Types of prestressed state of reinforced concrete. Important advantages and disadvantages of prestressed concrete. Ways to create prestress

Under prestressed understand reinforced concrete structures, elements, products in which preliminary, i.e. in the manufacturing process, are artificially created in accordance with the calculation of the initial tensile stresses in part or in all the working reinforcement and compression of all or part of the concrete.

Compression of concrete in prestressed structures to a predetermined value is carried out by pretensioned reinforcement, which tends to return to its original state after the release of the tensioning devices (Fig. 14). In this case, the slippage of reinforcement in concrete is excluded by their mutual natural adhesion, and if natural adhesion is insufficient, by special artificial anchoring of the ends of the reinforcement in concrete. The initial prestress of the reinforcement, created as a result of the artificial tension of the reinforcement, after the release of the tensioners is reduced due to the relative elastic compression of the concrete.

Over a long period of time, the prestress loss of reinforcement increases significantly due to shrinkage and creep of concrete and reinforcement, stress relaxation of reinforcement, and many other factors.

The essence of prestressed reinforced concrete structures is easy to trace, for example, by comparing diagrams of centrally tensioned elements, respectively, with prestressed and non-stressed reinforcement (Fig. 15). The reinforcement, trying to return to its original position, compresses the concrete with tension (Fig. 15, b).

In this case, the sample (Fig. 15, c) will shrink by the amount of elastic compression of concrete (for greater clarity, we assume that the loss of prestressing reinforcement from shrinkage and creep of concrete, creep of reinforcement, and stress relaxation of steel have not yet manifested themselves).

The established tensile prestress in the reinforcement (Fig. 15, a, point 2) will be balanced by the concrete precompression stress (Fig. 15, b and c).

With these prestresses in reinforcement and in concrete, the reinforced concrete element (see Fig. 15, c) enters the construction site.

Let us consider the fundamental difference between prestressed structures and structures without prestressing.

Even before the application of an external load in the reinforcement of prestressed structures, there are significant tensile prestresses (see Fig. 15, a, point 2), which compress the concrete of the elements (see Fig. 15, b and c).

External tensile force N(Fig. 15, d) causes a relative elongation of the prestressed element. As a result, the pre-compression of the concrete will be extinguished.

With increasing external load N e will increase up to the value of elastic compression of concrete.

With the magnitude of the external force N, equal to the strength of the prestressing of the reinforcement (Fig. 15, e), there is a complete repayment of the precompression of the concrete. With a further increase in the external load, tensile stresses will appear in the concrete, which will increase up to the design resistance (concrete tensile strength) (Fig. 15, e), in the same way as in reinforced concrete elements (see Fig. 15, a, curve III ), without prestressing. As soon as the relative elongation of the concrete reaches the limit value, a crack will appear in the prestressed element, as in the reinforced concrete element without prestressing.

Consequently, the crack resistance of prestressed structures is 2...3 times greater 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 limiting deformation of concrete tension. Dot 9 characterizes the formation of cracks in reinforced concrete structures, and the point 11 - in prestressed structures.

The higher the tension of the reinforcement and the stronger the compression of the concrete, the smaller the area 12... 13, where cracks form and open. When the points match 12 and 13 cracks in the prestressed element do not form until the rupture of the reinforcement. When a reinforced concrete element is stretched, concrete can deform together with reinforcement only within the section 0...9 (see Fig. 15, a), and throughout the section 9...13 and further, the formation of new cracks and the disclosure of old ones occur in it.

The strength of prestressed structures does not depend on the magnitude of prestressing reinforcement. That is why the strength calculation of any prestressed structures is no different from the strength calculation of reinforced concrete structures without prestressing.

All of the above allows us to conclude that the nature of prestressed structures is the same as that of reinforced concrete structures without prestressing. The creation of tensile prestresses in the reinforcement and compression of concrete before the application of operational loads does not significantly affect the basic physical and mechanical properties of reinforced concrete.

Prestressed structures are a general type of reinforced concrete structures, while reinforced concrete structures without prestressing are just a special case. At the same time, it should be borne in mind that the preliminary compression of concrete significantly increases the crack resistance of inclined sections and the reinforcing boundary and can significantly reduce the strength of the compressed section zone.

