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

When making reinforced concrete, steel reinforcement with high tensile strength is laid, then the steel is tensioned special device and the concrete mixture is laid. After setting, the pre-tensioning 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.

Methods of tensioning reinforcement:

Grants Pass, a prestressed concrete bridge in a botanical garden, Oregon, USA

By type of technology, the device is divided into:

• tension on the stops (before laying concrete in the formwork);
• tension on concrete (after laying and strengthening 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). Steel material (cable or reinforcement) is placed in a form for concreting in a case (corrugated thin-walled metal or plastic pipe). After production monolithic design The cable (reinforcement) is tensioned to a certain extent using special mechanisms (jacks). After that, liquid cement (concrete) mortar is pumped into the case with the cable (reinforcement). This ensures a strong connection between the bridge span segments.

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

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See what “Prestressed reinforced concrete” is in other dictionaries:

prestressed concrete- - [A.S. Goldberg. English-Russian energy dictionary. 2006] Topics: energy in general EN prestressed concrete ...

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prestressed reinforced concrete- Prefabricated or monolithic reinforced concrete structures, the reinforcement of which is stressed to a given design value [Terminological dictionary of construction in 12 languages ​​(VNIIIS Gosstroy USSR)] Topics construction products other EN prestressed... ... Technical Translator's Guide

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Prestressed concrete structures are those in which, before applying loads during the manufacturing process, significant compressive stresses are artificially created in the concrete by tensioning high-strength reinforcement. Initial compressive stresses are created in those areas of concrete that subsequently experience tension under the influence of loads. At the same time, the crack resistance of the structure increases and conditions are created for the use of high-strength reinforcement, which leads to savings in metal and a reduction in the cost of the structure.
The specific cost of reinforcement, equal to the ratio of its price (rub/t) to the calculated resistance Rs, decreases with increasing strength of the reinforcement. Therefore, high-strength reinforcement is much more profitable than hot-rolled reinforcement. However, it is impossible to use high-strength reinforcement in structures without prestressing, since with high tensile stresses in the reinforcement and corresponding elongation deformations, significant opening cracks appear in the tensile zones of concrete, depriving the structure of the necessary performance qualities.
The essence of prestressed reinforced concrete is the economic effect achieved through the use of high-strength reinforcement. In addition, the high crack resistance of prestressed reinforced concrete increases its rigidity and resistance dynamic loads, corrosion resistance, durability.
In a prestressed beam under load, concrete experiences tensile stresses only after the initial compressive stresses have subsided. In this case, the force causing the formation of cracks or their opening limited in width exceeds the load acting during operation. As the load on the beam increases to the maximum destructive value, the stresses in the reinforcement and concrete reach their maximum values.
Thus, reinforced concrete prestressed elements operate under load without cracks or with their opening limited in width, while structures without prestressing are operated in the presence of cracks and with large values deflections. This is the difference between prestressed and non-prestressed structures with the ensuing features of their calculation, design and manufacture.
In the production of prestressed elements, there are two possible ways to create prestress: tension on stops and tension on concrete. When tensioning on stops, before concreting the element, the reinforcement is inserted into the mold, one end of it is fixed in the stop, the other is tensioned with a jack or other device to a given controlled tension. After the concrete has acquired the required cubic strength, the reinforcement is released from the stops before compression. Reinforcement, when restoring elastic deformations under conditions of adhesion to concrete, compresses the surrounding concrete. With the so-called continuous reinforcement, the mold is placed on a pallet equipped with pins, the reinforcing wire is wound with a special winding machine onto tubes placed on the pins of the pallet with a given voltage value, and its end is secured with a die clamp. After the concrete gains the required strength, the product with the tubes is removed from the pallet pins, while the reinforcement compresses the concrete.
The rod reinforcement can be tensioned onto the stops using an electrothermal method. Rods with upset heads are heated electric shock up to 300-350 °C, put into the mold and secured at the ends in the stops of the molds. When the reinforcement is restored to its original length during the cooling process, it is pulled onto stops.
When tensioning concrete, first a concrete or weakly reinforced element is made, then, when the concrete reaches strength, a preliminary compressive stress is created in it. The prestressing reinforcement is inserted into channels or grooves left when the element is concreted, and is pulled onto the concrete. With this method, the stresses in the reinforcement are controlled after the concrete has been compressed. Channels that exceed the diameter of the reinforcement by 5-15 mm are created in concrete by laying extractable void formers (steel spirals, rubber hoses, etc.) or leaving corrugated steel tubes, etc. The adhesion of the reinforcement to the concrete is created after compression by injection - injection of cement into the channels test or solution under pressure. Injection is carried out through tees installed during the manufacture of the element - bends. If the prestressed reinforcement is located with outside element (ring fittings of pipelines, tanks, etc.), then its winding with simultaneous compression of concrete is carried out using special winding machines. In this case, after tensioning the reinforcement, it is applied to the surface of the element by shotcrete (under pressure) protective layer concrete.
Tensioning on stops, as a more industrial method, is the main method in factory production.

