Armature- used in construction are subject to various classifications. This is necessary in order to choose exactly what is needed for a specific design and type of work.

In addition to division by profile, diameter, and class, there is also a division into non-prestressed and prestressed reinforcement. This point is very important, since companies whose main business is the sale of rolled metal often forget about it. Stress inside the reinforcement achieved by pre-tensioning the reinforcement. It is important to note that with prestressing, hot-rolled reinforcement of classes from A600 to A1000 is used, as well as cold-deformed B500 and BP500, and rope Kr1400. Such reinforcement has a significant diameter and yield strength value. This is necessary so that the reinforcement does not lose its strength characteristics during prestressing and does not receive maximum permissible deformations.

Prestress in reinforcement is set in two ways.

1. The so-called bench method
2. Concrete compression method.

Bench method

The first is divided into several methods: mechanical, electrothermal, and electrothermo-mechanical, that is, mixed. Despite appearances, the technology of these methods is in many ways similar to each other. At mechanical method the reinforcement is pulled onto stops and stretched, after which concrete is poured into the mold, and when the transfer strength is reached, the reinforcement is released. When compressed, concrete slows down the reinforcement, preventing it from completely compressing. As a result, compressive forces appear in the reinforcement. This is extremely important for tensile elements. Electrothermal is in many ways similar to mechanical, only in this case electricity is supplied to the fittings and heated to high temperature, during the cooling process, the compression forces of the reinforcement are set. Mixed is the result of the simultaneous use of both mechanical and electrothermal.

Concrete compression method

The concrete compression method is a method in which the element gains its full mass without the use of reinforcement, but holes are left in it into which plastic tubes are inserted. Thus, the reinforcement is threaded through the holes and stretched, setting the tension. After this, the space in the holes between the reinforcement and the tube is sealed with concrete under pressure. This method is extremely effective for the manufacture of long-span structures, for example, trusses of industrial and special buildings. Prestressed elements have expanded construction possibilities.

1.4.1. Stressed reinforcement of structures. Prestressing in monolithic and prefabricated monolithic structures is created by the method of tensioning reinforcement on hardened concrete. In turn, according to the method of laying prestressed reinforcement, the method is divided into linear and continuous. With the linear method, channels (open or closed) are left in prestressed structures when concreting them. Once the concrete has acquired a given strength, reinforcing elements are placed in the channels and they are tensioned, transferring forces to the prestressed structure. The linear method is used to create prestress in beams, columns, frames, pipes, silos and many other structures. The continuous method involves winding endless reinforcing wire with a given tension along the contour of a concreted structure. In domestic construction, the method is used to prestress the walls of cylindrical tanks.

1.4.2. In linear reinforcement, prestressing elements are used in the form of individual rods, strands, ropes and wire bundles. Linear reinforcement includes: preparation of prestressing reinforcement elements; formation of channels for prestressed reinforcement elements; installation of prestressed reinforcement elements with anchor devices; tension of the reinforcement followed by injection of closed channels or concreting of open channels.

1.4.3. For rod reinforcement, hot-rolled steel of periodic profile of classes A-II, A-IIIb, A-IV4, At-IV, A-V, At-V, and At-VI and high-strength wire B-II and BP-N are used.

1.4.4. Preparation of rod elements (Fig. 1.4.1. a) consists of straightening, cleaning, cutting, butt welding and installation of anchors. To install anchors, steel shorts are welded to the ends of the rods (Fig. 1.4.1. b). The shorts have a thread onto which nuts are screwed, transmitting tensile loads through washers to the concrete.

1.4.5. Reinforcing non-unwinding strands and ropes are made from high-strength wire with a diameter of 1.5...5 mm. The industry produces three-, seven- and nineteen-wire strands (classes P-3, P-7 and P-19) with a diameter of 4.5... 15 mm (Fig. 1.4.1. c). Ropes are made from strands (Fig. 1.4.1. d, e).

Rice. 1.4.1. Prestressed linear reinforcement elements:
a - rod element; b - rod anchor; c - seven- and nineteen-wire strand; d - two- and three-strand rope (strand of 7 wires); d - double-strand rope (strand of 19 wires); e - sleeve anchor; g - sleeve-rod anchor;
1 - rod reinforcement; 2 - short piece with thread at the end; 3 - plate; 4 - nut; 5 - shank; 6 - beam reinforcement; 7 - sleeve

1.4.6. Strands and ropes come from factories wound on metal spools. They are unwound from reels and passed through correct devices, simultaneously cleaning from dirt and oil, and cutting to the required length. For anchoring strands (ropes), sleeve tips are used (Fig. 1.4.1. e). The sleeve is put on the prepared end of the strand (rope), pressed in with a press or jack, and then a thread is cut or rolled on its surface to attach the jack coupling, with the help of which the strand (rope) is tensioned.

