(prestressed reinforced concrete) is a building material designed to overcome the inability of concrete to resist significant tensile stresses. Structures made of prestressed reinforced concrete, in comparison with unstressed concrete, have significantly lower deflections and increased crack resistance, having the same strength, which makes it possible to bridge larger spans with an equal section of the element.
In the manufacture of reinforced concrete, reinforcement is laid from steel with high tensile strength, then the steel is stretched with a special device and the concrete mixture is laid. After setting, the pretensioning force of the loosened steel wire or rope is transferred to the surrounding concrete so that it is compressed. This creation of compressive stresses makes it possible to partially or completely eliminate the tensile stresses from the operating load.
Reinforcement tensioning methods:
Grants Pass, a prestressed reinforced concrete bridge in the Botanical Gardens, Oregon, USA
By the type of technology, the device is subdivided into:
- tension on stops (before placing concrete in the formwork);
- tension on concrete (after concrete laying and curing).
More often the second method is used in the construction of bridges with large spans, where one span is made in several stages (seizures). The material made of steel (cable or reinforcement) is placed in a mold for concreting into ducts (corrugated thin-walled metal or plastic pipe). After the manufacture of the monolithic structure, the cable (reinforcement) is pulled with special mechanisms (jacks) to a certain extent. After that, a liquid cement (concrete) solution is pumped into the duct with a cable (reinforcement). This ensures a strong connection of the bridge span segments.
While the tension on the stops implies only the straight-line shape of the tensioned reinforcement, an important distinguishing feature of the tension on concrete is the ability to tension the reinforcement of complex shape, which increases the efficiency of the reinforcement. For example, in bridges, reinforcing elements are lifted inside load-bearing reinforced concrete beams in sections above "bulls" supports, which allows them to more efficiently use their tension to prevent deflection.
Eugene Freycinet (France) and Viktor Vasilyevich Mikhailov (Russia) were at the origin of the creation of prestressed reinforced concrete.
Prestressed reinforced concrete is the main material of interfloor ceilings of high-rise buildings and protective containment of nuclear reactors, as well as columns and walls of buildings in areas of increased
Modern methods of frame construction use the technology of prestressing reinforced concrete structures. Prestressed structures- reinforced concrete structures, the stress in which is artificially created during manufacture, by tensioning a part or all of the working reinforcement (compression of a part, or all of the concrete).
Compression of concrete in prestressed structures by a predetermined value is carried out by tensioning the reinforcing elements, which, after their fixation and release of the tensioning devices, tend to return to their original state. At the same time, slipping of reinforcement in concrete is excluded by their mutual natural adhesion, or without adhesion of reinforcement to concrete - by special artificial anchoring of the ends of reinforcement in concrete.
Crack resistance of prestressed structures is 2 - 3 times higher than the crack resistance of reinforced concrete structures without prestressing. This is due to the fact that the preliminary compression of concrete by reinforcement significantly exceeds the ultimate deformation of concrete tension.
Prestressed concrete allows to reduce the consumption of scarce steel in construction by up to 50% on average. The preliminary compression of the stretched zones of concrete significantly delays the moment of cracking in the stretched zones of the elements, limits the width of their opening and increases the rigidity of the elements, practically without affecting their strength.
Advantages of prestressing technology for reinforced concrete
Prestressed structures turn out to be economical for buildings and structures with spans, loads and working conditions in which the use of reinforced concrete structures without prestressing is technically impossible, or causes excessive consumption of concrete and steel to provide the required rigidity and bearing capacity of structures.
The prestressing, which increases the rigidity and resistance of structures to the formation of cracks, increases their endurance when working under the influence of repeated loads. This is due to a decrease in the stress drop in reinforcement and concrete caused by a change in the magnitude of the external load. Correctly designed prestressed structures and buildings are safer to operate and more reliable, especially in seismic areas. With an increase in the percentage of reinforcement, the seismic resistance of prestressed structures in many cases increases. This is due to the fact that due to the use of stronger and lighter materials, the sections of prestressed structures in most cases turn out to be smaller in comparison with reinforced concrete structures without prestressing the same bearing capacity, and, therefore, more flexible and lightweight.
In the majority of developed foreign countries, prestressed reinforced concrete is used in ever-increasing volumes for the manufacture of floor structures and coatings for buildings for various purposes, a significant part of products used in engineering structures and in transport construction; the production of elements of external architectural design of buildings appeared.
