On April 22, a scientific-practical conference "Problems monolithic construction and ways to solve them. "The conference was attended by representatives of OJSC" NIIZhB "named after A.A. "GeroCrit", LLC BASF "Building Systems", etc.

The informative richness of the conference was very high, but there was not enough time to discuss the presented reports. It can be seen that a lot of questions have accumulated in this area, and representatives construction organizations, including those who are ready to discuss them.

We hope that the materials of this conference, published in a separate book by the State Unitary Enterprise "NIIMosstroy", will serve to improve work in the field of monolithic construction.

We bring to your attention the text of the report presented at the conference by the head of the Laboratory for testing building materials and structures, Dmitry Nikolaevich Abramov.

The main causes of defects in concrete structures

In my report, I would like to talk about the main violations of the technology for the production of reinforced concrete works, which the employees of our laboratory face at construction sites in the city of Moscow.

- early stripping of structures.

Due to the high cost of formwork in order to increase the number of cycles of its turnover, builders often do not comply with the concrete curing regimes in the formwork and remove the formwork at an earlier stage than is required by the project requirements. technological maps and SNiP 3-03-01-87. When dismantling the formwork, the value of the adhesion of the concrete to the formwork is of great importance: high adhesion makes stripping work difficult. Deterioration in the quality of concrete surfaces leads to defects.

- production of insufficiently rigid, deformable when placing concrete and insufficiently dense formwork.

Such formwork receives deformations during the laying of the concrete mixture, which leads to a change in the shape of the reinforced concrete elements. Deformation of the formwork can lead to displacement and deformation of reinforcing cages and walls, a change bearing capacity structural elements, the formation of protrusions and sagging. Violation of the design dimensions of structures leads to:

In case of their decrease

To reduce the bearing capacity

In the case of an increase, their own weight increases.

This kind of violation of observation technology in the manufacture of formwork in building conditions without proper engineering control.

- insufficient thickness or absence of a protective layer.

Observed in case of improper installation or displacement of the formwork or reinforcement cage, the absence of gaskets.

Serious defects of monolithic reinforced concrete structures weak control over the quality of structural reinforcement can result. The most common violations are:

- inconsistency with the design of structural reinforcement;

- poor quality welding of structural units and reinforcement joints;

- the use of highly corroded fittings.

- poor compaction of the concrete mix during laying in the formwork leads to the formation of cavities and cavities, can cause a significant decrease in the bearing capacity of the elements, increases the permeability of structures, promotes corrosion of the reinforcement located in the zone of defects;

- laying of exfoliated concrete mix does not allow obtaining uniform strength and density of concrete throughout the entire volume of the structure;

- using too hard concrete mix leads to the formation of cavities and cavities around the reinforcing bars, which reduces the adhesion of the reinforcement to the concrete and causes the risk of corrosion of the reinforcement.

There are cases of adhesion of the concrete mixture to the reinforcement and formwork, which causes the formation of cavities in the body of concrete structures.

- poor maintenance of concrete during its hardening.

During the maintenance of concrete, it is necessary to create such temperature and humidity conditions that would ensure the preservation of water in the concrete, which is necessary for the hydration of the cement. If the hardening process takes place at a relatively constant temperature and humidity, the stresses arising in the concrete due to volume changes and caused by shrinkage and temperature deformations will be negligible. Usually concrete is covered with plastic wrap or other protective coating. In order to prevent it from drying out. Overdried concrete has a much lower strength and frost resistance than normally hardened concrete; many shrinkage cracks appear in it.

When concreting in winter conditions in case of insufficient insulation or heat treatment, early freezing of concrete may occur. After thawing such concrete, it will not be able to gain the required strength.

Damage to reinforced concrete structures is divided into three groups according to the nature of the effect on the bearing capacity.

Group I - damages that practically do not reduce the strength and durability of the structure (surface cavities, voids; cracks, including shrinkage, with an opening of no more than 0.2 mm, as well as, in which, under the influence of temporary load and temperature, the opening increases by no more than 0 , 1mm; concrete chips without exposing reinforcement, etc.);

Group II - damages that reduce the durability of the structure (corrosion-hazardous cracks with an opening of more than 0.2 mm and cracks with an opening of more than 0.1 mm, in the area of working reinforcement of prestressed spans, including along sections under constant load; cracks with an opening of more than 0.3 mm under temporary load; sink voids and chips with bare reinforcement; surface and deep corrosion of concrete, etc.);

Group III - damages that reduce the bearing capacity of the structure (cracks not provided for by the calculation either in terms of strength or endurance; inclined cracks in the walls of the beams; horizontal cracks at the interfaces of the slab and superstructures; large cavities and voids in the concrete of the compressed zone, etc.).

Damage of the first group does not require urgent measures, they can be repaired by applying coatings at the current maintenance for preventive purposes. The main purpose of coatings in case of damage of group I is to stop the development of existing small cracks, prevent the formation of new ones, improve protective properties concrete and protect structures from atmospheric and chemical corrosion.

In case of damage of the II group, the repair provides an increase in the durability of the structure. Therefore, the materials used must have sufficient durability. Cracks in the zone of location of beams of prestressed reinforcement, cracks along the reinforcement are subject to compulsory sealing.

In case of damage of the III group, the bearing capacity of the structure is restored according to specific feature... The materials and technologies used must ensure the strength characteristics and durability of the structure.

To eliminate damages of the III group, as a rule, individual projects should be developed.

The constant growth in the volume of monolithic construction is one of the main trends that characterize the modern period of Russian construction. However, at present, a massive transition to construction from monolithic reinforced concrete can have negative consequences associated with a rather low level of quality of individual objects. Among the main reasons for the low quality of monolithic buildings being erected, it is necessary to highlight the following.

Firstly, most of the normative documents currently in force in Russia were created in the era of priority development of precast concrete construction, therefore, their focus on factory technologies and insufficient study of the issues of monolithic reinforced concrete construction are quite natural.

Secondly, most construction organizations lack sufficient experience and the necessary technological culture of monolithic construction, as well as low-quality technical equipment.

Thirdly, not created efficient system quality management of monolithic construction, including a system of reliable technological control quality of work.

The quality of concrete is, first of all, the compliance of its characteristics with the parameters in regulatory documents. Rosstandart approved and are in effect new standards: GOST 7473 “Concrete mixtures. Specifications ", GOST 18195" Concrete. Rules for the control and assessment of strength ". GOST 31914 "High-strength heavy and fine-grained concrete for monolithic structures" should come into force, the standard for reinforcement and embedded products should become effective.

The new standards, unfortunately, do not contain issues related to the specifics of legal relations between construction customers and general contractors, manufacturers of building materials and builders, although the quality of concrete work depends on each stage of the technical chain: preparation of raw materials for production, design of concrete, production and transportation of the mixture, laying and maintenance of concrete in the structure.

Ensuring the quality of concrete in the production process is achieved thanks to the complex different conditions: here and modern technological equipment, and the presence of accredited testing laboratories, and qualified personnel, and unconditional compliance with regulatory requirements, and the implementation of quality management processes.

Head of the Building Materials Testing Laboratory and

structures GBU "TSEIIS" -D.N. Abramov

The text of the report presented at the conference by the head of the Laboratory for testing of building materials and structures Dmitry Nikolaevich Abramov "The main causes of defects in concrete structures"

In my report, I would like to talk about the main violations of the technology for the production of reinforced concrete works that the employees of our laboratory encounter at construction sites in the city of Moscow.

- early stripping of structures.

Due to the high cost of formwork in order to increase the number of cycles of its turnover, builders often do not comply with the regimes of holding concrete in the formwork and remove the formwork at an earlier stage than is required by the project requirements with flow charts and SNiP 3-03-01-87. When dismantling the formwork, the value of the adhesion of the concrete to the formwork is of great importance: high adhesion makes stripping work difficult. Deterioration in the quality of concrete surfaces leads to defects.

- production of insufficiently rigid, deformable when placing concrete and insufficiently dense formwork.

