Measures to reduce the adhesion of formwork to concrete. Lubricants for removable formwork Reasons for adhesion of concrete to formwork

The adhesion of concrete to the formwork is affected by adhesion (adhesion) and shrinkage of concrete, surface roughness and porosity. With a large force of adhesion of concrete to the formwork, the formwork is complicated, the complexity of the work increases, the quality of the concrete surfaces deteriorates, the formwork panels prematurely wear out.

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

The strength (H) of adhesion of some formwork materials with concrete is as follows:

Therefore, to obtain high-quality surfaces, it is necessary to use claddings from textolite, hetinax, polypropylene or use waterproof plywood treated with special compounds. When the adhesion is small, the concrete surface is not broken and the formwork easily leaves. 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 surface quality is significantly reduced. Adhesion can be reduced by applying aqueous suspensions, hydrophobic lubricants, combined lubricants, and lubricants - concrete retarders to the formwork surface. The principle of action of aqueous suspensions and hydrophobic lubricants is based on the fact that a protective film forms 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 vinasse (SDB), soap-oil. Such lubricants plasticize the concrete of the adjacent area, and it does not collapse.

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

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

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10. DEFECTS OF MONOLITHIC REINFORCED CONCRETE STRUCTURES CAUSED BY VIOLATION OF THE TECHNOLOGY OF THEIR ESTABLISHMENT

The main violations of the technology of work leading to the formation of defects in monolithic reinforced concrete structures include the following:
   - manufacturing is not rigid enough, strongly deforming when laying concrete and not sufficiently dense formwork;
   - violation of the design dimensions of structures;
   - poor compaction of the concrete mixture when laying it in the formwork;
   - laying stratified concrete mix;
   - the use of too hard concrete mixture with dense reinforcement;
   - poor care of concrete in the process of hardening;
   - use of concrete with strength below design;
   - non-compliance with the design of reinforcing structures;
   - poor-quality welding of joints of fittings;
   - the use of heavily corroded fittings;
   - early formwork removal;
   - violation of the required sequence of stripping 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 take the form of strongly bent structures, vertical surfaces become convex. Deformation of the formwork can lead to displacement and deformation of the reinforcing cages and meshes and to a change in the bearing capacity of the elements. It should be borne in mind that the dead weight of the structure increases.
Loose formwork contributes to the outflow of cement mortar and the appearance in this connection of shells and caverns in concrete. Sinks and caverns also arise due to insufficient compaction of the concrete mixture when it is laid in the formwork. The appearance of shells and caverns causes a more or less significant decrease in the bearing capacity of elements, an increase in the permeability of structures, contributes to corrosion of the reinforcement located in the zone of shells and caverns, and can also cause the reinforcement to pull through in concrete.
   Reducing the design dimensions of the cross section of the elements leads to a decrease in their bearing capacity, an increase leads to an increase in the dead weight of the structures.
   The use of stratified concrete mix does not allow to obtain uniform strength and density of concrete throughout the entire volume of the structure and reduces the strength of concrete.
   The use of too rigid concrete mixture with dense reinforcement leads to the formation of shells and caverns around the reinforcing bars, which reduces the adhesion of the reinforcement to concrete and causes the risk of corrosion of the reinforcement.
   During the maintenance of concrete, such temperature and humidity conditions should be created that would ensure that the water required for the hydration of the cement is retained in the concrete. If the hardening process proceeds at a relatively constant temperature and humidity, stresses arising in concrete due to volume changes and caused by shrinkage and thermal deformation will be insignificant. Typically, concrete is coated with plastic wrap or other protective coating. Perhaps the use of film-forming materials. Concrete care is usually carried out within three weeks, and when using concrete heating - at the end of it.
   Poor care of concrete leads to overdrying of the surface of reinforced concrete elements or their entire thickness. Overdried concrete has significantly lower strength and frost resistance than normally hardened concrete; a lot of 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, 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, providing the necessary resistance to ice pressure and preserving subsequently at positive temperatures the ability to harden without significant deterioration in the properties of concrete are given in table. 10.1.