1.4.7. Wire bundles are made from high-strength wire. The wire is placed filling the entire section or along the circumference. In the first case, the beam is equipped with a sleeve anchor, and in the second - with a sleeve-rod anchor (Fig. 1.4.1. g).

1.4.8. The finished elements of strand and rope reinforcement are wound onto drum-type containers, and the anchors are lubricated with grease and wrapped in burlap.

1.4.9. To form channels for prestressed reinforcement elements, channel formers are installed in the structure prepared for concreting, the diameter of which is 10... 15 mm larger than the diameter of the rod or reinforcement beam. For this purpose they use steel pipes, rods, rubber hoses with a wire core, etc. Since channel formers are removed 2...3 hours after the structure is concreted, they, with the exception of the hoses, are turned every 15...20 minutes to avoid adhesion to the concrete around the axis.

1.4.10. For stressed reinforcement of large-sized structures, channels are arranged by laying thin-walled steel corrugated tubes that remain in the structure. After the concrete has reached its design strength, reinforcement is installed (stretched) into the channels.

1.4.11. Then the reinforcement is tensioned using single-action hydraulic jacks. These jacks consist (Fig. 1.4.2. a) of a cylinder, a piston with a rod, a gripper with replaceable nuts that allow tensioning of reinforcement with different diameters of anchoring devices, and a stop. After attaching the fittings to the gripper and supplying oil to the right cavity of the cylinder, the fittings are tensioned to the specified force. Then screw the anchor nut all the way into the structure, switch the right cavity to drain and supply oil to the left side. At this point the tension ends and the jack is disconnected.

1.4.12. To drive hydraulic jacks, mobile oil pumps are used. pumping stations, mounted on a trolley with a boom for hanging jacks (Fig. 1.4.2. b).

1.4.13. Tensioning of reinforcement and transfer of force to concrete is usually accompanied by: straightening of the reinforcing element (beam or rod); compression of concrete under support pads; friction between reinforcement and channel walls, etc.

Rice. 1.4.2. Prestressing of structures:
a - diagram of a single-action hydraulic jack; b - pumping station;
1 - cylinder; 2 - piston; 3 - rod; 4 - capture; 5 - jack stops; 6 - stand with bracket; 7 - hand winch; 8 - oil tank; 9 - control panel; 10 - electric motor; 11 - oil pump; 12 - pressure gauge

1.4.14. To eliminate these phenomena, which cause uneven tension along the length of the reinforcing element, the following operations are performed. First, the reinforcement is tensioned with a force not exceeding 0.1 of the required tension force of the beam (rod). In this case, the reinforcing bars are straightened and fit tightly to the channel walls. The support pads also fit tightly to the surface of the prestressing structure. A force equal to 0.1 of the calculated one is taken as a zero reference for further monitoring of tension using a pressure gauge and deformations.

1.4.15. In structures with a straight channel length of no more than 18 m, the reinforcement is strained on one side due to low friction forces. Stresses can also be equalized along the reinforcement by longitudinal vibration during the tensioning process. You can vibrate using a special device on a blind anchor.

1.4.16. When the length of straight channels exceeds 18 m and curved channels, the reinforcement is tensioned on both sides of the structure. First, using one jack, the reinforcement is tensioned to a force equal to 0.5 of the calculated one, and secured on the side of the structure from which it was tensioned. Then, on the other side of the structure, using another jack, the reinforcement is tensioned to 1.1 of the design force (1.1 is the coefficient of technological tension of the reinforcement). After keeping it in this state for 8...10 minutes, the tension value is reduced to the specified value and the second end of the prestressed reinforcement is secured. To eliminate the stress difference along the reinforcement, pulsating tension is sometimes used, i.e. this process is briefly repeated several times, successively increasing the amount of tension force, and then the excess force is released.

1.4.17. If there are several reinforcing elements in a cross-section of a structure, then tension begins with the element located closer to the middle of the section. If there are only two elements located at the edges, tension is carried out in steps or simultaneously with two jacks. With a large number of elements in the first, the tension will gradually decrease as the subsequent ones are tensioned as a result of increasing shortening of the concrete due to compression. These elements are then tightened again.

