The word "concrete" comes from the word "hormigo" ('flour porridge'),[1]​ and this in turn from "ant",[12]​ which comes from the Latin formīca.[13]​.

The term "concrete", classified in the RAE dictionary as Americanism,[3]​ is also originally from Latin: it comes from the word concretus, which means 'to grow together', or 'to unite'. Concretus is a compound word: from the prefix com- ('union') and the past participle of the verb crĕscere ('grow'). Its use in the Spanish language is transmitted through Anglo-Saxon culture[14]​ as an Anglicism (or semantic calque) to some countries in America, especially those closest to the United States, the original English word being concrete.[3]​ Etymologically, "concrete" is synonymous with "concrete" and "concretion", which is the union of various particles to form a mass.

Contenido

La historia del hormigón o concreto constituye un capítulo fundamental de la historia de la construcción"). Cuando se optó por levantar edificaciones utilizando materiales arcillosos o pétreos, surgió la necesidad de obtener pastas o morteros que permitieran unir dichos mampuestos para poder conformar estructuras estables. Inicialmente se emplearon pastas elaboradas con arcilla, yeso o cal, pero se deterioraban rápidamente ante las inclemencias atmosféricas. Se idearon diversas soluciones, mezclando agua con rocas y minerales triturados, para conseguir pastas que no se degradasen fácilmente. Así, en el Antiguo Egipto se utilizaron diversas pastas obtenidas con mezclas de yesos y calizas disueltas en agua, para poder unir sólidamente los sillares de piedra, como las que aún perduran entre los bloques calizos del revestimiento de la Gran Pirámide de Guiza.

Natural cement concrete

In Ancient Greece, around 500 BC. C., calcined limestone compounds were mixed with water and sand, adding crushed stones, broken tiles or bricks, giving rise to the first concrete in history, using volcanic tuffs extracted from the island of Santorini. The ancient Romans used volcanic earth or ash, also known as pozzolana, which contain silica and alumina, which, when chemically combined with lime, resulted in the so-called pozzolanic cement (obtained in Pozzuoli, near Vesuvius). By adding pieces of ceramic or other low-density materials (pumice stone) to their mass, they obtained the first lightweight concrete.[15]​ With this material, everything from pipes to port facilities was built, the remains of which still exist. Constructions stand out such as the various arches of the Roman Colosseum, the ribs of the vault of the Basilica of Maxentius, with spans of more than 25 meters,[16]​ the vaults of the Baths of Caracalla, and the dome of the Pantheon of Agrippa, about 43 meters in diameter, the one with the greatest span for centuries.[17]​.

After the fall of the Roman Empire, concrete was little used, possibly due to the lack of technical and human resources, the poor quality of lime firing, and the lack or remoteness of volcanic tuffs. There are no signs of its use in large works until the 13th century, when it was used again in the foundations of Salisbury Cathedral, or in the famous Tower of London, in England. During the Renaissance its use was scarce and very insignificant.

In some cities and large structures, built by the Mayans and Aztecs in Mexico or those of Machu Picchu in Peru, cementitious materials were used.[15]​.

In the 20th century, the desire for research was revived. John Smeaton, an engineer from Leeds, was commissioned to build a lighthouse for the third time on the Edystone cliff, on the Cornish coast, using stones joined with a calcined lime mortar to form a monolithic construction that would withstand the constant action of the waves and the humid winds; It was completed in 1759 and the foundation still remains.

19th century: Portland cement and reinforced concrete

Joseph Aspdin and James Parker patented Portland Cement in 1824, obtained from clayey limestone and coal calcined at high temperatures—named for its dark greenish-gray color, very similar to Portland stone. In 1845, Isaac Johnson obtained the prototype of modern cement made from a mixture of limestone and clay calcined at high temperature, until the formation of clinker; the industrialization process and the introduction of rotary kilns") led to its use for a wide variety of applications, towards the end of the 19th century.[18]​.

Concrete or concrete, due to its stone characteristics, supports compression forces well, but cracks with other types of stress (flexion, traction, torsion, shear); The inclusion of metal rods that supported these efforts led to the optimization of its characteristics and its widespread use in multiple engineering and architectural works.

The invention of reinforced concrete is usually attributed to the builder William Wilkinson"), who applied for a patent in 1854 for a system that included iron reinforcement for "the improvement of the construction of houses, warehouses and other fire-resistant buildings." The Frenchman Joseph Monier patented several methods in the 1860s, but it was François Hennebique who devised a convincing system of reinforced concrete, patented in 1892, which he used in the construction of a spinning factory in Tourcoing, Lille, in 1895.[19] Hennebique and his contemporaries based the design of their patents on experimental results, through load tests; the first theoretical contributions were made by prestigious German researchers, such as Wilhelm Ritter, who developed the theory of the "Ritter-Mörsch Lattice" in 1899. Fundamental theoretical studies were developed in the 20th century.

20th century: rise of the concrete industry

At the beginning of the century, the rapid growth of the cement industry arose, due to several factors: the experiments of the French chemists Louis Vicat and Le Chatelier and the German Michaélis, who managed to produce cement of homogeneous quality; the invention of the rotary kiln" for calcination and the tubular mill"); and the methods of transporting fresh concrete devised by Juergen Hinrich Magens, which he patented between 1903 and 1907. With these advances, Portland cement could be produced in large quantities and used advantageously in the construction industry.[15]​.

In 1901, Robert Maillart designed a 38-meter-span arch bridge over the Inn River in Switzerland, built with reinforced concrete box girders; between 1904 and 1906, he designed the Tavanasa bridge over the Rhine River, with a 51-meter span "Light (engineering)"), the largest in Switzerland. Claude A.P. In 1906, Turner built the Bovex building in Minneapolis (United States), with the first fungiform pillars (with large capitals).

In the 1920s, Le Corbusier demanded in Vers une Architecture a logical, functional and constructive production, stripped of the rhetoric of the past; In his design for Casa Domino, from 1914, the structure is made up of pillars and reinforced concrete slabs, allowing completely open facades and the free distribution of interior spaces.[20]​.

The hangars of Orly (Paris), designed by Freyssinet between 1921 and 1923, with 60 meters of span, 9 of arrow and 300 of length, are built with parabolic sheets of reinforced concrete, eliminating the functional division between walls and roof. In 1929, Frank Lloyd Wright built the first concrete skyscraper.

In the 1960s, concrete reinforced with fibers appeared, incorporated at the time of mixing, giving the concrete isotropy and increasing its qualities against bending, traction, impact, cracking, etc. During the 1970s, additives made it possible to obtain high-strength concretes, from 120 to more than 200 MPa; The incorporation of monomers generates concrete that is almost unassailable by chemical agents or indestructible by freeze-thaw cycles, providing multiple improvements in various properties of the concrete.

The great progress in the scientific study of the behavior of reinforced concrete and technological advances made possible the construction of taller skyscrapers, longer span bridges, wide decks and immense dams. Its use will be irreplaceable in public buildings that must accommodate crowds: stadiums, theaters, cinemas, etc. Many nations and cities will compete to erect the largest, or most beautiful, building as a symbol of their progress, which will normally be built of reinforced concrete.

The tallest buildings in the world have concrete and steel structures, such as the Petronas Towers in Kuala Lumpur, Malaysia (452 ​​meters, 1998), the Taipei 101 building in Taiwan (509 meters, 2004), or the Burj Khalifa in the city of Dubai (818 meters, 2009), in the 21st century.

21st century: environmental culture

The use of recycled materials as ingredients in concrete has gained popularity due to increasingly strict environmental legislation, as well as the progressive awareness of society. The most commonly used recycled ingredients are fly ash, a byproduct of coal-fired thermoelectric plants. The environmental impact of the cement industry is significant, but by using these new materials, the reduction of quarries and landfills (garbage) is possible, since they act as substitutes for cement), and reduce the amount necessary to obtain good concrete. Since one of the harmful effects on the environment is that the production of cement generates large volumes of carbon dioxide, cement replacement technology plays an important role in efforts to reduce carbon dioxide emissions. It is usually include certain catalysts in the mixtures that allow them to be 'self-cleaned', such as titanium dioxide.

It is also used to confine radioactive waste. Among them, the most important is that of the nuclear reactor that collapsed at the Chernobyl plant, which was covered in concrete to prevent radioactive leaks.

El concreto u hormigón es el material resultante de unir áridos con la pasta que se obtiene al añadir agua a un conglomerante.[21]​ El conglomerante puede ser cualquiera, pero cuando nos referimos al concreto, generalmente es un cemento artificial, y entre estos últimos, el más importante y habitual es el cemento portland.[21]​ Los áridos proceden de la desintegración o trituración, natural o artificial de rocas y, según la naturaleza de las mismas, reciben el nombre de áridos silíceos "Óxido de silicio (IV)"), calizos, graníticos, etc. El árido cuyo tamaño sea superior a 5 mm se llama árido grueso o grava, mientras que el inferior a 5 mm se llama árido fino "Arena (concreto)") o arena.[22]​ El tamaño de la grava influye en las propiedades mecánicas del concreto.

