Lectures / Concrete Curing; stripping of structures; Quality control

Stripping of monolithic concrete structures, timing and sequence.

Stripping structures, although it requires less working time than manufacturing or installing formwork, is still one of the main types of formwork work.
The suitability of formwork materials for further use largely depends on the quality of formwork. If the formwork is carelessly removed, the smooth surface of the sheathing is damaged, the sheathing boards and sometimes the frame are broken, and the fasteners are bent. As a result, the formwork must be repaired or even completely replaced in order to be reused. Therefore, stripping should be done carefully.

Stripping begins after the concrete reaches the required strength. Since the rate of concrete hardening mainly depends on the outside temperature and, in addition, different concrete structures require different strengths, the time after which stripping is carried out is set taking into account these factors.

Removal of the side elements of the formwork that do not bear the load from the weight of the structure is allowed only after the concrete has reached a strength that ensures the safety of the surface and edges of the corners when removing the formwork, unless otherwise specified in the construction design. Typically, the side surfaces are stripped in the summer 2-3 days after concreting, and often earlier. Reducing the curing time of concrete in the formwork speeds up concrete work, allows you to wrap the formwork faster and thereby use it more efficiently.

The load-bearing formwork of reinforced concrete structures is removed only after the concrete reaches a strength that ensures the integrity of the structure after stripping.

The required strength of concrete during stripping, depending on the magnitude of the actual load on the structure being stripped, is given in the table:

Required strength of concrete when stripping

Stripping of reinforced concrete structures and their partial loading can be allowed with lower concrete strength than indicated in the table, but with a strength of not less than 50% of the design and not less than 100 kg/cm2 when using reinforcement of classes A-I and A-II and 150 kg/cm2 cm2 - when using fittings of class A-III.

The posts and scaffolding supporting the formwork of load-bearing structures can be removed when the concrete columns reach the specified strength (see table above). Scaffolding and racks are removed only after removing the side formwork and inspecting the stripped structures and the columns supporting these structures. It is allowed to load the demoulded structure with the full design load only after the concrete has acquired its design strength.

Removal of formwork, which supports the weight of concrete of structures reinforced with load-bearing welded frames, is allowed only after the concrete of these structures reaches 25% of the design strength. Massive structures are stripped within a time period that is determined taking into account the required thermal hardening conditions of the massif provided for by the structure design.

Particular care is required when stripping arches and vaults, thin-walled structures (e.g. shell vaults), and beam structures with a span of more than 8 m. The sudden application of a load from its own weight (after removing the formwork and scaffolding) has an impact-like effect on the structure, which may lead to its destruction. Therefore, the removal of the formwork of the above-mentioned structures must be preceded by a smooth and uniform lowering of the supporting scaffolding. This process, called loosening, is carried out by loosening the wedges, lowering the jack screws, and releasing sand from the cylinders.

Unwinding is carried out in two, three or more steps depending on the length of the span and the weight of the structure.

The supports supporting the formwork of beam structures are lowered simultaneously along the entire span in proportion to the deflection of the structure from its own weight.

Before untwisting arches with strings equipped with couplings or other tensioning devices, first tighten the strings.

The opening of arches and ordinary vaults begins from the castle and proceeds symmetrically in both directions towards the heels.

The opening of the ceilings of round tanks, as well as the funnels of bunkers, is carried out by lowering the supports located in the center of the structure and moving them in concentric rows towards the perimeter of the structure. In this case, the supports located along each concentric row are lowered simultaneously.

When removing floor racks supporting the formwork of concrete floors of multi-story buildings, the following rules are followed:

• it is not allowed to remove the formwork posts of the floor located directly under the concrete floor;
• the formwork posts of the next underlying floor can be removed only partially, while under all beams and purlins with a span of 4 m or more, “safety posts” are left, located one from the other at a distance of no more than 5 m;
• the formwork posts of the remaining underlying floors can be completely removed if the concrete strength of these floors has reached the design level.

When stripping, use a set of tools consisting of wire cutters, wrenches and a set of crowbars of three types, length 1; 0.6 and 0.3-0.35 m. Crowbars have forked legs that serve as nail pullers. A large crowbar is used to perform operations that require great effort (for example, removing pressure and subcircular boards); middle - for knocking out wedges, removing shields, circling; small - to create gaps between the formwork elements, into which large crowbars are then inserted. A link of two workers requires a set of two large crowbars, two medium ones and one small one.

The formwork panels are removed using cranked formwork levers. The lever for removing panels arranged in two tiers consists of a metal rod 1, bent at a right angle, with two rollers 8. The rollers move along a thrust plate 4, fixed to the panel 6 of the upper tier. The short arm of the lever is connected to the clip 2, put on the purlin of the panel 3 of the lower tier, and the long arm has a loop 7 for the crane hook.

When lifting (position II), the corner of the rod 1, equipped with rollers 8, rests against the metal plate 4 of the upper panel 6. In this case, the end of the short bend presses on the girder of the lower panel 3 and tears the panel off the concrete, after which the formwork panel 5 suspended on the rod 1, is raised to a new position.

