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In all but the least critical applications, care needs to be taken to properly cure concrete, and achieve best strength and hardness. This happens after the concrete has been placed. Cement requires a moist, controlled environment to gain strength and harden fully. The cement paste hardens over time, initially setting and becoming rigid though very weak, and gaining in strength in the days and weeks following.
Even thought the cement reaction with water is completed over period of time (normally about around 240 minutes which is referred to as final setting time), the hardening of concrete and gain of strength is over a period of time 95% to 98% strength is achieved in 3 weeks or about 28 days. During this period concrete needs to be in a ideal conditions, controlled temperature and humid atmosphere, in practice this is achieved by spraying or ponding the concrete surface with water, thereby protecting concrete mass from ill effects of ambient conditions. The pictures to the right show two of many ways to achieve this, ponding – submerging setting concrete in water, and wrapping in plastic to contain the water in the mix.
Properly curing concrete leads to increased strength and lower permeability, and avoids cracking where the surface dries out prematurely. Care must also be taken to avoid freezing, or overheating due to the exothermic setting of cement. Improper curing can cause scaling, reduced strength and abrasion resistance and cracking.
Strength
Concrete has relatively high compressive strength, but significantly lower tensile strength (about 10% of the compressive strength). As a result, without compensating, concrete would almost always fail from tensile stresses – even when loaded in compression. The practical implication of this is that concrete elements subjected to tensile stresses must be reinforced with materials that are strong in tension. Concrete is most often constructed with the addition of steel reinforcement. The reinforcement can be by bars (rebar), mesh, or structural fibers, which provide the required tensile strength to concrete 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 can be non-destructive if carried out with equipment such as a Schmidt hammer, which is used to estimate concrete strength.
The ultimate strength of concrete is influenced by the water-cementitious 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 that with a higher ratio. The total quantity of cementitious materials (Portland cement, slag cement, pozzolans) can affect strength, water demand, shrinkage, abrasion resistance and density. All concrete will crack independent of whether or not it has sufficient compressive strength. In fact, high Portland cement content mixtures can actually crack more readily due to increased hydration rate. As concrete transforms from its plastic state, hydrating to a solid, the material undergoes shrinkage. Plastic shrinkage cracks can occur soon after placement but if the evaporation rate is high they often can actually occur during finishing operations, for example in hot weather or a breezy day. In very high strength concrete mixtures (greater than 10,000 psi) the crushing strength of the aggregate can be a limiting factor to the ultimate compressive strength. In lean concretes (with a high water-cement ratio) the crushing strength of the aggregates is not so significant.
Experimentation with various mix designs begins by specifying desired "workability" as defined by a given slump, "durability" requirements taking into consideration the weather exposure conditions (freeze-thaw) to which the concrete will be exposed in service, and the required "28 day compressive strength" as determined by properly molded standard-cured cylinder samples. 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 (and often chemical admixtures) to achieve the target water-cementitious ratio.
The internal forces in common shapes of structure, such as arches, vaults, columns and walls are predominantly compressive forces, with floors and pavements subjected to tensile forces. Compressive strength is widely used for specification requirement and quality control of concrete. The engineer knows his target tensile (flexural) requirements and will express these in terms of compressive strength.
Wired.com reported on April 13, 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.[14] 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).
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 increasingly 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 due to hydration 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 its originally placed volume.
Cracking
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 simple, using a large volume of concrete. He then realized that much of the concrete was very cracked, and could not be a part of the structure under compressive loads, yet the structure clearly worked. His later designs simply removed the cracked areas, leaving slender, beautiful concrete arches. The Salginatobel Bridge is an example of this.
Concrete cracks due to tensile stress induced by shrinkage or stresses occurring during setting or use. Various means are used to overcome this. Fiber reinforced concrete uses fine fibers distributed throughout the mix or larger metal or other reinforcement elements to limit the size and extent of cracks. In many large structures joints or concealed saw-cuts are placed in the concrete as it sets to make the inevitable cracks occur where they can be managed and out of sight. Water tanks and highways are examples of structures requiring crack control.