La pasta formada por cemento y agua es la que confiere al hormigón su fraguado y endurecimiento, mientras que el árido es un material inerte sin participación directa en el fraguado y endurecimiento del hormigón.[22]​ El cemento se hidrata en contacto con el agua, iniciándose diversas reacciones químicas de hidratación que lo convierten en una pasta maleable con buenas propiedades adherentes, que en el transcurso de unas horas, derivan en el fraguado y endurecimiento progresivo de la mezcla, obteniéndose un material de consistencia pétrea.

Una característica importante del hormigón es poder adoptar formas distintas, a voluntad del proyectista. Al colocarse en obra es una masa plástica que permite rellenar un molde, previamente construido con una forma establecida, que recibe el nombre de encofrado.[21]​.

Mechanical characteristics

The main structural characteristic of concrete is to resist compression forces very well. However, both its tensile and shear resistance are relatively low, which is why it should be used in situations where tensile or shear stresses are very low. To determine the resistance, mechanical tests (breakage tests) are prepared on concrete specimens.

To overcome this inconvenience, the concrete is "reinforced" by introducing steel bars, known as reinforced concrete, or reinforced concrete, allowing the shear and tensile forces to be supported with the steel bars. It is also common to have steel bars reinforcing areas or elements that are fundamentally compressed, as is the case of pillars. Attempts to compensate for the deficiencies of concrete in tension and shear led to the development of a new construction technique at the beginning of the 20th century, that of reinforced concrete. Thus, by introducing high-resistance wires stretched into the concrete before setting, it is compressed when setting, with which the tractions that would arise to resist external actions become decompressions of the previously compressed parts, resulting in very advantageous in many cases. For prestressing, steels with very high elastic limit are used, since the phenomenon called creep would nullify the advantages of prestressing. Subsequently, the convenience of introducing tensions in the steel deliberately and prior to the setting of the concrete of the structural piece was investigated, developing the techniques of "prestressed concrete" and "post-tensioned concrete").

Additives make it possible to obtain highly resistant concrete; The inclusion of monomers and additions for concrete provide multiple improvements in the properties of concrete.

When a reinforced concrete element is designed, the dimensions, type of concrete, quantity, quality, additives, additions and arrangement of the steel that must be provided are established based on the forces that each element must resist. A rational design, adequate dosage, mixing, placement, consolidation, finishing and curing, make concrete an ideal material to be used in construction, as it is resistant, durable, non-combustible, almost waterproof, and requires little maintenance. As it can be easily molded into a wide variety of shapes and acquire various textures and colors, it is used in a multitude of applications.

The main physical characteristics of concrete, in approximate values, are:.

Setting and hardening

The concrete paste is formed by mixing artificial cement and water and must completely absorb the aggregates. The main quality of this paste is that it sets and hardens progressively, both in air and under water.[23]​.

The setting and hardening process is the result of chemical hydration reactions between the cement components. The initial hydration phase is called setting and is characterized by the passage of the paste from the fluid state to the solid state. This is easily observed by simply pressing a finger on the surface of the concrete. Subsequently, the hydration reactions continue, reaching all the constituents of the cement, which cause the hardening of the mass and is characterized by a progressive development of mechanical resistance.[23]​.

Setting and hardening are nothing more than two conventionally separated states; In reality, there is only a single continuous hydration process.[23]​.

In Portland cement, the most common used in concrete, the first component to react is tricalcium aluminate with a rapid and short duration (up to 7-28 days). Then the tricalcium silicate, with an important initial contribution and continuous for quite some time. Next is the dicalcium silicate with a weak and very important initial contribution after 28 days.[23]​.

The physical phenomenon of hardening does not have defined phases. The cement is in powder form and its particles or grains are progressively hydrated, initially by contact of water with the surface of the grains, forming some crystalline compounds and a large part of microcrystalline compounds comparable to colloids that form a film on the surface of the grain. From then on, hardening continues to be dominated by these colloidal structures that surround the cement grains and through which hydration progresses to the core of the grain.[23]​.

The fact that the speed with which the mixed cement loses its fluidity and hardens can be regulated makes it a very useful product in construction. A rapid reaction of hydration and hardening would make it difficult to transport and facilitate easy installation by filling all the gaps in the formwork. A slow reaction would significantly postpone the development of mechanical resistance. In cement factories it is achieved by controlling the amount of gypsum added to the cement clinker. In the concrete plant, where the cement and water paste is mixed with the aggregates, products that regulate the setting time can also be added.[23]​.

Under normal conditions, normal concrete or Portland concrete begins to set between 30 and 45 minutes after it has rested in the molds and finishes setting after about 10 or 12 hours. Then the hardening begins, which takes a rapid pace in the first days until reaching the first month, and then increases more slowly until reaching the year where it practically stabilizes.[24]​ The following table shows the evolution of the compressive strength of a concrete taking the 28-day resistance as a unit, with figures being indicative:[25]​.

Endurance

In the previous design of the elements, the characteristic resistance (f) of the concrete is that which is adopted in all calculations as its compressive resistance, and assuming that the concrete to be executed will resist that value, the measurements of all the structural elements are sized.[26]​.

The design characteristic resistance (f) therefore establishes the lower limit, and it must be met that each batch of concrete placed has that resistance as a minimum. In practice, statistical resistance tests are carried out on the concrete that is placed and 95% of them must be greater than f, considering that with the current level of concrete technology, a defective fraction of 5% is perfectly acceptable.

The compressive strength of concrete is obtained in compression fracture tests of normalized cylindrical "Template (mechanical)" specimens carried out at 28 days of age and manufactured with the same batches put into work.[26]​ The Spanish Instruction (EHE) recommends using the following series of characteristic compressive strengths at 28 days (measured in Newton/mm²): 20; 25; 30, 35; 40; 45 and 50.[26]​ Therefore, concrete manufacturing plants usually supply concrete that guarantees these resistances.

Consistency of fresh concrete

Consistency is the greater or lesser ease that fresh concrete has to deform and consequently to occupy all the gaps in the mold or formwork. Different factors influence it, especially the amount of mixing water, but also the maximum size of the aggregate, the shape of the aggregate and its granulometry.[27]​.

The consistency is set before installation, analyzing which is the most appropriate for placement according to the compaction means available. This is a fundamental parameter in fresh concrete.

Among the tests that exist to determine consistency, the most used is the Abrams cone. It consists of filling a 30 cm high truncated conical mold with fresh concrete. The loss of height that occurs when the mold is removed is the measure that defines consistency.[27]​.

Concretes are classified by their consistency as dry, plastic, soft and fluid as indicated in the following table:[28]​.

Durability

The durability of concrete is defined in the Spanish EHE Instruction as the ability to behave satisfactorily against aggressive physical and chemical actions throughout the useful life of the structure, also protecting the reinforcement and metal elements embedded within it.[29]​.

Therefore, not only must we consider the effects caused by the loads and stresses, but also the physical and chemical conditions to which it is exposed. For this reason, the type of environment in which the structure will be found is considered and which may affect the corrosion of the reinforcement, aggressive chemical environments, areas affected by freeze-thaw cycles, etc.[29]​.

To guarantee the durability of the concrete and the protection of the reinforcement against corrosion, it is important to make concrete with reduced permeability, making a mixture with a low water/cement ratio, ideal compaction, an adequate cement weight and sufficient hydration of the cement, adding curing water to complete it. In this way, there are as few pores as possible and a poorly communicated internal capillary network, thus reducing attacks on the concrete.[29]​.

In cases of presence of sulfates in the soil or seawater, special cements must be used. To prevent corrosion of reinforcement, it is necessary to take care of the minimum coating thereof.[29]​.

In the Spanish Instruction (EHE), published in 1998, concrete is classified according to the following format, making it mandatory to refer to it in this way in plans and other project documents, as well as in manufacturing and installation:[30]​.

T: it will be called HM when it is mass concrete, HA when it is reinforced concrete and HP when it is prestressed concrete.

R: characteristic resistance of concrete expressed in N/mm².

C: initial letter of the type of consistency: S Dry, P plastic, B Soft, F Fluid and L Liquid.

TM: maximum size of the aggregate expressed in millimeters.

A: designation of the environment to which the concrete will be exposed.

Cement

Cements are products that, when mixed with water, set and harden, forming new compounds resulting from hydration reactions that are stable both in air and submerged in water.[32]​.

