Immediately before concreting the structure, the following necessary verification and preparatory work is performed. Check (leveling) all the main marks of the formwork, the correctness of the geometric dimensions of the formwork in plan and height, as well as the verticality of its elements. They check the reliability of the foundations under the supports and posts, the strength and correct installation of all parts of the formwork. The formwork is cleaned of debris and dirt, especially in the lower parts of the column boxes; all cleaning holes of the formwork are washed. The compliance with the drawings and the correctness of the reinforcement installation, the placement of joints and clamps, the density of their coverage of the working reinforcement, the strength of the binding and the presence of gaskets and concrete linings, ensuring the distances between the rods and the thickness of the concrete cover, are checked.

After checking all the elements and cleaning, the formwork is thoroughly poured with water from fire hoses, so that during concreting the wood does not suck water from the concrete mass. The gaps between the individual formwork boards, if they did not come together after soaking, are sealed with slats or bored with tow.

Before placing concrete in the foundations on the ground, prepare from a layer of crushed stone or gravel, carefully compacting it. On the section of the structure planned for concreting, roll tracks are installed. All transport devices, railways, lifting mechanisms and vibrators and other equipment must be in full working order, and their condition must be checked before each shift. Duty carpenters should be appointed to repair and relocate trolleys, troughs and other ancillary devices, and duty workers to clean and maintain the tracks. It should be especially noted that one of the main conditions for achieving large workings was the good condition of all equipment and tracks, as a result of which any delays in the supply of concrete to the place of laying were excluded.

Organizational preparation is of no less importance. establishing the necessary front of work, determining the number of workers in individual units of the brigade with the division of labor in accordance with the qualifications of the workers and with the full workload of each worker, the timely issuance of orders and the distribution of workers by jobs. The task for each day is explained to the brigade and together with it the order of its implementation is worked out.

By general rules Concrete paving for all types of structures requires that rigid and ductile concrete be poured using vibrators. Manual compaction of concrete is allowed only for small volumes of work and with very plastic concrete, used for concreting thin walls with dense reinforcement.

Before laying concrete mix a set of operations for the preparation of formwork, reinforcement, surfaces of previously laid concrete and the base are performed into the structure.

The formwork and supporting scaffolds are carefully inspected, checked for the reliability of the installation of racks, scaffolds and wedges under them, fasteners, as well as the absence of cracks in the formwork, the presence of embedded parts and plugs provided for by the project. The formwork is cleaned of debris and dirt.

Before placing the concrete mixture, the installed reinforcement structures are checked. They control the location, diameter, number of reinforcing bars, as well as the distance between them, the presence of dressings and welded tacks at the intersection of the bars. The distances between the rods must correspond to the design ones.

The design arrangement of reinforcing bars and meshes is ensured correct installation supporting devices: templates, clamps, stands, spacers and pads. It is forbidden to use linings made of rebar scraps, wooden blocks and crushed stone. Welded joints, knots and seams made during the installation of fittings are inspected from the outside. In addition, several samples of reinforcement cut from the structure are tested. The places of cutting and the number of samples are established in agreement with the representative of the technical supervision.

The distance from the reinforcement to the nearest formwork surface is checked by the thickness of the concrete cover indicated in the drawings of the concreted structure.

For reliable adhesion of the freshly laid concrete mixture to the reinforcement, the latter is cleaned of dirt, peeling rust and adhering pieces of mortar using a sandblaster or wire brushes.

For permanently joining previously set hardened concrete monolithic structures and prefabricated elements of precast-monolithic structures with new concrete horizontal surfaces of the hardened monolithic concrete and precast elements before placing the concrete mixture are cleaned of debris, dirt and cement film.

Before placing the concrete mixture on the ground, prepare the base. Vegetable, peat and other soils of organic origin are removed from it, dry loose soil is moistened. The bridges are filled with sand and compacted.

The readiness of the base for laying the concrete mixture is formalized by an act.

Methods for placing concrete mix

The laying of the concrete mixture should be carried out in such a way that the solidity of the concrete masonry, the design physical and mechanical parameters and the homogeneity of the concrete, its proper adhesion to the reinforcement and embedded parts and the complete (without any voids) filling the formwork space of the structure under construction with concrete are ensured.

The concrete mixture is laid by three methods: with compaction, casting (concrete mixtures with superplasticizers) and pressure laying. With each installation method, the basic rule must be observed - a new portion of the concrete mixture must be laid before the cement begins to set in the previously laid layer. This eliminates the need for the construction of working seams (see below) along the height of the structure.

As a rule, laying in structures that are small in terms of design (thin-walled, columns, walls, beams, etc.) is carried out immediately to the full height without interruption to exclude working seams.

Large structures (for example, massive foundation slabs) the concrete mix is ​​laid in horizontal layers and, as a rule, over the entire area. For multi-layer installation to ensure solidity concrete paving throughout the entire thickness of the structure, it is necessary to comply with the condition

where h is the thickness of the layer to be laid, m; Q is the intensity of concrete mix delivery, m3 / h; t is the maximum allowable time to overlap a layer of previously laid concrete, h; A is the area of ​​the structure to be concreted, m2.