Mortar mixers for fence production

To produce high-quality potting mixture, it is recommended to use special forced-type mortar mixers, which mix the mixture with special blades or scrapers in a fixed container.
This is explained by the fact that only these devices can ensure the production of the most homogeneous and, accordingly, high-quality mixture in minimal periods of time, unattainable when using manual labor. The result of the instant formwork method is the production of low-quality concrete products with an affordable market price for a wide range of consumers. The similarity of the exposure method with the instant formwork method lies in the use of vibration casting technology, while the main distinctive feature of the exposure method is the imparting of primary strength to the manufactured element in a matrix form, the formation of the front surface of which is carried out through the action of the response surface of the mold, which mirrors all the smallest elements of the product. It is logical that under these circumstances the mold must have a perfectly clean surface in order to convey a perfectly clean and smooth surface to the product.

Arrangement of formwork

The photo shows a diagram of the construction of a strip foundation

Formwork is a special structure consisting of blocks that must be fixed in a certain position in order to build a strong frame in order for concrete to be laid in the formwork to create a foundation. Removing it with your own hands or stripping it occurs after the poured solution reaches the required strength.

In order for this process to be carried out correctly, and you ultimately get a reliable, strong and durable structure, you need to know not only how to properly pour concrete into the formwork, but also at what strength of concrete you can remove the formwork so as not to harm it.

The method of constructing the frame and the timing of its removal directly depend on the purpose for which the cast product is being constructed.

It may be intended;

• To fill the foundation of a building;
• To create overlap between floors;
• For the construction of building walls;
• To implement monolithic construction methods;
• For arrangement of mine floors.

Panel installation diagram

Builders use various materials to construct formwork, the most popular of which are:

• Lumber or wood;
• Metal plates;
• Expanded polystyrene.

Lubricant for molds

Various industrial lubricants can be used to lubricate mold dies, the most effective of which at the moment are:

• "Lerossin";
• "Gravitann";
• K–222;
• "Separen."

Equipment for the production of concrete fences does not work equally well with all lubricants. It is not recommended to use used motor oil and various mixtures of mineral oils with heating or diesel fuel as lubricants for the production of Eurofences, since these products not only do not give the expected effect, but also significantly reduce the level of quality of manufactured products, in particular when demolishing finished products. products.

Requirements and technical standards for the removal of structures

According to the recommendations set out in SNiP 3.03.01-87, which describes enclosing and load-bearing structures, formwork is removed when the concrete reaches at least 70% of its final strength. At a constant positive temperature after pouring, stripping of foundations and floors is allowed at 50%. American technology standards indicate a recommended strength of 50% at standard air humidity.

The foundation should harden to 50-70% of the nominal hardness Source stroykarecept.ru

Most documents and technical requirements are based on certain temperature conditions. Based on them, they calculate when it is possible to remove the formwork from the concrete when it reaches maximum hardness.

Stripping depending on air temperature

Generally accepted provisions indicate that concrete gains 70% strength approximately 4 weeks after pouring. In practice, such indicators are achievable only at an air temperature of +5 degrees. In stable warm weather of +10 degrees, strip foundation demoulding occurs after 15 days from the date of installation of the structure. During particularly hot periods of +30 degrees or more, wooden shields can be removed as early as the 3rd day.

Important! Calculations refer to concrete grades M200 and M250, prepared from Portland cement M-500.

The rate of concrete hardening depending on temperature Source stroykarecept.ru When pouring a horizontal wide slab, it takes a longer time to gain optimal strength. Removal of formwork after concreting is carried out according to the following average daily temperature indicators:

• 1 degree – 28 days;
• 10 degrees – 21 days;
• 15 degrees – 15 days;
• 20 degrees – 9 days;
• 25 degrees – 7 days.

If the total length of the strip foundation or floor slab exceeds 6 m, the time frame for stripping is slightly shifted. This is explained by the need for concrete to gain greater strength. Recommended indicators for dismantling wooden structures are 80-85%.

Tips for dismantling

When installing formwork, it is necessary to ensure the integrity of the structure so that the solution does not spill beyond its boundaries. It can be removed after reaching 50-80% of the concrete strength, depending on the type of structure. At large facilities it is done in stages, in the reverse order of installation. During private construction, it is removed simultaneously from the entire monolith.

Dismantling the formwork is done carefully so as not to subject the poured structure to unnecessary load. Do not use heavy equipment as it can severely damage the surface. When removing, you must follow simple rules that will help preserve the surface of the poured solution, obtaining a strong, durable structure:

• Lubricate all elements in contact with the monolith with materials that reduce adhesion or cover with plastic film.
• The formwork should be removed manually to avoid damaging the fill and to be able to reuse it. If difficulties arise, a wooden wedge is carefully driven between the monolith and the shield.
• The solution sets better in the corners, so it is better to start work from them, remove from top to bottom.
• Supporting elements, columns, towers are the last to be freed from formwork.