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.
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 allowing remediation, repair, or if necessary, evacuation of an unsafe area.
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 long-duration forces is prone to creep. Short-duration forces (such as wind or earthquakes) do not cause 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.
Physical properties
The coefficient of thermal expansion of Portland cement concrete is 0.000008 to 0.000012 (per degree Celsius) (8-12 1/MK) [15]. The density varies, but is around 150 pounds per cubic foot (2400 kg/m³).[16]
Damage modes
Fire
Due to its low thermal conductivity, a layer of concrete is frequently used for fireproofing of steel structures. However, concrete itself may be damaged by fire.
Up to about 300 °C, the concrete undergoes normal thermal expansion. Above that temperature, shrinkage occurs due to water loss; however, the aggregate continues expanding, which causes internal stresses. Up to about 500 °C, the major structural changes are carbonation and coarsening of pores. At 573 °C, quartz undergoes rapid expansion due to Phase transition, and at 900 °C calcite starts shrinking due to decomposition. At 450-550 °C the cement hydrate decomposes, yielding calcium oxide. Calcium carbonate decomposes at about 600 °C. Rehydration of the calcium oxide on cooling of the structure causes expansion, which can cause damage to material which withstood fire without falling apart. Concrete in buildings that experienced a fire and were left standing for several years shows extensive degree of carbonation.
Concrete exposed to up to 100 °C is normally considered as healthy. The parts of a concrete structure that is exposed to temperatures above approximately 300 °C (dependent of water/cement ratio) will most likely get a pink color. Over approximately 600 °C the concrete will turn light grey, and over approximately 1000 °C it turns yellow-brown.[17] One rule of thumb is to consider all pink colored concrete as damaged, and to be removed.
Fire will expose the concrete to gases and liquids that can be harmful to the concrete, among other salts and acids that occur when gasses produced by fire come into contact with water.
Aggregate expansion
Various types of aggregate undergo chemical reactions in concrete, leading to damaging expansive phenomena. The most common are those containing reactive silica, that can react (in the presence of water) with the alkalis in concrete (K2O and Na2O, coming principally from cement). Among the more reactive mineral components of some aggregates are opal, chalcedony, flint and strained quartz. Following the reaction (Alkali Silica Reaction or ASR), an expansive gel forms, that creates extensive cracks and damage on structural members. On the surface of concrete pavements the ASR can cause pop-outs, i.e. the expulsion of small cones (up to 3 cm about in diameter) in correspondence of aggregate particles. When some aggregates containing dolomite are used, a dedolomitization reaction occurs where the magnesium carbonate compound reacts with hydroxyl ions and yields magnesium hydroxide and a carbonate ion. The resulting expansion may cause destruction of the material. Far less common are pop-outs caused by the presence of pyrite, an iron sulfide that generates expansion by forming iron oxide and ettringite. Other reactions and recrystallizations, e.g. hydration of clay minerals in some aggregates, may lead to destructive expansion as well.
Sea water effects
Concrete exposed to sea water is susceptible to its corrosive effects. The effects are more pronounced above the tidal zone than where the concrete is permanently submerged. In the submerged zone, magnesium and hydrogen carbonate ions precipitate about 30 micrometers thick layer of brucite on which a slower deposition of calcium carbonate as aragonite occurs. These layers somewhat protect the concrete from other processes, which include attack by magnesium, chloride and sulfate ions and carbonation. Above the water surface, mechanical damage may occur by erosion by waves themselves or sand and gravel they carry, and by crystallization of salts from water soaking into the concrete pores and then drying up. Pozzolanic cements and cements using more than 60% of slag as aggregate are more resistant to sea water than pure Portland cement.