1.4.18. The final operation is the injection of channels, which begins immediately after tensioning the reinforcement. To do this, use a solution of at least M300 with M400... 500 cement and clean sand. The solution is pumped with a mortar pump or pneumatic blower from one side of the channel. Injection is carried out continuously with an initial pressure of 0.1 MPa and a subsequent increase to 0.4 MPa. Stop injection when the solution begins to flow out from the other side of the channel.

1.4.19. Recently, a method without the installation of channels has been used; in this case, operations involving their injection are excluded. Before laying, reinforcing ropes or rods are coated with an anti-corrosion compound and then with fluoroplastic (Teflon), which has an almost zero coefficient of friction. When tensioned, the rope slides relatively easily in the concrete body.

Stressed reinforcement

In structures that work in bending (slabs, beams, purlins, etc.), tensile stresses appear under the influence of load and their own weight. To perceive them, it is necessary to place them in the stretchable zone a large number of fittings. Despite this and the provision of y for the reinforcement elements, cracks may be observed in the zones of maximum moment.

In order to increase the crack resistance and load-bearing capacity of reinforced concrete structures, as well as to more fully utilize the mechanical properties of reinforcing steel and reduce its consumption, prestressing reinforcement is used.

These are called prestressed reinforced concrete structures, in which in the expected tension zones, before the application of operational loads, an internal stress state is artificially created, expressed in compression of concrete and stretching of reinforcement. Before the concrete in a structure begins to work in tension, the pre-created compression must be extinguished.

Prestressed concrete structures offer many advantages over conventional reinforced concrete structures. Their performance qualities are higher, since due to crack resistance, rigidity and durability increase, and water resistance increases.

In prestressed reinforced concrete structures, they are effectively used high strength materials, which saves steel up to 40%, allows you to reduce the cross-sectional dimensions of structural elements by 20 - 30%, and reduce transportation costs.

It should be noted that in some cases it is advisable to use low-grade concrete, since prestressing brings the deformative properties of high-strength reinforcement and concrete closer together and ensures their joint operation at all stages of loading.

When producing prestressed elements, the following are required: technological processes, which make it possible not to waste excess material in elements due to the operating conditions of structures during the pre-operation period. For structures intended for manufacture, rational methods of production, transportation and assembly must be provided so that at these stages, before the structure is finally included in the building or structure, it is not subject to conditions more severe than under subsequent operational load.

In all cases of manufacturing prestressed elements, the reinforcement is tensioned using one of the following methods.

Pre-compression. The reinforcement is laid and tensioned (on stops) before concrete is laid. Stress control in reinforcement is carried out in in this case before compressing the concrete.

Subsequent compression. The reinforcement is placed in forms before laying concrete or in the channels of elements during the manufacturing process of the structure, but is tensioned (on concrete) after the concrete has gained sufficient strength in order to transfer the compression forces created by tension devices to it. In this case, tension control in the reinforcement is carried out during the concrete compression process.

In accordance with the accepted methods of stressed reinforcement of reinforced concrete structures, the reinforcement used in them is respectively called “pre-tensioned” and “post-tensioned”. For the manufacture of most types of structures, reinforcement with pre- or post-tensioning is used. Only in composite structures can both types of prestressed reinforcement be used, of which prestressed reinforcement is used in the manufacture of individual elements, and post-stressed reinforcement is used when assembling a structure from these elements.

In the manufacture of reinforced concrete structures with prestressed reinforcement, when concreting is carried out after tensioning the reinforcement, initial adhesion of the reinforcement to the concrete must be ensured, and control of the tension of the reinforcement

must be done before the concrete is compressed.

If post-tensioning is applied after the concrete has hardened, there is no adhesion to the concrete of the reinforcement located inside or outside the element; the tension of the reinforcement in this case is controlled after compression of the concrete.

The adhesion between the reinforcement and concrete is restored by subsequent concrete coating of the reinforcement of the element after tensioning the reinforcement.

Based on anchoring methods, prestressed reinforcement is divided into the following types:

a) unanchored from cold-drawn high-strength wire, hot-rolled, cold-drawn or low-alloy steel of periodic profile;

b) continuously wound from cold-drawn high-strength wire with the ends secured.

Post-tensioned reinforcement from single bars constant cross section, a bundle of wires made of cold-drawn or low-alloy steel is always anchored.

Methods of tensioning reinforcement - mechanical, electrothermal and electrothermomechanical. Reinforcement work in tension consists of preparing prestressed reinforcement and reinforcing elements, connecting, laying and tensioning the reinforcement.

PREPARATION AND CONNECTION OF STRESSED REINFORCEMENT.