Advice! If there is any doubt about when it will be possible to remove the formwork from the foundation or other element, it is better to do this after 28 days. Then the solution is guaranteed to receive the desired characteristics.

Formwork made from lumber should not be left for a long time in the autumn-winter period, since they begin to swell and warp, which leads to surface deformation.

The most suitable time to carry out the work is the end of autumn. At this time, the air becomes humid, and the difference between day and night temperatures decreases. If the foundation is cast, after removal it must stand all winter to compact and gain the required strength.

Technology of non-destructive stripping of vibro-compressed polystyrene concrete wall stones and blocks

Thermally efficient building materials in modern construction

Modern energy- and resource-saving construction places special demands on the building materials used.
Often, materials traditionally used in capital construction do not fully meet the increased requirements in terms of thermal efficiency of enclosing structures, energy saving and cost reduction of construction work.

In the context of a significant increase in energy prices, the need to construct buildings and structures that fully meet modern thermal engineering standards and requirements seems especially urgent.

The sharp increase in production volumes of wall materials with low thermal conductivity once again confirms the prospects of this direction. The undoubted leaders among thermally efficient wall materials are rightfully cellular concrete and concrete with lightweight aggregates. Small-piece wall stones, blocks, panels and slabs made from these materials are actively used in both low-rise and high-rise housing construction.

Low thermal conductivity, excellent environmental performance, low volumetric weight, ease of processing and, finally, the ability to significantly reduce the thickness of the wall, and, consequently, its cost, open up excellent prospects for the mass use of cellular concrete and concrete with light aggregates (for example, polystyrene concrete) in modern construction.

The undoubted advantages of non-autoclaved cellular concrete and low-density polystyrene concrete also include the possibility of preparing these materials on the construction site in close proximity to the place of use. Thus, the possibility of manufacturing both thermal insulation and structural materials of various densities directly on the construction site can significantly reduce transportation costs, eliminate the use of lifting equipment, and thereby significantly reduce the cost of construction work.

However, the scope of application of polystyrene concrete and foam concrete in construction is not limited to the organization of thermally efficient screeds and coatings. Small-piece wall stones and blocks based on low-density porous materials are actively used in various areas of capital construction.

The most widely used are wall blocks with dimensions of 598x295x188mm for laying with glue and partition blocks 100mm thick, with a volumetric weight of 400-700 kg/m3. It is this standard size of wall blocks that is mainly used in low-rise construction as a load-bearing wall material and as a self-supporting filler in multi-storey construction. Moreover, of particular interest are polystyrene concrete wall blocks with a volumetric weight of 350 to 600 kg/m3, suitable for the construction of building envelopes of limited number of floors (usually up to three floors inclusive).

Today, low-density polystyrene concrete is the clear leader in the market of thermally efficient materials in terms of the combination of basic physical and mechanical characteristics, price, availability of production and use in low-rise construction.

When compared with wall materials for similar purposes (foam concrete, aerated concrete), polystyrene concrete wins in almost all respects (strength, thermal conductivity, water absorption, frost resistance). In terms of thermal conductivity, polystyrene concrete wall blocks, suitable for the construction of load-bearing enclosing structures, are generally beyond competition.

So, with an average density of polystyrene concrete of 600 kg/m3, the thermal conductivity coefficient in a dry state is 0.145 W/(m*°C), the compressive strength class is B2.5 (strength grade M 35), and the frost resistance is F-50 F-100.

The production of polystyrene concrete is objectively a less expensive enterprise than the production of non-autoclaved foam concrete of similar density, and especially autoclaved aerated concrete. At the same time, the main problems that arise in the production of non-autoclaved foam concrete (unstable density of the resulting material, self-destruction of the material in molds, large shrinkage of the material upon drying, special requirements for the quality of the cement used) are not known to polystyrene concrete producers at all.

When supplying polystyrene concrete over a distance using mortar pumps of various designs, concrete with lightweight aggregates is a more stable material and less prone to delamination than non-pressurized foam concrete of similar density.

Foamed polystyrene granules are a completely ready-to-use product, the work with which, although quite specific, is generally familiar to builders and resembles working with any other lightweight aggregate (for example, expanded clay gravel and expanded clay sand).

When installing thermal insulation coatings, masonry and plaster mortars, ready-made foam polystyrene granules are added to the prepared mortar. After thorough mixing, the polystyrene concrete is ready and you can begin laying it.

However, if using polystyrene concrete mass directly on a construction site everything is more or less clear, the formation of small-piece polystyrene concrete wall blocks has its own specifics and requires a competent approach.

Traditionally, the formation of blocks from polystyrene concrete is carried out by the injection method; the vast majority of recommendations for the production of polystyrene concrete are based precisely on the injection method of molding products.

Polystyrene concrete of the required density prepared in a mortar mixer is loaded into cassette molds using a mortar pump or by gravity. After the material has gained stripping strength, the form is disassembled, the partitions are removed and the molded polystyrene concrete wall block is sent to the drying area. Cassette molds are cleaned of any remaining solution, lubricated and reassembled for refilling. Moreover, almost all of the above operations are performed manually, and the ability to automate these works requires very significant investments.

