Strengthening of reinforced concrete structures with a change in the design scheme. Reinforcement of reinforced concrete bending structures

Reinforcement of reinforced concrete beams much more difficult than metal, due to the fact that reinforced concrete is a composite material, where reinforcement works together with concrete. Often, the operating organization does not have project documentation, so the position of the working reinforcement has to be determined additionally.

There are two main ways to enhance or restore bearing capacity reinforced concrete beam structures:

amplification without changing the original design scheme;


strengthening with its change.

The first method is to increase the cross section of the element being reinforced, which is achieved by installing clamps or by installing special shirts, clips, overlays, extensions with the addition of reinforcement, and expansion of the supports. This leads to a decrease in the span, and, consequently, to a change in the design scheme. But it is also associated with an increase in the weight of the structure.

The second method consists in installing additional horizontal or trussed puffs with pre-tensioning or combined puffs, which changes the design scheme of the structure, but only slightly increases its weight.

Monolithic structures, not prefabricated, are especially needed to create a prestress along the lower chord of the beams during reinforcement. Precast concrete elements and monolithic (i.e. manufactured directly on construction site) cannot be equated in terms of bearing capacity, since in prefabricated structures, prestressing along the lower (stretched) chord is created under normal factory conditions with a quality guarantee. Under construction site conditions, this operation cannot be performed at all.

In terms of the quality of adhesion of concrete with reinforcement, in terms of reliability and durability, it is also impossible to put an equal sign between monolithic and prefabricated reinforced concrete structures. hardening monolithic structures at the construction site, as a rule, occurs in violation of technological requirements, and stripping is carried out until the concrete reaches the required strength.

At the same time, under factory conditions, during autoclave steaming, all components of the concrete mixture are involved in the process of binding reinforced concrete.

The elements used in this case are structurally simple, made of reinforcement or shaped steel outside the object being reconstructed, installed with minimal labor costs, immediately put into operation after installation and tension or increase in cross section without the use of other devices. They increase the initial bearing capacity of the bent elements by 2-2.5 times, do not disturb the interiors of the premises, can be hidden by a false ceiling, etc., take up little space and slightly increase the cross section or height of the structures.

Ways to strengthen reinforced concrete beams

a, c - concreting; b - clamps; d, d. f, g - embedding and pairing on supports

In order to ensure the joint operation of reinforced concrete with reinforced concrete, it is necessary, both in the design and in the performance of work, to pay attention to measures that increase the adhesion of old concrete to new. In particular, smooth contact surfaces are recommended to be sandblasted, knurled or wire brushed. Immediately prior to placing new concrete, the surface of the old concrete must be flushed with a high-pressure water jet. In this case, excess water in the form of puddles must be removed, since excessive moisture adversely affects adhesion. When constructing reinforced concrete casings of columns, the surface of existing concrete is washed with a jet of water under pressure.

	Reinforced concrete frame device Installation of metal corners
Reinforcement of a monolithic beam with a reinforced concrete cage 1 - reinforced beam; 2 - clip; 3 - plate; 4 - holes in the slab for the passage of clamps and the supply of concrete; 5 - mounting fittings of the holder; 6 - inclined rods of the holder; 7 - working fittings of the clip; 8 - clip clamps
	Installation of additional reinforcement on the polymer solution Installation of external sheet reinforcement on a polymer solution
Reinforcement of a monolithic beam with a reinforced concrete jacket 1 - reinforced beam; 2 - shirt; 3 - working fittings of the shirt; 4 - mounting fittings of the shirt; 5 - clamps; 6 - notch; 7 - coupler

shirts are more often used when reinforcing monolithic beams of ribbed floors. It is recommended to pay special attention to the anchoring of the transverse reinforcement at the ends of the cross section of the jackets. When strengthening columns, clamps must be welded to the reinforcement of the column being reinforced; in case of any difficulties, the column jacket must be designed to absorb the entire load. When strengthening the monolithic beams of ribbed floors, the clamps are removed through the slab through drilled holes and anchored with longitudinal reinforcing bars.

Reinforcement by extension means that the reinforced structure increases in height or width (from below, from the sides or from above the element being reinforced).

	Building beams from below with a significant increase in their bearing capacity
	Building beams from below with a slight increase in their bearing capacity
Reinforcement of beams with one-sided extension 1 - reinforced beam; 2 - building by means of shorties; 3 - extension by means of connecting elements; 4 - reinforcement of the reinforced beam; 5 - additional working fittings; 6 - shorty; 7 - connecting elements in welding

A characteristic feature of this method is the perception of shear stresses acting in the plane of contact of old concrete with new, special additional reinforcement welded to the reinforcement of the reinforced structure, previously exposed by chipping off the protective layer at the welding points.

Building is used to strengthen any iron concrete structures(both monolithic and prefabricated). Reinforcement of the upper flanges of prefabricated roof beams is carried out in case of replacement of roof slabs. When building, it is not recommended to use reinforcing bars with a diameter of less than 10 mm. When shearing a protective stand located in a compressed zone, a temporary decrease in the bearing capacity should be taken into account.

In some cases, to increase the bearing capacity of reinforced elements by building up, it is enough just to increase the amount of the main longitudinal reinforcement, for which it is recommended to chip protective layer not less than 0.5 of the diameter of the reinforcement and, by means of parallel welding through the short pieces of reinforcement with a diameter of 10 to 40 mm and a length of 50 to 200 mm, connect additional reinforcement to the existing one.

In the stretched zone of the reinforced elements, the short pieces are placed at a distance of 200 ... 1000 mm, in the compressed zone - at a distance of no more than 500 mm and no more than 20 diameters of the longitudinal reinforcement reinforcement. After welding, instead of a chipped protective layer, a new one is applied - in the form of cement plaster or shotcrete. In these cases, the cross section of the reinforced element increases slightly, in the range from 20 to 80 mm.

In case of breaks of reinforcing bars in bending elements, it is recommended to restore them by welding stressed overlays. First, it is necessary to support the reinforced structure with temporary supports, chop off the protective layer at the required length, weld the reinforcing rods (linings) at one end, heat with current, for example, from a welding transformer, weld the other end in a heated state, restore the damaged protective layer with plastic concrete on fine aggregate.

It is allowed to weld additional reinforcement from steels of classes A-I, A-II, A-III to existing reinforcement of the same classes. When fittings made of high-carbon steels of classes A-IV and above, as well as from ropes and strands, welding is not allowed.

In cases where conditions technological process allow constraint dimensions industrial premises, one of simple ways reinforcement of bending elements (beams, crossbars, frames, trusses, etc.) is the installation of additional rigid supports.

Strengthening the crossbar with an additional rigid support a - based on the underlying floor: 1 - reinforced crossbar; 2 - lining; 3 - shorties from round reinforcing bars; 4 - double-sided spacer, welded after the spacer of the half-braces; 5 - metal head; 6 - half-bracing; 7 - tightening at floor level; 8 - jack; 9 - welds; b - based on the clips of the column: 1 - reinforced crossbar; 2 - clip; 3 - struts; 4 - puff; 5 - tension clutch; 6 - metal head; 7 - straps; 8 - gaskets

Since, when making rigid supports on independent foundations, it is very difficult to completely avoid settlement of the support, it is in all cases desirable to install them on existing foundations, even if it is necessary to strengthen them. In these cases, rigid additional supports are made in the form of portals or struts.

Elements of additional rigid supports can be reinforced concrete and metal. It is recommended to make them in advance.

When making rigid supports in the form of connected racks with independent foundations, it is recommended to pay special attention to reducing the settlement of these foundations, for which it is necessary to pre-compress the soil under the sole. One of the ways to pre-compress the soil is to load the foundation with a load no less than the calculated one before the erection of the rack. To reduce the pressure on the soil under the sole of the new foundation, it is recommended to arrange a distribution sand and gravel pad.

When reinforcing the crossbar of the frame with additional rigid supports in the form of metal struts, the overhead metal parts in the lower corners must be fixed. After summing up the struts for a snug fit of the mating structures that ensure the effectiveness of the reinforcement, it is necessary to make a wedging in the upper node with wedge-shaped gaskets. It is possible to arrange struts with support on the metal clips of the columns.

When reinforcing the crossbars with additional rigid supports in the form of metal or reinforced concrete struts, the lifting of the reinforced crossbar can be carried out, for example, with a horizontally located jack. To facilitate the movement of expandable half-braces, it is necessary to lay metal linings and short pieces of round reinforcing steel in the gap between the reinforced crossbar and half-braces. After lifting the reinforced structure to the required value, spacers from profile metal, for example from channels, are welded on both sides of the half-braces, and the jack is removed. When reinforced with half-braces, in order to avoid overloading the columns below, the half-braces must be tied at the bottom with a special metal puff.

