Reinforced concrete
Reinforced concrete, an artificial building materials in concrete construction is a composite of the two components of concrete and reinforcing steel. A composite of both components is caused by the bonding to the cement binder and the ribbing of the round reinforcing steel.
Concrete compressive strength, in comparison to a tensile strength of only about 10%. On the other hand has a high tensile strength steel. The supporting principle in building materials reinforced concrete, therefore, is to strengthen claimed to train points of a component with steel (eg beams in the field range below), ie, to reinforce and exploit the compressive strength of concrete in the other areas (eg at the field level bar above). Please note that worn on pressure components (eg columns) of steel ( rebar ) is also used to increase the compressive strength, ie subjected to compressive stress.
- 5.1 carbonation
- 5.2 cracks
Meaning, application and components
Reinforced concrete is installed with over 100 million cubic meters in the most important building material in Germany. 12 % of German steel production are processed annually to around 6 million tonnes of reinforcing steel. The use of reinforced concrete instead of the unreinforced concrete is required when tensile stresses occur in a component that could lead to a sudden failure of the total capacity. Compared to other building materials such as steel, wood or plastic, its application is always useful, if not delicate and light support structures are necessary. As the use in the construction of bunkers shows, reinforced concrete is suitable with sufficient dimensions for extreme effects. Advantageously, in particular the non-flammability and high fire resistance. Limitations in the use of the building material resulting from the high weight of the concrete, which as dead load increases the required amount of steel and concrete in thin structures leads to cracking as a result of large deformations. In these cases, the use of a composite structure or prestressed concrete is suitable. The prestressed concrete differs from reinforced concrete by a systematic bias ( = pre-extension ) of the steel reinforcement, the so-called tendons. Thus an additional longitudinal force is applied external pressure, whereby the tensile stress are suppressed and the formation of cracks, therefore, the deformation of the components, is greatly reduced.
Typical reinforced concrete components include bending stressed components such as ceilings, beams or floor slabs. But even massive, large-volume components such as bridge abutments or retaining walls shall be made generally with this material.
History
In the 17th century, the mathematician Jakob I Bernoulli developed the principle of beam theory. He thus created the conditions necessary for the understanding of force development in components subjected to bending stress.
Basis of the development was the invention of the Roman cement in 1798 by the Englishman J. Parker, of the artificial hydraulic lime in 1817 by the Frenchman Louis -Joseph Vicat, the Portland cement by the Englishman Joseph Aspdin in 1824 and the discovery of the importance of sintering 1840 / 1844 by Vicat and Isaac Charles Johnson.
In the middle of the 19th century concrete components were first reinforced with steel inserts in France. Built in 1848 Joseph -Louis Lambot a boat of iron -reinforced cement mortar, which he patented in 1855. Since 1861 the gardener Joseph Monier turned planter in cement mortar ago, he with an iron braided reinforced so they will not so easily shattered. In 1867 he received a patent on it. The concept of reinforcing steel is used even today in various ways. Older names for reinforced concrete reinforced concrete (now also in Russian and Bulgarian usual) and Monierbeton. In 1861 published François Coignet principles for the use of reinforced concrete and put 1867 at the World Exhibition in Paris carriers and tubes of reinforced concrete from. In 1852, Coignet had a building with concrete and iron profiles built in Saint- Denis. The tenant farmers, Joseph -Louis Lambot reported in 1855 a patent for a new wooden building material which he " Ferciment " called. His patent, the following can be found: " My invention has a new product to the object, which is used to replace the wood in shipbuilding and wherever there is moisture at risk, [ ... ]. I give this network ( made of wire and rods ) a form that is adapted to the subject in the best possible extent, I want to make and then embed it in hydraulic cement or similar as bitumen, tar or their mixtures [ ... ]. "This patent was then extended by Coignet.
Parallel to the French engineers led the American lawyer Thaddeus Hyatt in 1855 experiments on the use of iron deposits in concrete by. In its basic patent of 1878 he wrote: " [ ... ] Hydraulic cements and concretes 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 support effect. The English builder William Boutland Wilkinson was already in 1854 a patent for reinforced concrete and used it in 1860 for ceilings in houses.
In Germany, acquired in 1885 Conrad Freytag and Gustav Adolf Wayss the Monierpatente. In the same year Wayss met the government architect Matthias Koenen, the subject line of the then under construction Reichstag building. After clearing out of concerns about the risk of corrosion, adhesion and different thermal expansions and because of attempts to Koenen decided to implement the new system for walls, ceiling panels and vaults. His findings led him to write a brochure that Wayss 1887 " The Monier system in its application to the entire construction industry" published under the title. Nevertheless, the building material is used primarily in the Reichstag building masonry bricks, which were used for foundations and piers as well as also for walls and vaulted ceilings.
