Reinforced concrete bending structures (beams, crossbars, crane beams, slabs, floors and coverings) are reinforced using the following fairly proven methods. When building up, the reinforced structure is increased in height or width (from below, from the sides and from above the reinforced element). A feature of this method is the perception of tangential stresses acting in the plane of contact of old concrete with new concrete, by special additional reinforcement welded to the reinforcement of the structure being strengthened.
Strengthening the existing structure, i.e. increasing its load-bearing capacity by building up leads to the joint work of the reinforced structure and the reinforcement structure, including them in the work in proportion to the rigidities. Extension is used to enhance reinforced concrete structures both monolithic (Fig. 3.2) and prefabricated (Fig. 3.3). Reinforcing bars are used 10 mm or more.

Reinforcement of bending elements instead of building them up with clips is allowed only in case of significant damage, for example, due to corrosion of reinforcement, since the reinforcement of bending elements is taken depending on the protective layer and the diameter of the longitudinal and transverse reinforcement and usually does not exceed 100 mm. When reinforcing a monolithic ribbed floor with a clip, it is necessary to punch holes in the floor slab to allow passage of clamps and supply of concrete mixture during concreting. Often, when constructing frames for beams, the slab is also concreted on top (Fig. 3.4, a).
The constructive solution in the form of a shirt, unlike a clip, is a concrete shell that is not closed on one side (see Fig. 3.4, a), in in this case with connection under the stove monolithic ceiling additional metal fin. Shirts are used in the same cases as clips, but only when it is not possible to cover the reinforced element from four sides.

Jackets are more often used when strengthening monolithic beams of ribbed floors. In this case, the clamps are brought out through the slab and anchored using longitudinal reinforcing bars. When reinforcing with a jacket only the damaged areas of the reinforced elements, it must be placed on undamaged parts of at least: the length of the anchorage of the longitudinal reinforcement of the jacket; five shirt wall thicknesses; edge width or diameter of the reinforced element and 500 mm. When reinforcing with jackets, reinforcement with a diameter of 8 mm or more is used for longitudinal rods and with a diameter of 6 mm for clamps.

In Fig. 3.5 and 3.6 compare methods of strengthening prefabricated and monolithic structures by building up and using a jacket. Sometimes, to increase the load-bearing capacity of reinforced elements by extension, it is enough just to increase the amount of main longitudinal reinforcement. To do this, the protective layer is removed to a depth of at least 0.5 diameters and additional reinforcement is built up by welding through short pieces of reinforcement 50...200 mm long. In the tensile zone, the shorts are placed every 200...1000 mm, in the compressed zone - at a distance of no more than 500 mm or 20 longitudinal reinforcement reinforcements. The reinforcement reinforcement is covered with cement plaster or gunite.

In the case of a significant increase in the cross-section, it is recommended to use specially welded connecting elements, for example 7 in Fig. 3.2 or 6 in Fig. 3.3. If reinforcing bars in bending elements break, it is recommended to restore them by welding prestressed overlays (Fig. 3.7). This operation requires preliminary strengthening of the structure with temporary supports. Welding of additional reinforcement is allowed only from steel classes A-I, A-II, A-III to existing reinforcement of the same classes.

An effective and fairly simple way to strengthen bending structures is to install additional rigid supports in the form of struts (Fig. 3.8) or vertical elements (Fig. 3.9). However, these solutions are limited by the conditions of the technological process, which does not allow for limited dimensions production premises.

Since it is very difficult to completely avoid settlement of the supports when making rigid supports on independent foundations, in all cases it is advisable to install them on existing foundations (Fig. 3.8, c), even if it is necessary to strengthen them. In these cases, rigid additional supports are made in the form of portals or in the form of struts. Reinforcement elements for rigid supports can be made of either reinforced concrete or metal.
If the reinforcement struts (Fig. 3.8, a and 3.8, b) are made of metal, in the lower nodes of the reinforcing system, overhead metal parts are connected by welding to the reinforcement of existing reinforced structures. After placing the struts in order to ensure a tight fit in the upper unit, the wedge is made using wedge-shaped spacers.
When making rigid supports in the form of connected racks with independent foundations (see Fig. 3.9), you should Special attention pay attention to reducing the settlement of these foundations, for which it is necessary to carry out preliminary compression of the soil under the sole. If the reinforced structure cannot be preliminarily unloaded, the installation of additional rigid supports must be accompanied by preliminary raising of the reinforced structure (see Fig. 3.8, b).
The lifting of the reinforced structure is carried out in various ways, depending on the design of additional supports and the designs of the reinforced elements. When reinforcing a prefabricated hinged frame, which is assembled on site from individual elements, hinges in the nodes and elastic gaskets between the reinforced crossbar and the reinforcement frame ensure the occurrence of two unloading forces of equal magnitude applied from bottom to top (see Fig. 3.8, b). The frame is stressed by lifting its racks with jacks, after which special metal spacers are placed in the gap between the frame racks and the existing support, and the jacks are removed.
When reinforcing the crossbars with prestressed precast reinforced concrete half-braces (see Fig. 3.8, a), the reinforced crossbar is lifted using a jack horizontally located in the upper unit. To facilitate the movement of the expanding half-braces, metal shorts made of round reinforcing steel are placed in the gap between the reinforced crossbar and the half-braces. After lifting the reinforced structure, the half-braces are connected to one another with a spacer made of profile metal by welding, and the jack is removed. To avoid overloading the columns from below, the half-braces at the bottom are tied with a special metal tie.

In addition to rigid additional supports, elastic additional supports are used to strengthen the bending elements, which are less restrictive to the dimensions of production premises. Additional elastic supports are usually created using metal trusses fixed to the same supports on which the reinforced structure rests. The elastic support for the reinforced element is created by a spacer between it and the reinforcement structure (Fig. 3.10), and has less rigidity than the reinforced concrete element being reinforced. In multi-storey buildings, if it is necessary to strengthen the crossbar of one of the floors, when the load-bearing structures of the overlying floor have a sufficient margin of safety, prestressed suspensions can be used (Fig. 3.11).
The compliance of supports of this type occurs due to their longitudinal deformation. The reactive unloading force is created by prestressing the strands, first with tension nuts, and finally with tension couplings. The loads from the tie rods are absorbed by the frame of the upper tier, to the racks of which the tie rods are secured, welding them to pre-arranged metal clips made of sheet steel.

To reduce bending moments in the elements of a multi-span multi-tier frame, cross prestressed connections made of flexible metal strands can be used (Fig. 3.12). The tension of such connections is carried out using turnbuckles or using a thermal method. Anchoring is carried out using special anchor clamps made of sheet metal, fixed to the columns. The specified connections can be installed along the height of the same frame only in different spans. For the same purposes, reinforcement with reinforced concrete braces with prestressed tie rods can be used (Fig. 3.13), when after installation of the brace the flexible metal ties are strained thermally on both sides the brace and the reinforcement element perceive both compressive and tensile forces.

To strengthen the bending elements of multi-span buildings, you can use the following solutions. Thus, when strengthening roof beams, outer pores are installed on intermediate supports (Fig. 3.14). To strengthen the bending elements, double-cantilever unloading brackets are also used, installed on intermediate supports (Fig. 3.15, 3.16).
When strengthening prefabricated beams, the branches of the brackets are triangular trusses. Their lower chord is usually made from an isosceles angle, and the upper chord and lattice can be made from either single angles or their round reinforcing bars (Fig. 3.15).

