Formwork. The main causes of defects in concrete structures Reasons for adhesion of concrete to formwork
The adhesion of concrete to the formwork reaches several kgf / cm2. This makes formwork difficult, degrades the quality of concrete surfaces and leads to premature wear of formwork panels.
The adhesion of concrete to the formwork is influenced by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the forming surface of the formwork.
By adhesion (adhesion) is understood the bond due to molecular forces between the surfaces of two dissimilar or liquid contacting bodies. In the period of contact of concrete with the formwork, favorable conditions are created for the manifestation of adhesion. Adhesive (adhesive), which in this case is concrete, is in a plastic state during installation. In addition, in the process of vibration compaction of concrete, its plasticity increases even more, as a result of which the concrete approaches the surface of the formwork and the continuity of contact between them increases.
Concrete adheres to wooden and steel formwork surfaces more strongly than to plastic ones, due to the poor wettability of the latter.
Wood, plywood, steel without processing and fiberglass are well wetted and the adhesion of concrete to them is quite large, with slightly wettable (hydrophobic) getinaks and textolite, the concrete adheres slightly.
The wetting angle of brushed steel is greater than that of raw steel. However, the adhesion of concrete to brushed steel is reduced slightly. This is explained by the fact that at the border of concrete and well-machined surfaces, the contact continuity is higher.
When applied to the surface of the oil film, it hydrophobizes, which sharply reduces adhesion.
Shrinkage adversely affects adhesion, and hence adhesion. The larger the shrinkage in the butt layers of concrete, the more likely the appearance of shrinkage cracks in the contact zone, weakening adhesion. By cohesion in the contact pair of the formwork - concrete, it should be understood the tensile strength of the connecting layers of concrete.
The surface roughness of the formwork increases its adhesion to concrete. This is because the rough surface has a larger actual contact area compared to a smooth one.
Highly supported formwork material also increases adhesion, since cement mortar, penetrating into the pores, under vibration compaction forms a point of reliable connection.
When removing the formwork, there can be three options for separation. In the first embodiment, the adhesion is very small, and the cohesion is quite large
In this case, the formwork comes off exactly along the contact plane. The second option is adhesion more than cohesion. In this case, the formwork comes off using an adhesive material (concrete).
The third option — adhesion and cohesion are approximately the same in size. The formwork comes off partly along the plane of contact of concrete with the formwork, partly along the concrete itself (mixed or combined separation).
With adhesive separation, the formwork is easily removed, its surface remains clean, and the concrete surface is of good quality. As a consequence of this, it is necessary to strive to ensure adhesion separation. To this end, the formwork surfaces of the formwork are made of smooth, poorly wettable materials or they are applied with lubrication and special release coatings.
Formwork lubricants depending on their composition, principle of operation and operational properties can be divided into four groups: aqueous suspensions; water repellent lubricants; lubricants - concrete setting retarders; combined lubricants.
Aqueous suspensions of powdered substances that are inert to concrete are simple and cheap, but not always effective in eliminating the adhesion of concrete to the formwork. The principle of operation is based on the fact that as a result of the evaporation of water from suspensions before concreting, a thin protective film is formed on the formwork surface of the formwork, which prevents the adhesion of concrete.
Most often, for the lubrication of the formwork, a lime-gypsum-coBVio suspension is used, which is prepared from gypsum gypsum (0.6-0.9 weight parts), lime test (0.4-0.6 weight parts), sulfite alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).
Suspension lubricants are erased with concrete mix and vibroconsolidation and contaminate concrete surfaces, as a result of which they are rarely used.
The most common hydrophobic lubricants are based on minsoal oils, emulsol EX or salts of fatty acids (soaps). After their application to the surface of the formwork, a hydrophobic film of a number of oriented molecules is formed (Fig. 1-1, b), which impairs the adhesion of the formwork material to concrete. The disadvantages of such lubricants are pollution of the concrete surface, high cost and fire hazard.
In the third group of lubricants, the properties of concrete are used to set in slow motion in thin joint layers. To slow down the hardening, molasses, tannin, etc. are introduced into the composition of the lubricants. The disadvantage of such lubricants is the difficulty in regulating the thickness of the concrete layer in which the setting is slowed down.
