# concrete

Cross section through concrete
Installation of ready-mixed concrete using a concrete pump
coated reinforced concrete (left), already set concrete in the finished state (right)
Flight through a stack of µCT images from a piece of concrete found on Montpellier beach . The calcium-based fillers (mussel and snail shells) have dissolved through long contact with salt water, leaving behind air pockets.
Concrete residential building in Winterthur, built in the 1930s

Beton [ beˈtõ ], [ beˈtɔŋ ] ( Austrian and partly Bavarian [ beˈtoːn ]; Swiss and alem. 1st syllable emphasized [ ˈbetɔ̃ ]), from the synonymous French. Wort béton , is a building material that is mixed as a dispersion with the addition of liquid from a binder and aggregates . The hardened concrete is also called artificial stone in some contexts .

Normal concrete contains cement as a binding agent and aggregate (previously aggregate ) as an aggregate. The added water (previously mixing water ) initiates the chemical setting process, i.e. H. the hardening one. In order to influence the workability and other properties of the concrete, concrete additives and concrete admixtures are added to the mixture . Most of the water is chemically bound. The mixture must therefore only be completely dried after it has hardened.

Fresh concrete can be viewed as a two-component system consisting of liquid cement paste and solid aggregate. Cement paste hardens to cement stone . This forms the matrix that surrounds the aggregate.

Today, concrete is mainly used as a composite material in combination with tensile reinforcement . The connection with reinforcing steel or prestressing steel results in reinforced concrete or prestressed concrete . More recent developments are fiber concrete with the addition of steel, plastic or glass fibers, as well as textile concrete that contains knitted fabrics made of alkali-resistant AR glass or carbon fibers .

## Basic properties and uses

Normal concrete usually has a compressive strength of at least 20 Newtons per square millimeter (N / mm²). Concrete with lower strength is used for the production of blinding layers , backfills, as well as in gardening and landscaping . High-performance concrete achieves strengths of over 150 N / mm².

Unreinforced concrete, on the other hand, can only absorb low tensile stresses without cracking , since its tensile strength is only around a tenth of its compressive strength. Tensile stresses are therefore usually absorbed by inserted rods or mats made of reinforcing steel , which have a tensile strength of over 400 N / mm². This combination has been shown to be beneficial for several reasons:

• Concrete and steel have a similar coefficient of thermal expansion , so that no temperature- related stresses occur in the composite material,
• the basic pH value of the concrete prevents corrosion of the steel ,
• In the event of a fire, concrete prevents the rapid temperature-related loss of strength of unprotected steel.

Typical areas of application for reinforced concrete:

Unreinforced concrete is used for heavyweight walls , curved gravity dams and other compact, massive components that are mainly subjected to pressure. Larger tensile stresses must either be structurally avoided, or breakage of the material must not pose a risk. This is the case, for example, with smaller prefabricated elements such as blocks for masonry construction or ( exposed aggregate concrete ) slabs in horticulture. Due to its low cost, any formability and a comparatively high density of around 2400 kg / m³, concrete is also used for counterweights on cranes and for breakwaters .

It is important to note the shrinkage of the component volume when it dries out or as a result of chemical processes. The degree of shrinkage depends on the composition of the starting material. A certain amount of creep occurs in all loaded components and describes the increasing deformation under load over time.

## Distinguishing features

Concrete can be differentiated based on various characteristics. Differentiations according to are common

• the dry bulk density in lightweight concrete , normal concrete and heavy concrete ,
• the strength, whereby the compressive strength plays the most important role,
• the place of manufacture in construction site or ready-mixed concrete,
• the intended use in, for example, waterproof concrete , underwater concrete ,
• the consistency in classes from stiff to (very) flowable,
• the type of compaction in vibrated concrete, stamped concrete, rolled concrete, flowing concrete, poured concrete, shotcrete, ...
• the type of aggregate in sand concrete, gravel concrete, gravel concrete, ...
• The hardening state in the fresh concrete that can still be processed , the green concrete that has already been installed and compacted , the young concrete that has already started to harden and finally the hardened hardened concrete ,
• the requirements for quality assurance in recipe concrete (production class R according to ÖNORM 4200 or class BI according to DIN 1045) and concrete according to the suitability test (production class E or class B II according to DIN).

Just like concrete, mortar is a mixture of a binding agent , aggregate and additives or additives. The difference lies in the size of the aggregate, which in the case of mortar may not be more than 4 mm in diameter. There is an overlap with spray plaster and masonry mortar , which in special cases can contain a maximum grain size of up to 16 mm, as well as with screed , which is usually mixed with 8 mm grain size.