World experience in using pre-voltage technology
In the world, monolithic reinforced concrete is predominantly prestressed. First of all, large-span structures, residential buildings, dams, energy complexes, TV towers and much more are erected in this way. TV towers made of monolithic prestressed reinforced concrete look especially impressive, becoming attractions of many countries and cities. The Toronto TV Tower is the world's tallest free-standing reinforced concrete structure. Its height is 555 m.
The trefoil tower cross-section has proven to be very successful for placement of prestressing reinforcement and concreting in slip formwork. The wind overturning moment for which this tower is designed is almost half a million ton meters with a dead weight of the tower's ground part of just over 60 thousand tons.
In Germany and Japan, egg-shaped reservoirs for treatment facilities are widely built from monolithic prestressed reinforced concrete. To date, such reservoirs have been built with a total capacity of more than 1.2 million cubic meters. Separate structures of this type have a capacity from 1 to 12 thousand cubic meters.
Abroad, monolithic slabs of increased span with reinforcement tension on concrete are becoming more and more widely used. In the USA alone, more than 10 million cubic meters of such structures are erected annually. A significant amount of such slabs is being constructed in Canada.
Recently, prestressing reinforcement in monolithic structures is increasingly used without adhesion to concrete, i.e. the channels are not injected, and the reinforcement is either protected from corrosion with special protective covers, or treated with anti-corrosion compounds. Thus, bridges, large-span buildings, high-rise buildings and other similar objects are erected.
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In addition to traditional construction purposes, monolithic prestressed reinforced concrete has found wide application in reactor vessels and containment shells for nuclear power plants. The total capacity of nuclear power plants in the world exceeds 150 million kW, of which the capacity of plants, reactor vessels and containment shells of which are built of monolithic prestressed reinforced concrete, is almost 40 million kW. Containment shells for nuclear power plants have become mandatory. It was the absence of such a shell that caused the Chernobyl disaster.
Offshore oil platforms are a prime example of the building capabilities of prestressed reinforced concrete. More than two dozen such grandiose structures have been erected in the world.
The Troll platform, built in 1995 in Norway, has a total height of 472 m, which is one and a half times higher than the Eiffel Tower. The platform is installed on a sea section with a depth of more than 300 m and is designed to withstand the impact of a hurricane storm with a wave height of 31.5 m. 250 thousand cubic meters were spent on its manufacture. high-strength concrete, 100 thousand tons of ordinary steel and 11 thousand tons of prestressing reinforcing steel. The estimated platform service life is 70 years.
Bridge construction has traditionally been an extensive area of application for prestressed reinforced concrete. In the USA, for example, more than 500 thousand reinforced concrete bridges with various spans have been built. Recently, more than two dozen cable-stayed bridges with a length of 600-700 m with central spans from 192 to 400 m have been built there. Extra-curricular bridges are built from pre-stressed reinforced concrete, which are built according to individual projects. Bridges with a span of up to 50 m are erected in a prefabricated version of reinforced concrete prestressed beams.
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| Bridge "Normandy" |
Advances in prestressed concrete bridge construction are also available in other countries. In Australia, in Brisbane, a girder bridge with a central span of 260 m was built, the largest among bridges of this type. The Barrnos de Luna cable-stayed bridge in Spain has a span of 440, the Anasis in Canada - 465, the Hong Kong bridge - 475 m. The arch bridge in South Africa has the largest span - 272 m. The world record for cable-stayed bridges belongs to the Normandy bridge. , where the span is 864 m. The Vasco de Gama bridge in Lisbon, built for the EXPO-98 World Exhibition, is not much inferior to it. The total length of this bridge is over 18 km. Its main supporting structures - pylons and spans - are made of concrete with a compressive strength of more than 60 MPa. The guaranteed service life of the bridge is 120 years according to the criterion of concrete durability (in Russia, in recent years, large-span bridges are more often built of steel).