Most effective combined lubricantsin which the properties of forming surfaces are used in combination with a delay in the setting of concrete in thin joint layers. Such lubricants are prepared in the form of so-called inverse emulsions. In addition to gndrofobizatora and setting retarders, some of them include plasticizing additives: sulphite-yeast vinasse (SDB), soap soap or TsNIPS additive. These substances during vibration compaction plasticize concrete in the butt layers and reduce its surface porosity.
ESO-GISI lubricants are prepared in ultrasonic hydrodynamic mixers (Fig. 1-2), in which mechanical mixing of the components is combined with ultrasonic. To do this, pour components into the mixer tank and turn on the mixer.
The installation for ultrasonic mixing consists of a circulation pump, a suction and pressure pipes, a junction box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The fluid supplied by the pump under an overpressure of 3.5-5 kgf / cm2 flows out at high speed from the nozzle of the vibrator and hits the wedge-shaped plate. In this case, the plate begins to vibrate at a frequency of 25-30 kHz. As a result, zones of intense ultrasonic mixing are formed in the liquid while dividing the components into tiny droplets. The mixing time is 3-5 minutes.
Emulsion lubricants are stable, they do not stratify within 7-10 days. Their use completely eliminates the adhesion of concrete to the formwork; they hold well on the forming surface and do not contaminate!
These greases and formwork can be applied with brushes, rollers and spray rods. With a large number of shields, a special device should be used to lubricate them.
The use of effective lubricants reduces the harmful effects on the formwork of certain factors.
For metal shields, the CE-3 enamel, which includes epoxy resin (4-7 parts by weight), methylpolysiloxane oil (1-2 parts by weight), lead litharge (2-4 parts by weight, is recommended as a release coating). ) and polyethylene polyamine (0.4-0.7 wt. h.). A creamy paste of these components is applied to a thoroughly cleaned and degreased metal surface with a brush or trowel. The coating hardens at 80-140 ° C for 2.5-3.5 hours. The turnover of such a coating reaches 50 cycles without repair.
For plank and plywood formwork at TsNIIOMTP a phenol-formaldehyde-based coating has been developed. It is pressed onto the surface of the panels at pressures of up to 3 kgf / cm2 and a temperature of + 80 ° C. This coating completely eliminates the adhesion of concrete to the formwork and can withstand up to 35 cycles without repair.
Despite the rather high cost (0.8-1.2 rub / m2), anti-adhesive protective coatings are more profitable than lubricants due to their multiple turnover.
It is advisable to use shields, the decks of which are made of getinax, smooth fiberglass or textolite, and the frame is made of metal corners. This formwork is wear-resistant, easy to remove and provides good quality concrete surfaces.
The adhesion of concrete to the formwork reaches several kgf / cm 2. This makes formwork difficult, degrades the quality of concrete surfaces and leads to premature wear of formwork panels.
The adhesion of concrete to the formwork is influenced by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the forming surface of the formwork.
By adhesion (adhesion) is understood the bond due to molecular forces between the surfaces of two dissimilar or liquid contacting bodies. In the period of contact of concrete with the formwork, favorable conditions are created for the manifestation of adhesion. The adhesive (adhesive), which in this case is concrete, is in a plastic state during installation. In addition, in the process of vibration compaction of concrete, its plasticity increases even more, as a result of which the concrete approaches the surface of the formwork and the continuity of contact between them increases.
Concrete adheres to wooden and steel formwork surfaces more strongly than to plastic ones, due to the poor wettability of the latter. The values \u200b\u200bof Kc for different types of formwork are: small-panel - 0.15, wooden - 0.35, steel - 0.40, large-panel (panels of small panels) - 0.25, large-panel - 0.30, volumetric - 0, 45, for block forms - 0.55.
Wood, plywood, steel without processing and fiberglass are well wetted and the adhesion of concrete to them is quite large, with slightly wettable (hydrophobic) getinaks and textolite, the concrete adheres slightly.
The wetting angle of brushed steel is greater than that of raw steel. However, the adhesion of concrete to brushed steel is reduced slightly. This is explained by the fact that at the border of concrete and well-machined surfaces, the contact continuity is higher.