The height of the brackets is taken to be equal to the height of the supporting part of the reinforced beams, and the length of the cantilever parts of the brackets is 1/4...1/6 of the span of the reinforced beams. If the length of the cantilever parts is short, you can completely abandon the lattice elements. The support elements of the bracket can be either vertical metal sheet 20...30 mm thick and 300...400 mm high, welded from below to the horizontal distribution gasket, or in the form of saddle-shaped pads installed on top of the beams and connected by welding.
The design of the support device depends on the tension method. When tensioned with bolts, it is a rigid plate passed under the bottom of the reinforced beam and bolted to the branches of the bracket (see Fig. 3.15). When tensioning with a jack with a tensioning device, tension control is carried out using the jack's pressure gauge. After tensioning, as a rule, fixing gaskets are installed. On intermediate supports, a bracket design can be adopted, assembled from two separate parts (see Fig. 3.16). After their installation, the upper stretched chords are welded over the support with overlays. Connections are created along the lower belt for the overall stability of the lower belt using special support pads. In case of anchorage violations, the extension of the longitudinal reinforcement of the support or bracket must be taken at a distance of at least 40 diameters of the rod reinforcement from the support sheet of the beam.

In cases where there is a need to carry out strengthening work without removing the temporary load, you can use a solution that involves installing additional prestressed reinforcement. Horizontal, truss, or a combination of both can be used as additional stressed reinforcement.
If anchor devices cannot be placed at the ends of the beams, they are welded in the support zone in places where the stresses in the reinforcement of the reinforced beam are low (Fig. 3.17). In this case, tension is produced thermally. To prevent sagging under the influence of their own weight, the reinforcement rods are secured using temporary hangers. After heating the rod, its free end is also welded. When reinforced with trussed reinforcement, its tension is carried out mechanically, i.e. by screwing the tension screws or placing more gaskets into the gap of the package (Fig. 3.18).

Both with the option of horizontal tension of additional reinforcement, and trussed or combined, it is possible to create tension in them by mutually tightening two or four rods with special tension bolts (Fig. 3.19, 3.20, 3.21). The coupling bolts have the form of a clamp with two threaded ends and a common washer. Tension is produced by simultaneously tightening the nuts at both ends of the clamps. Tension by mutual tightening does not require significant effort, since the stress in the tie bolts, which act as clamps, is 7...10 times less than the stress in the additional rods being tightened.

The advantage of this tension method, along with its simplicity, is the creation of uniform forces in all tightened rods as a result of self-regulation. Round rods of additional reinforcement are usually taken with a diameter of 18...40 mm. The perception of transverse forces when strengthening bending elements is mainly produced by increasing the cross-sectional area of ​​transverse and inclined reinforcement.

A less labor-intensive method is to strengthen it with vertical overhead clamps (Fig. 3.22). To do this, holes are first punched in the ceiling on both sides of the beam, gaskets are placed from the corners and clamps are placed over them, which have threaded ends. A strip steel backing is placed on these ends and the nuts are tightened. When tightening the clamps, the nuts should be tightened at both ends simultaneously. An option for reinforcement with vertical clamps is reinforcement using prestressed clamps (Fig. 3.23).

The design of prestressed clamps consists of: upper fastening angles suspended from the floor slab with bolts, lower fastening angles connected by welding strips; an even number of clamps and tie bolts with locking washers. After securing the clamps at the bottom and top, pre-stress is created by mutually tightening each two adjacent rods with tie bolts. The rods are tightened simultaneously on both sides of the reinforced beam.
When reinforcing beams with inclined overhead clamps (Fig. 3.24), instead of strip steel pads, corner pads are used, which are welded to the lower longitudinal reinforcement using short bars. After tightening the clamps, the protective layer is restored.

The ribs of the prefabricated covering slabs are strengthened by installing vertical overhead clamps, which connect both ribs together (Fig. 3.25). When reinforcing hollow-core slabs with round and oval holes, voids can be used. To do this, in the supporting sections of the slabs (1/4 of the span), holes are cut from above into which additional reinforcement cages are installed (Fig. 3.26), and the voids are concreted with plastic concrete on fine aggregate with or without the installation of an additional slab (Fig. 3.26, b) devices. To absorb both shear force and bending moment, the slabs are reinforced along their entire length.

When reinforcing hollow-core slabs on the outer supports, to prevent their shifting, the frames are installed so that they overlap the support. Then frames are installed along the ends of the slabs, which, after concreting, create a beam-frame, if necessary, along the perimeter of all walls. On intermediate supports, common frames are installed in the voids of the adjacent ends of the slabs.
The insufficient support area of ​​ribbed prefabricated slabs can be compensated by installing metal ties on intermediate supports that interconnect the ribs of slabs of adjacent spans (Fig. 3.27, a), and on the outer ones - by lengthening the supporting parts of the ribs (Fig. 3.27, b). If necessary, short consoles of columns can be strengthened by installing additional prestressed inclined or horizontal ties or clamps (Fig. 3.28). The rods are attached to the console with metal fasteners and tightened by screwing in the nuts.

Strengthening the capitals of beamless floors, along with the installation of reinforced concrete jackets, can be carried out by installing prestressed metal
spatial trusses (Fig. 3.29). The structure of the truss consists of a lower corner frame resting on a reinforced concrete support frame; an upper corner frame covering the reinforced capital along the perimeter, and four struts connecting the frames to each other. The concreted support frame on the column and the lower frame mounted on it with welded struts are connected by welding to the heated upper frame, which, when cooled, shortens and creates a preliminary compression in the struts. Dimensions of support clips, heating temperature top harness should be determined based on the load that the reinforcement must bear. The design forces in the elements of a spatial truss should be calculated as in a spatial statically determinate truss under the action of a given load.
Strengthening of slabs supported on the contour, along with the concrete, is carried out by installing spatial prestressed metal trusses, brought from below under the reinforced slab and suspended in the corners to the load-bearing elements of the contour with four bolts and four transfer crossbeams. The springs are installed in two mutually perpendicular planes along the diagonals of the slab at the same level (Fig. 3.30).

The upper chords of the truss are tightly pulled to the lower surface of the reinforced slab, which allows them to be included in joint work when prestressing the lower chords using a thermomechanical method. All installation and prestressing work can be done without unloading the reinforced slab.
Crane beams are reinforced in two ways - with a metal clip and external metal supports (Fig. 3.31) or with a metal clip and truss, similar to the combined reinforcement option shown in Fig. 3.21. Strengthening the fastenings of crane beams to columns is carried out by plates connected by welding to the embedded parts of the column.
The embedded parts on the column can be secured either with metal clamps on spring washers (Fig. 3.32, a) or with metal clips (Fig. 3.32, b).

There are two main methods of strengthening reinforced concrete structures:

By building up sections in the form of concrete blocks, clips and jackets. The nabetonka is made on one side, the frame on four, the shirt on three sides;

Unloading structures with changes in the static operating scheme.

The first method requires stopping the production process, as well as partial unloading of structures for the purpose of subsequent loading and inclusion of increased elements in the work. The method is highly labor-intensive, as it is associated with wet concreting processes and requires additional time to gain concrete strength, although it is a universal strengthening method for reinforced concrete structures.

The second method involves the inclusion of the unloading structure in joint work with the reinforced structure directly during its installation and does not require stopping production, which is often an important factor.

When repairing the protective layer of concrete, the following types of work are provided: sealing individual gouges and shells; replacement or restoration of the protective layer (partial or complete) .

During a complete replacement, the thickness of the protective layer can be increased, but in all cases it must be at least 3 cm in clearance for working fittings and at least 2 cm for clamps and non-working fittings. The protective layer of concrete is replaced in cases where its properties are reduced, the reinforcement is corroded, or the protective layer of concrete peels off. In these cases, the old protective layer must be completely removed, and the fittings must be cleaned of rust. For laying a new protective layer, ordinary concrete, but with fine fractions, is recommended.