When placing a concrete mixture with compaction, the calculated layer thickness must correspond (but not exceed) the depth of development established by the norms of the technical means of compaction used in these specific conditions.

In large areas it is sometimes impossible to cover the previous layer of concrete before the cement starts to set in it. In this case, a stepped laying method is used with the simultaneous laying of two or three layers. When laying in steps, there is no need to overlap the layers over the entire area of ​​the array. For the convenience of work, the length of the "step" is taken at least 3 m.

Concrete compaction

During preparation, transportation and laying, the concrete mixture is most often in a loose state; aggregate particles are loosely located and there is a free space between them, filled with air.

The purpose of the compaction process is to ensure high density and homogeneity of the concrete.

The main and most common method of compaction in monolithic masonry is vibration, based on the use of some properties of the concrete mixture.

A concrete mixture is a plastic-viscous body, which occupies, as it were, an intermediate position between solids and true fluids. The concrete mix resists shear, that is, it has a certain structural strength.

The concrete mixture belongs to the class of thixotropic systems, on which vibration compaction is based. Vibration reduces the adhesion force between the grains of the concrete mix. In this case, the concrete mixture loses its structural strength and acquires the properties of a viscous heavy liquid. The liquefaction process is reversible. At the end of vibration, the strength of the structure of the concrete mix is ​​restored.

Under the influence of vibration, the aggregate particles come into vibrational motion, the concrete mixture, as it were, liquefies, acquires increased fluidity and mobility. As a result, it is better distributed in the formwork and fills it, including the space between the reinforcing bars.

The concrete mix is ​​vibrated using internal (depth), surface and external vibrators. The working part of the internal vibrators, immersed in the concrete mixture, transmits vibrations to it through the body. Surface vibrators, mounted on the concrete mix to be compacted, transmit vibrations to it through the work platform. External vibrators, fixed on the formwork using a vise or other gripper, transmit vibrations to the concrete mix through the formwork.

The scope of application of various types of vibrators depends on the size and shape of the structure to be concreted, the degree of its reinforcement and the required intensity of concreting. Internal vibrators of the mace type are used to compact the concrete mixture laid in massive structures with varying degrees of reinforcement, and internal vibrators with a flexible shaft - in of various types densely reinforced structures.

Surface vibrators are used to compact only the upper layers of concrete and are used when concreting thin slabs and floors. External vibrators compact the concrete mixture in densely reinforced thin-walled structures.

Each type of vibrators has its own effective zone of compaction of the concrete mixture, characterized by the radius of action for the internal and external vibrators, and the thickness of the worked layer for the surface vibrators. So, depending on the power of the vibrator and the value of the generated amplitudes and frequency of oscillations, the radius of action of the internal vibrators is 15 ... 60 cm, external - 20. ... 40 cm, and the depth of the surface vibrators is 10 ... 30 cm.

The vibration method of compaction is most effective for moderately plastic concrete mixes with a mobility of 6 ... 8 cm. When more mobile mixes vibrate, delamination is observed.

The quality of the construction largely depends on the right choice optimal mode of vibration of concrete mix. If the vibration duration is insufficient, the concrete mix may be loosely laid, and if it is too long, it may delaminate. The duration of vibration in one position depends on the mobility of the concrete mix and the type of vibrator. Compaction of concrete mixtures with surface vibrators is carried out within 20 ... 60 s, deep - 20 ... 40 s, external - 50 ... 90 s. The duration of vibration of hard concrete mixes should not be less than the stiffness index of this mix. Visually, the duration of vibration can be set according to the following signs: the cessation of subsidence, the acquisition of a uniform appearance, the horizontal surface and the appearance of the cement milk mixture on the surface.

As each layer of concrete mix is ​​laid, the vibrator is rearranged from one position to another. The distance between the positions of the internal vibrators should not exceed one and a half radius of their action. When compaction of the layer to be laid, the inner vibrator is immersed 5 ... 8 cm into the underlying layer in order to work out the joint between the layers and ensure the solidity of the concrete. When relocating the surface vibrator, it is necessary that its working platform overlaps the adjacent vibrated area by at least 10 cm.

Sealing by bayonetting is carried out manually with the help of skimmers. Due to its laboriousness and low productivity, the method is used in exceptional cases when concreting thin-walled and densely reinforced structures, as well as when using highly mobile (with a cone draft of more than 10 cm) and cast mixtures in order to avoid their stratification during vibration.

Compaction by ramming is carried out by manual and pneumatic rammers when laying very rigid concrete mixtures in low-reinforced structures, as well as in cases where it is impossible to use vibrators due to the negative impact of vibration on the equipment located nearby. The mixtures are compacted in layers 10 ... 15 cm thick.

Construction of working seams

To ensure solidity, it is desirable to concrete the structure continuously. But this is possible only with insignificant volumes of work and in a relatively simple constructions... In all other cases, concreting interruptions are inevitable. If it is necessary to arrange breaks in the concreting of structures, they resort to the so-called working seams.

A working seam is the plane of the joint between hardened and new (freshly laid) concrete, formed due to a break in concreting. A working joint is formed when the subsequent layers of concrete mix are placed on the completely hardened previous ones. This usually happens during breaks in concreting from 7 hours.

