Concrete

Concrete is a construction material that consists, in its most common form, of Portland cement, construction aggregate (generally gravel and sand) and water.

Concrete does not solidify from drying after mixing and placement; the water reacts with the cement in a chemical process known as hydration. This water is absorbed by the cement, which hardens, bonding the other components together and eventually creating a stone-like material.

Concrete is used more than any other man made material on the planet^[1]. It is used to make pavements, building structures, foundations, motorways/roads, overpasses, parking structures, brick/block walls and footings for gates, fences and poles.

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 over two million workers in the United States alone. Over 55,000 miles of freeways and highways in America are made of this material. The People's Republic of China currently consumes 40% of world cement production.

History

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. In 1756, the British engineer John Smeaton pioneered the use of Portland cement in concrete, using pebbles and powdered brick as aggregate. 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 by product 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 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 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, and it was named after the limestone cliffs on the Isle of Portland in England because its color is similar to the stone quarried there. 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 w/c 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, while a higher w/c ratio yields a concrete with a lower strength. ^[2] 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.

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.

Admixtures

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. The most common types of admixtures are:

Accelerators speed up the hydration (hardening) of the concrete.
Retarders slow the hydration of concrete.
Air-entrainers add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles.
Plasticizers can be used to increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. Superplasticisers allow a properly designed concrete to flow in place even around congested reinforcing bars, see self consolidating concrete described below. Alternatively, they can be used to reduce the water content of a concrete (water reducers) while maintaining workability. This improves its strength and durability characteristics.
Pigments can be used to change the colour of concrete, for aesthetics.

Additions

Additions are very fine inorganic materials that usually have pozzolanic or latent hydraulic properties. They are added at the concrete mixer to improve the properties of concrete. The term is not used when the materials are added at the factory as constituents of blended cements.

Fly ash: A by product of coal fired elgenerating 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.
Ground granulated blast furnace slag (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: 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.

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 (or consistency, as it is known in Europe) 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 (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesireable gradiation 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. 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. Curing is the process of keeping concrete under a specific environmental condition until hydration is relatively complete. Good curing is typically considered to use a moist environment which 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 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 specimen geometry, the permeability of the concrete, curing length, and curing history.

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.

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, thay often can occur during finishing operations (for example in hot weather or a breezy day). Aggregate interlock and steel reinforcement in structural members often negates the effects of plastic shrinkage cracks, rendering them aesthetic in nature. Properly tooled control joints in slabs or saw cuts 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 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 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.

Elasticity

The modulus of elasticity of concrete is a funcion 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 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. Brickwork made of clay tends to expand for some time after manufacture of the bricks, and the relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface.

Cracking

Concrete is placed while 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. 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 its nature concrete shrinks and therefore cracks. 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.

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.

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.

Types of concrete

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

Regular concrete

Regular concrete is the lay term describing concrete that is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. This concrete can be produced to yield a varying strength from about 10 MPa to about 40 MPa, depending on the purpose, ranging from blinding to structural concrete respectively. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.

High-strength concrete

High-strength concrete is concrete with a compressive strength generally greater than 6,000 pounds/square inch (40 MPa). High-strength concrete is made by lowering the water-cement (w/c) ratio to 0.35 or lower. Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength at the cement-aggregate bond.

Low w/c ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used. To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.

In some applications of high-strength concrete the design criterion is the elastic modulus rather than the ultimate compressive strength.

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 self-compacting concretes (SCCs) are characterised by their extreme fluidity, behaving more like a thick fluid that is self-leveling, as opposed to the traditional concrete that needs consolidating, normally by vibration or packing.

SCCs are characterized by

extreme fluidity as measured by flow, typically between 700-750 mm, rather than slump
no need for vibrators to compact the concrete, which can be noisy
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 American 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".

