Formwork production. Measures to reduce the adhesion of concrete to formwork Causes of adhesion of concrete to formwork

K category: Concrete works

Measures to reduce the adhesion of concrete to formwork

Adhesion (sticking) and shrinkage of concrete, surface roughness and porosity affect the adhesion force of concrete with formwork. With a high adhesion force between concrete and formwork, work on formwork becomes more difficult, the labor intensity of work increases, the quality of concrete surfaces deteriorates, and formwork panels wear out prematurely.

Concrete adheres to wood and steel formwork surfaces much stronger than to plastic ones. This is due to the properties of the material. Wood, plywood, steel and fiberglass are well wetted, therefore the adhesion of concrete with them is quite high, with poorly wetted materials (for example, textolite, getinax, polypropylene), the adhesion of concrete is several times lower.

Therefore, to obtain high quality surfaces, one should use linings made of textolite, getinax, polypropylene or use waterproof plywood treated with special compounds. When the adhesion is low, the concrete surface is not disturbed and the formwork can be easily removed. With an increase in adhesion, the concrete layer adjacent to the formwork is destroyed. This does not affect the strength characteristics of the structure, but the quality of the surfaces is significantly reduced. Adhesion can be reduced by applying aqueous suspensions, hydrophobizing lubricants, combined lubricants, lubricants - concrete setting retarders to the surface of the formwork. The principle of action of aqueous suspensions and water-repellent lubricants is based on the fact that a protective film is formed on the surface of the formwork, which reduces the adhesion of concrete to the formwork.

Combined lubricants are a mixture of concrete setting retarders and water-repellent emulsions. In the manufacture of lubricants, they add sulfite-yeast stillage (SDB), soap naphtha. Such lubricants plasticize the concrete of the adjacent zone, and it does not collapse.

Lubricants - concrete setting retarders - are used to obtain a good surface texture. By the time of stripping, the strength of these layers is somewhat lower than that of the main mass of concrete. Immediately after stripping, the structure of the concrete is exposed by washing it with a jet of water. After such washing, a beautiful surface is obtained with a uniform exposure of coarse aggregate. Lubricants are applied to the formwork panels before installation in the design position by pneumatic spraying. This method of application ensures uniformity and constant thickness of the applied layer, and also reduces lubricant consumption.

For pneumatic application, sprayers or spray rods are used. More viscous lubricants are applied with rollers or brushes.

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

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

Adhesion (adhesion) is understood as the connection between the surfaces of two dissimilar or liquid contacting bodies due to molecular forces. During the period of contact between concrete and formwork, favorable conditions to show adhesion. The adhesive (adhesive), which in this case is concrete, is in a plastic state during the laying period. In addition, in the process of vibroconsolidation 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 wooden and steel formwork surfaces more strongly than to plastic ones due to the poor wettability of the latter. In table. 1-3 shows the values ​​of the normal adhesion of concrete with some formwork materials.

Wood, plywood, untreated steel and fiberglass are well wetted and the adhesion of concrete to them is quite large, concrete adheres slightly to poorly wetted (hydrophobic) getinax and textolite.

The contact angle of polished steel is greater than that of untreated steel. However, the adhesion of concrete to ground steel is only slightly reduced. This is explained by the fact that at the border of concrete and well-treated surfaces, the contact continuity is higher.

When an oil film is applied to the surface, it is hydrophobized (Fig. 1-1, b), which sharply reduces adhesion.

Shrinkage has a negative effect on adhesion and, consequently, on adhesion. The greater the shrinkage value in the butt layers of concrete, the more likely the appearance of shrinkage cracks in the contact zone, which weaken the adhesion. Under the cohesion in the contact pair formwork - concrete should be understood as the tensile strength of the butt layers of concrete.

The surface roughness of the formwork increases its adhesion to the concrete. This is because a rough surface has a larger actual contact area than a smooth one.

The highly porous formwork material also increases adhesion, as cement mortar, penetrating into the pores, during vibrocompaction forms points of reliable connection.