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

If all ice and snow have not been removed from the formwork prior to concreting, then shells and caverns appear in the concrete. An example is the construction of a boiler room in permafrost.
   The base of the boiler room was a monolithic reinforced concrete slab, in which the heads of piles immersed in the ground were embedded. A ventilated space was provided between the stove and the ground to isolate the soil from heat penetrating the boiler room floor. Releases of reinforcement were made from the top of the piles, around which ice was formed, not removed before concreting. This ice melted in the summer and the base plate of the building turned out to be supported only by the release of reinforcement from piles (Fig. 10.1). Reinforcing outlets from piles were deformed under the influence of the weight of the entire building and the base plate received large uneven precipitation.

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

Non-compliance with the project of concrete strength and reinforcement of structures, as well as poor-quality welding of reinforcement outlets and intersection of rods affects the strength, crack resistance, and stiffness of monolithic structures as well as similar defects in prefabricated reinforced concrete elements.
   Slight corrosion of the reinforcement does not affect the adhesion of the reinforcement to concrete, and, consequently, the operation of the entire 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 the elements due to a decrease in the section of reinforcement due to corrosion, an increase in the deformability of the elements and a decrease in crack resistance are observed.
The early dismantling of structures can lead to complete unsuitability of the structure and even its collapse during the dismantling process due to the fact that the concrete has not gained sufficient strength. The formwork 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 bending elements and the side surfaces of the columns. The last formwork can be removed only when the strength of the structures from the influence of its own weight and temporary load acting during the construction period is ensured. Submitted by N. N. Luknitsky, the removal of formwork of slabs with a span of up to 2.5 m can be carried out no earlier than concrete reaching 50% of the design strength, slabs with a span of more than 2.5 m and beams - 70%, long-span structures - 100%.
   When stripping vaulted structures, the circles at the castle must first be released, and then at the heels of the structure. At first, the crib circled to free from the heels, then the arch rests on the circling in its castle part, and the vault is not designed for such work.
   Currently, monolithic reinforced concrete structures are widely used, especially in multi-storey housing construction.
   Construction organizations, as a rule, do not have the appropriate formwork and rent it. Rental of formwork is expensive, so builders minimize the period of its turnover. Usually, formwork is done two days after concrete is laid. At such a pace of erection of monolithic structures, especially careful study of all stages of work is required: transportation of concrete mix, laying concrete in the formwork, moisture preservation in concrete, concrete warming up, concrete insulation, control of the heating temperature and concrete strength gain.
   To reduce the negative effect of the temperature difference of concrete, you should choose the minimum allowable temperature for heating concrete during formwork.
   For vertical structures (walls), the heating temperature of concrete can be recommended at 20 ° C, and for horizontal (floors) - 30 ° C. In St. Petersburg, within two days, the average air temperature is 20 ° C and, especially, 30 ° C. Therefore, concrete should be heated at any time of the year. Even in April and October, the author was not able to see the concrete heating at construction sites.
In winter, the concrete of the floors should be insulated when heated by laying a layer of effective insulation on top of the polyethylene film. And this is not done in many cases. Therefore, floor slabs, concreted in winter, have concrete strength 3-4 times lower from above than from below.
   When stripping in the middle of a section of a floor slab, temporary support is left in the form of a rack or a formwork section. Also, temporary supports should be installed before the stripping strictly vertically along the floors, which is also often not observed.
   Since the strength of concrete walls when stripping does not reach the design value, it is necessary to do an intermediate calculation to determine the number of floors that can be erected in winter.
   There is a large shortage of instructional literature on monolithic reinforced concrete, which affects its quality.

The text of the report presented at the conference by the head of the Laboratory for Testing Building Materials and Structures Dmitry Nikolaevich Abramov “The main causes of defects in concrete structures”

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

- early formwork removal.

Due to the high cost of formwork in order to increase the number of cycles of its turnover, builders often do not adhere to the concrete curing conditions in the formwork and carry out the formwork removal at an earlier stage than is required by the design cards and SNiP 3-03-01-87. When dismantling the formwork, the adhesion of concrete to the formwork is important when: large adhesion makes it difficult to dismantle. Deterioration in the quality of concrete surfaces leads to defects.

- manufacturing is not rigid enough, deforming when laying concrete and not sufficiently dense formwork.