When applied to the surface of the oil film, it hydrophobizes, which sharply reduces adhesion.
The surface roughness of the formwork increases its adhesion to concrete. This is because the rough surface has a larger actual contact area compared to a smooth one.
The highly porous formwork material also increases adhesion, since cement mortar, penetrating into the pores, forms vibration-tight points when vibro-compacted. When removing the formwork, there can be three options for separation. In the first embodiment, the adhesion is very small, and the cohesion is quite large.
In this case, the formwork comes off exactly along the contact plane. The other option is adhesion more than cohesion. In this case, the formwork comes off using an adhesive material (concrete).
The third option - adhesion and cohesion are approximately the same in value. The formwork comes off partly along the plane of contact of concrete with the formwork, partly along the concrete itself (mixed or combined separation).
With adhesive separation, the formwork is easily removed, its surface remains clean, and the concrete surface is of good quality. As a consequence of this, it is necessary to strive to ensure adhesion separation. To do this, the formwork surfaces of the formwork are made of smooth, poorly wettable materials or they are lubricated and special release coatings applied to them.
Lubricants for formwork, depending on their composition, principle of operation and operational properties, can be divided into four groups: aqueous suspensions; water repellent lubricants; lubricants - concrete setting retarders; combined lubricants.
Aqueous suspensions of powdered substances that are inert to concrete are simple and cheap, but not always effective in eliminating the adhesion of concrete to the formwork. The principle of operation is based on the fact that as a result of the evaporation of water from suspensions before concreting, a thin protective film is formed on the forming surface of the formwork, which prevents the adhesion of concrete.
Most often, lime-gypsum slurry is used to lubricate the formwork, which is prepared from gypsum gypsum (0.6-0.9 parts by weight), lime dough (0.4-0.6 parts by weight), sulphite-alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).
Suspension lubricants are erased by concrete mixture during vibration compaction and contaminate concrete surfaces, as a result of which they are rarely used.
The most common hydrophobic lubricants based on mineral oils, emulsol EX or salts of fatty acids (soaps). After their application to the surface of the formwork, a hydrophobic film of a number of oriented molecules is formed, which impairs the adhesion of the formwork material to concrete. The disadvantages of such lubricants are contamination of the concrete surface, high cost and fire hazard.
In the third group of lubricants, the properties of concrete are used to set in slow motion in thin joint layers. To slow down the setting, molasses, tannin, etc. are introduced into the composition of the lubricants. The disadvantage of such lubricants is the difficulty in controlling the thickness of the concrete layer.
Combined lubricants, which use the properties of forming surfaces in combination with a delay in the setting of concrete in thin joint layers, are most effective. Such lubricants are prepared in the form of so-called inverse emulsions. In addition to water repellents and setting retarders, some of them include plasticizing additives: sulphite-yeast vinasse (SDB), soap soap or TsNIPS additive. These substances during vibration compaction plasticize concrete in the butt layers and reduce its surface porosity.
ESO-GISI lubricants are prepared in ultrasonic hydrodynamic mixers in which mechanical mixing of the components is combined with ultrasonic. To do this, pour components into the mixer tank and turn on the mixer.
The installation for ultrasonic mixing consists of a circulation pump, a suction and pressure pipes, a junction box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The fluid supplied by the pump under an overpressure of 3.5-5 kgf / cm2 flows out at high speed from the nozzle of the vibrator and hits the wedge-shaped plate. In this case, the plate begins to vibrate at a frequency of 25-30 kHz. As a result, zones of intense ultrasonic mixing are formed in the liquid while dividing the components into tiny droplets. The mixing time is 3-5 minutes.
Emulsion lubricants are stable, they do not exfoliate for 7-10 days. Their use completely eliminates the adhesion of concrete to the formwork; they hold well on the forming surface and do not contaminate concrete.
Apply these lubricants to the formwork with brushes, rollers and spray rods. With a large number of shields, a special device should be used to lubricate them.
The use of effective lubricants reduces the harmful effects on the formwork of certain factors. In some cases, grease cannot be used. So, when concreting in sliding or climbing formwork, it is forbidden to use such lubricants due to their ingress into concrete and a decrease in its quality.
