The amount of adhesion between concrete and formwork reaches several kgf/cm2. This complicates stripping work, deteriorates the quality of concrete surfaces and leads to premature wear of formwork panels.

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

Adhesion (sticking) is understood as a bond caused by molecular forces between the surfaces of two dissimilar or liquid bodies in contact. During the period of contact between concrete and formwork, favorable conditions to demonstrate adhesion. The adhesive (adhesive), which in this case is concrete, is in a plastic state during the laying period. In addition, in the process of vibration compaction of concrete, its plasticity increases even more, as a result of which the concrete moves closer to the surface of the formwork and the continuity of contact between them increases.

Concrete sticks to wood and steel formwork surfaces more strongly than to plastic ones due to the latter's poor wettability.

When removing formwork, there can be three tearing options. In the first option, adhesion is very small, and cohesion is quite high. In this case, the formwork is torn off exactly along the contact plane. The second option is adhesion more than cohesion. In this case, the formwork is torn off along the adhesive material (concrete). The third option is that adhesion and cohesion are approximately the same in magnitude. The formwork comes off partially along the plane of contact between the concrete and the formwork, and partly along the concrete itself (mixed or combined tearing). With adhesive tearing, the formwork is easily removed, its surface remains clean, and the concrete surface is of good quality.

As a result, it is necessary to strive to ensure adhesive separation. To do this, the forming surfaces of the formwork are made of smooth, poorly wetted materials or lubricants and special anti-adhesive coatings are applied to them.

Formwork lubricants, depending on their composition, operating principle and operational properties, can be divided into four groups: aqueous suspensions; hydrophobic lubricants; lubricants - concrete set retarders; combined lubricants.

The use of effective lubricants reduces the harmful effects of certain factors on the formwork. In some cases, lubricants cannot be used. Thus, when concreting in sliding or climbing formwork, the use of such lubricants is prohibited due to their penetration into the concrete and a decrease in its quality. Anti-adhesive agents have a good effect protective coatings Based on polymers. They are applied to the forming surfaces of shields during their manufacture, and they withstand 20-35 cycles without re-application and repair. A phenol-formaldehyde-based coating has been developed for plank and plywood formwork. It is pressed onto the surface of the boards at a pressure of up to 3 kgf/cm2 and a temperature of + 80° C.

It is advisable to use boards whose decks are made of getinax, smooth fiberglass or textolite, and the frame is made of metal corners. This formwork is wear-resistant, easy to remove and provides good quality concrete surfaces.

The adhesion of formwork to concrete is influenced by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the formwork's forming surface. The adhesion value can reach several kg/cm2, which complicates formwork work, deteriorates the surface quality of the reinforced concrete product and leads to premature wear of the formwork panels.

Concrete sticks to wood and steel formwork surfaces more strongly than to plastic ones due to the latter's poor wettability.

Types of lubricants:

1) aqueous suspensions of powdery substances that are inert to concrete. When water evaporates from the suspension, a thin layer is formed on the surface of the formwork, which prevents the adhesion of concrete. more often a suspension of: CaSO 4 × 0.5H 2 O 0.6...0.9 wt is used. h., lime dough 0.4...0.6 parts by weight, LST 0.8...1.2 parts by weight, water 4...6 parts by weight. These lubricants are erased by the concrete mixture and contaminate concrete surfaces, so they are rarely used;

2) hydrophobic lubricants are most common based on mineral oils, emulsol or salts fatty acids(soap). After their application, a hydrophobic film is formed from a number of oriented molecules, which impairs the adhesion of the formwork to concrete. Their disadvantage: pollution concrete surface, high cost and fire hazard;

3) lubricants - retarders of concrete setting in thin butt layers. Molasses, tannin, etc. Their disadvantage is the difficulty of regulating the thickness of the concrete layer, in which setting slows down.

4) combined - the properties of the forming surfaces of the formwork are used in combination with retarding the setting of concrete in butt layers. They are prepared in the form of reverse emulsions; in addition to water repellents and retarders, plasticizing additives can be introduced: LST, soap naft, etc., which reduce the surface porosity of concrete in the butt layers. These lubricants do not delaminate for 7...10 days and are well retained. vertical surfaces and do not pollute concrete.

Installation of formwork .

The assembly of formwork forms from elements of inventory formwork, as well as the installation of volumetric-adjustable, sliding, tunnel and rolling formwork into the working position must be carried out in accordance with technological rules for their assembly. The forming surfaces of the formwork must be bonded with anti-adhesive lubricant.