For prestressed reinforcement up to 12 m in length, rods made of hot-rolled steel of classes A-600, A-800, A-1000 (A-IV, A-V, A-VI), steel reinforced by drawing of class A-400v, thermally strengthened steel of classes At- 600 and At-800, as well as high-strength wire of classes B-II, BP-II and steel ropes of classes K-7 and K-19. When the length of prestressing reinforcement is more than 12 m, hot-rolled and thermomechanically strengthened reinforcement of classes A-600, A-800, A-1000, At-600s and A-400v, high-strength wire and ropes are used, the same as for prestressing reinforcement up to 12 m in length. Rods made of steel of class A-400v are obtained by preliminary hardening by drawing of reinforcement of class A-400. Strengthening by drawing is carried out to values ​​​​corresponding to controlled elongation and stress. To control stretch strengthening, two samples are taken from each batch of rods for tensile testing.

The reinforcement blank includes connecting the rods into a “lash” up to 24 m long. Anchor heads are planted at the ends of such blanks (Fig. 1, 6), and then strengthening is carried out by drawing (Fig. 2). Reinforcement blanks are prepared on special semi-automatic lines equipped with machines for butt welding or crimping of the cage, a machine for cutting reinforcement, a stand for strengthening by mechanical drawing (Fig. 2), a hydraulic jack and other equipment. The productivity of such semi-automatic lines is up to 7 tons of reinforcement per shift. The lines are equipped with pneumatic systems

mathematics and automation, providing operation in automatic and manual modes. The line is serviced by two people - a welder and an operator.

Figure 1. Machine for planting anchors SMZh-128B

Figure 2. Installation for lengthening reinforcing bars SMZh-129B

You can connect the rods different ways depending on the steel class and rod diameter. Thus, rods made of A-600 and A-800 steel, which will subsequently be subjected to stretch hardening, are connected using resistance butt welding. To join rods made of difficult-to-weld steel of classes At-600, At800 At-1000, pressed-in connecting clips (couplings, sleeves) are used (Fig. 3a, 4). Reinforcing ropes are connected using a pressed sleeve (Fig. 3b), but they can also be connected with an overlap, along the entire length of which turns of knitting wire are placed in a dense row (Fig. 3c). You can connect the reinforcing wire using a coupling and a plug. This connection provides a coaxial, equal-strength joint (Fig. 5).

Figure 3. Joining of reinforcement elements. a - rod reinforcement using a pressed coupling; b - reinforcing ropes of strands with compression sleeve; c - lap, reinforcing ropes, strands wrapped with knitting wire; 1- coupling; 2 - rod; 3 - sleeve; 4 - rope, strand; 5 - wire winding

Figure 4. Connecting reinforcing bars by crimping with a coupling. Fittings intended for tension, as well as non-tensioned ones, are subject to

It is subject to pre-processing, which includes cleaning, straightening, welding and cutting. In addition, the prestressed reinforcement is subjected to additional processing or arrangement. This is the landing and installation of anchor heads. The heads of reinforcing bars are planted, for example, on the SMZh-128B installation (Fig. 1). In addition to force-planting the heads of the ends of rods and wire reinforcement, other methods of anchoring reinforcement can be used. Various anchoring devices at the ends of reinforcement products are shown in Figure 6.

Figure 5. Coaxial equal-strength joint. 1 - wires; 2 - plug; 3 - coupling; 4 - mounted heads.

Figure 6. Anchor devices at the ends of reinforcement products. a - with welded shorts: b - with a welded loop; c - with a welded plate; g - with a set head on a rod; d, f - with upset heads on high-strength wire; g - with a pressed and compressed bushing on the rod; h - with a pressed tube on a rope, strands; 1- rope, strand with tube assembly; 2 - tube blank.

More complex in manufacturing and design are anchor elements for reinforcement made of difficult-to-weld or non-weldable steels, as well as for tensioning several strands at the same time. Thus, on bench or aggregate-production production lines using high-strength heat-resistant wire with a diameter of 3-8 mm, unified prestressing elements (UNRAE), for example, are used. TsNIIOMTP designs with a slotted or perforated block (Fig. 7).

Anchor elements for reinforcement made of difficult-to-weld or non-weldable steels, as well as for tensioning several strands at the same time, are complex to manufacture and design. Thus, on bench or aggregate production lines using high-strength heat-resistant wire with a diameter of 3-8 mm, standardized prestressing elements with a slotted or perforated block are used (Fig. 7). The wire is pre-set according to size (length). In anchor blocks, the reinforcement is secured by placing heads on the ends of the wire. Depending on the number of wires fixed in the block, these reinforcing elements are unified by brand. For cold heading of reinforcing wire heads, SMZh-155 or SMZh-311 machines are used. When tensioning reinforcement on mold stops and on concrete, various anchor devices are used depending on the diameter and type of reinforcement (Table 1).