A similar molding method is also used in the production of wall blocks from non-autoclaved foam concrete.

The main pros and cons of molding wall blocks using injection molding technology in cassette molds are quite well known; we will dwell in detail only on the main ones:

DISADVANTAGES of injection molding technology for molding wall blocks

1. Disassembling and assembling cassette forms is a very labor-intensive task. The geometry, and accordingly the quality of the produced blocks, directly depends on the quality of the molds themselves, their preparation and assembly. With all the richness of choice of cassette forms offered, only a few models are theoretically capable of providing the maximum deviations in linear dimensions declared by the manufacturer during the mass production of wall blocks in real conditions of domestic production.
2. The required production space occupied for the production of wall blocks using injection molding technology is disproportionately large in relation to the modest volumes of products. The need to use lifting equipment to transport filled cassette forms to the drying area places special demands on production premises and their equipment. Serious difficulties when trying to significantly increase the volume of production of wall blocks using injection molding technology. For products to gain the required stripping strength, a significant holding time is required (usually from 8 to 24 hours), and the forming equipment cannot be reused (for example, when organizing production in two shifts).
3. The consumption of lubricating solutions necessary to prepare molds for pouring is only insignificant at first glance; when calculating costs, lubricants are a serious expense item that affects the total cost of production of wall blocks.
4. A large number of low-skilled workers employed in production, which is also associated with the need to carry out labor-intensive operations for maintenance and preparation of molds.
5. The high cost of high-quality cassette molds, capable of ensuring a deviation of the linear dimensions of products of no more than 2 mm, in the mass production of wall blocks. The need for constant monitoring of the condition of the molding equipment, its repair and, if necessary, replacement.
6. Increased cement consumption to ensure regulated strength of products both on the first day of normal hardening and on the 28th day. The high consumption of cement in the production of wall stones using casting technology is primarily due to the need to work at a high W/C ratio. As is known, the higher the content of unbound (free) water in concrete, the lower the strength of concrete products. Accordingly, in order to reduce cement consumption and reduce the time it takes for products to gain stripping strength, one should, if possible, strive to reduce the W/C ratio, which is difficult to achieve with the injection molding method.

ADVANTAGES of injection molding technology for molding wall blocks

1. At the moment, the injection molding method for forming wall blocks is the only way to organize such production with minimal initial costs.
2. Possibility of gradual retrofitting of production with cassette molds with increasing production volumes of wall blocks.

Thus, the method of forming polystyrene concrete wall blocks using injection molding technology cannot be considered optimal. Moreover, the injection molding method in the production of polystyrene concrete wall stones was borrowed from the production of wall blocks from non-autoclaved foam concrete.

But non-autoclaved foam concrete is a material that can only be molded using the injection molding method (cutting technology does not count, since the foam concrete mass for cutting is also formed in molds and only then supplied for cutting), and the molding of a polystyrene concrete wall block is also possible using the method of volumetric vibrocompression based on a rigid polystyrene concrete mixture, with the lowest possible W/C ratio.

In contrast to the injection molding method of molding products, the method of volumetric vibrocompression is the most promising, both in terms of reducing the cost of manufactured products and in terms of the ratio: product quality - initial costs for purchasing the necessary equipment.

Obtaining rigid polystyrene concrete mixtures in production usually does not present any difficulties. The only mandatory condition for obtaining rigid polystyrene concrete molding mixtures is the use of forced mixers capable of ensuring the most uniform distribution of polystyrene granules in the working solution.

Vibropressed polystyrene concrete wall blocks

The possibility of producing polystyrene concrete wall blocks using the method of volumetric vibrocompression allows us to bring the production of thermally efficient building materials to a qualitatively new level of development.

Reducing the W/C ratio when switching to rigid molding mixtures can significantly reduce cement consumption and, accordingly, the cost of production.

Molding wall blocks on replaceable technological pallets opens up the possibility of radically reducing the required production space. After immediate stripping, the products are moved on technological pallets to the drying area, and for small production volumes, the technological pallet together with a molded wall block with dimensions of 598x295x188 mm can be transported to the drying area manually, since its weight is about 12-20 kg. The molding of products using the method of volumetric vibrocompression occurs on one forming equipment, therefore the geometric dimensions of the products are completely identical, and possible dimensional deviations are fully within the requirements of the current GOST (+ - 2mm, for laying blocks on glue).

Thus, the method of volumetric vibrocompression of polystyrene concrete wall blocks allows us to produce high-quality, world-class products at moderate overhead costs using non-specialized production areas.

Moreover, in our opinion, the main producer of thermally efficient building materials in our country should be small enterprises in the construction industry that produce high-quality competitive products, the demand for which is constantly growing.

It is small enterprises with limited production and a small staff that are able to solve and are already solving the problem of modern energy-resource-saving construction in the regions using locally produced thermally efficient building materials.