## history

### Prehistory and Antiquity

Inside the dome of the Pantheon in Rome

Permanent lime mortar as a binding agent has already been proven on 10,000 year old building remains in today's Turkey . The Egyptians used quick lime when building the pyramids .

In the second half of the 3rd century BC, a concrete mixture of cement and brick splinters was developed in Carthage or Campania . This was first used towards the end of the Second Punic War in the construction of residential buildings in Rome. From this concrete mixture, the Romans developed the opus caementitium (opus = work, building; caementitium = aggregate , quarry stone), from whose name the word cement is derived. This building material, also known as Roman concrete or lime concrete, consisted of burnt lime, water and sand, the mortar (mortar), mixed with brick powder and volcanic ash , and was characterized by high compressive strength. Among other things, the aqueducts and the dome of the Pantheon in Rome , which has a diameter of 43 meters and is preserved to this day, were manufactured with it.

A major improvement developed by the Romans was the use of inert aggregates, which consisted mainly of remnants of burnt brick material and which have the property of not cracking when the temperature changes. This can still be observed today in places in North Africa (e.g. Leptis Magna , Cyrene ), where there are large screed areas that were built around 200-300 AD and which are still today despite large temperature differences between day and night are completely free of cracks.

### Modern times

The word concrete is taken from the synonymous French béton , this from the old French betun (mortar, cement), derived from the Latin bitumen (muddy sand, earth resin, mountain tar , putty). Bernard de Bélidor describes the production and use of concrete in his standard work "Architecture hydraulique" (Vol. 2, Paris 1753). The word then also appears in the German translation "Architectura hydraulica" (vol. 2, Augsburg 1769).

The development of concrete in modern times began in 1755 with the Englishman John Smeaton . In search of a water-resistant mortar, he carried out tests with burnt limes and clays and found that a certain amount of clay is necessary for self-hardening (hydraulic) lime.

Three inventions finally introduced modern concrete construction:

At first the concrete was not reinforced, but used as stamped concrete, similar to the pissé building. The oldest and also preserved building using this technology is the Villa Lebrun in Marssac-sur-Tarn , which the civil engineer François Martin Lebrun built for his brother.

In the middle of the 19th century, the first residential buildings made of concrete were built in Germany, such as the trainman's houses of the Upper Swabian Railway, some tenement houses in Berlin's Victoriastadt and the Villa Merkel .

A major leap in development was the invention of reinforced concrete by Joseph Monier (patent: 1867), which made it possible to manufacture components subject to tension, such as slabs and beams . Going back to Joseph Monier, reinforcing steel or reinforcing steel is still occasionally referred to as reinforcing iron .

In contemporary art , concrete is also used for monuments and sculptures ("artificial stone").

## Climate and environmental impact

Concrete production is responsible for around 6 to 9% of all man-made CO 2 emissions. There are two main reasons for this: the burning of the cement required for the production of concrete is very energy-intensive, but the greater part of the released carbon dioxide dissolves from the limestone as geogenic CO 2 during the burning process . 2.8 billion tons of cement, which contains an average of around 60% CaO, are produced worldwide. The release of the carbon dioxide bound in the lime results in emissions of at least two billion tons of CO 2 or 6% of the global annual CO 2 emissions, even with optimal process management. In Switzerland it is even 9% of all man-made emissions.

Sand is one of the most widely used building materials in the world, as it is one of the main components of concrete alongside water, gravel and cement. The worldwide mining of sand for the construction industry and especially concrete production is already leading to a noticeable shortage of the raw material sand. However, at least 95 percent of the world's sand, especially desert sand, is unsuitable for concrete production because the grains are ground too round and smooth. Concrete made from this would then not withstand any major acting forces.

In 2018, however, a process was patented that allows the use of desert sand and fine sand: For this purpose, the sand is rotated in a high-speed mixer at 1500 revolutions per minute , whereby a kind of stone meal with broken grain is produced (mixing at 50 revolutions per minute has been common up to now ). The stone meal is then processed into granules with mineral binders . Such a granulate creates a particularly resilient concrete that also requires 40% less cement. The use of desert sand is not worthwhile in Europe (since the transport costs exceed the material value from approx. 50 km), but in Germany alone hundreds of thousands of tons of previously unused fine sand are produced every year. The first two plants in Saudi Arabia and Egypt should be commissioned as early as spring 2020, from then on the benefits of the process could be assessed. Currently (2019) the Institute for Applied Building Research (IAB) in Weimar is testing the building material. If successful, the German Institute for Building Technology could approve concrete of this type for use in Germany on the basis of a certified test report from the Institute for Applied Building Research .