Construction joints are a weak point, so they should be arranged in sections where the joints of old and new concrete cannot negatively affect the strength of the structure. In the columns, working seams are allowed at the level of the top of the foundation, at the bottom of girders, beams or crane consoles, at the bottom of the column caps of non-girder floors; in frame structures - at the top of the haunch between the uprights and the crossbars of the frames. In beams, working seams are allowed within the middle part of the span. When concreting ribbed floors, one should be guided by the following: if concreting proceeds in a direction parallel to the secondary beams, the working seam is allowed within the middle third of the span of the beams; when concreting in a direction parallel to the main beams (girders) - within two middle quarters of the span of beams and slabs. In bezel-less ceilings, working seams are made in the middle of the slab span. Working seams in beams and slabs are formed in the form of a vertical cut.

The interrupted concreting can be resumed after the setting process is completed in the previously laid concrete mixture and the concrete acquires a strength of at least 1.5 MPa (it is able to perceive a slight dynamic effect without destruction).

The surface of the working seam should be perpendicular to the axis of the element, and in walls and slabs - their surface. To do this, install shields - limiters with slots for reinforcing bars, attaching them to the formwork shields.

For reliable adhesion of concrete in the working joint, the surface of the previously laid concrete is carefully processed: the edge of the set concrete is cleaned of the cement film and the coarse aggregate is exposed, wiping with wire brushes; blown with compressed air and rinsed with a stream of water. The concrete surface around the reinforcement outlets is especially carefully treated; the reinforcing bars are cleaned of the solution. The cleaned joint surface is covered with cement mortar having the same composition as the concrete mixture to be laid.

Laying concrete mix in various structures.

Technological methods for laying a concrete mixture are prescribed depending on the types of structures and the requirements for them, the composition of the concrete mixture used, the design features of the formwork, the methods of supplying the mixture to the places of laying. Taking these factors into account, practice has developed effective methods laying concrete mix, which are set out below for various types of the most massive structures.

In foundations and arrays, depending on the volume, depth, height and other features, the concrete mixture is laid according to the following technological schemes: with unloading the mixture from a transport device directly into the formwork from a movable bridge or overpass, using vibrating feeders and vibrating chutes, concrete pavers, concrete pumps, buckets using cranes.

When laying in low-reinforced foundations and arrays, rigid concrete mixtures are used with a cone draft of 1 ... 3 cm, in densely reinforced ones - with a cone draft of 4 ... 6 cm.

In stepped foundations with a total height of up to 3 m and an area of ​​the lower step up to 6 m2, the mixture is fed through the upper edge of the formwork, providing measures against the displacement of anchor bolts and embedded parts. During vibration compaction, the internal vibrators are immersed in the mixture through the open fans of the lower step and rearranged along the perimeter of the step towards the center of the foundation. Similarly, the concrete of the second and third stages is vibrated, after which they are smoothed. The concrete mix can be poured into the pylons immediately after the end of the paving in the steps. The mixture is fed into the pylon through the top of the formwork. Seals it with internal vibrators, lowering them from above.

When the height of the stepped foundations is more than 3 m and the area of ​​the lower step is more than 6 m2, the first portions of the concrete mix enter the lower step along the perimeter. Subsequently, the mixture is fed through the receiving hopper and link trunks. Vibration compaction of the mixture is carried out, as in the previous case, with internal vibrators.

In high pylons, the concrete mixture with a mobility of 4 ... 6 cm must be fed slowly and even with some interruptions (1 ... 1.5 hours) in order to exclude the squeezing out of the concrete laid in the steps through their upper open fans.

In massive foundations, perceiving dynamic loads (for example, under rolling, forging and pressing equipment), the concrete mixture is laid continuously. Their volume reaches 2.5 ... 3.0 thousand m3. The concrete mixture is supplied to them from overpasses, conveyors, concrete pumps or combined methods at a rate of up to 300 ... 350 m3 per shift. The mixture is supplied to hard-to-reach places of the massif and it is distributed over the area of ​​the foundation using vibrating chutes.

The concrete mixture is laid in massive foundations with dense reinforcement in horizontal layers 0.3 ... 0.4 m thick, compacting it with manual internal vibrators.

For device concrete preparations under the floors, a concrete mixture is used with a draft of a cone of 0 ... 2 cm. The area on which it is planned to arrange preparation is divided into strip cards 3 ... 4 m wide, installing lighthouse boards along their edges. Stripes-cards are concreted through one. The concrete mixture is placed in the intermediate strips after the concrete has hardened in the adjacent strips. Before concreting the intermediate strips, the lighthouse boards are removed; work seams are formed along these edges. The concrete mixture is unloaded to the place of concreting directly from the concrete truck (or supplied by concrete pumps). It is roughly leveled with shovels, and then compacted with a vibrating beam (vibrating screed). At the same time, the vibrating beam is held in one position until it descends with both ends on the beacon boards.

If concrete, cement or asphalt floors are assumed for the concrete preparation, then the surface of the preparation after driving the vibro-beam is left rough for better adhesion to the upper layers.