Such formwork receives deformations during the laying of the concrete mixture, which leads to a change in the shape of the reinforced concrete elements. Deformation of the formwork can lead to displacement and deformation of reinforcing cages and walls, a change in the bearing capacity of structural elements, the formation of protrusions and sagging. Violation of the design dimensions of structures leads to:

In case of their decrease

To reduce the bearing capacity

In the case of an increase, their own weight increases.

This kind of violation of observation technology in the manufacture of formwork in building conditions without proper engineering control.

- insufficient thickness or absence of a protective layer.

Observed in case of improper installation or displacement of the formwork or reinforcement cage, the absence of gaskets.

Poor control over the quality of structural reinforcement can lead to serious defects in monolithic reinforced concrete structures. The most common violations are:

- inconsistency with the design of structural reinforcement;

- poor quality welding of structural units and reinforcement joints;

- the use of highly corroded fittings.

- laying of exfoliated concrete mix does not allow obtaining uniform strength and density of concrete throughout the entire volume of the structure;

There are cases of adhesion of the concrete mixture to the reinforcement and formwork, which causes the formation of cavities in the body of concrete structures.

- poor maintenance of concrete during its hardening.

When concreting in winter conditions with insufficient insulation or heat treatment, early freezing of concrete can occur. After thawing such concrete, it will not be able to gain the required strength.

Damage to reinforced concrete structures is divided into three groups according to the nature of the effect on the bearing capacity.

Group I - damage that practically does not reduce the strength and durability of the structure (surface cavities, voids; cracks, including shrinkage, with an opening of no more than 0.2 mm, as well as in which, under the influence of temporary load and temperature, the opening increases by no more than 0 , 1mm; concrete chips without exposing reinforcement, etc.);

Group III - damages that reduce the bearing capacity of the structure (cracks not provided for by the calculation either in terms of strength or endurance; inclined cracks in the walls of beams; horizontal cracks in the joints of the slab and spans; large cavities and voids in the concrete of the compressed zone, etc. .).

Damage of the first group does not require urgent measures, they can be repaired by applying coatings at the current maintenance for preventive purposes. The main purpose of coatings in case of damage of group I is to stop the development of existing small cracks, prevent the formation of new ones, improve the protective properties of concrete and protect structures from atmospheric and chemical corrosion.

In case of damage to the III group, the bearing capacity of the structure is restored according to a specific characteristic. The materials and technologies used must ensure the strength characteristics and durability of the structure.

To eliminate damages of the III group, as a rule, individual projects should be developed.

Secondly, most construction organizations lack sufficient experience and the necessary technological culture of monolithic construction, as well as low-quality technical equipment.

Thirdly, an effective quality control system for monolithic construction has not been created, including a system of reliable technological control over the quality of work.

Ensuring the quality of concrete in the production process is achieved due to a complex of different conditions: there are modern technological equipment, and the presence of accredited testing laboratories, and qualified personnel, and unconditional compliance with regulatory requirements, and the introduction of quality management processes.

a. Filling the formwork with concrete

For concreting structures in sliding formwork, concrete mixtures are used on Portland cements of at least 400 grade with the beginning of setting not earlier than 3 hours and the end of setting not later than 6 hours. Based on the cement test data, the speed of concreting and lifting of the sliding formwork should be determined.

The slump of the cone of the concrete mixture used should be: when compaction with a vibrator 6-8 and manual compaction 8-10 cm, and W / C - no more than 0.5. The grain size of the coarse aggregate should be no more than / 6 smallest size the cross-section of the structure to be concreted, and for densely reinforced structures - no more than 20 mm.

The thickness of walls and beams erected in sliding formwork, as a rule, should not be less than 150 mm (the weight of concrete should be greater than the frictional forces), and the volume of concrete by 1 linear meter. m their height should not exceed 60 x 3.

Initially, the formwork is filled with a concrete mixture in two or three layers to a height equal to half of the formwork, in the course of no more than 3; b h. The second and third layers are laid only after the completion of the previous layer along the entire perimeter of the formwork. Further filling of the formwork is resumed only after the start of its lifting and ends no later than 6 hours later.

Before filling the formwork with concrete mixture to its full height, it is lifted at a speed of 60-70 mm / h.

b. Compaction process

After the initial filling of the formwork to its full height, with its further rise, the concrete mixture is laid continuously in layers up to 200 mm thick in thin walls (up to 200 mm) and no more than 250 mm in other structures. The laying of a new layer is made only after the completion of the laying of the previous layer before the start of its setting.

During concreting, the upper level of the mixture to be laid must be more than 50 mm below the top of the formwork panels.

The concrete mix is compacted with rod vibrators with a flexible shaft or manually with shurovs. The diameter of the vibrator tip should be 35 mm for wall thicknesses up to 200 mm and 50 mm for thicker walls.

In the process of compaction of the mixture, it is recommended to raise and lower the vibrator by 50-100 mm within the limits of the layer to be laid, while the tip of the vibrator should not rest against the formwork or reinforcement, and also should not reach the previously laid setting layer of concrete.

The rate of placing the concrete mixture and lifting the formwork should exclude the possibility of adhesion of the laid concrete to the formwork and ensure the strength of the concrete emerging from the formwork, sufficient to maintain the shape of the structure and at the same time, allowing easy floatation of the traces of the formwork on its surface.

c. Breaks in concreting

The intervals between raising the formwork should not exceed 8 minutes when using vibrators and 10 minutes when manually compacting the concrete mixture. The speed of lifting the formwork at an outside air temperature of +15, + 20 ° C and using Portland cement M 500 reaches 150-200 mm per hour.

In the process of concreting the walls in the sliding formwork, there can be concrete "breaks": the formwork carries with it a part of the immature concrete of the wall, as a result, shells are formed, and the reinforcement is exposed. The main reasons for "failures" are as follows: contamination of the formwork; non-observance of the taper of the formwork; long breaks during concreting.

In cases of a forced interruption in concreting, measures should be taken against adhesion of the laid concrete to the formwork; the formwork is slowly raised until a visible gap is formed between the formwork and the concrete, or it is periodically raised and lowered within one jack step (“step in place”). When resuming concreting, it is necessary to clean the formwork, remove the cement film from the concrete surface and rinse them with water.

In the process of concreting, traces of the movement of the formwork and small shells on the outer surface of the buildings to be concreted and inside the silos, bunkers and premises, immediately after the concrete comes out of the formwork, are rubbed with a 1: 2 cement mortar.

d. Mixing supply

Matting or tarpaulins are attached to the lower edges of the formwork to protect the fresh concrete from drying out (hypothermia), and in the summer it is regularly watered with water using an annular pipeline.

Window and door blocks in buildings and structures are installed in place during the movement of the formwork, for which they are pre-prepared (antiseptic, sheathed with tar paper) in accordance with the requirements of the project. To reduce the gaps between the walls of the formwork and the box of the block to 10 mm, slats are sewn to the box, which are subsequently removed. The fittings around the block are installed in accordance with the project.

Placement of concrete near the installed blocks is carried out simultaneously from both sides. After the formwork rises above the installed blocks, the temporary slats are removed.

Tower cranes, mine hoists, self-lifting cranes are used to supply the formwork with concrete mix, reinforcement, jack rods and other goods.

Concrete pumps and pneumatic blowers are also used to supply the mixture. At the end of the construction of the structure, the sliding formwork and all structures and equipment attached to it are dismantled in the order in which, after removal separate parts the stability and safety of the remaining elements is ensured.

The channels, in concrete, formed during the movement of the protective tubes, after removing the jack rods must be carefully sealed.

e. Prefabricated slabs

When erecting structures in winter conditions, concrete is heated in specially constructed greenhouses above the working floor and on external scaffolds using steam or electric heaters or infrared radiation.

Slabs of multi-storey floors, staircases and landings are concreted using additional inventory formwork or assembled from prefabricated elements. In the latter case, in the process of erecting a building or structure, the need for alterations and additional devices in the sliding formwork is eliminated.