When reinforcing the crossbars with additional rigid supports in the form of a strut system installed on one column, the reinforced structure is lifted by tensioning the metal tightening by means of a tension coupling. To install the strut system in the lower part of the column, you must first arrange the holder. After installing and tightening the struts, they are fixed in the lower part by welding metal strips to the struts. Reinforcement with rigid additional supports of this type to a lesser extent constrains the dimensions of industrial premises.

When reinforcing the crossbar of the frame with additional rigid supports in the form of metal struts, the overhead metal parts in the lower corners must be fixed. After summing up the struts for a snug fit of the mating structures that ensure the effectiveness of the reinforcement, it is necessary to make a wedging in the upper node with wedge-shaped gaskets. It is possible to arrange struts with support on the metal clips of the columns.

In cases where the reinforced structure cannot be preliminarily unloaded, the installation of additional rigid supports must necessarily be accompanied by a preliminary lifting of the reinforced structure. The lifting of the reinforced structure can be done in various ways and depends both on the design of additional supports and on the design of the reinforced elements.

	Summing up the unloading beams on the consoles
	Summing up unloading beams on clamps
Reinforcement with an additional elastic support (metal beam) a - on suspension brackets - coupling bolts: 1 - reinforced beam; 2 - reinforcing beam; 3 - coupling bolt; 4 - reference corner; 5 - gasket; b - on brackets; 1 - reinforced beam; 2 - reinforcing beam; 3 - metal holder of the column; 4 - brackets; 5 - wedge-shaped gaskets

When reinforcing the crossbars with additional rigid supports in the form of metal or reinforced concrete struts, the lifting of the reinforced crossbar can be carried out, for example, with a horizontally located jack. To facilitate the movement of expandable half-braces, it is necessary to lay metal linings and short pieces of round reinforcing steel in the gap between the reinforced crossbar and half-braces. After lifting the reinforced structure to the required value, spacers from profile metal, for example from channels, are welded on both sides of the half-braces, and the jack is removed. When reinforced with half-braces, in order to avoid overloading the columns below, the half-braces must be tied at the bottom with a special metal puff.

When reinforcing the crossbars with additional rigid supports in the form of a strut system installed on one column, the reinforced structure is lifted by tensioning the metal tightening by means of a tension coupling. To install the strut system in the lower part of the column, you must first arrange the holder. After installing and tightening the struts, they are fixed in the lower part by welding metal strips to the struts. Reinforcement with rigid additional supports of this type to a lesser extent constrains the dimensions of industrial premises.

	Suspension to unloading beams
	Installation of horizontal puffs from the corners
Reinforcement of bending elements with additional elastic supports a - metal beams on hangers; b - metal triangular trusses; 1 - unloaded element; 2 - unloading structure; 3 - suspension; 4 - support of the unloading structure; 5 - fixing bolt; 6 - gasket; 7 - holes filled with concrete after reinforcement

To reinforce the bending elements, additional elastic supports are also used, usually created with the help of metal trusses and beams, installed under the reinforced element on common or independent supports and perceiving the load through spacers located in the span between the reinforcing and reinforced element.

The inclusion of structures of additional elastic supports in the work can be carried out by pulling the supporting ends of the elastic supports to the element being strengthened during installation or using wedging gaskets. Spacer bolts can be installed instead of wedges.

When reinforcing bending elements multi-storey buildings elastic additional supports can be created by metal strands. The reactive unloading force is created by prestressing the tie rods, first by means of tension nuts, and finally by tension couplings. The load from the strands is perceived by the frame of the upper tier, to the racks of which they are attached.

For reinforcement mainly prefabricated roof beams for large spans and trusses under load, prestressed link chains may be recommended. The use of hinged-rod chains makes it possible to create a load opposite in sign in the form of a series of concentrated loads, the location and size of which are planned in advance depending on the outlines of the chains. The amplification effect (creation of reactive forces of given values) is achieved by tensioning a statically determinable chain.

Installation of unloading brackets	Installation of combined puffs made of reinforcing steel
Installation of puffs from the channel	Reinforced concrete jacket device
Reinforced concrete building device	Installation of tie-down collars at supports
Installation of tightened transverse rods at the supports	Installation of inclined rods at supports

The main elements when reinforcing in this way are: the hinge-rod chain itself, consisting of two identical branches on both sides of the reinforced beam (corners with cut vertical shelves at the bends, reinforcing bars up to 30 ... 36 mm in diameter or ropes); anchor devices in the form of welded plates made of sheet metal in the upper zone of the beams above the supports; hangers, usually made of round steel, or racks made of profile metal at the bends of the chain branches. Reinforcing bars are accepted from steel grades A-I, A-II, A-III, metal constructions- from steels Vst3sp, Vst3ps, Vst3kp. Welded joints must be made with great care.

	Installation of truss puffs from the corners
	Installation of trussed puffs made of reinforcing steel
Reinforcement of bending elements with a prestressed hinge-rod chain: a - reinforcement of a beam of a monolithic ribbed floor; b - reinforcement of the prefabricated roof beam; 1 - reinforced element; 2 - hinge-rod chain; 3 - rack; 4 - central pillar; 5 - metal clip of the anchor device

All elements of the chain are recommended to be manufactured in advance in accordance with the dimensions of the beam to be reinforced, carefully tested in field conditions. Chain elements should be installed in a certain sequence. Both branches of the chain are suspended from the anchor devices fixed on the beam with pre-attached hangers having screw threads at the ends and connecting strips. If, in addition to suspensions, racks are also required, then they are installed, leaving free space for the central suspension (rack).

When the nuts are tightened, all connecting straps of the hangers are tightly attracted to the reinforced beam, and the chain receives some tension, as a result of which the anchor devices are compressed and all intermediate nodes are crushed, which leads to the elimination or reduction of stress losses in the future. Then the tension is released and the nodes are set to the design position in accordance with the chain line. When fastening chain branches to anchor devices on bolts, it is possible to adjust the length of the chain, which allows you to install the chain in the design position with greater accuracy.

When designing the outline of a chain, it is recommended to take it in such a way that the tangents of the inclination angles of individual links, starting from the middle, are related to each other as 1:3:5, etc. Compliance with this condition leads to the fact that the forces (reactive forces) in all suspensions and struts will be approximately the same value, and the main tension can be produced at the location of the central suspension or strut. The magnitude of the effort is predetermined theoretically.

For the main tension of the chain branches at the location of the central suspension or rack, various methods are used. In cases where the chain is located above the bottom of the reinforced beam, i.e. suspension installation is required, tension can be carried out by tightening the nuts with a torque wrench using a jack with a pressure gauge resting against the bottom of the beam, and in other ways. The readings of the pressure gauge allow you to accurately determine the value of the unloading load. Regardless of the location of the chain relative to the reinforced beam, it is possible to tension its branches with a calibrated load, followed by fixing the assembly with a suspension or rack. When tensioned in this way, sufficient control is also provided.

To strengthen the bending elements of multi-span buildings or structures (prefabricated roof beams, secondary beams of monolithic ribbed slabs, etc.) in the support zones, two-cantilever prestressed unloading brackets installed on intermediate supports can be used.

When strengthening the prefabricated roof beams, both branches of the brackets are triangular trusses. The lower chord is made from one corner, while the upper chord and the grating can be made from both single corners and round reinforcing bars.

The height of the brackets is assumed to be equal to the height of the supporting part of the reinforced beams. The lengths of the cantilever parts of the brackets are recommended to be taken equal to 1/4 - 1/6 of the span of the reinforced beams. With a small length of the cantilever parts, it is possible to completely abandon the internal elements of the lattice.

Bracket parts are bracket branches, supporting elements (supporting sheet or saddle-shaped pads), connecting elements in the form of segments of angles or round rods, thrust devices attached under the bottom of the reinforced beam to the ends of the bracket branches. These devices serve to create supports for reinforced beams and may have different design depending on the method of tensioning the bracket.

The design of the thrust device depends on the tension method. When tensioned with bolts, it is a rigid element passed under the bottom of the reinforced beam and bolted to the branches of the bracket. Tension control is carried out by the deflection of the ends of the bracket. When a calibrated load is tensioned by a suspension, a rigid element of the thrust device is welded to the branches of the bracket, for which holes are provided in it or loops are welded. After tension, the fixing spacers are tightly placed in the gap between the bottom of the beam and the plate of the thrust device, and the loads are removed. Voltage losses are eliminated by suspending a load 10 ... 15% larger than the required unloading load. After tensioning with jacks installed between the stops suspended at the ends of the bracket and the bottom of the beam, fixing gaskets are also laid. Tension control is carried out by the manometer of the jack. Since this system is statically determined, freely rotating on the middle support, only one end of the bracket can be tensioned. The force at the other end in this case will also be known.

Unloading brackets can also be made in the form of solid beams made of rolled metal.

In cases where the reinforcement is caused by a violation of the anchoring of the longitudinal working reinforcement, the removal of the support or bracket from the supporting sheet of the beam must be at least 40 diameters with bar reinforcement of a periodic profile and at least 80 diameters with reinforcement made of high-strength wire.