The clean floor is concreted on lighthouse boards with the concrete mix compacted with a vibrating screed. Freshly laid concrete is thoroughly smoothed after 20 ... 30 minutes using hand tool or a special trowel. At this point, a thin film of water and cement laitance appears on the floor surface. Such a film is removed during smoothing. In 30 ... 40 minutes after smoothing, the concrete surface is treated with a metal trowel until gravel (crushed stone) grains are exposed. Such processing allows to obtain high-quality concrete floors with high abrasion and durability.

To give concrete floor high density and high hygienic qualities, its surface is iron. At the same time, dry cement is carefully rubbed into the surface of freshly laid wet concrete until a matte sheen appears. This operation is performed using steel trowels, trowels or trowels.

When placing a concrete mixture in massive, densely reinforced slabs of a large area (foundation slabs, bottoms of tanks and sedimentation tanks, etc.), the main technological requirement is the continuity of laying over the entire height of the slab (0.15 ... 1.5 m).

To carry out the laying process, the slabs are divided into cards. If the thickness of the slabs is less than 0.5 m, then the division into maps and the placement of concrete is carried out in the same way as for concrete preparations. With a greater thickness, the slabs are divided into parallel cards 5 ... 10 m wide, leaving between them dividing strips 1 ... 1.5 m wide.To ensure continuous laying of the mixture to the full height, the slab is divided into blocks without cutting the reinforcement, with block fencing metal meshes.

Maps are concreted in a row, i.e. one by one. The mixture is placed in the dividing strips with the hardened concrete of the cards after removing the formwork at their borders. The concrete mixture with a mobility of 2 ... 6 cm is fed to the cards by concrete pumps, using concrete pavers, overpasses, as well as cranes in buckets. It should be fed in the direction of the previously laid concrete, as if pressing the new portions to the laid ones.

In slabs of even large thickness, the concrete mixture is laid in one layer. At the same time, vibration compaction is somewhat difficult, since the internal vibrators need to be immersed in the mixture to a depth of 1.5 ... 2 times the length of the working part.

The concrete slabs are leveled along the lighthouses, the surface is smoothed with trowels, trowels or half-trowels. In places where walls adjoin, columns and pillars are supported, concrete is left rough with a device in individual cases corrugations and notches.

The peculiarity of laying a concrete mixture during the construction of walls and partitions depends on their thickness and height, as well as the type of formwork used.

When erecting walls in a collapsible formwork, the mixture is laid in sections with a height of no more than 3 m.In walls with a thickness of more than 0.5 m with weak reinforcement, a concrete mixture with a mobility of 4 ... 6 cm is served.When a length of more than 20 m, the walls are divided into sections of 7 ... 10 m and a separating formwork is installed at the border of the plots. The concrete mixture is fed directly into the formwork at several points along the length of the site by buckets, vibrating chutes, and concrete pumps. With a wall height of more than 3 m, link trunks are used, while the mixture is laid in horizontal layers 0.3 ... 0.4 m thick with obligatory vibration.

More mobile concrete mixtures (6 ... 10 cm) are placed in thin and densely reinforced structures of walls and partitions. With a wall thickness of up to 0.15 m, concreting is carried out in tiers up to 1.5 m high.On one side, the formwork is erected to the full height, and from the concreting side - to the height of the tier. This improves the quality and ensures the usability. Having laid the concrete mixture in the first tier, the formwork of the next one is built up, etc. When the concrete mixture is supplied by a concrete pump, the formwork can be set at once to the full height with the obligatory condition that the end of the concrete pipeline is buried in the concrete mixture being laid (the so-called "pressure concreting").

In columns up to 5 m in height with cross-sectional sides up to 0.8 m, without crossing clamps, the concrete mixture is laid immediately to the full height. The mixture is carefully loaded from above and compacted with internal vibrators. If the height of the columns is over 5 m, the mixture is fed through funnels along the trunks. In high and densely reinforced columns with crossing clamps, the mixture is laid in tiers up to 2 m with loading through windows in the formwork or special pockets. Sometimes, to supply the concrete mixture, the formwork of the columns is performed with removable shields, which are installed after the lower tier has been concreted.

In beams and slabs, monolithically connected to columns and walls, the concrete mixture is placed 1 ... 2 hours after the last layer (portion) is laid in vertical structures due to the need for the initial settlement of the mixture laid in them.

In beams (girders) and slabs of ribbed floors, the mixture is laid, as a rule, at the same time.

In beams with a height of more than 80 cm, the concrete mixture is laid in layers of 30 ... 40 cm with compaction with internal vibrators. In this case, the last layer of the mixture should be 3 ... 5 cm below the level of the bottom of the floor slab.

In the floor slabs, the concrete mixture is supplied immediately to the entire width with compaction with surface vibrators with a thickness of up to 0.25 m and internal vibrators with a greater thickness.

In arches and vaults with a span of less than 20 m, the concrete mixture is laid simultaneously from two sides - from the heels to the castle, and with a span of more than 20 m - in separate sections, symmetrically located relative to the middle. Separating strips with a width of 0.8 ... 1.2 m are left between the sections. At each section, the mixture is fed continuously. Begin laying the mixture from the areas adjacent to the supports. Then, in order to avoid bulging of the formwork at the top of the arch (vault), the mixture is placed in the castle section. After that, the concrete mixture is fed into the ordinary sections evenly from both sides of the structure. The mixture is placed in the dividing strips 6 ... 8 days after the shrinkage of the concrete of the main sections occurs. For strips, a rigid concrete mixture is used - a cone draft of 1 ... 3 cm.

