Concrete, Concrete

The word concrete comes from the Latin word "concretus" (meaning compact or condensed), the past participle of "concresco", from "com-" (together) and "cresco" (to grow).

Reinforced concrete and prestressed concrete are the most widely used modern kinds of concrete functional extensions.

Its widespread use in many Roman structures, a key event in the history of architecture termed the Concrete Revolution, freed Roman construction from the restrictions of stone and brick material and allowed for revolutionarily new designs designs both in terms of structural complexity and dimension.

Laid in the shape of arches, vault and dome, it quickly hardened into a rigid mass, free from many of the internal thrusts and strains which trouble the builders of similar structures in stone or brick. D.S. Robertson: Greek and Roman Architecture , Cambridge, 1969, p. 233 Modern tests show Opus caementicium similarly strong as modern Portland cement concrete in its compressive strength (ca. 200 kg/cm 2 ). Henry Cowan: The Masterbuilders, New York 1977, p. 56, ISBN 978-0471027409 However, due to the absence of reinforced steel, its tensile strength was far lower and its mode of application was also different: Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension. Robert Mark, Paul Hutchinson: "On the Structure of the Roman Pantheon", Art Bulletin , Vol. 68, No. 1 (1986), p. 26, fn. 5 The widespread use of concrete in many Roman structures has ensured that many survive to the present day. The Baths of Caracalla in Rome are just one example of the longevity of concrete, which allowed the Romans to build this and similar structures across the Roman Empire. Many Roman aqueducts and Roman bridges have masonry cladding to a concrete core, a technique they used in structures such as the Pantheon, the dome of which is concrete.

Portland cement was first used in concrete in the early 1840s. This version of history has been challenged however, as the Canal du Midi was constructed using concrete in 1670. http://www.allacademic.com/meta/p_mla_apa_research_citation/0/2/0/1/2/p20122_index.html Recently, 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, and, as it acts as a cement replacement, reduces the amount of cement required to produce a solid concrete.

Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened, and adding blood made it more frost-resistant http://www.djc.com/special/concrete/10003364.htm.

However, it is weak in tension as the cement holding the aggregate in place can crack, allowing the structure to fail. Reinforced concrete solves these problems by adding either metal reinforcing bars, glass fiber, or plastic fiber to carry tensile loads.

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.

These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cements).

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.

Therefore, equipment and methods should be capable of effectively mixing concrete materials containing the largest specified aggregate to produce uniform mixtures of the lowest slump practical for the work. Separate paste mixing has shown that the mixing of cement and water into a paste before combining these materials with aggregates can increase the compressive strength of the resulting concrete.

It is then added to a plasticizer admixture and mixed after that with aggregates in conventional concrete mixer. This paste can be used itself or foamed (expanded) for lightweight concrete.

Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration), and can be modified by adding chemical admixtures. 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 undesirable gradation 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.

The cone is placed with the wide end down onto a level, non-absorptive surface. It is then filled in three layers of equal volume, with each layer being tamped with a steel rod in order to consolidate the layer. 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).

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.

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.

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.

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.

Note that the value of E c found is in units of psi. For normalweight concrete (defined as concrete with a w c of 150 pcf) E c is permitted to be taken as 57000\sqrt.

The relative shrinkage and expansion of concrete and brickwork require careful accommodation when the two forms of construction interface.

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.

This is most common in concrete beam 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.

Concrete which is subjected to long-duration forces is prone to creep. Short-duration forces (such as wind or earthquakes) do not cause creep.

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.

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. Norwegian Building Research Institute, publication 24. Fire-damage to buildings. One rule of thumb is to consider all pink colored concrete as damaged that should be removed.

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.

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 a layer of brucite, about 30 micrometers thick, 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.

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.

Carbonation of concrete is a slow and continuous process progressing from the outer surface inward, but slows down with increasing diffusion depth.

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.

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 the weight of the next slab being constructed.

The compressive strength of a concrete is determined by taking standard molded, standard-cured cylinder samples.

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.

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.

After a concrete floor has been laid, floor hardeners (can be pigmented) are impregnated on the surface and a mould which may be textured to replicate a stone / brick or even wood is stamped on to give a superior textured surface finish. After sufficient hardening the surface is cleaned and generally sealed to give a protection. The wear resistance of stamped concrete is generally excellent and hence found in applications like parking lots, pavements, walkways etc.

Notable concrete-mixtures are: Ductal, concrete mixed with titanium oxide, ..

The steam will condense into water and will create low pressure, pulling out air from the concrete.

It is sometimes used for rock support, especially in tunneling. Shotcrete is also used for applications where seepage is an issue to limit the amount of water entering a construction site due to a high water table or other subterranean sources. This type of concrete is often used as a quick fix for weathering for loose soil types in construction zones.

When set, typically between 15% and 25% of the concrete volume is voids, allowing water to drain at around 5 gal/ft/ min (200 L/m/min) through the concrete.

It is possible to use a whole range of ultra-lightweight concretes which have a density and compressive strength very similar to that of wood. They are easy to work with, can be nailed with ordinary nails, cut with a saw, drilled with wood-working tools, easily repaired.

Variable density can be as low as 300 kg/m 3 http://www.litebuilt.com/table1.html although at this density it would have no structural integrity at all and would function as a filler or insulation use only. The variable density reduces strength http://www.litebuilt.com/table2.html to increase thermal http://www.litebuilt.com/table3.html and acoustical insulation by replacing the dense heavy concrete with air or a light material such as clay, cork granules and vermiculite. There are many competing products that use a foaming agent that resembles shaving cream to mix air bubbles in with the concrete. All accomplish the same outcome: to displace concrete with air.

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.

Polymer concrete is generally more expensive than conventional concretes.

According to this manufacturer its E-Crete branded concrete can be used in all applications where concrete is used today.

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.

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.

Source: Wikipedia > Concrete