Polymer-based release coatings give a good effect. They are applied to the forming surfaces of the panels during their manufacture, and they withstand 20-35 cycles without repeated application and repair.
A phenol-formaldehyde-based coating has been developed for plank and plywood formwork. It is pressed onto the surface of the panels at a pressure of up to 3 kgf / cm2 and a temperature of + 80 ° C. This coating completely eliminates the adhesion of concrete to the formwork and can withstand up to 35 cycles without repair.
Despite the rather high cost, anti-adhesive protective coatings are more profitable than lubricants due to their multiple turnover.
It is advisable to use shields, the decks of which are made of getinax, smooth fiberglass or textolite, and the frame is made of metal corners. This formwork is wear-resistant, easy to remove and provides good quality concrete surfaces.
Formwork It is called a form-forming temporary structure intended for molding monolithic concrete and reinforced concrete structures and consisting of the actual form, supporting scaffolding and fixing devices. The formwork must be stable and durable, ensure the correctness and immutability of the structure, the quality of the concrete surface, quickly assemble and disassemble, not create difficulties when installing reinforcement, laying and compacting the concrete mixture. When calculating the formwork, vertical and horizontal loads from the own mass of the formwork and scaffolding, concrete mix and reinforcement, working people and vehicles, vibration and dynamic loads that occur when unloading concrete mix into the formwork, as well as lateral pressure of the concrete mix, are taken into account. The side elements of the formwork are counted on the pressure of the concrete mixture, based on the fact that the pressure of this mass extends into the concrete no more than 1 m.
Depending on the material used, the formwork can be wooden, metal, wood-metal, reinforced concrete, reinforced cement, from synthetic or rubberized fabrics.
Wood formwork made of wood with a moisture content of not more than 25%. For the manufacture of elements of wooden formwork, boards, chipboards and fiberboards are used. Timber and wood-based materials can be made from coniferous and hardwood. The scaffold racks used for the formwork device are more than 3 m high, as well as the runs supporting the formwork, are made only of coniferous wood. For other elements of formwork and fixtures, hardwood is used - aspen, alder. In the manufacture of wood-metal shields, birch is used for cladding. For the deck of shields, waterproof bakelized plywood or fiberglass sheets are used. To reduce adhesion to concrete and improve the quality of front concrete surfaces, coating of the deck of the boards with films based on polymers is also used.
Additionally, see "Carpentry"
Metal formwork made of steel sheets with a thickness of 1.5-2 mm and rolling profiles; it must have quick disconnect connections. Metal parts of wood-metal formwork are also made of steel sheets. The mesh size of the metal mesh used as mesh formwork should not exceed 5x5 mm.
Reinforced concrete formwork It is a reinforced concrete slab-shell; these slabs are installed as formwork before concreting and are the outer part of the structure being erected, inextricably linked with it.
Reinforced cement formwork It is used in the form of cement-cement slabs 15-20 mm thick. Such slabs are made of fine-grained concrete reinforced with wire mesh. Before applying a layer of concrete, the mesh can be bent, giving the necessary curved profile to the concrete slab.
Pneumatic constructions formed by forcing air into the inner enclosed space of the shell of an airtight fabric; in this case, almost any shape can be imparted. Materials for the manufacture of inflatable formwork are technical textiles, synthetic materials, single-layer and multi-layer rubberized fabrics.
An important problem is the reduction of concrete adhesion to formwork. This adhesion depends on the adhesion (adhesion) and cohesion (tensile strength of the boundary layers at the formwork-concrete contact) of concrete, its shrinkage and the nature of the formwork surface of the formwork. Adhesion consists in the fact that during laying and vibration compaction the concrete mixture acquires plasticity properties and therefore the continuity of contact between it and the formwork increases. If the deck is made of slightly wettable (hydrophobic) materials, such as plastics, textolite, etc., and has a smooth surface, adhesion to the deck is negligible. If the deck is made of highly wettable (hydrophilic) materials, such as steel, wood, etc., has a rough surface or porous structure, the continuity and contact strength increase and, therefore, adhesion increases. If the adhesion is small and the cohesion is large, during the formwork, separation occurs along the contact plane and the forming surface of the formwork remains clean, and the front surfaces of the concreted structure are obtained in good quality.