Advantages.

In prestressed structures, it is possible to use highly economical high-strength bar reinforcement and high-strength wire reinforcement, which make it possible to reduce, on average, up to 50% the consumption of scarce steel in construction. Preliminary compression of the stretched concrete zones significantly delays the moment of crack formation in the stretched zones of the elements, limits their opening width and increases the rigidity of the elements, practically without affecting their strength.

Prestressed structures often turn out to be economical for buildings and structures with such spans, loads and operating conditions, under 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 the structures. The use of prestressing makes it possible to most rationally perform the joints of prefabricated structural elements by pressing them with prestressing reinforcement. At the same time, the consumption of additional metal in the joints is significantly reduced or the need for its use is completely eliminated.

Prestressing makes it possible to expand the use of prefabricated and precast-monolithic composite flow structures, in which high-strength concrete is used only in prefabricated prestressed elements, and the main or significant part of the structures is made of heavy or lightweight concrete that is not subjected to prestressing.

Prestressing, which increases the resistance of structures to the formation of cracks, increases their endurance when working under the influence of a repeatedly repeated load. 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. Properly designed prestressed structures are safe in operation, as they show significant deflections before failure, warning about the emergency state of structures.

With an increase in the percentage of reinforcement, the seismic resistance of prestressed structures in many cases increases (especially for T-sections with a flange in the compressed zone and lightweight concrete). This is explained by 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 compared to reinforced concrete structures without prestressing of the same bearing capacity, and therefore more flexible and lighter. The increase in seismic resistance is also facilitated by the spatial work of buildings and structures as a whole, obtained by compressing their individual parts with prestressed reinforcement. The most earthquake-resistant are stressed structures, which have a significant excess of the bearing capacity over the crack resistance limit.

Flaws.

Reinforced concrete structures with prestressed reinforcement have the following main disadvantages.

Prestressed structures are characterized by increased complexity of design and manufacture. They require greater care in the calculation and design, during manufacture, storage, transportation and installation, since even before the application of external loads in the sections of their elements, unacceptable compressive or tensile stresses can occur that can lead to an emergency condition. For example, in the ends of prestressed structures with a concentrated and uneven application of compression forces, longitudinal cracks can occur, which significantly reduce their bearing capacity. If you do not take into account the specific features of the creation of prestressing, then the working conditions under load of the entire structure or its individual parts may deteriorate.

Large forces transmitted by the prestressing reinforcement to the concrete of the structure at the time of release of the tensioning devices can lead to its complete destruction during compression or local damage, to slippage of the prestressing reinforcement due to a violation of its adhesion to concrete. Therefore, the standards require that it is mandatory to carefully check the strength of prestressed structures during the compression stage, during storage, transportation and installation, and to comply with the stipulated design requirements. Prestressed structures require complication and an increase in the metal consumption of the formwork, the labor intensity of reinforcement, and an increase in metal consumption for embedded parts and mounting fittings.

Due to the use of high-strength materials, the mass of prestressed structures is significantly less than the mass of reinforced concrete structures without prestressing, however, it remains higher than the mass of metal and especially wooden structures. The widespread introduction into practice of building structures made of lightweight and cellular concrete, reinforced cement, openwork thin-walled spatial, mesh and hanging structures makes it possible to significantly bring the mass of prestressed structures closer to the mass of metal structures.

The high heat and sound conductivity of reinforced concrete requires a complication of the design and the additional use of gaskets made of heat and sound insulating materials.

The strengthening of prestressed structures is not more difficult than the strengthening of reinforced concrete structures, but it is much more difficult than the strengthening of steel and especially wooden structures. The work on strengthening prestressed structures is very complex, laborious and expensive.

Prestressed structures are fireproof, but their fire resistance is lower than the fire resistance of reinforced concrete structures without prestressing. This is due to the fact that the critical temperatures to which prestressed reinforcement can be safely heated are lower compared to non-stressed reinforcement. For example, the strength of high-strength wire subjected to cold working (hardening), starting from a temperature of 200°C, noticeably decreases and at 600°C is about 2/3 of the original strength. Bar reinforcement of a periodic profile, strengthened by a hood, loses hardening at temperatures above 400 ° C. Thus, in case of fire, the fire resistance of prestressed structures will be ensured if the critical temperature for this type of reinforcement is not exceeded. This can only be achieved by increasing the protective layer of concrete.