K category: Reinforcement 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 on average for reinforcement). This is especially seriously reflected in horizontal structures that work in bending - slabs, beams, crossbars, etc. Under the influence of load, tensile stress appears here. Therefore, in the stretched zone of the section of a reinforced concrete structure it is necessary to place a large number of reinforcement, which increases the cross-sectional area and 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. At full use 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 disadvantages are largely eliminated in prestressed reinforced concrete structures.

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

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

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

The presence of prestress allows you to increase the load on the structure compared to a reinforced structure in the usual way, or at the same load, reduce the size of the structure, i.e., save concrete and steel.

The idea of ​​prestressing (compression) of tensile elements was first 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 work well in compression is fully used throughout the entire section. This makes it possible to reduce the cross-sections, and therefore the volume and weight of prestressed elements, by 20-30% and reduce the consumption of materials, in particular cement.

2. Thanks better use properties of reinforcing steel in prestressed structures, the consumption of reinforcement is reduced compared to conventional ones. Savings in reinforcement, especially effective and necessary when using steels with high tensile strength, reaches 40%.

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

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

Examples of the most common prefabricated prestressed structures are slabs for covering industrial buildings, crane beams, roof beams, etc.

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

When constructing prefabricated monolithic structures, individual prefabricated elements are connected in such a way that later during operation they work as one whole. This is done as follows.

When manufacturing prefabricated elements of a future prefabricated monolithic structure, reinforcement outlets are left behind. 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 forms one whole. Then reinforced seams(or joints) are filled with concrete, or, as they say, enclosed. After the concrete hardens at the joints and seams, a structure called prefabricated monolithic is obtained.

This method is often used in the designs of multi-story buildings (Fig. 1) and in spatial structures with curved outlines - vaults and domes.

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

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

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

Between reinforced mesh install special frames(Fig. 2, c). The relative position of the metal panels of the internal and external formwork and reinforcing mesh, and therefore the thickness of the protective layer of concrete, was fixed with bolts 9 with plastic tubes put on them (Fig. 2, c).

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

As prestressing reinforcement for the lower part and trunk of the tower, ropes with a diameter of 38 mm were used, 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 tension of the ropes was carried out 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 prestressing reinforcement for prestressed structures, it is advisable 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 made from it of the following types: – hot-rolled steel of a periodic profile of class A-Shv, strengthened by drawing; – hot-rolled steel of periodic profile of classes At-V and. At-VI, thermally strengthened; – hot-rolled steel of periodic profile of classes A-IV and A-V; – high-strength reinforcing wire, smooth and with a 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 reliable adhesion of the surface of the reinforcement to the surrounding concrete.

This explains the use of strands and ropes with complex shape surfaces.

Seven-wire strands are produced 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, ranging from 8 to 48. To preserve relative position wires inside the bundle, pieces of wire spirals are installed every 1-1.5 m. In the same places, the bundle is tied from the outside 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 where short wires 4 are installed along the length of the bundle, gaps remain through which the middle of the bundle is filled with solution. Multi-row bundles of groups of wires with a diameter of up to 8 mm (Fig. 3, c) are used in engineering structures, for example bridges. A package is a group of wires or strands arranged in several rows horizontally and vertically along a regular geometric grid.