When installing structures supporting the formwork, the following requirements are met:

1) the racks must be installed on foundations that have a supporting area sufficient to protect the concreted structure from unacceptable subsidence;

2) ties, screeds and other fastening elements should not interfere with concreting;

3) fastening of ties and braces to previously concreted reinforced concrete structures should be carried out taking into account the strength of the concrete at the time the loads from these fastenings are transferred to it;

4) the base for the formwork must be verified before its installation.

The formwork and surrounds of reinforced concrete arches and vaults, as well as the formwork of reinforced concrete beams with a span of more than 4 m, must be installed with a construction lift. The amount of construction lift must be at least 5 mm per 1 m span of arches and vaults, and for beam structures - at least 3 mm per 1 m span.

To install the beam formwork, an extendable clamp is placed on the upper end of the rack. Along the racks, purlins are installed on fork supports attached to the upper end of the rack, on which the formwork panels are installed. Sliding crossbars are also supported on the purlins. They can also be supported directly on the walls, but in this case, support sockets must be made in the walls.

Before installing the collapsible formwork, beacons are placed, on which marks are applied with red paint, fixing the position of the working plane of the formwork panels and supporting elements. Elements of formwork, supporting scaffolding and scaffolding should be stored as close as possible to the workplace in stacks of no more than 1...1.2 m by grade so as to ensure easy access to any element.

Shields, grips, racks and other elements must be lifted, as well as delivered to the workplace on a scaffold, in bags using lifting mechanisms, and the fastening elements must be supplied and stored in special containers.

The formwork is assembled by a specialized team and accepted by the foreman.

It is advisable to install and dismantle formwork using large-sized panels and blocks with maximum use means of mechanization. Assembly is carried out on assembly platforms with a hard surface. The panel and block are installed in a strictly vertical position using screw jacks mounted on struts. After installation, if necessary, install couplers, secured with a wedge lock on the contractions.

Formwork for structures over 4 m high is assembled in several tiers in height. The panels of the upper tiers are supported on the lower ones or installed on support brackets installed in concrete after dismantling the formwork of the lower tiers.

When assembling formwork with a curved outline, special tubular screeds are used. After assembling the formwork, it is straightened by tamping wedges sequentially in diametrically opposite directions.

Control questions

1. What is the main purpose of formwork for monolithic concreting? 2. What types of formwork do you know? 3. What materials can formwork be made from?

13. Reinforcement reinforced concrete structures

General information. Steel reinforcement for reinforced concrete structures is the most widespread type of high-strength rolled products with a tensile strength from 525 to 1900 MPa. Over the past 20 years, the volume of global production of reinforcement has increased approximately 3 times and reached more than 90 million tons per year, which is about 10% of all rolled steel produced.

In Russia in 2005, 78 million m3 of concrete and reinforced concrete were produced, volume of use steel reinforcement amounted to about 4 million tons, with the same pace of construction development and complete transition to ordinary reinforced concrete for reinforcement of classes A500 and B500 in our country in 2010 it is expected to consume about 4.7 million tons of reinforcing steel per 93.6 million m 3 of concrete and reinforced concrete.

Average consumption of reinforcing steel per 1 m 3 of reinforced concrete different countries world is in the range of 40...65 kg, for reinforced concrete structures manufactured in the USSR, the average consumption of reinforcing steel was 62.5 kg/m 3. Savings by switching to A500C steel instead of A400 are expected to be about 23%, while the reliability of reinforced concrete structures increases due to the elimination of brittle fracture of reinforcement and welded joints.

In the manufacture of prefabricated and monolithic reinforced concrete structures, rolled steel is used for the manufacture of reinforcement, embedded parts for assembling individual elements, as well as for mounting and other devices. The consumption of steel in the manufacture of reinforced concrete structures is about 40% of the total volume of metal used in construction. The share of rod reinforcement is 79.7% of the total volume, including: ordinary reinforcement - 24.7%, high-strength - 47.8%, high-strength - 7.2%; the share of wire reinforcement is 15.9%, including ordinary wire 10.1%, high-strength wire - 1.5%, hot-rolled wire - 1%, high-strength wire - 3.3%, the share of rolled wire for embedded parts is 4.4%.

Reinforcement installed according to calculations to absorb stress during the manufacturing, transportation, installation and operation of a structure is called working, and installed for structural and technological reasons is called assembly. Working and installation reinforcement is most often combined into reinforcement products - welded or knitted meshes and frames, which are placed in the formwork strictly in the design position in accordance with the nature of the operation of the reinforced concrete structure under load.

One of the main tasks solved in the production of reinforced concrete structures is to reduce steel consumption, which is achieved by using high-strength reinforcement. New types of reinforcing steels are being introduced for conventional and prestressed reinforced concrete structures, which are replacing low-performance steels.