The standards allow the use of prestressed structures made of heavy and lightweight concrete on a cement binder with systematic periodic exposure to elevated temperatures (the heating temperature should not change more than once a day by 30 ° C and once a week by 100 ° C) and stationary exposure to process temperatures up to 200 ° FROM. At high temperatures, the use of heat-resistant reinforced concrete is recommended.

Prestressed structures are characterized by insufficient corrosion resistance.

Corrosion of cement stone in concrete can occur due to:

1) leaching of lime from it with soft waters, causing the formation of white smudges on the concrete surface (“white death” of concrete);

2) the formation of soluble and water-borne products associated with exchange reactions when concrete is exposed to solutions of acids and some salts;

3) the formation of crystallizing salts in the pores and capillaries of concrete elements, for example, under the action of sulfate solutions, leading to cracking of the elements (cement bacillus). All three types of cement stone corrosion reduce the protective properties of concrete in relation to reinforcement and can cause dangerous corrosion of reinforcement.

Reinforcement corrosion can also occur due to insufficient cement content in concrete, the presence of harmful additives in it (for example, table salt), crack opening of more than 0.4 mm, insufficient thickness of the protective layer, low concrete density. Corrosion damage sharply reduces the bearing capacity and plastic properties of high-strength reinforcement, causes cracking of thermally hardened reinforcement, which causes sudden brittle fracture of prestressed structures.

The main measures to protect reinforced concrete from corrosion are as follows:

Preventing the formation of cracks or limiting their opening;

Limiting the degree of aggressiveness of the environment;

The use of dense and waterproof concretes on special sulfate-resistant cements;

Surface protection with a variety of polymeric materials, acid-resistant plaster, ceramic cladding, pasting and coating insulation;

Reinforcement overrun up to 10...20%; increase in the protective layer of concrete up to 25 mm.

Oil and its shoulder straps reduce the resistance of concrete to tension, compression and adhesion to reinforcement, as a result of which the concrete becomes permeable to liquids.

Vegetable and animal oils and fats, especially rancid ones, contain a fatty acid that saponifies the lime of concrete and forms a lime soap that destroys concrete.

Sugar, syrups, molasses form soluble salts with lime - sugars, which quickly destroy fresh concrete.

Alcohols in themselves are not harmful, but extracting water from concrete, dry it and stop the hardening process. The listed main disadvantages of reinforced concrete structures are insignificant in comparison with their numerous major advantages. The negative impact of many shortcomings can be significantly reduced by high-quality design, manufacture, installation and operation of reinforced concrete structures.

That is why, despite the short history of development (~ 135 years), they have become widespread in the construction of the most critical and unique buildings and structures. There is not a single area of capital construction in which modern reinforced concrete structures, and especially prestressed ones, could not be successfully used. With proper operation, reinforced concrete structures can serve for a long time without reducing the bearing capacity, because the strength of concrete increases over time and it reliably protects reinforcement from corrosion.

K category: Reinforcing works

About prestressed concrete

Reinforced concrete structures used in modern construction have some disadvantages. One of them is the large dead weight of reinforced concrete, equal to 2500 kg/m3 (including 100 kg/m3 of reinforcement on average). This is especially reflected in horizontal bending structures - slabs, beams, crossbars, etc. Tensile stress appears here under the action of a load. Therefore, a large amount of reinforcement has to be placed in the stretched zone of the section of the reinforced concrete structure, which increases the sectional area and the weight of the structure.

Another disadvantage of reinforced concrete structures is the incomplete use of the properties of reinforcing steel, in particular its tensile strength. With the full use of the strength of reinforcing bars, concrete gives cracks in the tension zone of structures, although the stress in the reinforcement does not exceed the yield strength. This is unacceptable during the operation of structures.

The mentioned shortcomings are largely eliminated in prestressed reinforced concrete structures.