There are several types of cements. The properties of each of them are closely associated with the chemical composition of their initial components, which is expressed in the form of their oxides, and depending on which ones they are, they will form different resulting compounds in the hydration reactions.[32]​.

Each type of cement is indicated for certain uses; Environmental conditions also determine the type and class of cement, affecting the durability of concrete. The types and names of cements and their components are standardized and subject to strict conditions. The Spanish standard establishes the following types: common cements, those resistant to sulfates, those resistant to seawater, those with low heat of hydration, white cements, those for special uses and those made of calcium aluminate. Common cements are the most important group and among them Portland is the usual one. In Spain, only cements legally marketed in the European Union can be used and are subject to the provisions of specific laws.[33]​.

In addition to the type of cement, the second factor that determines the quality of the cement is its class or 28-day compressive strength. This is determined in a standardized mortar and expresses the minimum resistance, which must always be exceeded in the manufacture of cement. It is not the same, nor should the resistance of cement be confused with that of concrete, since that of cement corresponds to standardized components and that of concrete will depend on each and every one of its components. But if the concrete is well dosed, the greater the resistance of the cement, the greater the resistance of the concrete.[32]​ The Spanish standard establishes the following resistance classes:[33]​.

Cement is in powder form and the fineness of its grinding is decisive in its binding properties, decisively influencing the speed of the chemical reactions of its setting and first hardening. When mixed with water, the cement grains are hydrated only to a depth of 0.01 mm, so if the grains were very thick, the hydration performance would be small as an inert core would remain inside. However, excessive fineness causes high shrinkage and heat of hydration. Furthermore, since resistance increases with fineness, a compromise solution must be reached; the cement must be finely ground, but not excessively.[32]​.

The storage of bulk cement is carried out in sealed silos that do not allow contamination of the cement and must be protected from humidity. For cements supplied in bags, storage must be carried out in covered, ventilated premises, protected from rain and sun.[33]​ Prolonged storage can cause the hydration of the finer particles due to weathering, losing their hydraulic value and resulting in a delay in setting and a decrease in resistance.[34]​.

Portland cement is obtained by calcining artificially prepared mixtures of limestone and clay at about 1500 °C. The resulting product, called clinker, is ground by adding an appropriate amount of setting regulator, which is usually natural gypsum stone.[35]​.

The average chemical composition of a portland, according to Calleja, is made up of 62.5% CaO (combined lime), 21% SiO (silica), 6.5% AlO (alumina), 2.5% FeO (iron) and other minorities. These four components are the main ones of cement, lime is basic and the other three are acidic. These components are not free in the cement, but combined forming silicates, aluminates and calcium ferrites, which are its hydraulic components or potential components. A medium type Portland cement clinker contains:[35]​.

The two main hydration reactions that cause the setting and hardening process are:

Tricalcium silicate is the active compound par excellence of cement as it develops a high initial strength and a high heat of hydration. It sets slowly and hardens quite quickly. In fast-hardening and high-resistance cements it appears in a higher proportion than usual.[35]​.

Dicalcium silicate is what develops long-term resistance in cement, it is slow in setting and hardening. Its chemical stability is greater than that of tricalcium silicate, which is why sulfate-resistant cements have a high content of dicalcium silicate.[35]​.

Tricalcium aluminate is the compound that governs setting and short-term resistance. Its chemical stability is good against seawater, but very weak against sulfates. In order to stop the rapid reaction of tricalcium aluminate with water and regulate the setting time of the cement, gypsum stone is added to the clinker.[35]​.

Tetracalcium aluminatoferrite") does not participate in the mechanical resistance, its presence is necessary due to the contribution of iron fluxes in the manufacture of clinker.[35]​.

In Spain there are so-called "Portland cements with active additions" that, in addition to the main components of clinker and gypsum stone, contain one of these additional components up to 35% of the weight of the cement: steel slag "Slag (metallurgy)"), silica fume, natural pozzolana, calcined natural pozzolana, siliceous fly ash, calcareous fly ash, calcined shale or limestone.[33]​.

High initial strength cements, those resistant to sulfates, those with low heat of hydration or white ones are usually special Portland cements and for them some of the four basic components of clinker are limited or enhanced.[36]​.

Steel cement is obtained by joint grinding of Portland clinker and setting regulator in a proportion of 5-64% with steel slag in a proportion of 36-95%.[33] It constitutes the family of cold cements. Slag is obtained by suddenly cooling the molten gangue from steelmaking processes in water; In this cooling the slag vitrifies and becomes hydraulically active due to its combined lime content. The slag on its own sets and hardens slowly, so Portland clinker is added to accelerate it.[36]​.

Pozzolanic cement is a mixture of Portland clinker and setting regulator in a proportion of 45-89% with pozzolana in a proportion of 11-55%.[33] Natural pozzolana has volcanic origin and although it does not have binding properties, it contains silica and alumina capable of fixing lime in the presence of water, forming compounds with hydraulic properties. Artificial pozzolana has analogous properties and is found in fly ash, diatomaceous earth or active clays.[36]​.

Aluminous cement is obtained by melting limestone and bauxite. The main constituent of this cement is monocalcium aluminate.[36]​.

Arid

Aggregates must have at least the same resistance and durability required for concrete. Soft limestone, feldspars, gypsum, pyrites or friable or porous rocks should not be used. For durability in aggressive environments, siliceous aggregates, those from the crushing of volcanic rocks or healthy, dense limestones, will be better.[37]​.

The aggregate that has the greatest responsibility in the whole is sand. According to Jiménez Montoya, it is not possible to make good concrete without good sand. The best sands are river sands, which are normally pure quartz, which ensures their resistance and durability.[37]​.

With natural rolled aggregates, the concrete is more workable and requires less mixing water than crushed aggregates, also having the guarantee that they are hard and clean stones. Crushed aggregates from crushing, having more fracture faces, cost more to put into use, but they lock better and this is reflected in greater resistance.[37]​.

If the rolled aggregates are contaminated or mixed with clay, it is essential to wash them to remove the jacket that surrounds the grains and that would reduce their adhesion to the concrete paste. Likewise, crushing aggregates are usually surrounded by crushing dust, which increases the fines in the concrete, requires more mixing water and will give lower resistance, which is why they are usually washed.[37]​.

The aggregates used in concrete are obtained by mixing three or four groups of different sizes to achieve an optimal granulometry. Three factors intervene in an adequate granulometry: the maximum size of the aggregate, the compactness and the content of fine grains. The larger the maximum size of the aggregate, the lower the cement and water needs will be, but the maximum size is limited by the minimum dimensions of the element to be built or by the separation between reinforcement, since these gaps must be filled by concrete and, therefore, by larger aggregates. In a mixture of aggregates, a high compactness is one that leaves few voids; It is achieved with mixtures low in sand and a large proportion of coarse aggregates, requiring little mixing water; Its great difficulty is to compact the concrete, but if sufficient means are available to do so, the result is very resistant concrete. Regarding the content of fine grains, these make the mixture more workable but require more mixing water and cement. In each case a compromise formula must be found taking into account the different factors. The Fuller and Bolomey parabolas give two families of granulometric curves widely used to obtain adequate dosages of aggregates.[37]​.

Water

The mixing water intervenes in the hydration reactions of the cement. The amount must be strictly necessary, since the excess that does not intervene in the hydration of the cement will evaporate and create voids in the concrete, reducing its resistance. It can be estimated that each liter of excess mixing water means canceling two kilos of cement in the mixture. However, an excessive reduction of water would cause a dry mixture, unwieldy and very difficult to place on site. Therefore, it is very important to properly set the amount of water.[38]​.

The characteristics of water for concrete must be evaluated so that it does not produce adverse reactions in the mixture, which is why physical-chemical analyzes must be carried out to guarantee its quality. In practice, a simple indicator is the potability of the water, with this we can determine if the water is suitable for use in the mixture or not.

During the setting and first hardening of the concrete, curing water is added to prevent desiccation and improve the hydration of the cement.[38]​.

Both the water intended for kneading and that intended for curing must be suitable to fulfill their function. It is very important that the curing water be suitable as it can more negatively affect the chemical reactions when the concrete is hardening. Normally, suitable water usually coincides with drinking water and a series of parameters that must be met are standardized. Thus, the regulations limit the pH, the content of sulfates, chlorine ion and carbohydrates.[38]​.

When a mass is excessively fluid or very dry, there is a danger of the segregation phenomenon occurring (separation of concrete into its components: aggregates, cement and water). It usually occurs when concrete is poured with material drops greater than 2 meters.[28]​.

Other minority components

The basic components of concrete are cement, water and aggregates; Other minor components that can be incorporated are: additions, additives, fibers, fillers and pigments.