For the manufacture of reinforcement, low-carbon, low or medium alloy open-hearth and converter steels are used various brands and structures, and, consequently, physical and mechanical properties with a diameter from 2.5 to 90 mm.

Reinforcement of reinforced concrete structures is classified according to 4 criteria:

– According to manufacturing technology, a distinction is made between hot-rolled rod steel, supplied in rods or coils depending on the diameter, and cold-drawn (made by drawing) wire.

– According to the method of strengthening, rod reinforcement can be strengthened thermally and thermomechanically or in a cold state.

– According to the shape of the surface, the reinforcement can be smooth, of a periodic profile (with longitudinal and transverse ribs) or corrugated (with elliptical dents).

– Based on the method of application, a distinction is made between reinforcement without prestressing and with prestressing.

Types of reinforcing steel. For reinforcement of reinforced concrete structures, the following is used: rod steel that meets the requirements of the standards: hot-rolled rod - GOST 5781, classes of this reinforcement are designated by the letter A; rod thermomechanically strengthened - GOST 10884, classes are designated At; wire made of low-carbon steel - GOST 6727, smooth is designated B, corrugated - BP; carbon steel wire for reinforcing prestressed reinforced concrete structures - GOST 7348, smooth is designated B, corrugated - BP, ropes according to GOST 13840, are designated by the letter K.

In the manufacture of reinforced concrete structures, it is advisable to use reinforcing steel with the highest mechanical properties to save metal. The type of reinforcing steel is chosen depending on the type of structure, the presence of prestress, manufacturing conditions, installation and operation. All types of domestic non-prestressing reinforcement are well welded, but limited weldable or non-weldable types of reinforcement are produced especially for prestressed reinforced concrete structures.

Hot rolled rod reinforcement. Currently, two methods are used to designate classes of bar reinforcement: A-I, A-II, A-III, A-IV, A-V, A-VI and, accordingly, A240, A300, A400 and A500, A600, A800, A1000. With the first designation method, one class can include different reinforcing steels with the same properties; with an increase in the class of reinforcing steel, its strength characteristics increase (conditional elastic limit, conditional yield strength, temporary resistance) and deformability indicators decrease (relative elongation after rupture, relative uniform elongation after rupture, relative narrowing after rupture, etc.). In the second method of designating classes of bar reinforcement, the numerical index denotes the minimum guaranteed value of the conditional yield strength in MPa.

Additional indices used to designate bar reinforcement: Ac-II - reinforcement of the second class, intended for reinforced concrete structures operated in the northern regions, A-IIIb - reinforcement of the third class, strengthened by drawing, At-IVK - heat-strengthened reinforcement of the fourth class, with increased durability to corrosion cracking, At-IIIS – tempered reinforcement III class weldable.

Rod reinforcement is available in diameters from 6 to 80 mm, reinforcement classes A-I and A-II with a diameter of up to 12 mm and class A-I II with a diameter of up to 10 mm inclusive can be supplied in rods or coils, the rest of the fittings are supplied only in rods with a length of 6 to 12 m, measured or of unmeasured length. The curvature of the rods should not exceed 0.6% of the measured length. Class A-I steel is made smooth, the rest is made with a periodic profile: class A-II reinforcement has two longitudinal ribs and transverse protrusions running along a three-way helical line. With a reinforcement diameter of 6 mm, protrusions are allowed along a single-lead screw line, and with a diameter of 8 mm, protrusions along a double-lead screw line are allowed. Reinforcement of class A-III and higher also has two longitudinal ribs and transverse protrusions in the form of a herringbone. The surface of the profile, including the surface of the ribs and protrusions, should be free of cracks, shells, rolling films and sunsets. In order to distinguish steel class A-III and higher, they are painted in various colors end surfaces of rods or mark steel with convex marks applied during rolling.

Currently, steel is also produced with a special screw profile - Europrofile (without longitudinal ribs, and transverse ribs in the form of a helical line, continuous or intermittent), which makes it possible to screw screw connecting elements - couplings, nuts - onto rods. With their help, reinforcement can be joined without welding anywhere and form temporary or permanent anchors.

Rice. 46. ​​Hot-rolled bar reinforcement of periodic profile:

a – class A-II, b – class A-III and higher.

For the manufacture of reinforcement, carbon steels are used (mainly St3kp, St3ps, St3sp, St5ps, St5sp), low and medium alloy steels (10GT, 18G2S, 25G2S, 32G2Rps, 35GS, 80S, 20KhG2Ts, 23Kh2G2T, 22Kh2G2AYu, 22Kh2G2R, 20Kh 2G2SR), change in carbon content and alloying elements regulate the properties of steel. Weldability of reinforcing steels of all grades (except 80C) is ensured chemical composition and technology. Carbon equivalent value:

Sequ = C + Mn/6 + Si/10

for welded steel from low-alloy steel A-III (A400) should be no more than 0.62.

