a. Fill the formwork with concrete

For concreting structures in the sliding formwork, concrete mixes are used on portland cement of at least 400 grade with the beginning of setting no earlier than 3 hours and the end of setting no later than 6 hours. Based on the cement test data, the speed of concreting and lifting of the sliding formwork should be determined.

The draft of the cone of the concrete mix used should be: when compacted with a 6-8 vibrator and manual compaction of 8-10 cm, and В / Ц - no more than 0.5. The grain size of coarse aggregate should be no more than / 6 of the smallest cross-sectional size of the concreted structure, and for densely reinforced structures - no more than 20 mm.

The thickness of the walls and beams erected in the sliding formwork, as a rule, should not be less than 150 mm (the weight of the concrete should be greater than the friction forces), and the volume of the concrete per 1 running. m of their height should not exceed 60 m3.

Initially, the formwork is filled with a concrete mixture of two or three layers to a height equal to half of the formwork, in the continuation of no more than 3. b h. The second and third layers are laid only after the previous layer has been laid around the entire perimeter of the formwork. Further filling of the formwork is resumed only after the beginning of its rise and ends no later than 6 hours.

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

b. Mixture compaction process

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

In the process of concreting, the upper level of the mix must be below the top of the formwork panels by more than 50 mm.

Concrete is compacted with rod vibrators with a flexible shaft or manually - shurovki. The diameter of the vibrator tip should be 35 mm with a wall thickness of up to 200 mm and 50 mm with a greater thickness.

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

The rate of laying the concrete mix and raising the formwork should exclude the possibility of adhesion of the laid concrete to the formwork and ensure the strength of the concrete coming out of the formwork sufficient to maintain the shape of the structure and at the same time allowing the rubbing of formwork on its surface to be easily rubbed.

c. Breaks in concreting

The intervals between the rise of the formwork should not exceed 8 minutes when using vibrators and 10 minutes when manually compacting the concrete mix. The speed of lifting the formwork at an outdoor 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 “breakdowns” of the concrete: the formwork carries along part of the loose concrete wall, as a result, sinks are formed, fittings are exposed. The main reasons for the "breakdowns" are the following: pollution of the formwork; failure to observe the taper of the formwork; long breaks during concreting.

In the event of a forced break in concreting, measures should be taken against the adhesion of the laid concrete to the formwork; the formwork slowly rises up to the formation of a visible gap between the formwork and concrete, or periodically rises and falls within the limits of one jack step (“step on the spot”). When resuming concreting, it is necessary to clean the formwork, remove the cement film from the surface of the concrete and rinse them with water.

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

d. Feed Mix

Sacks or a tarpaulin are attached to the lower edges of the formwork to protect fresh concrete from drying out (overcooling) and in summertime with a ring pipe regularly water it.

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 tile) in accordance with the requirements of the project. In order to reduce the gaps between the formwork walls and the box of the block to 10 mm, slats are sewn to the box and subsequently removed. Armature around the unit is installed in accordance with the project.

Laying concrete around the installed blocks is made simultaneously from two sides. After the formwork rises above the installed blocks, the temporary slats are removed.

Tower cranes, shaft hoists, and self-lifting cranes are used to supply concrete formwork, fittings, jacking rods and other goods to the formwork.

Concrete pumps and pneumatic blowers are also used to supply the mixture. Upon completion of the erection of the structure, the sliding formwork and all structures and equipment fixed on it are dismantled in the order in which the stability and safety of the remaining elements is ensured after the removal of the individual parts.

Channels, in concrete, formed by the movement of protective tubes, after removing the jacking rods should be carefully sealed.

e. Prefabricated floors

During the construction of structures in winter conditions, the concrete is heated in specially constructed greenhouses above the working floor and on outdoor scaffolds using steam or electric heaters or infrared radiation.

Multi-storey slabs, staircases and platforms are concreted using additional inventory formwork or mounted from prefabricated elements. In the latter case, in the process of erection of a building or structure, the need for alterations and additional devices in the sliding formwork is eliminated.

Prefabricated floors can be mounted with a tower crane after the walls have been erected "well" to the entire height of the building. In this case, the plates are based on special inventory, removable brackets fixed on the walls slightly below a number of small openings in the wall. Through the openings, reinforcing rods are joined, which are joined with releases from the floor slabs. The joining of external walls with floor slabs is carried out with the help of wall punches. This technology ensures the continuity of concreting, fast and high-quality wall construction.

Monolithic floors can be concreted after the construction of the walls of the building "well". Shields inventory formwork and supporting devices (metal telescopic racks and sliding bolts) are transferred from floor to floor with a tower crane or manually.

Monolithic floors can also be concreted with the use of descending suspended formwork mounted on a special platform. This method is especially effective if concrete pumps or pneumosuperchargers are used to supply the concrete mix.

f. Concreting Slabs

Concreting floors with a lag from concreting the walls on the 1-2 floors of the construction of buildings is complicated by the need for frequent stops when lifting the sliding formwork.

The method of combined cyclic concreting of walls and ceilings is that the concreting of walls in the sliding formwork stops each time at the mark of the next floor. Empty wall formwork is displayed above this mark so that there is a gap between the bottom of the sliding formwork and the bottom of the overlap mark equal to the thickness of the future overlap. At the same time, the boards of the formwork of external walls, as well as the formwork, forming the inner surface of elevator shafts and other cells that do not have overlaps, are made more in height than the boards of the rest of the formwork. Concreting of slabs is made by panel or sectional formwork with the shields of the working floor removed after stopping and alignment of the sliding formwork.

The construction of buildings and structures with a height of 40-50 m in monolithic reinforced concrete using the sliding formwork method is based on the basic technical and economic indicators at the level of construction of precast concrete structures, and the construction of high-rise civil buildings has several advantages: reduced construction time; reducing the complexity and estimated cost of construction, including by reducing the specific capital investments in the base of the construction industry; increase of reliability, durability and rigidity of structures due to solidity and lack of joints, which is especially valuable during construction in seismic areas, on mine workings and subsiding soils.

g. Construction of high-rise buildings

In recent years, a new way of erecting high-rise structures made of monolithic reinforced concrete in a sliding formwork of a coreless system consisting of hydraulic or pneumatic support-lifting devices in our country has been developed and introduced in our country.