Prefabricated ceilings can be erected with a tower crane after the walls have been erected with a "well" to the entire height of the building. In this case, the slabs rest on special inventory, removable brackets fixed to the walls slightly below the row of small openings in the wall. Reinforcing bars are passed through the openings, which are joined with the outlets from the floor slabs. Docking of the outer walls to the floor slabs is carried out with the help of strips in the walls. This technology ensures the continuity of concreting, fast and quality construction walls.

Monolithic ceilings can be concreted after the walls of the building have been erected with a "well". Inventory formwork panels and supporting devices (metal telescopic racks and sliding beams) are transferred from floor to floor by a tower crane or manually.

Monolithic floors can also be concreted using a descending suspended formwork mounted on a special platform. This method is especially effective if concrete pumps or pneumatic blowers are used to supply the concrete mixture.

f. Floor concreting

Concreting of ceilings with a 1-2 storey lag behind the concreting of walls. The process of erecting buildings is complicated by the need for frequent stops when lifting the sliding formwork.

The method of combined cyclic concreting of walls and floors is that the concreting of walls in a sliding formwork stops each time at the level of the next floor. The empty wall formwork is brought out above this mark so that a gap remains between the bottom of the sliding formwork and the level of the bottom of the slab, equal to the thickness of the future slab. In this case, the formwork panels of the outer walls, as well as the formwork that form inner surface elevator shafts and other cells that do not have overlaps are made higher in height than the shields of the rest of the formwork. The slabs are concreted in the panel or sectional formwork with the panels of the working floor removed after stopping and aligning the sliding formwork.

The erection of buildings and structures with a height of 40-50 m in monolithic reinforced concrete by the sliding formwork method according to the main technical and economic indicators is at the level of construction from prefabricated reinforced concrete structures, and the construction of high-rise civil buildings has a number of advantages: reduction in construction duration; reduction of labor intensity and estimated cost of construction, including by reducing specific capital investments in the base of the construction industry; increasing the reliability, durability and rigidity of structures due to the solidity and absence of joints, which is especially valuable during construction in seismic regions, on mine workings and subsiding soils.

g. Construction of high-rise buildings

In recent years, our country has developed and implemented new way erection of high-rise structures made of monolithic reinforced concrete in a sliding formwork of a rodless system consisting of hydraulic or pneumatic support-lifting devices that provide reliable support by compressing the erected part of the walls with special grippers and creating supporting friction forces.

Based on the proposals of Donetsk PromstroyNIIproekt, a pilot production model of a movable formwork was created, consisting of two (lower and upper) supporting-lifting sections of walking action with support on the walls of the structure being erected, electromechanical worm-screw lifts, forms of sliding formwork and frames for fastening. With the help of this formwork, the towers of the transport galleries of the blast furnace ore warehouse were erected at the construction site of the Zaporozhye Iron Ore Combine.

The towers to be erected have an outer diameter of 6 m and a height of 14 m, and the thickness of the walls is 300 mm. The construction of one tower was carried out by a team of five people. The average speed of concreting reached 0.3 m / h at the value of the machine speed of lifting the formwork in the process of laying and compacting the concrete mixture of 0.6. m / h. In this case, the lower section of the lifting device rested on concrete of 10-12 hours strength. The step of the lifting sections of 2 m allowed continuous concreting for 6-6.5 hours.

h. Climbing formwork

Climbing formwork is used in the construction of structures of variable cross-section in height, including chimneys, hyperbolic cooling towers, television towers and other tall objects. The main element of this formwork is a mine hoist with a working platform, to which a set of movable external and internal formwork is attached.

The design of the lift allows you to periodically build it up from above or grow it up from below. After each cycle of installing the formwork panels, reinforcement and placing the concrete mixture, the next lifting of the working platform and the rearrangement of the formwork are carried out.

Formwork for chimneys up to 320 m high consists of external and internal panels, bearing rings, framing (support) frame, radial movement mechanisms, a work platform, suspended scaffolding, as well as a rack-mounted mine hoist with a lifting head assembled from 2.5-meter tubular sections and equipped with a cargo cage and a cargo-passenger elevator.

The lifting head, installed on a hoist with a lifting capacity of 25 and 50 tons, rises at a speed of up to 3 mm / s when the formwork is moved to the next tier. The working step of lifting the formwork is 2.5 m.

i. Concreting the pipe barrel

The formwork consists of two shells - outer and inner, which are assembled from panels made of 2 mm thick sheet steel, bolted together.

The external formwork of chimneys consists of rectangular and trapezoidal panels with a height of 2.5 m. The combination of these panels will make it possible to obtain a tapered surface of the chimney.

The outer formwork is suspended from the bearing ring, which, when the pipe perimeter decreases, is replaced with a new one of a smaller diameter.

For the convenience of placing concrete, the internal formwork is assembled from panels of 1250x550 mm in size.

Concreting the pipe barrel: work organization diagram; development of the external climbing formwork of the conical chimney; rectangular panels; trapezoidal panels; c - panel of the inner shell of the formwork; covered canopy; protective overlap; mine hoist; lining platform; clip; working platform; dispensing hopper; cargo cage bucket; lifting head; cargo-passenger elevator; telpher; cargo cage; Crane beam; strip pad; strip steel lugs; steel strips; steel sheet 2 mm thick.

To stiffen the panels, overlays are welded to their upper and lower edges, with the help of which the panels are assembled in height. On the outside of the panels, lugs are welded into which reinforcing bars 10-14 mm are laid, forming a row of elastic horizontal rings.

j. Erection of cooling tower shells

Shields are installed in two (sometimes three) tiers. The formwork of the second tier is installed after placing the concrete in the formwork of the first tier. 8-12 hours after placing the concrete in the second tier, the outer formwork is removed and installed in the next highest position. After installing the reinforcement of the third tier, the lower tier of the inner formwork is removed and rearranged higher. Then the cycle repeats. Reinforcement is installed manually with separate rods.

The concrete mix is fed by the bucket of the cargo cage into the receiving hopper located on the working site, then into the movable hopper of the concrete paver and from there - along the trunk into the formwork. The concrete mix is compacted with deep vibrators with a flexible shaft.

The rate of concreting of chimney shafts at an outside air temperature of 15-20 ° C reaches 1-1.5 m / day.

The erection of the cooling tower shells is carried out with the help of a unit, which is a lattice (expandable) tower, on the swivel head of which rotating booms are mounted, to which the climbing formwork panels are attached, as well as working cradles.

Concrete mix is supplied to the upper platform of the cradle in a vibrating bowl by a hoist moving along the boom. Concreting is carried out in tiers by analogy with concreting chimneys.

2. Methods for concreting structures

a. Slipform concreting

Special methods of concreting structures. Concreting in sliding formwork is used in the construction of chimney walls, working towers of elevators and silos, headframes, water towers, as well as frames of multi-storey buildings. Structural elements buildings and structures erected in sliding formwork must be vertical, which is dictated by the main feature of sliding formwork.

The method of concreting monolithic reinforced concrete buildings and structures in sliding formwork is a highly organized and complex-mechanized, flow-speed construction process. Formwork device, reinforcement, laying and compacting of the concrete mixture, concrete stripping are carried out overlapping and continuously in the process of formwork lifting (SNiP N1-V.1-70).

Sliding formwork includes: formwork panels, jack frames, working floor with a canopy along the outer contour of the formwork, suspended scaffolding, formwork lifting equipment.

Formwork panels are made with an inventory height of 1100-1200 mm from following materials: steel sheet not less than 1.5 mm thick; planed wooden planks not less than 22 mm thick; waterproof plywood 8 mm thick; 7 mm bakelized plywood or 3 mm fiberglass. In some cases, wood-metal panels are made, in which the frame is made of rolled steel profiles, and the sheathing is made of planed boards or plywood. Circles for fastening formwork panels are usually made of rolled steel profiles.

b. Construction of atypical structures

Metal formwork panels are used in the construction of a number of structures of the same type (silos, chimneys, tanks), when the side walls perceive the high pressure of the freshly laid concrete mixture and, in addition, multiple turnover of the formwork panels is ensured.