The essence of prestressing (Fig. 1) is as follows. The working reinforcement of the structure is tensioned before concreting and concreting is carried out in the tensioned state. After the concrete seizes, hardens and acquires the necessary strength, the pulling force is removed. In this case, the reinforcing steel tends to shrink again (shrink in length) and transfers part of the compressive forces to the surrounding concrete.

Thus, the concrete in the manufactured prestressed structure, even before it is installed in the structure and transferred to it various operational loads, is already subjected to compressive stress, or, as they say, an internal stress state is artificially created in the structure, characterized by concrete compression and reinforcement tension.

Before concrete in a prestressed structure, perceiving the design (operational) load, begins to work in tension, the pre-created compression must first be extinguished in it.

The presence of prestressing makes it possible to increase the load on the structure in comparison with a structure reinforced in the usual way, or, at the same load value, to reduce the dimensions of the structure, i.e. save concrete and steel.

For the first time, the idea of prestressing (compression) of elements working in tension was proposed in 1861 by the Russian scientist, academician A.V. Gadolin for gun barrels.

The advantages of prestressed reinforced concrete structures over conventional ones are as follows.

1. The ability of concrete to perform well in compression is fully utilized throughout the entire section. This makes it possible to reduce the cross-sections and, consequently, the volume and weight of prestressed elements by 20-30% and reduce the consumption of materials, in particular cement.

2. Due to the better use of the properties of reinforcing steel in prestressed structures, compared to conventional structures, the consumption of reinforcement is reduced. Reinforcement savings, which are especially effective and necessary when using steels with high tensile strength, reach 40%.

3. Structures with prestressed reinforcement (stress-reinforced) are characterized by high crack resistance, which protects the reinforcement from rusting. This is of great importance for structures under constant pressure of water or any other liquids and gases (pipes, dams, tanks, etc.).

4. Due to the reduction in the volume and weight of stress-reinforced reinforced concrete elements, the use of prefabricated structures is facilitated.

Examples of the most common precast prestressed structures are industrial building roof slabs, crane beams, roof beams, etc.

The use of prestressing is effective not only in prefabricated, but also in monolithic and precast-monolithic reinforced concrete structures. Prefabricated monolithic structures consist of prefabricated prestressed elements that absorb forces together with concrete and reinforcement, which are additionally laid after the installation of prefabricated elements in the design position.

When erecting prefabricated monolithic structures, individual prefabricated elements are connected in such a way that in the future they work as a single unit during operation. This is done in the following way.

In the manufacture of prefabricated elements of the future precast-monolithic structure, they leave reinforcement outlets. During the installation of these elements, additional reinforcing bars are placed in the seams between them and welded to the outlets so that the reinforcement of adjacent elements is one. Then the reinforced seams (or joints) are filled with concrete, or, as they say, monolithic. After the concrete has hardened in the joints and seams, a structure is obtained, called prefabricated-monolithic.

This method is often used in the construction of multi-storey buildings (Fig. 1) and in spatial structures with curvilinear outlines - vaults and domes.

Rice. 1. Joint of reinforcement of prefabricated purlins and slabs of a multi-storey industrial building with the laying of three-row reinforcing shorts into columns: 1 - joint of the short with the outlets of the reinforcement of the girders, 2 - reinforcing short, 3 - reinforcement laid in the seams between the prefabricated slabs

An example of a unique monolithic reinforced concrete structure, implemented by Soviet builders for the first time in world practice, is the Ostankino television tower (Fig. 2, a) in Moscow.

The total height of the tower is 525 m. The lower tier, up to the level of 17.5 m, consists of ten separate reinforced concrete pillars. Above this mark, up to the mark of 63 m, individual supports are combined into a reinforced concrete cone with a solid wall. From mark 63 to mark 385 rises the reinforced concrete shaft of the tower with a diameter of 18 and 8.2 m, respectively, with walls 40 to 35 cm thick (Fig. 2, b). The walls of the shaft are reinforced with a double mesh of steel 35GS of a periodic profile with a reinforcement intensity of up to 230 kg/m3.

Special frames are installed between the reinforced meshes (Fig. 2, c). The mutual position of the metal panels of the inner and outer formwork and reinforcing meshes, and, consequently, the thickness of the concrete protective layer, were fixed by bolts 9 with plastic tubes put on them (Fig. 2, c).