Based on the proposals of the Donetsk PromstroyNIIproekt, a pilot production sample of a mobile formwork consisting of two (lower and upper) supporting and lifting sections of a walking action supported by walls of the erected structure, electromechanical worm-screw elevators, forms of sliding formwork and frames for fastening was created. With the help of this formwork, tower supports were built for the transport galleries of the blast-ore warehouse on the construction of Zaporozhye Iron Ore Combine.

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

h. Climbing formwork

Lifting-adjustable 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 adjustable external and internal formwork is attached.

The design of the lift allows you to periodically increase it from above or grow it from below. After each cycle of installation of formwork panels, reinforcement and laying of the concrete mix, the next lifting of the working platform and shifting the formwork is carried out.

Chimney formwork with a height of up to 320 m. Consists of exterior and interior panels, support rings, framing (support) frame, radial movement mechanisms, working platform, suspended scaffolding, and a rack mount hoist with a lifting head, assembled from 2.5-meter tubular sections and equipped with a cargo cage and cargo-passenger elevator.

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

i. Concreting pipe barrel

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

The external formwork of chimneys consists of rectangular and trapezoidal shields with a height of 2.5 m. The combination of these shields will provide an opportunity to get a conical surface of the pipe.

Suspended external formwork to the supporting ring, which, while reducing the perimeter of the pipe is replaced by a new smaller diameter.

For the convenience of laying concrete, the internal formwork is assembled from 1250x550 mm shields.

Concreting the barrel pipe: the scheme of work; sweep of the external lifting-adjustable formwork of a conical chimney; rectangular panels; trapezoidal panels; c - panel of the inner shell of the formwork; covered canopy; protective overlap; mine lift; lining pad; clip; working platform; distributing bunker; cargo cage bucket; lifting head; passenger lift; telpher; cargo crate; Cathead; strip lining; steel strip eyes; steel strips; steel sheet 2 mm thick.

To stiffen the panels, lining is welded to their upper and lower edges, with which panels are assembled in height. Eyes are welded to the outer side of the shields, into which reinforcement rods of 10-14 mm are formed, forming a series of elastic horizontal rings.

j. Construction of cooling tower

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. After 8-12 hours after placing the concrete in the second tier, the external formwork is removed and placed in the next highest position. After the reinforcement of the third tier is installed, the lower tier of the inner formwork is removed and rearranged higher. Then the cycle repeats. Install reinforcement with individual rods manually.

The concrete mix is ​​supplied with a cargo cage bucket into the receiving bunker located on the working platform, then into the mobile bunker of the concrete paver and from there to the trunk of the formwork. Compact the concrete mix with deep shaft vibrators with a flexible shaft.

The speed of concreting of the trunks of chimneys at an outside air temperature of 15-20 ° C reaches 1-1.5 m / day.

The construction of the shells of cooling towers is carried out with the help of an aggregate representing a lattice (stackable) tower, on the rotary head of which the rotating booms are mounted, to which the shields of the load-adjustable formwork are attached, as well as the working cradles.

The concrete mix is ​​fed to the upper platform of the bassinet in a vibro-tiller by a hoist moving along an arrow. Concreting is carried out in tiers by analogy with the concreting of chimneys.

2. Methods of concreting structures

a. Concreting in sliding formwork

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

The method of concreting monolithic reinforced concrete buildings and structures in a sliding formwork is a highly organized and complex-mechanized, flow-speed construction process. The formwork device, reinforcement, laying and compaction of the concrete mix, the demolding of concrete are combined and continuously in the process of lifting the formwork (SNiP H1-B.1-70).

Sliding formwork includes: formwork shields, jacking frames, working floor with a visor along the outer contour of the formwork, suspended scaffolding, equipment for lifting the formwork.

Shuttering boards perform inventory height of 1100-1200 mm from the following materials: steel sheet with a minimum thickness of 1.5 mm; planed wooden planks with a minimum thickness of 22 mm; waterproof plywood 8 mm thick; 7 mm thick tank-lined plywood or 3 mm thick fiberglass. In some cases, wood-metal shields are made, in which the framework is made of steel rolled sections, and the skin is made of planed boards or plywood. Circling for fastening formwork panels, as a rule, are made of steel rolled sections.

b. Construction of non-standard facilities

Metal formwork shields are used in the construction of a number of similar structures (silos, chimneys, tanks), when the side walls perceive a large pressure of fresh concrete, and, moreover, the formwork shields are recurred.

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

Promising are formwork shields made of waterproof plywood and fiberglass. They are durable and lighter than shields made of other materials, but so far more expensive than them.

For the construction of non-standard structures used non-inventory wooden formwork. By design, sliding formwork shields are used in two types: large-block and small-block.

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

In the small-block boards, only metal circles rigidly interconnected, forming the skeleton of the walls, and the formwork panels are hung on the circle without being fastened together.

3. Concreting grounds and floors

a. Concrete preparation

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

Concrete preparations are arranged mainly in single-storey industrial workshops for cement and asphalt floors, cast-iron slab floors, end wood checkers and other types of floors with a thickness of 100-300 mm over the prepared and leveled ground. For concrete foundations, rigid concrete mixtures of grades 100, 200 and 300 are usually used.

Concrete and cement-sand floor coverings are made up to 40 mm thick of concrete or mortar for preparation. In high-rise buildings, reinforced concrete floors are usually used as a base.

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

Before starting the preparation of concrete preparation, all underground work on the foundation, canals, tunnels, etc. should be completed, the backfill sinuses, backfilling and soil compaction should be backfilled.

Preparation of the soil basis. With dense soils, the concrete mix is ​​placed directly on the planned soil. Bulk and with a disturbed structure of the soil in the grounds should be compacted in a mechanized way. In places inaccessible to sealing mechanisms of the ground, the thickness of the soil compacted by hand tampers should not exceed 0.1 m.

b. Floor concreting techniques

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

Before laying the concrete preparation, a layer of crushed stone or gravel 60-150 mm thick is rolled or rolled into the surface of the base of weak soils onto it. Before installing floors on water-saturated clay, loamy and silt soil, lower the groundwater level and dry the base until the design bearing capacity is restored. On heaving soils, flooring should be carried out in accordance with the project guidelines.

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

Receptions concreting floors and bases. Before concreting on the level, lighthouse boards are installed in such a way that their upper face is at the level of the surface of concrete preparation (Fig. 14, a). The distance between the boards depends on the length of the vibrating rail and is usually 3-4 m. They fix the lighthouse boards with the help of wooden stakes driven into the ground. Concrete floors and foundations strip through one, starting with the most remote from the passage of places.

c. Concreting preparations

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

The concrete mix is ​​compacted by a vibrating rail, which is a metal beam (channel, I), on which one or two electric motors from the surface vibrator are fixed.