On steep sections of arches or vaults, in order to prevent the concrete mixture from slipping when vibrating, concreting is carried out in a double-sided formwork, the outer shields of which are increased during the process.

The bases and surfaces of the working joints, prepared for laying the concrete mixture, must meet the following requirements:

Natural and artificial foundations of non-rocky soils must retain the physical and mechanical properties provided for by the project;

Rock foundations should be made of non-weathering rock;

Rocky bases and surfaces of working seams must be cleaned of debris, dirt, oils, snow and ice, washed and free from water on the surface;

Concrete foundations and work joints on horizontal and inclined surfaces must be cleaned of cement film (vertical surfaces are cleaned if required in the project).

When cleaning the concrete surface from the cement film, damage to the concrete is not allowed. When cleaning the surface, the strength of concrete must be at least, MPa:

Cleaning with water or air jet - 0.3;

Cleaning with a mechanical metal brush - 1.5;

Hydro-sandblasting or cleaning with a power cutter - 5.

Before the start of laying the concrete mixture, check and accept all structures and their elements that are closed in the course of subsequent work, the correct installation and fastening of the formwork and supporting structures. Before concreting, the formwork is cleaned of debris and dirt, and the reinforcement is cleaned of rust.

The surfaces of wooden, plywood and metal formwork are coated with grease, which should not deteriorate appearance and strength properties of structures.

The mobility of concrete mixtures pumped through pipelines is assigned taking into account technical characteristics used concrete pumps and pneumatic blowers, in all cases it must be at least 4 cm.

For any type of concrete mixture supply to reinforced structures, the free drop height should not exceed 2 m, and when supplying to the ceiling - 1 m. The permissible height of dumping the concrete mixture into the formwork of columns with cross-sectional sides of 0.4-0.8 m and in the absence of crossing reinforcement clamps should be no more than 5 m.

The permissible height of the free throwing of the concrete mixture into the formwork of unreinforced structures is established by the construction laboratory on the basis of production experience and must ensure the homogeneity and strength of the concrete, as well as the safety of the base and formwork. The extreme limit - the dumping height should not exceed 6 m. From a greater height, the concrete mix is ​​lowered along inclined chutes or vertical trunks.

The concrete mixture is laid in the concreted structure in horizontal layers of the same thickness without breaks, with a consistent laying direction in one direction in all layers.

The laying of the concrete mixture by the stepwise method with the simultaneous laying of 2-3 layers can be allowed if it is permissible by PPR.

The thickness of the concrete layer to be laid is determined depending on the compaction means. When using heavy suspended vertically arranged vibrators, the layer thickness should be 5-10 cm less than the length of the working part of the vibrator. When using vibrators located at an angle to the vertical (up to 35 °), the layer thickness should be equal to the vertical projection of the length of the working part of the vibrator. The thickness of the layer to be laid when using manual deep vibrators should not exceed 1.25 times the length of the working part of the vibrator.

When compacting the concrete mixture with surface vibrators, the layer thickness should not exceed 25 cm in unreinforced structures and structures with single reinforcement, and 12 cm in structures with double reinforcement. powerful vibrators increase the thickness of the concrete layer to be laid.

When compacting the concrete mixture, the following rules must be observed:

The step of rearranging the deep vibrators should not exceed one and a half radius of their action;

The immersion depth of the deep vibrator in the concrete mixture should ensure its deepening into the previously laid layer by 5-10 cm;

The step of rearranging the surface vibrators should ensure that the vibrator pad overlaps the boundaries of the vibrated area by 10 cm;

The support of vibrators during operation on reinforcement and embedded parts of structures to be concreted, as well as on rods and other elements of its fastening, is not allowed.

At each position, the duration of vibration is established by experience, while it must ensure sufficient compaction of the concrete mixture, the main features of which are: the cessation of its settling, the appearance of cement laitance on the surface and the cessation of the release of air bubbles.

When concreting massive structures, the following rules must be observed: if the array is divided into blocks, the closing blocks should be concreted only after the concrete of the blocks to be joined has shrunk and cooled; foundations that absorb the dynamic effects of the equipment should be concreted without interruption.

When concreting massive structures with individual stones more than 15 cm in size ("raisins"), the following rules must be observed: the dimensions of individual stones should not exceed 1/3 of the smallest size of the concreted structure; the ratio of the individual dimensions of the stone should not exceed 1: 2.5, the stones should be at least 30 cm away from the formwork and not come into contact with reinforcement and embedded parts; the distance between individual stones must be at least 20 cm.

The strength of the stones should not be lower than the minimum strength of the coarse aggregate for concrete of this class, frost resistance - not lower than that provided for by the project for the concrete of the structure.

When placing concrete mix in columns and walls, the following rules must be observed:

The height of columns, pillars and walls, concreted without interruption, should not exceed 5 m - for columns, 3 m - for walls and partitions, 2 m - for columns with cross-sectional sides less than 0.4 m and columns of any cross-section with crossing clamps, and also for walls and partitions less than 0.15 m thick; at a greater height of the sections of columns and walls, concreted without working seams, it is necessary to arrange breaks for the settlement of the concrete mixture;

The duration of the break to ensure the concrete slump should be at least 40 minutes, but not exceed 2 hours;

Frame structures are concreted with a break between concreting columns (pillars) and frame girders.