Tensioning of reinforcement when reinforcing prestressed structures is carried out in two ways - before or after concreting.

Tension on forms or stops. When reinforcing using this method, the reinforcing bars are tensioned before laying the concrete mixture. Tensile forces, sometimes reaching several tens of tons in value, are absorbed by the powerful structure of the steel mold in which the product is manufactured, or by special stops of the stand, which is why this method is called the bench method. The structure is concreted with tensioned reinforcement. When the tension devices are removed after the concrete has cured, compression of the concrete is achieved by the adhesion between the tending to compress reinforcing bars and the surrounding hardened concrete.

The reduction in length upon compression is shown in conventional scale, since it is invisible to the eye.

At this method control of tension (and therefore stress) of the reinforcement is carried out before compression of the concrete.

Tension of reinforcement on concrete. In this case, the tension force of the reinforcement is perceived not by the form, but by the hardened concrete. This method is used mainly for reinforcing structures assembled from individual blocks. The method of tensioning concrete allows you to assemble large-sized structures (up to 30 m or more in length) at the site of their installation from separate, easily transportable smaller parts. The tension of the reinforcement is controlled during the concrete compression process. Compression can be carried out only after the hardened concrete has accumulated strength sufficient to withstand the forces created by tension devices.

Apply various ways reinforcement tension: mechanical - using special jacks; electrothermal, which uses the property of a steel rod to elongate when heated, and electrothermo-mechanical, which is a combination of the first two.

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

- About prestressed concrete

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, key words that reflect the essence of the material are highlighted. To identify the role of each of the highlighted concepts, let us consider in more detail the essence of each of them.

Concrete is fake diamond, which, like any stone material, has a fairly high compression resistance, and its tensile resistance 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, let's consider how the strength of concrete is used in a bending element (Fig. 1a). When a beam bends 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 the beam is loaded, the stresses in the tensile zone reach the tensile strength of concrete R bt, the outermost fiber will rupture, i.e. the first crack will appear. This will be followed by brittle failure, i.e. beam fracture. Stresses in the compressed zone of concrete sbc at the moment of destruction will be only 1/10 ¸ 1/15 of the compressive strength of concrete Rb, i.e. the strength of concrete in the compressed zone will be used by 10% or less.

For example reinforced concrete beams with reinforcement, let's consider how the strength of concrete and reinforcement is used here. The first cracks in the tensile zone of concrete will appear at almost 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 cracks appear, the tensile force in the section with the crack will be absorbed by the reinforcement, and the beam will be able to withstand an increasing load. Failure 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 compressive strength of concrete. In this case, initially, when the yield strength s flow 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 compressive strength. Rb. Since the stress level 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 larger load ( N in Fig. 1-b). Conclusion- the feasibility of reinforced concrete lies in the fact that tensile forces are absorbed by reinforcement, and compressive forces are absorbed by concrete. Hence, main purpose of fittings in reinforced concrete is that it is she who must absorb tension due to the insignificant tensile strength of concrete. By means of reinforcement, the load-bearing capacity of a bending element, compared to concrete, can be increased by more than 20 times.

The joint deformation of concrete and reinforcement installed in it is ensured by adhesion forces that occur during 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 (obviously, the share of this adhesion component is small); secondly, due to compression of the reinforcement by concrete due to its shrinkage during hardening; thirdly, due to the mechanical engagement of concrete on the periodic (corrugated) surface of the reinforcement. Naturally, for periodic profile reinforcement this component of adhesion is the most significant, therefore the adhesion of periodic profile reinforcement to concrete is several times higher than that for reinforcement with a smooth surface.

