concrete
Concrete (from French béton) is a solid building material created by the hardening of a concrete mixture, or a building material created by mixing a binder, filler and water, or other additives and admixtures, which acquires the properties of a solid substance after solidification.
Composition
It consists of a mixture of:
binders e.g. cement, but also lime, asphalt, tar, magnesite (xylolite floors), macromolecular resins (plastic concrete), gypsum (sound insulation partitions) and so on. Concrete with a high cement content is called rich, with a low cement content lean.
fillers e.g. inorganic natural materials such as sand and aggregate, but also artificial ones: cinder, slag, ceramsite and some organic materials, e.g. sawdust, wood shavings, cork, peat, etc. Fillers can have different fractions.
water
ingredients
admixtures
Features
Due to its properties, cement concrete (CB) is among the most used construction materials. It has high compressive strength, but low tensile and shear strength. Compressive strength ranges from 5 to 50 MPa as required. Concrete has the properties of natural stone, durability and strength (3-60 MPa in pressure, about ten times less in tension). To increase tensile strength and load-bearing capacity, it is reinforced with steel rods, wires and nets (reinforced concrete). Its strength and properties change with the composition and processing of the mixture. Strength is also its disadvantage, because the reconstruction of concrete structures is very laborious. The strength depends mainly on the fraction of sand or gravel. Vibrated concrete with a small fraction of sand is the strongest and most expensive. It is easy to shape and allows to create various shapes with the help of formwork within static options. It is resistant to chemicals, high temperatures, is durable and has volume stability. The durability of concrete is great both in water and in air.
Concrete hardens in about 28 days. However, the hardening does not end after that, concrete hardens with the right humidity almost indefinitely (even under water). Steel STN11xxx (structural steels, concrete steel) is mainly used as reinforcement. The connection is made possible by almost the same thermal expansion of steel and concrete.
The disadvantage of concrete is its weight, which depends on the materials used (especially aggregate). For plain concrete, the specific weight ranges from 2,000 to 2,400 kg/m3 and for reinforced concrete from 2,300 to 2,600 kg/m3. With prefabs, the assistance of lifting mechanisms is necessary, which increases construction costs. Thermal conductivity also has a negative effect on the construction and depends on the density of the concrete. The volume of concrete changes during solidification and hardening, while cracks appear, these are removed by expansion joints.
History
The use of concrete did not become widespread until the 20th century. Plain concrete, however, was used in earlier times. The Persians, the Carthaginians, but especially the ancient Romans, used concrete as a building material, which is evidenced by the still-preserved structures such as roads and aqueducts that date from the 1st century AD. Concrete fell out of use for many centuries after the fall of the Roman Empire.
Concrete did not reappear until 1774, when John Smeaton used it to repair the foundations of the lighthouse at Edyston, England. Smeaton added iron slag to the usual combination of quicklime, sand and water. Thus, the first modern concrete was created.[1]
Concrete was not used on a wider scale until 50 years after the introduction of Portland cement production. The patent for its production was obtained in 1824 by the English stonemason Joseph Aspdin from Leeds in Portland County. The origin of the name Portland cement comes from there. Englishman Johnson, who discovered the process of burning limestone and clay, contributed to the introduction of industrial cement production in 1845. Thanks to the production of Portland cement, in the second half of the 19th century, remarkable engineering structures were built from plain concrete, especially in France. The idea of combining concrete with iron elements was arrived at in 1849 by the Frenchman Joseph-Louis Lambot while building a boat, when he was applying cement mortar to a wire mesh. This boat was used for 60 years. Six years later, he was granted a patent for building ships with reinforced concrete. In 1861, the Frenchman Francois Coignet published the first book about concrete and its practical use in construction. The book was called Les betóns aglomérés appliqués l´art de construire and it described the application of reinforced concrete in building structures. His own reinforced concrete house (Paris from 1862) still exists today.[1] Right after that, the American lawyer Hyatt in 1877 published the discovery of the advantage of using reinforced concrete in bending stress and in connection with fire
Concrete processing
Due to high demands, concrete is produced industrially in concrete plants and transported to the destination by concrete mixers. For large constructions (highways, bridges), concrete plants are built close to the construction site. At the destination, the concrete is poured into the pre-prepared formwork, creating a monolith. For more difficult-to-reach places, concrete is also transported using a concrete pump or baskets that are lifted using cranes. The concrete pump is a specially modified truck with a hydraulic arm in lengths of 20 to 70 meters, on which a steel pipe with a diameter of 100 and 125 mm is attached, and with the help of hydraulic pistons it pushes the concrete mixture into the pipe. Smaller quantities of concrete can also be laboriously processed by hand at the point of use.