Beams and floor slabs are concreted simultaneously, and monolithically connected to columns and walls - 1-2 hours after concreting the columns and walls. If the cross-sectional height of the beams is more than 0.8 m, they can be concreted separately from the slabs.

When placing the concrete mixture in the vertical sliding formwork, the following conditions must be met:

The concrete mixture must be laid in even layers with a thickness of 23-30 cm;

The speed of placing the concrete mixture should ensure filling the formwork within 2.5-3.5 hours to a height equal to at least half the height of the formwork;

Each new layer of concrete mix should be laid after the completion of the previous one, before it begins to set;

The formwork is raised after filling the entire perimeter of the structure to be concreted with the concrete mixture to a level 5 cm below the top of the formwork panels and at a speed that excludes both the adhesion of the concrete to the formwork and its sliding upon leaving the formwork;

The concreted part of the structure with a height of no more than 10 m should be surveyed in order to correct its position.

In flat unreinforced structures (platforms, preparation for floors), the concrete mixture is laid in strips 3-4 m wide every one, Filling the intermediate strips after the concrete has hardened in adjacent strips.

In the arches and vaults, the concrete mixture is laid from the heels to the castle. In this case, the planes of the working seams limiting the concreting strips should be perpendicular to the surfaces of arches and vaults.

The duration of the breaks at which the device of working seams is required is determined by the laboratory, depending on the type of structure, the cement used and the temperature of concrete hardening. The laying of the concrete mixture after such breaks is allowed after the concrete has acquired a strength of at least 1.5 MPa.

The surface of the seams arising from the laying of the concrete mixture with breaks must be perpendicular to the axis of the columns and beams to be concreted, the surface of the slabs and walls. It is allowed to arrange working seams when concreting: columns - at the level of the top of the foundation, the bottom of the girders, beams or crane consoles, the top of the crane beams, the bottom of the column capitals; beams of large sizes, monolithically connected to the slabs, - 2-3 cm below the mark of the lower surface of the slab, and if there are heaps in the slab - at the mark of the bottom of the slab; flat slabs - anywhere parallel to the smaller side of the slab; ribbed slabs in a direction parallel to the secondary beams; individual beams - within the middle third of the span of the beams; massifs, arches, vaults, reservoirs, bunkers, hydraulic structures, bridges and other complex engineering structures and structures - in the places indicated in the projects.

Prestressed structures must be concreted without interruption; construction of working seams is allowed only with the appropriate design instructions.

Immediately before concreting, the formwork must be cleaned of debris and dirt, and the reinforcement from peeling rust. Gaps in wooden, plywood and metal formwork should be coated with grease, and the surfaces of concrete, reinforced concrete and reinforced concrete formwork should be moistened. The surface of the previously laid concrete must be free of cement film and moistened or covered with cement mortar.

Concrete supply and placement.

Concrete mixtures should be laid in horizontal layers of the same thickness (~ 0.3x0.5 m) without breaks with the direction of laying in one direction in all layers. The next layer of concrete must be laid before the concrete of the previous layer begins to set. The upper level of the laid concrete mix must be 50x70mm below the top of the formwork panels.

The permissible height of free dumping of the concrete mixture is taken according to # M12293 2 871001100 4120950664 78 1943371100 971260270 3996115668 1343157494 492301028 2631723650 table from SNiP 3.03.01-87 # S. With a higher dumping height of the mixture, in order to avoid its stratification, its descent into vertical structures should be carried out along vibrating chutes, inclined trays or chutes, which provide a slow slide of the mixture into the formwork.

When organizing work seams, their surface should be perpendicular to the axis of the columns to be concreted or the surface of the walls. Working seams (in agreement with the design organization) may be arranged during concreting:

The resumption of concreting is allowed when the concrete reaches a strength of at least 1 5 kg / cm.

In the process of concreting and at the end of it, take measures to prevent adhesion of plugs, formwork elements and temporary fasteners to the concrete

Concrete compaction.

Compaction of the concrete mix is ​​carried out by vibration using deep vibrators. The step of permutation of deep vibrators should not exceed 1.5 of their radius of action. The greatest thickness of the laid layer should not exceed 1.25 times the length of the working part of the vibrator, and when the vibrator is located at an angle of up to 35 °, the layer thickness should be equal to the vertical projection of its working part. The immersion depth of the vibrator in the concrete mixture should ensure its deepening into the previously laid layer by 510 cm.

In places where reinforcement, embedded products or formwork interfere with the proper compaction of the concrete mixture with vibrators, it should be additionally compacted with a bayonet.

When compacting the concrete mixture, it is necessary to ensure that the vibrators do not come into contact with the frame reinforcement. It is not allowed to rest the vibrators on fittings, embedded products, rods and other formwork fastening elements.

Concrete curing and maintenance.

During the hardening period, the concrete must be protected from atmospheric precipitation or moisture loss. Subsequently, maintain a temperature and humidity regime with the creation of conditions that ensure an increase in its strength.

