gain can be realized with the inclusion of pozzolans and supplementary cementitious materials (SCM's) such as fly ash and/or slag cement. As strength gain depends on the type of mixture, its constituents, the use of standard curing, proper testing and care of cylinders in transport, etc. it becomes imperative to equally rely on testing the fundamental properties of concrete... in its fresh, plastic state.

Concrete is typically sampled while being placed, with testing protocols requiring that test samples be cured under laboratory conditions (standard cured). Additional samples may be field cured (non-standard) for the purpose of early stripping strengths, ie. form removal, evaluation of curing, etc. but the standard cured cylinders comprise acceptance criteria. Concrete tests measure the "plastic" (unhydrated) properties of concrete prior to, and during placement. As these properties affect the hardened compressive strength and durability of concrete (resistance to freeze-thaw) , the properties of slump (workability), temperature, density and age are monitored to ensure the production and placement of 'quality' concrete. Tests are performed per ASTM International or CSA (Canadian Standards Association) and European methods and practices. Technicians performing concrete tests MUST be certified. Structural design and material properties are often specified in accordance with ACI International code (www.concrete.org) under the "prescription" or "performance" purchasing options per ASTM C94 (www.astm.org).

Compressive strength tests are conducted using an instrumented hydraulic ram to compress a cylindrical sample to failure. Tensile strength tests are conducted either by three-point bending of a prismatic beam specimen or by compression along the sides of a cylindrical specimen.

Use of concrete in structures

Mass concrete structures

These include gravity dams such as the Hoover Dam and the Three Gorges Dam and large breakwaters

Reinforced concrete

Reinforced concrete contains steel reinforcing that is designed and placed in structural members at specific positions to cater for all the stress conditions that the member is required to accommodate.

Post-tensioned concrete structures

Main article: Prestressed concrete.

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 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.

References

External links

Cement and Concrete Institute - South Africa
History of Concrete
EcoSmart Concrete A non-profit organization dedicated to reduce the GHG signature of concrete
ACI A non-profit scientific and technical society which provides knowledge and information for the best use of concrete.
Eco-Cement – A new type of cement which incorporates magnesium oxide (magnesia) and wastes to make it environmentally sustainable
Effect of curing on the tensile strength of medium to high strength concrete
Cement Concrete & Aggregates Australia - Concrete basics (PDF)
Bendable Concrete - Latest technology/invention with a bendable concrete that has elastic properties, good for quake prone countries.
The Post-Tensioning Institute

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