When reinforcing roof beams of multi-span buildings, it is recommended to use simultaneously different design solutions for the outer and middle spans. For extreme spans, prestressed truss can be used, for medium spans, prestressed load-bearing brackets can be used.

For monolithic and prefabricated bending elements, in cases where it is necessary to carry out work in the shortest possible time without removing the live load, a reinforcement method can be recommended by installing additional prestressed reinforcement.

Additional reinforcement can be both horizontal and trussed. It is also possible to install horizontal and truss reinforcement at the same time. As a result of the installation of additional reinforcement with its preliminary stress, the stress-strain state of the reinforced beams changes. Prestressing includes additional reinforcement in joint work with a reinforced beam, which can be considered as a bending structure with an increased reinforcement area, the additional part of which does not adhere to concrete, and with a variable working height.

The method of reinforcement by installing additional prestressed reinforcement has several varieties, which differ from each other in the anchoring of additional reinforcement and the method of its tension.

Additional reinforcement can be tensioned mechanically, electrothermally or electrothermomechanically.

With the mechanical method, the tension of the prestressing reinforcement is carried out using jacks, torque wrenches, tension bolts, tie-down clamps that attract the strands to each other, as well as special reinforcing devices of the trussed or lever type.

One of the ways to fix additional reinforcement in cases where anchor devices cannot be placed at the ends of the beam is to weld them to the existing reinforcement. In these cases, the protective layer is chipped off on small areas in support zones, i.e. where the stresses in the reinforcement of the reinforced beam are insignificant. It is necessary to weld short pieces to the exposed working reinforcement, the diameter of which is slightly larger than the thickness of the protective layer, while the transverse reinforcement rods or clamps should not be broken. Reinforcing bars are welded to the short stacks. The tension in this case is produced by a thermal method.

The reinforcing rods are installed in the design position with the help of temporary hangers, the number of which should be specified in such a way as to prevent sagging under their own weight. The rods should be as straight as possible. One end of the rod is welded to the shorty, and the other remains free. Rod include in electrical circuit and heated to the calculated temperature. The free end is pressed against the shorty and welded. During the welding process, until the weld is completely cooled, it is necessary to maintain a constant design temperature. To prevent bulging of the longitudinal reinforcement in the places of welding of additional stressed reinforcement, it is desirable to place the short pieces next to one of the clamps from the side of the span.

Gaskets, linings, and other parts, when reinforced with trussed reinforcement and its tension, must be installed at the points of bending of the rods between the lower edge of the reinforced beam and the trussed rods. The designs of these elements depend on the method of tensioning the rods, the distance between the bottom face of the beam being reinforced and the reinforcement rods, and the width of the beam being reinforced.

In a separate group of reinforcement with additional reinforcement, puffs can be combined and recommended for use, strained by mutual tightening of two or four rods with special coupling bolts. Coupling bolts should be in the form of a collar with two threaded ends and a common washer. Tension is produced by simultaneously tightening the nuts at both ends of this clamp. Tension by mutual tightening is characterized by simplicity and does not require significant efforts, since the stresses in the tie bolts (clamps) are 7 ... 10 times less than the stresses in the additional rods being tightened. This method allows you to create uniform forces in all tightened rods (two or four), i.e. ensures their self-regulation. Tightening can be done with one or two tie bolts, with or without intermediate spacers. The vertical rods of the tie-down anchors are passed through the anchoring holes drilled in the ceiling.

Strengthening of the stretched zone is carried out increase in the cross-sectional area of ​​the working reinforcement reinforced structure by installing additional reinforcement in this area ensuring its joint work with the design. The joint operation of additional reinforcement with a reinforced structure is ensured by:

welding to existing fittings ;

bonding to the concrete of the tension zone .

Ensuring joint operation of additional reinforcement by welding to existing reinforcement

welding additional tensile reinforcement to the existing reinforcement of the reinforced structure, depending on the condition and thickness of the protective layer, as well as the possibility of increasing the cross-sectional dimensions, the following is performed: directly lap joint with a beating of the protective layer along the length of the additional reinforcement (Fig. 8.2, a); with the help of shorties diameter exceeding the thickness of the protective layer (Fig. 8.2, b, in,); using staples(Fig. 8.2, G). After welding in the design position, additional reinforcement is concreted.

Rice. 8.2. Strengthening the stretched zone of structures by welding additional reinforcement: a- lap joint; b- by means of shorty from the side of the stretched zone; in– by means of short pieces on the side of the side protective layer; G- with staples

Welding of additional reinforcement to the existing prestressed reinforcement, as well as to the support not brought over the edge to the required length of the non-stressed reinforcement of the reinforced structure, is not allowed.

The protective layer of concrete in the places of welding of additional reinforcement, short pieces or staples is beaten off by at least half the diameter of the existing reinforcement. Existing fittings at the welding points must be cleaned of rust, dust and other contaminants to bare metal.

As additional working reinforcement, rod reinforcement of a periodic profile or smooth, as well as rolled profiles, are used.

The short pieces and sections of the connection of brackets from rod reinforcement are taken 50 ... 200 mm long and placed along the length of the structure “in a row” with a distance between them along the rods of at least 20, where  is the larger diameter of the rods to be welded.

In order to reduce the concentration of stresses, embrittlement of the metal and weakening of the cross section when making welds, the presence of burns and melting from arc welding on the surface of the working rods is not allowed. Burns should be cleaned with an abrasive wheel along the rod. When strengthening the structure under load, welding of additional reinforcement is carried out in two passes symmetrically in the direction from the ends of the structure to the middle. Welding of additional reinforcement to the existing reinforcement of the reinforced structure, unloaded during the performance of reinforcement work, is allowed to be performed in one pass.

Welding of additional reinforcement to the existing reinforcement of the reinforced structure without its preliminary unloading is not allowed if the stresses in the working reinforcement of the most unfavorable section of the structure exceed 85% of its yield strength. The stresses in the reinforcement of the reinforced structure are determined under the actual loads, the actual strength of concrete and reinforcement, the cross-sectional area of ​​the reinforcement minus the section of the welded rod of the reinforced structure.

When strengthening the structure without unloading, it is advisable to prestress the additional reinforcement by thermal, mechanical or combined thermomechanical methods. At thermal method, an additional rod is pre-welded with one end to the existing reinforcement, then the rod is heated and its second end is welded. At electrothermal method for heating through the rod, current is passed from the welding transformer. The magnitude of the prestress is controlled by the elongation of the rod or the temperature of its heating. The required elongation of the additional rod is determined by the formula

,where – required prestress, is the length of the rod between the inner ends of the welds; is the elastic modulus of the reinforcement.

The required heating temperature of additional fittings is determined by the formula

,where
- coefficient of thermal expansion for reinforcing steel; - temperature environment at the moment of tension of the armature. Heating temperature should not exceed 400 With.

At mechanical Using the prestressing method, an additional rod, welded at one end to the existing reinforcement, is welded from the opposite end with a tensioner in the form of a bolt and nut, and a stop in the form of a pipe segment with an inner diameter slightly larger than the bolt diameter is welded to the existing reinforcement. After fixing the ends, additional reinforcement is welded to the existing one along the length. After tensioning the additional reinforcement, the tensioner is cut off and reused. To create a pre-tension, it is possible to use a turnbuckle included in the stressed rod.

To facilitate mechanical tension, the additional rods are simultaneously heated ( thermomechanical way). The magnitude of the prestress is controlled by the elongation of the rod.

The value of the prestress of additional reinforcement is taken within

The maximum prestress value for wire reinforcement must not exceed
.In order to reduce the deflection and increase the crack resistance of the reinforced structure, the prestressing value of the additional reinforcement is assumed to be maximum.

The loss of prestress in additional reinforcement is determined by , as for structures with reinforcement tension on concrete.

Ensuring the joint work of additional reinforcement by gluing the tension zone to the concrete

When ensuring the joint operation of additional reinforcement and the reinforced structure gluing with the help of polymer solutions (Fig. 8.3), additional sheet and profile reinforcement is placed on the surface, and rod reinforcement is placed in specially prepared grooves or in a layer of polymer solution. In addition, additional working reinforcement can be placed in prefabricated reinforced concrete reinforcement elements glued to the tension zone of the structure. In the case of exposure to aggressive environments, given the high protective properties of polymer solutions, it is advisable to simultaneously perform coatings on the surface of the reinforced structure. Steel sheets are protected with flame retardant and anti-corrosion compounds. Additional reinforcement in the tension zone is installed along the entire length of the structure or at the calculated length in accordance with the diagram of internal forces.