Accordingly, the equipment used in the production of polystyrene concrete wall blocks must meet several conditions:

With high quality products and a productivity of at least 5-12 m3 of wall blocks (stones) per shift, a set of equipment should be available to small businesses with a limited budget.

The technology for producing wall blocks should be accessible and easily reproducible, regardless of the distance of production from the center. Accordingly, it is necessary to rely on the local raw material base, eliminate the use of scarce additives, and use fairly common materials, take into account that the quality of cement in individual regions is not the same, and the level of automation of small production should be sufficient to ensure constantly high quality of products, but not excessive .

The above requirements are fully met by technological equipment manufactured in Tula.

Thus, the Borets type brick making machine, designed for the production of polystyrene concrete wall blocks and stones, is designed taking into account the basic requirements for equipment for the production of building materials in small enterprises.

Above we wrote about the main advantages of the technology for molding products from polystyrene concrete using the method of volumetric vibrocompression compared to injection molding technology. However, the specifics of polystyrene concrete production impose special requirements on brick-and-mortar equipment.

A large number of foamed polystyrene granules, evenly distributed in a cement-sand matrix, dictates its own methods of vibroforming the material, which are completely different from the traditionally used technology for producing vibro-pressed concrete stones. The rules for selecting the composition of the molding sand also differ significantly.

Features of vibration molding of polystyrene concrete mixtures

The fact is that the proportion of cement and sand in polystyrene concrete is not significant, but the content of spherical granules of foamed polystyrene in the mixture, on the contrary, is high. And, if vibration compaction of the cement-sand component under the loading influence of the punch plate of a brick-making press leads to uniform compaction of the concrete mixture, then excessive compaction of the polystyrene concrete mixture under the influence of the forming punch plate causes elastic deformation of foamed polystyrene granules. Polystyrene granules deformed during compaction after stripping the product restore their original shape, which leads to the destruction of the molded polystyrene concrete wall block on the technological pallet.

The principle of vibration compaction of a cement-sand mixture is to fill intergranular voids, compact the material, and increase the strength of products.

However, a decrease (up to almost complete filling) of intergranular voids causes an increase in the average density of the material and, accordingly, a deterioration in its thermal insulation properties.

Traditionally, in the practice of producing sand-cement wall stones, the method of molding voids in the body of the stone is used. The voids come in different configurations and volumes, but their purpose remains the same - reducing the volumetric weight and increasing the thermal insulation properties of the material.

At the same time, the production of wall stones with large voids requires more precise compliance with the technological production regulations and places increased demands on the accuracy of selecting the composition of concrete in terms of determining the W/C ratio and selecting the granulometric composition of the filler (for example, sand of different size moduli).

The load-bearing capacity of hollow wall stones is lower than that of solid stones with similar brand strength of products. The percentage of defects in the production of hollow-core wall stones is usually higher than in the molding of solid products, which is primarily due to the reduced wall thickness of the hollow-core wall stone.

And, finally, the maintenance-free service life of the forming equipment (punch, matrix) of a brick making press for the production of solid wall stones is much longer than the service life of the equipment for the production of hollow-core products.

When molding wall stones (blocks) from polystyrene concrete or from concrete on other lightweight aggregates (for example, sawdust concrete), taking into account that the reduction in volumetric weight for a material based on lightweight aggregate is achieved precisely through the use of a large volume of the lightest aggregate, and the thermal conductivity coefficient of such materials is quite low, the possibility of producing hollow wall stones based on polystyrene concrete seems unpromising. Especially when you consider that vibrocompressed polystyrene concrete, having a volumetric weight of about 400-600 kg/m3, is a much less durable material than sand-cement wall stones and, if molding the voids of sand-cement wall stone is an absolute necessity, then in the production of polystyrene concrete stones At low densities, the presence of voids is not so important.

The increased percentage of defects in the molding of hollow polystyrene concrete wall stones (given the low strength of freshly molded low-density polystyrene concrete) also suggests that the production of solid wall stones and blocks from polystyrene concrete is a more convenient enterprise in terms of the selection of working compositions, molding, technological movements, loading and delivery of material to the consumer.

The void formers located in the forming matrix only prevent the uniform distribution of the polystyrene concrete mixture. As a result, unfilled areas may appear, which, of course, is a molding defect. The production of solid wall blocks and stones from polystyrene concrete allows not only to produce material with a volumetric weight of 400-600 kg/m3 and a dry thermal conductivity coefficient of 0.10-0.145 W/(M * oC), which fully complies with the requirements of GOST R 51263-99, but also opens up the possibility of producing this material in non-specialized areas with simplified production regulations.

Thus, abandoning the molding of polystyrene concrete hollow wall stones and blocks in favor of solid products allows not only to produce a material that fully complies with the requirements of the current GOST, but also to minimize the percentage of defects during molding, moving and transporting finished products. There is also another important condition for high-quality molding of wall blocks and stones from polystyrene concrete. The fact is that, as mentioned above, the formation of a rigid polystyrene concrete mixture should be carried out with minimal influence of the upper load (punch), and the vibration compaction of the mixture itself should not be excessive. In other words, when forming a polystyrene concrete mixture, the punch plate functions not as a mechanism for compacting the mixture, but as a surface-forming plate.