The very existence of reinforced concrete and its good durability were made possible thanks to favorable combination some important physical and mechanical properties of concrete and steel reinforcement, namely:

1) when concrete hardens, it adheres firmly to steel reinforcement and under load, both of these materials are deformed together;

2) concrete and steel have similar values ​​of linear thermal expansion coefficients. That is why when temperature changes environment within the range of +50 o C ¸ -70 o C there is no disruption of adhesion between them, since they are deformed by the same amount;

3) concrete protects reinforcement from corrosion and direct fire. The first of these circumstances ensures the durability of reinforced concrete, and the second ensures 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 necessary 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 impact of its shortcomings on the performance of the structure.

TO merits (positive properties) reinforced concrete include:

1. Durability - with correct operation reinforced concrete structures can serve indefinitely without deterioration bearing capacity.

2. Good resistance to static and dynamic loads.

3. Fire resistance.

4. Low operating costs.

5. Cheap and good performance.

To the main disadvantages of reinforced concrete relate:

1. Significant dead weight. This disadvantage is eliminated to some extent by using lightweight aggregates, as well as by using progressive hollow-core and thin-walled structures (that is, by choosing a rational cross-sectional shape and outline of the structures).

2. Low crack resistance of reinforced concrete (from the example discussed above it follows that there should be cracks in tensile concrete during the operation of the structure, which does not reduce the load-bearing capacity of the structure). This disadvantage can be reduced by using prestressed reinforced concrete, which serves radical means increasing its crack resistance (the essence of prestressed reinforced concrete is discussed in topic 1.3 below.

3. Increased sound and thermal conductivity of concrete in some cases requires additional costs for thermal or sound insulation of buildings.

4. The 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 loads on them increase.

Prestressed reinforced concrete: its essence and methods of creating prestress

Sometimes the formation of cracks in structures in which operating conditions are unacceptable (for example, in tanks, pipes, structures exposed to aggressive environments). To eliminate this disadvantage of reinforced concrete, prestressed structures are used. In this way, it is possible to avoid the appearance of cracks in concrete and reduce the deformation of the structure during operation.

Let's consider short definition 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 the section of the structure that experiences tension during operation (Fig. 2).

As a rule, initial compressive stresses in concrete are created using pre-tensioned high-strength reinforcement

This increases the crack resistance and rigidity of the structure, and also creates conditions for the use of high-strength reinforcement, which leads to savings in metal and a reduction in the cost of the structure.

The unit cost of reinforcement decreases with increasing reinforcement strength. Therefore, high-strength reinforcement is much more profitable than conventional reinforcement. 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 tensile zones of concrete will significantly open, thereby reducing the required performance qualities of the structure.

Advantages prestressed reinforced concrete over conventional concrete is, first of all, its high crack resistance; increased structural rigidity (due to reverse bending obtained when compressing the structure); better resistance dynamic loads; corrosion resistance; durability; as well as certain economic effect, achieved by using high-strength reinforcement.

In a prestressed beam under load (Fig. 2), concrete experiences tensile stresses only after the initial compressive stresses have been extinguished. Using the example of two beams, it can be seen that cracks in a prestressed beam form at a higher load, but the failure 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.

When producing prestressed reinforced concrete structures in a factory, there are two possible options: circuit diagrams creating prestress in reinforced concrete:

prestressing with tensioning of reinforcement on stops and on concrete.

When pulling on the stops the reinforcement is placed into the mold before the element is concreted, one end is fixed to the stop, the other is tensioned with a jack or other device to a controlled tension. Then the product is concreted, steamed and after the concrete has acquired the necessary cubic strength to absorb compression Rbp the reinforcement is released from the stops. The reinforcement, trying to shorten within the limits of elastic deformations, if there is adhesion to the concrete, drags it along with it and compresses it (Fig. 3-a).

