Reinforced concrete , an artificial building material in reinforced concrete construction, a form of solid construction , is a composite material made up of the two components concrete and reinforcing steel . The bond between the two components is created by gluing the cement binder to the ribs of the round reinforcing steel.
Compared to compressive strength, concrete only has a tensile strength of around 10%. Steel, on the other hand, has a high tensile strength . The load-bearing principle for reinforced concrete as a building material is therefore to reinforce areas of a component subject to tensile stress with steel (e.g. in the case of beams in the field area below), i.e. to reinforce, and to use the compressive strength of the concrete in the other areas (e.g. with bars in the field area above). In the case of components that are heavily stressed by pressure (e.g. supports), the steel (reinforcement) is also used to increase the compressive strength, i.e. it is subjected to pressure.
If the reinforced concrete is also provided with tendons, it is called prestressed concrete .
Significance, application and components
Reinforced concrete is the most important building material in Germany with over 100 million cubic meters built up every year. 12% of German steel production is processed into around 6 million tons of reinforcing steel every year. The use of reinforced concrete instead of unreinforced concrete is necessary if tensile stresses occur in a component that could lead to a sudden failure of the overall load-bearing capacity. Compared to other building materials such as steel, wood or plastic , its use is always useful when no delicate and light load-bearing structures are required. As the use in the construction of bunkers shows, reinforced concrete with sufficient dimensions is also suitable for extreme effects. The non-combustibility and high fire resistance are particularly advantageous . Limits in the use of the building material result from the high dead weight of the concrete, which increases the required amount of reinforcing steel as a dead load and leads to large deformations in slim structures due to the formation of cracks. In these cases, the use of a composite structure or prestressed concrete is more suitable. The prestressed concrete differs from reinforced concrete by a systematic bias (= pre-strain) of the steel inserts, the so-called tendons . In this way, an additional external longitudinal compressive force is applied, as a result of which the tensile stresses are overpressed and the formation of cracks and thus component deformation is greatly reduced.
Typical reinforced concrete components include components subject to bending stress, such as ceilings , beams or floor slabs . But also massive, large-volume components such as bridge piers or retaining walls are usually made with this material.
The dimensioning and manufacture of reinforced concrete was regulated in Germany in DIN 1045; since the introduction of the Eurocodes, the regulations have been standardized throughout Europe in DIN EN 1992 Eurocode 2.
In the middle of the 19th century, concrete components were reinforced with steel inserts for the first time in France. In 1848 Joseph-Louis Lambot built a boat out of iron-reinforced cement mortar, which he patented in 1855. The gardener Joseph Monier had been making planters from cement mortar since 1861 , which he reinforced with an iron mesh so that they would not break easily. In 1867 he received a patent for it . The term reinforcement iron is still used variously today. Older names for reinforced concrete are reinforced concrete (still common today in Russian and Bulgarian) and Monierbeton (the term reinforced concrete was not used until 1940). As early as 1861, François Coignet published principles for the use of reinforced concrete and exhibited beams and tubes made of reinforced concrete at the World Exhibition in Paris in 1867 . As early as 1852, Coignet had built a building with concrete and iron profiles in Saint-Denis . In 1855, the landlord Joseph-Louis Lambot registered a patent for a new wood construction material, which he called "Ferciment". The following can be deduced from his patent specification: “My invention is about a new product which is used to replace wood in shipbuilding and wherever it is at risk from moisture [...]. I give this network (made of wire and rods) a shape that is as close as possible to the object I want to manufacture and then embed it in hydraulic cement or something similar such as bitumen, tar or their mixtures [...] then expanded by Coignet.
In parallel with the French engineers, the American lawyer Thaddeus Hyatt carried out experiments on the use of iron inserts in concrete since 1855. In his basic patent from 1878 he wrote: “[…] Hydraulic cements and concrete are combined with metal bars and rods, so as to form slabs, beams and arches. The tensible strength of the metal is only utilized by the position, in which it is placed in slabs, beams etc. […]. “ Hyatt had recognized the load-bearing effect. The English building contractor William Boutland Wilkinson also received a patent for reinforced concrete in 1854 and used it for ceilings in houses around 1860. Another pioneer of reinforced concrete in the USA was Ernest Leslie Ransome , who built the first skyscraper entirely made of reinforced concrete in the USA in 1903 (Ingalls Building, Cincinnati ), in 1884 received a patent for his helical reinforcing bars for reinforced concrete, and in 1890/91 the first reinforced concrete bridges (small pedestrian underpasses) in the United States in Golden Gate Park in San Francisco and published a book on reinforced concrete structures in 1912.
In Germany, Conrad Freytag and Gustav Adolf Wayss acquired the Monier patents in 1885. In the same year Wayss met the government master builder Matthias Koenen , who was the site manager for the Reichstag building at the time. After eliminating concerns about the risk of corrosion, adhesive strength and different thermal expansion as well as on the basis of tests, Koenen decided to use the new system for walls, ceiling tiles and vaults. His findings prompted him to write a brochure, which Wayss published in 1887 under the title “The Monier System in Its Application to the Entire Building Industry”. However, the building material primarily used in the Reichstag building was masonry bricks, which were used for foundations and pillars as well as for walls and vaulted ceilings.