To repair minor damage to the protective layer, manual plastering techniques using a trowel are used. After about an hour, the laid concrete (mortar) is moistened with water, sprinkled with dry cement and smoothed using a trowel, wooden or metal trowel. At the same time, the depth of the gouged areas of the surface being prepared for repair should not disappear towards the edge of the gouge; it should be at least 1 cm everywhere. The transition from the gouge site to the undamaged protective layer should be made by a step at an angle of 90°.

For a large volume of work, the most effective method of applying concrete is shotcrete, which achieves a very dense, durable protective layer.

When preparing the surface for concreting, single cracks with an opening width of more than 1 mm are cut into a rectangle to a depth and caulked with concrete. In places where there are large concrete spalls and exposed reinforcement, an additional reinforcing mesh with a cell size of 2.5 to 10 cm and a wire diameter of 0.5 to 6 mm is installed, with the newly installed mesh being attached to the main reinforcement of the structure.

To increase the adhesion forces between new and old concrete, it is recommended to use a layer of epoxy-thiokol adhesive K-153. When restoring the protective layer using an epoxy-thiokol layer, the concrete must be laid until the adhesive loses its tack.

Depending on the degree of development of cracks, the following methods of repairing structures are used:

Device protective films and coatings for repairing cracked surfaces with cracks opening up to 0.2 mm;

Sealing cracks (filling them with waterproof elastic materials) for repairing structures with cracks opening more than 0.3 mm;

Surface sealing of cracks (installation of a sealing lining that covers the crack and strengthens the section with the crack) for repairing structures with through cracks with an opening of more than 0.2 mm;

Strength sealing (sealing the crack cavity with an adhesive composition) for repairing structures with cracks opening more than 0.3 mm. Covering repaired surfaces with films is intended to protect concrete and structure surfaces from atmospheric and chemical corrosion.

The installation of protective films and coatings is carried out by painting concrete surface polymer cement paints or synthetic varnishes.

Sealing cracks with highly elastic materials without restoring the solidity of the structure is intended to block the access of moisture and other agents that cause corrosion to the reinforcement, ensuring its safety. Cracks are sealed with elastic materials in the form of mastic using syringes.

Strength sealing is recommended, if necessary, to restore the solidity of the structure simultaneously with the elimination of cracks. Strength sealing can be done by injecting epoxy or cement mortar into the crack cavity. Before injection, holes must be made and nipples installed in them through which the adhesive composition is supplied. After installing the nipples, the crack on the concrete surface is sealed using a fiberglass sticker, which prevents the adhesive from leaking out. Injection begins from the lower nipple.

The most common way to strengthen structures is to increase the cross-sections by installing all-round clips or unilateral extensions. This method allows you to obtain a significant increase in the load-bearing capacity of both intact and heavily damaged elements. When strengthening reinforced concrete structures by one-sidedly increasing the cross-section, additional reinforcement is welded to the old one using bends, shorts, inclined and vertical clamps. It is recommended to weld clamps and shorts using electric welding with double flank seams.

If there is local damage in the form of single or concentrated cracks over a short length, local reinforcement of the structure is carried out using one of the following methods: installation of local four-sided reinforced concrete cages well reinforced with clamps, bent and longitudinal reinforcement; arrangement of metal clips from vertical tension clamps.

If there are vertical or oblique cracks, longitudinal distribution angles are located under the clamps, covering the damaged part of the beam. The clamps are covered with shotcrete over a metal mesh or concreted.

When strengthening columns, the cage is reinforced with longitudinal rods and stirrups or spiral reinforcement. The cage can be concreted in the formwork or shotcreted; the thickness of the walls during conventional concreting should be at least 10 cm and with shotcreting - 5 cm. It is recommended to chip off the corners of the reinforced column. At the top and bottom of the column at a length equal to the largest dimension cross section columns, the clamp spacing is halved. If there are local damages or defects in the columns, a reinforcing collar can be installed within the damaged area with a bypass in both directions for a length of 50 cm, but not less than the larger cross-sectional size.

When strengthening reinforced concrete structures by building up elements, it is necessary to chop off the protective layer of concrete at the welding points on the side of the section intended for strengthening and expose the longitudinal rods of the existing reinforcement to half of their cross-section.

After this, the concrete surface is washed with a stream of water under pressure. If for some reason it is not possible to create pressure, the concrete surface, after notching with a chisel and brushing, is blown with air so that no dust remains on it and washed with water.

The concrete surface must be damp until the moment when a layer of new concrete is applied to it. Immediately before concreting, puddles of water must be removed from horizontal surfaces of old concrete. After this, the concrete surface is covered with a layer of plastic cement mortar with a composition of 1:2, 1-2 mm thick. New concrete must be laid no later than 1.5 hours after laying the mortar.

Exposed reinforcement bars must be thoroughly cleaned with steel brushes, sandblasting, etc. to remove dirt, rust or scale. If the rods of the old reinforcement are significantly damaged by corrosion, the damaged film is removed with a chisel or hammer, after which they are cleaned with a steel brush and welded to the new reinforcement. Before concreting, reinforcement bars are painted cement mortar 1:2 layer thickness 1-2 mm.

Connections of steel reinforcing bars (Fig. 2.1) with damage to the existing reinforcement by corrosion or overstressing of the reinforcement, are eliminated as follows:

Rice. 2.1 Connection of reinforcing bars during welding during reinforcement: a, b - using overlays; c - overlap; 1 - working rod; 2 - butt plate; 3 - weld

The formwork must be designed in such a way that it is possible to gradually increase it along the height of the reinforced beams and columns. When constructing the formwork, the necessary gaps and holes in it must be provided, as well as special trays for laying concrete and its compaction. The laid concrete or applied layer of shotcrete is provided with appropriate temperature and humidity care.

Strengthening compressed elements

Strengthening reinforced concrete columns (Fig. 2.2) with insufficient load-bearing capacity as a result of destruction of concrete and significant corrosion of reinforcement, loss of load-bearing capacity of reinforcement produced by making a reinforced concrete frame or by building up.

Rice. 2.2 Reinforcement of reinforced concrete columns: a - reinforced concrete frame; b - increasing the section; 1 - reinforced column; 2 - additional longitudinal reinforcement reinforcement; 3 - fill with concrete; 4 - additional transverse reinforcement reinforcement in the form of a spiral; 5 - existing longitudinal reinforcement of the column; 6 - connecting rods for welding; 7 - additional transverse reinforcement for strengthening columns

For amplification, according to Fig. 2.2 a, additional working reinforcement (2) and additional transverse reinforcement in the form of a spiral (4) with a diameter of at least 6 mm are installed, while the protective layer is first chipped off to at least the diameter of the working reinforcement. The distance between the spiral turns in the axes is taken to be 50-70 mm. The spiral covers all working reinforcement reinforcement and the existing longitudinal reinforcement of the column. After installing the reinforcement, the column is concreted in formwork or using shotcrete.

When strengthening a column using the method of increasing the section (Fig. 2.2 b), the protective layer is first chopped off by at least 0.5 times the diameter of the reinforcement. Then, through special connecting (6) rods made of reinforcement with a diameter of 10-40 mm and a length of 50 to 200 mm, the existing reinforcement is connected by welding to the reinforcement reinforcement (2). Transverse rods are also welded to the new reinforcement with a pitch of no more than 500 mm and no more than 20 diameters of the longitudinal reinforcement.

After installing the reinforcement, the section is concreted.

Reinforcement of reinforced concrete columns with a steel frame (Fig. 2.3) with a decrease in load-bearing capacity due to various damages carried out using prestressed struts (Fig. 2.3 a), which are included in joint work with the reinforced column, which allows monitoring the degree of their condition. In this case, measures should be taken to ensure the stability of the coating and safety precautions.