When tensioning reinforcement on concrete (Fig. 3-b) first, a concrete or lightly reinforced element is made, then after the concrete reaches strength Rbp create a preliminary compressive stress in it. This is done as follows: the prestressed reinforcement is inserted into the channels or grooves left when concreting the element, and tensioned using a jack, resting directly on the end of the product. In this case, concrete compression occurs already during the process of tensioning the reinforcement. With this method, the stress in the reinforcement is controlled after the concrete has been compressed. Channels in concrete that exceed the diameter of the reinforcement by (5¸15) mm are created by laying subsequently removed void formers (steel spirals, rubber tubes, etc.). The adhesion of the reinforcement to the concrete is achieved due to the fact that after compression it is injected (cement paste or mortar is pumped into the channels under pressure through tees - bends - installed 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 its winding with simultaneous compression of the concrete is performed with special winding machines. In this case, a protective layer of concrete is applied to the surface of the element after tensioning the reinforcement.

Stop tensioning is a more industrial method in factory production. Tensioning on concrete is mainly used for large-sized structures, created directly at the site of their construction.

Reinforcement tension on the stops can be carried out not only using a jack, but also using an electrothermal method. To do this, the rods with the upset heads are heated by electric current to 300 - 350°C, inserted into the mold and secured in the mold stops. When the initial length is restored during cooling, the reinforcement becomes stretched. The reinforcement can also be tensioned using the electrothermo-mechanical method (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 to make: foundations, columns, roofing and floor slabs, Wall panels, beams and trusses, crane beams, i.e. almost all frame elements of single- and multi-story buildings.

Special structures during the construction of industrial and civil complexes - retaining walls, bunkers, silos, tanks, pipelines, power line supports, etc.

In hydraulic engineering and road construction Reinforced concrete is used to make dams, embankments, bridges, roads, runways, etc.

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

Concrete is used in various types construction. The name "pre" does not mean that this type The concrete was put under tension before the floor above it was built. However, instead of buckling under pressure, it manages to become stronger and is able 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 that will help us understand this better.

What is prestressed concrete?

Concrete in its normal state is extremely high level 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 integrity strength. Therefore, if ordinary concrete is used for the construction of floors, it will sag under the pressure of compression on it, and will eventually crack and crumble. To eliminate this drawback, the prestressing method is used. In its most basic form, prestressing is accomplished as follows.

A series of steel cables are tensioned by applying a pulling force at their ends, and placed in a concrete block. The liquid concrete is then poured into the molds and hardens, causing a bond between it and steel cables inside. After this, 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 use in structures. Since the concrete is stressed before it is used, it is called prestressed concrete.

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

Advantages and disadvantages of prestressed concrete

Advantages

1) high tensile strength and crack resistance

Regular concrete slab, if put under tension, sags down under the pressure of the weight. In this situation, top part The slab 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 able to bear heavy loads without cracking or failing.

2) Below depth

Due to its high strength, prestressed concrete can be used to build structures that have significantly less depth than reinforced concrete structures. This has two main advantages. When 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 structural depths is that they weigh less, 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 have a longer lifespan compared to reinforced concrete. When constructing buildings, this means that fewer columns will be needed to support the slabs, and the spacing between them can be significantly greater. For bridges, the use of prestressed concrete can allow engineers to build long bridge, which will not fail under load.

4) fast and reliable construction

Prestressed concrete blocks are manufactured commercially in several standard shapes and sizes. These are known as prefabricated blocks. Since they are professionally manufactured, they have a very good build quality and at the same time, they provide the full strength of the benefits of precast concrete. They can be directly delivered to the construction site and used for quick completion construction work. Structures built using these blocks are known to have best quality, and longer operation.

Flaws

1) Great complexity of the building

Prestressing of concrete at construction site- it is labor-intensive and difficult process. One must have in-depth knowledge of each step that is involved along with complete knowledge of the use of various equipment. Precast concrete structures are manufactured once and are difficult to change and hence the complexity of the initial planning also increases. Moreover, since the probability of error is very low, great care must be taken during construction.

2) Increased construction costs

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 additional tensile strength, prestressed concrete may be unnecessary, since 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 inspected to ensure that it has been subjected to the appropriate stress. Too much attention is also bad and can cause damage to the concrete, making it weaker.

Prestressed concrete structures provide superior tensile strength compared to normal and even reinforced concrete, but they are complex in construction 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 the design specification requires it.