Rod thermomechanically strengthened reinforcement is also divided into classes according to mechanical properties and operational characteristics: At-IIIC (At400C and At500C), At-IV (At600), At-IVC (At600C), At-IVK (At600K), At-V (At800), At-VK (At800K), At-VI (At1000 ), At-VIK(At1000K), At-VII(At1200). The steel is made of a periodic profile, which can be like a hot-rolled rod class A-Sh, or as shown in Fig. 46 with or without longitudinal and transverse crescent-shaped ribs, smooth reinforcement can be produced upon request.

Reinforcing steel with a diameter of 10 mm or more is supplied in the form of bars of measured length; welded steel can be supplied in bars of unmeasured length. Steel with a diameter of 6 and 8 mm is supplied in coils; delivery in coils of steel At400S, At500S, At600S with a diameter of 10 mm is allowed.

For welded reinforcing steel At400C carbon equivalent:

Seq = C + Mn/8 + Si/7

should be at least 0.32, for At500S steel - at least 0.40, for At600S steel - at least 0.44.

For reinforcing steel of classes At800, At1000, At1200, stress relaxation should not exceed 4% per 1000 hours of exposure at an initial force of 70% of the maximum force corresponding to the temporary resistance.

Rice. 47. Rod steel thermomechanically hardened with periodic profile

a) – crescent-shaped profile with longitudinal ribs, b) – crescent-shaped profile without longitudinal ribs.

Reinforcing steel of classes At800, At1000, At1200 must withstand without destruction 2 million stress cycles, which is 70% of the tensile strength. The stress range for smooth steel should be 245 MPa, for periodic steel – 195 MPa.

For reinforcing steel of classes At800, At1000, At1200, the conditional elastic limit must be at least 80% of the conditional yield strength.

Reinforcing wire It is made by cold drawing with a diameter of 3–8 mm or from low-carbon steel (St3kp or St5ps) - class V-1, Vr-1 (Vr400, Vr600); wire of class Vrp-1 with a crescent profile is also produced, or from carbon steel grades 65... 85 class V-P, Vr-P (V1200, Vr 1200, V1300, Vr 1300, V1400, Vr 1400, V1500, Vr 1500). The numerical indices of the class of reinforcing wire with the last designation correspond to the guaranteed value of the conditional yield strength of the wire in MPa with a confidence probability of 0.95.

Example symbol wire: 5Вр1400 – wire diameter is 5 mm, its surface is corrugated, the nominal yield strength is not less than 1400 MPa.

Currently, the domestic hardware industry has mastered the production of stabilized smooth high-strength wire with a diameter of 5 mm with increased relaxation ability and low-carbon wire with a diameter of 4...6 mm of class BP600. high-strength wire is manufactured with a standardized straightness value and cannot be straightened. A wire is considered straight if loose laying a segment with a length of at least 1.3 m on the plane forms a segment with a base of 1 m and a height of no more than 9 cm.

Table 3. Regulatory Requirements to the mechanical properties of high-strength wire and reinforcing ropes

Type of reinforcement and its diameter	Standards of mechanical properties according to GOST 7348 and GOST 13840
,MPa	Error! The object cannot be created from edit field codes., MPa	E.10 -5 MPa	, %	%
No less	No more
B-II 3i 5 1 mm			2,00	4,0	8/2,5 1
B-II 4,5,6 mm			2,00	4,0	-
B-II 7 mm			2,00	5,0	-
B-II 8 mm			2,00	6,0	-
K7 6,9,12 mm			1,80	4,0	8,0
K7 15 mm			1,80	4,0	-

Notes: 1 – 5 1 and 2.5 1 refers to stabilized wire with a diameter of 5 mm,

2 – – the value of stress relaxation is given after 1000 hours of exposure at a voltage = 0.7% of the initial stress.

Reinforcing ropes made from high-strength cold-drawn wire. For best use strength properties of the wire in the rope, the laying pitch is taken to be maximum, ensuring the non-unwinding of the rope - usually within 10–16 rope diameters. K7 ropes are made (from 7 wires of the same diameter: 3,4,5 or 6 mm) and K19 (10 wires with a diameter of 6 mm and 9 wires with a diameter of 3 mm), in addition, several ropes can be twisted: K2×7 – sets of 2 seven-wire ropes, K3×7, K3×19.

Regulatory requirements for the mechanical properties of high-strength wire and reinforcing ropes are given in table.

Hot-rolled rods of classes A-III, At-III, At-IVC and BP-I wire are used as non-stressed working reinforcement. Possible use fittings A-I I, if the strength properties of reinforcement of higher classes are not fully used due to excessive deformations or opening of cracks.