When concreting preparations and floor coverings, each vibrated area should be overlapped by a vibrating rail, respectively, by 150 mm and half its width.

Methods of concreting floors and foundations: the scheme of concreting the base under the floors; hand tools for smoothing concrete surfaces; laid base; preparation under the base; stakes; side formwork; rubber belt scraper for removing cement milk; ironing device; taters; ironing board; rubber tape.

Depending on the working conditions of the work, the concrete mix is ​​laid in the bases by concrete pavers in two ways: “from itself” when the unit moves behind the concrete front, and the concrete in the area of ​​operation of the unit manages to gain strength necessary for its movement, and “to itself” when the mechanism moves ahead of the concreting front, as the concrete does not have time to gain the necessary strength.

d. Concrete production

The first method is preferable, since it creates a wide scope of work to prepare the foundation. In the second method, the preparatory work ahead of the laying of the concrete mix on one plot, the length of which is equal to the radius of the mechanism.

In unheated premises in the concrete preparation, every two lanes arrange longitudinal and 9-12 m along the length of the strips transverse temperature-shrinkable seams, which divide the concreted area into separate slabs with dimensions of 6X9-9X12 m.

Longitudinal seams are performed by installing planed boards plastered with hot bitumen, or boards wrapped with roofing sheets. After the concrete has set, the boards are removed and the seams are filled with bitumen. Arrange the seams also by coating with bitumen a layer of 1.5-2.0 mm lateral faces of the strips before laying the concrete mix into adjacent spaces.

For the formation of transverse deformation seams (half-joints), metal strips 60–180 mm wide and 5–7 mm thick are used, which in the process of concreting are laid in preparation for 7–3 of their width and then removed after 30–40 min. The resulting recesses after the final hardening of the concrete are cleaned and poured with grade III bitumen or cement mortar.

e. Concrete Substrate Surface

In places of interruption in the concreting of bases and floors, it is not allowed to install a vibrating rail near the edge of the laid layer, since this will cause the concrete mixture to slip and stratify. Therefore, at the end of the work shift in the places of the planned break in concreting a partition wall is installed and the last portion of the concrete mix is ​​leveled and vibrated along it.

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

At an early age of concrete for this purpose use mechanical steel brushes. With high strength of concrete using pneumatic tools, grooves of 5–8 mm depth every 30–50 mm are applied to its surface. This allows you to get a rough surface of the underlying layer and to provide better adhesion to the upper layer.

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

Before concreting the coating, lighthouse wooden slats or metal framing corners are fastened to the surface of the concrete base. The concrete mix is ​​compacted by vibrating rails, and the surface of the concrete is leveled with a wooden slat moved across the strip.

f. Cement Milk

Cement milk, which protrudes to the surface when compacting concrete foundations and floor coverings, is removed with a rubber scraper.

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

To give the concrete floor a high abrasion resistance, its surface approximately 30 minutes after the final leveling is treated with a metal scraper, exposing grains of gravel. If high abrasion resistance is not required, then a cement floor is made from the mortar on concrete preparation.

If necessary, devices of a double-layer floor should first lay the lower layer between the lighthouse boards and compacted with a pad vibrator or an obliquely mounted vibrolath, 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 performed.

e. Iron surface of concrete

For large amounts of work, the surface of a clean concrete floor in the initial period of hardening is rubbed with a CO-64 (or OM-700) machine, consisting of a 600-mm trowel disc, an electric motor and a control handle. Rotating at a speed of 140 rpm, the trowel disk levels and smooths down the concrete floor surface. Productivity of the machine is 30 m2 / h.

Iron surface concrete is used to give the floor a high density. It lies in the fact that dry and sifted cement is rubbed into the surface of wet concrete until a smooth gloss appears on it. Dry concrete surfaces before ironing moistened with water. Iron can be done manually with steel trowels or with a CO-64 trowel.

A variety of concrete floors are mosaic, made from a mixture, which consists of: white or colored Portland cement, marble, granite or basalt crumb and mineral dye. Mosaic layer with a thickness of 1.5-2 cm is placed, as a rule, on the underlying layer of cement mortar of approximately the same thickness. The monochrome fields are limited and the patterns provided for by the project are carried out with the help of strips of glass, copper or brass embedded in the underlying layer of the solution. These strips are exposed in such a way that their upper edges serve as beacons when laying and leveling the mosaic layer.

Trim the surface of mosaic floors with electric machines after the hardening of the concrete (after 2-3 days or more). After the first polishing, the flaws found on the floor surface are filled with a painted cement-sand mortar. Then the floor is polished with smaller abrasives, treated with polishing powders and glossy with a tinting machine.

4. Concreting of columns

a. Formwork rectangular columns

Columns as an element of the frame of buildings and structures are 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 boards (wooden, metal or combined). The lateral pressure of the concrete mixture is perceived clamps, compressing the box. Clamps perform inventory metal with a large formwork and wooden turnover - with a small number of revolutions. The holes in the straps of the metal clamp for fastening wedges allow them to be used for columns of different sections. To clean the box in the bottom of one of the shields arranged a temporary hole. Block concreting is also used for concreting the columns.

Typical unified boards and formwork panels are attached to the reinforcement blocks with tie bolts and tied together with cords. The formwork of low columns is fixed in two mutually perpendicular directions by inclined pitches (braces). With a height of columns of more than 6 m, the formwork box is attached to specially arranged forests.

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

b. Concreting columns

For round columns, special metal block forms are made.

The observance of the thickness of the protective layer in the columns is ensured by special cement pads, which, prior to concreting, are attached to the reinforcement rods by the binding wire embedded in the gaskets during their manufacture.

Concreting of columns with transverse dimensions from 400 to 800 mm in the absence of intersecting collars is performed from above without interruption in sections up to 5 m high. sections of height not exceeding 2 m.

Column formwork: box assembled; inventory metal clamp; wooden clamp on the wedges; detail of the wooden clamp assembly; box; metal inventory clamp; wedges fastening collars; frame under the formwork of the column; door cleaning hole; covering shields; openings for wedges; embedded shields; thrust dies.

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

c. Frame construction

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

When concreting them from above, the lower part of the formwork of the columns is recommended to be initially filled to a height of 100-200 mm with a cement mortar of composition 1: 2-1 = 3 in order to prevent large aggregate of aggregate without solution at the base of the column. When dropping from above portions of the concrete mix, large aggregate particles are embedded in this solution, forming a mixture of normal composition.