When removing the formwork, there can be three options for separation. In the first variant, the adhesion is very small, and the cohesion is quite large. In this case, the formwork is torn off exactly along the contact plane, the second option - adhesion is greater than cohesion. In this case, the formwork is torn off along the adhesive material (concrete).

The third option - adhesion and cohesion are approximately the same in their values. The formwork is torn off partly along the plane of contact between the concrete and the formwork, partly along the concrete itself (mixed or combined separation).

With adhesive detachment, the formwork is easily removed, its surface remains clean, and the concrete surface has good quality. As a result, it is necessary to strive to ensure adhesive separation. To do this, the forming surfaces of the formwork are made of smooth, poorly wetted 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; hydrophobic lubricants; lubricants - concrete setting retarders; combined lubricants.

Aqueous suspensions of powdered substances inert to concrete are a simple and cheap, but not always effective means for eliminating concrete adhesion to formwork. The principle of operation is based on the fact that as a result of the evaporation of water from suspensions before concreting, a thin protective film is formed on the forming surface of the formwork, which prevents concrete from sticking.

More often than others, a lime-gypsum suspension is used to lubricate the formwork, which is prepared from semi-aqueous 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 vibrocompaction and contaminate concrete surfaces, as a result of which they are rarely used.

The most common hydrophobizing lubricants based on mineral oils, EKS emulsol 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 (Fig. 1-1, b), which impairs 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 to set slowly in thin butt layers are used. To slow down the setting, molasses, tannin, etc. are introduced into the composition of the lubricants. The disadvantage of such lubricants is the difficulty in controlling the thickness of the concrete layer in which it slows down "* Setting.

The most effective are combined lubricants, which use the properties of the forming surfaces in combination with the slowdown in the setting of concrete in thin butt layers. Such lubricants are prepared in the form of so-called inverse emulsions. In some of them, in addition to water repellents and setting retarders, plasticizing additives are introduced: sulfite-yeast vinasse (SDB), soap naft or TsNIPS additive. These substances, when vibrocompacted, plasticize the concrete in the butt layers and reduce its surface porosity.

The composition of some combined lubricants such as inverse emulsions and the conditions for their use are indicated in Table. 1-4.

The ultrasonic mixing plant consists of a circulation pump, suction and pressure pipelines, a junction box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The liquid supplied by the pump under excess pressure of 3.5-5 kgf/cm 2 flows out at high speed from the vibrator nozzle 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 tiny droplets. The duration of mixing is 3-5 minutes.

Emulsion lubricants are stable, they do not delaminate within 7-10 days. Their use completely eliminates the adhesion of concrete to the formwork; they adhere well to the forming surface and do not contaminate concrete.

These lubricants can be applied to the formwork with brushes, rollers and spray rods. With a large number of shields, a special device should be used to lubricate them (Fig. 1-3).

The use of effective lubricants reduces the harmful effects of certain factors on the formwork. In some cases, lubricants cannot be used. So, when concreting in a sliding or climbing formwork, it is forbidden to use such lubricants because they get into the concrete and reduce its quality.

Anti-adhesion protective coatings based on polymers give a good effect. They are applied to the forming surfaces of the shields during their manufacture, and they withstand 20-35 cycles without reapplication and repair. Such coatings completely eliminate the adhesion of concrete to the formwork, improve the quality of its surface, and also protect the wooden formwork from getting wet and warping, and the metal formwork from corrosion.

For metal shields, SE-3 enamel is recommended as an anti-adhesive coating, which includes epoxy resin (4-7 parts by weight), methyl polysiloxane oil (1-2 parts by weight), lead litharge (2-4 parts by weight). .) and polyethylenepolyamine (0.4-0.7 parts by weight). A creamy paste of these components is applied to a thoroughly cleaned and degreased metal surface with a brush or spatula. The coating hardens at 80-140°C for 2.5-3.5 hours. The turnover of such a coating reaches 50 cycles without repair.

