Interesting and useful information about building materials and technologies. Measures to reduce the adhesion of concrete to formwork Reasons for concrete products sticking to formwork
Candidates of technical Sciences Y. P. BONDAR (TSNIIEP housing) Y. S. OSTRINSKY (NIIES)
To find methods for concreting in sliding formwork for walls less than 12-15 ohms thick, the interaction forces between formwork and concrete mixtures prepared with dense aggregates, expanded clay and slag pumice were studied. With the existing technology of concreting in sliding formwork, this is the minimum permissible wall thickness. For molded concrete, expanded clay gravel from the Beskudnikovsky plant with crushed sand from the same expanded clay and slag pumice made from melts from the Novo-Lipetsk Metallurgical Plant with a line obtained by crushing slag lemza were used.
Expanded clay concrete grade 100 had vibration compaction, measured on N. Ya. Spivak’s device, 12-15 s; structure factor 0.45; volumetric mass 1170 kg/m3. Slag pumice concrete grade 200 had a vibration compaction time of 15-20 s, a structure factor of 0.5, and a volumetric mass of 2170 kg/m3. Heavy concrete grade 200 at volumetric mass 2400 kg/m3 was characterized by a standard cone draft of 7 cm.
The forces of interaction between sliding formwork and concrete mixtures were measured on a test setup, which is a modification of the Casarande device for measuring single-plane shear forces. The installation is made in the form of a horizontal tray, filled concrete mixture. Test slats made of wooden blocks, sheathed along the surface of contact with the concrete mixture with strips of roofing steel, were laid across the tray. Thus, the test slats simulated steel slip 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 horizontal movement of the slats on the concrete were recorded. General form installation is given in Fig. 1.
Based on the results of the tests, the dependence of the interaction forces between the steel sliding formwork and the concrete mixture m on the magnitude of the concrete pressure on the formwork a (Fig. 2), which is linear in nature, was obtained. The angle of inclination of the graph line relative to the abscissa axis characterizes the angle of friction of the formwork on concrete, which makes it possible to calculate the friction forces. The value cut off by the graph line on the ordinate axis characterizes the adhesion forces of the concrete mixture and formwork m, independent of pressure. The friction angle of the formwork on concrete does not change when the duration of fixed contact increases from 15 to 60 minutes, the magnitude 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.
Grip strength heavy concrete And steel formwork 15 minutes after compaction of the mixture does not exceed 2.5 g/m2, or 25 kg/m2 of the contact surface. This amounts to 15-20% of the generally accepted value of the total interaction force between heavy concrete and steel formwork (120-150 kg/m2). The main part of the effort comes from friction forces.
The slow growth of adhesion forces during the first 1.5 hours after concrete compaction is explained by the insignificant number of new formations during the setting of the concrete mixture. According to research, during the period from the beginning to the end of setting of the concrete mixture, a redistribution of mixing water occurs in it between the binder and aggregates. Neoplasms develop mainly after setting is completed. A rapid increase in the adhesion of sliding formwork to the concrete mixture begins 2-2.5 hours after compaction of the concrete mixture.
Specific gravity adhesive forces in the total interaction forces between heavy concrete and steel sliding formwork are about 35%. The main share of efforts comes from friction forces, determined by the pressure of the mixture, which changes over time under concreting conditions. To test this assumption, the shrinkage or swelling of freshly molded concrete samples 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, smooth surface which was in the same plane with the vertical edge. After compacting the concrete and removing the sample from the vibrating table, the vertical faces of the cube were freed from the side walls of the mold, and within 60-70 minutes, the distances between the opposite vertical faces were measured using a messenger. The measurement results showed that freshly molded concrete, immediately after compaction, shrinks, the value of which is higher, the greater the mobility of the mixture. The total value of bilateral settlement reaches 0.6 mm, i.e. 0.4% of the sample thickness. In the initial period after forming, swelling of freshly laid concrete does not occur. This is explained by contraction in the initial stage of concrete setting during the process of water redistribution, accompanied by the formation of hydrate films that create high surface tension forces.
The operating principle of this device is similar to that of a conical plastometer. However, the wedge-shaped shape of the indenter makes it possible to use the design scheme of a viscous-flowing mass. The results of experiments with a wedge-shaped indenter showed that To varies from 37 to 120 g/cm2 depending on the type of concrete.