Cement concrete cannot be processed at low temperatures. Water, which is part of the mixture, freezes and the chemical reaction between the binder and the aggregate does not take place as a result. If the concrete is exposed to high temperatures during hardening (e.g. direct sunlight), it must be treated with water so that cracks do not occur when the admixture water evaporates. On the other hand, if the concrete is exposed to rain, it must be protected from water so that the binder does not wash away.
Types of admixtures
Concrete admixtures are one of the components of concrete. These are fine powder substances that are added to the concrete mixture already during mixing. The volume of admixtures in the fresh concrete mix ranges from 10 to 40% by weight of cement. Admixtures in concrete have the task of improving the technological properties of fresh concrete, as well as the mechanical properties of hardened concrete.
Fly ash
increases the amount of mixing water
increases the long-term strength of concrete
reduces the depth of concrete carbonation
Siliceous substances, flyaways
increases the amount of mixing water
affects the porous structure of concrete (porosity increases absorption)
increases the strength of concrete
increases the resistance of concrete to aggressive environments (suitable for the water management and chemical industry)
Color pigments
they serve to color concrete
Concrete additives
Concrete admixtures are one of the components of concrete. These are chemical compounds that are added to the concrete mixture already during mixing. The volume of additives in the fresh concrete mix ranges from 2-50 g/m3.
Plasticizing additives
they reduce the surface tension of concrete
they improve the workability of the concrete mixture (longer transport distance)
they reduce the required amount of mixing water
increase the compressive strength of the hardened mixture by up to 10%
Stabilizing additives
they reduce water mixing
they reduce the height of the water elevation
they increase the density of the hardened cement, reduce the porosity, prevent rising and absorption of water
Aeration additives
they create a large number of microscopic pores
they provide resistance to repeated freezing and thawing and de-icing salts
Retarding additives
they slow down the solidification and hardening of the concrete mixture
Accelerating additives
they accelerate the solidification and hardening of the concrete mixture
Other ingredients
corrosion inhibitors
biocidal additives
gas-forming additives (for creating aerated concrete)
fulfilling the adhesive function (they increase the cohesion of the new mixture with already hardened concrete)
Types of concrete
Dry concrete is concrete that is in a loose state and can be transported without the help of mixers. Unlike normally processed concrete mix, a small percentage of water is added to dry concrete. It is most often used in the construction of highways as a base concrete (stabilization) under an asphalt carpet. When placing dry concrete, it is poured directly from the truck bed into the paver, which places evenly thick layers. After storage, it is rolled and sprayed with water for better strength.
Plain concrete is concrete without added reinforcement. It is mainly used as a base concrete. Its opposite is reinforced concrete.
Reinforced steel concrete: reinforced concrete
Prestressed concrete is prefabricated reinforced concrete. Such concrete is produced in such a way that the reinforcement is mechanically stretched to the required values in the formwork. Concrete mixture is added to the formwork and the whole mixture is vibrated in the formwork. After solidification, the stretched steel reinforcement is released and prestressed. It has higher compressive strength and is mainly used in the construction of bridges.
Reinforced concrete is concrete with an admixture of steel fibers that increase its compressive strength. It is used for floors with high load (industrial floors) and for reinforced surfaces (parking lots).
Asphalt concrete (AB) is an aggregate of different fractions coated with asphalt. It is used in the construction of covering layers of roads (bitumen covers).
Special species:
Refractory concrete – the refractoriness depends on the aggregate used, when higher quality aggregate is used it ranges up to 800°C and for reinforced concrete up to 600°C.
Fair-faced concrete – is concrete with surface treatment, especially on facades or visible structures.
Lightweight types: are concretes mixed with perlite or granulated polystyrene - EPS. It is mainly used to lighten and insulate the floors of multi-story family houses.
Types: aerated concrete, aerated concrete/drainage concrete (for drainage systems), perlite concrete
The use
It is mainly used in construction for the creation of load-bearing structures, but also in gardening and sculpture. Concrete is also used for paving, formwork blocks, slates, various prefabs such as beams, panels, columns, curbs, retaining walls, gutters, barriers, garages, shafts, cesspools, etc.
Exposed corroded reinforcement in concrete