In 2020, researchers developed a concrete-like material (“living building material”, LBM) that does not release any carbon dioxide (CO 2 ) during production . Instead, the greenhouse gas is even bound. The material is based on a mixture of sand and gelatine, in which bacteria (genus: Synechococcus ) mineralize the greenhouse gas by means of photosynthesis in the form of calcium carbonate (CaCO 3 ). The material is just as stable as ordinary mortar (strength: ∼3.5 MPa , this corresponds to the minimum strength of Portland cement base). The researchers do not see the material as a complete replacement for cement, but rather possible uses, for example in structures with low loads such as paving, facades and temporary civil and military structures. Interestingly, the metabolic activity of the microorganisms could be controlled by adjusting the temperature and the humidity. In the study, 9-14% of the microorganisms were still living in the solid material after 30 days at 50% relative humidity.

## Fresh concrete

Concrete that has not yet hardened is called fresh concrete. The cement paste, i.e. the mixture of water, cement and other fine-grained components, has not yet set. This means that the fresh concrete can still be processed, that is, it can be shaped and in some cases flowable . During the setting of the cement paste, the concrete is called young concrete or green concrete . After the cement paste has set, the concrete is called hardened concrete.

### Ingredients and composition

The composition of a concrete is defined in a concrete recipe before industrial production , which is created through empirical values ​​and tests. The composition depends, among other things, on the desired strength class, the environmental conditions to which the later component will be exposed, the desired processability and, if necessary, also on architectural aspects, such as B. the color scheme. Accordingly, depending on the application, cement, water, aggregate, concrete additives and concrete additives are mixed in a certain ratio.

In non-industrial production, mainly in DIY , concrete is mixed using simple rules of thumb. Such a rule of thumb is, for example: 300 kg of cement, 180 l of water and 1890 kg of aggregate result in one cubic meter of concrete that roughly corresponds to strength class C25 / 30. However, the information is not sufficient to reliably estimate the hardened concrete properties. Both the cement and the aggregate can, depending on the product selected, have a significant effect on the strength, which is why such simple recipes cannot be used in the construction of load-bearing structures.

### consistency

Spreading test to check the consistency of very flowable concrete

The consistency of the fresh concrete describes how flowable or stiff the fresh concrete is. It must be selected accordingly in advance so that the concrete can be conveyed, installed and practically completely compacted without significant segregation. The relevant fresh concrete property is the workability. The fresh concrete consistency must be determined before construction begins and must be adhered to during construction.

The standardized consistency ranges from "(very) stiff", through "plastic", "soft" and "very soft" to "(very) flowable". Measured values ​​are linked to the consistency ranges , which can be checked and controlled using standardized, construction site-appropriate methods, such as the spreading test , the setting test and the compaction test. The subsequent admixing of water to the finished fresh concrete, e.g. B. on arrival at the construction site improves the flow properties, but is not permitted according to German regulations, as this has a negative impact on the water-cement ratio (w / c ratio) and consequently the hardened concrete properties. However, superplasticizers may be added to ready- mixed concrete on site in order to improve the workability. The maximum permissible amount is 2 l / m³, which turns plastic concrete into easily flowable concrete.

The installation conditions determine the required consistency. For components with complex geometries or high degrees of reinforcement, a more flowable concrete tends to be necessary. The delivery of fresh concrete also determines the required consistency. If, for example, a concrete is to be conveyed with a concrete pump , the concrete consistency should be at least in the plastic range, i.e. H. Slump class F2, better F3.

### Installation and compaction

Compaction with a bottle shaker

Concrete must be installed as soon as possible after mixing or delivery and compacted using suitable equipment. The air pockets are driven out by the compression, so that a dense concrete structure with few air pores is created. Vibrating, slinging, tamping, poking, spraying and rolling are suitable compaction methods depending on the concrete consistency and the paving method. Nowadays the internal vibrator (also called “bottle” or “immersion vibrator”) is usually used as a compacting device on building construction sites . External vibrators ("formwork vibrators ") can also be used when manufacturing high components or with very tight reinforcement . When paving concrete for streets or hall floors, compaction with the help of vibratory screeds is common. Vibrating tables are used in the precast plant .