Analytical calculations of the pressure of a layer of concrete mixture 25 ohms thick in sliding formwork showed that the mixtures of the adopted compositions, after they have been compacted by vibration, do not exert active pressure on the formwork skin. The pressure in the “sliding formwork - concrete mixture” system is caused by elastic deformations of the panels under the influence of the hydrostatic pressure of the mixture during its compaction by vibration.
The interaction of sliding formwork panels and compacted concrete at the stage of their joint work is quite well modeled by the passive resistance of a viscoplastic body under the influence of pressure from the vertical retaining wall. Calculations have shown that with the unilateral action of the formwork shield on the concrete mass, in order to displace part of the mass along the main sliding planes, increased pressure is required, significantly exceeding the pressure that occurs under the most unfavorable combination of conditions for laying and compacting the mixture. When formwork panels are pressed on both sides of a vertical layer of concrete of limited thickness, the pressure forces required to displace the compacted concrete along the main sliding planes acquire the opposite sign and significantly exceed the pressure required to change compression characteristics mixtures. 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 sliding formwork in layers 25-30 cm thick, does not exert pressure on the formwork panels and is capable of absorbing elastic pressure from them that occurs during compaction by vibration.
To determine the interaction forces arising during the concreting process, measurements were carried out on a full-size model of the sliding formwork. A sensor with a membrane made of high-strength phosphor bronze was installed in the molding cavity. Pressures and forces on lifting rods in the static position of the installation were measured with an automatic pressure meter (AID-6M) during vibration and lifting of the formwork using an N-700 photooscilloscope with an 8-ANCh amplifier. The actual characteristics of the interaction of steel sliding formwork with various types of concrete are given in the table.
During the period between the end of vibration and the first rise 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 between the sliding formwork and the concrete mixture, it is necessary to reduce or completely eliminate the pressure between the formwork panels and the compacted concrete. This problem is solved by the proposed concreting technology using intermediate removable panels (“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 panels of the sliding formwork (Fig. 5) and immediately after laying and compacting the concrete, they are removed from it one by one.
The gap (2 mm) remaining between the concrete and the formwork, after removing the shields, protects the formwork shield, which straightens after an elastic deflection (usually 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 their given shape. This allows thin walls to be concreted in slip formwork.
Fundamental moldability thin walls with the help of liners, it was tested during the construction of natural wall fragments 7 cm thick, made of expanded clay concrete, slag pumice concrete and heavy concrete. The results of trial moldings showed that lightweight concrete mixtures better correspond to the features of the proposed technology than mixtures using dense aggregates. This is due to the high sorption properties of porous aggregates, as well as the cohesive structure of lightweight concrete and the presence of a hydraulically active dispersed component in light sand.
Heavy concrete (albeit to a lesser extent) also exhibits the ability to maintain the verticality of freshly formed surfaces with its mobility no more than 8 cm. When concreting civil buildings with thin interior walls and partitions using the proposed technology, two to four pairs of liners with a length of 1.2 to 1.6 m, ensuring concreting of walls with a length of 150-200 m. This will significantly reduce concrete consumption compared to buildings erected using the accepted technology, and increase the economic efficiency of their construction.
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 iron production technology concrete works faced by our laboratory staff 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 remove formwork at an earlier stage than the requirements of the project technological maps and SNiP 3-03-01-87. When dismantling formwork, the amount of adhesion between concrete and formwork is important: high adhesion makes formwork removal difficult. Deterioration in quality concrete surfaces, leads to the occurrence of defects.
- production of insufficiently rigid formwork that deforms when laying concrete and is not dense enough.
Such formwork undergoes deformation during the laying of the concrete mixture, which leads to a change in shape reinforced concrete elements. Deformation of the formwork can lead to displacement and deformation of reinforcement frames and walls, changes in the bearing capacity of structural elements, and the formation of protrusions and sagging. Violation of the design dimensions of structures leads to:
If they decrease
To reduce load-bearing capacity
In case of increase, their own weight increases.
This type of violation of observation technology during the manufacture of formwork under construction conditions without proper engineering control.
- insufficient thickness or absence of a protective layer.
Observed when incorrect installation or displacement of the formwork or reinforced frame, lack of gaskets.
To serious defects of monolithic reinforced concrete structures may result from poor control over the quality of reinforcement of structures. The most common violations are:
- non-compliance with the structural reinforcement design;
- poor quality welding structural units and reinforcement joints;
- use of heavily corroded reinforcement.