Another pioneer of reinforced concrete construction was the engineer François Hennebique, the 1892 also received a patent for reinforced concrete and both the bridge and in the housing set milestones, including the invention of the T-beam. The licensed by him " Hennebique system " was, inter alia, by Eduard Züblin and Max Pommer taken, which - as Hennebique itself - according to this method the end of the 19th century built the first pure reinforced concrete structures in Europe and is not confined to parts of the building.
A little later, Emil Mörsch brought a first scientifically-based representation of the mode of action of reinforced concrete. Which was published in 1902. These led Emil Mörsch by one of the first comprehensive series of tests. He was finally 1916-1948 Professor of Structural Analysis of the massive structures, arched bridges and reinforced concrete structure at the Technical University of Stuttgart, where it has helped to shape the design method for reinforced concrete decisive. In 1920 came the introduction of the concept of reinforced concrete. 1942 was followed by the renaming of the German committee for reinforced concrete in German Committee for Reinforced Concrete and accordingly the replacement of DIN 1045 from 1937 regulations of the German committee for reinforced concrete in 1943 by the provisions of the German Committee for Reinforced Concrete.
Monier Built in 1875 at Chazelet his first reinforced concrete bridge, which had 16.5 m span and in Switzerland created in 1890 on the site of the Jura - Cement - works in Möriken -Wildegg a factory channel a 37.2 m wide spanning arch bridge after the Monier system .. in the 1890s, the first bridges were built with a concrete iron girders for a system of Joseph Melan in Europe and the United States, 1899 with the George bridge in Meiningen, the first in Germany. The 1900 shared Pont Camille de Hogues is known worldwide as the first major reinforced concrete bridge. It was designed by Hennebique; the first spans of 100 m -conquering arch bridges Ponte del Risorgimento 1911 and Langwieser Viaduct 1914 were also constructed according to his system. 1942 reached the Martín -Gil Viaduct 210 m, 1964, the Gladesville Bridge 305 m and 1980 the Krk bridge 390 m. Since 1995, the Wanxian Bridge with 420 m has the largest concrete arch.
Among the first Stahlbetonhochbauten in Germany include the building of the " Royal Anatomy " in Munich, built in 1905-1907 by the architect Max Littmann. In the U.S. in 1902 was the 16-story Ingalls Building in Cincinnati the first skyscraper and 1903-1904 with the Packard Automobile Plant, the first factory building in reinforced concrete.
The opened in 1956 Stuttgart TV tower was built as the first large radio tower in the world in reinforced concrete and has since served as a model for numerous other radio and television towers.
Components
Concrete
Concrete is an artificial stone made of cement, concrete aggregate (sand and gravel or crushed stone ), optionally additives and water. This building material is to be produced cheaper than metallic materials (eg steel), depending on the consistency particularly suitable for solid, large-volume components when certain boundary conditions such as the heat of hydration or segregation special attention by dumping heights relatively easy to shape and because of its relatively low price be. An important application is the construction in the water (immediate water impact by hydraulic curing possible), in which case soft water or chemical exposure are of particular importance.
Its mechanical properties are characterized by a relatively high compressive strength and a low tensile strength ( about 10% of the compressive strength ).
Reinforcing steel
Rebar, also known as reinforcing steel, is a special, nowadays ribbed or profiled steel rod with a high tensile strength ( = 500 N/mm2). It is installed in the formwork of the component and then concreted. Thus, the reinforcing bars are located in the finished concrete part at the scheduled place and not move during concreting, they are using binding wire fixed to one another to a basket ( zusammengerödelt ). When pouring the concrete, the concrete is poured, the rebar is completely enveloped by the concrete, causing the bond between the two materials. To ensure a minimum thickness of concrete between the steel reinforcement and the outer surface of the concrete part, be incorporated between the reinforcement and the lower or side formwork spacers of a suitable material (plastic, concrete ), and concrete.
Spacer or support
The concrete must the reinforcing steel for corrosion protection with a certain level specified in the standards, which include coverage. These supports and spacers are installed. They represent the distance between the reinforcing steel and the formwork and the concrete surface so that subsequent safe.