Monolithic reinforced concrete prestressing technology in Russia
In Russia, these products account for more than a third of the total production of prefabricated elements. Abroad, non-formwork molding of slab structures on long stands is very widespread. There, the usual practice is the production of slabs with a span of up to 17 m, a section height of 40 cm for a load of up to 500 kgf / m2. In Finland, hollow-core reinforced concrete slabs under the same load are produced with a cross-sectional height of even 50 cm with a span of up to 21 m, that is, the use of prestressing allows the production of prefabricated elements of a qualitatively different level. The tension of rope reinforcement at such stands, as a rule, is group tension with a jack capacity of 300-600 tons. Today, various systems without formwork molding have been developed on long stands Spyrol, Spankrit, Spandek, Max Roth, Partek and others, characterized by high productivity, used reinforcement, technological requirements for concrete, the shape of the cross-section of the panels and other parameters. On stands up to 250 m long, a slab is made at a speed of up to 4 m / min, 6 slabs can be concreted in height in a package. The width of the slabs reaches 2.4 m, with a maximum span of 21 m. Spencrit slabs alone are used in the USA over 15 million m2 annually.
At one time, long stands for form-less molding using the Max Roth technology appeared in Russia as well. However, this technology has not gained further acceptance. In the structural systems of buildings that are widely used in our country, the connection of elements is carried out through embedded parts. In slabs made on long stands, as a rule, by extrusion, the possibilities for placing embedded parts are limited. However, for precast-monolithic buildings, slabs without embedded parts can find the widest distribution, which is the case abroad, especially in the Scandinavian countries and the USA.
Later, the Partek lines appeared in Russia (at the ZhBK-17 plant in Moscow, St. Petersburg, Barnaul), which indicates the emergence of demand for such plates. The improvement of the structural systems of buildings will certainly give an impetus to the development of technology for the production of panel products.
The protracted Russian stagnation in the field of application of prestressed reinforced concrete is partly due to the fact that we have not received proper study and application of prestressed structures with tension of reinforcement on concrete, including in building conditions.
"Enerprom" begins to develop this direction and offers a number of equipment of its own design for the implementation of this technology.
The essence of reinforced concrete. Its advantages and disadvantages
Reinforced concrete is a complex building material consisting of concrete and steel fittings, deforming together up to the destruction of the structure.
In the above definition, keywords are highlighted that reflect the essence of the material. To identify the role of each of the highlighted concepts, let us consider in more detail the essence of each of them.
Concrete is an artificial stone which, like any other stone material, has a sufficiently high resistance to compression, and its tensile strength is 10 - 20 times less.
Steel reinforcement has a fairly high resistance to both compression and tension.
Combining these two materials in one allows you to rationally use the advantages of each of them.
For example concrete beams, consider how the strength of concrete is used in a bending element (Fig. 1a). When the beam is bent above the neutral layer, compressive stresses arise, and the lower zone is stretched. The maximum stresses in the sections will be in the extreme upper and lower fibers of the section As soon as, when loading the beam, the stresses in the tensile zone reach the ultimate tensile strength R bt, the edge fiber will break, i.e. the first crack will appear. This will be followed by brittle destruction, i.e. fracture of the beam. Stresses in the compressed zone of concrete s bc at the moment of destruction will be only 1/10 ¸ 1/15 of the ultimate strength of concrete in compression R b, i.e. the strength of the concrete in the compressed zone will be used by 10% or less.
For example reinforced concrete beams with reinforcement consider how the strength of concrete and reinforcement is used here. The first cracks in the tensile zone of concrete will appear at practically the same load as in the concrete beam. But, unlike a concrete beam, the appearance of a crack does not lead to the destruction of a reinforced concrete beam. After the appearance of cracks, the tensile force in the section with a crack will be absorbed by the reinforcement, and the beam will be able to absorb the increasing load. The destruction of a reinforced concrete beam will occur only when the stresses in the reinforcement reach the yield point, and the stresses in the compressed zone reach the ultimate strength of concrete in compression. At the same time, at first, when the yield point s tek is reached in the reinforcement, the beam begins to bend intensively due to the development of plastic deformations in the reinforcement. This process continues until the concrete in the compressed zone is crushed when it reaches its ultimate compressive strength R b. Since the level of stresses in concrete and reinforcement in this state is much higher than the value R bt, then this means that it must be caused by a greater load ( N in fig. 1-b). Output- the expediency of reinforced concrete is that the tensile forces are perceived by the reinforcement, and the compressive ones - by the concrete. Hence, main purpose of fittings in reinforced concrete consists in the fact that it is she who must perceive tension due to the insignificant tensile strength of concrete. By means of reinforcement, the bearing capacity of a bent element, in comparison with a concrete one, can be increased by more than 20 times.