- poor compaction of the concrete mixture during laying into the formwork leads to the formation of cavities and cavities, can cause a significant decrease in the load-bearing capacity of elements, increases the permeability of structures, and promotes corrosion of reinforcement located in the defect zone;
-laying laminated concrete mixture does not allow obtaining uniform strength and density of concrete throughout the entire volume of the structure;
- use of too hard concrete mixture leads to the formation of cavities and cavities around the reinforcing bars, which reduces the adhesion of the reinforcement to the concrete and causes the risk of corrosion of the reinforcement.
There are cases of 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 process.
When caring for concrete, it is necessary to create such temperature-humidity conditions that would ensure that the water necessary for hydration of cement is retained in the concrete. If the hardening process takes place at a relatively constant temperature and humidity, the stresses arising in the concrete due to changes in volume and caused by shrinkage and temperature deformations will be insignificant. Usually the concrete is covered plastic film or other protective coating. In order to prevent it from drying out. Overdried concrete has significantly less strength and frost resistance than normally hardened concrete; many shrinkage cracks appear in it.
When concreting in winter conditions If insulation or heat treatment is insufficient, early freezing of the concrete may occur. After thawing, such concrete will not be able to gain the necessary strength.
Damage to reinforced concrete structures is divided according to the nature of its impact on bearing capacity into three groups.
Group I - damage that practically does not reduce the strength and durability of the structure (surface cavities, voids; cracks, including shrinkage ones, with an opening of no 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 ,1mm; concrete chips without exposing reinforcement, etc.);
Group II - damage that reduces the durability of the structure (corrosion-dangerous 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 the working reinforcement of prestressed spans, including along areas under constant load; cracks with an opening of more than 0.3 mm under temporary load load; shell voids and chips with exposed reinforcement; surface and deep corrosion of concrete, etc.);
Group III - damage that reduces the load-bearing capacity of the structure (cracks not included in the calculations either in terms of strength or endurance; inclined cracks in the walls of beams; horizontal cracks in the interfaces of the slab and spans; large cavities and voids in the concrete of the compressed zone, etc. .).
Damage of group I does not require urgent measures; they can be eliminated by applying coatings during routine 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, and improve protective properties concrete and protect structures from atmospheric and chemical corrosion.
In case of damage of group II, repair ensures an increase in the durability of the structure. Therefore, the materials used must have sufficient durability. Cracks in the area where bundles of prestressed reinforcement are located and cracks along the reinforcement are subject to mandatory sealing.
In case of damage of group III, the load-bearing capacity of the structure is restored according to a specific feature. The materials and technologies used must ensure strength characteristics and durability of the structure.
To eliminate group III damage, as a rule, individual projects must be developed.
Constant growth in volumes monolithic construction is one of the main trends characterizing the modern period of Russian construction. However, at present, a massive transition to construction from monolithic reinforced concrete may 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, 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 reinforced concrete, so their focus on factory technologies and insufficient elaboration of the issues of construction from monolithic reinforced concrete are completely natural.
Secondly, most construction organizations There is no sufficient experience and the necessary technological culture of monolithic construction, as well as poor-quality technical equipment.
Thirdly, 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 compliance of its characteristics with the parameters in regulatory documents. Rosstandart has approved and is in force new standards: GOST 7473 “Concrete mixtures. Specifications", GOST 18195 "Concrete. Rules for monitoring and assessing strength." GOST 31914 “High-strength heavy and fine-grained concrete for monolithic structures", should become a valid standard for reinforcement and embedded products.
The new standards, unfortunately, do not contain issues related to the specifics of legal relations between construction customers and general contractors, manufacturers of building materials and builders, although the quality of concrete work depends on each stage of the technical chain: preparation of raw materials for production, design of concrete, production and transportation of the mixture, laying and maintaining concrete in structures.
Ensuring the quality of concrete during the production process is achieved thanks to a complex various conditions: here and modern technological equipment, and the presence of accredited testing laboratories, and qualified personnel, and unconditional implementation regulatory requirements, and implementation of quality management processes.
On April 22, the 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" named after. A.A. Gvozdeva, LLC "GEOStrom", OJSC "Moscow IMET", State Budgetary Institution "TsEIIS", State Unitary Enterprise "NIIMosstroy", OJSC "MonArch", LLC "GeroCrit", LLC BASF " Building systems" and etc.
The information content of the conference was very high, but there was not enough time to discuss the presented reports. It is clear 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 the State Unitary Enterprise "NIIMosstroy", will serve to improve work in the field of monolithic construction.
We bring to your attention the text of the report presented at the conference by the head of the Laboratory for Testing Building Materials and Structures, Dmitry Nikolaevich Abramov.