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

In case of their decrease

To decrease bearing capacity

In case of increase to increase of their own weight.

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

- insufficient thickness or lack of a protective layer.

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

Serious defects in monolithic reinforced concrete structures can be caused by poor quality control of reinforcing structures. The most common are violations:

- inconsistency with the design of reinforcing structures;

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

- the use of highly corroded fittings.

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

- laying of the 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 caverns 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 adhesion of concrete mixture to reinforcement and formwork, which causes the formation of cavities in the body of concrete structures.

- poor maintenance of concrete in the process of hardening.

During the maintenance of concrete, temperature-moist conditions should be created that would ensure that the water required for the hydration of the cement is retained in the concrete. If the hardening process proceeds at a relatively constant temperature and humidity, stresses arising in concrete due to volume changes and caused by shrinkage and thermal deformation will be insignificant. Typically, concrete is coated with plastic wrap or other protective coating. In order to prevent it from drying out. Overdried concrete has significantly lower strength and frost resistance than normally hardened concrete; a lot of 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, he will not be able to gain the necessary strength.

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

Group I - damages that practically do not reduce the strength and durability of the structure (surface shells, voids; cracks, including shrinkage, with openings not exceeding 0.2 mm, and also, under the influence of a temporary load and temperature, the opening increases by no more than 0 , 1mm; concrete chips without exposure of reinforcement, etc.);

Group II - damages that reduce the durability of the structure (corrosive cracks with an opening of more than 0.2 mm and cracks with an opening of more than 0.1 mm, in the area of \u200b\u200bworking reinforcement of prestressed spans, including along sections under constant load; cracks with an opening of more than 0.3 mm under temporary load; voids of 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 not provided for by calculation of either strength or endurance; inclined cracks in the walls of the beams; horizontal cracks in the joints of the slab and spans; large shells and voids in the concrete of the compressed zone, etc. .).

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

In case of damage of group II, repair provides an increase in the durability of the structure. Therefore, the materials used must have sufficient durability. Cracks in the area of \u200b\u200bthe arrangement of bundles of prestressed reinforcement, cracks along the reinforcement are subject to obligatory sealing.

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

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

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

Firstly, the majority of regulatory documents currently in force in Russia were created in the era of priority development of prefabricated reinforced concrete construction, 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 lack sufficient experience and the necessary technological culture of monolithic construction, as well as poor-quality technical equipment.

Thirdly, an effective quality management system for monolithic construction has not been created, 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 are operating new standards: GOST 7473 “Concrete mixtures. Specifications ", GOST 18195" Concrete. Rules of control and strength assessment. " GOST 31914 “High-strength heavy and fine-grained concrete for monolithic structures” should come into force, the standard for reinforcing and embedded products should become effective.

The new standards, unfortunately, do not contain issues related to the specifics of legal relations between construction customers and general contractors, manufacturers of building materials and builders, although the quality of concrete work depends on each stage of the technical chain: preparation of raw materials for production, 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 variety of conditions: here, modern technological equipment, the presence of accredited testing laboratories, qualified personnel, unconditional compliance with regulatory requirements, and the implementation of quality management processes.

The adhesion of concrete to the formwork reaches several kgf / cm2. This makes formwork difficult, degrades the quality of concrete surfaces and leads to premature wear of formwork panels.

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

By adhesion (adhesion) is understood the bond due to molecular forces between the surfaces of two dissimilar or liquid contacting bodies. In the period of contact of concrete with the formwork, favorable conditions are created for the manifestation of adhesion. Adhesive (adhesive), which in this case is concrete, is in a plastic state during installation. In addition, in the process of vibration compaction 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.

Wood, plywood, steel without processing and fiberglass are well wetted and the adhesion of concrete to them is quite large, with slightly wettable (hydrophobic) getinaks and textolite, the concrete adheres slightly.

The wetting angle of brushed steel is greater than that of raw steel. However, the adhesion of concrete to brushed steel is reduced slightly. This is explained by the fact that at the border of concrete and well-machined surfaces, the contact continuity is higher.

When applied to the surface of the oil film, it hydrophobizes, which sharply reduces adhesion.