Additives and additions can be used as components of concrete, provided that through the appropriate tests, it is justified that the added substance in the proportions and conditions provided produces the desired effect without excessively disturbing the remaining characteristics of the concrete or representing a danger to the durability of the concrete or to the corrosion of the reinforcement.[39]​.

Additions are inorganic, pozzolanic or latent hydraulic materials that, finely ground, can be added to concrete at the time of its manufacture, in order to improve some of its properties or give it special properties. The EHE only includes the use of fly ash and silica fume, determining its limitations. It is composed of limestone crushed into very small pieces like powder, and other materials such as HQR chemicals (herqiros) among others.

Additives are substances or products that are incorporated into concrete, before or during mixing, producing the modification of some of its characteristics, its usual properties or its behavior. The EHE establishes a proportion of no more than 5% of the weight of the cement and other conditions.

Regulations

In the 19th century, the resistance of reinforced concrete structural elements was calculated experimentally. Navier, at the beginning of the 19th century, raised the need to know and establish the limits to which structures behaved elastically, without permanent deformations, in order to obtain reliable physical-mathematical models and coherent formulas. Subsequently, given the complexity of concrete behavior, it was required to use methods based on the calculation of probabilities to achieve more realistic results. During the first half of the 20th century, structural elements were calculated using the allowable stress method.

During the 1960s, the theory of structural safety began to be developed with respect to Limit States, establishing maximum values ​​in the deflections and in the cracking of structural elements, limiting the risks.

The concept of Limit State had its rise in the 1970s, as a set of requirements that a structural element had to satisfy to be considered suitable. The regulations focused on two types: Service limit states and Solicitation limit states.

The regulations of the 1970s, in order to simplify the complex calculations of probabilities, established Safety coefficients, depending on the quality of the materials, the control of the execution of the work and the difficulty of the project. The Load increase coefficients or actions, and the Resistance decrease coefficients of the material components were introduced.[40]​.

In the middle of the century the regulations had dozens of pages, in the century they have hundreds. The introduction of computer programs makes very complex, fast calculations and more precise solutions possible. The regulations place special emphasis on ultimate states of service (cracking, deformations) behavior (construction details) and durability (coatings, qualities), limiting the experimental resolution with multiple conditions. Thus, Eurocode 1 establishes usual and accidental situations (such as earthquakes), which imply Partial safety coefficients for the most varied stresses and resistances. Some specific regulations by geographic area are EHE (Spain), Eurocode 2 (Europe), ASCE/SEI (United States).

Calculation and project

Before constructing any concrete element, the loads to which it will be subjected must be calculated and, based on them, the dimensions of the elements and quality of concrete, the arrangement and quantity of reinforcement in them will be determined.

The calculation of a concrete structure consists of several stages. First, a series of simplifications are made to the real structure, transforming it into an ideal calculation structure. Afterwards, the loads that the structure will support are determined, considering at each point the combination of loads that produces the most unfavorable effect. Finally, each of the sections is sized so that it can withstand the most unfavorable stresses.

Once the structure has been calculated, the project is drawn up, which is the set of documents that is used to carry out the work and details the elements to be built. The calculations carried out are included in the project. It also includes the plans showing the dimensions of the elements to be executed, the classification of the concretes planned and the resistant characteristics of the steels to be used.

Manufacturing

It is very important to achieve the optimal mixture in the precise proportions of aggregates of different sizes, cement and water. There is no optimal mixture that works for all cases.[41]​ To establish the appropriate dosage in each case, the mechanical resistance, factors associated with manufacturing and installation, as well as the type of environment to which it will be subjected must be taken into account.[42]​.

There are many methods for pre-measuring concrete, but they are only indicative. The final proportions of each of the components are usually established through laboratory tests, making corrections to what was obtained in the theoretical methods.[43]​.

The basic aspects that must be determined are briefly noted:

Once the most appropriate dosage has been determined, in the concrete plant the components, water in volume, must be measured, while cement and aggregates are measured in weight.[45]​.

The materials are mixed in a concrete mixer or mixer to achieve a homogeneous mixture of all the components. The aggregate must be well enveloped by the cement paste. To achieve this homogeneity, first half of the water is poured, then the cement and sand simultaneously, then the coarse aggregate and finally the rest of the water.[45]​.

For transportation to the place of use, procedures that do not vary the quality of the material must be used, normally concrete mixer trucks. The time elapsed should not be more than an hour and a half since it was mixed.[46]​ If upon reaching where the concrete is to be placed, it has begun to set, it should be discarded.[45]​.

Putting into work

The reinforcements must be clean and fastened to the formwork and to each other so that they maintain the planned position without moving during the pouring and compaction of the concrete. To do this, shims or spacers are placed in sufficient numbers to maintain the rigidity of the assembly.[47]​.

The distances between the various reinforcing bars must maintain a minimum separation that is standardized to allow correct placement of the concrete between the bars so that there are no gaps or gaps during the compaction of the concrete.[47]​.

Likewise, the free space between the steel bars and the formwork, called covering, must maintain a minimum separation, also standardized, that allows this space to be filled with concrete. This space is controlled by means of separators that are placed between the reinforcement and the formwork.[47]​.

The formwork must contain and support the fresh concrete during its hardening, maintaining the desired shape without deforming. They are usually made of wood or metal and are required to be rigid, resistant, watertight and clean. During assembly, they must be well secured so that no movements occur during the subsequent consolidation of the concrete.[48]​.

Before reusing a formwork, it must be cleaned well with wire brushes, removing any remains of mortar that may have adhered to the surface. To facilitate stripping, release products are usually applied to the formwork; These must be free of substances harmful to the concrete.[48]​.

The pouring of fresh concrete inside the formwork must be carried out avoiding segregation of the mixture. To do this, you must avoid pouring it from a great height, up to a maximum of two meters of free fall, and you must not move the mass horizontally.[49]​.

It is placed in layers or horizontal layers of reduced thickness to allow good compaction (up to 40 cm in mass concrete and 60 cm in reinforced concrete). The different layers or layers are consolidated successively, each layer locking with the previous one with the compaction medium used and without the previous layer having begun to set.[49]​.

To achieve a compact concrete, eliminating its voids and to obtain a complete closure of the mass, there are several consolidation systems. Chopping with a bar, which is carried out by introducing it successively, requires concrete of soft and fluid consistency and is carried out in works of little resistance. Compaction by repeated pounding with a tamper is used in layers 15 or 20 cm thick and with a lot of horizontal surface. Compaction by vibration is common in resistant concretes and is appropriate in dry consistencies.[49]​.

The most used vibrator is the needle vibrator, a metal cylinder with a diameter of 35 to 125 mm whose frequency varies between 3,000 and 12,000 cycles per minute. The needle is placed vertically in the mass of fresh concrete, introduced into each layer until the tip penetrates the previous layer and taking care not to touch the reinforcement as the vibration could separate the mass of concrete from the reinforcement. Through vibration, the air contained in the uncompacted concrete is reduced, which is estimated to be in the order of 15 to 20% to 2-3% after vibrating.[49]​.

World cement production was more than 2.5 billion tons in 2007. Estimating a cement dosage between 250 and 300 kg of cement per cubic meter of concrete, this means that 8,000 to 10,000 million cubic meters could be produced, which is equivalent to 1.5 cubic meters of concrete per person. No construction material has been used in such quantities and in the future there does not seem to be another construction material that can compete with concrete in volume magnitude.[50]​.

This massive production of concrete means that most of the structures and buildings in the world are partly made of this material. Respect for the environment, concrete is 100% recyclable and has a "CO sink effect", that is, it has the ability to absorb carbon dioxide from the atmosphere, thus reducing the amount of it in the air, which makes it a star resource in terms of sustainability. The quality of being a thermal insulator allows global energy consumption to be reduced as a result of temperature peaks, minimizing the energy cost of buildings, as well as collaborating in the reduction of greenhouse gases involved in energy production. Its high resistance gives buildings and bridges a lot of strength against fire or natural phenomena such as earthquakes, improving the service of structures and infrastructures and their level of social security, which provides them with prolonged durability, allowing their conservation with few maintenance costs and, therefore, their economic viability.[52]​.

The German philosopher Anselm Jappe criticizes the massive use of concrete in his 2020 book "Concrete. Weapon of mass construction of capitalism". It reviews its problems of deterioration, the excessive extension of its use, due to its low price, in all sectors of engineering and architecture, the abandonment of other traditional materials, the loss of professions and knowledge related to masonry. He begins his book with the chapter "A bridge collapses" about the collapse of the Morandi Bridge in 2018 in Genoa.[53]​ He mentions, among others, what he considers the disaster of the planned city of Chandigarh, in India, due to the deterioration of the concrete, designed by Le Corbusier, whom he considers a disastrous architect.[54][55]​.