Wooden and wood-metal shields have less rigidity and turnover, but at the same time, less cost compared to metal ones. They are used in the construction of residential and civil buildings, where the wall thickness does not exceed 200 mm, as well as in dry and hot climates to protect concrete from overheating.

Formwork panels made of waterproof plywood and fiberglass are promising. They are strong and lighter than shields made of other materials, but still more expensive than them.

Non-inventory wooden formwork is used for the construction of atypical structures. By design inventory shields sliding formwork is used in two types: large-block and small-block.

In large-block shields, metal circles are rigidly fastened to the skin. These shields are strong, durable and relatively easy to assemble.

In small-block panels, only metal circles are rigidly connected to each other, forming a frame of the walls, and formwork panels are hung on the circles without fastening to each other.

3. Concreting of foundations and floors

a. Concrete preparations

Concrete floors and foundations (preparation) are widely used in industrial and civil buildings.

Concrete preparations are arranged mainly in one-storey industrial workshops for cement and asphalt floors, floors made of cast iron slabs, end wooden blocks and other types of floors with a thickness of 100-300 mm on prepared and leveled soil. For concrete substrates, rigid concrete grades 100, 200 and 300 are usually used.

Concrete and cement-sand floor coverings are made up to 40 mm thick from concrete or mortar according to preparation. In multi-storey buildings, the base is usually reinforced concrete floors.

The scope of work on the construction of single-layer concrete floors in one-story buildings includes: preparation of soil foundations; installation of lighthouse boards; reception, leveling of concrete mix; surface grouting or ironing.

Before the start of the concrete preparation, all underground work on the construction of foundations, canals, tunnels, etc. should be completed, backfilling of the pits sinuses, leveling and compaction of the soil should be completed.

Preparation of the soil base. With dense soils, the concrete mixture is laid directly on the graded soil. Bulk and disturbed soils in the foundations must be compacted in a mechanized way. In places inaccessible to compaction mechanisms, the thickness of the soil layer compacted by hand rammers should not exceed 0.1 m.

b. Floor concreting techniques

Soils subject to significant settlement are replaced or strengthened. In the latter case, the concrete preparation is reinforced with a mesh.

Into the surface of the base weak soils before laying concrete preparation on it, a layer of crushed stone or gravel with a thickness of 60-150 mm is rammed or rolled by rollers. Before installing floors on clay, loamy and dusty soils saturated with water, it is necessary to lower the level groundwater and dry the base until the design load capacity is restored. On heaving soils, flooring should be carried out in accordance with the instructions of the project.

It is prohibited to plan and compact soil mixed with frozen soil, as well as snow and ice. It is also not allowed to install concrete floors on frozen soils.

Techniques for concreting floors and foundations. Before concreting, lighthouse boards are installed on the level so that their upper edge is at the level of the surface of the concrete preparation (Fig. 14, a). The distance between the boards depends on the length of the vibrating screed and is usually 3-4 m. The lighthouse boards are fixed with wooden stakes driven into the ground. The floors and bases are concreted in strips every other, starting from the places farthest from the passage.

c. Concreting preparations

The intermediate strips are concreted after the concrete of the adjacent strips has hardened. Before concreting the intermediate strips, the lighthouse boards are removed. The length of the stripes is taken as long as possible. A layer of concrete mixture in preparation before leveling and compaction should exceed the level of the lighthouse boards by 2-3 cm.

The concrete mix is compacted with a vibrating screed, which is a metal beam (channel, I-beam), on which one or two electric motors are attached from a surface vibrator.

When concreting preparations and floor coverings, each vibrated section should be covered by a vibrating screed, respectively, by 150 mm and by half of its width.

Techniques for concreting floors and foundations: scheme of concreting the base for floors; hand tools for smoothing concrete surfaces; laid base; preparation for the base; stakes; side formwork; scraper with rubber band for removing laitance laitance; trowel; half-scraper; ironing board; rubber band.

Depending on the conditions of the work, the concrete mixture is poured by the concrete pavers into the foundations in two ways: “from oneself”, when the unit moves behind the concreting front, and the concrete in the area of the unit has time to gain the strength necessary for its movement, and “on itself”, when the mechanism moves in front of the concreting front, since the concrete does not have time to gain the required strength.

d. Concrete mix production

The first method is preferable, since it creates a wide scope of work for preparing the base. In the second method, the preparatory work is ahead of the laying of the concrete mixture by one plot, the length of which is equal to the radius of the mechanism.

In unheated rooms in a concrete preparation, longitudinal and after 9-12 m along the length of the strips, transverse temperature-shrinkage joints are arranged every two strips, which break the concreted area into separate slabs with dimensions of 6X9-9X12 m.

Longitudinal seams are made by installing planed boards coated with hot bitumen, or boards wrapped in tar paper. After the concrete has set, the boards are removed and the seams are filled with bitumen. The seams are also arranged by coating the side edges of the strips with bitumen with a layer of 1.5-2.0 mm before placing the concrete mixture into adjacent spaces.

For the formation of transverse expansion joints(half seams) metal strips 60-180 wide and 5-7 mm thick are used, which, during the concreting process, are laid in the preparation for 7th of their width and then removed after 30-40 minutes. The formed recesses after the final hardening of the concrete are cleaned and filled with grade III bitumen or cement mortar.

e. Surface of concrete foundations

In places where there is a break in the concreting of foundations and floors, it is not allowed to install a vibrating screed at the edge of the laid layer, as this will result in slumping and delamination of the concrete mixture. So at the end work shift in the places of the planned break in concreting, a partition of boards is installed and the last portion of the concrete mixture is leveled and vibrated along it.

The surface of concrete bases before laying on it continuous floor coverings on a cement binder or from piece materials on a cement-sand mortar must be cleaned of debris and cement film.

V early age concrete, mechanical steel brushes are used for this purpose. With a high strength of concrete, using a pneumatic tool, grooves with a depth of 5-8 mm are applied to its surface every 30-50 mm. This results in a rough surface of the sub-base and better adhesion to the sub-base.

Concrete or cement-sand floor coverings consist of a 20-40 mm layer of concrete or mortar and are concreted similarly to preparation in strips 2-3 m wide every one.

Before concreting the coating, lighthouse wooden slats or metal framing corners are fixed on the surface of the concrete base. The concrete mix is compacted with vibrating screeds, and the concrete surface is leveled with wooden slats moved across the strip.

f. Cement milk

Cement laitance that has come out from the compaction of concrete substrates and floor coverings is removed with a rubber band scraper.

For small volumes of work, the surface of the concrete floor is finally finished with an ironing board or rubberized tarpaulin tape, the length of which should be 1-1.5 m longer than the width of the concreted strip. The ends of the tape are attached to rollers that serve as handles, the width of the tape is 300-400 mm. Smooth the compacted concrete mix 25-30 minutes after laying. When the tape is moved alternately across and along the strip, the protruding thin film of water is removed from the concrete surface and the concrete floor is pre-smoothed. The final leveling of the surface is carried out after 15-20 minutes with shorter strokes of the tape.

To give the concrete floor a high abrasion resistance, its surface is treated with a metal trowel approximately 30 minutes after the final leveling, exposing the grains of crushed stone. If high abrasion resistance is not required, then a cement floor from the solution is arranged on the concrete preparation.

If it is necessary to install a two-layer floor at once, the lower layer is first laid between the beacon boards and compacted with a platform vibrator or an obliquely installed vibrating screed, then with a break of no more than 1.5-2 hours (for better connection of the lower layer with the upper one), a clean floor is made.

e. Concrete surface ironing

With large volumes of work, the surface of a clean concrete floor in the initial period of hardening is rubbed with a SO-64 (or OM-700) machine, consisting of a trowel disk with a diameter of 600 mm, an electric motor and a control handle. Rotating at 140 rpm, the trowel disc smooths and smoothes the concrete floor. Machine productivity 30 m2 / h.