Bacterial corrosion
Bacteria themselves do not have noticeable effect on concrete. However, anaerobic bacteria (Thiobacillus) in untreated sewage tend to produce hydrogen sulfide, which is then oxidized by aerobic bacteria present in biofilm on the concrete surface above the water level to sulfuric acid which dissolves the carbonates in the cured cement and causes strength loss. Concrete floors lying on ground that contains pyrite are also at risk. Using limestone as the aggregate makes the concrete more resistant to acids, and the sewage may be pretreated by ways increasing pH or oxidizing or precipitating the sulfides in order to inhibit the activity of sulfide utilizing bacteria.
Chemical damage
Carbonation
Carbon dioxide from air can react with the calcium hydroxide in concrete to form calcium carbonate. This process is called carbonation, which is essentially the reversal of the chemical process of calcination of lime taking place in a cement kiln. Carbonation of concrete is a slow and continuous process progressing from the outer surface inward, but slows down with increasing diffusion depth. Carbonation has two effects: it increases mechanical strength of concrete, but it also decreases alkalinity, which is essential for corrosion prevention of the reinforcement steel. Below a pH of 10, the steel's thin layer of surface passivation dissolves and corrosion is promoted. For the latter reason, carbonation is an unwanted process in concrete chemistry. Carbonation can be tested by applying Phenolphthalein solution, a pH indicator, over a fresh fracture surface, which indicates non-carbonated and thus alkaline areas with a violet color.
Chlorides
Chlorides, particularly calcium chloride, have been used to shorten the setting time of concrete.[18] However, calcium chloride and (to a lesser extent) sodium chloride have been shown to leach calcium hydroxide and cause chemical changes in Portland cement, leading to loss of strength,[19] as well as attacking the steel reinforcement present in most concrete.
Sulphates
Sulphates in solution in contact with concrete can cause chemical changes to the cement, which can cause significant microstructural effects leading to the weakening of the cement binder.
Leaching
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Physical damage
Damage can occur during the casting and de-shuttering processes. The corners of beams for instance, can be damaged during the removal of shuttering because they are less effectively compacted by means of vibration (improved by using form-vibrators). Other physical damage can be caused by the use of steel shuttering without base plates. The steel shuttering pinches the top surface of a concrete slab due to weight of the next slab being constructed.
Hydration=
Cement hydration
By the process of hydration - reaction with water - Portland cement, mixed with sand gravel and water, produces the synthetic rock we call concrete. Concrete is as essential a part of the modern world as are electricity or computers.
Other pages on this web site describe how Portland cement is made and what is in it. Here, we'll discuss what happens when water is added to cement.
Cement clinker is anhydrous - without water - having come from a hot kiln. Cement powder is also anhydrous if we ignore the small amount of water in any gypsum added at the clinker grinding stage.
The process by which cement reacts with water is termed 'hydration.' In cement, this involves many different reactions, often occurring at the same time. As the reactions proceed, the products of the cement hydration process gradually bond together the individual sand and gravel particles, and other components of the concrete, to form a solid mass.
The hydration process – reactions Portland cement is composed largely of four types of minerals: alite, belite, aluminate (C3A) and a ferrite phase (C4AF). For more information on the composition of cement clinker, see the clinker pages. Also present are small amounts of clinker sulfate (sulfates of sodium, potassium and calcium) and also gypsum, which was added when the clinker was ground up to produce cement powder.
When cement and water are mixed together, the reactions which occur are mostly exothermic – heat is produced. We can get an indication of the rate at which the minerals are reacting by monitoring the rate at which heat is evolved using a technique called conduction calorimetry. An illustrative example of the heat evolution curve produced by cement is shown below. Cement heat evolution curve from conduction calorimetry Three principal reactions occur:
Almost immediately on adding water, some of the clinker sulphates and gypsum dissolve, producing an alkaline, sulfate-rich solution.