The main causes of defects in concrete structures
In my report, I would like to talk about the main violations of the technology for the production of reinforced concrete work that employees of our laboratory encounter at construction sites in 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 remove formwork at an earlier stage than is provided for by the project requirements in technological maps and SNiP 3-03-01-87. When dismantling formwork, the amount of adhesion between concrete and formwork is important: high adhesion makes formwork removal difficult. Deterioration in the quality of concrete surfaces leads to the occurrence of defects.
- production of insufficiently rigid formwork that deforms when laying concrete and is not dense enough.
Such formwork undergoes deformation during the laying of the concrete mixture, which leads to a change in the shape of the reinforced concrete elements. Deformation of the formwork can lead to displacement and deformation of reinforcement frames and walls, changes in the bearing capacity of structural elements, and the formation of protrusions and sagging. Violation of the design dimensions of structures leads to:
If they decrease
To reduce load-bearing capacity
In case of increase, their own weight increases.
This type of violation of observation technology during the manufacture of formwork under construction conditions without proper engineering control.
- insufficient thickness or absence of a protective layer.
Observed when the formwork or reinforced frame is incorrectly installed or displaced, or when gaskets are missing.
Poor control over the quality of reinforcement of structures can lead to serious defects in monolithic reinforced concrete structures. The most common violations are:
- non-compliance with the structural reinforcement design;
- poor-quality welding of structural units and reinforcement joints;
- use of heavily corroded reinforcement.
- poor compaction of the concrete mixture during laying into the formwork leads to the formation of cavities and cavities, can cause a significant decrease in the load-bearing capacity of elements, increases the permeability of structures, and promotes corrosion of reinforcement located in the defect zone;
-laying laminated concrete mixture does not allow obtaining uniform strength and density of concrete throughout the entire volume of the structure;
- use of too hard concrete mixture leads to the formation of cavities and cavities around the reinforcing bars, which reduces the adhesion of the reinforcement to the concrete and causes the risk of corrosion of the reinforcement.
There are cases of 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 process.
When caring for concrete, it is necessary to create such temperature-humidity conditions that would ensure that the water necessary for hydration of cement is retained in the concrete. If the hardening process takes place at a relatively constant temperature and humidity, the stresses arising in the concrete due to changes in volume and caused by shrinkage and temperature deformations will be insignificant. Typically, concrete is covered with plastic film or other protective coating. In order to prevent it from drying out. Overdried concrete has significantly less strength and frost resistance than normally hardened concrete; many shrinkage cracks appear in it.
When concreting in winter conditions with insufficient insulation or heat treatment, early freezing of the concrete may occur. After thawing, such concrete 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 impact on the load-bearing capacity.
Group I - damage that practically does not reduce the strength and durability of the structure (surface cavities, voids; cracks, including shrinkage ones, with an opening of no 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 ,1mm; concrete chips without exposing reinforcement, etc.);
Group II - damage that reduces the durability of the structure (corrosion-dangerous 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 the working reinforcement of prestressed spans, including along areas under constant load; cracks with an opening of more than 0.3 mm under temporary load load; shell voids and chips with exposed reinforcement; surface and deep corrosion of concrete, etc.);
Group III - damage that reduces the load-bearing capacity of the structure (cracks not included in the calculations either in terms of strength or endurance; inclined cracks in the walls of beams; horizontal cracks in the interfaces of the slab and spans; large cavities and voids in the concrete of the compressed zone, etc. .).
Damage of group I does not require urgent measures; they can be eliminated by applying coatings during routine 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 damage of group II, repair ensures an increase in the durability of the structure. Therefore, the materials used must have sufficient durability. Cracks in the area where bundles of prestressed reinforcement are located and cracks along the reinforcement are subject to mandatory sealing.
In case of damage of group III, the load-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 group III damage, as a rule, individual projects must be developed.
The constant growth in the volume of monolithic construction is one of the main trends characterizing the modern period of Russian construction. However, at present, a massive transition to construction from monolithic reinforced concrete may have negative consequences associated with the rather low level of quality of individual objects. Among the main reasons for the low quality of constructed 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 reinforced concrete, so their focus on factory technologies and insufficient elaboration of the issues of construction from monolithic reinforced concrete are completely natural.
Secondly, most construction organizations do not have 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 control of the quality of work.