Shotcrete

Shotcrete uses compressed air to shoot (cast) concrete onto (or into) 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. ^[3]

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

Pervious concrete

With pervious or "no fines" concrete, carefully controlled amounts of water and cementitious materials are used to create a paste that forms a thick coating around aggregate particles. A pervious concrete mixture contains little or no sand (fines), creating a substantial void content. Using sufficient paste to coat and bind the aggregate particles together creates a system of highly permeable, interconnected voids that drains quickly. Typically, between 15% and 25% voids are achieved in the hardened concrete, and flow rates for water through pervious concrete are typically around 480 in./hr (0.34 cm/s, which is 5 gal/ft²/ min or 200 L/m²/min), although they can be much higher. Both the low mortar content and high porosity also reduce strength compared to conventional concrete mixtures, but sufficient strength for many applications is readily achieved.

Pervious concrete pavement is a unique and effective means to address important environmental issues and support sustainable growth. By capturing rainwater and allowing it to seep into the ground, porous concrete is instrumental in recharging groundwater, reducing stormwater runoff, and meeting US Environmental Protection Agency (EPA) stormwater regulations. The use of pervious concrete is among the Best Management Practices (BMPs) recommended by the EPA, and by other agencies and geotechnical engineers across the country, for the management of stormwater runoff on a regional and local basis. This pavement technology creates more efficient land use by eliminating the need for retention ponds, swales, and other stormwater management devices. In doing so, pervious concrete has the ability to lower overall project costs on a first-cost basis.

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. See also aerated autoclaved 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 large heavy rollers typically used in earthwork. The concrete mix achieves a high density and cures over time into a strong monolithic block. ^[4] Roller-compacted concrete is typically used for concrete pavement, but has also been used to build concrete dams, as the low cement content causes less heat to be generated while curing than typical for conventionally placed massive 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 a long 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 up to 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, that is, form removal, evaluation of curing, etc. but the standard cured cylinders comprise acceptance criteria. Concrete tests can 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.

Concrete recycling

When structures made of concrete are to be demolished, concrete recycling is a common method of disposing of the rubble. Concrete debris was once routinely shipped to landfills for disposal, but recycling has a number of benefits that have made it a more attractive option in this age of greater environmental awareness, more environmental laws, and the desire to keep construction costs down.

Pieces of concrete collected from demolition sites are put through a crushing machine, often along with asphalt, bricks, and rocks. Crushing facilities accept only uncontaminated concrete, which must be free of trash, wood, paper and other such materials. Metals such as rebar are accepted, since they can be removed with magnets and other sorting devices and melted down for recycling 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 the strength, and is not allowed in many jurisdictions.

Recycling concrete provides environmental benefits, as recycling concrete saves landfill space and using recycled concrete as aggregate reduces the need for gravel mining.

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. Concrete that is poured all at once in one block (so that there are no weak points where the concrete is "welded" together) is used for tornado shelters.

Reinforced concrete structures

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.

Prestressed concrete structures

Reinforced concrete structures are normally very heavy and they have to be designed to carry their own weight as well as the superimposed design loads. The high compressive forces found in concrete columns present few problems, but the tensile stresses found in slabs and beams present design challenges to engineers. Prestressed concrete provides a way to overcome the combined tensile stresses, due to own weight and design loads in beams and slabs, by introducing a compressive stress in the structural element prior to the superimposed design loads coming into play. The net effect in a properly designed prestressed structural element is a stress condition that satisfies the stress limits in the concrete for both compression and tension.

The prestressing is achieved by utilising steel tendons or bars that are subjected to a tensile force prior to casting the concrete, in pre-tensioned concrete, or only later once the concrete has cured, in post-tensioned concrete.

References

^ The Skeptical Environmentalist: Measuring the Real State of the World, by Bjorn Lomborg, p 138.
^ http://www.olemiss.edu/courses/engr313/engr314/materials.html
^ http://www.shotcrete.org/
^ http://www.cement.org/pavements/pv_rcc.asp

External links

History 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)
Do It Yourself guide on how to pour concrete
Guide Specification for High Performance Concrete for Bridge Elements

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[1] The Skeptical Environmentalist: Measuring the Real State of the World, by Bjorn Lomborg, p 138.

[2] ttp://www.olemiss.edu/courses/engr313/engr314/materials.html

[3] ttp://www.shotcrete.org/

[4] ttp://www.cement.org/pavements/pv_rcc.asp

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