Structural behavior
The bond between the concrete and the reinforcing steel caused by the adhesion of the binder cement ( bonding ), by the friction between steel and concrete ( composite friction ) and by the form-fitting generated due to the ribbing of the reinforcing steel ( shear bond ). In uncracked concrete, the expansion of the two materials are the same. This condition, without relative movement between concrete and steel, is also referred to as a perfect composite.
Unreinforced concrete fails in tension (eg flexural ) because of its brittleness without announcing cracking abruptly. This is in comparison to the compressive stress even at low load, because the tensile strength is small. For this reason, the tensile-stressed areas of the concrete are provided with reinforcing steel, which is set in concrete. Since the concrete on train can not follow the high strain of the steel, he rips in the tensile zone. In the area of crack only the reinforcing steel is effective. Train or biegezugbeanspruchte components can therefore be dimensioned and manufactured to the component failure preceded by announcing an intensive cracking and significant deformations. For the realistic calculation of the deformations, the calculation method of structural analysis to be extended, such as with the non-linear frame analysis. For components which are subjected to pressure, steel inserts can increase the carrying capacity in compression.
Are steel and concrete, an equal thermal expansion coefficient ( 10-5 K-1 on the reinforced concrete standards), as a result of temperature changes in approximately equal thermal expansion of the two materials, and thus causes no significant residual stresses in the composite material of reinforced concrete.
Durability of reinforced concrete
Carbonation
A prerequisite for the durability of the composite material in the alkaline medium with a pH of 12 to 14 This is caused by the conversion of limestone into calcium hydroxide during hydration of the concrete and provides with adequate concrete cover long-term protection of the reinforcing steel from corrosion -proof ( see also concrete corrosion). With a pH of less than 10 this protection is the so-called passivation, no longer exists. Starting from the concrete surface, the alkalinity of the concrete and thus the thickness of the passivation layer is reduced by the reinforcing steel with time due to moisture and carbon dioxide, the so-called carbonation decreases. Cracks in reinforced concrete component can facilitate this process. When rebar corrodes, its volume increases and builds up a pressure on the surrounding concrete. This can widen any cracks, which in turn speeds up the corrosion process, and finally has a spalling of the concrete result.
For improved corrosion protection reinforcing steel can be coated with epoxy or hot-dip galvanized. The use of stainless steel and FRP reinforcement is possible. The reinforcement elements mentioned are subject to technical approval in Germany. A list of officially approved reinforcement elements leads the German Institute for Building Technology. Stainless steel costs depending on the quality about 10 times of normal BSt 500 reinforcing steel.
To protect against corrosion of the reinforcing steel due to carbonation or chloride penetration can also be a Cathodic protection with impressed current anode (actually only one polarization ) can be controlled via a rectifier with a protection current, to be applied. This can for example come in bridge for use.
The proof of the durability of reinforced concrete structures based on a period of 50 years.
Cracks
Cracks in concrete structures are part of the structural behavior and therefore usually no shortage, if the crack widths do not exceed the defined as acceptable by the standards or values no crack-free surface has been agreed. Cracks can have three causes in principle:
Cracks in composite reinforced concrete in most cases ( inevitably ) permitted depending on environmental conditions and usage of the component sees the Euro Code 2, for example, limiting the width to 0.1 to 0.4 mm in front. The Swiss standard SIA 262 limits the stresses in the reinforcing steel at up to 50% of the yield strength.
A constructive measure against excessive crack widths is the insertion of a sufficient finely divided ( many thin rather than fewer thick steel) reinforcement, while not preventing the cracks, but ensures that instead of a few, wide cracks correspondingly more but narrow and thus safer crack arise. This measure increases the stability of the device and improves the optical impression.
For special components, such as floor slabs of gas stations, which must be performed free of cracks, this is ensured by corresponding component geometries and expansion joints or by biasing. The influence of the reinforcement to ensure freedom from cracks is of minor importance.
Of the unavoidable structural cracks surface cracks can be distinguished, which are fundamentally undesirable and often have concrete technological reasons, such as an unfavorable fresh concrete composition ( eg with excessive hydration heat development ), a not proper concrete placement and insufficient treatment of the fresh concrete surface.
Mounting parts
In addition to the reinforcing steel, other components are encased in concrete schedule. These are referred to as built-in components. They serve mostly the mounting of components on the reinforced concrete component, such as steel structures. These include anchor plates and anchor rails. Other fixtures, such as dowel bars or rope loops, replace a geometrically complex and demanding concrete steel reinforcement through a special for the stress of the concrete developed " steel construction ".