During installation, care must be taken to ensure that the concrete does not segregate, i.e. This means that larger grains settle at the bottom and a water or water cement layer forms on the surface. Fresh concrete must therefore not be dropped into the formwork from a great height. The concrete must be guided into the formwork using slides, downpipes or hoses, so that the maximum free fall height does not exceed 1.5 m. In order to be able to compact well afterwards, the concrete must also be installed in layers of a maximum height of 50 cm. Only after one layer has been compacted does the next follow.

Separation, so that an aqueous cement slurry forms on the surface, can also occur if the vibration duration is too long. The separation of water on the concrete surface after installation is also known as "bleeding". The segregation has a particularly detrimental effect on the strength and durability of the concrete. With correct compaction and a suitable consistency, only a thin layer of fine mortar forms on the surface. In the rest of the concrete body, the rock grains are almost evenly distributed.

When placing the fresh concrete, the concrete temperature should be between +5 ° C and +30 ° C, otherwise special measures are required. In winter this can e.g. B. be the heating of the formwork with fans. Cooling of the concrete may be necessary in summer.

### Aftercare

Post-treatment of the fresh concrete is necessary to protect the concrete surface against drying out and thus to ensure a closed, tight and durable concrete surface. For this, there must also be enough water in the areas of the concrete close to the surface for the cement to hydrate. In particular, this must not evaporate from sunlight, frost and / or wind . If the water in the concrete evaporates too quickly, this can lead to damage in the concrete, so-called shrinkage cracks, shortly after the concrete is poured. There are various post-treatment methods to ensure the hardening process.

Depending on the properties of the concrete, the environmental conditions that correspond to the exposure classes and the climatic boundary conditions, the necessary duration of the aftertreatment can be between a day, a week or even more. Basically, it is best to start post-treatment as early as possible after concreting and to carry out these measures for as long as possible in order to obtain components with optimal hardened concrete properties.

#### Post-treatment methods in summer

In summer, evaporation is the biggest problem. So water has to be supplied and / or kept from evaporating, e.g. B. by placing a waterproof cover, using special post-treatment films or by continuously spraying or flooding with water. Furthermore, the rapid drying of the concrete z. B. by leaving it in the formwork , by sealing with plastic sheeting or by applying film-forming aftertreatment agents. By measuring the so-called capillary pressure of the concrete, conclusions can be drawn regarding a sufficient amount of water for hardening. However, such a measurement is more likely to be used in test laboratories.

#### Post-treatment methods in winter

In winter, if there is frost, the fresh concrete must have a minimum temperature of +10 ° C throughout the first three days after concreting. After three days under these conditions, the concrete has hardened to such an extent that freezing water can no longer cause frost splitting. Maintaining the temperature is achieved z. B. by a cover with a heat insulating film or by the cover with a heated construction tent. Here, too, it must be ensured that the concrete has enough water to hydrate the cement.

### Hardening

The cement acts as a binding agent to hold the other components together. The strength of the concrete is created by the exothermic reaction of the crystallization of the clinker components of the cement with absorption of water. Crystal needles grow , which are tightly interlocked. The crystal growth lasts for months, so that the final strength is only achieved long after the concrete has been cast. However, as in DIN 1164 ( strength classes of cement ), it is assumed that the standard strength is reached after 28 days under normal temperature and humidity conditions. In addition to this hydraulic reaction , the so-called pozzolanic reaction also develops with silicate aggregates .

## Properties of hardened concrete

The hardened fresh concrete is called hardened concrete.

### Strength classes

The compressive strength is one of the most important characteristics of the concrete. The DIN 1045-2 (structures made of concrete, reinforced concrete and prestressed concrete) requires an assessment by the examination after 28 days storage in water based on cubes of 15 cm edge length (test cubes) or 30 cm long cylinders with 15 cm diameter in front. The regulations for the geometry and storage of the test pieces are not uniformly regulated worldwide and have also changed in the individual generations of standards. Based on the determined compressive strength, which can vary in the component, the concrete can be assigned to the strength classes. A C12 / 15 has the characteristic cylinder compressive strength of 12 N / mm² and a characteristic cube compressive strength of 15 N / mm². The C in the nomenclature stands for English concrete (German: concrete ). In the course of the harmonization of the European standards, these concrete strength classes are standardized across Europe in the current generation of standards. In the following table the designations according to the old DIN 1045 are given in the last column for information.