For mounting hinges of prefabricated elements, hot-rolled steel of class Ac-II grade 10GT and A-I brands VSt3sp2, VSt3ps2. If the installation of reinforced concrete structures occurs at temperatures below minus 40 0 ​​C, then the use of semi-quiet steel is not allowed due to its increased cold brittleness. Rolled carbon steel is used for embedded parts and connecting linings.

For prestressed reinforcement of structures up to 12 m in length, it is recommended to use bar steel of classes A-IV, A-V, A-VI, strengthened by drawing A-IIIb, and thermomechanically strengthened classes At-IIIC, At-IVC, At-IVK, At-V , At-VI, At-VII. For elements and reinforced concrete structures longer than 12 m, it is advisable to use high-strength wire and reinforcing ropes. For long structures, it is allowed to use welded rod reinforcement, joined by welding, classes A-V and A-VI. Non-weldable reinforcement (A-IV grade 80C, as well as classes At-IVK, At-V, At-VI, At-VII) can only be used in measured lengths without welded joints. The rod reinforcement with a screw profile is joined by screwing on threaded couplings, with the help of which temporary and permanent anchors are also installed.

Into iron concrete structures, intended for operation at low negative temperatures, the use of reinforcing steels subject to cold brittleness is not allowed: at operating temperatures below minus 30 0 C, class A-II steel grade VSt5ps2 and class A-IV grade 80 C cannot be used, and at temperatures below minus 40 0 ​​C The use of steel A-III grade 35GS is additionally prohibited.

For the manufacture of welded mesh and frames, cold-drawn wire of class BP-I with a diameter of 3-5 mm and hot-rolled steel of classes A-I, A-II, A-III, A-IV with a diameter of 6 to 40 mm are used.

The reinforcing steel used must meet the following requirements:

– have guaranteed mechanical properties under both short-term and long-term loads, maintain strength properties and ductility when exposed to dynamic, vibration, alternating loads,

– ensure constant geometric dimensions of the section, profile along the length,

- welds well with everyone types of welding,

– have good adhesion to concrete – have a clean surface; during transportation, warehousing, and storage, measures must be taken to prevent the steel from becoming dirty and wet. If necessary, the surface of steel reinforcement must be cleaned by mechanical means,

– high-strength steel wire and ropes must be supplied in coils of large diameter so that the unwinding reinforcement is straight, mechanical straightening this steel is not allowed,

– reinforcing steel must be corrosion-resistant and must be well protected from external aggressive influences by a layer of dense concrete of the required thickness. The corrosion resistance of steel increases with a decrease in its carbon content and the introduction of alloying additives. Thermo-mechanically hardened steel is prone to corrosion cracking, so it cannot be used in structures operated in aggressive conditions.

Preparing non-prestressing reinforcement .

The quality of reinforcement in monolithic reinforced concrete structures and its location are determined by the required strength and deformation properties. Reinforced concrete structures are reinforced with individual straight or bent rods, meshes, flat or spatial frames, as well as by introducing dispersed fiber into the concrete mixture. The reinforcement must be located exactly in the design position in the concrete mass or outside the concrete contour with subsequent coating cement-sand mortar. Connections of steel reinforcement are mainly made using electric welding or twisting with knitting wire.

Compound reinforcement works includes manufacturing, enlarged assembly, installation into formwork and fixation of reinforcement. The main volume of reinforcement is manufactured centrally at specialized enterprises; it is advisable to organize the production of reinforcement in the conditions of a construction site at mobile reinforcement stations. The production of reinforcement includes the following operations: transportation, receipt and storage of reinforcing steel, straightening, cleaning and cutting of reinforcement supplied in coils (except for high-strength wire and ropes, which are not straightened), joining, cutting and bending of rods, welding of meshes and frames, if necessary – bending meshes and frames, assembling spatial frames and transporting them to the formwork.

Butt joints are made by crimping couplings in a cold state (and high-strength steels - at a temperature of 900...1200 0 C) or welding: contact butt welding, semi-automatic arc under a layer of flux, arc electrode or multi-electrode welding in inventory forms. When the diameter of the rods is more than 25 mm, they are fastened by arc welding.

Spatial frames are made on conductors for vertical assembly and welding. The formation of spatial frames from bent meshes requires less labor, metal and electricity, and ensures high reliability and manufacturing accuracy.

The reinforcement is installed after checking the formwork; the installation is carried out by specialized units. To install a protective layer of concrete, gaskets made of concrete, plastic, and metal are installed.

When reinforcing prefabricated monolithic reinforced concrete structures, for reliable connection, the reinforcement of the prefabricated and monolithic parts is connected through outlets.