When concreting structures in winter time measures for the maintenance of concrete, the procedure and timing of their implementation, control over their implementation and the timing of the stripping of structures should be established by the PPRk.

The movement of people on the concrete structures and the installation of the formwork of the overlying structures is allowed after the concrete reaches a strength of at least 15 kg / cm.

2.6. The peculiarity of laying concrete mix during the construction of walls and partitions

The peculiarity of laying a concrete mixture during the construction of walls and partitions depends on their thickness and height, as well as the type of formwork used.

When erecting walls in a collapsible formwork, the mixture is laid in sections with a height of no more than 3 m. A concrete mixture with a mobility of 4 .6 cm is fed into walls with a thickness of more than 0.5 m with a weak reinforcement. For a length of more than 20 m, the walls are divided into sections of 7 .10 m and on the border of the plots a dividing formwork is installed. The concrete mixture is fed directly into the formwork at several points along the length of the site by buckets, vibrating chutes, and concrete pumps. With a wall height of more than 3 m, link trunks are used, while the mixture is laid in horizontal layers 0.3 to 0.4 m thick with obligatory vibration. It is not recommended to apply the mixture to one point, as this will form inclined loose layers that reduce the surface quality and the homogeneity of the concrete.

More mobile concrete mixtures (6.10 cm) are placed in thin and densely reinforced structures of walls and partitions. With a wall thickness of up to 0.15 m, concreting is carried out in tiers up to 1.5 m high.On one side, the formwork is erected to the full height, and from the concreting side - to the height of the tier. This improves the quality and ensures the usability. Having laid the concrete mixture in the first tier, the formwork of the next one is increased, etc.

3. Requirements for the quality of work

Quality control and acceptance of structures.

3.1. A concrete log should be kept on site every shift.

3.2. When accepting concreted structures, in accordance with the requirements of the current state standards, determine:

The quality of concrete in terms of strength, and, if necessary, frost resistance, water resistance and other indicators specified in the project;

Surface quality;

Availability and compliance with the project of holes, openings and channels;

The presence and correctness of the expansion joints;

The admissibility of deviations of structures according to # M12293 0 871001100 4120950664 24255 1943371100 406179787 2717138943 1343157494 492301028 2631723650 Table 11 # S (# M12291 871001100СНиП 3.03.01-87 # S *), in particular, certificates, documents for .), confirming the quality of the materials, products and semi-finished products used.

3.3. Acceptance of finished concrete and reinforced concrete structures or parts of structures should be formalized with an act of survey of hidden works or an act of acceptance of critical structures.

Table 1 - Table of tolerances

Parameter	Limit deviations
Foundations
Walls and columns supporting prefabricated beam structures
Walls of buildings and structures erected in sliding formwork, in the absence of intermediate floors	1/500 of the height of the structure, but not more than 100 mm
Walls of buildings and structures erected in sliding formwork, in the presence of intermediate floors	1/1000 of the height of the structure, but not more than 50 mm
Deviation of horizontal planes for the entire length of the verified area
Local unevenness of the concrete surface when checking with a two-meter rail, except for supporting surfaces
Length or span of elements
Cross-sectional dimensions of elements
Elevations of surfaces and embedded products that serve as supports for steel or precast concrete columns and other prefabricated elements
The slope of the supporting surfaces of foundations when supporting steel columns without grout
Anchor bolt location: inside the support contour in height

The foundation is an integral part of any building and structure. When erecting it, it is important to prevent technological errors, because carelessness and non-observance of the construction stages can lead to serious consequences. Despite the serious responsibility, building foundations is an easy process that even a beginner can handle. Concreting the foundation will allow you to get a solid, reliable foundation for buildings and structures, which will last for more than a dozen years. But in order for the structure to turn out with the required strength characteristics, you should pay attention to the main subtleties that are present in the process of concreting.

To erect concrete foundation, you will need the following tools and materials:

• welding machine;
• grater;
• pegs;
• steel rods;
• a concrete mixer or an empty container for preparing a solution;
• shovel;
• rope or cord for marking;
• hoe;
• building level;
• cement;
• sand;
• water;
• beams for the construction of formwork.

Excavation

Before concreting, the construction site is prepared and a set of measures necessary for the construction of the foundation is carried out. The first stage includes earthworks. To do this, you will need pegs and a rope, which will mark the area. After the territory is marked out, they begin to dig a trench. The depth of the trench is made such that it is 30 centimeters greater than the depth of soil freezing. The bottom of the dug trench is filled with a layer of sand or gravel.

The process of erecting the foundation, although easy, still requires certain power loads and time, therefore it is recommended to hire several more people who will help speed up the construction. You can make your work easier by using the services of special equipment. The tractor will quickly and efficiently dig a trench even in places with hard ground.

Formwork creation

As a basis for the concrete mix, formwork from wooden boards or prefabricated panels is installed. When installing the formwork, you should adhere to the following subtleties:

• Parts of the formwork must be accurately installed and checked with a building level, which will exclude distortions and slopes of the base.
• The quality of the surface of the boards, as well as the presence of slots in the structure being erected, can entail a number of unpleasant consequences. Therefore, possible gaps between the boards are eliminated even before the laying of the working mixture.
• To achieve the required formwork rigidity, boards with a thickness of at least five centimeters are used, which are able to withstand heavy loads during the process of concreting the structure.
• To give the formwork increased rigidity, more struts should be used, placing them with a smaller pitch.
• Before erecting the formwork, you need to dig a trench, fill it up and level it.