Joint deformation of concrete and reinforcement installed in it is ensured by adhesion forces that arise during the hardening of the concrete mixture. In this case, adhesion is formed due to several factors, namely: firstly, due to the adhesion (gluing) of the cement paste to the reinforcement (it is obvious that the share of this component of adhesion is small); secondly, due to the compression of the reinforcement with concrete due to its shrinkage during hardening; thirdly, due to the mechanical engagement of concrete on the periodic (grooved) surface of the reinforcement. Naturally, for reinforcement with a periodic profile, this component of adhesion is the most significant, therefore, the adhesion of reinforcement of a periodic profile with concrete is several times higher than that for reinforcement with a smooth surface.
The very existence of reinforced concrete and its good durability turned out to be possible due to the beneficial combination of some important physical and mechanical properties of concrete and steel reinforcement, namely:
1) concrete, when hardened, firmly adheres to steel reinforcement and under load, both of these materials are deformed together;
2) concrete and steel have close values of the coefficients of linear thermal expansion. That is why when the ambient temperature changes within +50 o C ¸ -70 o C, there is no violation of adhesion between them, since they are deformed by the same amount;
3) concrete protects reinforcement from corrosion and direct action of fire. The first of these circumstances ensures the durability of reinforced concrete, and the second - its fire resistance in the event of a fire. The thickness of the protective layer of concrete is determined precisely from the conditions for ensuring the required durability and fire resistance of reinforced concrete.
When using reinforced concrete as a material for building structures, it is very important to understand the advantages and disadvantages of the material, which will allow it to be used rationally, reducing the adverse effect of its shortcomings on the performance of the structure.
TO merits(positive properties) of reinforced concrete include:
1. Durability - with proper operation, reinforced concrete structures can serve indefinitely without reducing the bearing capacity.
2. Good resistance to static and dynamic loads.
3. Fire resistance.
4. Low operating costs.
5. Cheapness and good performance.
To the main disadvantages of reinforced concrete relate:
1. Significant dead weight. This disadvantage is to some extent eliminated when using lightweight aggregates, as well as when using progressive hollow and thin-walled structures (that is, by choosing a rational shape of sections and outlines of structures).
2. Low crack resistance of reinforced concrete (from the example considered above, it follows that there should be cracks in tensioned concrete during operation of the structure, which does not reduce the bearing capacity of the structure). This disadvantage can be reduced by using prestressed reinforced concrete, which serves as a radical means of increasing its crack resistance (the essence of prestressed reinforced concrete is discussed in topic 1.3 below.
3. Increased sound and heat conductivity of concrete in some cases requires additional costs for heat or sound insulation of buildings.
4. Impossibility of simple control to check the reinforcement of the manufactured element.
5. Difficulties in strengthening existing reinforced concrete structures during the reconstruction of buildings, when the load on them increases.
Prestressed reinforced concrete: its essence and methods of creating prestressing
Sometimes the formation of cracks in structures in which it is unacceptable due to the operating conditions (for example, in tanks; pipes; structures operating under the influence of aggressive media). To eliminate this disadvantage of reinforced concrete, prestressed structures are used. Thus, it is possible to avoid the appearance of cracks in the concrete and to reduce the deformation of the structure during the operation stage.
Consider a brief definition of prestressed reinforced concrete.
A reinforced concrete structure is called prestressed, in which, during the manufacturing process, significant compressive stresses are created in the concrete of that section of the structure that undergoes tension during operation (Fig. 2).
Typically, the initial compressive stresses in concrete are created using pre-tensioned high-strength reinforcement.
This increases the fracture toughness and rigidity of the structure, as well as creates conditions for the use of high-strength reinforcement, which leads to savings in metal and a decrease in the cost of the structure.
The specific cost of reinforcement decreases with an increase in the strength of the reinforcement. Therefore, high-strength fittings are much more profitable than conventional ones. However, it is not recommended to use high-strength reinforcement in structures without prestressing, since at high tensile stresses in the reinforcement, cracks in the tensioned zones of concrete will significantly open, while reducing the required performance of the structure.
Advantages prestressed reinforced concrete before the usual one - this is, first of all, its high crack resistance; increased rigidity of the structure (due to the reverse bend obtained when the structure is compressed); better resistance to dynamic loads; corrosion resistance; durability; as well as a certain economic effect achieved by the use of high-strength reinforcement.