The quality of concrete is, first of all, the compliance of its characteristics with the parameters in regulatory documents. Rosstandart has approved and is in force new standards: GOST 7473 “Concrete mixtures. Technical conditions", GOST 18195 "Concrete. Rules for monitoring and assessing strength." GOST 31914 “High-strength heavy and fine-grained concrete for monolithic structures” should come into force, and the standard for reinforcement 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, design of concrete, production and transportation of the mixture, laying and maintaining concrete in structures.
Ensuring the quality of concrete during the production process is achieved thanks to a set of various conditions: here is modern technological equipment, the presence of accredited testing laboratories, qualified personnel, unconditional compliance with regulatory requirements, and the introduction of quality management processes.
Head of the Laboratory for Testing of Building Materials and
structures of the State Budgetary Institution "TsEIIS" -D.N. Abramov
The amount of adhesion between concrete and formwork reaches several kgf/cm2. This complicates stripping work, deteriorates the quality of concrete surfaces and leads to premature wear of formwork panels.
The adhesion of concrete to formwork is influenced by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the formwork's forming surface.
Adhesion (sticking) is understood as a bond caused by molecular forces between the surfaces of two dissimilar or liquid bodies in contact. During the period of contact between concrete and formwork, favorable conditions are created for adhesion to occur. The adhesive (adhesive), which in this case is concrete, is in a plastic state during the laying period. In addition, in the process of vibration compaction of concrete, its plasticity increases even more, as a result of which the concrete moves closer to the surface of the formwork and the continuity of contact between them increases.
Concrete sticks to wood and steel formwork surfaces more strongly than to plastic ones due to the latter's poor wettability.
When removing formwork, there can be three tearing options. In the first option, adhesion is very small, and cohesion is quite high. In this case, the formwork is torn off exactly along the contact plane. The second option is adhesion more than cohesion. In this case, the formwork is torn off along the adhesive material (concrete). The third option is that adhesion and cohesion are approximately the same in magnitude. The formwork comes off partially along the plane of contact between the concrete and the formwork, and partly along the concrete itself (mixed or combined tearing). With adhesive tearing, the formwork is easily removed, its surface remains clean, and the concrete surface is of 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.
Formwork lubricants, depending on their composition, operating principle and operational properties, can be divided into four groups: aqueous suspensions; hydrophobic lubricants; lubricants - concrete set retarders; combined lubricants.
The use of effective lubricants reduces the harmful effects of certain factors on the formwork. In some cases, lubricants cannot be used. Thus, when concreting in sliding or climbing formwork, the use of such lubricants is prohibited due to their penetration into the concrete and a decrease in its quality. Good effect give anti-adhesive protective coatings Based on polymers. They are applied to the forming surfaces of shields during their manufacture, and they withstand 20-35 cycles without re-application and repair. A phenol-formaldehyde-based coating has been developed for plank and plywood formwork. It is pressed onto the surface of the boards at a pressure of up to 3 kgf/cm2 and a temperature of + 80° C.
It is advisable to use boards whose decks 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 force of concrete to formwork is influenced by adhesion (sticking) and shrinkage of concrete, roughness and porosity of the surface. With a high adhesion force between concrete and formwork, the work of stripping becomes more complicated, the labor intensity of the work increases, the quality of concrete surfaces deteriorates, and the formwork panels wear out prematurely.
Concrete sticks to wood and steel formwork surfaces much more strongly 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 to them is quite high; with weakly wetted materials (for example, textolite, getinax, polypropylene) the adhesion of concrete is several times lower.
The adhesion force (N) of some formwork materials to concrete is as follows:
Therefore, to obtain surfaces High Quality You should use cladding made of textolite, getinax, polypropylene, or use waterproof plywood treated with special compounds. When adhesion is low, the concrete surface is not disturbed and the formwork comes off easily. As adhesion increases, 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, water-repellent lubricants, combined lubricants, and concrete retarding lubricants to the surface of the formwork. The principle of operation of aqueous suspensions and water-repellent lubricants is based on the fact that on the surface of the formwork a protective film, which reduces the adhesion of concrete to formwork.
Combined lubricants are a mixture of concrete set retarders and water-repellent emulsions. When making lubricants, sulfite-yeast stillage (SYD) and soap naft are added to them. Such lubricants plasticize the concrete of the adjacent area, and it does not collapse.
Lubricants - concrete set retarders - are used to obtain a good surface texture. By the time of formwork, the strength of these layers is slightly lower than the bulk of the concrete. Immediately after stripping, the structure of the concrete is exposed by washing it with a stream 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 layer applied and also reduces lubricant consumption.
For pneumatic application, sprayers or spray rods are used. More viscous lubricants are applied with rollers or brushes.