Concrete is compacted in columns with internal vibrators with a flexible or rigid shaft. Compaction with external vibrators attached to the formwork of columns of small cross section is less effective and almost never used.

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

Concreting of columns in accordance with SNiP III-B.1-70 is made to the full height without working joints. Allowed the construction of joints: at the top of the foundation, at the bottom of the girders and beams or crane cantilevers and the top of crane girders.

d. Concreting frame structures

It is allowed to arrange seams in the columns of non-beam floors either at the very bottom of the columns or at the bottom of the capitals. Concrete capitals simultaneously with the slab.

The surface of the working joints, arranged when laying the concrete mix intermittently, should be perpendicular to the axis of the concreted columns.

Concreting of frame structures should be done with a break between the laying of the concrete mixture in the columns (pillars) and frame beams. 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 circular, curvilinear, polygonal and straight.

When concreting walls and partitions, the following types of formwork are used: typical unified boards and panels of folding-adjustable formwork, block forms, roll-up, sliding, sliding and sliding formwork.

Disassembled-adjustable small-plate formwork is installed in two steps: first, on one side, the entire height of the wall or partition, and after the installation of the reinforcement, on the other. If the wall thickness is more than 250 mm, special inventory materials are installed for the second side formwork.

The height of the wall is set on the wind, otherwise - in the process of concreting. In the formwork installed to the full height of the wall, openings are provided for supplying the concrete mixture through them to the structure.

5. Concreting walls

a. Design wall thickness

The formwork of walls up to 6 m high is mounted from mobile platforms or light scaffoldings. At higher altitude forests are made. Wall formwork is fixed with struts or braces, coupling bolts or wire ties.

To comply with the design thickness of the walls in the passage of screeds installed concrete or wooden struts. The latter are removed in the process of concreting.

Disassembled-adjustable large-block formwork is installed in the process of concreting walls. This allows you to limit the formwork kit only two tiers. All the work of a full cycle of concreting walls in this formwork is performed in the following sequence: scaffolding (scaffolding) is first installed or built up, then the working seam of concreting is processed and reinforcement is installed, after which the formwork is moved from the lower tier to the upper one. The cycle of concreting one tier ends with the laying and compaction of the concrete mix and the subsequent aging of concrete in the formwork.

Block form for formwork: fixing collar number 1; reinforced concrete tape; bedding; screw jack; block formwork; fencing element for the 1st tier of concreting; paneling; fixing collar number 2; working flooring; fencing element for the 2nd tier of concreting; inventory insert; sliding rack; double wooden wedge.

b. Formwork block forms

Block forms of formwork are used when concreting walls of considerable height and length, i.e., when their repeated use is ensured. The block form of the Kharkivorktekhstroy trust structure consists of blocks, panels, additional and fasteners.

The rigidity of the blocks is provided by horizontal contractions and supporting trusses, which simultaneously serve as scaffolding. For installation, adjustment and dismantling of formwork, supporting trusses are equipped with jacking devices. Sizes of ordinary blocks 3X8,3X2 and 1,5x3 m.

Rolling formwork design Donetsk PromstroyNIIproekt: trolley; column; beam; winch lifting shields; formwork shield; clamps; stairs; crawlers; clamping device; flooring; fencing; bunker.

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

Assembled formwork blocks can be disassembled into individual shields, if necessary. The block form formwork is rearranged in the process of concreting. When concreting the walls of constant and variable cross-section, a rolled formwork is used (including moving horizontally on skids).

c. Wall construction

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

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

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

With a greater height of the wall sections, concreted without working joints, it is necessary to arrange breaks of not less than 40 minutes, but not more than 2 hours for precipitating the concrete mix and preventing the formation of sedimentary cracks.

If there is a window or door in the concreted wall, the concreting should be interrupted at the level of the upper edge of the opening or, if possible, make a working joint in this place. Otherwise, sedimentary cracks form around the corners of the mold. When feeding a concrete mix from a height of more than 2 m, link trunks are used.

When concreting, the lower part of the formwork of the walls is first filled with a layer of cement mortar 112-1: 3 in order to prevent the formation of porous concrete at the bottom of the walls with a large aggregate aggregation.

When concreting the walls of tanks for storing liquids, the concrete mix should be laid continuously to the entire height in layers with a thickness not exceeding 0.8 the length of the working part of the vibrators. In exceptional cases, the working joints formed must be very carefully processed before concreting.

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

6. Concreting beams, slabs, arches

a. Concreting ribbed floors

Concreting beams, slabs, arches, arches and tunnels. Beams and slabs, floors are usually concreted in folding-retractable formwork of typical unified boards and panels. Beams and girders are concreted also in block forms.

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

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

Concreting ribbed floors: a general view of scaffolding and ribbed floor forms; the location of the working joints when concreting ribbed ceilings in the direction parallel to the secondary beams; the same, the main beams; formwork beams; slab formwork; circled; formwork girder; column formwork; sliding racks; clamping boards; coasters; frieze boards; slab formwork shields; circled; podkruzhnye boards; side shields; bottom: rack head; the working position of the seam (arrows indicate the direction of concreting).

b. Formwork beamless overlap

The boards of the formwork slab are laid with an edge on the rounded planks, which in turn rest on the sub-boards, nailed to the stitched planks of the side shields of the beam and supported by supports.

To fasten the round and side shields around the perimeter of the plate, fascia boards are laid, which also facilitate the removal of the plate. With the height of the beams more than 500 mm, the side shields of the formwork are additionally reinforced with wire strands and temporary struts.

The distance between the racks and the circles is determined by calculation. Support racks fasten in mutually perpendicular directions inventory cords or braces.

The formwork of flat slab consists of the formwork of columns, capitals and slabs. Slab formwork consists of two types of boards stacked in circles between frieze boards sewn on tops of racks. To support the circle are arranged pair runs from the boards, based on the rack. Shields capitals one side based on the formwork of the columns, and on the outer contour are supported by circles.

When installing suspended formwork slabs for precast concrete or metal beams are arranged metal loops-pendants, laid out on beams with a given step. Into these hinges they install round-the-boards, on which the circles and slabs of slab formwork are supported.

c. Protective layer

Concreting floors (beams, girders and slabs) is usually done 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 bottom surface, and if there are wut in the slab - at the bottom of the slab (SNiP Sh-V.1-70 ).