In a prestressed beam under load (Fig. 2), concrete undergoes tensile stresses only after the initial compressive stresses have been canceled. The example of two beams shows that cracks in a prestressed beam are formed at a higher load, but the breaking load for both beams is close in value, since the ultimate stresses in the reinforcement and concrete of these beams are the same. The deflection of the prestressed beam is also much less.
In the production of prestressed reinforced concrete structures in the factory, two basic schemes for creating prestressing in reinforced concrete are possible:
prestressing with tensioning of reinforcement on stops and on concrete.
When pulling on the stops the reinforcement is brought into the mold before the element is concreted, one end of it is fixed on the stop, the other is pulled with a jack or other device to a controlled tension. Then the product is concreted, steamed, and after the concrete has acquired the required cubic strength for the perception of compression R bp the fittings are released from the stops. The reinforcement, trying to shorten within the limits of elastic deformations, in the presence of adhesion to concrete, carries it along and compresses it (Fig. 3-a).
When pulling reinforcement onto concrete (Fig. 3-b) first, a concrete or weakly reinforced element is made, then when concrete reaches strength R bp create a preliminary compressive stress in it. This is done in the following way: the prestressed reinforcement is inserted into the channels or grooves left during the concreting of the element, and tensioned with a jack, resting directly on the end of the product. In this case, the compression of concrete occurs already in the process of tensioning the reinforcement. With this method, the stresses in the reinforcement are controlled after the end of the concrete compression. Channels in concrete, exceeding the diameter of the reinforcement by (5 - 15) mm, are created by laying the subsequently extracted void formers (steel spirals, rubber tubes, etc.). The adhesion of the reinforcement to the concrete is achieved due to the fact that after compression they are injected (cement paste or mortar is injected into the channels under pressure through the tees - branches laid down during the manufacture of the element). If the prestressing reinforcement is located on the outside of the element (ring reinforcement of pipelines, tanks, etc.), then coiling it with simultaneous compression of concrete is performed with special coiling machines. In this case, after tensioning the reinforcement, a protective concrete layer is sprayed onto the surface of the element.
Thrust tensioning is a more industrial method in factory production. Tension on concrete is used primarily for large-scale structures created on site.
Rebar tension the stops can be carried out not only with a jack, but also electrothermally. To do this, rods with upset heads are heated with an electric current to 300 - 350 ° C, brought into the mold and fixed in the mold stops. When the initial length is restored during the cooling process, the reinforcement is stretched. The armature can also be tensioned by the electrothermomechanical method (it is a combination of the first two methods).
Reinforced concrete is used in almost all areas of industrial and civil construction:
In industrial and civil buildings, reinforced concrete is used for: foundations, columns, roof and floor slabs, wall panels, beams and trusses, crane beams, i.e. almost all elements of the frames of one- and multi-storey buildings.
Special structures for the construction of industrial and civil complexes - retaining walls, bunkers, silos, tanks, pipelines, power line supports, etc.
In hydraulic engineering and road construction, dams, embankments, bridges, roads, runways, etc. are made of reinforced concrete.
The main advantages of reinforced concrete are: high strength, fire resistance, durability, ease of shaping. A concrete beam (figure below), which undergoes tension below the neutral axis and compression above it during bending, has a low bearing capacity due to the weak resistance of concrete to tension. At the same time, the strength of concrete in the compressed zone is not fully utilized. In this regard, unreinforced concrete is not recommended for use in structures designed to work in bending or tension, since the dimensions of such elements would be prohibitively large.
Concrete structures are used mainly when they work in compression (walls, foundations, retaining structures, mustache, etc.) and only sometimes when working in bending at low tensile stresses that do not exceed the tensile strength of concrete.
Reinforced concrete structures, reinforced in the stretched zone with reinforcement, have a significantly higher bearing capacity. So, the bearing capacity of a reinforced concrete beam (Fig. Below) with reinforcement laid at the bottom is 10-20 times greater than the bearing capacity of a concrete beam of the same dimensions. In this case, the strength of the concrete in the compressed zone of the beam is fully utilized.
Operating schemes of elements under load
Steel rods, wires, rolled profiles, as well as fiberglass, synthetic materials, wooden bars, bamboo trunks are used as reinforcement.