The main compaction of the polystyrene concrete mixture is carried out by applying vibration pulses to the walls of the matrix and compacting the material under its own weight, with minimal pressure from the punch plate.

Thus, a fairly light vibration compaction of the material forms the structure of the wall block capable of maintaining the shape of the product when stripping and moving, but does not lead to excessive compaction of the cement-sand component of the mixture with almost complete filling of the voids between the granules of foamed polystyrene.

The microvoids that remain unfilled increase the thermal resistance of the material, reduce the bulk density and help save material.

Moreover, material savings with moderate vibration compaction in some cases amount to 20-25% (the volume of voids between polystyrene granules unfilled with sand and cement). Thus, the volume of unfilled voids during the production of polystyrene concrete wall blocks practically corresponds to the volume of voids formed during the production of hollow sand-cement wall stones.

Accordingly, the technology of volumetric vibrocompression of polystyrene concrete mixture is a kind of compromise between the need to increase the strength of the material (increasing strength is necessary to ensure instant demoulding of products) and maintaining minimum values ​​of density and thermal conductivity.

To ensure these conditions, a strictly dosed supply of vibration pulses to the product being formed is required, as well as an original scheme for non-destructive formwork of the molded wall stone or block.

The first part of these necessary conditions can be easily reproduced on non-specialized brick-making equipment after minor modifications. It is enough to reduce the intensity of vibration pulses and lighten the weight of the punch plate as much as possible, thereby eliminating over-compaction of the molded mixture and, accordingly, reducing the likelihood of self-destruction of the product after stripping as a result of elastic deformation of polystyrene foam granules. With the second condition, everything is much more complicated.

The scheme for stripping products, traditionally used in vibropress equipment for the production of building materials using the method of volumetric vibrocompression, is the upward movement of the forming matrix relative to the molded product. The molded product (wall stone, paving slabs, etc.) is located on a technological pallet, a punch plate holds it on top, and the matrix, moving upward, comes off the product.

When the die has virtually no contact with the molded product, the punch plate is also separated from the product and rises up.

Thus, the forming equipment of the concrete block press remains in the upper position, and the molded product on the technological pallet is ready to be moved to the drying area. A similar pattern of movement of forming equipment has proven itself to be excellent in the production of products based on heavy sand concrete; most of the produced brick making machines are built according to this pattern.

However, when molding products using light and especially light aggregates, this scheme is clearly not optimal and blindly copying it from the technology of molding heavy concrete to the technology of producing lightweight concrete of especially low densities would undoubtedly be a mistake.

When trying to mold products from polystyrene concrete with a density of 400-600 kg/m3 using standard (originally intended for molding products from heavy concrete) vibration-pressing equipment, even after recommended modifications (reducing vibration intensity, reducing the mass of the punch), destruction of the molded products on the technological pallet is observed after immediate stripping.

The destruction of polystyrene concrete blocks and stones on the technological pallet is mainly observed when the matrix comes off the product or when the punch plate is lifted, which is explained by the low strength of freshly molded products.

A way out of the current situation, when standard brick-making equipment is either not at all able to mold a product from a polystyrene concrete mixture that meets the requirements of the current GOST, or when the percentage of defective products during formwork and technological transportation is disproportionately high, could be the creation of specialized brick-pressing equipment designed for molding specifically lightweight concrete in general and polystyrene concrete with a volumetric weight of 400-600 kg/m3 in particular.

Accordingly, the molding of polystyrene concrete requires the use of specialized vibropress equipment. The main difference between such a concrete blocking press, which is capable of providing non-destructive demoulding of products made from low-density polystyrene concrete mixture, is the original scheme for instant demoulding of molded products.

On a blocking press of the “Borets” , the molding of the polystyrene concrete mixture occurs outside the movable matrix. To feed the molded product on the technological pallet, a pusher is used, which ensures the most careful movement of the molded products to the supply line. In this case, the destruction of wall blocks and stones during demoulding is completely eliminated.

The Borets type brick making machine is equipped with two electromechanical vibrators and a lever suspension of the punch plate, which ensures guaranteed production of vibrocompressed polystyrene concrete blocks and stones of the main standard sizes, with a volumetric mass of 350 kg/m3, fully complying with the requirements of GOST R 51263-99.

The authors of the series of articles “Construction Pilot” are employees of Veksler M.V. Lipilin A.B.