Rice. Fig. 2. Ostankino television tower in Moscow: a - general view, b - section of the tower shaft, c - detail of the installation of formwork and reinforcement in the wall of the tower shaft; d - supports, 1 - conical part of the tower, 3 - reinforced concrete shaft, 4 - service premises, 5 - restaurant, 6 - steel antenna, 7 - inner formwork panels, 8 - outer formwork panels, 9 - bolt, 10 - reinforcing mesh, 11 - frame, 12 - plastic tube of the turret barrel

Ropes with a diameter of 38 mm were used as prestressing reinforcement of the lower part and the trunk of the tower, located in eight tiers from the foundation to mark 385. The length of the ropes passing in the channels inside the walls ranges from 154 to 344 m. The ropes were tensioned using hydraulic jacks; the tension force reached 69 tf. In total, 1040 tons of reinforcing steel were laid in the tower structure.

Rice. 3. Sections of wire reinforcing bundles: a - loose at the ends, b - fixed at the ends, c - multi-row, d - from groups of wires; 1 - prestressing wires of the bundle, 2 - knitting wire, 3 - spiral, 4 - short wires, 5 - central wire, 6 - tube, 7 - solution, 8 - group of wires, 9 - additional wires

As prestressed reinforcement for prestressed structures, it is expedient to use reinforcing steel with higher mechanical characteristics; this achieves the greatest savings in reinforcement, reducing the cross section and weight of the structure.

Therefore, prestressed structures are reinforced, as a rule, with high-strength reinforcing steel and products from it of the following types: - hot-rolled steel of a periodic profile of classes At-V and. At-VI, thermally hardened; - hot-rolled steel of a periodic profile of classes A-IV and A-V; - high-strength reinforcing wire, smooth and periodic profile of classes B-II and Vr-P; wire strands; wire ropes; bundles (Fig. 3) and packages of high-strength wire. For prestressed structures, it is very important to ensure that the surface of the reinforcement adheres well to the surrounding concrete.

This explains the use of strands and ropes with a complex surface shape as prestressing reinforcement.

Seven-wire strands are made from wires with a diameter of 1.5-5 mm. Multi-strand ropes are made from wires with a diameter of 1-3 mm. The bundle consists of wires located around the circumference, in an amount from 8 to 48. To maintain the relative position of the wires inside the bundle, segments of wire spirals are installed every 1-1.5 m. In the same places from the outside, the bundle is pulled together with a knitting wire (Fig. 3, a, c, d). The bundles fixed at the ends (Fig. 3, b) consist of 8-24 wires. In places of installation of short wires 4, gaps remain along the length of the bundle, through which the middle of the bundle is filled with a solution. Multi-row bundles of groups of wires up to 8 mm in diameter (Fig. 3, c) are used in engineering structures, such as bridges. The package is a group of wires or strands arranged in several rows horizontally and vertically along a regular geometric grid.

Reinforcement tension when reinforcing prestressed structures is performed in two ways - before or after concreting.

Tension on forms or stops. When reinforcing by this method, the reinforcing bars are tensioned before laying the concrete mix. Tension forces, sometimes reaching several tens of tons in magnitude, are perceived by a powerful steel mold structure in which the product is made, or by special bench stops, therefore this method is called bench. The structure is concreted with tensioned reinforcement. When the tensioners are removed after the concrete has set, compression of the concrete is achieved by bonding between the tending to contract rebars and the surrounding hardened concrete.

The decrease in length during compression is shown on a conventional scale, since it is imperceptible to the eye.

With this method, the control of the tension (and, consequently, the stress) of the reinforcement is carried out before the compression of the concrete.

Reinforcement tension on concrete. In this case, the tensile force of the reinforcement is perceived not by the form, but by the hardened concrete. This method is mainly used for reinforcing structures assembled from separate blocks. The method of tensioning on concrete makes it possible to assemble large-sized structures (up to 30 m long or more) at the place of their installation from separate, easily transportable parts of a smaller size. The tension of the reinforcement is controlled during the compression of concrete. Compression can be performed only after the hardened concrete has accumulated strength sufficient to absorb the forces created by the tensioners.