The use of dispersed reinforcement in the production of fiber-reinforced concrete makes it possible to increase strength, crack resistance, impact strength, frost resistance, wear resistance, and water resistance.

The adhesion of concrete to formwork reaches several kgf/cm2. This complicates stripping work, deteriorates the quality of concrete surfaces and leads to premature wear of formwork panels.

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

Adhesion (sticking) is understood as a bond caused by molecular forces between the surfaces of two dissimilar or liquid bodies in contact. During the period of contact between concrete and formwork, favorable conditions are created for adhesion to occur. Adhesive), which in this case is concrete, is in a plastic state during the laying period. In addition, in the process of vibration compaction of concrete, its plasticity increases even more, as a result of which the concrete moves closer to the surface of the formwork and the continuity of contact between them increases.

Concrete sticks to wood and steel formwork surfaces more strongly than to plastic ones due to the latter's poor wettability.

Wood, plywood, untreated steel and fiberglass are well wetted and the adhesion of concrete to them is quite large; concrete has little adhesion to weakly wettable (hydrophobic) getinax and textolite.

The contact angle of ground steel is greater than that of untreated steel. However, the adhesion of concrete to polished steel is reduced slightly. This is explained by the fact that at the interface between concrete and well-treated surfaces the contact continuity is higher.

When an oil film is applied to the surface, it becomes hydrophobized, which sharply reduces adhesion.

Shrinkage negatively affects adhesion and, consequently, adhesion. The greater the shrinkage in the butt layers of concrete, the more likely it is that shrinkage cracks will appear in the contact zone, weakening adhesion. Cohesion in the formwork-concrete contact pair should be understood as the tensile strength of the butt layers of concrete.

The roughness of the formwork surface increases its adhesion to concrete. This happens because a 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, during vibration compaction it forms points of reliable connection.

When removing formwork, there can be three tearing options. In the first option, adhesion is very small, and cohesion is quite large

In this case, the formwork is torn off exactly along the contact plane. The second option is adhesion more than cohesion. In this case, the formwork is torn off along the adhesive material (concrete).

The third option is that adhesion and cohesion are approximately the same in magnitude. The formwork comes off partially along the plane of contact between the concrete and the formwork, and partly along the concrete itself (mixed or combined tearing).

With adhesive tearing, the formwork is easily removed, its surface remains clean, and the concrete surface is of good quality. As a result, it is necessary to strive to ensure adhesive separation. To do this, the forming surfaces of the formwork are made of smooth, poorly wetted materials or lubricants and special anti-adhesive coatings are applied to them.

Formwork lubricants Depending on their composition, principle of action and operational properties, they can be divided into four groups: aqueous suspensions; hydrophobic lubricants; lubricants - concrete set retarders; combined lubricants.

Aqueous suspensions of powdered substances, inert to concrete, are a simple and cheap, but not always effective, means for eliminating the adhesion of concrete to 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 layer is formed on the forming surface of the formwork. protective film, preventing concrete from sticking.

Most often, a lime-gypsum-coBVio suspension is used to lubricate formwork, which is prepared from semi-aqueous gypsum (0.6-0.9 parts by weight), lime paste (0.4-0.6 parts by weight), sulfite- alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).

Suspension lubricants are erased by the concrete mixture during vibration compaction and contaminate concrete surfaces, as a result of which they are rarely used.

The most common hydrophobic lubricants are based on mineral oils, EX emulsol or fatty acid salts (soaps). After their application to the surface of the formwork, a hydrophobic film is formed from a number of oriented molecules (Fig. 1-1, b), 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.

The third group of lubricants uses the properties of concrete to set slowly in thin butt layers. To slow down setting, molasses, tannin, etc. are added to lubricants. The disadvantage of such lubricants is the difficulty of regulating the thickness of the concrete layer in which setting slows down.

Most effective combined lubricants, which use the properties of forming surfaces in combination with retarding the setting of concrete in thin butt layers. Such lubricants are prepared in the form of so-called reverse emulsions. In some of them, in addition to hydrophobizers and retarders, plasticizing additives are introduced: sulfite-yeast stillage (SYD), soap naft or TsNIPS additive. During vibration compaction, these substances plasticize the 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 mixing. To do this, pour the components into the mixer tank and turn on the mixer.

The ultrasonic mixing unit consists of circulation pump, suction and pressure pipelines, distribution box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The liquid supplied by the pump under overpressure 3.5-5 kgf/cm2, flows out of the vibrator nozzle at high speed and hits the wedge-shaped plate. In this case, the plate begins to vibrate at a frequency of 25-30 kHz. As a result, zones of intense ultrasonic mixing are formed in the liquid with the simultaneous division of components into tiny droplets. Mixing duration is 3-5 minutes.