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

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

Floor slabs are concreted in the direction of secondary beams. The protective layer in the plates, beams and runs is formed with the help of special pads of cement mortar or clamps. As the structures are concreted, the reinforcement is shaken slightly with metal hooks, ensuring that a protective layer of the required thickness is formed under the reinforcement.

d. Concreting Slabs

Concrete mixes in slabs up to 250 mm thick with single reinforcement and up to 120 mm thick with double reinforcement are compacted with surface vibrators, and in thicker slabs - depth ones.

When concreting flat seams, working seams may be arranged anywhere parallel to the smaller side of the slab. In ribbed floors, when concreting is parallel to the direction of the main beams, the working joint 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 working joints in beams and slabs should be perpendicular to the direction of concreting. Therefore, boards are installed on the edge in the designated areas of the break of the concreting of slabs, and in beams - shields with holes for reinforcement.

Temperature joints in the ceiling are arranged on the consoles of the columns or by installing paired columns, ensuring free movement in the seam of the beams in a horizontal plane on the metal support sheet.

When concreting ceilings in multi-storey frame buildings at the level of each ceiling, the receiving platforms are arranged, and conveyors and vibration chutes are installed inside the building to supply the concrete mix after it is lifted by a crane to the installation site.

e. Vaults and arches

In the process of concreting of coatings, ceilings and individual beams it is not allowed to load them with concentrated loads that exceed the allowable values ​​specified in the design of the work.

Vaults and arches of small extent are concreted in a folding-adjustable small-piece or large-panel formwork supported by racks. For concreting of the arches and arches of a great extent, inventory roller formwork mounted on a trolley is used. On the lower part of the formwork installed hoisting and lowering circles, 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 clamping the formwork in the arch during its swelling. Lifting and lowering the circle 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: breaks simultaneously from both sides of the supports (toes) to the middle of the arch (the castle), which ensures the safety of the design form of the formwork. If there is a danger of buckling the formwork at the vault in the process of concreting the side parts, it is temporarily loaded.

Rolled roof formwork: transverse section; lengthwise cut; tightening arch-diaphragm; retractable racks; hand hoists.

7. The process of concreting complex structures

a. Massive arches and vaults

The long vaults are divided along the length into limited areas of concreting with working seams located perpendicular to the generator of the vault. Laying concrete in limited areas is the same as in the vaults of a small extent, i.e., symmetrically from the heels to the castle.

Massive arches and vaults with a span of more than 15 m are concreted with strips parallel to the longitudinal axis of the arch. Laying of the concrete mix into strips is also performed symmetrically on both sides from the heels to the vault lock.

The gaps between the strips and sections of long-length vaults are left approximately 300-500 mm wide and are concreted with hard concrete 5–7 days after the end of the concreting of the strips and sections, i.e. when the main concrete is laid.

With steep arches, the pillars at the supports are concreted in double-sided formwork, with the second (upper) formwork being installed with separate shields during the concreting process.

Concrete mix is ​​compacted in massive arches and vaults with internal vibrators with a flexible or rigid shaft, depending on the degree of reinforcement, in thin-walled vaults - with surface vibrators. Concrete tightening of arches and arches with tensioning devices should be done after tightening these devices and uncoiling. Hard puffs without tensioning devices are allowed to be concreted simultaneously with the concreting of the coating.

b. Tunnels and pipes

Tunnels and pipes are concreted in open trenches and under the ground in disassembled-adjustable and rolling mobile formwork. Mobile wooden formwork of a tunnel through a curvilinear outline with a cross-section of up to 3 m consists of curved panels in the form of curvilinear circles, sheathed with planed boards, waterproof plywood or sheet steel on the boardwalk. Racks supporting the working floor are sewn to the outer shield circles. Internal formwork consists of two shields, the bottom of which rests on paired wedges, and the top is bolted in the vault lock.

External and internal formwork are connected to each other with coupling bolts. The length of the shields is usually taken to be 3 m, the mass of the formwork reaches 1.5 tons. They move the outer and inner forms with a winch along wooden rails. The external formwork can also be rearranged to a new place by a crane. Rolling wooden formwork of the construction of Ing. V. B. Oak for concreting tunnels and collectors of rectangular section consists of sections 3.2 m long.

The inner formwork section consists of four steel U-shaped frames, sheathed with planed boards, plywood or sheet steel. Each frame consists of two side racks and two: half-gates, interconnected by three hinges. The extreme frames of the formwork section are in the middle of a single sliding rack of pipes, tightening screw jacks. The frames are supported by means of middle pillars and sliding horizontal beams on a trolley moving along the track.

c. Tunneling arches

The outer formwork section consists of five frames with struts and split bolts. Racks of frames from the inside are sheathed with boards. External formwork is fastened with internal bolts, passed through the removable girders. 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 external formwork is rearranged usually by crane. When dismantling, the coupling bolts are removed, the bolt joints are separated: the outer form frames, after which the formwork is removed. To remove the inner formwork using jacking devices located in the outermost racks, lower the semi-anvils with ceiling: shields.

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

The vaults of tunnel structures are concreted simultaneously from two sides from the toes to the castle by radial layers. The castle is concreted in sloping layers along the shelhyg vault, while the formwork is laid as it is concreted in short sections - from round to round.

In the powerful arches of tunnel structures, the working joints to be arranged should be radial. The desired direction of the seam 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

Tunnel finishes should be concreted in parallel with the tunneling, as in this case, the overall construction of the tunnel is reduced. However, for small sizes of the tunnel cross-section, due to constrained conditions, the finish is erected at the end of the tunneling of the entire tunnel or separate sections between the intermediate faces.

Tunnel trim is concreted either continuously over the entire cross section of the excavation, or in parts in the following sequence: tunnel tray, arch and walls or vice versa.

For the formwork, the concrete mixture is supplied from the end or through the hatches in the formwork using concrete pumps or pneumosuperchargers. The concrete mix can also be supplied to the side walls and the tunnel tray by tipping trolleys using distribution chutes.

Concrete mix is ​​compacted in layers with deep vibrators through windows in the formwork or external vibrators attached to the formwork.

If the walls of the tunnel finishes are concreted after the roof (the “arch” method), the formwork is removed from the bottom surface of the foot of the roof before concreting and the surface is thoroughly cleaned. Concrete walls in horizontal layers with a simultaneous build-up of formwork to a point less than the mark of the bottom of the heel by up to 400 mm. The space between the fifth arch and the adjacent wall is filled with stiff concrete mix and carefully sealed. Previously at the site of junction lay the tube for the subsequent injection of cement mortar.