Compressive strength classes for normal concrete according to Eurocode 2 and designation according to the old DIN 1045
Over-
wachungs-
class
Strength strength
keits-
class
Cylinder compressive strength (N / mm²) Average
tensile strength
${\ displaystyle f_ {ctm}}$
(N / mm²)
Average
modulus of elasticity (N / mm²)
${\ displaystyle E_ {cm}}$
Designation
according to the old
DIN 1045
characteristic
${\ displaystyle f_ {ck, cyl} / f_ {ck, cube}}$
Average
${\ displaystyle f_ {cm}}$
1 C8 / 10 * 008/10 00- 0,- 0000- B10
C12 / 15 012/15 020th 1.6 27000 B15
C16 / 20 016/20 024 1.9 29000 B0-
C20 / 25 020/25 028 2.2 30000 B25
C25 / 30 025/30 033 2.6 31000 B0-
2 C30 / 37 030/37 038 2.9 33000 B35
C35 / 45 035/45 043 3.2 34000 B45
C40 / 50 040/50 048 3.5 35000 B0-
C45 / 55 045/55 053 3.8 36000 B55
C50 / 60 050/60 058 4.1 37000 B0-
3 C55 / 67 055/67 063 4.2 38000 B65
C60 / 75 060/75 068 4.4 39000 B75
C70 / 85 070/85 078 4.6 41000 B85
C80 / 95 080/95 088 4.8 42000 B95
C90 / 105 090/105 098 5.0 44000 B0-
C100 / 115 100/115 108 5.2 45000 B0-
1. a b c No normative definition of measurands.
2. No corresponding strength class defined in the old DIN 1045.

### Young's modulus, shear modulus and Poisson's ratio

Stress-strain relation of concrete for different strengths

The modulus of elasticity of the concrete depends to a large extent on the concrete aggregates used . To simplify matters, it can be determined in the linear-elastic state of stress (ie a maximum of 40% of the strength) depending on the concrete strength according to the Eurocode using the empirical equation . Thus, the modulus of elasticity for the concrete strength classes from C12 / 15 to C50 / 60 according to Eurocode is between 27,000 N / mm² and 37,000 N / mm². ${\ displaystyle \ textstyle E_ {cm} = 22,000 \ cdot \ left ({\ frac {f_ {cm}} {10}} \ right) ^ {0.3}}$

The Poisson's ratio varies in the range of the use of voltages, depending on the concrete composition, age of concrete and concrete moisture between 0.15 and 0.25. According to the standards, the influence of 0.2 can be taken into account for non-cracked concrete. For cracked concrete, the Poisson's ratio is to be set to zero.

The shear modulus can be approximated as in isotropic materials, are calculated from Young's modulus and Poisson's ratio.

### Bulk density

The density of the concrete depends on the aggregate. In normal concrete, the dry bulk density is between 2000 and 2600 kg / m³. Usually 2400 kg / m³ can be used. Concretes above 2600 kg / m³ are referred to as heavy concrete , below 2000 kg / m³ as lightweight concrete .

Lightweight concrete has porous lightweight aggregates such as expanded clay or pumice . It is normatively divided into the bulk density classes 1.0 - 1.2 - 1.4 - 1.6 - 1.8 - 2.0, which correspond to the bulk densities between 1000 and 2000 kg / m³. Reinforced concrete has a gross density increased by approximately 100 kg / m³.

### Composite zone

Setting behavior of normal-strength concrete, the bond zone is clearly visible

A weak point in the structure of the hydrated concrete is the bond zone between cement stone and aggregate. Due to the accumulation of ettringite and portlandite (CH, calcium hydroxide) on the edges of the aggregate, no solidifying CSH phases can form. This results in reduced strength in this area. By adding pozzolans , the portlandite is converted into CSH phases via the pozzolanic reaction . Pozzolans are highly silicate additives such as microsilica or fly ash . The highly alkaline environment partially dissolves them and initiates a reaction with the calcium hydroxide (CH) to form CSH without additional water absorption:

• 2SiO 2 + 3Ca (OH) 2 → 3CaO + 2SiO 2 + 3H 2 O

or short:

• S + CH → CSH.

This is particularly important in the development and manufacture of high-strength and ultra- high-strength concrete .

### Pores in the concrete

In addition to strength, the porosity of the concrete is an important quality criterion . The various types of pores differ from one another, sometimes greatly, in terms of their formation and effect. Basically, the strength decreases proportionally with increasing capillary, air and compression porosity. A reduction in the modulus of elasticity can also be demonstrated.