Adhesion forces can be reduced by using hydrophobic materials for the formwork surfaces by applying special lubricants and anti-adhesive hydrophobic coatings to the deck surface. Combined lubricants in the form of so-called inverse emulsions are most practical. In addition to water repellents and setting retarders, plasticizing additives are introduced into them, which plasticize the concrete in the zone of contact with the formwork and facilitate its separation.
The design of the formwork should provide sufficient strength, reliability and ease of installation and dismantling of its elements, the possibility of enlarged assembly and wide variation in layout with their minimum nomenclature. In terms of turnover, they distinguish non-inventory formwork, used only for one structure, and inventory, that is, reusable. Inventory formwork can be collapsible and movable.
Inventory collapsible formwork assembled from shields, boxes, large inventory racks and other elements. Collapsible formwork is designed so that it is possible to dismantle the side surfaces, beams, girders and columns, regardless of the bottoms of the girder beams and girders, which can be dismantled only after the concrete has reached the formwork strength specified in the project. After disassembling, the formwork is cleaned, if necessary, repaired and reused. The main elements of wooden or combined collapsible formwork are panel shields made of boards with a thickness of 25-30 mm and upholstered with water-resistant plywood or from boards with board upholstery on the forming side of roofing steel, plastic, etc. The sizes and weight of the formwork elements must allow their manual installation .
Column foundation formwork arrange from rectangular boxes, which are collected from the outer and inner shields. The outer shields are 20–25 cm longer than the inner shields and have special stop bars to which the inner shields are attached; wire ties are attached to the outer shields, which perceive the spacer pressure of the freshly laid concrete mixture. The formwork of the columns is a shield, fastened in the form of a box with metal or wooden clamps, installed after 0.4-0.7 m.
Wooden formwork of girders and beams consists of a bottom, which rests on the heads of the supporting struts, and side shields. The shuttering formwork panels are installed on the circles, which are based on the undercircle boards, nailed to the stitching strips of the side shields.
To maintain the formwork forms arrange forests. When the formwork height is up to 6 m, telescopic inventory wood-metal or metal racks are used. To increase the bearing capacity, telescopic racks are grouped using inventory links of 3 or 4 pcs.
When installing walls with a thickness of up to 15 cm, rack racks are installed on one side of the partition and one wall is assembled from the shields, after which the partition is reinforced to the full height. Then, the rack-racks are installed from the front of the work, which is sealed with shields to a height of 1 m. The shields are expanded as the concrete is concreted.
The unified collapsible formwork is different from the usual inventory with a large interchangeability of elements, it has increased rigidity and inventory devices (contractions, lock joints, etc.) that facilitate installation. Such formwork can be wooden, wood-metal (combined) or steel. Steel formwork is made of corners, channels and sheet steel with a thickness of 2 mm. With good operation, it can be used up to 200 times, while the turnover of wooden inventory formwork is no more than 10-15 cycles. The design of a unified formwork allows you to assemble large-sized panels with an area of \u200b\u200bup to 35 m 2, as well as rigid formwork or reinforcing-formwork blocks. The use of panel or block formwork for large-sized structures and for large volumes of work allows you to approximately halve the complexity and significantly reduce the time of formwork.
Sliding and rolling formwork relate to the so-called mobile formwork systems.