Structures are reinforced not only when they work in tension and bending, but also in compression (Fig. Above). Since steel has high tensile and compressive resistance, its inclusion in compressed members significantly increases their bearing capacity. The joint work of materials with different properties, such as concrete and steel, is ensured by the following factors:
- the adhesion of reinforcement to concrete, which occurs during the hardening of the concrete mixture; due to adhesion, both materials deform together;
- close in value coefficients of linear temperature deformations (for concrete 7 · 10 -6 -10 · 10 -6 1 / deg, for steel 12 · 10 -6 1 / deg), which excludes the appearance of initial stresses in materials and slippage reinforcement in concrete with temperature changes up to 100 ° С;
- reliable protection of steel, enclosed in dense concrete, from corrosion, direct action of fire and mechanical damage.
A feature of reinforced concrete structures is the possibility of cracking in the tensioned zone under the action of external loads. The opening of these cracks in many structures during the operation stage is small (0.1-0.4 mm) and does not cause corrosion of the reinforcement or disruption of the normal operation of the structure. However, there are structures and structures in which, according to operational conditions, the formation of cracks is unacceptable (for example, pressure pipelines, trays, tanks, etc.) or the width of the opening must be reduced. In this case, those zones of the element in which tensile forces appear under the action of operational loads are subjected to intensive compression in advance (before the application of external loads) by pre-tensioning the reinforcement. Such structures are called prestressed. The preliminary compression of structures is carried out mainly in two ways: by tensioning the reinforcement on the stops (before concreting) and on concrete (after concreting).
In the first case, before concreting the structure, the reinforcement is pulled and fixed on the stops or ends of the form (Fig. Below). Then the element is concreted. After the concrete has acquired the necessary strength to absorb the pre-compression forces (transfer strength), the reinforcement is freed from the stops and, in an effort to shorten, it compresses the concrete. The transfer of force to the concrete occurs due to the adhesion between the reinforcement and the concrete, as well as by means of special anchor devices located in the concrete of the structure, if the adhesion is insufficient.
In the second case, a concrete or weakly reinforced element with channels or grooves is first made (Fig. Below). When the concrete reaches the required transfer strength, reinforcement is inserted into the channels (grooves), tightened with the stop of the tension attachment on the end of the element and anchoring. Thus, the concrete is compressed. To create adhesion of reinforcement to concrete, cement or cement-sand mortar is injected into the channels. If the prestressing reinforcement is located on the outer surface of the element (ring reinforcement of pipelines, reservoirs, etc.), then its winding with simultaneous compression of concrete is carried out with special winding machines. After tensioning the reinforcement, a protective layer of concrete is applied to the surface of the element by gunning. The tension of the reinforcement can be done by mechanical, electrothermal, combined and physicochemical methods.
Pre-stressing methods
a - tension on the stops; b - tension on concrete; I - tension of the reinforcement and concreting of the element; II, IV - ready-made element; III - element during tensioning of the reinforcement; 1 - emphasis; 2 - jack; 3 - anchor
With the mechanical method, the fittings are tensioned with hydraulic or screw jacks, winding machines and other mechanisms. With the electrothermal method, the armature is heated with an electric current to 300-350 ° C, put into a mold and fixed on stops. During the cooling process, the reinforcement is shortened and receives preliminary tensile stresses. The combined tensioning method combines electrothermal and mechanical reinforcement tensioning methods carried out simultaneously. With the physicochemical method, the tension of the reinforcement is achieved as a result of the expansion of concrete prepared on a special stress cement (NC) during its hydrothermal treatment.
The reinforcement embedded in concrete prevents an increase in its volume and stretches, and compressive stresses arise in concrete. The reinforcement is tensioned on the stops by mechanical, electrothermal or combined methods, and on concrete - only mechanically.
The main advantage of prestressed structures is high crack resistance. When a prestressed element is loaded with an external load, the pre-created compressive stresses are extinguished in the concrete of the tensile zone, and only then tensile stresses arise. The higher the strength of concrete and steel, the more pre-compression can be created in the element.
The use of high-strength materials allows to reduce the consumption of reinforcement by 30-70% in comparison with non-stressed reinforced concrete. Concrete consumption and weight of the structure are also reduced. In addition, the high crack resistance of prestressed structures increases their rigidity, water resistance, frost resistance, resistance to dynamic loads, and durability.
The disadvantages of prestressed reinforced concrete include the fact that the process is a significant labor-intensive manufacturing of structures. In addition, there is a need for the use of special equipment and highly qualified workers.
The stress-strain states of pre-stressed elements after the formation of cracks in the concrete of the tensile zone are similar to elements without pre-stress.