Rice. 2.3 Reinforcement of reinforced concrete columns with a steel frame: a - reinforcement using prestressed spacers from corners at the time of manufacture and in finished form; b - strengthening of a part of the column with a steel frame made from corners at the point of destruction; 1 - reinforced column; 2 - corners; 3 - mounting mounting bolts; 4 - connecting strips; 5 - tension mounting bolts; 7 - felled concrete to working reinforcement; 8 - reinforcement embedded part; 9 - short piece welded to the working fittings 10

Spacers are made from corners. Using fastening bolts (3), the corners (2) are installed on the column. They are installed with a bend in the middle of the height, resting the upper and lower ends against sufficiently strong structures (foundations, floor beams). Stops (4) are placed at the ends of the corners.

The spacers are brought into tension by straightening them using tension bolts (5) to a vertical position. The spacers are secured in a tense state with transverse strips. Subsequently, the cutouts in the corners are reinforced with overlays.

Reinforcement of a partially damaged column can be carried out with a steel cage (Fig. 2.3 b) for an incomplete length of the column. The cage is made of corners connected by strips and welded to the embedded parts (8). The embedded parts (8) must be welded to the working reinforcement of the reinforced concrete column.

Additional fastening of vertical steel ties to reinforced concrete columns (Fig. 2.4) in case of insufficient load-bearing capacity of the columns, the need to fasten other structures during reconstruction is made by installing a steel clamp welded from steel plates. The clamp is assembled using coupling bolts, and the connections are fastened to a gusset welded to the clamp.

Rice. 2.4 Additional fastening of vertical steel ties to reinforced concrete columns: 1 - reinforced concrete column; 2 - clamp; 3 - vertical steel connections; 4 - coupling bolts; 5 - gusset for fastening ties

Strengthening foundations by making cages made of concrete or reinforced concrete and increasing the supporting area (Fig. 2.5) in case of insufficient bearing capacity of the foundation, exceeding the design pressure on the foundation and uneven settlement of the foundation produced by building up the existing foundation (7), having previously dug it to the base. In the old foundation, dowels (4) are installed or grooves are cut out to ensure that the old and new concrete work together. Steel ties (6) connect the old foundation to the reinforced concrete frame (2). The height of the dowels is taken based on ensuring the transfer of transverse forces from the frame to the existing foundation.

Rice. 2.5 Strengthening a brick or concrete foundation: a - strip brick foundation; b - separate reinforced concrete foundation; 1 - wall; 2 - reinforced concrete frame; 3 - longitudinal reinforcement; 4 - keys; 5 - crushed stone compacted into the ground; 6 - steel ties; 7 - existing foundation; 8 - new fittings; 9 - new concrete; 10 - cutting surface of the existing foundation; 11 - column

Strengthening bendable elements

To join existing and new reinforcement reinforcement, joint plates are used or lap joints are used. In welded seams the following is accepted: seam thickness 0.25d, seam width - 0.5d. The cross-section of the butt plate must be equal in strength to the joint rod.

Strengthening prefabricated reinforced concrete slabs (Fig. 2.6), which have concrete in the flanges of the ribbed slab destroyed throughout its entire thickness or lack of adhesion of the working reinforcement to the concrete, exposure of the rods of the working reinforcement is performed as follows: if there is a destroyed bottom flange in a hollow-core slab (Fig. 2.6 a), additional reinforcement cages (3) are installed in the hollow channels to strengthen it, followed by embedding these channels with concrete. The number of frames and monolithic hollow channels depends on the degree of damage to the slab and the load on it. When strengthening the ribs of prefabricated slabs, the seams between the slabs are concreted and reinforcement cages are installed in them (6) (Fig. 2.6 b). Reinforcement is also carried out by one-sided extension (Fig. 2.6 c) with the installation of additional reinforcement (7), welded to the existing one through shorts (8) with a diameter of 10-40 mm in increments of 200 to 1000 mm.

Rice. 2.6 Strengthening the ribs of prefabricated slabs: a - by embedding additional frames in hollow channels; b - concreting the seam between the slabs; c - one-sided extension from below; 1 - multi-hollow panel; 2 - a groove made in the shelf along the void channel; 3 - additional reinforcement cage; 4 - monolithic concrete; 5 - reinforced plate; 6 - reinforcement in the seam; 7 - additional fittings; 8 - short ones; 9 - reinforcement of slab ribs

Strengthening the supports of precast reinforced concrete slabs (Fig. 2.7), which have insufficient support area for the precast slabs, is performed by installing additional steel support elements (3).

On intermediate supports (Fig. 2.7 a) metal beams reinforcement elements (3) protrude on both sides of the support and are common to slabs of adjacent spans.

The longitudinal ribs of adjacent slabs rest on a common traverse of additional supporting elements (4).

On the outer supports (Fig. 2.7 b), additional supporting elements protrude to one side and have a large overhang. They are attracted to the slab anchor bolts (6).

Rice. 2.7 Strengthening the supports of prefabricated slabs: a - reinforcement on the middle supports; b - reinforcement on the outer supports; 1 - existing slab; 2 - beam; 3 - metal reinforcement beam; 4 - transverse beam; 5 - anchor corners; 6 - anchor bolts

Strengthening the upper flange of reinforced concrete beams (Fig. 2.8), which has insufficient load-bearing capacity, damage with exposure of the reinforcement of the upper flange, made by building up with reinforced concrete (Fig. 2.8 a, b) and using a steel cage (Fig. 2.8 c).

Additional longitudinal reinforcement of the “extensions” is connected to the existing longitudinal reinforcement of the overhangs using shorts or clamps. Concreting is carried out in formwork with careful compaction of the concrete.

The steel frame consists of two channels (4), covering the sides of the overhangs of the upper shelf, pressed to them with bolts (5). The space between the channels above the upper edge of the shelf is carefully sealed.

Rice. 2.8 Strengthening the upper flange of reinforced concrete beams: a - three-sided extension of the upper flange; b - building up a shelf along the top; c - reinforcement with steel structures; 1 - additional frame; 2 - concrete on small crushed stone; 3 - concrete frame connected to the upper reinforcement of the beam; 4 - channels; 5 - coupling bolts; 6 - hole for a bolt in the beam wall

Strengthening reinforced concrete beams (Fig. 2.9), which have deep and significant damage to reinforced concrete structures with exposure of reinforcement and loss of its adhesion to concrete, is carried out by one-sidedly increasing the section from the side of the stretched zone (Fig. 2.9 a, c).

Additional longitudinal reinforcement (4) is connected to the existing reinforcement of the reinforced beam using connecting elements (2) or shorts.

Connecting elements (2) with a diameter of 10-30 mm are used; shorts (5) with a diameter of 10-40 mm in increments of 200-1000 mm.

After welding work, the surface is prepared and the section being built up is concreted.

Rice. 2.9 Strengthening reinforced concrete beams: a, c - by one-sided extension; d, b - details of options for welding reinforcement fittings; 1 - reinforced beam; 2 - connecting elements; 3 - reinforcement of the reinforced beam; 4 - additional working fittings; 5 - short ones; 6 - concrete reinforcement; 7 - chipped concrete

Reinforcement of a T-beam with steel trusses (Fig. 2.10) when the load-bearing capacity of the beam is reduced due to corrosion of concrete and reinforcement is carried out by installing an anchor device at the ends of the beam, to which the truss bolts are welded with a design weld. Gaskets from two corners are installed on the beam at the tightening level. The tightening tension is carried out using two lanyards.

Rice. 2.10 Strengthening reinforced concrete beams with steel sprengel: 1 - reinforced concrete beam; 2 - steel truss tightening; 3 - insulating gasket

Strengthening the tensile elements of the lattice of reinforced concrete trusses (Fig. 2.11), which have significant damage to the tensile elements of the trusses, reducing their load-bearing capacity, is performed with pre-stressed puffs (2).