Sliding the (movable) formwork system is used for concreting tall structures with a compact perimeter and a plan shape that is unchanged in height. Sliding formwork consists of formwork panels suspended from a jack U-shaped frame, jacks, oil pipelines, a working platform and suspended scaffolds. Jacking frames are the main bearing element, they are suspended formwork, scaffolds, desktop. Sliding formwork usually has a height of 1.1-1.2 m and covers the concrete structure along the external and internal contours. With a circular cross section of the structure, the sliding formwork consists of two concentrically arranged walls attached to the inner and outer circles. The formwork has a taper (the width of the form on the top is 6 ^ -8 mm less than the bottom), which facilitates its lifting, and is usually performed all-metal, which gives it greater rigidity and increases turnover. The formwork is lifted with the help of jacks resting on the jack support rods installed inside the formwork of the erected structure. Jacks, rising along the jack rods, carry the formwork behind them. The working floor of the mold block is wooden, it is laid on lightweight metal girders and fixed to the U-shaped frames racks. If necessary, scaffolds are hung from them, from which they wipe the concrete surface or perform other work. For the safety of work on the outer contour of the movable formwork, fences of the working floor with a height of 1 m are arranged, and for the protection of workers located on the external hanging scaffolds, visors. The lifting speed depends on the strength acquired by the concrete, allowing for demolition and precluding the possibility of adhesion of concrete to the formwork. The walls of small block formwork have more flexibility than large block formwork. The shields of this formwork at a height of 1.1 m have a width of 0.5-0.65 m. They are hung on a circle, assembled in frames. In the stacks of large-block formwork, the circles form one unit with the sheathing of the shield. A steel shield 2 mm thick is welded by intermittent welding to the upper side angle and to the vertical stiffeners - corners. The upper and lower circles from corner steel are welded to stiffeners. Shields are interconnected using pads and bolts. The length of the shields is from 0.5 to 2.5 m, the height is 1.1 m.
Rolling Formwork It is a formwork with a mechanical device for formwork and folding into transport position. The formwork is mounted on shields or trolleys and moved along the rail track. Depending on the design of the scaffolding support scaffolds, all types of rolling (horizontally moved) formwork can be divided into two groups: with scaffolds that are constant in height and with lifting and lowering scaffolds. The former are used for concreting smooth surfaces without ribs and diaphragms, and the latter, if any. Then, in the first case, the formwork is moved with a slight separation from the concrete or lowering it with jacks, wedges or other devices, and in the second with a winch and chain hoists or hoists. The correct position of the axes of the formwork is checked after each rearrangement. The following requirements are imposed on rolling formwork:
the structural elements that make up each section of the formwork must be reliably connected to each other so that when rearranging the design section of the concrete structure is not distorted;
formwork constructions should provide the possibility of its rapid separation from the concreted parts of the structure, unimpeded movement to a new position of the exact installation for re-concreting.
Climbing formwork consists of two conical shells - external and internal - suspended from radial rails, which are attached to an annular frame suspended on hinges from a mine hoist. Shells are assembled from panels made of sheet steel 2 mm thick, which are fastened together by bolts. The panels of the outer shell are of two types - rectangular and trapezoidal, so that the shell takes on the shape of a cone. The panels of the inner shell are half the height, they are hung in two tiers. All panels of the inner shell and formwork are rectangular. “Ears” are welded from the inside of these panels, into which reinforcing bars with a diameter of 14 mm are laid, forming four rows of closed elastic horizontal rings. The structure is concreted in tiers. After the concrete in the next tier reaches the required strength, the formwork is rearranged to the upper tier. In this case, the formwork is adjusted in the radial direction. As you move up along the concreting of the formwork, the circumference of the form decreases due to the removal of the shell panels after each lifting of the formwork.
Climbing formwork can be used instead of movable (sliding) formwork, if in the latter it is difficult to organize concrete concreting of structures due to the small amount of work or for other reasons.
Constructed climbing formwork should provide:
the possibility of changing the cross section of the concrete structure in accordance with the project when moving the formwork in height;
strictly defined position of the formwork and reliable fastening of its elements during rearrangements;
the possibility of unhindered lifting of people and the supply of materials to the working area during the construction process.
When moving the climbing formwork, the displacement of its longitudinal axis relative to the axis of the structure is allowed no more than 10 mm.
Block form It is a large-sized spatial frame structure consisting of shields and fixtures designed for mechanized installation and dismantling. According to the design, the block forms are one-piece of rigid all-removable forms and detachable. The former are removed using jacks from the concreted foundation without disassembling due to the conicity of the forming surfaces, the latter - with the help of special angle locks connecting the formwork panels and tear-off devices, which, when stripping, ensure the separation of the forming planes from the concrete.