Reinforcement of the concrete surface is used to give the floor an increased density. It consists in the fact that dry and sifted cement is rubbed into the surface of wet concrete until an even shine appears on it. Dry concrete surfaces are moistened with water before ironing. Ironing can be done manually with steel trowels or with a SO-64 trowel.

A variety of concrete floors are mosaic, made from a mixture that includes: white or colored Portland cement, marble, granite or basalt chips and mineral dye. A mosaic layer with a thickness of 1.5-2 cm is laid, as a rule, on an underlying layer of cement mortar of approximately the same thickness. The limitation of single-color fields and the implementation of the patterns provided for by the project is carried out using strips-veins made of glass, copper or brass, embedded in the underlying layer of the solution. These strips are exposed in such a way that their upper ribs serve as beacons when laying and leveling the mosaic layer.

The surfaces of mosaic floors are finished with electric machines after the concrete has hardened (after 2-3 or more days). After the first sanding, the flaws found on the floor surface are putty with painted cement-sand mortar. Then the floor is sanded with finer abrasives, treated with polishing powders and polished using a polishing machine.

4. Concreting of columns

a. Formwork for rectangular columns

Columns as an element of the frame of buildings and structures are of rectangular, polygonal and circular cross-section. The height of the columns reaches 6-8 m and more.

The formwork of rectangular columns is a box of two pairs of panels (wooden, metal or combined). The lateral pressure of the concrete mix is perceived by the clamps that clamp the box. Clamps are made of inventory metal with a high turnover of the formwork and wooden - with a low number of revolutions. The holes in the strips of the metal clamp for the fastening wedges allow them to be used for columns of different cross-sections. To clean the box, a temporary hole is made in the lower part of one of the shields. For concreting the columns, block forms are also used.

Typical unified shields and formwork panels are attached to the reinforcement blocks with tie bolts and pulled together with straps. The formwork of low columns is fixed in two mutually perpendicular directions by inclined braces (braces). With a column height of more than 6 m, the formwork boxes are attached to specially arranged scaffolding.

After installing the formwork of the column, every 2-3 m in height, holes of 500x500 mm are arranged and working platforms for the production of concrete work. The formwork of high columns can be mounted only from three sides, and from the fourth, it can be increased during the concreting process.

b. Column concreting

For columns of circular cross-section, special metal block-forms are made.

Compliance with the thickness of the protective layer in the columns is ensured by special cement gaskets, which, before concreting, are attached to the reinforcement rods with a knitting wire embedded in the gaskets during their manufacture.

Concreting of columns with transverse dimensions from 400 to 800 mm in the absence of crossing clamps is carried out from above without interruption in sections up to 5 m high.Columns with section sides less than 400 mm and columns of any section with intersecting clamps, which contribute to the stratification of the concrete mixture when it falls, are concreted from the side sections with a height of no more than 2 m.

Column formwork: assembled box; inventory metal clamp; wooden clamp on wedges; detail of the wooden clamp assembly; box; stock metal clamp; wedges holding the clamps; frame for column formwork; cleaning hole door; covering boards; holes for wedges embedded shields; persistent dies.

At a greater height of the sections of the columns, concreted without working seams, it is necessary to arrange breaks for the concrete mixture to settle. The duration of the break should be at least 40 minutes and not more than 2 hours.

c. Frame constructions

In cases where the columns are part of the frame structure and above them, beams or purlins with dense reinforcement are located, it is allowed to first concrete the columns, and then, after installing the reinforcement, beams and purlins.

When concreting them from above, it is recommended to initially fill the lower part of the column formwork to a height of 100-200 mm with a cement mortar of the composition 1: 2-1 = 3 to prevent the accumulation of coarse aggregate without mortar at the base of the column. When a portion of the concrete mixture is dropped from above, large aggregate particles are embedded in this solution, forming a mixture of normal composition.

The concrete mix is compacted in the columns with internal vibrators with a flexible or rigid shaft. Sealing with external vibrators attached to the formwork of small columns is less effective and practically not used.

In order to avoid the formation of cavities in the process of concreting the columns (especially corners), it is very useful to tap wooden hammer outside at or slightly below the layer of concrete to be laid.

Concreting of columns in accordance with SNiP III-B.1-70 is carried out to the full height without working seams. It is allowed to arrange working seams: at the level of the top of the foundation, at the bottom of the girders and beams or crane consoles and the top of the crane beams.

d. Concreting of frame structures

In the columns of non-girder floors, it is allowed to arrange seams either at the very bottom of the columns, or at the bottom of the capitals. The capitals are concreted simultaneously with the floor slab.

The surface of the working seams, which are made during the laying of the concrete mixture at intervals, must be perpendicular to the axis of the columns to be concreted.

Concreting of frame structures should be carried out with a break between placing the concrete mixture in the columns (pillars) and crossbars of the frames. Working seams are arranged a few centimeters below or above the junction of the frame crossbar to the rack.

Walls (including partitions) are of constant and variable cross-section, vertical and inclined, in terms of round, curved, polygonal and straight.

When concreting walls and partitions, the following types of formwork are used: standard unified shields and panels of collapsible formwork, block forms, rolling climbing, sliding and sliding formwork.

Collapsible and adjustable small-panel formwork It is installed in two steps: first, on one side, to the entire height of the wall or partition, and after installing the reinforcement, on the other. If the wall thickness is more than 250 mm, special inventory forms are installed on the second side.

The height of the wall is installed on a fan, otherwise - in layers during the process of concreting. In the formwork installed to the full height of the wall, holes are provided for feeding the concrete mixture through them into the structure.

5. Wall concreting

a. Design wall thickness

Wall formwork up to 6 m high is mounted from mobile platforms or light scaffolds. At a higher height, forests are arranged. The wall formwork is fastened with struts or braces, tie bolts or wire ties.

To comply with the design wall thickness, concrete or wooden spacers are installed in the places where the screeds pass. The latter are removed during the concreting process.

The collapsible large-block formwork is installed in layers during the process of concreting the walls. This allows us to restrict ourselves to a set of formwork for only two tiers. All works full cycle Concreting of walls in this formwork is carried out in the following sequence: first, scaffolds (scaffolds) are installed or built up, then the working seam of the concreting is processed and reinforcement is installed, after which the formwork is rearranged from the lower tier to the upper one. The cycle of concreting one tier ends with the laying and compaction of the concrete mixture and subsequent curing of the concrete in the formwork.

Formwork block: fixing clamp No. 1; reinforced concrete tape; bedding; screw jack; formwork block; element of the fence for the 1st tier of concreting; formwork panel; fixing clamp No. 2; working floor; element of the fence for the 2nd tier of concreting; inventory insert; sliding rack; double wooden wedge.

b. Formwork block

Block-forms of formwork are used when concreting walls of considerable height and length, that is, when their multiple use is ensured. The block-form of the construction of the Kharkovorgtekhstroy trust consists of blocks, panels, additional and fastening elements.

The rigidity of the blocks is ensured by horizontal scrapes and support trusses, which also serve as scaffolds. For installation, alignment and dismantling of the formwork, the supporting trusses are equipped with jacking devices. The dimensions of the ordinary blocks are 3X8.3X2 and 1.5x3 m.

Rolling formwork structures of Donetsk PromstroyNIIproekt: trolley; Column; beam; shield lifting winch; formwork shield; clamps; ladder; sliders; clamping device; flooring; fencing; bunker.

The deck of blocks, panels and add-ons is assembled from small-sized boards made of 45X45x5 mm corners and 3 mm thick sheet steel. In the ribs of the frame of the panels there are holes with a diameter of 13 mm for attaching the panels to each other.