The word "concrete" comes from the word "hormigo" ('flour porridge'),[1]​ and this in turn from "ant",[12]​ which comes from the Latin formīca.[13]​.

The term "concrete", classified in the RAE dictionary as Americanism,[3]​ is also originally from Latin: it comes from the word concretus, which means 'to grow together', or 'to unite'. Concretus is a compound word: from the prefix com- ('union') and the past participle of the verb crĕscere ('grow'). Its use in the Spanish language is transmitted through Anglo-Saxon culture[14]​ as an Anglicism (or semantic calque) to some countries in America, especially those closest to the United States, the original English word being concrete.[3]​ Etymologically, "concrete" is synonymous with "concrete" and "concretion", which is the union of various particles to form a mass.

Contenido

La historia del hormigón o concreto constituye un capítulo fundamental de la historia de la construcción"). Cuando se optó por levantar edificaciones utilizando materiales arcillosos o pétreos, surgió la necesidad de obtener pastas o morteros que permitieran unir dichos mampuestos para poder conformar estructuras estables. Inicialmente se emplearon pastas elaboradas con arcilla, yeso o cal, pero se deterioraban rápidamente ante las inclemencias atmosféricas. Se idearon diversas soluciones, mezclando agua con rocas y minerales triturados, para conseguir pastas que no se degradasen fácilmente. Así, en el Antiguo Egipto se utilizaron diversas pastas obtenidas con mezclas de yesos y calizas disueltas en agua, para poder unir sólidamente los sillares de piedra, como las que aún perduran entre los bloques calizos del revestimiento de la Gran Pirámide de Guiza.

Natural cement concrete

In Ancient Greece, around 500 BC. C., calcined limestone compounds were mixed with water and sand, adding crushed stones, broken tiles or bricks, giving rise to the first concrete in history, using volcanic tuffs extracted from the island of Santorini. The ancient Romans used volcanic earth or ash, also known as pozzolana, which contain silica and alumina, which, when chemically combined with lime, resulted in the so-called pozzolanic cement (obtained in Pozzuoli, near Vesuvius). By adding pieces of ceramic or other low-density materials (pumice stone) to their mass, they obtained the first lightweight concrete.[15]​ With this material, everything from pipes to port facilities was built, the remains of which still exist. Constructions stand out such as the various arches of the Roman Colosseum, the ribs of the vault of the Basilica of Maxentius, with spans of more than 25 meters,[16]​ the vaults of the Baths of Caracalla, and the dome of the Pantheon of Agrippa, about 43 meters in diameter, the one with the greatest span for centuries.[17]​.

After the fall of the Roman Empire, concrete was little used, possibly due to the lack of technical and human resources, the poor quality of lime firing, and the lack or remoteness of volcanic tuffs. There are no signs of its use in large works until the 13th century, when it was used again in the foundations of Salisbury Cathedral, or in the famous Tower of London, in England. During the Renaissance its use was scarce and very insignificant.

In some cities and large structures, built by the Mayans and Aztecs in Mexico or those of Machu Picchu in Peru, cementitious materials were used.[15]​.

In the 20th century, the desire for research was revived. John Smeaton, an engineer from Leeds, was commissioned to build a lighthouse for the third time on the Edystone cliff, on the Cornish coast, using stones joined with a calcined lime mortar to form a monolithic construction that would withstand the constant action of the waves and the humid winds; It was completed in 1759 and the foundation still remains.

19th century: Portland cement and reinforced concrete

Joseph Aspdin and James Parker patented Portland Cement in 1824, obtained from clayey limestone and coal calcined at high temperatures—named for its dark greenish-gray color, very similar to Portland stone. In 1845, Isaac Johnson obtained the prototype of modern cement made from a mixture of limestone and clay calcined at high temperature, until the formation of clinker; the industrialization process and the introduction of rotary kilns") led to its use for a wide variety of applications, towards the end of the 19th century.[18]​.

Concrete or concrete, due to its stone characteristics, supports compression forces well, but cracks with other types of stress (flexion, traction, torsion, shear); The inclusion of metal rods that supported these efforts led to the optimization of its characteristics and its widespread use in multiple engineering and architectural works.

The invention of reinforced concrete is usually attributed to the builder William Wilkinson"), who applied for a patent in 1854 for a system that included iron reinforcement for "the improvement of the construction of houses, warehouses and other fire-resistant buildings." The Frenchman Joseph Monier patented several methods in the 1860s, but it was François Hennebique who devised a convincing system of reinforced concrete, patented in 1892, which he used in the construction of a spinning factory in Tourcoing, Lille, in 1895.[19] Hennebique and his contemporaries based the design of their patents on experimental results, through load tests; the first theoretical contributions were made by prestigious German researchers, such as Wilhelm Ritter, who developed the theory of the "Ritter-Mörsch Lattice" in 1899. Fundamental theoretical studies were developed in the 20th century.

20th century: rise of the concrete industry

At the beginning of the century, the rapid growth of the cement industry arose, due to several factors: the experiments of the French chemists Louis Vicat and Le Chatelier and the German Michaélis, who managed to produce cement of homogeneous quality; the invention of the rotary kiln" for calcination and the tubular mill"); and the methods of transporting fresh concrete devised by Juergen Hinrich Magens, which he patented between 1903 and 1907. With these advances, Portland cement could be produced in large quantities and used advantageously in the construction industry.[15]​.

In 1901, Robert Maillart designed a 38-meter-span arch bridge over the Inn River in Switzerland, built with reinforced concrete box girders; between 1904 and 1906, he designed the Tavanasa bridge over the Rhine River, with a 51-meter span "Light (engineering)"), the largest in Switzerland. Claude A.P. In 1906, Turner built the Bovex building in Minneapolis (United States), with the first fungiform pillars (with large capitals).

In the 1920s, Le Corbusier demanded in Vers une Architecture a logical, functional and constructive production, stripped of the rhetoric of the past; In his design for Casa Domino, from 1914, the structure is made up of pillars and reinforced concrete slabs, allowing completely open facades and the free distribution of interior spaces.[20]​.

The hangars of Orly (Paris), designed by Freyssinet between 1921 and 1923, with 60 meters of span, 9 of arrow and 300 of length, are built with parabolic sheets of reinforced concrete, eliminating the functional division between walls and roof. In 1929, Frank Lloyd Wright built the first concrete skyscraper.

In the 1960s, concrete reinforced with fibers appeared, incorporated at the time of mixing, giving the concrete isotropy and increasing its qualities against bending, traction, impact, cracking, etc. During the 1970s, additives made it possible to obtain high-strength concretes, from 120 to more than 200 MPa; The incorporation of monomers generates concrete that is almost unassailable by chemical agents or indestructible by freeze-thaw cycles, providing multiple improvements in various properties of the concrete.

The great progress in the scientific study of the behavior of reinforced concrete and technological advances made possible the construction of taller skyscrapers, longer span bridges, wide decks and immense dams. Its use will be irreplaceable in public buildings that must accommodate crowds: stadiums, theaters, cinemas, etc. Many nations and cities will compete to erect the largest, or most beautiful, building as a symbol of their progress, which will normally be built of reinforced concrete.

The tallest buildings in the world have concrete and steel structures, such as the Petronas Towers in Kuala Lumpur, Malaysia (452 ​​meters, 1998), the Taipei 101 building in Taiwan (509 meters, 2004), or the Burj Khalifa in the city of Dubai (818 meters, 2009), in the 21st century.

21st century: environmental culture

The use of recycled materials as ingredients in concrete has gained popularity due to increasingly strict environmental legislation, as well as the progressive awareness of society. The most commonly used recycled ingredients are fly ash, a byproduct of coal-fired thermoelectric plants. The environmental impact of the cement industry is significant, but by using these new materials, the reduction of quarries and landfills (garbage) is possible, since they act as substitutes for cement), and reduce the amount necessary to obtain good concrete. Since one of the harmful effects on the environment is that the production of cement generates large volumes of carbon dioxide, cement replacement technology plays an important role in efforts to reduce carbon dioxide emissions. It is usually include certain catalysts in the mixtures that allow them to be 'self-cleaned', such as titanium dioxide.

It is also used to confine radioactive waste. Among them, the most important is that of the nuclear reactor that collapsed at the Chernobyl plant, which was covered in concrete to prevent radioactive leaks.

El concreto u hormigón es el material resultante de unir áridos con la pasta que se obtiene al añadir agua a un conglomerante.[21]​ El conglomerante puede ser cualquiera, pero cuando nos referimos al concreto, generalmente es un cemento artificial, y entre estos últimos, el más importante y habitual es el cemento portland.[21]​ Los áridos proceden de la desintegración o trituración, natural o artificial de rocas y, según la naturaleza de las mismas, reciben el nombre de áridos silíceos "Óxido de silicio (IV)"), calizos, graníticos, etc. El árido cuyo tamaño sea superior a 5 mm se llama árido grueso o grava, mientras que el inferior a 5 mm se llama árido fino "Arena (concreto)") o arena.[22]​ El tamaño de la grava influye en las propiedades mecánicas del concreto.