Soon after mixing, the (C3A) phase - the most reactive of the clinker minerals - reacts with the water to form an aluminate-rich gel (Stage I on the heat evolution curve above). The gel reacts with sulfate in solution to form small rod-like crystals of ettringite. (C3A) hydration is a strongly exothermic reaction but it does not last long, typically only a few minutes and is followed by a period of a few hours of relatively low heat evolution. This is called the dormant, or induction period (Stage II).
The first part of the dormant period – up to perhaps half-way through - corresponds to when concrete can be placed. As the dormant period progresses, the paste becomes too stiff to be workable.
At the end of the dormant period, the alite and belite in the cement start to hydrate, with the formation of calcium silicate hydrate and calcium hydroxide. This corresponds to the main period of cement hydration (Stage III), during which time concrete strengths increase. The cement grains react from the surface inwards, and the anhydrous particles become smaller. (C3A) hydration also continues, as fresh crystals become accessible to water.
The period of maximum heat evolution occurs typically between about 10 and 20 hours after mixing and then gradually tails off. In a mix containing Portland cement as the only cementitious material, most of the strength gain has occurred within about a month. Where the cement has been partly-replaced by other materials, such as fly ash, strength growth may occur more slowly and continue for several months or even a year. Final strengths may exceed those from Portland-cement-only mixes.
Ferrite hydration also starts quickly as water is added, but then slows down, probably because a layer of iron hydroxide gel forms, coating the ferrite and acting as a barrier, preventing further reaction.
Cement hydration products The products of the reaction between cement and water are termed 'hydration products.' In concrete (or mortar or other cementitious materials) made using Portland cement only as the cementitious material there are four main types of hydration product:
Calcium silicate hydrate: this is the main hydration product and is the main source of concrete strength. It is often abbreviated, using cement chemists' notation, to 'C-S-H,' the dashes indicating that no strict ratio of SiO2 to CaO is inferred. The Si/Ca ratio is somewhat variable but typically approximately 0.45-0.50.
Calcium hydroxide - Ca(OH)2: often abbreviated, using cement chemists' notation, to 'CH.' CH is formed mainly from alite hydration. Alite has a Ca:Si ratio of 3:1 and C-S-H has a Ca/Si ratio of approximately 2:1, so excess lime is available from alite hydration and this produces CH.
Ettringite: ettringite is present as rod-like crystals in the early stages of cement hydration. The chemical formula for ettringite is [Ca3Al(OH)6.12H2O]2.2H2O] or, mixing cement notation and normal chemistry notation, C3A.3CaSO4.32H2O.
Monosulfate: monosulfate tends to occur in the later stages of hydration, after a few days. Usually, it replaces ettringite, either fully or partly. The chemical formula for monosulfate is C3A.CaSO4.12H2O. Both ettringite and monosulfate are compounds of C3A, CaSO4 (anhydrite) and water, in different proportions.
AFm and AFt phases: monosulfate is one of a group of minerals called ‘AFm’ phases. Ettringite is a member of a group known as AFt phases. The general definitions of these phases are somewhat technical, but ettringite is an AFt phase because it contains three (t-tri) molecules of anhydrite when written as C3A.3CaSO4.32H2O and monosulfate is an AFm phase because it contains one (m-mono) molecule of anhydrite when written as C3A.CaSO4.12H2O.
Important points to note about AFm and AFt phases are that:
* They contain a lot of water, especially the AFt phases. * They contain different ratios of sulfur to aluminium. * The aluminium can be partly-replaced by iron in both AFm and AFt phases. * The sulfate ion in AFm phases can be replaced by other anions; a one-for-one substitution if the anion is doubly-charged(eg: carbonate, CO22-) or one-for-two if the substituent anion is singly-charged (eg: hydroxyl, OH- or chloride, Cl-). The sulfate in ettringite can be replaced by carbonate or, probably, partly replaced by two hydroxyl ions.