Fixed formwork (formwork-shell) is a thin-walled form, which serves as a formwork for concreting, and then its cladding. Fixed formwork works together with monolithic concrete and is included in the design design section. Depending on the purpose, fixed formwork is made of heat-insulating reinforced concrete and reinforcing plates, asbestos-cement plastic sheets, expanded polystyrene, etc. It is most economical to use fixed formwork when it also plays the role of waterproofing and insulation.
Pneumatic (inflatable) formwork represents a variety of collapsible and permutable. It is made from rubberized and other special fabrics. Pneumatic formwork in the form of a shell is spread out and fixed. When injected into an enclosed space of air, the shell takes a given shape. After reaching the formwork strength, air is released from the shell, and the structure is freed from the formwork.
Struggling of structures is carried out with the safety of formwork. Support racks should be removed only after removing the side formwork and inspection of the dismantled structures. Stripping of bearing reinforced concrete structures is allowed after concrete reaches at least 70% of its strength. It is allowed to load the dismantled structure with full rated load only after concrete reaches its design strength. Structures concreted in winter should be dismantled after confirming the required strength by testing control samples; after removal of thermal protection, not earlier than the concrete cools down to a temperature of +5 ° С.
Formwork care and formwork lubrication provide formwork turnover. Shields of inventory formwork, as well as supporting elements-contractions, racks, crossbars, girders and similar fastenings - clamps, clamps, locks, etc. after each revolution must be cleaned of cement with metal brushes and scrapers. The use of hammers or other percussion instrument to clean the formwork elements from the solution is strictly prohibited. The use of inventory formwork provides for the mandatory lubrication of the deck of the boards and its thorough cleaning from the remains of cement mortar after each revolution. The grease should not leave oily stains (in some cases, when concreting foundations and structures filled with soil or protected by waterproofing, this requirement may not be observed), the grease should not impair the strength properties of the surface layers of reinforced concrete structures, the grease components should not be volatile and harmful to health substances. When using lubricants for formwork of vertical surfaces, they must have sufficient viscosity and adhesive properties to remain on a vertical surface for 24 hours at a temperature of +30 ° С.
Formwork is carried out in strict accordance with the working drawings. The formwork project is part of the general construction project and consists of:
marking drawings of the most characteristic, often repeated or complex formwork designs. The drawings show the location of the individual elements of the formwork in plan, section, facade or in the development;
technological maps of work;
schemes for organizing formwork, interconnected with other types of work, in which it is necessary to provide: breakdown by grips, direction of movement of the formwork sets, the speed of turnover of the kit on individual grips or blocks when concreting complex structures and structures; specifications of elements and the total volume of the formwork kit.
On the diagram of the organization of formwork, in addition to the image of concrete structures and structures with an indication of the volume of formwork, a list of hoisting mechanisms is placed, storage areas, as well as linear work schedules are indicated.
Quality control of formwork consists in determining:
compliance of the forms and geometrical dimensions of the formwork with the working drawings;
coincidence of the axes of the formwork with the center axes of structures and structures;
the accuracy of the marks of individual formwork planes or leader in formwork areas;
vertical and horizontal formwork planes;
the correct installation of embedded parts, plugs, etc .;
the density of joints and joints of formwork elements with extras in place, with previously laid concrete or preparation.
Hello dear readers! All our and your questions are answered today by master Vadim Aleksandrovich. Today we will talk about the features of pouring concrete into the formwork.
Hello Vadim Alexandrovich!
Hello! First of all, I want to say that this work is quite complicated and very responsible, and it is better to entrust the professionals to fill the floors and load-bearing walls than to try to do it yourself. Let's get to your questions.
1. Do I need to somehow prepare the formwork and reinforcement?
The formwork is lubricated with a special aqueous emulsion lubricant (Emulsol) in order to separate the formwork from the hardened concrete. Although there were cases at a construction site when they were poured into an ungreased formwork and then it was torn off. Also, the formwork is pulled together with special screeds that are inserted into the tubes between the shields.
2. Is the method of filling horizontal forms different from vertical?
Virtually no different. Verticals are a little harder to tamp.
3. Please tell us how to pour concrete.
The method of pouring is determined by the project (TCH). It is desirable to fill the entire formwork immediately, pouring layers is undesirable, otherwise you will have to make notches with a perforator for better adhesion of the layers. Vertical forms must be filled in whole.