La pasta formada por cemento y agua es la que confiere al hormigón su fraguado y endurecimiento, mientras que el árido es un material inerte sin participación directa en el fraguado y endurecimiento del hormigón.[22]​ El cemento se hidrata en contacto con el agua, iniciándose diversas reacciones químicas de hidratación que lo convierten en una pasta maleable con buenas propiedades adherentes, que en el transcurso de unas horas, derivan en el fraguado y endurecimiento progresivo de la mezcla, obteniéndose un material de consistencia pétrea.

Una característica importante del hormigón es poder adoptar formas distintas, a voluntad del proyectista. Al colocarse en obra es una masa plástica que permite rellenar un molde, previamente construido con una forma establecida, que recibe el nombre de encofrado.[21]​.

Mechanical characteristics

The main structural characteristic of concrete is to resist compression forces very well. However, both its tensile and shear resistance are relatively low, which is why it should be used in situations where tensile or shear stresses are very low. To determine the resistance, mechanical tests (breakage tests) are prepared on concrete specimens.

To overcome this inconvenience, the concrete is "reinforced" by introducing steel bars, known as reinforced concrete, or reinforced concrete, allowing the shear and tensile forces to be supported with the steel bars. It is also common to have steel bars reinforcing areas or elements that are fundamentally compressed, as is the case of pillars. Attempts to compensate for the deficiencies of concrete in tension and shear led to the development of a new construction technique at the beginning of the 20th century, that of reinforced concrete. Thus, by introducing high-resistance wires stretched into the concrete before setting, it is compressed when setting, with which the tractions that would arise to resist external actions become decompressions of the previously compressed parts, resulting in very advantageous in many cases. For prestressing, steels with very high elastic limit are used, since the phenomenon called creep would nullify the advantages of prestressing. Subsequently, the convenience of introducing tensions in the steel deliberately and prior to the setting of the concrete of the structural piece was investigated, developing the techniques of "prestressed concrete" and "post-tensioned concrete").

Additives make it possible to obtain highly resistant concrete; The inclusion of monomers and additions for concrete provide multiple improvements in the properties of concrete.

When a reinforced concrete element is designed, the dimensions, type of concrete, quantity, quality, additives, additions and arrangement of the steel that must be provided are established based on the forces that each element must resist. A rational design, adequate dosage, mixing, placement, consolidation, finishing and curing, make concrete an ideal material to be used in construction, as it is resistant, durable, non-combustible, almost waterproof, and requires little maintenance. As it can be easily molded into a wide variety of shapes and acquire various textures and colors, it is used in a multitude of applications.

The main physical characteristics of concrete, in approximate values, are:.

Setting and hardening

The concrete paste is formed by mixing artificial cement and water and must completely absorb the aggregates. The main quality of this paste is that it sets and hardens progressively, both in air and under water.[23]​.

The setting and hardening process is the result of chemical hydration reactions between the cement components. The initial hydration phase is called setting and is characterized by the passage of the paste from the fluid state to the solid state. This is easily observed by simply pressing a finger on the surface of the concrete. Subsequently, the hydration reactions continue, reaching all the constituents of the cement, which cause the hardening of the mass and is characterized by a progressive development of mechanical resistance.[23]​.

Setting and hardening are nothing more than two conventionally separated states; In reality, there is only a single continuous hydration process.[23]​.

In Portland cement, the most common used in concrete, the first component to react is tricalcium aluminate with a rapid and short duration (up to 7-28 days). Then the tricalcium silicate, with an important initial contribution and continuous for quite some time. Next is the dicalcium silicate with a weak and very important initial contribution after 28 days.[23]​.

The physical phenomenon of hardening does not have defined phases. The cement is in powder form and its particles or grains are progressively hydrated, initially by contact of water with the surface of the grains, forming some crystalline compounds and a large part of microcrystalline compounds comparable to colloids that form a film on the surface of the grain. From then on, hardening continues to be dominated by these colloidal structures that surround the cement grains and through which hydration progresses to the core of the grain.[23]​.

The fact that the speed with which the mixed cement loses its fluidity and hardens can be regulated makes it a very useful product in construction. A rapid reaction of hydration and hardening would make it difficult to transport and facilitate easy installation by filling all the gaps in the formwork. A slow reaction would significantly postpone the development of mechanical resistance. In cement factories it is achieved by controlling the amount of gypsum added to the cement clinker. In the concrete plant, where the cement and water paste is mixed with the aggregates, products that regulate the setting time can also be added.[23]​.

Under normal conditions, normal concrete or Portland concrete begins to set between 30 and 45 minutes after it has rested in the molds and finishes setting after about 10 or 12 hours. Then the hardening begins, which takes a rapid pace in the first days until reaching the first month, and then increases more slowly until reaching the year where it practically stabilizes.[24]​ The following table shows the evolution of the compressive strength of a concrete taking the 28-day resistance as a unit, with figures being indicative:[25]​.

Endurance

In the previous design of the elements, the characteristic resistance (f) of the concrete is that which is adopted in all calculations as its compressive resistance, and assuming that the concrete to be executed will resist that value, the measurements of all the structural elements are sized.[26]​.

The design characteristic resistance (f) therefore establishes the lower limit, and it must be met that each batch of concrete placed has that resistance as a minimum. In practice, statistical resistance tests are carried out on the concrete that is placed and 95% of them must be greater than f, considering that with the current level of concrete technology, a defective fraction of 5% is perfectly acceptable.

The compressive strength of concrete is obtained in compression fracture tests of normalized cylindrical "Template (mechanical)" specimens carried out at 28 days of age and manufactured with the same batches put into work.[26]​ The Spanish Instruction (EHE) recommends using the following series of characteristic compressive strengths at 28 days (measured in Newton/mm²): 20; 25; 30, 35; 40; 45 and 50.[26]​ Therefore, concrete manufacturing plants usually supply concrete that guarantees these resistances.

Consistency of fresh concrete

Consistency is the greater or lesser ease that fresh concrete has to deform and consequently to occupy all the gaps in the mold or formwork. Different factors influence it, especially the amount of mixing water, but also the maximum size of the aggregate, the shape of the aggregate and its granulometry.[27]​.

The consistency is set before installation, analyzing which is the most appropriate for placement according to the compaction means available. This is a fundamental parameter in fresh concrete.

Among the tests that exist to determine consistency, the most used is the Abrams cone. It consists of filling a 30 cm high truncated conical mold with fresh concrete. The loss of height that occurs when the mold is removed is the measure that defines consistency.[27]​.

Concretes are classified by their consistency as dry, plastic, soft and fluid as indicated in the following table:[28]​.

Durability

The durability of concrete is defined in the Spanish EHE Instruction as the ability to behave satisfactorily against aggressive physical and chemical actions throughout the useful life of the structure, also protecting the reinforcement and metal elements embedded within it.[29]​.

Therefore, not only must we consider the effects caused by the loads and stresses, but also the physical and chemical conditions to which it is exposed. For this reason, the type of environment in which the structure will be found is considered and which may affect the corrosion of the reinforcement, aggressive chemical environments, areas affected by freeze-thaw cycles, etc.[29]​.

To guarantee the durability of the concrete and the protection of the reinforcement against corrosion, it is important to make concrete with reduced permeability, making a mixture with a low water/cement ratio, ideal compaction, an adequate cement weight and sufficient hydration of the cement, adding curing water to complete it. In this way, there are as few pores as possible and a poorly communicated internal capillary network, thus reducing attacks on the concrete.[29]​.

In cases of presence of sulfates in the soil or seawater, special cements must be used. To prevent corrosion of reinforcement, it is necessary to take care of the minimum coating thereof.[29]​.

In the Spanish Instruction (EHE), published in 1998, concrete is classified according to the following format, making it mandatory to refer to it in this way in plans and other project documents, as well as in manufacturing and installation:[30]​.

T: it will be called HM when it is mass concrete, HA when it is reinforced concrete and HP when it is prestressed concrete.

R: characteristic resistance of concrete expressed in N/mm².

C: initial letter of the type of consistency: S Dry, P plastic, B Soft, F Fluid and L Liquid.

TM: maximum size of the aggregate expressed in millimeters.

A: designation of the environment to which the concrete will be exposed.

Cement

Cements are products that, when mixed with water, set and harden, forming new compounds resulting from hydration reactions that are stable both in air and submerged in water.[32]​.