Monosulfate gradually replaces ettringite in many concretes because the ratio of available alumina to sulfate increases with continued cement hydration. On mixing cement with water, most of the sulfate is readily available to dissolve, but much of the C3A is contained inside cement grains with no initial access to water. Continued hydration gradually releases alumina and the proportion of ettringite decreases as that of monosulfate increases.
If there is eventually more alumina than sulfate available, all the sulfate will be as monosulfate, with the additional alumina present as hydroxyl-substituted AFm phase. If there is an excess of sulfate, the cement paste will contain a mixture of monosulfate and ettringite. Near the concrete surface, carbonation will release sulfate as carbonate ions replace sulfate in the ettringite and monosulfate phases.
Hydrogarnet: hydrogarnet forms mainly as the result of ferrite or C3A hydration. Hydrogarnets have a range of compositions, of which C3AH6 is the main phase forming from normal cement hydration and then only in small amounts. A wider range of hydrogarnet compositions can be found in autoclaved cement products.
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 (1450 psi) to about 40 MPa (5800 psi), 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.
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 concrete 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 ensure the highest strength.
High-strength concrete
High-strength concrete has a compressive strength generally greater than 6,000 pounds per square inch (40 MPa = 5800 psi). 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.
High-performance concrete
High-performance concrete (HPC) is a relatively new term used to describe concrete that conforms to a set of standards above those of the most common applications, but not limited to strength. While all high-strength concrete is also high-performance, not all high-performance concrete is high-strength. Some examples of such standards currently used in relation to HPC are:
- Ease of placement
- Compaction without segregation
- Early age strength
- Long-term mechanical properties
- Permeability
- Density
- Heat of hydration
- Toughness
- Volume stability
- Long life in severe environments
Self-consolidating concretes
During the 1980s a number of countries including Japan, Sweden and France developed concretes that are self-compacting, known as self-consolidating concrete in the United States. This self-consolidating concrete (SCCs) is characterized by:
- extreme fluidity as measured by flow, typically between 650-750 mm on a flow table, rather than slump(height)
- no need for vibrators to compact the concrete
- placement being easier.
- no bleed water, or aggregate segregation
- Increased Liquid Head Pressure, Can be detrimental to Safety and workmanship
SCC can save up to 50% in labor costs due to 80% faster pouring and reduced wear and tear on formwork.
As of 2005, self-consolidating concretes account for 10-15% of concrete sales in some European countries. In the US 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 plasticizer instead of older naphthalene based polymers, and viscosity modifiers to address aggregate segregation.
Shotcrete
Shotcrete (also known by the trade name Gunite) uses compressed air to shoot 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 tunneling. Shotcrete is also used for applications were seepage is an issue to limit the amount of water entering a construction site due to a high water table or other sub-terranian sources. This type of concrete is often used as a quick fix for weathering for loose soil types in construction zones.
There are two application methods for shotcrete.
- 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.
- 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.[20]
Pervious concrete
Pervious concrete contains a network of holes or voids, to allow air or water to move through the concrete.
It is formed by leaving out some or all of the fine aggregate (fines), the remaining large aggregate then is bound by a relatively small amount of cement paste. When set, typically between 15 and 25% of the concrete volume are voids, allowing water to drain at around 5 gal/ft²/ min or 200 L/m²/min) through the concrete.
Pervious concrete allows water to drain naturally through roadway or other structures, reducing the amount of artificial drainage needed, and allowing the water to naturally replenish groundwater
It can significantly reduce noise, by allowing air squeezed between vehicle tires and the roadway to escape.
Cellular concrete
Aerated concrete produced by the addition of an air entraining agent to the concrete (or a lightweight aggregate like expanded clay pellets or cork granules and vermiculite) is sometimes called Cellular concrete.