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10. DEFECTS OF MONOLITHIC REINFORCED CONCRETE CONSTRUCTIONS CAUSED BY VIOLATION OF THEIR CONSTRUCTION TECHNOLOGY

The main violations of the technology of work, leading to the formation of defects in monolithic reinforced concrete structures, include the following:
   - production of insufficiently rigid, strongly deformed when laying concrete and insufficiently dense formwork;
   - violation of design dimensions of structures;
   - poor compaction of the concrete mix during its laying in the formwork;
   - laying of stratified concrete mix;
   - the use of too hard concrete mix with thick reinforcement;
   - poor care of concrete in the process of hardening;
   - the use of concrete strength below the design;
   - discrepancy to the project of reinforcement of structures;
   - poor-quality welding of fittings;
   - the use of highly corrosive reinforcement;
   - early demolition of the structure;
   - violation of the required sequence of stripping of vaulted structures.

The manufacture of insufficiently rigid formwork, when it receives significant deformations during the laying of the concrete mixture, leads to the appearance of large changes in the shape of reinforced concrete elements. In this case, the elements get the appearance of strongly arched structures, the vertical surfaces acquire convexity. Deformation of the formwork can lead to the displacement and deformation of the reinforcement cages and grids and changes in the bearing capacity of the elements. It should be borne in mind that the own weight of the structure increases.
Loose formwork contributes to the leakage of cement mortar and the appearance of shells and cavities in the concrete. Sinks and caverns are also caused by insufficient compaction of the concrete mix when it is laid in the formwork. The appearance of sinks and cavities causes a more or less significant reduction in the bearing capacity of elements, an increase in the permeability of structures, contributes to corrosion of reinforcement located in the zone of sinks and cavities, and can also cause rebar in concrete.
   Reducing the design dimensions of the cross section of elements leads to a decrease in their bearing capacity, an increase to an increase in their own weight of structures.
   The use of stratified concrete mix does not allow to obtain a uniform strength and density of concrete throughout the entire structure and reduces the strength of concrete.
   The use of too rigid concrete mixture with thick reinforcement 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 danger of reinforcement corrosion.
   During the maintenance of concrete, it is necessary to create such temperature and humidity conditions that would ensure the preservation in the concrete of the water necessary for the hydration of cement. If the curing process takes place at a relatively constant temperature and humidity, the stresses that occur in the concrete due to volume changes and are caused by shrinkage and temperature distortions will be insignificant. Typically, the concrete is covered with plastic wrap or other protective coating. Perhaps the use and film-forming materials. Caring for concrete is usually carried out within three weeks, and in the application of heating of concrete - after its completion.
   Poor maintenance of concrete leads to overdrying of the surface of reinforced concrete elements or their entire thickness. The overdried concrete has much lower strength and frost resistance than normally hardened, many shrinkage cracks arise in it.
   When concreting in winter conditions, with insufficient insulation or heat treatment, early freezing of concrete may occur. After thawing such concrete, he will not be able to gain the necessary strength. The final compressive strength of concrete subjected to early freezing, can reach 2-3 MPa or less.
   The minimum (critical) strength of concrete, which provides the necessary resistance to ice pressure and the ability to harden later at positive temperatures without significant deterioration of the properties of concrete, is given in Table. 10.1.

Table 10.1. The minimum (critical) strength of concrete that concrete should acquire by the time of freezing is minimal (available only when downloading the full version of the book in Word doc format)

If all ice and snow were not removed from the formwork before concreting, then sinks and cavities arise in the concrete. An example is the construction of a boiler room in permafrost conditions.
   The base of the boiler house was a monolithic reinforced concrete slab into which the heads of the piles immersed in the ground were embedded. A ventilated space was provided between the slab and the ground to isolate the soil from the heat penetrating the boiler room floor. Outlets of reinforcement were made from the top of the piles, around which ice was formed that was not removed before concreting. This ice melted in the summer, and the base plate of the building was supported only on releases of reinforcement from piles (Fig. 10.1). The reinforcement outlets from the piles were deformed by the weight of the entire building and the base plate received large uneven sediments.

Fig. 10.1. State diagram of the monolithic base plate of the boiler room (a - during concreting; b - after the ice melted in the formwork has melted): 1 - monolithic plate; 2 - ice left in the formwork; 3 - pile reinforcement; 4 - pile (available only when downloading the full version of the book in Word doc format)

The inconsistency of the project with the strength of concrete and reinforcement of structures, as well as poor-quality welding of rebar outlets and the intersection of rods affects the strength, crack resistance, and rigidity of monolithic structures as well as similar defects in precast concrete elements.
   Minor corrosion of the reinforcement does not affect the adhesion of the reinforcement to the concrete, and, consequently, the work of the whole structure. If the reinforcement is corroded so that the corrosion layer peels off the reinforcement upon impact, then the adhesion of such reinforcement to concrete deteriorates. At the same time, along with a decrease in the bearing capacity of elements due to a decrease in reinforcement section due to corrosion, an increase in the deformability of the elements and a decrease in crack resistance are observed.
Early demolition of structures can lead to complete unsuitability of the structure and even its collapse during the process of demoulding because the concrete has not gained sufficient strength. The removal time is mainly determined by the temperature conditions and the type of formwork. For example, the formwork of the side surfaces of walls, beams can be removed much earlier than the formwork of the lower surfaces of the bent elements and the side surfaces of the columns. The last formwork can be removed only when the strength of the structures is ensured from the impact of its own weight and temporary load acting during the construction period. According to N. N. Luknitsky, the removal of the formwork of slabs with a span of up to 2.5 m can be accomplished not earlier than 50% of the design strength with concrete, slabs with a span of more than 2.5 m and beams - 70%, large-span structures - 100%.
   When dismantling the vaulted structures, they should first be released circling at the castle, and then at the heels of the structure. The nursery circled first to release at the heel, then the vault rests on the circling in its castle part, and the vault is not designed for such work.
   At present, monolithic reinforced concrete structures have become widespread, especially in high-rise housing construction.
   Construction companies, as a rule, do not have the appropriate formwork and rent it. Renting formwork is expensive, so the builders minimize the duration of its turnover. Usually, demolding is done two days after the laying of concrete. At this rate of construction of monolithic structures, very careful study of all stages of work is required: transportation of concrete mix, laying concrete into formwork, retaining moisture in concrete, heating the concrete, insulating the concrete, monitoring the heating temperature and strength gain of concrete.
   To reduce the negative effect of the temperature drop of the concrete, the minimum permissible temperature of the concrete should be selected during demolding.
   For vertical structures (walls), the temperature of heating of concrete can be recommended 20 ° С, and for horizontal (floors) - 30 ° С. Under the conditions of St. Petersburg for two days, the average air temperature is 20 ° С and, moreover, 30 ° С does not exist. Therefore, the concrete should be heated at any time of the year. Even in April and October, the author did not manage to see the concrete being heated at construction sites.
In winter, the concrete overlap should be heated when heated by laying a layer of effective insulation over a polyethylene film. And in many cases this is not done. Therefore, floor slabs, concreted in winter, have concrete strength from above 3-4 times less than below.
   When demolding in the middle of the floor slab section, temporary support is left in the form of a rack or formwork section. Also temporary supports should be installed before stripping strictly vertically on the floors, which is also often not respected.
   Since the concrete strength of the walls during demoulding does not reach the design value, it is necessary to do their intermediate calculation to determine the number of floors that can be erected in winter.
   There is a large shortage of instructive literature on monolithic reinforced concrete, which is reflected in its quality.