Reinforced concrete structures are the basis of modern construction. However, they have significant flaws associated, first of all, with insufficient load capacity and the formation of cracks in the stone under operational loads. Improvement in the technology of manufacturing concrete products and steel reinforcement has led to the creation of prestressed reinforced concrete, which has a number of advantages.
Definition
Prestressed reinforced concrete structures are construction products, the concrete of which, at the stage of creation, is forced to receive the initial design compressive stress. It is created due to the preliminary formation of tensile stress in the working high-strength reinforcement and its compression of concrete in those areas that are to experience tension (deflection) during operation. Compressing, the reinforcement does not slip, as it is adhered to the material or is held by anchoring the reinforcement at the ends of the products. Thus, the tensile stress that the reinforced concrete composition acquires with the help of reinforcement balances the tension of the preliminary compression of the stone.
Advantages
Prestressed reinforced concrete for a long time postpones the time of the beginning of the formation of splits in the products, working for deflection, reduces the depth of their opening. At the same time, the products acquire increased rigidity without decreasing their strength.
Prestressed reinforced concrete beams tend to work well in compression and deflection, having the same strength along the length, which allows you to increase the width of overlapped spans. In such structures, the dimensions of the cross-section are reduced, therefore, the volume and weight of the component elements (by 20 - 30%), as well as the consumption of cement, are reduced. A more rational use of the properties of steel makes it possible to reduce (rod and wire) up to 50%, especially from high-strength grades (A-IV and higher), which have a significant tensile strength. The chemical neutrality of concrete to steel helps to protect the reinforcement from corrosion. At the same time, increased crack resistance protects stressed reinforcement from rusting in structures that are under constant pressure of water, other liquids, and gases.
The building construction methods used in frame construction are based on the technology of prestressing reinforced concrete structures during construction. The tension reinforcement, pressing the concrete of the assembly units, ensures their practical docking by significantly reducing the metal consumption at the joints. Prefabricated and precast-monolithic products from reinforced concrete stressed structures can consist of abutting parts with the same cross-section, which are made of unstressed lightweight (heavy) concrete at the edges, and the loaded fragment is prestressed reinforced concrete. Such products have increased endurance by compensating for repetitive dynamic influences.
This property makes it possible to damp changes in stresses in concrete and reinforcement caused by fluctuations in external loads. The increased seismic resistance of buildings is increased due to the high structural stability of stressed reinforced concrete, which compresses their individual fragments. The prestressed structure provides greater safety, since its destruction is preceded by an out-of-limit deflection, signaling that the structure has exhausted its strength.
disadvantages
The state of prestressing in the material is achieved by special equipment, accurate calculations, labor-intensive design and costly production. Products require careful storage, transportation and installation, which do not cause their emergency state even before use.
Concentrated loads can promote longitudinal cracks, which reduce the load-bearing capacity. Miscalculations in design and production technology can cause complete destruction of the created reinforced concrete product on the slipway. Prestressed structures require high-strength metal-consuming formwork, increased consumption of steel for embedments and reinforcement.
Large values of sound and thermal conductivity require the placement of compensating materials in the stone body. Such reinforced concrete structures provide a lower fire resistance threshold (due to the lower critical heating temperature of prestressed reinforcing steel) compared to conventional reinforced concrete. The prestressed concrete structure is critically affected by leaching, solutions of acids and sulfates, salts, leading to corrosion of cement stone, cracking and corrosion of reinforcement. This can lead to a sharp decrease in the bearing capacity of the steel and sudden brittle fracture. Also, the disadvantages include the significant weight of the products.
Materials for structures
Reinforced concrete is a multicomponent material, the main components of which are concrete and steel reinforcement. Their quality parameters are determined by special design requirements for structural elements at the site of use.
Concrete
Concrete casting molds with prestressing rods. Prestressing in reinforced concrete is ensured by using heavy compositions of average density from 2200 to 2500 kg / m3, which have axial tensile strength classes higher than Bt0.8, strength from B20 and more, waterproof grades from W2 and higher, frost resistance from F50 ... Requirements for the products guarantee concrete with a standard strength not lower than the established one with a probability of 0.95 (in 95% of cases). The mixture must age at least 28 days before the material is prestressed. In the early stages of operation, concrete stone is able to partially lose its stressed quality due to a general decrease in steel stress (up to 16%). The coefficient of reliability of the material in tension and compression in limiting states is set for serviceability not less than 1.0.