There are several types of cements. The properties of each of them are closely associated with the chemical composition of their initial components, which is expressed in the form of their oxides, and depending on which ones they are, they will form different resulting compounds in the hydration reactions.[32]​.

Each type of cement is indicated for certain uses; Environmental conditions also determine the type and class of cement, affecting the durability of concrete. The types and names of cements and their components are standardized and subject to strict conditions. The Spanish standard establishes the following types: common cements, those resistant to sulfates, those resistant to seawater, those with low heat of hydration, white cements, those for special uses and those made of calcium aluminate. Common cements are the most important group and among them Portland is the usual one. In Spain, only cements legally marketed in the European Union can be used and are subject to the provisions of specific laws.[33]​.

In addition to the type of cement, the second factor that determines the quality of the cement is its class or 28-day compressive strength. This is determined in a standardized mortar and expresses the minimum resistance, which must always be exceeded in the manufacture of cement. It is not the same, nor should the resistance of cement be confused with that of concrete, since that of cement corresponds to standardized components and that of concrete will depend on each and every one of its components. But if the concrete is well dosed, the greater the resistance of the cement, the greater the resistance of the concrete.[32]​ The Spanish standard establishes the following resistance classes:[33]​.

Cement is in powder form and the fineness of its grinding is decisive in its binding properties, decisively influencing the speed of the chemical reactions of its setting and first hardening. When mixed with water, the cement grains are hydrated only to a depth of 0.01 mm, so if the grains were very thick, the hydration performance would be small as an inert core would remain inside. However, excessive fineness causes high shrinkage and heat of hydration. Furthermore, since resistance increases with fineness, a compromise solution must be reached; the cement must be finely ground, but not excessively.[32]​.

The storage of bulk cement is carried out in sealed silos that do not allow contamination of the cement and must be protected from humidity. For cements supplied in bags, storage must be carried out in covered, ventilated premises, protected from rain and sun.[33]​ Prolonged storage can cause the hydration of the finer particles due to weathering, losing their hydraulic value and resulting in a delay in setting and a decrease in resistance.[34]​.

Portland cement is obtained by calcining artificially prepared mixtures of limestone and clay at about 1500 °C. The resulting product, called clinker, is ground by adding an appropriate amount of setting regulator, which is usually natural gypsum stone.[35]​.

The average chemical composition of a portland, according to Calleja, is made up of 62.5% CaO (combined lime), 21% SiO (silica), 6.5% AlO (alumina), 2.5% FeO (iron) and other minorities. These four components are the main ones of cement, lime is basic and the other three are acidic. These components are not free in the cement, but combined forming silicates, aluminates and calcium ferrites, which are its hydraulic components or potential components. A medium type Portland cement clinker contains:[35]​.

The two main hydration reactions that cause the setting and hardening process are:

Tricalcium silicate is the active compound par excellence of cement as it develops a high initial strength and a high heat of hydration. It sets slowly and hardens quite quickly. In fast-hardening and high-resistance cements it appears in a higher proportion than usual.[35]​.

Dicalcium silicate is what develops long-term resistance in cement, it is slow in setting and hardening. Its chemical stability is greater than that of tricalcium silicate, which is why sulfate-resistant cements have a high content of dicalcium silicate.[35]​.

Tricalcium aluminate is the compound that governs setting and short-term resistance. Its chemical stability is good against seawater, but very weak against sulfates. In order to stop the rapid reaction of tricalcium aluminate with water and regulate the setting time of the cement, gypsum stone is added to the clinker.[35]​.

Tetracalcium aluminatoferrite") does not participate in the mechanical resistance, its presence is necessary due to the contribution of iron fluxes in the manufacture of clinker.[35]​.

In Spain there are so-called "Portland cements with active additions" that, in addition to the main components of clinker and gypsum stone, contain one of these additional components up to 35% of the weight of the cement: steel slag "Slag (metallurgy)"), silica fume, natural pozzolana, calcined natural pozzolana, siliceous fly ash, calcareous fly ash, calcined shale or limestone.[33]​.

High initial strength cements, those resistant to sulfates, those with low heat of hydration or white ones are usually special Portland cements and for them some of the four basic components of clinker are limited or enhanced.[36]​.

Steel cement is obtained by joint grinding of Portland clinker and setting regulator in a proportion of 5-64% with steel slag in a proportion of 36-95%.[33] It constitutes the family of cold cements. Slag is obtained by suddenly cooling the molten gangue from steelmaking processes in water; In this cooling the slag vitrifies and becomes hydraulically active due to its combined lime content. The slag on its own sets and hardens slowly, so Portland clinker is added to accelerate it.[36]​.

Pozzolanic cement is a mixture of Portland clinker and setting regulator in a proportion of 45-89% with pozzolana in a proportion of 11-55%.[33] Natural pozzolana has volcanic origin and although it does not have binding properties, it contains silica and alumina capable of fixing lime in the presence of water, forming compounds with hydraulic properties. Artificial pozzolana has analogous properties and is found in fly ash, diatomaceous earth or active clays.[36]​.

Aluminous cement is obtained by melting limestone and bauxite. The main constituent of this cement is monocalcium aluminate.[36]​.

Arid

Aggregates must have at least the same resistance and durability required for concrete. Soft limestone, feldspars, gypsum, pyrites or friable or porous rocks should not be used. For durability in aggressive environments, siliceous aggregates, those from the crushing of volcanic rocks or healthy, dense limestones, will be better.[37]​.

The aggregate that has the greatest responsibility in the whole is sand. According to Jiménez Montoya, it is not possible to make good concrete without good sand. The best sands are river sands, which are normally pure quartz, which ensures their resistance and durability.[37]​.

With natural rolled aggregates, the concrete is more workable and requires less mixing water than crushed aggregates, also having the guarantee that they are hard and clean stones. Crushed aggregates from crushing, having more fracture faces, cost more to put into use, but they lock better and this is reflected in greater resistance.[37]​.

If the rolled aggregates are contaminated or mixed with clay, it is essential to wash them to remove the jacket that surrounds the grains and that would reduce their adhesion to the concrete paste. Likewise, crushing aggregates are usually surrounded by crushing dust, which increases the fines in the concrete, requires more mixing water and will give lower resistance, which is why they are usually washed.[37]​.

The aggregates used in concrete are obtained by mixing three or four groups of different sizes to achieve an optimal granulometry. Three factors intervene in an adequate granulometry: the maximum size of the aggregate, the compactness and the content of fine grains. The larger the maximum size of the aggregate, the lower the cement and water needs will be, but the maximum size is limited by the minimum dimensions of the element to be built or by the separation between reinforcement, since these gaps must be filled by concrete and, therefore, by larger aggregates. In a mixture of aggregates, a high compactness is one that leaves few voids; It is achieved with mixtures low in sand and a large proportion of coarse aggregates, requiring little mixing water; Its great difficulty is to compact the concrete, but if sufficient means are available to do so, the result is very resistant concrete. Regarding the content of fine grains, these make the mixture more workable but require more mixing water and cement. In each case a compromise formula must be found taking into account the different factors. The Fuller and Bolomey parabolas give two families of granulometric curves widely used to obtain adequate dosages of aggregates.[37]​.

Water

The mixing water intervenes in the hydration reactions of the cement. The amount must be strictly necessary, since the excess that does not intervene in the hydration of the cement will evaporate and create voids in the concrete, reducing its resistance. It can be estimated that each liter of excess mixing water means canceling two kilos of cement in the mixture. However, an excessive reduction of water would cause a dry mixture, unwieldy and very difficult to place on site. Therefore, it is very important to properly set the amount of water.[38]​.

The characteristics of water for concrete must be evaluated so that it does not produce adverse reactions in the mixture, which is why physical-chemical analyzes must be carried out to guarantee its quality. In practice, a simple indicator is the potability of the water, with this we can determine if the water is suitable for use in the mixture or not.

During the setting and first hardening of the concrete, curing water is added to prevent desiccation and improve the hydration of the cement.[38]​.

Both the water intended for kneading and that intended for curing must be suitable to fulfill their function. It is very important that the curing water be suitable as it can more negatively affect the chemical reactions when the concrete is hardening. Normally, suitable water usually coincides with drinking water and a series of parameters that must be met are standardized. Thus, the regulations limit the pH, the content of sulfates, chlorine ion and carbohydrates.[38]​.

When a mass is excessively fluid or very dry, there is a danger of the segregation phenomenon occurring (separation of concrete into its components: aggregates, cement and water). It usually occurs when concrete is poured with material drops greater than 2 meters.[28]​.

Other minority components

The basic components of concrete are cement, water and aggregates; Other minor components that can be incorporated are: additions, additives, fibers, fillers and pigments.