4. How to connect layers if nevertheless we fill with layers? Well, we didn’t have enough concrete for pouring the whole thing.
As I said, we make notches with a puncher for hardened concrete.
5. What are the secrets to evenly filling?
There are no secrets, there are general rules: We fill it in different places and not in one, scatter it with shovels in all its shape, then - ram it with a vibrator to a smooth glossy surface in order to remove all voids and concrete uniformly filled the formwork. However, if the concrete is of poor quality, but it is very necessary to fill it, then you cannot use a vibrator - all the water will flow out and the concrete will not seize. 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 affect the fill?
A thick solution is difficult to evenly distribute and compact. Before pouring, add water to the mixer. Too liquid - and again bad, when tamping all the water will flow out and the concrete will not seize. If we do it ourselves, then we add cement and sand, if we are brought ready, we are sent to the factory due to non-compliance.
7. I heard that concrete heats up when solidified. Is this a problem and is it necessary to deal with it?
Yes, this is a problem and it needs to be fought. In the heat, it is necessary to pour the formwork with cold water, otherwise the concrete will crack. And in the cold, on the contrary, we warm up.
8. If we do not keep track and the concrete is cracked, how to fix it?
Small cracks are permissible, the maximum crack size is indicated in the design documentation, if the size is exceeded, then we take a jackhammer and beat off. Otherwise, it will fall apart after a while. After all, cracks significantly reduce the strength of the structure.
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When working with monolithic-type structures made of reinforced concrete, it is worth paying attention to the features of concrete adhesion to formwork, where the value reaches several kg per square centimeter. Due to the clutch, the stripping of the reinforced concrete structure will be more complicated, in addition, this process can degrade the concrete surface itself, namely, its quality. And the formwork panels can even collapse before the specified time. To prevent this from happening, ubts.kiev.ua is now available, which solves all these problems.
Due to the factors described below, concrete adheres to formwork:
concrete undergoes adhesion and cohesion;
concrete shrinks;
the formwork adjacent to the reinforced concrete structure may have a rough or porous surface.
At the moment when the concrete is laid, its condition is plastic, therefore it is considered an adhesive substance, due to which there is a process called adhesion (when concrete adheres to the formwork). When the material is compacted, the plasticity index of concrete can increase, as a result of which it is adjacent to the surface of the formwork.
The adhesion process can be different, depending on the material used to produce the formwork surface: more concrete will adhere to wood and steel. Plastic products, because of their less wettability, are less likely to adhere to concrete.
If plywood, steel, wood or fiberglass materials are not pre-processed, they will be easily wetted, which will ensure high adhesion to concrete. Less significant coefficient of adhesion with getinaks and textolite, as they belong to the category of hydrophobic materials.
The wettability can be reduced by treating the surface, which is the application of an oil film on it, as a result of which the adhesion process will be significantly reduced. Due to shrinkage, not only adhesion can decrease, but adhesion: because of the large shrinkage, it is likely that shrinkage cracks will appear in the contact zone, which affects the adhesion weakening.
If formwork of a monolithic concrete structure is required, three methods are now available, thanks to which the detachable formwork is detached:
high cohesion and low adhesion. In this situation, the formwork needs to be torn off along the contact plane;
adhesion level exceeds cohesion. The formwork will be torn off according to the material that is adhesive (concrete);
approximate equality between adhesion and cohesion. This situation involves the separation of the mixed (combined) type.
The first option is the most optimal, since it allows you to easily remove the formwork, keeping its surface clean, as well as maintaining the quality of the concrete itself. In this regard, the adhesive separation must be provided more often than others. It is available in such situations:
when the forming formwork surface is made of a smooth material that is poorly wetted;
the forming surface was treated with special grease or special release coatings.
Formwork grease must meet the following requirements:
after its use, oil stains should not be left on the concrete surface;
the contact layer of concrete should not become less durable;
high level of fire safety;
the composition should not include volatile substances that are dangerous to human health;
the ability to stay on the surface (vertical and horizontal) during the day at an air temperature of +30 degrees Celsius.