Shrinkage adversely affects adhesion, and hence adhesion. The larger the shrinkage in the butt layers of concrete, the more likely the appearance of shrinkage cracks in the contact zone, weakening adhesion. By cohesion in the contact pair of the formwork - concrete, it should be understood the tensile strength of the connecting layers of concrete.

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.

Highly supported formwork material also increases adhesion, since cement mortar, penetrating into the pores, under vibration compaction forms a point of reliable connection.

When removing the formwork, there 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 along the contact plane. The second option is adhesion more than cohesion. In this case, the formwork comes off using an adhesive material (concrete).

The third option — adhesion and cohesion are approximately the same in size. The formwork comes off partly along the plane of contact of concrete with the formwork, partly along the concrete itself (mixed or combined separation).

With adhesive separation, the formwork is easily removed, its surface remains clean, and the concrete surface is of good quality. As a consequence of this, it is necessary to strive to ensure adhesion separation. To this end, the formwork surfaces of the formwork are made of smooth, poorly wettable materials or they are applied with lubrication and special release coatings.

Formwork lubricants  depending on their composition, principle of operation and operational properties can be divided into four groups: aqueous suspensions; water repellent lubricants; lubricants - concrete setting retarders; combined lubricants.

Aqueous suspensions of powdered substances that are inert to concrete are simple and cheap, but not always effective in eliminating the adhesion of concrete to the 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 formwork surface of the formwork, which prevents the adhesion of concrete.

Most often, for the formwork lubrication, a lime-gypsum-coBVio suspension is used, which is prepared from semi-aquatic gypsum (0.6-0.9 weight parts), lime test (0.4-0.6 weight parts), sulfite alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).

Suspension lubricants are erased with concrete mix and vibroconsolidation and contaminate concrete surfaces, as a result of which they are rarely used.

The most common hydrophobic lubricants are based on minsoal oils, emulsol EX or salts of fatty acids (soaps). After their application to the surface of the formwork, a hydrophobic film of a number of oriented molecules is formed (Fig. 1-1, b), which impairs the adhesion of the formwork material to concrete. The disadvantages of such lubricants are pollution of the concrete surface, high cost and fire hazard.

In the third group of lubricants, the properties of concrete are used to set in slow motion in thin joint layers. To slow down the hardening, molasses, tannin, etc. are introduced into the composition of the lubricants. The disadvantage of such lubricants is the difficulty in regulating the thickness of the concrete layer in which the setting slows down.

Most effective combined lubricantsin which the properties of forming surfaces are used in combination with a delay in the setting of concrete in thin joint layers. Such lubricants are prepared in the form of so-called inverse emulsions. In addition to gndrofobizatora and setting retarders, some of them include plasticizing additives: sulphite-yeast vinasse (SDB), soap soap or TsNIPS additive. These substances during vibration compaction plasticize concrete in the butt layers and reduce its surface porosity.

ESO-GISI lubricants are prepared in ultrasonic hydrodynamic mixers (Fig. 1-2), in which mechanical mixing of the components is combined with ultrasonic. To do this, pour components into the mixer tank and turn on the mixer.

The installation for ultrasonic mixing consists of a circulation pump, a suction and pressure pipes, a junction box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The fluid supplied by the pump under an overpressure of 3.5-5 kgf / cm2 flows out 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 while dividing the components into tiny droplets. The mixing time is 3-5 minutes.

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

These greases and formwork can be applied with brushes, rollers and 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 certain factors.

For metal shields, the CE-3 enamel, which includes epoxy resin (4-7 parts by weight), methylpolysiloxane oil (1-2 parts by weight), lead litharge (2-4 parts by weight, is recommended as a release coating). ) and polyethylene polyamine (0.4-0.7 wt. h.). Creamy paste of these components is applied to a thoroughly cleaned and degreased metal surface with a brush or trowel. 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 at TsNIIOMTP a phenol-formaldehyde-based coating has been developed. It is pressed onto the surface of the panels 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 rather high cost (0.8-1.2 rubles / m2), 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.