Various methods of reinforcement tension are used: mechanical - with the help of special jacks; electrothermal, which uses the property of a steel bar to elongate when heated, and electrothermomechanical, which is a combination of the first two.

There are methods of laying prestressed reinforcement: linear, in which individual rods, wire bundles or packages of precisely measured length are laid, and the method of continuous laying (winding) of reinforcement directly from the bay onto the pins of a rotating pallet or using a moving winding machine.

- About prestressed concrete

Prestressing concrete to increase its strength is a modern way to increase the strength of concrete structures. In this article, we list the advantages and disadvantages of prestressed concrete.

Concrete is used in various types of construction. The name "preliminarily" does not mean that the concrete was stressed before the floor above it was built. However, instead of buckling under pressure, it manages to become stronger, and it acquires the ability to withstand much greater stresses than ordinary concrete.

But how to do that. What are the advantages and disadvantages of prestressed concrete? Let's find out the answers to these questions, which will help to understand this better.

What is prestressed concrete?

Concrete in its normal state has an extremely high level of compressive strength. This makes it possible to use it to create structures that must bear compressive loads. For example, it is used to create columns and supports to support various structures in large buildings.

However, compared to its compressive strength, concrete has almost no holistic strength. Therefore, if ordinary concrete is used to build floors, it will sag under pressure when compressed on it, and eventually crack and crumble. To eliminate this shortcoming, the prestressing method is used. In its most basic form, prestressing is done as follows.

A series of steel cables are tensed by applying a pulling force at their ends and placed in a concrete block. Then, the liquid concrete is poured into the molds and hardens, which causes bonding between it and the steel cables inside. After that, the cables try to restore their original shape, they pull the concrete with them, creating compression. This stresses the internal particles of concrete, strengthening it and making it an excellent material for structural use. Since concrete is stressed before it is used, it is called prestressed concrete.

Prestressed concrete has a large amount of strength, both in compression and in tension. It is used to build long bridges, building slabs, etc.

Advantages and disadvantages of prestressed concrete

Advantages

1) high tensile strength and crack resistance

An ordinary concrete slab, when placed under tension, sags downward under the pressure of the weight. In this position, the upper part of the plate is compressed, and its bottom is under tension. Since concrete can withstand large amounts of compression, the top of the slab is able to withstand such a load. However, concrete is weak in terms of tensile strength. At the bottom, the slab begins to crack until the entire slab collapses down.

Prestressed concrete has a high tensile strength and is therefore capable of bearing heavy loads without cracking or sinking.

2) Below depth

Due to its high strength, prestressed concrete can be used to build structures that have a much shallower depth compared to reinforced concrete structures. This has two main benefits. If it is used for building boards, it does not take up much space, and additional usable space becomes available, especially in multi-storey buildings. The second advantage of lower structure depths is that they are lighter, and the load-bearing columns in buildings can also be made smaller, saving on construction costs and effort.

3) Duration

Prestressed concrete can be used to build structures that last longer than reinforced concrete. In the construction of buildings, this means that fewer columns will be needed to support the slabs, and the distance between them can be much greater. For bridges, the use of prestressed concrete can allow engineers to build a long bridge that will not collapse under load.

4) fast and reliable construction

Prestressed concrete blocks are manufactured commercially in several standard shapes and sizes. They are known as prefabricated blocks. Because they are professionally made, they have a very good build quality and at the same time they provide all the power of precast concrete. They can be directly delivered to the construction site and used to quickly complete construction work. Structures built with these blocks are known to be of better quality and longer life.

Flaws

1) Greater complexity of the building

Prestressing concrete at a construction site is a time-consuming and complex process. One must have a thorough knowledge of each step that is involved along with a thorough knowledge of the use of various equipment. Precast concrete structures are produced once, they are difficult to change, and therefore the complexity of the initial planning also increases. Also, since the probability of error is very low, great care must be taken when building.

2) Increase in construction cost

Prestressed concrete requires knowledge and specialized equipment, which can be expensive. Even the cost of reinforced concrete blocks is significantly higher than reinforced blocks. In residential construction, for added tensile strength, prestressed concrete may not be necessary, as plain reinforced concrete is much cheaper and strong enough to meet all load requirements.