Rules for pouring formwork

Let's consider common types of concrete work in private construction, for which you can make formwork yourself:

• Pouring a blind area, garden path, car area and other similar “horizontal” concrete work can be done in formwork made of new or used red brick. Brick formwork for pouring concrete is laid out along the contour of the future structure in a shallow trench (50-60 mm) at the building level. The space between the brick “contours” is filled with concrete. In this case, the brick remains in the trench and performs the function of protecting the contours of the concrete structure from destruction.
• Pouring the foundation of the building. As a rule, this is a strip foundation poured into a trench of the appropriate depth and width. In this case, the function of the main formwork is performed by the walls of the trench. If, for one reason or another, it is necessary to raise the upper section of the foundation above the zero point (soil surface), concrete is poured into formwork made of boards at least 25 mm thick and bars 40x40 or 50x50 mm. Shields are made from boards and bars. The panels are installed along the outer and inner perimeter of the foundation and are connected to each other with steel pins. Concrete pouring work is in progress. The air temperature will tell you when to remove the formwork after pouring concrete. At an air temperature of 20-25 degrees Celsius, the structure is dismantled after 3-4 days. At a temperature of 5-10 degrees, stripping is carried out in 7-10 days.
• Pouring concrete walls. In this case, homemade adjustable formwork is used from wooden panels about 70 cm high, first supported by the base of the building. After pouring and setting the first “belt”, the sewn sheets are rearranged higher and rested on the already poured “belt”, and so on until all the walls are “forced”. The outer shield belt is connected to the inner belt using two rows of steel threaded adjustable pins. In this case, the lower row of studs serves as support for the base and the next “belts” of the wall.
• Filling the ceiling. Homemade formwork for pouring a floor slab consists of two structures. These are wooden panels, fixed in one way or another along the outer perimeter of the walls and an additional horizontally located wooden panel, placed indoors at ceiling height using powerful logs. This is a rather complex, but viable design. The main problem is to correctly and reliably position the horizontal shield. Otherwise, it will either fall under the load of concrete or collapse.

Concrete hardening daily

The data in the following table has been calculated and averaged for concrete of the indicated classes and is of a advisory nature. It is clear that the plate is designed for optimal concrete hardening conditions, which is not always the case in practice.

Using the table, you can navigate by the gained strength of concrete, based on the time elapsed from concreting, indicated in days, and the average daily air temperature during the maturation period. Average daily means that day and night temperatures need to be added and their average found. It is clear that the data in the last column +30⁰С for central Russia is unlikely to be needed. Numbers expressed in decimal fractions show the proportion of concrete strength compared to the grade.

number of days of hardening

Once again: these tables are calculated based on the optimal moisture conditions for concrete hardening, in other words, close to ideal, that is, these indicators are maximum. One asterisk indicates the strength gain data at which the structure can be stripped, but not loaded with subsequent work. Two asterisks indicate the strength achieved by concrete, at which the following work can begin - for example, laying brick walls.

And these calculations are just recommendations. And it is better to remove the formwork later than earlier. In practice, it is always better not to rush with formwork, this allows you to get better quality concrete structures.

Production of concrete fences

The production of such a widely demanded product in modern construction as concrete panels, the so-called “Euro fence”, is carried out using the vibrating cassette casting method according to 2 different technological schemes:

1. Instant demoulding;
2. Expositions.

Despite the seemingly minor differences between these casting methods, the difference in the end result is enormous. Since the instant formwork method involves the use of the most durable form-matrices, precise selection of the consistency of the filling mixture, mandatory uniformity of filler size and the presence in production of a large number of special formwork pallets.

The material for the manufacture of mold-matrices is fiberglass and a powerful metal frame, which makes this product as durable as possible with load-bearing capacity, practically unlimited working life and a weight of at least 50 kg. Despite the considerable weight, working with these forms is not particularly difficult, but requires the performer to be in good physical shape and a certain dexterity.

Production of Euro fences using the instant formwork method

Equipment for the production of concrete fences using the instant formwork method provides that the number of formwork pallets must be equal to the number of concrete sections produced per work shift multiplied by two.

The technological process of manufacturing Euro fences using the instant formwork method can be divided into the following stages:

• A fully prepared matrix mold is installed on a functioning vibrating table, into which the required volume of concrete mixture is loaded, after which the excess concrete is cut off using a special rule designed to level the pouring mixture and thereby bring its level to the same level as the edge of the matrix mold;
• After that, reinforcing elements are placed on the vibrating pouring mixture, followed by their recessing under the influence of gravity to the intended center of the product;
• Upon completion of the vibration compaction procedure, determined by the absence of air bubbles on the surface of the potting mixture, two physically strong performers with a deft movement turn the matrix mold with the finished product onto the stripping pallet for its subsequent setting and strength gain;
• Then the matrix mold is thoroughly washed from adhering concrete, lubricated no less thoroughly and placed on a vibrating table for the manufacture of the next product.

How does concrete mature?

The ripening of concrete refers to a chemical process, as a result of which the physical properties of the solution change, which acquires the desired hardness. In this case, water participates in the cement hydration reaction, which is irreversible.