The assembled formwork blocks, if necessary, can be disassembled into separate panels. The block-form of the formwork is rearranged in layers during the concreting process. When concreting walls of constant and variable cross-section, rolling formwork is used (including horizontally moved on skids).

c. Construction of walls

Concreting of structures can be carried out in layers with continuous or cyclical movement of the formwork, as well as along the grips to the entire height of the wall. The rolling formwork of the structure of Donetsk PromstroyNIIproekt consists of two metal panels 6-8 m long and 1.3 m high. The frame of the panels is made of a corner, and the deck is made of sheet steel 6 mm thick. Formwork size 6700X X 5400X3900 mm, weight 800 kg. With the help of special devices - sliders - the shields are attached to the guide columns of the portal.

The portal columns at the bottom are supported by a trolley, and at the top they are connected by a beam, which allows the columns to be spread to the required width (up to 600 mm). The shields are moved perpendicular to the surface of the structure to be concreted with a screw device, and the lift is carried out on cables through fixed blocks fastened to the connecting beams. The movement of the formwork along the wall to be concreted is carried out using double-sided winches.

The construction of walls in sliding and climbing formwork is discussed below, among the special methods of erection of structures.

When concreting walls, the height of sections erected without interruption should not exceed 3 m, and for walls less than 15 cm thick - 2 m.

d. Concrete supply

With a greater height of the wall sections, concreted without working seams, it is necessary to arrange breaks lasting at least 40 minutes, but not more than 2 hours to settle the concrete mixture and prevent the formation of sedimentary cracks.

If there is a window or doorway in the wall to be concreted, the concreting should be interrupted at the level of the upper edge of the opening or, if possible, arrange a working seam in this place. Otherwise, sedimentary cracks are formed near the corners of the mold. When conveying a concrete mixture from a height of more than 2 m, link trunks are used.

The lower part of the wall formwork during concreting from above is first filled with a layer of cement mortar of the composition 112-1: 3 in order to avoid the formation of porous concrete with an accumulation of coarse aggregate at the base of the walls.

When concreting the walls of tanks for storing liquids, the concrete mixture should be laid continuously to the entire height in layers no more than 0.8 of the length of the working part of the vibrators. In exceptional cases, the formed working seams must be very carefully processed before concreting.

The walls of large tanks are allowed to be concreted with vertical sections, followed by processing and filling the vertical construction joints with concrete. The joints of the walls and bottom of the tanks are made in accordance with the working drawings.

6. Concreting beams, slabs, vaults

a. Concreting ribbed slabs

Concreting beams, slabs, vaults, arches and tunnels. Beams and slabs, floors are usually concreted in a collapsible formwork from standard unified shields and panels. Beams and girders are also concreted in block forms.

The ribbed slab formwork is made of small-piece wooden boards supported by wood-metal sliding racks at a height of up to 6 m and specially arranged scaffolding at a height of more than 6 m.

The formwork of the beam is made of three panels, one of which serves as the bottom, and the other two - as side rails of the surfaces. The side panels of the formwork are fixed at the bottom with clamping boards sewn to the head of the rack, and at the top with the formwork of the slab.

Concreting of ribbed slabs: general view of scaffolding and ribbed slab formwork; the location of the working seams when concreting ribbed floors in a direction parallel to the secondary beams; the same for the main beams; formwork of beams; slab formwork; circled; girder formwork; column formwork; sliding racks; pressure boards; coasters; frieze boards; slab formwork panels; circled; sub-kruzhny boards; side shields; bottom: head of the rack; working position of the joint (arrows show the direction of concreting).

b. Beamless slab formwork

The slabs of the formwork flooring are laid with an edge on the circles of the boards, which, in turn, rest on the sub-circle boards, nailed to the stitching strips of the side boards of the beam and supported by supports.

To fix the circles and side shields, fascia boards are laid along the perimeter of the slab, which also facilitate the stripping of the slab. With a beam height of more than 500 mm, the side panels of the formwork are additionally reinforced with wire ties and temporary struts.

The distance between the posts and the circles is determined by calculation. The supporting racks are fastened in mutually perpendicular directions with inventory straps or braces.

The formwork of a flat slab consists of a formwork for columns, capitals and a slab. The slab formwork consists of two types of panels, laid in circles between frieze boards sewn onto the tops of the posts. To support the circles, paired runs are arranged from boards resting on racks. The shields of the capitals rest on one side of the formwork of the columns, and along the outer contour they are supported by circles.

When installing the suspended formwork of floor slabs on prefabricated reinforced concrete or metal beams, metal suspension loops are arranged, laid out on the beams with a predetermined step. In these hinges, supercircular boards are installed, on which the circles and formwork boards of the slab rest.

c. Protective layer

Concreting of floors (beams, purlins and slabs) is usually carried out simultaneously. Beams, arches and similar structures with a height of more than 800 mm are concreted separately from the slabs, arranging working seams 2-3 cm below the level of the lower surface, and if there are haunches in the slab - at the level of the bottom of the slab haunch (SNiP Sh-V.1-70 ).

In order to prevent sedimentary cracks, the concreting of beams and slabs monolithically connected to the columns and walls should be made 1-2 hours after the concreting of these columns and walls.

The concrete mix is placed in beams and girders in horizontal layers, followed by compaction with vibrators with a flexible or rigid shaft - in powerful or weakly reinforced beams. In the floor slabs, the concrete mixture is laid along the lighthouse strips, which are installed on the formwork with the help of linings in rows every 1.5-2 m. After concreting, the strips are removed, and the formed recesses are smoothed out. In case of double reinforcement of floor slabs, the leveling and compaction of the concrete mixture is carried out from the movable flooring so as not to bend the upper reinforcement.

The floor slabs are concreted in the direction of the secondary beams. The protective layer in slabs, beams and purlins is formed with the help of special gaskets made of cement mortar or retainers. As the structures are concreted, the reinforcement is lightly shaken with the help of metal hooks, making sure that a protective layer required thickness.

d. Floor concreting

Concrete mix in slabs up to 250 mm thick with single reinforcement and up to 120 mm thick with double reinforcement is compacted by surface vibrators, in slabs of greater thickness - by deep ones.

When concreting flat seams, it is allowed to arrange construction joints anywhere parallel to the smaller side of the slab. In ribbed ceilings, when concreting parallel to the direction of the main beams, the working seam should be arranged within two middle quarters of the span of the girder and slabs, and when concreting parallel to the secondary beams, as well as individual beams, within the middle third of the span of the beams.

The surface of the construction joints to be made in beams and slabs must be perpendicular to the direction of concreting. Therefore, in the planned breaks in the concreting of the slabs, boards are installed on the edge, and in the beams - shields with holes for reinforcement.

Expansion joints in ceilings are arranged on the column consoles or by installing paired columns, ensuring free movement in the beams seam in horizontal plane on a metal backing sheet.

When concreting floors in multi-storey frame buildings, receiving platforms are arranged at the level of each floor, and conveyors and vibrating chutes are installed inside the building to supply the concrete mixture after lifting it with a crane to the place of laying.

e. Vaults and arches

In the process of concreting coatings, ceilings and individual beams, it is not allowed to load them with concentrated loads exceeding the permissible ones specified in the project for the production of works.

Vaults and arches of short length are concreted in a collapsible small-piece or large-panel formwork supported by racks. For concreting vaults and long arches, an inventory rolling formwork mounted on a trolley is used. On the lower part of the formwork, lifting and lowering circles are installed, carrying a two-layer sheathing, consisting of boards laid with a gap of 10 mm and waterproof plywood. The gap between the boards reduces the risk of the formwork being pinched in the vault when it swells. The lifting and lowering of the circles is done with the help of hoists and blocks, and the entire formwork is moved along the rails with the help of a winch.

Vaults and arches of a small span should be concreted without: interruptions simultaneously from both sides of the supports (heels) to the middle of the vault (castle), which ensures the preservation of the design form of the formwork. If there is a danger of bulging of the formwork at the vault lock during the concreting of the side parts, it is temporarily loaded.

Rolling vault formwork: cross section; lengthwise cut; tightening of the diaphragm arch; retractable racks; hand hoists.