A distinction is made between the following types of pores:

• Gel pores (Ø approx. 0.1-10 nm)
The physically bound mixing water, which is called gel water, is stored in gel pores. Since the same proportion of water is always converted into gel water, its formation cannot be avoided.
• Shrink pores (Ø approx. 10 nm)
Since the hydration reaction products have a smaller volume than the starting materials, shrinkage occurs. Shrinkage pores are formed. Their creation cannot be avoided either.
• Capillary pores (Ø 10 nm - 100 µm)
With w / c values> 0.42, water that is not required for hydration remains in the concrete, which dries out over time and leaves capillary pores. These are responsible for transport processes and have a strong influence on the strength and modulus of the material. Your total volume can be controlled by choosing a favorable w / c value.
• Air pores (Ø 1 µm - 1 mm)
The mixing process causes air to enter the cement gel, which forms air pores. They provide an alternative space for freezing water and thus increase the frost resistance of the concrete. Targeted influencing of the proportion of air pores is possible using air-entraining agents .
• Compaction pores (Ø> 1 mm)
Compaction pores are caused by insufficient compaction of the concrete after installation. Due to their size, they can significantly influence the strength of the material. An exposed concrete surfaces compression spores are also undesirable - visual, tactile and because dirt intercalates in the outstanding pores.

### Building physical properties

A water vapor diffusion resistance number between 70 (wet) and 150 (dry) can be assumed for concrete . The thermal conductivity is around 2.1 W / (m · K) for normal concrete, the specific heat capacity is 1000 J / (kg · K). However, both values ​​are heavily dependent on the aggregate. The coefficient of thermal expansion is the reinforced concrete according to standards 10 -5 / K (for example, DIN 1045-1. 2001-07). However, this can vary between 6 and 14 · 10 −6 / K depending on the type of concrete aggregate , the cement content and the moisture content of the concrete . The moisture content is 25 liters of water per cubic meter of concrete at 23 ° C and 50% relative humidity and 40 l / m³ at 80% relative humidity. All these concrete properties are also significantly temperature-dependent and only apply approximately well below 100 ° C.

### Working capacity

The possible energy absorption of a concrete component until failure is called the work capacity. The graph of the stress-strain diagram is also called the working line for concrete . The work capacity is defined as the area under the work line and thus includes all elastic and irreversible deformation components .

## Monitoring classes

To check the relevant properties of fresh and hardened concrete, the concrete is divided into three monitoring classes. This results in the scope and frequency of the tests, which is regulated in DIN 1045-3. Concrete of the monitoring classes 1, 2 and 3 is u. a. to be checked by the company carrying out the work and a recognized monitoring body. The tests in monitoring class 1 are only used for self-monitoring by the company carrying out the work. Monitoring class 2 is used for concretes with increased requirements such as B. WU , prestressed, underwater and radiation protection concrete , etc. applied. Tests are carried out with at least three test specimens every 3rd day of concreting or every 300 m³. In monitoring class 3, the test is carried out at least every concreting day or every 50 m³.

## Types of concrete

A type of concrete is a precisely defined mixture that is repeatedly produced according to a concrete recipe . Suppliers usually have their own varieties that are ordered by customers. In the case of large construction projects, the construction companies often compile their own types of concrete in a list of types in consultation with the client and the supplier. These concretes are then "tailor-made" for a construction site and its special features.

## Types of concrete

All concretes can be differentiated according to their production, their type of installation or their special properties. A concrete does not necessarily belong to only one type . One and the same product is usually assigned to several categories. For example, every concrete is either ready-mixed or construction site concrete. Depending on the properties, these concretes are then e.g. B. Air-entrained concretes, high-strength concretes, etc. The names used for the concretes in use are listed in the list of concretes in use.

## Durability, damage and repair

Concrete column with exposed reinforcement

For permanent concrete structures, the required properties and stability under the planned loads must be constant over the expected service life with normal maintenance costs. For the concrete to be sufficiently durable, the concrete composition ( water- cement ratio and cement content), the strength class, the compaction and the subsequent treatment of the concrete are important.

Concrete is a chemically unstable building material. Various internal and external influences can have a lasting effect on the durability of concrete. Due to the typical use of concrete in conjunction with reinforcement made of steel , further the durability of concrete influencing factors, such as insufficient covering of the reinforcement steel by concrete result. The exposure classes are therefore used to classify the chemical and physical environmental conditions to which the concrete is exposed, from which the requirements for the composition of the concrete to be used and, in the case of reinforced concrete, the minimum concrete cover follow.

The following damage mechanisms can occur:

Surface protection systems , such as paints or the impregnation of concrete surfaces with a water repellent, serve to improve durability and can be applied directly after production or as part of a concrete repair work to extend the service life.