For plank and plywood formwork, TsNIIOMTP has developed a coating based on phenol-formaldehyde. It is pressed onto the surface of the panels at a pressure of up to 3 kgf / cm 2 and a temperature of + 80 ° C. This coating completely eliminates the adhesion of concrete to the formwork and withstands up to 35 cycles without repair.

Despite the rather high cost (0.8-1.2 rubles / m 2), anti-adhesive protective coatings are more profitable than lubricants due to their multiple turnover.

It is advisable to use shields, the decks of which are made of getinax, smooth fiberglass or textolite, and the frame is made of metal corners. This 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 features of the adhesion of concrete to formwork, where the value reaches several kg per square centimeter. Due to the adhesion, the stripping of the concrete structure will be more difficult, in addition, this process may degrade the concrete surface itself, namely, its quality. And the formwork panels can even collapse before the specified time. To prevent this from happening, ubts.kiev.ua is now available, which solves all these problems.

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

At the moment when the concrete is laid, its state is plastic, so it is considered an adhesive, due to which a process called adhesion takes place (when the concrete sticks to the formwork). When the material is compacted, the plasticity index of the concrete may increase, as a result of which it adheres to the formwork surface.

The adhesion process can be different, depending on the material that was used to produce the formwork surface: concrete will adhere more strongly to wood and steel. Plastic products, due to their less wettability, are the least likely to adhere to concrete.

If plywood, steel, wood or fiberglass materials are not pre-treated, they will be easily wetted, which will provide good adhesion to concrete. Less significant coefficient of adhesion with getinax and textolite, since they belong to the category of hydrophobic materials.

Wetting can be reduced by surface treatment, which is the application of an oil film on it, as a result of which the adhesion process will significantly decrease. Due to shrinkage, not only adhesion can be reduced, but adhesion: 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 it is required to strip a structure made of concrete of a monolithic type, then three methods are now available, thanks to which the separation is made removable formwork:
high cohesion and low adhesion. In this situation, formwork separation along the contact plane is required;
the level of adhesion exceeds cohesion. The formwork will be torn off using a material that is adhesive (concrete);
approximate equality between adhesion and cohesion. This situation suggests a separation of a mixed (combined) type.

The first option is the most optimal, since it allows you to easily remove the formwork, keeping its surface clean, and also maintain the quality of the concrete itself. In this regard, adhesive separation should be provided more often than others. It is available in the following situations:
when the forming formwork surface is made of a smooth material that is poorly wetted;
the forming surface was treated with a special lubricant or special anti-adhesion 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 include volatile substances that are hazardous to human health;
the ability to stay on the surface (vertical and horizontal) during the day at an air temperature of +30 degrees Celsius.

On April 22, a scientific and practical conference "Problems of monolithic construction and ways to solve them" was held at the State Unitary Enterprise "NIIMosstroy". The conference was attended by representatives of JSC "NIIZHB" them. A.A. Gvozdev, GEOStrom LLC, Moscow IMET OJSC, CEIIS State Budgetary Institution, NIIMosstroy State Unitary Enterprise, MonArch OJSC, GeroKrit LLC, BASF Construction Systems LLC, etc.

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

We hope that the materials of this conference, published as a separate book by SUE "NIIMosstroy", will serve to improve the 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 Testing Laboratory building materials and designs by 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 of reinforced concrete work, faced by employees of our laboratory at construction sites city ​​of Moscow.

- early demoulding 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 conditions for curing concrete in the formwork and strip structures at an earlier stage than the requirements of the project technological maps and SNiP 3-03-01-87. When dismantling the formwork, the amount of adhesion of concrete to the formwork is of great importance when: a large adhesion makes it difficult to remove the formwork. The deterioration of the quality of concrete surfaces leads to defects.

- production of insufficiently rigid, deformable when laying 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. Formwork deformation 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:

If they decrease

To a decrease in bearing capacity

In case of increase to increase their own weight.

This type of violation of the monitoring technology during the manufacture of formwork in construction conditions without proper engineering control.

- insufficient thickness or absence of a protective layer.

It is observed with improper installation or displacement of the formwork or reinforcement cage, the absence of gaskets.

