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. In tab. 1-3 shows the values ​​of normal adhesion of concrete with some formwork materials.

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 a film of oil, it is water-repellent (Fig. 1-1, b), which drastically reduces adhesion.

Shrinkage adversely affects the adhesion, and hence the adhesion. The greater the amount of shrinkage in the joint layers of concrete, the more likely the appearance of shrinkage cracks in the contact zone, weakening the adhesion. Under cohesion in the contact pair of formwork - concrete, you should understand the tensile strength of the joint 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.

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 the 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.

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 formwork surface, a hydrophobic film is formed from a number of oriented molecules (Fig. 1-1, b), 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, which slows down "* Seizure.

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.

The composition of some combination lubricants of the type of inverse emulsions and the conditions for their use are shown in Table. 1-4.

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 overpressure of 3.5–5 kgf / cm 2 expires 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 the smallest droplets. The 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 6-tone.

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 for their lubrication (Fig. 1-3).

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.

Anti-adhesive protective coatings on c) are more effective. They are applied to the forming surfaces of the boards during their manufacture, and they withstand 20-35 cycles without reapplication and repair. Such coatings completely eliminate the adhesion of concrete to a form, improve the quality of its surface, and also protect wooden formwork from wetting and warping, and metal formwork from corrosion.

For metal panels, the enamel SE-3 is recommended as an anti-adhesive coating. .) and polyethylene polyamine (0.4-0.7 wt. h.). Creamy paste from 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 this coating reaches 50 cycles without repair.

For the plank and plywood formwork in the Central Scientific Research Institute of Aviation and Timber Industry, 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. 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 (0.8-1.2 rubles / m 2), anti-adhesive protective coatings are more profitable than lubricants due to their repeated turnover.

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

In my report I would like to tell about the main violations of the production technology of reinforced concrete work faced by the staff of our laboratory on the construction sites of the city of Moscow.

- early demolition 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 of curing concrete in the formwork and demolish structures at an earlier stage than it is provided for by the project requirements with technological maps and SNiP 3-03-01-87. When dismantling the formwork, the amount of adhesion of the concrete to the formwork is important in case of: large adhesion it is difficult to dismantle the work. Deterioration in the quality of concrete surfaces leads to defects.

- production is not enough rigid, deformed when laying concrete and not dense formwork.

Such formwork receives deformation during the laying of the concrete mix, which leads to a change in the shape of reinforced concrete elements. The deformation of the formwork can lead to the displacement and deformation of the reinforcement cages and walls, changes in the bearing capacity of the structural elements, the formation of projections and sagging. Violation of the design dimensions of structures results:

In the case of their reduction

To reduce the carrying capacity

In the case of increasing to increase their own weight.

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

- insufficient thickness or lack of a protective layer.

Observed with improper installation or offset formwork or reinforced frame, no gaskets.

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

- discrepancy to the project of reinforcement of structures;

- poor-quality welding of structural components and fittings;

- the use of highly corrosive reinforcement.

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

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

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

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

- poor care of concrete in the process of hardening.

During the care of the concrete, it is necessary to create such temperature-wet 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. In order to prevent it from drying out. 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 damage of reinforced concrete structures is divided into three groups according to the nature of the influence on the carrying capacity.

Group I - damage that practically does not reduce the strength and durability of the structure (surface sinks, voids; cracks, including shrinkage, with a disclosure of not more than 0.2mm, and also, which, under the influence of temporary load and temperature, the disclosure increases by no more than 0 , 1mm; chipped concrete without exposing reinforcement, etc.);

Group II - damage, reducing the durability of the structure (corrosion-hazardous cracks more than 0.2 mm open and cracks more than 0.1 mm open, in the area of ​​working reinforcement of prestressed spans, including along sections under constant load; cracks more than 0.3 mm open under temporary load, emptiness of the shell and chips with exposed rebar, surface and deep corrosion of concrete, etc.);

Group III - damage, reducing the bearing capacity of the structure (cracks, not provided for by calculation of either strength or endurance; inclined cracks in the walls of beams; horizontal cracks in the mates of the slab and span structures; large shells and voids in the concrete of the compressed zone, etc. .).

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

In case of damages of group II, the repair provides increased durability of the structure. Therefore, the materials used must have sufficient durability. Cracks in the area of ​​the bundles of prestressed reinforcement, cracks along the reinforcement are subject to mandatory sealing.

At damages of the III group restore the bearing ability of a design on a specific sign. Applied materials and technologies should provide strength characteristics and durability of the structure.

For the elimination of damages of group III, as a rule, individual projects should be developed.

The constant growth of monolithic construction is one of the main trends characterizing 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 poor 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 priority development of construction from precast concrete, therefore their focus on factory technologies and insufficient development of monolithic reinforced construction are natural.

Secondly, most construction companies 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, including a system of reliable technological quality control of works, has not been created.

The quality of concrete is, first of all, the compliance of its characteristics with the parameters in regulatory documents. Rosstandart approved and the new standards are in force: GOST 7473 “Concrete mixes. Technical conditions ", GOST 18195" Concretes. Rules of control and evaluation of strength. GOST 31914 “High-strength heavy and fine-grained concretes for monolithic structures” should come into force, the standard for reinforcing and embedded products should become the current one.

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

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

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.

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

Hello Vadim Aleksandrovich!

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

1. Is it necessary to prepare formwork and reinforcement in some way?

The formwork is smeared with a special water-based lubricant (Emulsol) in order to separate the formwork from hardened concrete. Although at the construction site there were cases when they poured into an ungreased formwork and then it was torn off. Also the formwork is tightened with special ties, which are inserted into the tubes between the shields.

2. Is the method of pouring horizontal forms from vertical?

Virtually no different. Vertical is a little harder to tamp.

3. Please tell us how to pour concrete.

The method of casting is determined by the project (TCH). It is desirable to fill the entire formwork at once, pouring layers is undesirable, otherwise you will have to make notches with a punch for better adhesion of the layers. Vertical forms must be poured entirely.

4. How to merge layers if you still fill with layers? Well, we did not have enough concrete to fill the whole.

As I said, we make the notches punch on frozen concrete.

5. What are the secrets of a uniform fill?

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

6. How does the density of the solution to fill?

A thick solution is difficult to evenly distribute and tamp. Before pouring, add water to the mixer. Too liquid - and again it is bad, when compacting all the water will flow out and the concrete will not catch. If we do it ourselves, then we add cement and sand, if we are brought ready, then we are sent to the plant because of inconsistencies.

7. I heard that the concrete heats up when it freezes. Is this a problem and should it be dealt with?

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

8. If we did not follow and the concrete cracked, how to fix it?

Small cracks are permissible, the maximum crack size is specified in the project documentation, if the size is exceeded - then take a jackhammer and repel. Otherwise, it will fall apart by itself in some time. After all, cracks significantly reduce the strength of the structure.

Many thanks for the consultation Vadim Alexandrovich. We and our readers are very grateful to you.