R
is. 8.3. Reinforcement of the stretched zone of the structure by gluing additional reinforcement: 1 - reinforced structure; 2 - pit; 3 - anchor; 4 - sheet reinforcement; 5 - polymer solution; 6 - corner; 7 - channel; 8 - groove; 9 - bar reinforcement; 10 - coating from a polymer solution; 11 - precast concrete element; 12 - fiberglass; 13 - thin sheet with stampings; 14 - anchor plate

To increase the efficiency of anchoring of additional sheet reinforcement, anchor connections are used in the form of segments of rod reinforcement of a periodic profile, welded to the sheet and anchored in holes pre-drilled in concrete, filled with a polymer solution, or steel sheets glued along the side faces of the reinforced structure.

When reinforcing the stretched zone by gluing additional reinforcement, it is advisable to maximize the unloading of the reinforced structure or prestress the additional reinforcement.

As additional working reinforcement, glued in the stretched zone of the reinforced structure, rod reinforcement, reinforcing

ropes, rolled sheets with a thickness of 3...20 mm, rolled profiles in the form of channels, angles, as well as non-metallic fittings based on glass, basalt, carbon and other fibers.

Work on strengthening the stretched zone of structures by gluing additional reinforcement or precast concrete elements with additional reinforcement is carried out in the following sequence. The bonded surfaces of the reinforcement elements and the reinforced structure are prepared. Steel sheets on the inside are cleaned of rust, scale and degreased with acetone. Bonded concrete surfaces of the reinforced structure and the precast concrete element must be free of protrusions, chipped ribs, grease stains, dirt and dust. Surfaces previously exposed to aggressive media are washed with clean water and dried. If aggressive environments were acidic, then after washing the surfaces are neutralized with alkaline compounds and again washed and dried. With a large amount of work, the surfaces are subjected to sandblasting and dedusting with the help of hair brushes and blowing with compressed, oil-free and moisture-free air. Cracks are injected. Grooves for placing rod reinforcement are cut using diamond and hard-alloy mechanized tools. Then the reinforcement elements are installed in the design position and fixed with the help of temporary fasteners (supports, clamps, clamps, etc.).

Polymer solution for embedding rod reinforcement in grooves and anticorrosive coating of the surface is applied manually, by pouring or spraying. The polymer solution in the grooves between the sheet reinforcement or the reinforced concrete prefabricated element is injected through a fitting screwed into the hole of the reinforcement element. In this case, the gaps along the perimeter of the seam are pre-sealed with a polymer solution of the same composition with the addition of a filler.

When using additional reinforcement in the form of channels, before installing the channel in the design position, the required amount of polymer solution is placed on the inner surface of the profile. Then the channel is raised to the design position and attracted to the structure using temporary mounting clamps. Excess polymer solution is squeezed out into the gaps between the side faces of the reinforced structure and the profile shelves.

When reinforcing prefabricated multi-hollow floor panels, voids are used to accommodate additional reinforcement. Additional reinforcement can be in the form of separate rods with clamps to provide a protective layer or frames. Additional reinforcement is installed in the voids through holes punched from the side of the upper or lower faces of the slab, and the voids are filled with concrete using concrete pumps (Fig. 8.4).

Rice. 8.4. Reinforcement of multi-hollow floor panels by installing additional reinforcement: 1 - slab; 2 - welded frame; 3 - concrete

In order to reduce the consumption of materials when reinforcing multi-hollow panels, additional reinforcement may not be installed along the entire length of the panel, and voids may not be filled to the full extent. To do this, slots are made at the ends of the reinforcement zone from the side of the upper or lower face of the slab, clamps are installed on the reinforcement, reinforcement is inserted into the voids in the middle zone of the panel, temporary restrictive plates are installed, through the slots, using nozzles, the voids between the restrictive plates are filled with a polymer solution, after hardening of which , the restrictive plates are removed, and the slots are sealed (Fig. 8.5).

R
is. 8.5. Strengthening the stretched zone of multi-hollow panels by installing additional reinforcement: a- when arranging slots on top of the slab; b- when arranging slots at the bottom of the slab, 1 - reinforced slab, 2 - slot, 3 - additional reinforcement, 4 - retainer, 5 - restrictive plate, 6 - branch pipe, 7 - polymer solution

The thickness of the polymer solution layer is determined from the strength condition of the contact weld and must be at least 3, where  is the diameter of the additional reinforcement.

Slots are made in the support zones of reinforced prefabricated multi-hollow panels, temporary restrictive plates are installed in the form of a circle with a diameter equal to the diameter of the void, with a slot for reinforcement. Then the reinforcing bar is mounted and the support zones of the voids are concreted. After the concrete has gained strength, the reinforcement is strained with tension bolts, which are mounted through holes from the side of the lower face. At the same time, formwork is installed under the holes from the side of the lower edge. Then the remaining void space is filled with a concrete mixture, after which the formwork is removed and the protruding ends of the tension bolts are cut off (Fig. 8.6).

R
is. 8.6. Reinforcement of prefabricated hollow-core slabs with prestressed reinforcement: a- slabs at the moment of prestressing of reinforcement; b- reinforced slab, 1 - reinforced slab, 2 - additional reinforcement, 3 - temporary restrictive plate, 4 - concrete, 5 - tension bolt, 6 - formwork

Additional reinforcement to reinforce the stretched zone of prefabricated panels can be installed in the expanded joint between the slabs with subsequent concreting. At the same time, joint operation of additional reinforcement with reinforced panels should be ensured by means of notches, dowels on the side faces of adjacent plates, as well as the use of polymer solutions with high adhesive properties.

Precast concrete reinforcement elements (conventional and prestressed) must be designed for the loads acting during the period of manufacture, transportation and installation in accordance with. The concrete class of the reinforcement elements must not be lower than the actual strength of the concrete of the reinforced structure. The thickness of a precast concrete element with additional reinforcement is assumed to be at least 50 mm. The number of prefabricated reinforced concrete elements placed along the width of the section of the reinforced structure can be one or more.

To strengthen reinforced concrete structures, a large number of methods have been developed:

an increase in the geometric dimensions of the cross sections of structural elements, which is accompanied by an increase in the own weight of the structures and an increase in the construction height;

· installation of external screeds, supports, belts, trusses, leading to a change in the architectural appearance of structures and significant time and material costs;

gluing metal plates or welding them.

So, reinforcement of reinforced concrete structures, by sticking composite materials allows to significantly increase their bearing capacity and rigidity, as well as extend the life of the entire structure. The following main advantages of the material should be noted here:

joint work of the external reinforcement element with the reinforced structure at all stages of its loading (such work is ensured by reliable adhesive bonding);

high durability and resistance to corrosion;

high mechanical characteristics(strength and modulus of elasticity) of the materials that make up the reinforcement system;

high relative elongation of reinforcement materials;

Ease of installation and low dead weight.

Ways to strengthen reinforced concrete structures with a change in the static scheme of their work.

Strengthening features with this method is performed

1. Changing the place of transfer of loads with the help of distribution forces in the form of beams that change the place of concentrated loads and reduce the bending moment

2. Increasing the degree of static uncertainty by arranging additional supports by installing additional connections in order to ensure spatial work and continuity.

Complements the supports - rigid in the form of racks with rigid functions or struts of suspensions that transfer the load to the existing f-t.

To include additional rigid supports in the joint work at the interfaces of the reinforced structures, they are prestressed with jacks or with the help of wedges. Complementary elastic supports are less effective but also less constrain the dimensions of the room. Elastic supports include supports whose settlement cannot be neglected. They are installed using beams with a system of trusses that are located from the bottom on top or on the sides of the reinforced features. For bending large-span structures, multi-span buildings are effectively made of additional elastic connections in the form of elastic brackets made of rolled profiles.

3. Increasing the degree of internal static indeterminacy by the device for tightening the spacers of mates of hinged rod chains

when ensuring the joint work of an additional stretched ar-ry with a reinforced design, fixing it at the ends of an additional ar-ra plays the role of a puff. Its fastening at the ends is carried out with the help of anchor devices. Depending on the place of fastening of the ar-ry on the construction, there are different horizontal trussed puffs.

Ways to strengthen beams and crossbars with puffs

a - horizontal; b - trussed; in - combined; 1 - tension bolt; 2 - stop washer; 3 - tie-puffs; 4 - support anchor from the channel; 5 - linings from a round rod; 6 - hole in the slab, sealed after the installation of the anchor; 7 - corner stop; 8 - corner stop anchors; 9 - tie-puffs; 10 - strip steel linings

Reinforcement of columns with spacers

a - compressed; b - non-centrally compressed; I - coupling bolts; 1 - stops az corners; 3 - slats; 4 - spacers; 5 - tension bolt; 6 - strips welded after the installation of spacers

Amplification methods reinforced concrete slabs building up

Scope of stone and armo stone structures. Materials for the manufacture of masonry, their physical and mechanical properties. Strength and deformation characteristics of masonry. The main factors affecting the strength of the masonry. Calculation of elements of stone structures for central and eccentric compression.