Another pioneer of reinforced concrete construction was the civil engineer François Hennebique , who also received a patent on reinforced concrete in 1892 and set milestones in both bridge and residential construction, including the invention of the plate beam . The "System Hennebique" licensed by him was u. a. Taken over by Eduard Züblin and Max Pommer , who - like Hennebique himself - erected the first pure reinforced concrete structures in Europe using this method at the end of the 19th century and were not limited to building parts. The first building in Germany to be built in reinforced concrete according to François Hennebique's system is the extension of the Carl Gottlieb Röder music printing company in Leipzig, built by the Eisenbetonbau Max Pommer company in 1898/99 under the architect Max Pommer. Nikolai Beleljubski and Artur Loleit were pioneers of reinforced concrete construction in Russia .
A little later, Emil Mörsch published a first scientifically based presentation of the mode of action of reinforced concrete. It was published in 1902. Emil Mörsch was one of the first to carry out extensive series of tests. After all, from 1916 to 1948 he was professor of statics of massive structures, arched bridges and reinforced concrete construction at the Technical University of Stuttgart , where he had a decisive influence on the design methods for reinforced concrete. In 1920 the term reinforced concrete was introduced. In 1942 the German Committee for Reinforced Concrete was renamed the German Committee for Reinforced Concrete and accordingly the replacement of DIN 1045 from 1937 provisions of the German Committee for Reinforced Concrete by the provisions of the German Committee for Reinforced Concrete in 1943 .
Monier built his first reinforced concrete bridge in 1875 near Chazelet , which had a span of 16.5 m, and in Switzerland a 37.2 m wide arched bridge based on the Monier system was built in 1890 on the site of the Jura-Cement works in Möriken-Wildegg over a factory canal . In the 1890s, the first bridges with cast iron girders in Europe and the United States were built according to a system by Joseph Melan ; in 1899, the Georgsbrücke in Meiningen was the first in Germany. The Pont Camille-de-Hogues , which was opened in 1900, is considered the world's first larger reinforced concrete bridge. It was designed by Hennebique; the arched bridges Ponte del Risorgimento in 1911 and Langwieser Viaduct in 1914, which were for the first time overcoming spans of 100 m, were also constructed according to his system. In 1942 the Martín Gil Viaduct reached 210 m, in 1964 the Gladesville Bridge 305 m and in 1980 the Krk Bridge reached 390 m. Since 1995, the Wanxian Bridge has had the largest concrete arch at 420 m.
One of the first reinforced concrete structures in Germany was the “ Royal Anatomy ” building in Munich , built from 1905 to 1907 according to plans by the architect Max Littmann . In the USA, the first high -rise building was built in 1902 with the 16-story Ingalls Building in Cincinnati, and the first reinforced concrete factory was built in 1903–1904 with the Packard automobile plant .
The Stuttgart television tower , which opened in 1956, was the first large radio tower in the world to be built using reinforced concrete and has served as a model for numerous other radio and television towers ever since.
Concrete is an artificial rock made of cement , aggregates ( sand and gravel or chippings ), possibly additives and water . This building material is cheaper to produce than metallic building materials (such as steel), depending on its consistency, can be shaped relatively easily and, because of its relatively low price, is particularly suitable for massive, large-volume components when certain boundary conditions, such as e.g. B. special attention should be paid to the heat of hydration or segregation due to bulk heights. An important area of application is also the construction in water (immediate exposure to water through hydraulic hardening possible), whereby soft water or chemical loads have to be considered.
Its mechanical properties are characterized by a relatively high compressive strength and low tensile strength (approximately 10% of the compressive strength).
Reinforcing steel, also known as reinforcing steel, is a special, nowadays ribbed or profiled round steel with a high tensile strength ( = 500 N / mm²). This is built into the formwork of the component and then concreted in. So that the reinforcing bars in the finished concrete part located at the scheduled place and not move during concreting, they are using binding wire with each other to a basket fixed ( zusammengerödelt ). When pouring the concrete, concreting, the reinforcing steel is completely enveloped by the concrete, which creates the bond between the two building materials. In order to ensure a minimum concrete thickness between the steel reinforcement and the outer surface of the concrete part, spacers made of suitable material (plastic, concrete) are installed between the reinforcement and the lower or side formwork and embedded in concrete.
Spacer or support
The concrete must enclose the reinforcing steel for corrosion protection with a certain cover specified in the standards. For this purpose, supports and spacers must be built in. These ensure the distance between the reinforcing steel and the formwork and thus the subsequent concrete surface.
The bond between the concrete and the reinforcing steel is created by the adhesion of the cement binding agent (adhesive bond), by the friction between steel and concrete (friction bond) and by the form fit (shear bond) created as a result of the ribbing of the reinforcing steel. In non-cracked reinforced concrete, the expansion of the two building materials is the same. This state, without any relative shifts between concrete and steel, is also known as a perfect bond.