The fastening of the tie rods in the nodes can be carried out by welding to gussets secured with bolts and clamps (Fig. 2.11, node A), or by welding to the corners, pulled by anchor bolts to the truss chord (Fig. 2.11, node B).

When tensioning the tightening (2) with nuts, the ends of the tightening with threads are made from short pieces with a diameter exceeding the diameter of the tightening by 4 mm. The connection of the shorts with the tightening must be done by welding, subject to the condition that the joint has equal strength to the base metal of the tightening section. The height of the tension nuts must be at least 1.5 times the thread diameter.

Rice. 2.11 Strengthening the tensile elements of the truss lattice: 1 - compressed chord; 2 - pre-stressed puffs; 3 - elements of anchor devices; 4 - bolt; 5 - anchor bolt

Reinforcement of a reinforced concrete beam of rectangular cross-section with steel trusses (Fig. 2.12) with a decrease in the load-bearing capacity of the beam due to corrosion of concrete and reinforcement is carried out by installing anchor devices at the ends of the beam (in support units), to which the truss bolts are welded with a design weld.

Rice. 2.12 Strengthening a reinforced concrete beam with a steel sprengel: 1 - reinforced element; 2 - steel corners; 3 - metal strips

Previous

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

There are two main ways to strengthen or restore the load-bearing capacity of reinforced concrete beam structures:

strengthening without changing the original design;


strengthening with its change.

The first method is to increase the cross-section of the reinforced element, which is achieved by installing clamps or installing special jackets, 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 is to install additional horizontal or truss tie-downs with pre-tensioning or combined tie-downs, which changes the design design of the structure, but only slightly increases its weight.

When strengthening, monolithic structures that are not prefabricated especially need to create prestress along the lower chord of beams. Prefabricated reinforced concrete elements and monolithic ones (i.e., manufactured directly on the construction site) cannot be equated in terms of load-bearing capacity, since in prefabricated structures, pre-stress along the lower (stretched) chord is created under normal factory conditions with a guarantee of quality. Under construction site conditions, this operation cannot be performed at all.

In terms of the quality of adhesion of concrete to reinforcement, reliability and durability - it is also impossible to put an equal sign between monolithic and prefabricated reinforced concrete structures. Hardening of monolithic structures on a 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 from reinforcement or shaped steel outside the reconstructed facility, installed with minimal labor costs, and immediately put into operation after installation and tensioning or increasing the cross-section without the use of other devices. They increase the initial load-bearing capacity of bending elements by 2-2.5 times, do not disturb the interiors of the premises, and can be hidden suspended ceiling etc., take up little space and slightly increase the cross-section or height of structures.

Methods for strengthening reinforced concrete beams

a, c - concrete coating; b - clamps; g, d. f, g - by embedding and mating on supports

To ensure the joint work of the concrete of the reinforced structure with the reinforced concrete, it is necessary, both during the design and during the execution of work, to pay attention to measures that help increase the adhesion of old concrete to new. In particular, it is recommended to sandblast, cut or wire brush smooth contact surfaces. Immediately before laying new concrete, the surface of the old one should be washed with a jet of water under pressure. In this case, excess water in the form of puddles must be removed, since excessive moisture negatively affects adhesion. When installing reinforced concrete column frames, the surface existing concrete washed with a jet of water under pressure.

	Construction of a reinforced concrete frame Installation of metal corners
Strengthening a monolithic beam with a reinforced concrete frame 1 - reinforced beam; 2 — clip; 3 - plate; 4 - holes in the slab for passing clamps and supplying concrete; 5 — mounting fittings of the cage; 6 — inclined rods of the cage; 7 — working fittings of the cage; 8 — clip clamps
	Installation of additional reinforcement on a polymer solution Installation of external sheet reinforcement using a polymer solution
Strengthening a monolithic beam with a reinforced concrete jacket 1 - reinforced beam; 2 - shirt; 3 - working fittings of the jacket; 4—jacket mounting fittings; 5 - clamps; 6 - notch; 7 - screed

Shirts are more often used when strengthening monolithic beams of ribbed floors. It is recommended to pay special attention to the anchoring of transverse reinforcement at the ends of the cross-section of the jackets. When strengthening columns, the clamps must be welded to the reinforcement of the reinforced column; in case of any difficulties, the column jacket must be designed to bear the entire load. When reinforcing monolithic beams of ribbed floors, clamps are routed through the slab through drilled holes and are anchored using 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 reinforced element).

	Increasing beams from below with a significant increase in their load-bearing capacity
	Increasing beams from below with a slight increase in their load-bearing capacity
Strengthening beams with one-sided extension 1 - reinforced beam; 2 - extension through shortening; 3 - extension using connecting elements; 4 - reinforcement of the reinforced beam; 5 - additional working fittings; 6 - short ones; 7 - connecting elements for welding

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

Building up is used to strengthen any reinforced 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 up, it is not recommended to use reinforcing bars with a diameter of less than 10 mm. When chipping a protective column located in a compressed zone, a temporary reduction in load-bearing capacity should be taken into account.

In some cases, to increase the load-bearing capacity of reinforced elements by building up, it is enough just to increase the amount of main longitudinal reinforcement, for which it is recommended to chop off the protective layer by at least 0.5 of the diameter of the reinforcement and by parallel welding through 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 tensile zone of reinforced elements, the shorts 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, a new protective layer is applied to replace the chipped protective layer - in the form of cement plaster or shotcrete. In these cases, the cross-section of the reinforced element increases slightly, ranging from 20 to 80 mm.

	Connecting unloading racks
	Connection of unloading portal frames
	Construction of a reinforced concrete frame
Reinforcement with additional rigid support a - connected metal stand: 1 - reinforced structure; 2 - separate foundation for additional support; 3 — metal stand; 4 — fastening elements; b - connected metal portal: 1 - reinforced structure; 2 — supplied metal portal; 3 - covering metal clamp; 4 - gaskets; c - metal struts: 1 - reinforced crossbar; 2 — metal struts; 3 - tightening at floor level; 4 — wedge-shaped gaskets; 5 - support corner; 6 — fixing bolts

In case of ruptures of reinforcing bars in bending elements, it is recommended to restore them by welding stressed linings. First, you should support the structure to be reinforced with temporary supports, chop off the protective layer to the required length, weld the reinforcement rods (lining) at one end, heat them 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.

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

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

Reinforcement of the crossbar with additional rigid support a - resting on the underlying floor: 1 - reinforced crossbar; 2 - lining; 3 - short pieces of round reinforcing bars; 4 - double-sided spacer, welded after the half-braces have been spaced; 5 — metal headband; 6 - half-brace; 7 — tightening at floor level; 8 - jack; 9 - welds; b - resting on the column frames: 1 - reinforced crossbar; 2 — clip; 3 - struts; 4 - tightening; 5 - tension coupling; 6 — metal headband; 7 - strips; 8 — gaskets

Since when making rigid supports on independent foundations it is very difficult to completely avoid settlement of the supports, in all cases it is advisable 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 or metal. It is recommended to prepare them in advance.

When making rigid supports in the form of supported racks that have 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 base. One of the ways to pre-compress the soil is to load the foundation with a load no less than the design load before erecting the rack. To reduce the pressure on the ground, it is recommended to install a distribution sand and gravel cushion under the base of the new foundation.

When reinforcing the frame crossbar with additional rigid supports in the form of metal struts, the overhead metal parts in the lower corners must be secured. After placing the struts for a tight fit of the mating structures, ensuring the effectiveness of the reinforcement, it is necessary to make a wedge in the upper node using wedge-shaped spacers. It is possible to install struts supported by metal frames of columns.