The adhesion of concrete to the formwork reaches several kgf / cm 2. This makes formwork difficult, degrades the quality of concrete surfaces and leads to premature wear of formwork panels.
  The adhesion of concrete to the formwork is influenced by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the forming surface of the formwork.
  By adhesion (adhesion) is understood the bond due to molecular forces between the surfaces of two dissimilar or liquid contacting bodies. In 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 plastic state during installation. In addition, in the process of vibration compaction 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. The values \u200b\u200bof Kc for different types of formwork are: small-panel - 0.15, wooden - 0.35, steel - 0.40, large-panel (panels of small panels) - 0.25, large-panel - 0.30, volumetric - 0, 45, for block forms - 0.55.
  Wood, plywood, steel without processing and fiberglass are well wetted and the adhesion of concrete to them is quite large, with poorly wettable (hydrophobic) getinaks and textolite, concrete adheres slightly.
  The wetting angle of brushed steel is greater than that of raw steel. However, the adhesion of concrete to brushed steel is reduced slightly. This is explained by the fact that at the border of concrete and well-machined surfaces, the contact continuity is higher.
  When applied to the surface of the oil film, it hydrophobizes, which sharply 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 cement mortar, penetrating into the pores, forms vibration-tight points when vibro-compacted. When removing the formwork, there 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 along the contact plane. The other option is adhesion more than cohesion. In this case, the formwork comes off using an adhesive material (concrete).
  The third option - adhesion and cohesion are approximately the same in value. The formwork comes off partly along the plane of contact of concrete with the formwork, partly along the concrete itself (mixed or combined separation).
  With adhesive separation, the formwork is easily removed, its surface remains clean, and the concrete surface is of good quality. As a consequence of this, it is necessary to strive to ensure adhesion separation. To do this, the formwork surfaces of the formwork are made of smooth, poorly wettable materials or they are lubricated and special release coatings applied to them.
  Lubricants for formwork, depending on their composition, principle of operation and operational properties, can be divided into four groups: aqueous suspensions; water repellent lubricants; lubricants - concrete setting retarders; combined lubricants.
  Aqueous suspensions of powdered substances that are inert to concrete are simple and cheap, but not always effective in eliminating the adhesion of concrete to the 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 the adhesion of concrete.
  Most often, lime-gypsum slurry is used to lubricate the formwork, which is prepared from gypsum gypsum (0.6-0.9 parts by weight), lime dough (0.4-0.6 parts by weight), sulphite-alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).
  Suspension lubricants are erased by concrete mixture during vibration compaction 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 their application to the surface of the formwork, a hydrophobic film of a number of oriented molecules is formed, 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 are used to set in slow motion in thin joint 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 in controlling the thickness of the concrete layer.
  Combined lubricants, which use the properties of forming surfaces in combination with a delay in the setting of concrete in thin joint layers, are most effective. Such lubricants are prepared in the form of so-called inverse emulsions. In addition to water repellents and setting retarders, some of them include plasticizing additives: sulphite-yeast vinasse (SDB), soap soap or TsNIPS additive. These substances during vibration compaction plasticize concrete in the butt layers and reduce its surface porosity.
  ESO-GISI lubricants are prepared in ultrasonic hydrodynamic mixers in which mechanical mixing of the components is combined with ultrasonic. To do this, pour components into the mixer tank and turn on the mixer.
  The installation for ultrasonic mixing consists of a circulation pump, a suction and pressure pipes, a junction box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The fluid supplied by the pump under an overpressure of 3.5-5 kgf / cm2 flows out 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 while dividing the components into tiny droplets. The mixing time is 3-5 minutes.
  Emulsion lubricants are stable, they do not exfoliate for 7-10 days. Their use completely eliminates the adhesion of concrete to the formwork; they hold well on the forming surface and do not contaminate concrete.
  Apply these lubricants to the formwork with brushes, rollers and 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 certain factors. In some cases, grease cannot be used. So, when concreting in sliding or climbing formwork, it is forbidden to use such lubricants due to their ingress into concrete and a decrease in its quality.
  Polymer-based release coatings give a good effect. They are applied to the forming surfaces of the panels during their manufacture, and they withstand 20-35 cycles without repeated application and repair.
  A phenol-formaldehyde-based coating has been developed for plank and plywood formwork. It is pressed onto the surface of the panels 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 rather high cost, 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.