Unreinforced concrete fails suddenly under tensile stress due to its brittleness without any forewarning crack formation. In comparison to the compressive load, this happens even with low loads, because the tensile strength is low. For this reason, the tensile stressed areas of the concrete are provided with reinforcing steel that is set in concrete. Since the concrete cannot follow the large expansions of the steel when it is pulled, it cracks in the tensile area. In the area of a crack, only the reinforcement steel is then effective. Components subject to tensile or flexural stress can therefore be dimensioned and manufactured in such a way that component failure is announced in advance by intensive crack formation and significant deformations. For a realistic calculation of the deformations, the calculation methods of the structural analysis are extended, for example with the non-linear structural analysis . For components that are subjected to pressure, steel inserts can increase the load-bearing capacity under pressure.
Steel and concrete have the same coefficient of thermal expansion (10 −5 K −1 according to the reinforced concrete standards), which results in approximately the same thermal expansion of the two materials in the event of temperature changes and thus does not cause any significant internal stresses in the reinforced concrete composite.
Durability of reinforced concrete
A prerequisite for the durability of the composite material is the alkaline medium with a pH of 12 to 14 this is caused by the conversion of limestone into calcium hydroxide during the hydration of the concrete and provides with sufficient concrete cover a long-term protection of the reinforcing steel against corrosion safely (see also concrete corrosion ). With a pH value of less than 10, this protection, the so-called passivation , is no longer available. Starting from the concrete surface, moisture and carbonic acid reduce the alkalinity of the concrete and thus the thickness of the passivation layer around the reinforcing steel over time, whereby the so-called carbonation rate decreases. Cracks in the reinforced concrete component can promote this process.
As soon as reinforcement steel corrodes, its volume increases and pressure is built up on the surrounding concrete. This can widen any cracks, which in turn accelerates the corrosion process and ultimately causes the concrete to flake off.
Reinforcing steel can be hot-dip galvanized or coated with epoxy for improved protection against corrosion . The use of stainless steel and GRP reinforcement is also possible. The reinforcement elements mentioned require a building authority approval in Germany . According to the general building inspectorate approval (abZ) for hot-dip galvanized reinforcing steel, it is possible to reduce the concrete cover by up to 10 mm when using hot-dip galvanized reinforcing steel in exposure classes XC1 to XC4.
The German Institute for Structural Engineering keeps a list of reinforced elements approved by the building authorities . Depending on the quality, stainless steel costs around 10 times as much as normal BSt 500 reinforcement steel.
To protect the reinforcing steel from corrosion as a result of carbonation or chloride penetration, cathodic corrosion protection with an impressed current anode, which is controlled by rectifiers with a protective current (actually only one polarization), can be used. This can be used, for example, in bridge construction .
The proof of the durability of reinforced concrete components is based on a period of 50 years.
Cracks in reinforced concrete components are part of the load-bearing behavior and are therefore usually not a defect , provided that the crack widths do not exceed the values defined as permissible in the standards and no crack-free area has been agreed. In principle, cracks can have three causes:
- Direct influences: Tensile stresses resulting from the load (e.g. dead weight, traffic load) of the structure exceed (i.e. in the tensile zone of a structural component cross-section given by the static system, structural component geometry and load) the tensile stress that can be absorbed there by the building material (approx. 10 of the compressive stress that can be absorbed).
- Indirect influences: Concrete has complex properties, during the hardening the concrete volume "shrinks" and under constant load it suffers plastic deformations, it "creeps". A hindrance to the deformation leads to constrained internal forces that activate the reinforcing steel in the tension zones, such as the direct action.
- Internal stresses : When the concrete shrinks, the heat exchange on the concrete surface results in an uneven temperature distribution over the cross section, which leads to tensile stresses on the surface and, if the tensile strength of the concrete is exceeded, cracks.
Cracks are generally (inevitably) permissible in reinforced concrete composite material; depending on the environmental conditions and use of the component, Eurocode 2, for example, provides for a width limitation of 0.1 to 0.4 mm. The Swiss standard SIA 262 limits the stresses in the reinforcement steel to up to 50% of the yield point.
A constructive measure against cracks that are too large is the insertion of sufficient, finely distributed reinforcement (many thin instead of less thick steels), which does not prevent the cracks, but ensures that instead of a few wider cracks, there are correspondingly more, but narrow and therefore less harmful ones Cracks appear. This measure increases the durability of the component and improves the visual impression.
In the case of special components , such as the floor slabs of gas stations , which have to be free of cracks, this is ensured by means of appropriate component geometries and expansion joints or by prestressing. The influence of the reinforcement to ensure freedom from cracks is of secondary importance.
A distinction must be made between the unavoidable structural cracks and surface cracks, which are fundamentally undesirable and often have concrete technological reasons, such as an unfavorable fresh concrete composition (e.g. with excessive hydration heat development ), improper concrete paving and inadequate post-treatment of the fresh concrete surface.
In addition to the reinforcing steel, other construction elements are also set in concrete as planned. These are called built-in parts. They are mostly used to fasten components to the reinforced concrete component, such as steel structures. These include anchor plates and anchor channels . Other built-in parts, such as dowel strips or cable loops, replace a geometrically difficult and complex reinforcing steel reinforcement with a "steel construction" specially developed for the stress on the concrete.
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