	Installation of horizontal reinforcement steel ties
	Installation of additional thermally stressed reinforcement

When reinforcing the crossbars with additional rigid supports in the form of metal or reinforced concrete struts, lifting the reinforced crossbar can be done, for example, with a horizontal jack. To facilitate the movement of the expanding half-braces, it is necessary to place metal pads and shorts made of round reinforcing steel into the gap between the reinforced crossbar and the half-braces. After lifting the reinforced structure to the required amount On both sides of the half-braces, spacers made of profile metal, for example from channels, are welded, 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 tie.

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 tie using a tension coupling. To install the braced system at the bottom of the column, it is necessary to first arrange a cage. After installing and tightening the struts, they are secured in the lower part by welding metal strips to the struts. Reinforcement with rigid additional supports of this type reduces the dimensions of production premises to a lesser extent.

When reinforcing the frame crossbar with additional rigid supports in the form of metal struts, the overhead metal parts in the lower corners must be secured. After placing the struts for a tight fit of the mating structures, ensuring the effectiveness of the reinforcement, it is necessary to make a wedge in the upper node using wedge-shaped spacers. It is possible to install struts supported by metal frames of columns.

In cases where the reinforced structure cannot be preliminarily unloaded, the installation of additional rigid supports must necessarily be accompanied by preliminary lifting of the reinforced structure. 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 unloading beams on consoles
	Connecting unloading beams to clamps
Reinforcement with additional elastic support (metal beam) a - on hangers - coupling bolts: 1 - reinforced beam; 2 - reinforcing beam; 3 - coupling bolt; 4 - support corner; 5 - gasket; b - on brackets; 1 - reinforced beam; 2 - reinforcing beam; 3 - metal frame 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, lifting the reinforced crossbar can be done, for example, with a horizontal jack. To facilitate the movement of the expanding half-braces, it is necessary to place metal pads and shorts made of round reinforcing steel into the gap between the reinforced crossbar and the half-braces. After lifting the reinforced structure to the required amount, spacers made of 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 tie.

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 tie using a tension coupling. To install the braced system at the bottom of the column, it is necessary to first arrange a cage. After installing and tightening the struts, they are secured in the lower part by welding metal strips to the struts. Reinforcement with rigid additional supports of this type reduces the dimensions of production premises to a lesser extent.

	Suspension to unloading beams
	Installing horizontal corner ties
Reinforcement of bending elements with additional elastic supports a - metal beams on suspensions; 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 strengthen 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 taking the load through spacers located in the span between the reinforcing and reinforced elements.

The inclusion of structures of additional elastic supports in the work can be done by pulling the supporting ends of the elastic supports to the reinforced element during installation or by using wedging pads. Instead of wedging gaskets, spacer bolts can be installed.

When reinforcing bending elements multi-storey buildings elastic additional supports can be created with metal cords. The reactive unloading force is created by prestressing the strands, 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.

To enhance mainly prefabricated covering beams For large spans and trusses under load, prestressed hinge-bar chains may be recommended. The use of hinge-rod chains makes it possible to create a load of opposite sign in the form of a series of concentrated loads, the location and magnitude of which are planned in advance depending on the outlines of the chains. The amplification effect (creation of reactive forces of given magnitudes) is achieved by tensioning a statically definable chain.

Installation of unloading brackets	Installation of combined reinforcement steel ties
Installation of channel ties	Construction of a reinforced concrete jacket
Reinforced concrete extension device	Installation of tie clamps at supports
Installation of tie-down cross bars at supports	Installation of inclined rods at supports

The main elements when strengthening using this method are: the hinge-rod chain itself, consisting of two identical branches on both sides of the reinforced beam (corners with trimmed vertical flanges at the bend points, reinforcing bars up to 30...36 mm in diameter or ropes); anchor devices in the form of welded sheet metal plates in the upper zone of the beams above the supports; pendants, usually made of round steel, or racks made of profile metal in places where chain branches bend. Reinforcing bars are accepted from steel classes A-I, A-II, A-III, metal constructions— from steels VSt3sp, VSt3ps, VSt3kp. Welded connections must be made with special care.

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

It is recommended to make all elements of the chain in advance in accordance with the dimensions of the reinforced beam, carefully tested under natural conditions. The circuit elements should be installed in a certain sequence. Both branches of the chain with pre-attached pendants with screw threads at the ends and connecting strips are suspended from the anchor devices fixed to the beam. If, in addition to pendants, racks are also required, then they are installed, leaving free space for the central suspension (rack).

When the nuts are tightened, all connecting bars of the suspensions 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 installed in the designed position in accordance with the catenary line. When attaching chain branches to bolted anchor devices, 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 angles of inclination 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 (reaction forces) in all suspensions and racks will be approximately the same value, and the main tension can be produced at the location of the central suspension or rack. The magnitude of the effort is preliminarily determined theoretically.

For the main tension of the chain branches at the location of the central suspension or rack, various ways. In cases where the chain is located above the bottom of the reinforced beam, i.e. installation of a suspension is required; tension can be carried out by tightening the nuts with a torque wrench using a jack with a pressure gauge resting on the bottom of the beam, and in other ways. The pressure gauge readings make it possible to accurately determine the magnitude 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 stand. This method also provides sufficient control when tensioning.

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

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

Strengthening the prefabricated roof beam a - hinged-rod chain: 1 - reinforced beam; 2 - hinge-rod chain; 3 - anchor device; 4 - tension bolt; 5 — support lining; 6 — connecting support channel; 7 — stiffener;	g - girth systems: I - extreme spans - truss; II - medium spans - unloading brackets; 1 - reinforced beam; 2 - support; 3 - truss; 4 — stops; 5 - corner; 6 — connecting coupler;
b - prestressed unloading brackets: 1 - reinforced beam; 2 — corners of the lower belt of the bracket; 3 — bracket rods; 4 - column; 5 - tension bolts; 6 - connections along the lower belt; 7 — headband; 8 - coating slabs; 9 — distribution gasket; 10 - support sheet; c - outriggers: 1 - reinforced beam; 2 - support; 3 - double-cantilever metal beams; 4 — outrigger table; 5 — connecting rod;	d - sprengel: 1 - reinforced beam; 2 - truss; 3 - anchor device; 4 - support channel; 5 — lining with a ball socket; 6 — support sheet; 7 - gasket made of round rods; 8 — square lining; 9 - nut welded into the support sheet; 10 - tension screw; 11 — package of metal gaskets

The height of the brackets is assumed to be equal to the height of the supporting part of the reinforced beams. It is recommended to take the lengths of the cantilever parts of the brackets equal to 1/4 - 1/6 of the span of the reinforced beams. If the length of the cantilever parts is short, you can completely abandon the internal elements of the lattice.

The parts of the bracket are branches of the brackets, supporting elements (support sheet or saddle-shaped pads), connecting elements in the form of segments of corners or round rods, thrust devices attached under the bottom of the reinforced beam to the ends of the branches of the bracket. These devices serve to create supports for reinforced beams and can have different designs 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 tensioning a calibrated load with a suspension, the rigid element of the thrust device is welded to the branches of the bracket, for which holes are provided in it or hinges are welded. After tensioning, locking spacers are tightly placed in the gap between the bottom of the beam and the plate of the thrust device, and the weights are removed. Voltage losses are eliminated by suspending a load 10...15% greater 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 spacers are also placed. Tension control is carried out using the jack pressure gauge. Since this system is statically definable, freely rotating on the middle support, tension can only be applied to one end of the bracket. 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 anchorage of the longitudinal working reinforcement, the offset of the support or bracket from the support sheet of the beam must be at least 40 diameters for bar reinforcement of a periodic profile and at least 80 diameters for reinforcement made of high-strength wire.

When strengthening roof beams of multi-span buildings, it is recommended to simultaneously apply various design solutions for the outer and middle spans. For extreme spans, a prestressed truss can be used; for middle spans, prestressed unloading brackets can be used.