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

- non-compliance with the design of structural reinforcement;

- poor-quality welding of structural units and joints of reinforcement;

- use of heavily corroded reinforcement.

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

- laying of stratified concrete mix does not allow to obtain uniform strength and density of concrete throughout the entire volume of the structure;

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

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

- poor care of concrete during its hardening.

During curing of concrete, it is necessary to create such temperature-moist conditions that would ensure the preservation of water in the concrete, which is necessary for the hydration of the cement. If the curing 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 thermal deformations will be insignificant. Concrete is usually covered with plastic sheeting or other protective coating. To prevent it from drying out. Overdried concrete has significantly less strength and frost resistance than normally hardened concrete, and many shrinkage cracks appear in it.

When concreting in winter conditions, with insufficient insulation or heat treatment, early freezing of concrete may occur. After thawing such concrete, it will not be able to gain the necessary 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 pits, voids; cracks, including shrinkage cracks, with an opening of not more than 0.2 mm, and also, in which, under the influence of temporary load and temperature, the opening increases by no more than 0 , 1 mm; concrete chips without reinforcement exposure, etc.);

Group II - damage that reduces the durability of the structure (corrosion cracks with an opening of more than 0.2 mm and cracks with an opening of more than 0.1 mm, in the zone of working reinforcement of prestressed superstructures, including along sections under constant load; cracks with an opening of more than 0.3 mm under temporary load, voids in the shell and chips with exposed reinforcement, surface and deep corrosion of concrete, etc.);

Group III - damage that reduces the bearing capacity of the structure (cracks that are not provided for by the calculation either in terms of strength or endurance; inclined cracks in the walls of beams; horizontal cracks in the junctions of the slab and superstructures; large shells and voids in the concrete of the compressed zone, etc. .).

Group I damage does not require urgent action, they can be eliminated by applying coatings at the current maintenance for preventive purposes. The main purpose of coatings for group I damage 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 group II damage, the repair provides an increase in the durability of the structure. Therefore, the materials used must have sufficient durability. Mandatory sealing is subject to cracks in the area of ​​the beams of prestressed reinforcement, cracks along the reinforcement.

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

To eliminate damages of group III, 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 the construction of monolithic reinforced concrete may have Negative consequences associated with a rather low level of quality of individual objects. Among the main reasons for the low quality of erected monolithic buildings, the following should be highlighted.

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

Secondly, most construction organizations do not have sufficient experience and the necessary technological culture of monolithic construction, as well as low-quality technical equipment.

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

The quality of concrete is, first of all, the correspondence of its characteristics to the parameters in regulatory documents. Rosstandart approved and have new standards: GOST 7473 “Concrete mixes. Specifications”, GOST 18195 “Concrete. Rules for the control and evaluation of strength. GOST 31914 “High-strength heavy and fine-grained concrete for monolithic structures”, the standard for reinforcing and embedded products should become valid.

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, concrete design, 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 through a set of various conditions: modern technological equipment, the availability of accredited testing laboratories, qualified personnel, unconditional compliance with regulatory requirements, and the introduction of quality management processes.

Head of the Laboratory for Testing Building Materials and

designs of GBU "CEIIS" -D.N. Abramov

Tech Candidates. Ya. P. BONDAR (TsNIIEP housing) Yu. S. OSTRINSKY (NIIES)

To find ways of concreting in the 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 sliding formwork, this is the minimum allowable wall thickness. For molding concrete, expanded clay gravel from the Beskudnikovsky plant with crushed sand from the same expanded clay and slag pumice made from melts of the Novo-Lipetsk Metallurgical Plant with a fishing line obtained by crushing slag lemza were used.

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

The interaction forces of the sliding formwork with concrete mixtures were measured on a test rig, which is a modification of the Casa Rande device for measuring single-plane shear forces. The installation is made in the form of a horizontal tray filled with concrete mix. Across the tray, test rails were laid from wooden bars, sheathed on the surface of contact with the concrete mixture with strips of roofing steel. Thus, the test rails simulated a steel sliding formwork. The slats were kept on the concrete mixture under weights of various sizes, simulating the pressure of concrete on the formwork, after which the forces causing the horizontal movement of the slats along the concrete were recorded. General form installation is given in fig. one.