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

These lubricants can be applied to the formwork using brushes, rollers and spray rods. At large quantities shields, a special device should be used to lubricate them.

The use of effective lubricants reduces harmful effects on the formwork of some factors.

For metal panels, SE-3 enamel is recommended as an anti-adhesive coating, which contains epoxy resin(4-7 parts by weight), methylpolysiloxane oil (1-2 parts by weight), lead litharge (2-4 parts by weight) and polyethylene polyamine (0.4-0.7 parts by weight). A creamy paste of these components is applied to a thoroughly cleaned and degreased metal surface with a brush or spatula. The coating hardens at 80-140 ° C for 2.5-3.5 hours. The turnover of such a coating reaches 50 cycles without repair.

For board and plywood formwork TsNIIOMTP has developed a coating based on phenol-formaldehyde. It is pressed onto the surface of the boards at a pressure of up to 3 kgf/cm2 and a temperature of +80° C. This coating completely eliminates the adhesion of concrete to the formwork and can withstand up to 35 cycles without repair.

Despite the 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 panels whose decks are made of getinax, smooth fiberglass or textolite, and the frame is made of metal corners. This formwork is wear-resistant, easy to remove and provides good quality concrete surfaces.

K category: Concrete works

Measures to reduce the adhesion of concrete to formwork

The adhesion force of concrete to formwork is influenced by adhesion (sticking) and shrinkage of concrete, roughness and porosity of the surface. With a high adhesion force between concrete and formwork, the work of stripping becomes more complicated, the labor intensity of the work increases, the quality of concrete surfaces deteriorates, and the formwork panels wear out prematurely.

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

Therefore, to obtain surfaces High Quality You should use cladding made of textolite, getinax, polypropylene, or use waterproof plywood treated with special compounds. When adhesion is low, the concrete surface is not disturbed and the formwork comes off easily. As adhesion increases, the concrete layer adjacent to the formwork is destroyed. This does not affect the strength characteristics of the structure, but the quality of the surfaces is significantly reduced. Adhesion can be reduced by applying aqueous suspensions, water-repellent lubricants, combined lubricants, and concrete retarding lubricants to the surface of the formwork. The principle of operation of aqueous suspensions and water-repellent lubricants is based on the fact that a protective film is formed on the surface of the formwork, which reduces the adhesion of concrete to the formwork.

Combined lubricants are a mixture of concrete set retarders and water-repellent emulsions. When making lubricants, sulfite-yeast stillage (SYD) and soap naft are added to them. Such lubricants plasticize the concrete of the adjacent area, and it does not collapse.

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

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

- Measures to reduce the adhesion of concrete to formwork

When working with monolithic reinforced concrete structures, you have to deal with adhesion to the formwork, the value of which can reach several kgf/cm2. Cohesion not only makes it difficult to strip the reinforced concrete structure, but also leads to deterioration in quality concrete surface, as well as to premature wear of the formwork panels.

The adhesion of concrete to formwork is determined by the influence of the following factors:

• adhesion and cohesion of concrete;
• shrinkage of concrete;
• roughness and porosity of the surface of the formwork adjacent to the reinforced concrete structure.

During the laying period, concrete is in a plastic state and is an adhesive (adhesive), due to which adhesion occurs (sticking of concrete to the formwork). During the compaction process, the plasticity of concrete increases, it moves closer to the surface of the formwork and the continuity of contact between the concrete and the formwork panels increases.

The material from which the formwork surface is made also affects adhesion: concrete sticks to wooden and steel surfaces more strongly than to plastic ones, since the latter have less wettability.

Without special treatment, plywood, wood, steel, and fiberglass are well wetted, which creates a fairly strong adhesion to concrete. And getinax and textolite are weakly wettable (hydrophobic), so concrete adheres to them slightly.

When processing the forming surface and applying an oil film to it, the wettability is significantly reduced (hydrophobic), which significantly reduces adhesion.

Shrinkage reduces adhesion and cohesion: why more shrinkage in butt layers of concrete, the more likely it is that shrinkage cracks will appear in the contact zone, which weakens adhesion.

Cohesion in the contact pair “formwork and concrete” is the tensile strength of butt layers of concrete.

There are three possible options separation removable formwork when stripping a monolithic concrete structure:

1. option 1: adhesion is low and cohesion is high. In this case, it comes off exactly along the contact plane;
2. option 2: adhesion is greater than cohesion. The formwork will be torn off along the adhesive material (concrete);
3. option 3: adhesion is approximately equal to cohesion. In this case, a (combined) tearing is observed, in which the formwork comes off partly along the plane of contact between the concrete and the formwork, and partly along the concrete itself.