Types of structures:

STONE

b) Ceilings (arches, vaults, lintels of the shell)

c) Pillars, piers

ARMOCONE

a) Structures with transverse reinforcement

b) Structures with longitudinal reinforcement

c) Structures with transverse and longitudinal

d) Structures with prestressing reinforcement

Classification of stone materials
Origin:
- natural;
- artificial.
To size:
- blocks (height > 500 mm);
- small stones (height< 200 мм).
By material:
- artificial: clay, silicate, concrete, lightweight concrete, cellular;
- natural: granite, limestone (butt), tuff, etc.

Brand of brick and mortar

The strength of the masonry depends on the brand of stone and the brand of mortar, but the strength of bricks and compression is used slightly.

During compression, individual stones in the masonry work for bending and shearing, therefore the brand of brick is determined from its compressive and bending strength. Bending and shearing of individual bricks occurs due to the uneven density of the mortar in the seam

The strength of the masonry is affected by the shape of the surface of the brick and the thickness of the seam; the smoother the brick and the thinner the seam, the stronger the masonry.

The size of the masonry section (wall thickness) influences: with a decrease in the size of the masonry section, its strength increases. This is partly due to a decrease in the number of seams.

The strength of the masonry is affected by the difference in the deformation properties of the brick

The strength of the masonry increases with time as a result of the increase in the strength of the solution.

The compressive strength of the masonry is not affected by the bonding system and the adhesion of the mortar to the brick.

Masonry deformity.

In masonry, the following deformations are distinguished:

Volumetric, arising in all directions, due to shrinkage of the mortar and stone or from temperature changes;

Power, developing mainly along the direction of the force.

Shrinkage deformations of the masonry depend on the material of the masonry. Temperature deformations of the masonry also depend on the material of the masonry and the coefficient of linear expansion of the masonry.

Under the action of a load (force deformations), the masonry is an elastic-plastic material. Starting with small stresses in the masonry, in addition to elastic, plastic deformations also develop. Therefore, force deformations will depend on the nature of the load application and can be of 3 types:
- deformations at a single loading for a short time. load

Deformations under long-term load

Deformations under repeated loads.
Calculation for central compression

m g is the coefficient of influence of duration, which is expressed in the increase in deflections due to creep.

The coefficient of longitudinal bending (is found according to the tables depending on the flexibility and elastic characteristics of the masonry

The choice of one or another method of strengthening building structures depends on the terms of reference for the reconstruction of a building or structure, which includes a change in space-planning solutions, loads and operating conditions. The main reasons for strengthening reinforced concrete structures are given in Table. 1, and ways to increase the bearing capacity of structures - in table. 2.

Tab. 1. The main reasons for strengthening reinforced concrete structures

Tab. 2. Ways to increase the bearing capacity of structures

One of the most effective ways to strengthen reinforced concrete columns is the installation of reinforced concrete and metal clips. In bending elements, the clips are used with significant corrosion of the reinforcement.

Reinforced concrete cage consists of reinforcement and a thin layer of concrete covering the reinforced element from four sides (beams, crossbars, columns).

Most simple type are reinforced concrete cages with conventional longitudinal and transverse reinforcement without connection between the casing reinforcement and the reinforcement of the reinforced column. With this method of strengthening, it is important to ensure the joint work of "old" and "new" concrete, which is achieved by thoroughly cleaning the concrete surface of the reinforced structure with a sandblaster, notching or processing with metal brushes, as well as washing under pressure immediately before concreting. To increase adhesion and protection of concrete and reinforcement in aggressive operating conditions, the use of polymer concrete is recommended.

The thickness of the casing of the columns is determined by the calculation and design requirements (the diameter of the longitudinal and transverse reinforcement, the size of the protective layer, etc.). As a rule, it does not exceed 300 mm. The area of ​​the working longitudinal reinforcement is also determined by calculation.

With local reinforcement, the clip is extended beyond damaged area for a length not less than the length of the anchoring of the reinforcement, as well as not less than twice the width of the larger face of the column, but not less than 400 mm. To improve the adhesion of "new" and "old" concrete, it is recommended to perform an adhesive coating of polymeric materials.

The transverse reinforcement of a reinforced concrete cage can be made in the form of a spiral winding of wire with a diameter of at least 6 mm. More efficient (but also more labor-intensive) are reinforced concrete clips, in which the connection between the existing and additional reinforcement is provided. These clips are recommended for severe damage to existing reinforcement or concrete cover. In this case, the reinforcement of the reinforced structure is thoroughly cleaned to bare metal, the destroyed clamps are restored by punching transverse grooves in the concrete, installing new clamps in them and connecting them with longitudinal reinforcement.

Additional longitudinal reinforcement is welded to the existing one with the help of connecting shorts, which, in order to avoid burns, are made of class A-I reinforcement with a diameter of 10-16 mm and are located at a distance of at least 20 diameters of longitudinal reinforcement in a checkerboard pattern.

If it is impossible to make a closed cage, for example, when a column is adjacent to a wall, it is recommended to install "shirts" - concrete blocks that are not closed on one side. With this method of reinforcement, it is necessary to ensure reliable anchoring of the transverse reinforcement at the ends of the cross section of the "jackets". In columns, this is done by welding collars to the existing reinforcement.

When reinforcing locally damaged sections with "shirts", as well as when reinforcing with clips, they must be extended to undamaged parts of the structure for a length of at least 500 mm, as well as at least the length of the anchoring of the longitudinal reinforcement, at least the width of the edge of the element or its diameter and at least five times jacket wall thickness.

The effectiveness of the inclusion of a metal clip in the operation of the column depends on the tightness of the fit of the corners to the body of the column and the prestressing of the transverse bars. For a snug fit of the corners, the concrete surface along the faces of the columns is carefully leveled by chipping off irregularities and caulking with cement mortar. The connection strips are prestressed thermally. To do this, the strips are welded on one side to the corners of the clip, then heated gas burner up to 100-120 ° C and in a heated state, the second end of the strips is welded. The closure of the slats is carried out symmetrically from the average height of the column belt. When the bars cool, the cross sections of the column are compressed, which leads to an increase in the bearing capacity.

The metal cage consists of corner profile posts, connecting strips, support pads (Fig. 1).

Rice. 1. Strengthening the column with a metal clip:

1 - overlap; 2 - strengthened column; 3 - clip;

4 - angle to i-racks; 5 - transverse strips; 6 - support bars

An effective means of reinforcing the outer columns is the installation of prestressed metal struts. Single-sided or double-sided struts are metal cages with prestressed posts located on one or both sides of the columns. The former are used to increase the bearing capacity of eccentrically compressed columns with large and small eccentricities, the latter - for centrally and eccentrically compressed columns.

Prestressed single-sided struts consist of two angles connected to each other by metal strips. In the upper and lower zones of the spacers, special strips with a thickness of at least 15 mm are welded, which transfer the load to the stop angles and have a cross-sectional area equal to the cross-section of the spacers. The strips are installed in such a way that they protrude beyond the ends of the corners of the spacers by 100-120 mm, and are provided with two holes for the tie bolts. Thrust angles must be installed in such a way that their inner edges coincide with the outer edge of the columns. To do this, the protective layer of concrete in the upper and lower zones of the column is chipped off and the stop angles are installed on the cement mortar strictly horizontally. Before installing the spacers in the design position, a cut is made in the side shelves of the corners in the middle of their height and their slight bend is made. The weakening of the cross-section of the corners in the place of the cutout is compensated by welding additional strips, in which holes for the tie bolts are provided.

The spacers are prestressed by placing them in a vertical position by tightening the nuts of the tension bolts. At the same time, it is necessary to ensure a snug fit of the corners to the body of the column, as well as their joint work, combining the spacers by welding metal strips to them. The step of the bars is taken equal to the minimum size of the section of the column. After welding the strips, the coupling mounting bolts are removed, and the weakened sections of the spacers are reinforced with additional metal plates.

To effectively engage the spacers in the work, it is enough to create in them a preliminary stress of the order of 40-70 MPa, which is ensured by the calculated elongation when straightening the corners.

Building - an increase in the cross section of reinforced structures from above, below and from the sides with a layer of monolithic reinforced concrete (beam, crossbar, column, walls, floor slab).

Reinforcement of columns by concreting (Fig. 2) is performed in the following sequence:

Rice. 2. Reinforcement of the column with concreting: 1 - existing column;

2 - reinforced concrete "shirt"

The calculation determines the number and diameter of the working reinforcement and clamps or spiral reinforcement. The new reinforcement is connected to the old one;

Formwork is installed and concreting is carried out. For better adhesion of old and newly laid concrete, the surface of the column is thoroughly cleaned, notched and washed with water under pressure. The minimum thickness of the "jacket" should be sufficient to accommodate the reinforcement and the protective layer (50 mm), and when shotcrete - 30 mm.