3) the need for quality control and inspection

The procedure used for prestressing must be checked and approved by quality control specialists. Each prestressed concrete structure must be checked to ensure that it has been subjected to the correct stress. Too much attention is also bad, and it can damage the concrete, making it weaker.

Prestressed concrete structures provide superior tensile strength compared to normal and even reinforced concrete structures, but they are complex to construct and more expensive. For low stress applications such as building floors, using prestressed concrete is impractical. Therefore, the decision to use prestressed concrete should only be made if required by the project specification.

Modern methods of frame construction use the technology of prestressing reinforced concrete structures. Prestressed structures- reinforced concrete structures, the tension 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 to a given 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, the slippage 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 the reinforcement in concrete.

The crack resistance of prestressed structures is 2–3 times greater 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 limiting deformation of concrete tension.

Prestressed concrete allows on average up to 50% to reduce the consumption of scarce steel in construction. Preliminary compression of the stretched concrete zones significantly delays the moment of crack formation in the stretched zones of the elements, limits their opening width and increases the rigidity of the elements, practically without affecting their strength.

Advantages of reinforced concrete prestressing technology

Prestressed structures turn out to be economical for buildings and structures with such spans, loads and operating conditions under which the use of reinforced concrete structures without prestressing is technically impossible, or causes excessive consumption of concrete and steel to ensure the required rigidity and bearing capacity of the structures.

Prestressing, which increases the rigidity and resistance of structures to the formation of cracks, increases their endurance when working under the influence of a repeatedly repeated load. 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. Properly designed prestressed structures and buildings are safer and more reliable, especially in seismic zones. With an increase in the percentage of reinforcement, the seismic resistance of prestressed structures in many cases increases. This is explained by 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 compared to reinforced concrete structures without prestressing of the same bearing capacity, and, consequently, more flexible and lighter.

In most developed foreign countries, structures of floors and coverings of buildings for various purposes, a significant part of the products used in engineering structures and in transport construction, are made in increasing volumes from prestressed reinforced concrete; production of elements of external architectural design of buildings appeared.

World experience in the use of prestressing technology

In the world, monolithic reinforced concrete is mostly prestressed. First of all, large-span structures, residential buildings, dams, energy complexes, TV towers and much more are being built in this way. TV towers made of monolithic prestressed reinforced concrete look especially impressive, having become the sights 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 cross section of the tower in the form of a trefoil proved to be very successful for placing prestressing reinforcement and concreting in a sliding formwork. The wind overturning moment for which this tower is designed is almost half a million tons, with its own weight of the ground part of the tower just over 60 thousand tons.

In Germany and Japan, egg-shaped tanks for treatment plants are widely built from monolithic prestressed reinforced concrete. To date, such tanks have been built with a total capacity of more than 1.2 million cubic meters. Separate structures of this type have a capacity of 1 to 12 thousand cubic meters.

Abroad, monolithic ceilings of an increased span with reinforcement tension on concrete are increasingly being used. In the USA alone, more than 10 million cubic meters of such structures are built annually. A significant amount of such ceilings is being built in Canada.

Recently, prestressed reinforcement in monolithic structures is increasingly used without adhesion to concrete, i.e. channels are not injected, and the reinforcement is either protected from corrosion with special protective shells or treated with anti-corrosion compounds. This is how bridges, large-span buildings, high-rise buildings and other similar objects are erected.

In addition to traditional construction purposes, monolithic prestressed concrete has found wide application for reactor vessels and containment shells of nuclear power plants. The total capacity of nuclear power plants in the world exceeds 150 million kW, of which the power of stations, reactor vessels and protective shells of which are built of monolithic prestressed reinforced concrete, is almost 40 million kW. Protective shells for nuclear power plant reactors have become mandatory. It was the absence of such a shell that caused the Chernobyl disaster.

A prime example of the building possibilities of prestressed concrete are offshore oil platforms. More than two dozen such grandiose structures have been erected in the world.

Built in 1995 in Norway, the Troll platform has a total height of 472 m, which is one and a half times higher than the Eiffel Tower. The platform is installed in a sea area with a depth of more than 300 m and is designed to withstand 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 prestressed reinforcing steel. Estimated service life of the platform is 70 years.