The installation of the formwork begins with driving the pegs located along the entire perimeter into the ground surface. Next, they proceed to fastening the boards with nails. It is important to monitor the evenness of the structure being erected, otherwise the distortions will affect the foundation. Dismantling of the formwork is carried out after the concrete is completely dry. This will happen a couple of days after laying the working mixture.

Preparation of the solution

To provide the foundation with high strength, a concrete solution with high quality characteristics should be prepared. Cement is purchased with grades not lower than M-200. To prepare it you will need:

• crushed stone;
• cement;
• sand;
• water;
• if necessary, plasticizers that are capable.

To make the mixture, you need to pour dry ingredients into a concrete mixer: three parts of sand, one part of cement and five parts of crushed stone. The dry components are mixed and poured with water, there should be enough water to obtain a solution with a consistency resembling thick sour cream. Mixing the components of the mixture in a concrete mixer will take about five minutes, if there is no opportunity to purchase such equipment, then the working mixture is diluted in a large-volume container. But then it should be borne in mind that preparing the solution without special equipment will take a lot of time.

It is necessary to make a concrete mixture immediately before starting to pour the foundation.

Concreting methods

When concreting the base for buildings and structures, reinforcement with steel rods, the diameter of which is 0.8-1.2 centimeters, should be carried out. A mesh is constructed from steel rods, while observing the dimensions of the cell - 150x150 millimeters. We concretize the structure only after the reinforcing mesh has been installed in the formwork cavity. Masonry concrete mortar can be done in two ways:

• Manual method. Hand-built foundations involve several people pouring the finished mortar. In this case, on construction site there should be two concrete mixers that feed cement from both sides.
• Using a mixer. The laying of concrete mix using a mixer is carried out several times faster than the manual method. This method of erecting a foundation is expensive, but with the best finished result.

Manual way

Manual preparation of concrete mix for the foundation.

To concrete small structures, a manual method is used, for the implementation of which a team of several workers is required. If you try to concrete it alone, you can harm the construction process and all the work will be in vain.

You should also purchase two concrete mixers, which are installed on both sides of the concreting object. Three people are assigned to each equipment. The process of pouring a solution of sand, cement and crushed stone should be carried out quickly, since the concrete composition is able to quickly gain strength. The concrete solution is fed along the entire perimeter of the foundation up to 40 centimeters thick.

After pouring the mixture, it should be carefully tamped with a deep vibrator. will get rid of air bubbles in the mixture and distribute it over the entire depth of the structure. Next, they start laying the next layer of concrete mortar, which should also be compacted. Pouring of each new layer is carried out two hours after the concreting of the previous one has finished. Filling the last layer requires special care, namely, giving the surface a special evenness. You can make an even layer using a building level or other special devices for leveling a massive surface.

The poured concrete will reach its strength characteristics after a month, during which it is important to provide optimal conditions for the concrete solution to harden. Cover the concrete with insulating materials to prevent direct sunlight and atmospheric precipitation.

Using a mixer

Pouring the foundation with a mixer involves the use of a vehicle that will pour concrete mortar around the entire perimeter of the foundation in short time... This method is more costly than manual, therefore, before choosing a method of concreting, you should determine the priority aspects for yourself.

It includes the supply of concrete to the formwork by special ones. It is worth considering the speed and have time to level the flooded surface with a vibrator or rag.

Construction of working seams

Concreting seam.

It is recommended to fill the foundations with concrete without long breaks. Laying of subsequent layers of the mixture should not have a break of more than three hours. But often it is not possible to adhere to such rules and then it is necessary to organize the arrangement of working seams.

Construction joints are the boundary between the poured concrete layers, and during their manufacture, one should adhere to technological process... If the technology is violated, then the foundation for buildings and structures will not acquire the required quality characteristics.

When installing construction joints, it is important to achieve maximum adhesion of the concrete layers to the joints. For this, the hardened surface must be prepared for subsequent work. The surface is cleaned of dust, snow, debris or water. Then remove, which is formed on the hardened solution. Cement milk forms a film that has a loose fragile structure, which leads to a decrease in the adhesion of the solution to the hardened layer. Removal of the film is carried out using a water or air jet, which is supplied under high pressure. Thus, the film is removed from the hardened concrete surface. In addition to removing the film with a high pressure jet, milling machines are used. It is possible to destroy the cement film with acid, which is applied to a hard layer and left for several hours.

The laying of subsequent layers of concrete mortar is started only after removing the film from the frozen surface. Next, the joints are insulated and waterproofed, which will prevent premature destruction of the base of buildings and structures.

Conclusion

For high-quality and durable foundation construction, you should adhere to the production technology and the sequence of stages. Compliance with all the rules and recommendations for will lead to the creation of a reliable foundation for buildings and structures, eliminating the need to create working seams.

But if, nevertheless, the creation of seams is inevitable, then the surface should be carefully prepared before proceeding with the laying of concrete mortar.