Cement contains four main components responsible for its properties:

• Tricalcium silicate is the main component that takes part in hydration, as a result of which the temperature of the composition increases and it gains strength.
• The dicalcium silicate continues to harden over several years, resulting in a stronger monolith.
• Tricalcium aluminate is responsible for the rate of hardening in the first day after laying the mortar.
• Quadricalcium aluminoferrite - necessary for hardening at the final stage of setting.

Concrete ripening occurs within 28 days at a temperature of about 20⁰C. But these are ideal conditions that rarely occur. In reality, they depend on the temperature and humidity of the environment. The maturation process occurs in two stages. First comes setting, which occurs from several hours to a day. The set concrete loses its mobility and retains its shape. To slow down this process, use the property of thixotropy - slowing down the setting when the solution is stirred. It is used for transportation in automixers. Before setting, concrete can be compacted and its geometric configuration changed without deteriorating its characteristics.

Strengthening of concrete occurs over a long period of time; it is believed that it does not stop for several years. Within 28 days, even under ideal conditions, the solution reaches up to 98% of the calculated value. During this period, chemical reactions take place that transform the soft solution into a durable monolith.

Brand of concrete for the production of fences

The grade of concrete used for the production of Euro fences must be no less than M300 with a water-cement ratio of no less than 0.35, and crushed stone used as a filler with a fraction of no more than 5 mm. In addition, for the production of high-quality concrete Eurofence panels, the use of a special plasticizer is a prerequisite, while the use of additives that accelerate the process of strengthening of products and various coloring pigments depends on the personal preferences of each specific customer or manufacturer.

After how many days to remove it, taking into account the air temperature?

To decide when exactly to remove wooden formwork, you need to take into account one main factor, namely the ambient temperature. According to this, at different times of the year the setting period will be different. As a result, basically all construction work related to pouring the foundation is carried out in the summer.

When calculating the temperature, it is not the maximum or minimum value during the day that is taken into account, but the average daily value. Depending on the specific weather conditions, the time for removing the created formwork from the concrete floor is calculated. You definitely shouldn’t rush into stripping too much, since certain unaccounted factors can somewhat slow down the process of crystallization of the concrete solution.

Formwork for monolithic construction of foundations, walls, ceilings

Formwork is an important element of any monolithic construction. It allows you to plan buildings of any scale and complexity, despite the fact that, in comparison with other technologies, construction time is significantly reduced, and the quality and reliability of objects is much higher. Based on the purpose of the structure, monolithic formwork can be used to create:

• walls, foundations, bridge supports, pipes and elevator shafts;
• overpasses, bridge structures and floors;
• non-standard architectural solutions, for example, a spherical surface (in this case, a beam-transom formwork system is constructed);
• flights of stairs, etc.

1ST FORMWORK COMPANY offers profitable purchases of formwork in Moscow and other cities of the country. Available - new and used products. Renting is also possible. Additionally, delivery services to the site are provided.

Scope of application of formwork for monolithic works

Based on the purpose of the structure, monolithic formwork can be used as:

• vertical surface for walls, foundations, bridge supports, pipes and elevator shafts;
• horizontal surface for creating overpasses, bridge structures and floors;
• a curved surface for pouring non-standard architectural solutions, for example, a spherical surface (in this case, a beam-transom formwork system is constructed);
• staircases are built according to individual designs on the basis of durable formwork.

Installing monolithic structures is quite simple - it does not require much labor or the use of expensive equipment. As a result, the installed formwork forms the foundation, walls, ceilings and other elements of the structure quickly and really efficiently.

Characteristics of formwork for monolithic construction from the company “1st FORMWORK COMPANY”:

• high strength, load resistance, reliability;
• made from quality raw materials;
• ease of assembly and disassembly;
• smoothness of the working surface, which ensures minimal adhesion to concrete;
• formwork systems for reusable use are equipped with durable elements that, if used correctly and carefully dismantled, can participate in an unlimited number of production cycles;
• all structures are manufactured in accordance with the requirements of strict building standards;
• do not absorb moisture from the concrete mixture, so the hardening process is not disrupted;
• favorable cost of formwork for monolithic construction.

Low prices for turnkey formwork from the manufacturer

Buying permanent formwork in Moscow or renting used formwork for monolithic construction from 1st FORMWORK COMPANY LLC is profitable and reliable. We always strive for long-term cooperation, selecting the best conditions for purchasing equipment individually for each client.

We offer to buy formwork for monolithic works in the most convenient way for the customer:

• purchase new equipment;
• buy used formwork for monolithic construction;
• equipment rent;
• formwork leasing;
• trade-in.

Our own production allows us to thoroughly monitor and guarantee the quality of our products. For each type of formwork, appropriate certificates are provided.

Thanks to modern technologies, we have reduced unnecessary costs in the production process. In addition, we are a direct supplier. Taken together, this allows us to set an affordable price for formwork for monolithic construction.

If you still have a question about where to buy formwork for monolithic construction in Moscow, or are looking for the best deal, you should personally contact the specialists of 1st FORMWORK COMPANY LLC. They will advise you in detail, make a free preliminary calculation of the cost of formwork for monolithic construction and select the most profitable option for cooperation.