For concrete repair also include all activities where damage (cracks, flaking, etc.) corrected and the original protective properties of the concrete are possible restored or improved. The repair work is carried out by specialized concrete repairers .

In bridge construction , especially on motorway bridges, cathodic corrosion protection (KKS) is carried out using an impressed current anode. For this purpose, an anode grid made of coated titanium is applied to the surface to be protected and about 2 cm to 3 cm is injected with shotcrete . The shotcrete serves as an electrolyte . The current is fed into the reinforcement via a rectifier, thus providing cathodic protection. The measure is continuously checked with an automatic monitoring system.

These two presentations give a good explanation of the KKS:

• General cathodic corrosion protection a-bau.co.at (PDF; 55 KB)
• Cathodic corrosion protection in bridge construction a-bau.co.at (PDF; 2.4 MB)

## Built-in parts

To reduce the weight of concrete parts, so-called displacement bodies are installed. This has the effect that voids are created and less concrete is required. This is often used in panel constructions.

In the past, parts made of polystyrene foam and other foams were used for this purpose , which today are no longer permitted due to the adverse effects of fires. At present, spheres or cube-shaped elements made of polyethylene or polypropylene are used, which can save up to a third of the concrete and consequently its own weight. So are large components such. B. Roof constructions with spans of up to 19 meters possible.

Due to major structural damage in the past, the installation of displacement bodies in bridge constructions is no longer permitted in Germany .

## Other materials known as “concrete”

The term “concrete” is also used in connection with other building materials and is intended to describe their high strength or their composition principle.

### Aerated concrete

Aerated concrete (formerly aerated concrete ) is a mineral material that is produced by chemical foaming of a mortar mixture. The alkaline mortar suspension reacts with the formation of gas with powders of base metals such as. B. aluminum . Aerated concrete contains almost no aggregates. Compared to conventional concrete, aerated concrete has low strength and low thermal conductivity due to its low bulk density .

### Fiber concrete

Fiber concrete is an extension of the artificial building material concrete. Fibers are added to the concrete during production in order to improve the material properties such as tensile , compressive and shear strength and the breaking and cracking behavior. In contrast to concrete, this means that fiber concrete can better absorb tensile forces. This leads, among other things, to the possibility of producing any body shape in a statically load-bearing manner.

### Concrete glass

Concrete glass is a glass block that is arranged horizontally and has high compressive strength.

### Asphalt concrete

Asphalt concrete is a name for a mixture of bitumen and aggregate. The part of the name "concrete" refers to the "concrete principle" of the mixture, ie, as with concrete as a building material, various aggregate sizes are evenly distributed in asphalt concrete and completely encased by binding agent.

### Mineral concrete

Mineral concrete is a name for a highly compacted mineral mixture, usually with the use of a high proportion of broken grain. The grading curve is to be built up according to Fuller's parabola , the optimal water content for compaction is to be set. Segregation must be avoided during installation. Without a binding agent, mineral concrete becomes a highly stable building material that is used, for example, in road surfaces. The common product is the granular gravel base layer with 0 to 32 mm as frost protection material according to ZTV T-StB 95.

### Sulfur concrete

Sulfur concrete is a mixture of quartz sand , limestone or crushed stone to which 15–20% sulfur is added as a binding agent . The sulfur is modified beforehand with dimeric cyclopentadiene and stabilized as plastic sulfur. Compared to concrete, sulfur concrete has a higher compressive and tensile strength as well as early strength and is significantly more corrosion-resistant to acids and salt solutions. The disadvantages are the softening at temperatures above 120 ° C and the flammability. Its use has been restricted in Germany so far. The use of sulfur concrete is important wherever aggressive chemicals and substances harmful to groundwater are used, stored or transferred.

### Synthetic resin concrete

Synthetic resin concrete uses - just like synthetic resin mortar - unsaturated polyester resin , methacrylate resin or epoxy resin as binding agent. Cement is not needed for strength. Here hardeners and accelerators are used in small quantities. The principle, areas of application and processing are typical for concrete. Because it hardens quickly, the concrete is ideal for repairs. Compared to cement-bound concrete, the result is a significantly higher tensile strength and a lower modulus of elasticity.

## Appearance

Concrete, the surface of which is still visible after completion of the building, is generally referred to as exposed concrete . In a narrower sense, exposed concrete describes concrete surfaces with a special design quality.

In the Brutalist architectural style , on the other hand, raw, unrefined concrete was used as a creative medium. The buildings of brutalism are more impressive because of their coarse structure, ie their cubature , than because of the surface quality.