When reinforcing reinforced concrete structures, a number of technological processes are performed: preparing the surface of the reinforced structure, installing reinforcement and formwork, laying and compacting the concrete mixture, caring for concrete during the period of achieving the required strength and dismantling the formwork. The surface of the reinforced structure is prepared to ensure reliable adhesion of the concrete of the reinforcement layer to it. In this case, the following operations are performed: removal of the surface of the protective layer and removal of concrete peelings; cleaning of reinforcement from surface corrosion; blowing with compressed air and moistening the surface. The removal of the protective layer of concrete and the removal of its delaminations is carried out using a mechanized tool (electric planer hammers IE-4207 and IE-4210, chipping hammers IP-4119, EP-1027, EP-1056, etc.).

Reinforcement cleaning from rust is recommended to be carried out by waterjet treatment, using equipment for shotcrete, and quartz sand or sand-gravel mixture with a moisture content of up to 6% as a working mixture. During hydroabrasive processing, the pressure ratio is observed compressed air(on the compressor receiver) and water supplied to the nozzle 4: 0.5. To clean fittings from rust when strengthening structures in cramped conditions, it is effective to use a small-sized sandblaster with a vacuum gun operating on the principle of an ejector. With small amounts of work, for cleaning fittings from rust, pneumatic hand-held angular metal brushes IP-2104 (brush weight 4 kg, compressed air pressure in the pneumatic system 0.6 MPa) are used.

It is most expedient to carry out laying of the concrete mixture when strengthening concrete structures using installations for concrete pneumatic spraying: with a reinforcement layer thickness of up to 80 mm - shotcrete using a cement gun; with a thickness of a layer of reinforcement of massive structures up to 250 mm and its total surface of at least 10-15 m 2 - sprayed concrete using concrete-syringe machines. A feature of these plants is the supply of dry concrete mixture through hoses using compressed air, which is mixed with water at the outlet of the end nozzle. The concrete mixture is ejected from the nozzle at a speed of 50-70 m/s and forms a dense layer on the surface. The machines perform four processes simultaneously: they transport the concrete mixture to the place of laying, mix it with water, spray and compact. When using these installations, formwork is completely excluded, labor costs and terms of work are significantly reduced, which is especially important during reconstruction. Sprayed concrete has increased strength and adhesion, and also provides enhanced protective functions and improves the performance of structures compared to conventional concrete.

For shotcreting structures in cramped conditions, it is effective to use the SB-117 cement gun, for applying sprayed concrete - SB-67 and SB-68 installations. The thickness of the applied layer of sprayed concrete at a time is 50-70 mm, the distance between the nozzle and the surface to be concreted is 1 - 1.2 m. .9 and 0.6 MPa (for SB-117), a water tank, mobile scaffolds or auto-hydraulic lifts for working at height. Dry concrete mixes are supplied centrally: for volumes of work up to 2.5 m 3 - in bags, for large volumes of work - in specialized containers.

With an increase in the load on the consoles of the columns, they are reinforced with horizontal or inclined strands (Fig. 3).

Rice. 3. Reinforcement of consoles with ties:

1 - reinforced console; 2 - supporting elements; 3 - stops from the corners; 4 - strands;

5 - anchors; 6 - stops from channels

Prestressing is created by tightening the nuts or by tightening the clamps together. Unloading of consoles is also used with the help of additional metal brackets or special supports in the form of channels (corners), which are attached to the column using prestressed strands.

REINFORCEMENT OF ROOF STRUCTURES

With an additional load on the truss trusses and beams, it often becomes necessary to strengthen them as a whole or individual elements and assemblies. Most effective ways amplifications are shown in fig. 12.

The reinforcement consists of two identical (hinged-rod) chains on both sides of the structure, anchor devices in the upper zone on supports, round steel hangers or profile metal racks located at the bends of the chain branches.

Branches are usually made from corners, the vertical shelves of which are cut at the bends of the chains, as well as from reinforcing bars with a diameter of up to 36 mm or high-strength wire ropes. Anchors are made from sheet or profile steel. Reinforcing elements of reinforcement are accepted in classes A-I, A-P, A-III, K7, K19, metal structures made of steels Vst3sp, Vst3ps and VstZkp. The prestressing of the hinge-rod system is carried out by tightening the nuts with a wrench or a jack.

Rice. 1. Ways to strengthen the metal trusses of the coating:

a) prestressed hinge-rod chains by tightening the nuts;

b) reinforcement of truss nodes with metal clamps made of sheet steel or reinforced concrete;

c) trussed puffs from corners or I-beams and corners;

1 - single-tier reinforcement within the height of the trusses; 2 - the same below the truss belt; 3 - hinged-rod chains; 4 - horizontal strands; 5 - reinforcement clamps; 6 - concrete;

7 - sprengel; 8 - support device; 9 - spacer; 10 - tension screws

Rice. 2. Methods for reinforcing roof beams:

a) summing up unloading racks, frames, struts, etc.:

1 - reinforced beam; 2 - additional support; 3 - supporting element from the channel;

4 - metal wedges for putting the rack into operation;

6) reinforced concrete build-up:

1 - reinforced beam; 2 - reinforced concrete building; 3 - longitudinal reinforcement reinforcement; 4 - reinforcing shorties; 5 - bare reinforcement of the beam (in increments of 1 m);

c) the device of a reinforced concrete clip:

1 - reinforced beam; 2 - reinforced concrete slabs; 3 - reinforced concrete clip; 4 - beam surface prepared for concreting (cleaning, notching, washing with water);

5 - holes punched in the shelves of concrete slabs

Reinforcement of compressed truss belts is carried out by installing metal clips made of sheet or profile metal. Reinforcement of the lower belt is carried out by prestressed puffs. The supporting parts of the anchor devices of the puffs are made of plates with a thickness of 10-24 mm, reinforced with ribs. To include puffs in the work of farms, it is necessary to create a prestress of about 15-20 MPa in them. Anchor devices must fit snugly against the supporting parts of the trusses, for which, in some cases, a layer of grade 25 cement mortar is made between the base plates and concrete.

The stretched braces of the truss are reinforced with prestressed puffs, which are fastened to the truss nodes by welding to shaped parts or support corners. The end sections of the puffs are provided with threaded shorts, and the diameter of the shorts must exceed the diameter of the puffs by at least 4 mm.

The metal clips of the compressed elements of the trusses are included in the work due to the expansion forces that arise when an additional load is applied to the truss. If it is necessary to unload the compressed elements of the trusses, prestressed one-sided or two-sided struts are performed. The struts abut against special clips made of sheet steel installed in the truss nodes.

To reinforce the rafter beams, trussed puffs from corners or I-beams and corners are recommended. The prestressing of the sprengel is necessary for the reliable inclusion of the sprengel in the operation of the beam. The truss tightening includes two side corners, which are attached to anchor boxes installed on the cement mortar along the ends of the beam (Fig. 3). The prestressing of the sprengel is carried out by mutual tightening of the horizontal corners of the lower chord with the help of special bolts.

Rice. 3. Strengthening the truss beam with a prestressed truss from the corners:

I - reinforced element; 2 - inclined strand; 3 - corner of the lower belt; 4 - compensating pads; 5 - mounting hangers; 6 - horizontal strand of sprengel

The lower horizontal part of the sprengel can be made of an I-beam or a channel. In this case, the prestressing of the sprengel is carried out by pulling the I-beam from the beam with the help of tension screws, and first the screws are simultaneously tightened in the places where the strands are bent, and then the middle bolt. After tightening, the bolts are welded to the lower sprengel belt to prevent their untwisting.

Prestressing can also be carried out with the help of hydraulic jacks suspended from the truss at the points of bending of the strands.

Fixing the prestress is carried out by filling the gap between the lower chord of the beam and the I-beam with cement mortar or special linings from pieces of strip steel.

After the reinforcement is completed, all metal parts are painted with a protective varnish or enamel.

The sequence of reinforcement of reinforced concrete structures is shown in fig. 4.

Rice. 4. The sequence of reinforcement of reinforced concrete structures

Strengthening reinforced concrete trusses located in emergency condition, is performed by unloading them and transferring forces to additional steel trusses installed on both sides of the emergency truss using mounting beams (winches, blocks).

The transfer of load from the roof slabs to the installed trusses is carried out by uniform wedging, which eliminates the gaps between the support posts of the installed trusses and the longitudinal ribs of the roof slabs. Wedging is carried out simultaneously along both farms from the middle to the edges. Further, gaps are formed between the coating slabs and the emergency truss.

STRENGTHENING THE STRUCTURES OF PLATES

Monolithic floor slabs can be reinforced by extension, i.e. concreting an additional reinforced concrete slab over the existing one, as well as adding additional supports in the form of monolithic reinforced concrete or metal beams.

Precast concrete hollow core slabs can be reinforced using hollow spaces. From above, in the channel location zone, a shelf is pierced and a reinforcing cage is installed. Then the channel is filled with plastic concrete on fine gravel and the slab is calculated taking into account additional reinforcement (Fig. 10).