Methods for creating prestress in ties of reinforced concrete structures
Installation of heated tightening on stops	Welding heated tightening to bare reinforcement
Installation of hydraulic jacks between the structure and the tightening	Installation of hydraulic jacks under tightening
Tightening the couplings	Tightening the bolts
Tightening the nuts	Wedging with plates
Tightening the clamps	Bolt tension

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

Additional reinforcement can be either horizontal or trussed. It is also possible to install horizontal and trussed reinforcement simultaneously. As a result of installing additional reinforcement with its preliminary tension, the stress-strain state of the reinforced beams changes. Prestressing includes additional reinforcement in joint work with the strengthened beam, which can be considered as a bending structure with an increased area of ​​reinforcement, the additional part of which has no adhesion to the concrete, and with a changing working height.

Strengthening secondary beams with prestressed relief brackets 1 - reinforced beam; 2 — corners of the lower belt of the bracket; 3 — bracket rods; 4 — connecting strips; 5 - gasket installed after the bracket is tensioned; 6 — pendants; 7 - junction of strands; 8 - lining; 9 — support pads; 10 - holes sealed with asphalt; 11 — nabetonka	1 - reinforced beam; 2 - beam reinforcement; 3 - additional prestressed reinforcement; 4 - shorty

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

Tensioning of additional reinforcement can be done mechanically, electrothermal or electrothermo-mechanically.

At mechanically The tension of the prestressed reinforcement is carried out using jacks, torque wrenches, tension bolts, tie clamps that attract the strands to each other, as well as special reinforcing devices of the truss or lever type.

One of the ways to secure 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 in small areas in the support zones, i.e. where the stresses in the reinforcement of the reinforced beam are insignificant. It is necessary to weld short rods to the exposed working reinforcement, the diameter of which is slightly larger than the thickness of the protective layer, and the transverse reinforcement rods or clamps should not be damaged. Reinforcing reinforcing bars are welded to the short ones. In this case, tension is produced thermally.

Strengthening a beam with additional prestressed reinforcement a - horizontal tightening: 1 - reinforced beam; 2 — horizontal tightening; 3 - anchor corner; 4 - vertical anchor rods; 5 - tension bolt; 6 — washer; 7 - hole to be sealed after installing the anchor; b - intermediate struts: 1 - reinforced beam; 2 - puffs; 3 — intermediate struts; 4 - tension bolt; 5 - anchor device; c - trussed tightening: 1 - reinforced beam; 2 — truss puffs; 3 - lining; 4 - shorty; 5 - tension bolt; 6 — washer; 7 - anchor channel; 8 - hole to be sealed after installing the anchor; d - combined puffs: 1 - reinforced beam; 2 — horizontal tightening; 3 — truss puffs; 4 - lining; 5 - short ones; 6 — corner of the anchor for horizontal tightening; 7 - vertical anchor rods; 8 — tension bolt; 9 — washer; 10 — channel of the truss anchor; 11 - hole to be sealed after installing the anchor

Reinforcement rods are installed in the design position using temporary hangers, the number of which should be set so 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 short end, and the other remains free. The rod is connected to the electrical circuit and heated to the design temperature. The free end is pressed against the short end and welded. During the welding process until the seam has completely cooled, it is necessary to maintain a constant design temperature. To prevent buckling of the longitudinal reinforcement in places where additional stressed reinforcement is welded, it is advisable to place short bars next to one of the clamps on the side of the span.

Gaskets, linings, and other parts when strengthening with truss reinforcement and its tension must be installed in the places where the rods are bent between the lower edge of the reinforced beam and the truss rods. The designs of these elements depend on the method of tensioning the rods, the distance between the bottom edge of the reinforced beam and the reinforcement rods, and the width of the reinforced beam.

Tightenings can be combined into a separate group of reinforcement with additional reinforcement and recommended for use, tensioned by mutually tightening two or four rods with special tightening bolts. The coupling bolts should look like a clamp 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 effort, 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 tightening bolt or two, with or without intermediate spacers. Vertical rods of tie-down anchors are passed through holes drilled in the ceiling for anchoring.

The stretched zone is strengthened increasing the cross-sectional area of ​​the working reinforcement reinforced structure by installing additional reinforcement in this area ensuring its collaboration with the structure. The joint operation of additional reinforcement with the reinforced structure is ensured by:

welding to existing reinforcement ;

gluing the stretched area to concrete .

Ensuring the joint operation of additional reinforcement by welding to existing reinforcement

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

Rice. 8.2. Strengthening the tensile zone of structures by welding additional reinforcement: A– lap joint; b– by means of shorts from the side of the stretched zone; V– by means of shorts from the side of the protective layer; G– using staples

Welding additional reinforcement to existing prestressed reinforcement, as well as non-prestressed reinforcement of the structure being strengthened that is not extended beyond the edge of the support to the required length, is not allowed.

The protective layer of concrete in places where additional reinforcement, shorts or staples are welded is knocked off by at least half the diameter of the existing reinforcement. Existing fittings in welding areas must be cleaned of rust, dust and other contaminants to bare metal.

As additional working reinforcement, periodic or smooth bar reinforcement is used, as well as rolled profiles.

The shorts and sections for connecting brackets from rod reinforcement are taken with a length of 50...200 mm and are placed “staggered” along the length of the structure with a distance between them along the rods of at least 20, where  is the larger diameter of the welded rods.

In order to reduce stress concentration, metal embrittlement 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 smoothed out with an abrasive wheel along the shaft. When strengthening a 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 structure being strengthened, unloaded during reinforcement work, may be performed in one pass.

Welding additional reinforcement to the existing reinforcement of the structure being strengthened without first unloading it 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 cross-section of the welded rod of the reinforced structure.

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

,Where – required prestress, – length of the rod between the inner ends of the welds; – modulus of elasticity of 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 tensioning the reinforcement. Heating temperature should not exceed 400 WITH.

At mechanical In the prestressing method, a tension device in the form of a bolt with a nut is welded to an additional rod, welded at one end to the existing reinforcement, at the opposite end, and a stop in the form of a piece of pipe with an internal diameter slightly larger than the diameter of the bolt is welded to the existing reinforcement. After securing the ends, additional reinforcement is welded to the existing one along its length. After tensioning the additional reinforcement, the tensioning device is cut off and reused. To create pre-tension, it is possible to use a turnbuckle included in the prestressing rod.

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

The amount of prestressing of additional reinforcement is taken within the limits

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

Prestress losses in additional reinforcement are determined by , as for structures with reinforcement tensioned on concrete.

Ensuring the joint operation of additional reinforcement by gluing the tensile zone to the concrete

While ensuring the joint operation of additional reinforcement and the reinforced structure gluing Using 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 service reinforcement can be placed in precast concrete reinforcement elements bonded to the tension zone of the structure. In the case of exposure to aggressive environments, taking into account the high protective properties of polymer solutions, it is advisable to simultaneously apply coatings to the surface of the reinforced structure. Steel sheets are protected with fire retardant and anti-corrosion compounds. Additional reinforcement in the tension zone is installed along the entire length of the structure or to the design length in accordance with the diagram of internal forces.

R
is. 8.3. Strengthening the tensile zone of a 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 – rod reinforcement; 10 – coating from a polymer solution; 11 – prefabricated reinforced concrete element; 12 – fiberglass; 13 – thin sheet with stampings; 14 – anchor plate

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

When strengthening a tensile 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 tensile zone of the reinforced structure, rod reinforcement, reinforcing bars are used.

ropes, sheet metal 3...20 mm thick, rolled profiles in the form of channels, angles, as well as non-metallic reinforcement based on glass, basalt, carbon and other fibers.