### Architectural concrete

The term architectural concrete is occasionally used by specialized providers in the construction industry to denote exposed concrete that has particularly high design requirements in terms of surface structure and quality.

### Colored concrete

Colored concrete contains pigments that change its color.

## literature

Wiktionary: Concrete  - explanations of meanings, word origins, synonyms, translations
Commons : Concrete  - collection of images, videos and audio files

## Individual evidence

1. a b c G. Stehno: Building materials and building material testing . Springer-Verlag, 2013, p. 93ff. (books.google.de)
2. ^ Frank Kolb: Rome. The history of the city in ancient times. CH Beck. Munich 2002, ISBN 3-406-46988-4 , p. 230.
3. newscenter.berkeley.edu
4. ↑ Word meaning and etymology (French)
5. ^ Ferdinand Werner : François Martin Lebrun and the first concrete house. In: INSITU . Volume 8, No. 1, 2016, pp. 75-88 (79).
6. Sand is becoming scarce from the commonplace commodity to the sought-after raw material.
7. a b Michael Gassmann: Raw material: Two Germans know how to make concrete out of desert sand . January 22, 2019 ( welt.de [accessed October 29, 2019]).
8. Patent DE102017006720 : Building material granulate , method for producing a building material granulate based on mineral grains and its use. Published on June 21, 2018 , inventors: Helmut Rosenlöcher, Dagmar Tretbar.
9. Knapper Sand: Munich startup helps construction industry. August 18, 2019, accessed October 29, 2019 .
10. ^ Daniela Albat: Researchers develop living concrete . In: scinexx | The knowledge magazine . January 16, 2020 ( scinexx.de [accessed April 23, 2020]).
11. Chelsea M. Heveran, Sarah L. Williams, Jishen Qiu, Juliana Artier, Mija H. Hubler: Biomineralization and Successive Regeneration of Engineered Living Building Materials . In: Matter . tape 2 , no. 2 , February 5, 2020, p. 481–494 , doi : 10.1016 / j.matt.2019.11.016 .
12. Placing the concrete. German cement and concrete industry, accessed March 20, 2014 .
13. Compaction of the concrete. German cement and concrete industry, accessed March 20, 2014 .
14. Bleeding of the concrete. InformationsZentrum Beton of the German cement and concrete industry, accessed on March 20, 2014 .
15. Concrete technical data - fresh concrete temperature. (No longer available online.) HeidelbergCement, archived from the original on October 28, 2014 ; accessed on March 20, 2014 .
16. a b c d aftercare. German cement and concrete industry, accessed March 20, 2014 .
17. ^ A b Association of German Cement Works eV (Hrsg.): Zement-Merkblatt Betontechnik B8 3.2011 - Post treatment of concrete . March 2011 ( PDF, approx. 657 kB ).
18. Concrete classes in comparison - old and new names and their assignment. (PDF; 44 kB) (No longer available online.) Ingenieurbüro Suess, archived from the original on July 2013 ; Retrieved July 15, 2013 .
19. a b DIN EN 1992-1-1: Eurocode 2: Dimensioning and construction of reinforced concrete and prestressed concrete structures - Part 1-1: General dimensioning rules and rules for building construction . German version EN 1992-1-1: 2004 + AC: 2010, p. 27 ff.
20. DIN 1045-1 Structures made of concrete, reinforced concrete and prestressed concrete - Part 1: Dimensioning and construction, Paragraphs 3.1.4 to 3.1.5
21. www.vdz-online.de Compendium cement concrete 2-3_Design criteria PDF file
22. Simone Hempel: Script BAUSTOFFE - PART 3, Exercise 3-5, Concrete - Structure, Hydration, Porosity , Institute for Building Materials (TU Dresden), edition of WS 07/08
23. Peter Grübl, Helmut Weigler, Sieghart Karl: Concrete: Types, Production and Properties , John Wiley & Sons, 2002, p. 388.
24. beton.org: Monitoring concrete on construction sites
25. Ralf Steudel, Hans-Joachim Mäusle: Liquid sulfur - a raw material with a complicated composition, In: Chemistry in our time. 14th year, No. 3, 1980, p. 79,
26. Diploma thesis on sulfur concrete
27. ^ Hans-Gustav Olshausen: VDI-Lexikon civil engineering . Springer-Verlag, Berlin / Heidelberg 1991, ISBN 978-3-662-30425-9 , pp. 650 .
 This version was added to the list of articles worth reading on September 2, 2005 .