Figure 7. Unified prestressing reinforcement elements. a - with a perforated anchor block; b - with slotted anchor block; 1

Anchor block; 2 - high-strength wire; 3 - spiral clamp; 4 - planted heads.

Table 1

Clamp type		Armature	Purpose
	For reinforcement bars
		Periodic pro-	When tensioning reinforcement
		fillet with a diameter of 12 -
			on mold stops
VNIIStroineft		The same, diameter 12
Factory "Barrika-		The same, with a diameter of 16

Construction process technology				Lecture 7.3.1
		For wire reinforcement
Wedge single			High strength wire	When pulling on the stops
			loca is smooth and perio-
				forms and stands
			dic profile
Rod group			High strength wire	When pulling on the stops
			loca is smooth and perio-
			dic profile
				For automatic
			High strength wire	fastening of fittings at
			lock or strand	continuous reinforced
For beam art-				When tensioned on concrete

LAYING AND TENSIONING REINFORCEMENT

There are two main ways of laying reinforcement into forms, stands or finished structures, where it is subsequently tensioned, namely: linear and continuous.

Linear laying of reinforcement is the laying of rods or wire of a finite length into a device for tensioning reinforcement.

Single rods are usually laid in molds or stands and secured in single rod clamps. Groups of rods or wires are preliminarily combined into packages in which the ends of the reinforcement are secured in one clamping device for a package or bundle suitable for transportation, installation in pre-prepared channels of reinforced concrete structures or protective metal tubes.

Tensioning of reinforcement in the form of single rods, bundles or bundles of wire is carried out using hydraulic jacks (Fig. 13) of various types.

Continuous laying consists of winding wire with preliminary or final tension on pins or contours installed on pallets or stands, depending on the layout of the reinforcement in the product.

Winding and tensioning of reinforcement is carried out by special machines.

For all laying methods and methods of tensioning reinforcement, deviations from the specified control voltage should not be more than 5%.

For the production of short-length products (up to 12 m), the method of pre-tensioning reinforcement is widely used. The production of such parts is carried out on stands or in molds in a factory way. In some cases, when using this method, structures of greater length are made.

The method of post-tensioning reinforcement is expedient, effective and is used for the manufacture of structures longer than 12 m. With this method, composite structures assembled on the construction site from blocks are successfully manufactured.

LINEAR LAYING OF FARMING TENSIONS.

In the manufacture of structures in molds, reinforcement in the form of individual rods is mainly used. However, in some cases, when manufacturing structures in molds, reinforcement is used in the form of a package or bundle.

The process of laying and tensioning reinforcement in the form of individual bars is that the previously cleaned and straightened reinforcing bars are installed in clamps located on the mold tooling; after they are tensioned, the same clamps secure the reinforcement to the form, and in this form the form follows the rest of the production cycle of stress-reinforced structures. Before removing the finished product, the clamps are disassembled, freeing the stressed reinforcement bars. In this case, compression of the reinforced concrete element occurs.

Figure 8. Schemes of stands for the manufacture of prestressed structures: A - diagram of a batch stand; B - diagram of a broaching stand.

IN In the case of using a package of wires, the process of manufacturing parts remains unchanged and differs in that after tensioning the package of wires, it is secured by installing metal spacers between the mold equipment and the clamp body, which combines a number of rods into one package or bundle.

The method of producing reinforced concrete structures on stands has become widely used. There are two types - batch and broaching stands (Fig. 8). The fundamental difference between the schemes of these stands lies in the method of preparing a package of wires and transporting it to the forming site of the stand.

IN package stands wire from coils 9 enters the pulling conveyor8, where it is cut to the required length and then secured in a clamp3, forming a package of 2 wires. Prepared packages from the pulling conveyor are transported to the forming platform1 to the stops4 of the stand, where the package with clamps is fixed in the stop6

and tension 5 devices of the stand. The reinforcement is tensioned using a hydraulic jack 7.