7. The process of concreting complex structures

a. Massive arches and vaults

Long vaults are divided in length into limited concreting areas with working seams located perpendicular to the generatrix of the vault. Placement of concrete in limited areas is carried out in the same way as in vaults of short length, that is, symmetrically from the heels to the castle.

Massive arches and vaults with a span of more than 15 m are concreted in strips parallel to the longitudinal axis of the vault. The concrete mixture is laid in strips also symmetrically on both sides from the heels to the vault lock.

The gaps between the strips and sections of long vaults are left with a width of about 300-500 mm and are concreted with a rigid concrete mixture 5-7 days after the end of concreting of the strips and sections, that is, when the main concrete is placed.

With steep arches, the sections at the supports are concreted in a double-sided formwork, and the second (upper) formwork is installed with separate shields during concreting.

The concrete mix is compacted in massive arches and vaults by internal vibrators with a flexible or rigid shaft, depending on the degree of reinforcement, in thin-walled vaults - by surface vibrators. Tightening of vaults and arches with tensioning devices should be concreted after tightening these devices and uncoiling the coverings. Rigid tightening without tensioning devices is allowed to be concreted simultaneously with the concreting of the pavement.

b. Tunnels and pipes

Tunnels and pipes are concreted in open trenches and underground in a collapsible and rolling mobile formwork. The movable wooden formwork of a passage tunnel with a curvilinear outline with a cross-section of up to 3 m consists of shields in the form of curvilinear circles, sheathed planed boards, waterproof plywood or sheet steel over the boardwalk. Racks supporting the working floor are sewn to the circles of the outer shields. The inner formwork consists of two panels, the bottom of which is supported by paired wedges, and the top is bolted into the vault lock.

External and internal formwork are connected with tie bolts. The length of the boards is usually taken equal to 3 m, the mass of the formwork reaches 1.5 tons. The outer and inner formwork is moved with a winch along wooden guides. The outer formwork can also be moved to a new location with a crane. Rolling timber formwork by Ing. V. B. Oak for concreting tunnels and collectors rectangular section consists of sections 3.2 m long.

The internal formwork section consists of four U-shaped steel frames, sheathed with planed boards, plywood or sheet steel. Each frame consists of two side struts and two: semi-rails, connected by three hinges. The outer frames of the formwork section have one sliding rack in the middle of the pipes, which are pulled together by screw jacks. The frames are supported by means of middle racks and retractable horizontal beams on a trolley moving along a rail track.

c. Tunnel vaults

The external formwork section consists of five frames with struts and split ledgers. Frame stands with inside are sheathed with boards. The outer formwork is fastened with the inner bolts passed through the removable purlins. The formwork allows concreting tunnels with a width of 2100-2800 mm and a height of 1800-2200 mm: The mass of one section of the formwork reaches 3 tons.

The outer formwork is usually repositioned with a crane. When stripping the formwork, the tie bolts are removed, the joints of the girders are disconnected: the frames of the outer formwork, after which the formwork is removed. To remove the internal formwork using the jacking devices available in the outer racks, the semi-rails with ceiling shields are lowered.

Concreting of tunnels is carried out, as a rule, in two stages: first, the bottom, and then the walls and ceilings (roof) of the tunnel.

The vaults of tunnel structures are concreted simultaneously from both sides from the heels to the castle in radial layers. The castle is concreted in inclined layers along the arch of the arch, while the formwork is laid as concreting in short sections - from circling to circling.

In strong vaults of tunnel structures, the working seams to be arranged must be radial. The required direction of the joint surfaces is ensured by the installation of formwork: shields. Before concreting the castle, the cement film from the surface: the concrete must be removed.

d. Tunnel finishes

It is advisable to concrete tunnel finishes in parallel with the tunneling, since in this case the overall duration of the tunnel construction is reduced. However, with small dimensions of the cross-section of the tunnel, due to the constrained conditions, the finishing is erected at the end of the penetration of the entire tunnel or individual sections between the intermediate faces.

Tunnel finishing is concreted either continuously over the entire cross-section of the mine, or in parts in the following sequence: tunnel tray, vault and walls, or vice versa.

For the formwork, the concrete mixture is fed from the end or through the hatches in the formwork using concrete pumps or pneumatic blowers. The concrete mix can also be fed into the side walls and the tunnel chute using tilting trolleys using distribution chutes.

The concrete mix is compacted layer by layer with deep vibrators through the windows in the formwork or with external vibrators attached to the formwork.

If the walls of the finishing of the tunnel are concreted after the vault (the "supported vault" method), then before concreting the formwork is removed from the lower surface of the vault heels and the surface is thoroughly cleaned. The walls are concreted in horizontal layers with the simultaneous build-up of the formwork to a mark less than the mark of the bottom of the vault heel by up to 400 mm. The space between the fifth vault and the adjoining wall is filled with a hard concrete mix and carefully compacted. Tubes are preliminarily laid at the junction for the subsequent injection of cement mortar.

Technician candidates Sciences Ya.P. BONDAR (TsNIIEP dwellings) Yu.S. OSTRINSKY (NIIES)

To find ways of concreting in sliding formwork of walls with a thickness of less than 12-15 ohms, the forces of interaction between the formwork and concrete mixtures prepared on dense aggregates, expanded clay and slag pumice were studied. With the existing technology of concreting in slip formwork, this is minimal. permissible thickness walls. For molded concrete, we used expanded clay gravel from the Beskudnikovsky plant with crushed sand from the same expanded clay and slag pumice made from the melts of the Novo-Lipetsk metallurgical plant with fishing line obtained by crushing slag pumice.

Expanded clay concrete grade 100 had vibration compaction, measured on N. Ya. Spivak's device, 12-15 s; structural factor 0.45; bulk density 1170 kg / m3. Slagopemzobetoi grade 200 had a vibration compaction of 15-20 s, a structural factor of 0.5, and a bulk density of 2170 kg / m3. Heavy concrete grade 200 at bulk density 2400 kg / m3 was characterized by a standard cone draft of 7 cm.

The forces of interaction of the sliding formwork with concrete mixtures were measured on a test setup, which is a modification of the Kazaranda device for measuring single-plane shear forces. The installation is made in the form of a horizontal tray filled with concrete mixture. Test strips made of wooden blocks sheathed along the surface of contact with the concrete mixture with strips of roofing steel were laid across the tray. Thus, the test battens simulated a steel sliding formwork. The slats were kept on a concrete mixture under weights of various sizes, simulating the pressure of concrete on the formwork, after which the efforts were recorded, causing the horizontal movement of the slats on the concrete. General form installation is given in Fig. 1.

Based on the results of the tests carried out, the dependence of the interaction forces between the steel sliding formwork and the concrete mix t on the value of the concrete pressure on the formwork a (Fig. 2), which is linear, was obtained. The angle of inclination of the graph line with respect to the abscissa axis characterizes the angle of friction of the formwork on concrete, which makes it possible to calculate the friction forces. The value cut off by the graph line on the ordinate axis characterizes the adhesion forces of the concrete mixture and the formwork m, which do not depend on pressure. The angle of friction of the formwork on concrete does not change with an increase in the duration of stationary contact from 15 to 60 minutes, the magnitude of the adhesion forces increases by a factor of 1.5-2. The main increase in adhesion forces occurs during the first 30-40 minutes with a rapid decrease in the increment over the next 50-60 minutes.

The bond strength of heavy concrete and steel formwork 15 minutes after the compaction of the mixture does not exceed 2.5 g / ohm2, or 25 kg / m2 of the contact surface. This is 15-20% of the generally accepted value of the total force of interaction between heavy concrete and steel formwork (120-150 kg / m2). The main part of the effort falls on the share of friction forces.

The slow growth of adhesion forces during the first 1.5 hours after concrete compaction is explained by the small number of new formations during the setting of the concrete mixture. According to research, in the period from the beginning to the end of the setting of the concrete mixture, there is a redistribution of mixing water in it between the binder and aggregates. Neoplasms develop mainly after the end of setting. The rapid growth of the adhesion of the sliding formwork to the concrete mixture begins 2-2.5 hours after the concrete mixture has been compacted.