Rice. 10. Reinforcement of precast hollow core slabs:

I - reinforced plate; 2 - support; 3 - additional reinforcement cage;

4 - reinforced concrete

When strengthening only the supporting part of the slab, the frames are located on a part of its span, and if necessary, strengthening along normal and inclined sections - along the entire length of the slab.

Strengthening the supporting parts of hollow core slabs with insufficient area of ​​their support is recommended to be carried out according to the following schemes:

For extreme supports - by installing reinforcing cages in the channels with their removal beyond the ends of the slabs to the required length, then installing vertical frames parallel to the ends of the slabs, concreting the anchor beam and supporting sections of the slab voids;

For intermediate supports - by installing common vertical frames into pre-punched holes in support zones, adjacent slabs and subsequent concreting of channels with additionally installed reinforcement. In this case, the plates work as continuous structures.

The longitudinal ribs of prefabricated reinforced concrete ribbed slabs are reinforced by adding additional metal supports that reduce the span of the ribs, additional metal beams, which are included in the work with the help of a wedge; truss structures.

An effective way to strengthen the longitudinal ribs of the slabs along normal sections is to install additional reinforcing cages in the joints between the slabs and concreting the joints.

It is possible to build up longitudinal ribs with additional reinforcement while ensuring its connection with the existing working reinforcement.

If it is impossible to make concrete to reinforce the slabs supported along the contour, it is recommended to bring a prestressed spatial truss under the slabs (Fig. 11), which consists of two mutually intersecting flat trusses at the same level, the upper chords of which are tightly fitted to the lower plane of the slab, and the lower the belts are prestressed mechanically or thermomechanically.

During operation, the sprengel must be protected from corrosion, and, if necessary, closed with a false ceiling.

Rice. 11. Reinforcement of the prefabricated plate, supported along the contour,

spatial sprengel:

1 - reinforced plate; 2 - element of the carrier circuit; 3 - spatial sprengel;

4 - upper belt; 5 - lower belt; 6 - intermediate racks; 7 - central pillar;

8 - bolts for suspension of the sprengel; 9 - transfer traverses

To strengthen the support of prefabricated floor and roof slabs on the crossbars and building structures, it is recommended to bring metal tables from the corners under the supports, fixing them with bands or clips to adjacent structures or the upper belt of the crossbars and truss structures (Fig. 12, 13).

Rice. 12. Support table options, if available

embedded details:

1 - crossbar; 2 - plate; 3 - embedded part in the crossbar; 4-support table

Rice. 13. Strengthening the support of plates:

1 - crossbar; 2 - plate; 3 - fastening the strand to the plate; 4 - inclined strand-5 - thrust table; 6 - stiffeners; 7 - clamps; 8 - corner "support table

INSTALLATION OF ADDITIONAL INSERTS AND REINFORCEMENT OF JOINTS

It is often necessary to install additional embedded parts or restore structures missed during the manufacture of structures.

At the same time, one should distinguish between structural embedded parts to which large loads are not transferred, and those that perceive significant bending moments and tearing forces.

The first group includes embedded parts for fixing elements that are installed on load-bearing structures (covering slabs on beams and trusses, beams and trusses on columns, self-supporting walls and Wall panels to columns, etc.). These embedded parts experience compressive or slight shear forces and are easily fixed using a special metal clamp.

For example, to fix a supporting metal sheet on the surface of a reinforced concrete element (Fig. 14), it is enough to chop off the protective layer at two corner reinforcing bars, weld round shorts or strip steel ribs to them, and to the latter - a sheet (corner) of a new embedded part.

Rice. 14. Installation of parts on the upper plane:

I - chipped concrete zone; 2 - shorty-lining from a round rod;

3 - welds; 4 - additional embedded part; 5 - corner

element armature; 6 - transverse frame rods

If it is necessary to make an embedded part flush with the surface of the concrete in the protective layer, a groove is cut, the width of which exceeds the width of the embedded part by 10-20 mm, and the depth - the thickness of the plate by 5-10 mm. The plate is pressed into a fresh cement mortar and welded through short pads to the working reinforcement of the frame.

The method of installing structural embedded parts using metal clamps is less laborious (Fig. 15), although it requires more steel consumption. Such embedded parts are made locally from pre-prepared and fitted elements.

Fig, 15. Installing parts using clamps:

1 - side planks collar; 2 - the front plate of the clamp; 3 - welds; 4 - coupling bolt; 5 - stiffeners; 6 - hole in the wall of the beam for the passage of screed concrete

When arranging rigid joints of crossbars with columns, as well as in case of defects in reinforcement protrusions (misalignment, reduction in diameter and quantity of reinforcement), female clamps are recommended, the area of ​​\u200b\u200bwhich is equal to the design section of the joint. During reconstruction, it often becomes necessary to anchor additional reinforcement or install new embedded parts in an existing reinforced concrete structure. In these cases, it is recommended to drill wells in concrete with a perforator to a depth of at least 20 reinforcement diameters and seal reinforcement in them with epoxy glue or by vibro-caulking with a rigid cement mixture. On epoxy glue, it is possible to fix reinforcement of a smooth and periodic profile to the horizontal and vertical plane of concrete, as well as to the lower plane located at an angle of 45 ° to the horizon. It is allowed to fix the reinforcement on the cement mortar only on the horizontal plane of the concrete. A washer is welded to the anchor shorty at the end, the hole is caulked with cement mortar using a special vibrocompactor. The anchoring of the rods in the body of concrete is carried out at a distance of at least 5 diameters from each other and at the same distance from the face of the concrete.

Reconstruction buildings or structures are referred to as partial or complete rebuilding undertaken on existing production or residential areas in order to modernize the technological process or in connection with the need to improve the functional or aesthetic qualities of the object during its operation. Renovation should be distinguished from plant expansion, which is the construction of new facilities adjacent to existing production facilities.

Reconstruction problem industrial enterprises acquires special relevance. Practice shows that in the reconstruction of enterprises, capital investments pay off 2-3 times faster than in new construction. The investment itself can be significantly reduced. The choice of a reconstruction method is a complex engineering task, since the designer is limited by the existing construction conditions, space-planning and design solutions, work conditions and other factors.

Measures for the reconstruction of the building, depending on the volume of material labor costs, can be divided into three categories:

• ? small reconstruction, which consists in restoring or increasing the bearing capacity of structures by strengthening them without changing the space-planning solution and without stopping or with a partial stop of the technological process;
• ? medium reconstruction, involving the replacement of individual structures, increasing the level of coverage, etc., i.e. with a partial change in the space-planning solution and a possible complete stop of the technological process;
• ? full reconstruction, in which the existing building must be demolished and a new building must be erected, corresponding to the adopted space-planning decision and the project for placing new equipment.

Gain structures can be carried out according to two schemes: the construction of new unloading or replacement structures, which fully or partially perceive additional loads; increase in the bearing capacity of existing structures. In turn, an increase in the bearing capacity of structures can be carried out: without change and with a change in the design scheme and stress state; using special amplification methods.

The calculation of reinforced concrete reinforcement structures is carried out taking into account actual characteristics strength and reinforcement of materials. Reinforcing concrete should be taken one class higher than the conditional strength class of concrete of the reinforced element, but not lower than B15 for ground structures and B 12.5 for foundations. In addition, under aggressive operating conditions, the class of concrete must meet the necessary density and resistance, corresponding to the requirements of the operating environment. The solution for sealing holes, protective plaster, etc. is accepted not lower than grade 150.

When strengthening concrete and reinforced concrete structures with build-up, “shirts” and clips, Portland cement of a grade of at least 400 should be used. 10 °C/h).

The minimum thickness of the protective layer of concrete for prestressed reinforcing reinforcement is assumed to be 20 mm. It is recommended to place the most critical reinforcement nodes outside the zones of constant moisture.

Reinforced concrete structures of elements of a building or structure are reinforced in two main ways: by increasing the size of the elements and by changing the design scheme. The build-up reinforces monolithic and prefabricated floor slabs, beams, columns and other structures. The purpose of changing the structural scheme is to create more favorable working conditions for the structure (in terms of the forces acting in it).

Compressed zones of bent and eccentrically compressed elements can be reinforced with shotcrete up to 30 mm thick, which is applied to the cleaned and washed concrete surface old concrete, wrapped with a mesh with a cell of 30-60 mm from a wire with a diameter of 1-2 mm, attached to the structure with dowels using a construction gun. With careful observance of the above recommendations, reliable adhesion of "new" and "old" concrete is ensured, as a result, the cross section of the structure and, as a result, its bearing capacity increase. A more significant increase in the bearing capacity of the elements can be achieved by increasing the cross-sectional area of ​​​​the reinforcement (section increase).