IN broaching stand, coils with wire are installed on a trolley 9, moving from one stand to another. The number of coils corresponds to the number of wires in the product. In addition, the stand has a special trolley8 for pulling the wire package along the forming platform1 of the stand during its formation. After fixing

Technology of construction processes Lecture 7.3.1

By placing one end of all the wires into clamp 3 and securing the clamp to the trolley, the package is pulled along the stand to the length of its working part. The wire is pulled as the cart moves from one end of the stand to the other. When the cart is in the second extreme position, the second clamp is installed and the package is cut off from the wires coming from the coils.

The package is installed in the tensioning 5 and thrust 6 devices installed in the structures 4, after which it is tensioned with a hydraulic jack 7. There are schemes of broaching stands where four wires are pulled several times, providing the required number of wires for the product. Only four wires are also tensioned sequentially.

To ensure uniform tension in the wires of the package within permissible deviations, it is necessary to have reliably operating clamps that do not allow individual wires of the package to slip or bite.

CONTINUOUS WINDING AND TENSIONING OF REINFORCEMENT Continuous winding of reinforcement is also carried out on forms or stands.

On the forms intended for winding continuous reinforcement, pins or a contour with folding jaws are installed for winding reinforcement onto them according to a given pattern.

The mold with pins (Fig. 9) is intended for the manufacture of flat structures; it consists of a pallet1, side equipment2, pins3, onto which stressed reinforcement4 is wound.

Figure 9. Form with pins for continuous winding of prestressed reinforcement.

The mold with a contour (Fig. 10) is intended for the manufacture of beam structures; it consists of a pallet 1, a contour rod 2 and folding jaws 3.

Figure 10. Form with a contour for continuous winding of stressed reinforcement.

Continuous winding of reinforcement onto pins or mold contours is carried out by special machines. Stands on which structures with continuous winding of reinforcement are manufactured are also equipped with a system of pins for working according to a given pattern.

Winding of stressed reinforcement on stands has not yet become widespread.

The pin for winding stressed reinforcement (Fig. 11) is a glass 3 in which a rod is installed, on one side ending in a conical part 2, onto which reinforcement 6 is wound, and on the other side ending in a T-shaped head 4.

Figure 11. Pin diagram.

The head of the pin in relation to the mirror of tray 1 occupies two positions: upper - when the reinforcement is wound and lower - when, after hardening, the conical part of the pin is removed from the hardened product.

The lower and upper positions of the pin are fixed with a finger 5 installed in the rod of the pin. The glass with the pin is installed in the mold and secured with a nut 7.

Winding of stressed reinforcement onto forms with pins is carried out in the following sequence. The free end of the wire is fixed to one of the pins, after which the reinforcement is wound according to a given program. Having finished winding, secure the second end of the reinforcement. After hardening, the pins are removed from the product using special pressing tools. In this case, stress is transferred from the reinforcement to the concrete.

Figure 12. Diagram of a circuit with folding jaws for winding stressed reinforcement

The crossbars are manufactured on a pallet 1 with a special contour 2 (Fig. 12), at the ends of which folding cheeks 3 are installed, ensuring simultaneous winding of the reinforcement onto two crossbars.

Before winding the reinforcement, the folding jaws are in the upper position and the free end of the wire is fixed to the contour rod.

After winding the first row of reinforcement, one strip is dropped from each side of the contour and wound onto the second row of reinforcement, and so on, until the end of winding with a given number of rows of reinforcement and with a certain amount of wire in each row.

Having secured the second end, the wire is cut, and the tray with the contour passes through the necessary molding stations and is sent to the hardening chamber.

After the product has hardened, the reinforcement extending beyond the onboard equipment is trimmed, and the finished product is removed from the pallet.

Reinforcement after subsequent tension For subsequent tension, reinforcement is prepared in the form of rods or

beams, the design of which corresponds to the anchoring devices used in the products and the equipment used for tension.

There are two methods for post-tensioning reinforcement. The first is when the adhesion of the reinforcement to concrete after hardening is not restored, and the second is when this adhesion is restored by subsequent concrete coating of the reinforcement. In products where the adhesion of reinforcement to concrete is not restored, reinforcement is used in the form of individual rods.

The process of laying and tensioning such reinforcement occurs as follows. The reinforcing bar, pre-lubricated with bitumen, is placed in the mold, after which the concrete is placed, compacted, finished and hardened. After removing the hardened product, the reinforcement is tensioned and secured. A layer of bitumen protects the reinforcement from adhesion to the concrete during the formation of the product.

The production of products with stressed reinforcement with the mandatory restoration of adhesion between concrete and reinforcement is developing in two directions. First -

Strength concrete surface during tension it is significantly inferior to this parameter during compression. Under excessive overload, the beam collapses due to the receipt of a certain level of stress in the tensile area even before the strength of the compressed area is exhausted. Destruction occurs quickly, along with the formation of crack-like defects in the concrete in the middle of the span or under load.