Bridge building has traditionally been an extensive area of application for prestressed concrete. In the USA, for example, more than 500,000 reinforced concrete bridges with various spans have been built. Recently, more than two dozen cable-stayed bridges 600-700 m long with central spans from 192 to 400 m have been built there. Out-of-class bridges are being built from prestressed reinforced concrete, which are built according to individual projects. Bridges with a span of up to 50 m are built in a prefabricated version from reinforced concrete prestressed beams.

Bridge "Normandie"

Achievements in bridge building from prestressed reinforced concrete 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 cable-stayed bridge "Barrnos de Luna" in Spain has a span of 440, "Anasis" in Canada - 465, the bridge in Hong Kong - 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 World Expo-98, is not much inferior to it. The total length of this bridge crossing exceeds 18 km. Its main load-bearing 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 durability of concrete (in Russia, in recent years, large-span bridges are more often built of steel).

Technology of prestressing monolithic reinforced concrete in Russia

In Russia, these products account for more than a third of the total production of prefabricated elements. Abroad, formless molding of slab structures on long stands is widely used. There, the usual practice is the production of slabs with a span of up to 17 m, a section height of 40 cm under a load of up to 500 kgf/m2. In Finland, reinforced concrete hollow-core slabs for the same load are produced with a section 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 on such stands, as a rule, is group with a jack power of 300-600 tons. Today, various systems for formworkless molding on long stands "Spirol", "Spancrete", "Spandek", "Max Roth", "Partek" have been developed and others, characterized by high performance, applied reinforcement, technological requirements for concrete, cross-sectional shape of 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 a package in height. The width of the slabs reaches 2.4 m, with a maximum span of 21 m. Spancrete slabs alone are used in the USA for more than 15 million m2 annually.

At one time, long stands for formworkless molding using the Max Rot technology appeared in Russia. However, this technology has not received further distribution. In the structural systems of buildings widely used in our country, the elements are connected through embedded parts. In slabs produced on long stands, as a rule, by extrusion, the possibilities for placing embedded parts are limited. However, for prefabricated monolithic buildings, slabs without embedded parts can find the widest distribution, which is the case abroad, especially in the Scandinavian countries and in 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. Improving the structural systems of buildings, of course, will give impetus to the development of technology for the production of plate products.

The protracted Russian stagnation in the field of application of prestressed reinforced concrete is also partly due to the fact that we have not received due study and application of prestressed structures with reinforcement tension on concrete, including in building conditions.

Enerprom begins to develop this area and offers a range of equipment of its own design for the implementation of this technology.

prestressed concrete (prestressed concrete) is a building material designed to overcome the inability of concrete to resist significant tensile stresses. Structures made of prestressed reinforced concrete, compared to non-stressed ones, have significantly lower deflections and increased crack resistance, having the same strength, which makes it possible to cover large spans with an equal section of the element.

In the manufacture of reinforced concrete, steel reinforcement with high tensile strength is laid, then the steel is stretched with a special device and the concrete mixture is laid. After setting, the pretensioning force of the released steel wire or cable is transferred to the surrounding concrete, so that it is compressed. This creation of compressive stresses makes it possible to partially or completely eliminate tensile stresses from the load.

Reinforcement tension methods:

Grants Pass, Prestressed Concrete Botanical Garden Bridge, Oregon, USA

According to the type of technology, the device is divided into:

tension on stops (before placing concrete in the formwork);
tension on concrete (after laying and curing of concrete).

More often, the second method is used in the construction of bridges with large spans, where one span is made in several stages (captures). The steel material (cable or reinforcement) is placed in a mold for concreting in a case (corrugated thin-walled metal or plastic pipe). After the manufacture of a monolithic structure, the cable (reinforcement) is tensioned to a certain extent by special mechanisms (jacks). After that, a liquid cement (concrete) mortar is pumped into the case with a cable (reinforcement). Thus, a strong connection of the bridge span segments is ensured.

Eugene Freycinet (France) and Viktor Vasilievich Mikhailov (Russia) were at the origins of the creation of prestressed reinforced concrete

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See what "Prestressed concrete" is in other dictionaries:

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