Specific gravity adhesion forces in the total value of the interaction forces of heavy concrete and steel sliding formwork is about 35%. The main part of the efforts falls on the friction forces determined by the mixture pressure, which changes over time under concreting conditions. To test this assumption, the shrinkage or swelling of freshly molded concrete specimens was measured immediately after vibration compaction. During the molding of concrete cubes with an edge size of 150 mm, a textolite plate was placed on one of its vertical edges, the smooth surface of which was in the same plane with the vertical edge. After the concrete was compacted and the sample was removed from the vibrating table, the vertical sides of the cube were freed from the side walls of the mold, and the distance between the opposite vertical sides was measured using a tool for 60-70 Min. The measurement results showed that freshly formed concrete immediately after compaction shrinks, the value of which is the higher, the greater the mobility of the mixture. The total value of the bilateral settlement reaches 0.6 mm, i.e., 0.4% of the specimen thickness. In the initial period after molding, swelling of freshly laid concrete does not occur. This is due to contraction at the initial stage of concrete setting in the process of water redistribution, accompanied by the formation of hydrate films that create large surface tension forces.

The principle of operation of this device is similar to the principle of operation of a conical plastometer. However, the wedge-shaped shape of the indenter makes it possible to use the design scheme of a viscous-flowing mass. The results of experiments with a wedge-shaped indenter showed that To varies from 37 to 120 g / cm2, depending on the type of concrete.

Analytical calculations of the pressure of a layer of concrete mixture with a thickness of 25 ohms in the sliding formwork showed that the mixtures of the accepted compositions after compaction by vibration do not exert active pressure on the formwork plating. The pressure in the system "sliding formwork - concrete mix" is due to the elastic deformations of the panels under the influence of the hydrostatic head of the mix in the process of its compaction by vibration.

The interaction of sliding formwork panels and compacted concrete at the stage of their joint operation is sufficiently well modeled by passive resistance of a viscoplastic body under the influence of pressure from the side of the vertical retaining wall. Calculations have shown that with the one-sided action of the shuttering board on the concrete mass), to displace a part of the massif along the main sliding planes, an increase in pressure is required, which significantly exceeds the pressure that occurs with the most unfavorable combination of conditions for laying and compaction of the mixture. With double-sided pressing of the shuttering panels on a vertical-layer of concrete of limited thickness, the pressing forces required to displace the compacted concrete to the main sliding planes acquire the opposite sign and significantly exceed the pressure required to change the compression characteristics of the mixture. Re-loosening of the compacted mixture under the action of double-sided compression requires such a high pressure that is unattainable with concreting in slip formwork.

Thus, the concrete mixture, laid according to the rules of concreting in a sliding formwork in layers 25-30 cm thick, does not exert pressure on the formwork panels and is able to perceive from their side the elastic pressure arising from the vibration compaction process.

To determine the interaction forces arising during the concreting process, measurements were carried out on a full-size sliding formwork model. A sensor with a high-strength phosphorous bronze membrane was installed in the molding cavity. The pressures and forces on the lifting rods in the static position of the installation were measured with an automatic pressure meter (AID-6M) in the process of vibrating and lifting the formwork using an N-700 photooscilloscope with an 8-ANCh amplifier. The actual characteristics of the interaction of steel sliding formwork with various types of concrete are shown in the table.

In the period between the end of vibration and the first raising of the formwork, a spontaneous decrease in pressure occurred. which was held unchanged until the formwork began to move upward. This is due to the intense shrinkage of the freshly formed mixture.

To reduce the forces of interaction of the sliding formwork with the concrete mixture, it is necessary to reduce or completely eliminate the pressure between the formwork panels and the compacted concrete. This problem is solved by the proposed concreting technology using intermediate removable shields ("liners") from a thin (up to 2 mm) sheet material... The height of the liners is greater than the height of the molding cavity (30-35 ohm). The liners are installed in the molding cavity close to the sliding formwork panels (Fig. 5) and immediately after the concrete is laid and compacted, the concrete is removed one by one from it.

The gap (2 mm) remaining between the concrete and the formwork, after removing the shields, protects the formwork shield straightening after elastic deflection (usually not exceeding 1-1.5 mm) from contact with vertical surface concrete. Therefore, the vertical edges of the walls, freed from the liners, retain the shape given to them. This allows thin walls to be concreted in slip formwork.

The fundamental possibility of forming thin walls with the help of liners was tested when erecting natural fragments of walls 7 cm thick, made of expanded clay concrete, slag concrete and heavy concrete. The results of trial moldings showed that lightweight concrete mixtures better correspond to the features of the proposed technology than mixtures based on dense aggregates. This is due to the high sorption properties of porous aggregates, as well as the solid structure of lightweight concrete and the presence of a hydraulically active dispersed component in light sand.

Heavy concrete (albeit to a lesser extent) also exhibits the ability to maintain the verticality of freshly formed surfaces with its mobility of no more than 8 cm.When concreting civil buildings with thin walls and partitions inside the apartment, using the proposed technology, two to four pairs of liners with a length of 1.2 to 1.6 m, providing concreting of walls with a length of 150-200 m. This will significantly reduce the consumption of concrete in comparison with buildings erected according to the adopted technology, and increase the economic efficiency of their construction.

Hello dear readers! Master Vadim Alexandrovich answers all our and your questions today. Today we will talk about the features of pouring concrete into the formwork.

Hello Vadim Alexandrovich!

Hello! First of all, I want to say that this work is quite difficult and very responsible, and the pouring of the floors and load-bearing walls it is better to entrust it to professionals than to try to do it yourself. Let's get down to your questions.

1. Do I need to somehow prepare the formwork and reinforcement?

The formwork is lubricated with a special water-based lubricant (Emulsol) in order to separate the formwork from the hardened concrete. Although at the construction site there were cases when they poured it into an unlubricated formwork and then tore it off. Also, the formwork is pulled together with special ties, which are inserted into the tubes between the panels.

2. Is the method of filling horizontal shapes different from vertical ones?

Practically the same. Vertical ones are a little more difficult to tamp.

3. Please tell us how concrete should be poured.

The method of pouring is determined by the project (TKP) It is desirable to pour the entire formwork at once, pouring in layers is undesirable, otherwise you will have to make notches with a perforator for better adhesion of the layers. Be sure to fill in the vertical forms entirely.

4. How to connect layers if we fill them with layers? Well, we didn’t have enough concrete for pouring it entirely.

As I said, we make notches with a perforator on the hardened concrete.

5. What are the secrets of even pouring?

There are no secrets, there are general rules: We fill in different places and not in one, scatter with shovels over the entire shape, then we tamp with a vibrator to a smooth glossy surface in order to remove all voids and the concrete evenly filled the formwork. However, if the concrete is of poor quality, but it really needs to be poured, then you cannot use the vibrator - all the water will flow out and the concrete will not grab. In this case, you just need to knock on the formwork. But try to avoid such cases - build for yourself.

6. How does the density of the mortar affect the filling?

A thick solution is difficult to evenly distribute and tamp. Before pouring, add water to the mixer. Too liquid - and again bad, when tamping all the water will flow out and the concrete will not grab. If we do it ourselves, then we add cement and sand, if we are delivered ready-made, then we send it to the plant due to a discrepancy.

7. I've heard that concrete heats up when it cures. Is this a problem and should we fight it?

Yes, this is a problem and must be dealt with. In the heat, it is imperative to water the formwork with cold water, otherwise the concrete will crack. And in the cold, on the contrary, we warm it up.

8. If we didn't keep track and the concrete cracked, how to fix it?

Small cracks are acceptable maximum size cracks indicated in project documentation, if the size is exceeded, then we take a jackhammer and beat it off. Otherwise, it will fall apart after a while. After all, cracks significantly reduce the strength of the structure.

Thank you very much for the consultation, Vadim Alexandrovich. We and our readers are very grateful to you.