The most effective means that makes it possible to use steel without reducing performance characteristics reinforced concrete structures, is the stress resulting from the tension of the rods and the compression of the cement mortar.

Purpose of structures

IN construction work For the formation of reinforced concrete frames, prestressed reinforcement is increasingly being used. The peculiarity of its tension is that the working element at the beginning of the concreting process experiences tension as a result of electrothermal or mechanical (jacking) action. Upon completion of hardening concrete mixture this tension is removed, due to which it, trying to return to its original shape, transfers a residual compressive force to the concrete.

This type of reinforced concrete materials, in comparison with traditional ones, is able to withstand many times increased loads, has increased crack resistance and rigidity. These qualities make it possible to reduce the direct cross-section of the frames, thus reducing the consumption of auxiliary elements: reinforcement and cement mixture.

Reinforced concrete beams obtained as a result of the use of prestressed reinforcement are actively used in the production of complex reinforced concrete structures for the residential and civil construction sector (interfloor floors, main parts of staircases, balconies), in the design of water stations, railway sleepers, cylinder tanks, containers for silage and other things.

Production of fittings

Flat and volumetric frames are produced at reinforcement welding plants and in specialized workshops equipped with highly efficient equipment. At such plants, rationalization consists of producing a large-scale assembly of reinforcement elements, taking into account the permissible dimensions of transport and the lifting characteristics of installation and commissioning units.

By forming prestressing reinforcement, an initial compression is created in concrete along the perimeter of the frame or only in a specific area where tensile stresses are present. The degree of compression should be greater than the tensile stresses that arise in the concrete layer during its operation (about 55 kgf/cm2). The compression of the concrete pad is carried out due to the energy of the elastic aftereffect, which forms the stressed state of the structure.

Prestressing reinforcement elements are made from high-strength wire, as well as single-bar or hot-rolled steel. The choice of finished product is influenced by the category of equipment on which it is stretched.

When producing reinforced concrete products with reinforcement tension, 1-axis and volumetric compression of the cement mixture is used. 1-axis is carried out with a wire bundle or special rods located along the longitudinal axis of the product. Volumetric - by winding a loaded wire in different directions. In addition, wire entanglement on the finished product is allowed, but further protection of the structural elements with a sufficient concrete layer is required.

The mechanical method of tensioning reinforcement involves stretching it under a longitudinal load created by jacking units. The product is initially tensioned to a force equal to 50% of the design value. Then it is increased by another 10% and held for 5 minutes. After this, the degree of interference is reduced to the design level.

The electrothermal method is that the reinforcement is stretched as a result of electric heating to a certain temperature. At the end of the thermal treatment, the hot rod is installed in stops, which effectively prevent its length from shortening when cooling. Then, after the concrete has completely hardened, the fasteners are removed from the reinforcing bars, and the tension force transfers to the concrete plane.

For this tensioning method, devices with parallel and sequential force on several rod supports are used. Compared to the previous method, this technique requires less complex equipment and is not as labor-intensive.

The principle of forming stressed reinforcement is carried out subject to certain laws.

1. By fastening rods with a radius of 1.2-1.5 mm with a cement mixture. When using larger reinforcement elements, fastening is carried out as a result of the formation of additional roughness on it, by wrapping it with special strands of 2-3 wire materials or by using variable-profile structures.
2. As a result of fastening the rods to concrete, which is reinforced with auxiliary anchor units.
3. Due to the stressed effect on concrete by anchor units placed at the ends of the reinforcement.

Advantages of prestressed reinforced concrete frames

• . significant reduction in the amount of steel (30-45%);
• . increased resistance to cracking, preventing corrosive destruction of structures operating in aggressive environments and requiring additional impermeability to liquids and gases;
• . increasing the level of rigidity and decreasing deflection;
• . reducing the volume of concrete mixture and the weight of structures as a result of using high-quality cement;
• . minimization of dimensions cross sections functional parts of the structure and rationalization of their use.

Stressed concrete is a modern building material that is gaining popularity.

Tense resists significant stress much better.

It allows you to overcome one of the main disadvantages of the conventional one - the inability to resist significant stresses. Structures made from this material have a number of advantages over structures made from conventional material:

• have less deflection;
• have increased crack resistance;
• allow you to overlap large plots with the same section of the element.

Prestressed material has a number of advantages:
has less deflection;
has increased resistance to cracks;
with the same cross-section it covers much larger areas.