The amount of adhesion of concrete to formwork reaches several kgf / cm 2. This makes demolishing work difficult, degrades the quality of concrete surfaces and leads to premature wear of the formwork panels.

The adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the forming surface of the formwork affect the adhesion of concrete to the formwork.

Under the adhesion (sticking) understand the bond due to molecular forces between the surfaces of two dissimilar or liquid contiguous bodies. During the period of contact of concrete with the formwork, favorable conditions are created for the manifestation of adhesion. The adhesive (adhesive), which in this case is concrete, is in a ductile state during laying. In addition, in the process of vibrocompaction of concrete, its plasticity increases even more, as a result of which the concrete approaches the surface of the formwork and the continuity of contact between them increases.

Concrete adheres to the wooden and steel surfaces of the formwork stronger than to the plastic, due to the weak wettability of the latter.

When removing the formwork can be three options for separation. In the first embodiment, the adhesion is very small, and the cohesion is large enough. In this case, the formwork comes off exactly along the plane of contact. The second option is adhesion more than cohesion. In this case, the formwork comes off on the adhesive material (concrete). The third option - adhesion and cohesion in their values ​​are about the same. The formwork comes off partly along the plane of contact of the concrete with the formwork, partly along the concrete itself (mixed or combined separation). With adhesive tear, the formwork is removed easily, its surface remains clean, and the surface of the concrete is of good quality.

As a consequence, it is necessary to strive to ensure adhesive separation. For this, the forming surfaces of the formwork are made of smooth, poorly wettable materials, or lubricants and special anti-adhesive coatings are applied to them.

Lubricants for formwork, depending on their composition, principle of operation and performance properties can be divided into four groups: aqueous suspensions; water-repellent greases; lubricants - concrete retarders; combined lubricants.

The use of effective lubricants reduces the harmful effects on the formwork of some factors. In some cases, the use of lubricants is impossible. Thus, when concreting in sliding or form-folding formwork, it is prohibited to use such lubricants due to their penetration into concrete and a decrease in its quality. A good effect is provided by anti-adhesive protective coatings on the basis of polymers. They are applied to the forming surfaces of the boards during their manufacture, and they withstand 20-35 cycles without reapplication and repair. For board and plywood formwork, a coating based on phenol-formaldehyde was developed. It is pressed onto the surface of the boards at a pressure of up to 3 kgf / cm 2 and a temperature of + 80 ° C.

It is advisable to use shields, the decks of which are made of getinaks, smooth fiberglass or textolite, and the frame is made of metal corners. Such formwork is wear-resistant, easy to remove and provides good quality concrete surfaces.