According to the results of the tests, the dependence of the forces of interaction between the steel sliding formwork and the concrete mixture 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 y-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 the fixed contact from 15 to 60 minutes, the value of the adhesion forces increases by 1.5-2 times. 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 adhesion force of heavy concrete and steel formwork 15 minutes after 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 interaction force of heavy concrete and steel formwork (120-150 kg/m2). The main part of the effort falls on the friction forces.

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

The share of cohesive forces in the total 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 pressure of the mixture, which, under concreting conditions, changes with time. To test this assumption, the shrinkage or swelling of freshly molded concrete specimens was measured immediately after vibration compaction. During the formation of concrete cubes with an edge size of 150 mm, a textolite plate was placed on one of its vertical faces, the smooth surface of which was in the same plane with the vertical face. After the concrete was compacted and the sample was removed from the vibrating table, the vertical faces of the cube were freed from the side walls of the mold, and within 60–70 min, the distances between the opposite vertical faces were measured using a messureur. The measurement results showed that freshly molded concrete shrinks immediately after compaction, the value of which is the higher, the greater the mobility of the mix. The total value of bilateral upset reaches 0.6 mm, i.e. 0.4% of the sample thickness. AT initial period after molding, there is no swelling of freshly laid concrete. This is due to contraction in the initial stage of concrete setting during the redistribution of water, accompanied by the formation of hydrated films that create high 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 calculation 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 ohm in a sliding formwork showed that the mixtures of the adopted compositions after their compaction by vibration do not exert active pressure on the formwork skin. The pressure in the "sliding formwork - concrete mixture" system is due to the elastic deformation of the panels under the influence of the hydrostatic head of the mixture 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 quite well modeled by the passive repulsion of a viscoplastic body under pressure from a vertical retaining wall. Calculations have shown that with a one-sided action of the formwork shield on the concrete mass, in order to shift a part of the array along the main sliding planes, an increase in pressure is required, significantly exceeding the pressure that occurs with the most unfavorable combination of conditions for laying and compacting the mixture. With double-sided pressure of formwork panels on a vertical layer of concrete of limited thickness, the pressure forces necessary to shift the compacted concrete along the main sliding planes acquire the opposite sign and significantly exceed the pressure required to change the compression characteristics of the mixture. The reverse loosening of the compacted mixture under the action of bilateral compression requires such high pressure, which is unattainable when concreting in sliding formwork.

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

To determine the interaction forces that occur during the concreting process, measurements were carried out on a life-size sliding formwork model. A sensor with a high-strength phosphor 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 by an automatic pressure gauge (AID-6M) during the vibration and lifting of the formwork by an N-700 photooscilloscope with an 8-ANCH amplifier. The actual characteristics of the interaction of steel sliding formwork with different types of concrete are given in the table.

In the period between the end of the vibration and the first lifting 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 molded mixture.

To reduce the forces of interaction of the sliding formwork with the concrete mix, 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") made of thin (up to 2 mm) sheet material. The height of the liners is greater than the height of the molding cavity (30-35 ohms). The liners are installed in the molding cavity close to the sliding formwork panels (Fig. 5) and, immediately after laying and compaction, the concrete is removed from it one by one.

The gap (2 mm) remaining between the concrete and the formwork, after the removal of the shields, protects the formwork shield, straightening after elastic deflection (as a rule, not exceeding 1-1.5 mm) from contact with the vertical surface of the 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 sliding formwork.

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

Heavy concrete (although to a lesser extent) also shows the ability to maintain the verticality of freshly molded surfaces with its mobility not exceeding 8 cm. 1.6 m, providing concreting of walls with a length of 150-200 m. This will significantly reduce the consumption of concrete compared to buildings erected according to the accepted technology, and increase economic efficiency their construction.