In conventional reinforced concrete, the mortar associated with the reinforcement is subjected to strong tension, which can lead to failure of the layer due to its sensitivity to tension. Cracks may form on the surface before the element is subjected to maximum load. The appearance of cracks is fraught with certain unpleasant consequences. For example, the fact that the material will not fulfill its protective function and the reinforcement, interacting with environment, will be subject to corrosion and then destruction.

In the manufacture of this material, steel reinforcement is laid, which has high tensile strength. The reinforcement is tensioned using a special device, then the mixture is laid. After the mixture begins to harden, the tension force of the reinforcement frame is transferred to the solution, which becomes compressed. These manipulations make it possible to reduce or completely eliminate tensile stress from the load on the structure, since the force that in ordinary reinforced concrete caused the appearance of cracks on the surface, in prestressed concrete only reduces the compression created by the stressed reinforcement.

There are several main methods of tensioning reinforcement:

• electrothermomechanical - a combination of the following two methods;
• electrothermal - carried out using electric current, which increases the temperature of the reinforcement and, as a result, stretches to a certain size;
• mechanical - carried out using jacks (hydraulic or screw).

As a rule, a prestressing element is designed so that it is not subject to tensile stress during operation. If such an element is subjected to a stress greater than the average, but less than the yield strength of the reinforcement, then after removing the load it can almost completely recover, that is, the cracks in it will disappear.

Requirements for fittings

The tension reinforcement must be made of high-strength wire.

The reinforcement used for prestressing must have certain characteristics that will allow it to withstand the required loads. Steel reinforcement must be able to withstand high voltage stretching, that is, not stretching under prolonged strain.

If the reinforcement does not have this property, then the prestress will decrease, as a result of which the prestressed element will have the same properties as a regular one. Thus, this material will not be able to withstand the loads for which it is designed. For manufacturing it is necessary to use not ordinary steel, but high-strength wire, which is made in a special way, allowing to significantly reduce its fluidity.

Required qualities

1 - shape;
2 - fittings;
3- stops.

To get the most high performance it is necessary to use one that has a certain set of properties. The optimal solution will be the use of a high-strength solution. To prepare it, it is necessary to exercise control throughout the entire cooking process in order to eliminate deviations that could lead to a decrease in its strength.

The highest strength can be achieved by using hard and fatty mixtures. Vibrators are usually used for styling.

Properties to be aware of include shrinkage due to moisture loss and creep under load. These properties can cause the structure to contract, which can cause prestressed concrete to lose its advantages over conventional concrete over time. To avoid the consequences of these material properties, it is necessary to subject the reinforcement to greater prestress than originally intended.

In the initial period of operation, the loss of prestress is higher than in the later period. Overall, the voltage loss can be around 16%.

Pre-tensioning of reinforcement

Hydraulic stops are used to tension reinforcement in production.

The pre-tensioning method involves laying and tensioning the reinforcement first, and then covering it with mortar. The tension of the heavy-duty steel reinforced wire is maintained until the concrete is strong enough. After this, the wire is cut, and its tension is transferred to the mixture due to adhesion to it. Due to this, the concrete is subjected to compressive stress, and production is completed.

This method is generally not used for monolithic structures directly to construction site, its main area of ​​application is the production of prefabricated elements in industrial settings.

In factory conditions, most effective way production of prestressed concrete is the so-called long line system. Using this method, reinforced wire is placed between the anchor plates and then tensioned. The transverse walls must be positioned at a distance corresponding to the planned length of the beams being manufactured.

In the process of applying this method, the tension force is transferred to the formwork of the element.

Pre-tensioning is used for manufacturing monolithic slabs directly at the construction site.

When using this method, it is better to use individual forms. This has the following advantages:

• it becomes possible to vary the sizes of products;
• in case of piece production, if the reinforcement loses tension, only one element will deteriorate.

During the manufacturing process, it is necessary to check randomly selected products.

Post tension

This method differs from the previous one in that during its application the reinforcement is protected from adhesion by a special shell or placed after it has hardened in special holes or recesses. The reinforcing elements are tensioned onto stops, which are installed at the ends of the structure, and the tension is carried out immediately after hardening.

A vibrator is used for pouring.

The application of this method has its own characteristics. The applied force is increased to the calculated value and then decreased until it reaches zero. This procedure is repeated the required number of times until the desired elongation is achieved. The reinforcement is brought to a certain elongation, and not stress, due to the fact that wire friction occurs inside the structure, which reduces the stress.