With a slight increase in the cross section of the reinforcement, the protective layer is cut off, the reinforcement is exposed and short pieces with a diameter of 10–40 mm and a length of 50–200 mm are welded to it with an intermittent seam in increments of 200–1000 mm - for tensioned rods and no more than 20 diameters of longitudinal reinforcement, but no more 500 mm - for compressed ones (Fig. 17.1a). Additional longitudinal reinforcement is welded to the short stacks, which can be used of the same classes. With reinforcement of class A600 and above from high-strength wire and ropes, as well as with severe corrosion of reinforcement, the use of welding is not allowed, and reinforcement of structures by extension is not recommended.

After installing additional reinforcement, it is shotcrete or sealed cement plaster, while the size of the section of the element increases by 20-80 mm. With a greater thickness of the build-up, vertical and inclined connecting elements are used.

To increase the adhesion of "old" and "new" concrete, a notch is made on the surface of the reinforced element before building, which is thoroughly cleaned of dust and dirt with water under pressure. The minimum diameter of the reinforcement when building up is 10 mm. If more powerful amplification is required, outer corner half-shells are arranged.

In order to work together with a reinforced concrete structure, the reinforcement steel structures must be welded to the existing reinforcement. For this purpose, corner bars reinforcing cage they are exposed in limited areas 6-12 cm long in increments of 60-120 cm (Fig. 17.16). Short reinforcing bars are welded to the reinforcement, the diameter of which is taken such that they are flush with the outer edges of the section. Then, clip strips are welded to the short spacers, tightly adhering to the concrete body. Clips from the corners are welded directly to the connecting strips of the clips.

Short spacers can be replaced with diagonal ribs made of sheet steel (Fig. 17.1c). The gaps between the branches of the cage and the concrete body are filled with a 1:2 or 1:3 cement mortar on expanding or non-shrinking cements, then the reinforcement elements are covered with perchlorovinyl enamel on the ground to match the color of the structure.

Reinforcements by adding reinforcement, as well as in the form of cages and semi-cages, can also be recommended if errors in reinforcement are found during the manufacture of structures or underestimation of the design class of concrete.

Rice. 17.1. Schemes for reinforcing beams with half-shells:

• a) adding bar reinforcement; b) reinforcement of the outer corner cage welded to the existing reinforcement; in) detail of welding of the corner with the help of diagonal ribs made of sheet steel;
• 1 - welds; 2 - additional reinforcement reinforcement; 3 - reinforced element; 4 - chipped concrete of the protective layer with its subsequent restoration; 5 - protective covering; 6 - transverse rods of the extreme welded frames; 7 - rods - gaskets - shorties; 8 - corner rods of the extreme welded frames; 9 - connecting strips of the clip;
• 10 - side sheet gaskets; 11 - clip corners; 12 - space filled with cement mortar; 13 - sheet diagonal gasket

A common way to reinforce bent reinforced concrete elements is the installation of "shirts" - concrete blocks that are not closed on one side. This method is recommended when reinforcing beams of ribbed floors, etc. (Fig. 17.2). In case of violation of the anchoring of the longitudinal reinforcement, the removal of the bracket supports from the ends of the beam is taken at least 40 diameters with bar reinforcement and at least 80 with reinforcement made of high-strength wire. When reinforcing floor beams of multi-span buildings, combined schemes: in the extreme span - pre-stressed truss, in the middle - pre-stressed relief brackets.

One of the simplest ways to strengthen bending monolithic and precast concrete structures, carried out without unloading them, is to install additional reinforcement, which can have a horizontal or trussed shape (Fig. 17.3).

Rice. 17.2.

• 1 - reinforced beam; 2 - working fittings; 3 - clamps; 4 - coupler;
• 5 - notch; 6 - mounting fittings "shirt"; 7 - "shirt"

If it is necessary to strengthen the bent element in a local area, a protective layer of concrete is chipped off in the support zone, where the stresses in the longitudinal reinforcement are insignificant, and short pieces are welded to the existing reinforcement, the diameter of which slightly exceeds the thickness of the protective layer. Then reinforcement reinforcement is welded to one of the shorties.

Rice. 17.3.

• a) linear; b) trussed;
• 1 - connecting elements; 2 - reinforced beam; 3 - prestressing reinforcement; 4 - tensioning device; 5 - inclined branches of the support device; 6 - connecting elements

An effective way to strengthen reinforced concrete columns is to install reinforced concrete or metal clips.

Reinforcement with clips is especially rational for columns with little flexibility (

The simplest type of reinforced concrete cages are cages with conventional longitudinal and transverse reinforcement without connection between the cage reinforcement and the reinforcement of the reinforced column (Fig. 17.4).

With this method of strengthening, it is important to ensure the joint work of the “old” and “new” concrete, which is achieved by thoroughly cleaning the surface of the concrete of the reinforced structure with a sandblaster, notching or processing with metal brushes, as well as washing under pressure immediately before concreting. To improve adhesion and protection of concrete and reinforcement in aggressive operating conditions, the use of polymer concrete is recommended.

The thickness of the casing of the columns is determined by the calculation and design requirements (the diameter of the longitudinal and transverse reinforcement, the thickness of the protective layer, etc.). As a rule, it does not exceed 300 mm. The area of ​​the working longitudinal reinforcement is also determined by calculation, its diameter is taken to be at least 16 mm for rods working in compression, and 12 mm for rods working in tension. Transverse reinforcement with a diameter of at least 6 mm for knitted frames and 8 mm for welded frames is installed in increments of 15 diameters of longitudinal reinforcement and not more than three times the thickness of the clip, but not more than 200 mm. In places of stress concentration, the step of the clamps decreases.

Rice. 17.4.

• 1 - reinforced column; 2 - clip; 3 - longitudinal reinforcement;
• 4 - transverse reinforcement of the clip; 5 - rigid longitudinal clip;
• 6 - support angles

With local reinforcement, the clip is extended to the limits of the damaged area for a length of at least five times its thickness and at least the length of the reinforcement anchoring, as well as at least two times the width of the larger face of the column, but not less than 400 mm. With local reinforcement, in order to improve the adhesion of "new" and "old" concrete, it is recommended to perform an adhesive coating of polymeric materials.

Additional longitudinal reinforcement is welded to the existing one with the help of connecting shorts, which, in order to avoid burns, are made of reinforcement of class A240 with a diameter of 10-16 mm and are located at a distance of at least 20 diameters of longitudinal reinforcement in a checkerboard pattern.

If it is impossible to make a closed cage, for example, when a column is adjacent to a wall, it is recommended to install "shirts" - concrete blocks that are not closed on one side. With this method of reinforcement, it is necessary to ensure reliable anchoring of the transverse reinforcement at the ends of the cross section of the "jackets". In columns, this is done by welding clamps to the column reinforcement.

When reinforcing local damaged areas with “jackets”, as well as when reinforcing with clips, they must be extended to undamaged parts of the structure for a length of at least 500 mm, and also not less than the length of the anchoring of the longitudinal reinforcement, at least five times the thickness of the “jacket” wall. For structural reasons, the diameter of the longitudinal and transverse reinforcement of the “shirts” is taken to be at least 8 mm, with knitted frames, the minimum diameter of the collars is 6 mm.

If it is impossible to increase the section of the columns and the deadlines for the production of reinforcement work are recommended, metal clips from the corners installed along the faces of the columns and the connecting strips between them. The effectiveness of the inclusion of a metal clip in the operation of the column depends on the tightness of the fit of the corners to the body of the column and the prestressing of the transverse bars.

For a snug fit of the corners, the surface of the concrete along the faces of the columns is carefully leveled by chipping off irregularities and baking with cement mortar. The connection strips are prestressed thermally. To do this, the strips are welded on one side to the corners of the clip, then they are heated with a gas burner to 100-120 ° C and the other end of the strips is welded in a heated state. The slats are closed symmetrically from the middle belt height of the column. When the bars cool down, the cross sections of the column are compressed, which significantly increases its load-bearing capacity.

When replacing crane equipment with equipment of greater carrying capacity, columns and crane consoles are reinforced or additional racks are installed that carry only the crane load. Crane beams are usually subject to replacement. In cases where the lifting capacity of cranes increases slightly, this can be compensated by replacing the steel bridges of the cranes with lighter aluminum ones. In all cases of reconstruction of buildings associated with an increase in payloads or loads from the dead weight of structures, it is necessary to pay the most serious attention to ensuring the bearing capacity of the foundations and existing foundations and, if necessary, to strengthen them.

Reconstruction of industrial buildings is carried out on average in 10-20 years. The service life of industrial buildings and structures made of reinforced concrete during normal operation is usually 50-100 years, i.e. the obsolescence of the structure occurs much earlier than the physical one. For these two indicators, it is necessary to provide for the development of a particular technology based on scientific forecasts at the design stage. Therefore, it is necessary to strive for the design of buildings with a flexible structural scheme, the smallest possible number of intermediate supports, etc. This opens up the possibility for engineers to widely use progressive structures (thin-walled spatial coatings, etc.).