In the first (adhesive) tearing option, the formwork is easily removed, its surface remains clean, and the concrete surface has a good quality. Therefore, it is important to ensure adhesive separation. This is achieved by the following methods:

• formwork forming surfaces are made of smooth, poorly wetted materials

• formwork lubricants, emulsions and special anti-adhesive coatings are applied to the forming surfaces.

Requirements for formwork lubricants:

• should not leave oily stains on the concrete. The exceptions here are structures that are subsequently covered with earth/covered or waterproofed;
• do not reduce the strength of the contact layer of concrete;
• Fire safety;
• absence of volatile substances harmful to health;
• must be kept on inclined and vertical surfaces at a temperature of 30 o C for at least 24 hours.

Types of lubricants

Concrete surface using different formwork lubricants

Depending on the composition, principle of action and operational properties, formwork lubricants can be divided into four groups:

1. aqueous suspensions;
2. hydrophobic lubricants;
3. lubricants - concrete set retarders;
4. combined lubricants.

Aqueous suspensions

obtained from powdered substances inert to concrete. These are simple and cheap, but not always effective means, eliminating the adhesion of concrete to the formwork. Their principle of operation is based on the fact that the suspension evaporates, and a thin protective film is formed on the forming surface of the formwork, which prevents concrete from sticking to the deck.

The most commonly used aqueous slurry is lime-gypsum slurry. To prepare it, mix semi-aqueous gypsum (0.6-0.9 parts by weight), lime paste (0.4-0.6 parts by weight), sulfite-alcohol stillage (0.8-1.2 parts by weight). hours) and water (4-6 parts by weight).

During vibration compaction, suspension lubricants are washed away by the concrete and contaminate the concrete surface. Therefore in monolithic construction they are used quite rarely.

Hydrophobizing lubricants

are made on the basis of mineral oils, EX emulsol or salts of fatty acids (in other words, based on soaps). When processing a deck, a water-repellent lubricant creates a thin water-repellent (hydrophobic) film of a layer of oriented molecules on its forming surface. Water-repellent lubricants are common in monolithic construction, but they have a number of disadvantages: high cost, contamination of the concrete surface, fire hazard.

Concrete retarders

The third group of lubricants. To slow down the setting of concrete, tannin, molasses, etc. are introduced into the composition of such lubricants. Their disadvantage is that it is difficult to regulate the thickness of the concrete layer in which the setting slows down.

Combined lubricants - inverse emulsions

The most effective means for improving the quality of the resulting concrete surface monolithic design and increasing the service life (turnover) of removable construction formwork. Such lubricants are prepared in the form of inverse emulsions. In addition to water repellents and set retarders, some of them also contain plasticizing agents, for example, soap naft, sulfite-yeast stillage (SYD), etc. During vibration compaction, plasticizers plasticize the concrete in the butt layers, thereby significantly reducing its surface porosity.

Emulsion lubricants are stable. They do not delaminate within 7-10 days. When using them, the adhesion of concrete to the formwork is completely eliminated. They also adhere well to the deck surface and do not contaminate the concrete.

Composition of formwork lubricants

To lubricate formwork, emulsions (such as water-soap-kerosene; water-oil) and suspensions (such as clay-oil; water-chalk; cement-oil-water) are usually used. The compositions are prepared in repair shops or obtained in finished form from reinforced concrete factories, house-building factories, etc.

For panel formwork used in the construction of underground reinforced concrete structures, bitumen-kerosene lubricants are universal. They are obtained by dissolving low-grade bitumen in kerosene. These lubricants are suitable for both metal, plank and plastic decks. It is also recommended to use petrolatum-solar, petrolatum-kerosene, and paraffin-solar lubricants for plank decks.

Components	Composition, weight. h.		Cooking equipment
Laundry soap		Horizontal surfaces of wood, combined and steel formwork (including thermosetting). Vertical surfaces of wooden and wood-metal formwork.	Vibrating dispersant
Laundry soap
Laundry soap Solar oil		Steel formwork
		Wooden, combined and steel formwork(including thermoactive)	Saturator
		Wood and steel formwork	Mixer with heater
Oil BM- I, BM-II		Formwork forms for pouring underground structures building
Laundry soap			Vibrating dispersant
Soda Ash EX emulsion		Horizontal surfaces of steel formwork forms	Saturator

The procedure for applying lubricant to the formwork:

Formwork lubricant consumption

Consumption depends on the method of application to the deck surface, the outside temperature, the consistency of the lubricant, and the time interval between installing the formwork and laying the concrete.

Approximate consumption:

Material from which the shield deck is made	Application on a horizontally inclined surface		Application on a vertical surface
	with a pistol		with a pistol
Summer time
Plastic, steel