The amount of adhesion of concrete to formwork reaches several kgf / cm 2. This makes demolishing work difficult, degrades the quality of concrete surfaces and leads to premature wear of the formwork panels.
  The adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the forming surface of the formwork affect the adhesion of concrete to the formwork.
  Under the adhesion (sticking) understand the bond due to molecular forces between the surfaces of two dissimilar or liquid contiguous bodies. During the period of contact of concrete with the formwork, favorable conditions are created for the manifestation of adhesion. The adhesive (adhesive), which in this case is concrete, is in a ductile state during laying. In addition, in the process of vibrocompaction of concrete, its plasticity increases even more, as a result of which the concrete approaches the surface of the formwork and the continuity of contact between them increases.
Concrete adheres to the wooden and steel surfaces of the formwork stronger than to the plastic, due to the weak wettability of the latter. The values ​​of Ks for different types of formwork are: small-shield - 0.15, wooden - 0.35, steel - 0.40, large-panel (panel of small panels) - 0.25, large-panel - 0.30, reversible - 0, 45, for block forms - 0.55.
  Wood, plywood, steel without treatment and fiberglass are well wetted and the adhesion of concrete to them is quite large, with concrete slightly wetted with poorly wettable (hydrophobic) getinax and textolite.
  Wetting angle grinded steel more than untreated. However, the adhesion of concrete to ground steel is reduced slightly. This is explained by the fact that on the border of concrete and well-treated surfaces the continuity of contact is higher.
  When applied to the surface of the oil film, it is water-repellent, which drastically reduces adhesion.
  The surface roughness of the formwork increases its adhesion to concrete. This is because the rough surface has a larger actual contact area compared to a smooth one.
  The highly porous formwork material also increases adhesion, since the cement mortar, penetrating into the pores, forms a point of reliable connection when vibrated. When removing the formwork can be three options for separation. In the first embodiment, the adhesion is very small, and the cohesion is quite large.
  In this case, the formwork comes off exactly on the plane of contact. Nonetheless, adhesion is greater than cohesion. In this case, the formwork comes off on the adhesive material (concrete).
  The third option - adhesion and cohesion in their values ​​are about the same. The formwork comes off partly along the plane of contact of the concrete with the formwork, partly along the concrete itself (mixed or combined separation).
  With adhesive tear, the formwork is removed easily, its surface remains clean, and the surface of the concrete is of good quality. As a consequence, it is necessary to strive to ensure adhesive separation. For this, the forming surfaces of the formwork are made of smooth, poorly wettable materials, or lubricants and special anti-adhesive coatings are applied to them.
  Lubricants for formwork, depending on their composition, principle of operation and performance properties can be divided into four groups: aqueous suspensions; water-repellent greases; lubricants - concrete retarders; combined lubricants.
Aqueous suspensions of powdered substances that are inert to concrete are simple and cheap, but not always an effective means to eliminate the adhesion of concrete to the formwork. The principle of operation is based on the fact that as a result of evaporation of water from suspensions prior to concreting, a thin protective film is formed on the forming surface of the formwork, preventing adhesion of concrete.
  The lime-gypsum slurry, which is prepared from semi-aquatic gypsum (0.6-0.9 wt. H.), Lime dough (0.4-0.6 wt. H.), Sulfite-alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).
  Suspension lubricants are erased by the concrete mixture during vibroplate and contaminate concrete surfaces, as a result of which they are rarely used.
  The most common hydrophobic lubricants based on mineral oils, emulsol EX or salts of fatty acids (soaps). After they are applied to the surface of the formwork, a hydrophobic film is formed from a number of oriented molecules, which degrades the adhesion of the formwork material to concrete. The disadvantages of such lubricants are contamination of the concrete surface, high cost and fire hazard.
  In the third group of lubricants, the properties of concrete are used to set slowly in thin butt layers. To slow down the setting, molasses, tannin, etc. are introduced into the composition of the lubricants. The disadvantage of such lubricants is the difficulty of controlling the thickness of the concrete layer.
  The most effective combined lubricants, which use the properties of the forming surfaces in combination with the slow setting of concrete in the thin butt layers. Such lubricants are prepared in the form of so-called inverse emulsions. In addition to water-repellents and retarders, plasticizing agents are added to some of them: sulphite-yeast bard (SDB), mylonaphs or TsNIPS additive. These materials during plastic compaction plasticize the concrete in the butt layers and reduce its surface porosity.
  ESO-GISI lubricants are prepared in ultrasonic hydrodynamic mixers in which mechanical mixing of components is combined with ultrasonic. For this purpose, components are poured into the mixer tank and the mixer is switched on.
The installation for ultrasonic mixing consists of a circulation pump, suction and pressure pipelines, a junction box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The fluid supplied by the pump under an excess pressure of 3.5-5 kgf / cm2, expires at high speed from the nozzle of the vibrator and hits the wedge-shaped plate. In this case, the plate begins to vibrate at a frequency of 25-30 kHz. As a result, zones of intense ultrasonic mixing are formed in the liquid with simultaneous division of the components into the smallest droplets. Duration of mixing 3-5 minutes
  Emulsion lubricants are stable, they are not stratified within 7-10 days. Their application completely eliminates adhesion of concrete to the formwork; they keep well on the forming surface and do not contaminate the concrete.
  It is possible to apply these lubricants on the formwork with brushes, rollers and with the help of spray rods. With a large number of shields, a special device should be used to lubricate them.
  The use of effective lubricants reduces the harmful effects on the formwork of some factors. In some cases, the use of lubricants is impossible. Thus, when concreting in sliding or form-folding formwork, it is prohibited to use such lubricants due to their penetration into concrete and a decrease in its quality.
  A good effect is provided by anti-adhesive protective coatings on the basis of polymers. They are applied to the forming surfaces of the boards during their manufacture, and they withstand 20-35 cycles without reapplication and repair.
  For board and plywood formwork, a coating based on phenol-formaldehyde was developed. It is pressed onto the surface of the boards at a pressure of up to 3 kgf / cm2 and a temperature of + 80 ° C. This coating completely eliminates the adhesion of concrete to the formwork and can withstand up to 35 cycles without repair.
  Despite the relatively high cost, anti-adhesive protective coatings are more profitable than lubricants due to their multiple turnaround.
  It is advisable to use shields, the decks of which are made of getinaks, smooth fiberglass or textolite, and the frame is made of metal corners. Such formwork is wear-resistant, easy to remove and provides good quality concrete surfaces.

When working with monolithic structures made of reinforced concrete, it is worth paying attention to the adhesion characteristics of concrete with formwork, where the value reaches several kg per square centimeter. Because of the adhesion, the stripping of the reinforced concrete structure will be more complicated, moreover, this process can degrade the concrete surface itself, namely, its quality. And the formwork panels may even collapse ahead of time. To prevent this from happening, ubts.kiev.ua is now available, which solves all these problems.

Due to the factors described below, the concrete adheres to the formwork:
  concrete undergoes adhesion and cohesion;
  shrinkage of concrete occurs;
  formwork adjacent to the structure of reinforced concrete may have a rough or porous surface.

At that moment, when the concrete is laid, its condition is plastic, therefore it is considered an adhesive substance, due to which a process occurs, referred to as adhesion (when the concrete sticks to the formwork). When the material is compacted, the indicator of plasticity of the concrete may increase, in consequence of which it is adjacent to the surface of the formwork.

The adhesion process can be different, depending on the material used to produce the forming formwork surface: the concrete will adhere more strongly to wood and steel. Plastic products due to their less wettability, least of all adhesive with concrete.

If plywood, steel, wood or fiberglass materials are not pre-treated, they will be easily wetted, which will ensure high adhesion to concrete. A less significant coefficient of adhesion with getinaks and textolite, as they belong to the category of hydrophobic materials.

To reduce the wettability by surface treatment, which is the application of an oil film on it, in consequence of which the adhesion process is significantly reduced. Due to shrinkage, not only adhesion, but adhesion can be reduced: due to large shrinkage, there is a high probability that shrinkage cracks will appear in the contact zone, which affects the weakening of adhesion.

If you want to dismantle the structure of concrete monolithic form, now there are three ways available, through which the detachable formwork is detached:
  a large indicator of cohesion and low adhesion. In this situation, it is necessary to take formwork along the contact plane;
  adhesion level exceeds cohesion. The formwork will be torn off according to the material that is adhesive (concrete);
approximate equality between adhesion and cohesion. This situation involves the separation of the mixed (combined) type.

The first option is the most optimal, as it allows you to easily remove the formwork, keeping its surface clean, and also to maintain the quality of the concrete itself. In this regard, the adhesive separation should be provided more often than others. It is available in such situations:
  when the forming formwork surface is made of a smooth material that is poorly wettable;
  The forming surface was treated with a special lubricant or special anti-adhesive coatings.

Formwork lubricant must meet the following requirements:
  after its use, oil stains should not be left on the concrete surface;
  the contact layer of concrete should not become less durable;
  high level of fire safety;
  the composition should not contain volatile substances that are hazardous to human health;
  the ability to stay on the surface (vertical and horizontal) for a day at an air temperature of +30 degrees Celsius.