Additives and additions can be used as components of concrete, provided that through the appropriate tests, it is justified that the added substance in the proportions and conditions provided produces the desired effect without excessively disturbing the remaining characteristics of the concrete or representing a danger to the durability of the concrete or to the corrosion of the reinforcement.[39]​.

Additions are inorganic, pozzolanic or latent hydraulic materials that, finely ground, can be added to concrete at the time of its manufacture, in order to improve some of its properties or give it special properties. The EHE only includes the use of fly ash and silica fume, determining its limitations. It is composed of limestone crushed into very small pieces like powder, and other materials such as HQR chemicals (herqiros) among others.

Additives are substances or products that are incorporated into concrete, before or during mixing, producing the modification of some of its characteristics, its usual properties or its behavior. The EHE establishes a proportion of no more than 5% of the weight of the cement and other conditions.

Regulations

In the 19th century, the resistance of reinforced concrete structural elements was calculated experimentally. Navier, at the beginning of the 19th century, raised the need to know and establish the limits to which structures behaved elastically, without permanent deformations, in order to obtain reliable physical-mathematical models and coherent formulas. Subsequently, given the complexity of concrete behavior, it was required to use methods based on the calculation of probabilities to achieve more realistic results. During the first half of the 20th century, structural elements were calculated using the allowable stress method.

During the 1960s, the theory of structural safety began to be developed with respect to Limit States, establishing maximum values ​​in the deflections and in the cracking of structural elements, limiting the risks.

The concept of Limit State had its rise in the 1970s, as a set of requirements that a structural element had to satisfy to be considered suitable. The regulations focused on two types: Service limit states and Solicitation limit states.

The regulations of the 1970s, in order to simplify the complex calculations of probabilities, established Safety coefficients, depending on the quality of the materials, the control of the execution of the work and the difficulty of the project. The Load increase coefficients or actions, and the Resistance decrease coefficients of the material components were introduced.[40]​.

In the middle of the century the regulations had dozens of pages, in the century they have hundreds. The introduction of computer programs makes very complex, fast calculations and more precise solutions possible. The regulations place special emphasis on ultimate states of service (cracking, deformations) behavior (construction details) and durability (coatings, qualities), limiting the experimental resolution with multiple conditions. Thus, Eurocode 1 establishes usual and accidental situations (such as earthquakes), which imply Partial safety coefficients for the most varied stresses and resistances. Some specific regulations by geographic area are EHE (Spain), Eurocode 2 (Europe), ASCE/SEI (United States).

Calculation and project

Before constructing any concrete element, the loads to which it will be subjected must be calculated and, based on them, the dimensions of the elements and quality of concrete, the arrangement and quantity of reinforcement in them will be determined.

The calculation of a concrete structure consists of several stages. First, a series of simplifications are made to the real structure, transforming it into an ideal calculation structure. Afterwards, the loads that the structure will support are determined, considering at each point the combination of loads that produces the most unfavorable effect. Finally, each of the sections is sized so that it can withstand the most unfavorable stresses.

Once the structure has been calculated, the project is drawn up, which is the set of documents that is used to carry out the work and details the elements to be built. The calculations carried out are included in the project. It also includes the plans showing the dimensions of the elements to be executed, the classification of the concretes planned and the resistant characteristics of the steels to be used.

Manufacturing

It is very important to achieve the optimal mixture in the precise proportions of aggregates of different sizes, cement and water. There is no optimal mixture that works for all cases.[41]​ To establish the appropriate dosage in each case, the mechanical resistance, factors associated with manufacturing and installation, as well as the type of environment to which it will be subjected must be taken into account.[42]​.

There are many methods for pre-measuring concrete, but they are only indicative. The final proportions of each of the components are usually established through laboratory tests, making corrections to what was obtained in the theoretical methods.[43]​.

The basic aspects that must be determined are briefly noted:

Once the most appropriate dosage has been determined, in the concrete plant the components, water in volume, must be measured, while cement and aggregates are measured in weight.[45]​.

The materials are mixed in a concrete mixer or mixer to achieve a homogeneous mixture of all the components. The aggregate must be well enveloped by the cement paste. To achieve this homogeneity, first half of the water is poured, then the cement and sand simultaneously, then the coarse aggregate and finally the rest of the water.[45]​.

For transportation to the place of use, procedures that do not vary the quality of the material must be used, normally concrete mixer trucks. The time elapsed should not be more than an hour and a half since it was mixed.[46]​ If upon reaching where the concrete is to be placed, it has begun to set, it should be discarded.[45]​.

Putting into work

The reinforcements must be clean and fastened to the formwork and to each other so that they maintain the planned position without moving during the pouring and compaction of the concrete. To do this, shims or spacers are placed in sufficient numbers to maintain the rigidity of the assembly.[47]​.

The distances between the various reinforcing bars must maintain a minimum separation that is standardized to allow correct placement of the concrete between the bars so that there are no gaps or gaps during the compaction of the concrete.[47]​.

Likewise, the free space between the steel bars and the formwork, called covering, must maintain a minimum separation, also standardized, that allows this space to be filled with concrete. This space is controlled by means of separators that are placed between the reinforcement and the formwork.[47]​.

The formwork must contain and support the fresh concrete during its hardening, maintaining the desired shape without deforming. They are usually made of wood or metal and are required to be rigid, resistant, watertight and clean. During assembly, they must be well secured so that no movements occur during the subsequent consolidation of the concrete.[48]​.

Before reusing a formwork, it must be cleaned well with wire brushes, removing any remains of mortar that may have adhered to the surface. To facilitate stripping, release products are usually applied to the formwork; These must be free of substances harmful to the concrete.[48]​.

The pouring of fresh concrete inside the formwork must be carried out avoiding segregation of the mixture. To do this, you must avoid pouring it from a great height, up to a maximum of two meters of free fall, and you must not move the mass horizontally.[49]​.

It is placed in layers or horizontal layers of reduced thickness to allow good compaction (up to 40 cm in mass concrete and 60 cm in reinforced concrete). The different layers or layers are consolidated successively, each layer locking with the previous one with the compaction medium used and without the previous layer having begun to set.[49]​.

To achieve a compact concrete, eliminating its voids and to obtain a complete closure of the mass, there are several consolidation systems. Chopping with a bar, which is carried out by introducing it successively, requires concrete of soft and fluid consistency and is carried out in works of little resistance. Compaction by repeated pounding with a tamper is used in layers 15 or 20 cm thick and with a lot of horizontal surface. Compaction by vibration is common in resistant concretes and is appropriate in dry consistencies.[49]​.

The most used vibrator is the needle vibrator, a metal cylinder with a diameter of 35 to 125 mm whose frequency varies between 3,000 and 12,000 cycles per minute. The needle is placed vertically in the mass of fresh concrete, introduced into each layer until the tip penetrates the previous layer and taking care not to touch the reinforcement as the vibration could separate the mass of concrete from the reinforcement. Through vibration, the air contained in the uncompacted concrete is reduced, which is estimated to be in the order of 15 to 20% to 2-3% after vibrating.[49]​.

World cement production was more than 2.5 billion tons in 2007. Estimating a cement dosage between 250 and 300 kg of cement per cubic meter of concrete, this means that 8,000 to 10,000 million cubic meters could be produced, which is equivalent to 1.5 cubic meters of concrete per person. No construction material has been used in such quantities and in the future there does not seem to be another construction material that can compete with concrete in volume magnitude.[50]​.

This massive production of concrete means that most of the structures and buildings in the world are partly made of this material. Respect for the environment, concrete is 100% recyclable and has a "CO sink effect", that is, it has the ability to absorb carbon dioxide from the atmosphere, thus reducing the amount of it in the air, which makes it a star resource in terms of sustainability. The quality of being a thermal insulator allows global energy consumption to be reduced as a result of temperature peaks, minimizing the energy cost of buildings, as well as collaborating in the reduction of greenhouse gases involved in energy production. Its high resistance gives buildings and bridges a lot of strength against fire or natural phenomena such as earthquakes, improving the service of structures and infrastructures and their level of social security, which provides them with prolonged durability, allowing their conservation with few maintenance costs and, therefore, their economic viability.[52]​.

The German philosopher Anselm Jappe criticizes the massive use of concrete in his 2020 book "Concrete. Weapon of mass construction of capitalism". It reviews its problems of deterioration, the excessive extension of its use, due to its low price, in all sectors of engineering and architecture, the abandonment of other traditional materials, the loss of professions and knowledge related to masonry. He begins his book with the chapter "A bridge collapses" about the collapse of the Morandi Bridge in 2018 in Genoa.[53]​ He mentions, among others, what he considers the disaster of the planned city of Chandigarh, in India, due to the deterioration of the concrete, designed by Le Corbusier, whom he considers a disastrous architect.[54][55]​.