- See also: Aerated autoclaved concrete
Cork-cement composites
Cork granules are obtained during production of bottle stoppers from the treated bark of Cork oak or Quercus suber trees.[21] These trees are mainly found in Portugal, Spain and North Africa.[22] Portugal is the largest cork producing country, followed by Spain. The waste cork granules have a density of about 300 kg/m³, which is lower than that of most of the lightweight aggregates used for making lightweight concrete. It has been found that cork granules do not significantly influence cement hydration. However, cork dust can influence hydration.[23] Cork cement composites have several advantages over standard concrete, such as lower thermal conductivities, lower densities and good energy absorption characteristics. These composites can be made of density from 400 to 1500 kg/m³, compressive strength from 1 to 26 MPa, and flexural strength from 0.5 to 4.0 MPa.
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.[24] 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.
Glass concrete
The use of recycled glass as aggregate in concrete has become popular in modern times, with large scale research being carried out at Columbia University in New York. This greatly enhances the aesthetic appeal of the concrete. Recent research findings have shown that concrete made with recycled glass aggregates have shown better long term strength and better thermal insulation due to its better thermal properties of the glass aggregates. [25]
Asphalt concrete
Strictly speaking, asphalt is a form of concrete as well, with bituminous materials replacing cement as the binder.
Rapid strength concrete
This type of concrete is able to develop high resistance within few hours after being manufactured. This feature has advantages such as removing the formwork early and to move forward in the building process at record time, repair road surfaces that become fully operational in just a few hours.
Rubberized concrete
While "rubberized asphalt concrete" is common, rubberized Portland cement concrete ("rubberized PCC") is still undergoing experimental tests, as of 2007[26] [27] [28] [29].
Polymer concrete
Polymer concrete is concrete which uses polymers to bind the aggregate. Polymer concrete can gain a lot of strength in a short amount of time. For example, a polymer mix may reach 5000 psi in only four hours. Polymer concrete is generally more expensive than conventional concretes.
Geopolymer or green concrete
Geopolymer concrete is a greener alternative to ordinary Portland cement made from inorganic aluminosilicate (Al-Si) polymer compounds that can utilise 100% recycled industrial waste (e.g. fly ash and slag) as the manufacturing inputs resulting in up to 80% lower carbon dioxide emissions. Greater chemical and thermal resistance, and better mechanical properties, are said to be achieved by the manufacturer at both atmospheric and extreme conditions.[30]
Similar concretes have not only been used in Ancient Rome (see Roman concrete) as mentioned but also in the former Soviet Union in the 1950s and 1960s. Buildings in the Ukraine are still standing after 45 years so that this kind of formulation has a sound track record.[31]
Limecrete
Limecrete or lime concrete is concrete where cement is replaced by lime.[32]
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 (SCMs) 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 proactively 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, material design and properties are often specified in accordance with ACI American Concrete Institute) code (www.concrete.org); with test methods, production and delivery 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 or cubic 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
Concrete recycling is an increasingly common method of disposing of concrete structures. Concrete debris was once routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws, and economic benefits.
Concrete, which must be free of trash, wood, paper and other such materials is collected from demolition sites and put through a crushing machine, often along with asphalt, bricks, and rocks.
Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled 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 strength and is not allowed in many jurisdictions. On March 3, 1983, a government funded research team (the VIRL research.codep) approximated that almost 17% of worldwide landfill was by-products of concrete based waste.
Recycling concrete provides environmental benefits, conserving landfill space and use as aggregate reduces the need for gravel mining.
Use of concrete in structures
Mass concrete structures
These include gravity dams such as the Itaipu, 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
Prestressed concrete is a form of reinforced concrete which builds in compressive stresses during construction to oppose those found when in use. This can greatly reduce the weight of beams or slabs, by better distributing the stresses in the structure to make optimal use of the reinforcement.
For example a horizontal beam will tend to sag down. If the reinforcement along the bottom of the beam is prestressed, it can counteract this.
In pre-tensioned concrete, the prestressing is achieved by using steel or polymer tendons or bars that are subjected to a tensile force prior to casting, or for post-tensioned concrete, after casting.
See also
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