Work to strengthen the tensile zone of structures by gluing additional reinforcement or precast reinforced concrete elements with additional reinforcement is carried out in the following sequence. Prepare the bonded surfaces of the reinforcement elements and the reinforced structure. Steel sheets are cleaned from rust and scale from the inside and degreased with acetone. The bonded concrete surfaces of the reinforced structure and the precast reinforced concrete element must be free of protrusions, chipped ribs, grease stains, dirt and dust. Surfaces previously exposed to aggressive environments are washed with clean water and dried. If the aggressive environment was acidic, then after washing the surfaces are neutralized with alkaline compounds and washed and dried again. For large volumes of work, surfaces are subjected to sandblasting and dust removal using hair brushes and blowing with compressed air, free of oil and moisture. The cracks are injected. Grooves for placement of rod reinforcement are cut using diamond and carbide mechanized tools. Then the reinforcement elements are installed in the design position and fixed using temporary fasteners (supports, clamps, clamps, etc.).

The polymer solution for embedding rod reinforcement in grooves and anti-corrosion coating of the surface is applied manually, by pouring or spraying. The polymer solution in the grooves between sheet reinforcement or reinforced concrete prefabricated element is injected through a fitting screwed into the hole of the reinforcement element. In this case, the gaps around the perimeter of the seam are pre-sealed with a polymer solution of the same composition with the addition of 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 lifted to the design position and pulled to the structure using temporary mounting clamps. Excess polymer solution is squeezed into the gaps between the side edges of the reinforced structure and the profile flanges.

When strengthening prefabricated hollow-core 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 into the voids through holes punched from the upper or lower edges of the slab, and the voids are filled with concrete using concrete pumps (Fig. 8.4).

Rice. 8.4. Strengthening hollow-core floor panels by installing additional reinforcement: 1 – slab; 2 – welded frame; 3 – concrete

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

R
is. 8.5. Strengthening the tensile zone of hollow-core panels by installing additional reinforcement: A– when creating cracks on top of the slab; b– when making slots at the bottom of the slab, 1 – reinforced slab, 2 – slot, 3 – additional reinforcement, 4 – retainer, 5 – limit plate, 6 – pipe, 7 – polymer solution

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

In the support zones of reinforced prefabricated hollow-core panels, slots are made, 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 installed and the supporting void zones are concreted. After the concrete gains strength, the reinforcement is strained with tension bolts, which are mounted through holes on the side of the bottom edge. In this case, formwork is installed under the holes from the bottom edge. Then the remaining void space is filled concrete mixture, after which the formwork is removed and the protruding ends of the tension bolts are cut off (Fig. 8.6).

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

Additional reinforcement to strengthen the tension zone of precast panels can be installed in the expanded joint between the slabs, followed by concreting. In this case, the joint operation of additional reinforcement with the reinforced panels must be ensured by installing notches, dowels on the side edges of adjacent slabs, as well as the use of polymer solutions with high adhesive properties.

Prefabricated reinforced concrete reinforcement elements (conventional and prestressed) must be designed for loads acting during manufacture, transportation and installation in accordance with. The concrete class of reinforcement elements must be no lower than the actual strength of the concrete of the reinforced structure. The thickness of a precast reinforced concrete element with additional reinforcement is taken to be at least 50 mm. The number of prefabricated reinforced concrete elements placed along the cross-sectional width of the reinforced structure can be one or more.

Nikolai Mikhailovich Onufriev
Doctor of Engineering science professor
Strengthening reinforced concrete structures of industrial buildings and structures
Publishing house of literature on construction
Leningrad 1965 Moscow

The book examines various practical ways reinforcement of reinforced concrete structures, performed as during reconstruction industrial facilities, in the process of their major re-equipment, and during the modernization of production facilities carried out during the operational period without stopping the enterprises.
At the same time, methods for calculating the reinforcement of reinforced concrete structures, including statically indeterminate systems taking into account the redistribution of forces, are given, calculation formulas and tables are given to simplify the design of such reinforcements along with calculation examples, and numerous reinforcements of industrial structures produced in kind are also illustrated.
The experimental studies carried out are briefly summarized and recommendations for strengthening structures, developed on the basis of long-term experience of such reconstruction work, are given.
The book is intended for practical guide engineers and technicians, during design, etc. performing work to strengthen and partially correct reinforced concrete structures.
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Accepted notations 3
Introduction 5
Chapter I. Methods of strengthening reinforced concrete structures 7
§ 1. Contribution of domestic science and technology to solving issues related to strengthening reinforced concrete structures 7
§ 2. Varieties of methods for strengthening reinforced concrete structures 9
Chapter II. Unloading and replacement structures 12
§ 3. Unloading and replacement structures during reconstruction of industrial facilities 12
§ 4. Features of the calculation of unloading structures 21
§ 5. Examples of implemented reinforcements using unloading structures 33
Chapter III. Designs of clips, shirts and extensions 37
§ 6. Reinforcement through structures in the form of clips, shirts and extensions (concrete) 37
§ 7. Features of designing reinforcements for elements of reinforced concrete structures using clips, jackets and extensions 50
§ 8. Examples of reinforced structures with clips, jackets and extensions (betons) 61
Chapter IV. Additional rigid supports 68
§ 9. Strengthening bending elements using additional rigid supports 68
§ 10. Features of the design and calculation of additional rigid supports 76
§ 11. Examples of implemented reinforcements using additional supplied rigid supports 83
Chapter V. Additional elastic supports 89
§ 12. Strengthening bending elements using intermediate elastic supports 89
§ 13. Calculation formulas for the design of elastic-supporting reinforcement structures 89
§ 14. Features of the design and calculation of elastic-supporting reinforcement structures 110
§ 15. Examples of reinforcements performed using elastic-supporting structures 128
Chapter VI. Prestressed reinforcement tightening 132
§ 16. Strengthening bending elements using prestressed horizontal, truss and combined ties 132
§ 17. Calculation formulas and tables for the design of prestressed horizontal, truss and combined reinforcement tightening 148
§ 18. Features of the design and calculation of prestressed reinforcement tightening various types 185
§ 19. Examples of completed reinforcement structures using various types of prestressing ties 203
Chapter VII. Strengthening statically indeterminate structures with strings 215
§ 20. Methods for designing reinforcements using prestressed ties for bending statically indeterminate reinforced concrete structures 215
§ 21. Examples of designing ties when reinforcing statically indeterminate structures 218
Chapter VIII. Prestressed reinforcement struts 241
§ 22. Strengthening columns using prestressed reinforcement struts 241
§ 23. Features of the design and calculation of prestressed struts for strengthening columns 247
§ 24. Examples of reinforced columns with prestressed struts 263
Chapter IX. Some strengthening techniques in special cases of reconstruction 270
§ 25. Methods of strengthening various cantilever structures 270
§ 26. Strengthening bendable structures to absorb lateral forces 279
§ 27. Examples of reinforcements of bending elements for transverse force 283
§ 28. Strengthening foundations using prestressed reinforcement and preliminary compression of the soil 285
§ 29. Increasing the rigidity of structures in order to strengthen them 289
§ 30. Strengthening long-span structures for crack resistance 293
Chapter X. Experimental research 296
§ 31. Experimental studies of reinforcements through shirts and extensions 296
§ 32. Experimental studies of reinforcements through prestressing tightening on statically determinate and indeterminate structures in laboratory and production conditions 300
§ 33. Experimental studies of prestressed reinforcement struts 311
§ 34. Experimental studies of reinforcement using prestressed transverse rods 315
Chapter XI. Technical and economic issues and recommendations for strengthening structures 320
§ 35. Some technical and economic indicators of the effectiveness of reinforcements with elastic support systems and prestressed structures of tie rods and struts 320
§ 36. Recommendations for strengthening elements of reinforced concrete structures 325
Appendix 339
Literature 340