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10. DEFECTS OF MONOLITHIC REINFORCED CONCRETE STRUCTURES CAUSED BY VIOLATION OF THEIR CONSTRUCTION TECHNOLOGY

The main violations of the work production technology, leading to the formation of defects in monolithic reinforced concrete structures, include the following:
- production of insufficiently rigid, highly deformable when laying concrete and insufficiently dense formwork;
- violation of the design dimensions of structures;
- poor compaction of the concrete mixture when it is placed in the formwork;
- laying of exfoliated concrete mix;
- the use of too rigid concrete mix with thick reinforcement;
- poor care of concrete during its hardening;
- the use of concrete with a strength below the design;
- non-compliance with the design of structural reinforcement;
- poor-quality welding of reinforcement joints;
- use of heavily corroded reinforcement;
- early demoulding of the structure;
- violation of the required sequence of stripping of vaulted structures.

The production of insufficiently rigid formwork, when it receives significant deformations during the laying of the concrete mix, leads to large changes in the shape of reinforced concrete elements. In this case, the elements take on the appearance of strongly sagging structures, the vertical surfaces acquire bulges. Formwork deformation can lead to displacement and deformation of reinforcing cages and meshes and a change in the bearing capacity of the elements. It should be borne in mind that the own weight of the structure increases in this case.
Loose formwork contributes to the outflow of cement mortar and, as a result, the appearance of shells and cavities in concrete. Shells and cavities also occur due to insufficient compaction of the concrete mixture when it is placed in the formwork. The appearance of shells and cavities causes a more or less significant decrease in the bearing capacity of the elements, an increase in the permeability of structures, promotes corrosion of the reinforcement located in the zone of shells and cavities, and can also cause reinforcement to be pulled through in concrete.
A decrease in the design dimensions of the section of elements leads to a decrease in their bearing capacity, an increase - to an increase in the own weight of structures.
The use of exfoliated concrete mixture does not allow obtaining uniform strength and density of concrete throughout the entire volume of the structure and reduces the strength of concrete.
The use of a too rigid concrete mix with thick reinforcement leads to the formation of shells and cavities around the reinforcing bars, which reduces the adhesion of the reinforcement to concrete and causes the risk of corrosion of the reinforcement.
During the care of concrete, it is necessary to create such temperature and humidity conditions that would ensure the preservation of water in the concrete, which is necessary for the hydration of the cement. If the curing process takes place at a relatively constant temperature and humidity, the stresses arising in the concrete due to volume changes and caused by shrinkage and thermal deformations will be insignificant. Concrete is usually covered with plastic sheeting or other protective coating. It is also possible to use film-forming materials. Concrete care is usually carried out within three weeks, and when concrete heating is used, after its completion.
Poor maintenance of concrete leads to overdrying of the surface of reinforced concrete elements or their entire thickness. Overdried concrete has significantly less strength and frost resistance than normally hardened concrete, and many shrinkage cracks appear in it.
When concreting in winter conditions, with insufficient insulation or heat treatment, early freezing of concrete may occur. After thawing such concrete, it will not be able to gain the necessary strength. The final compressive strength of concrete subjected to early freezing can reach 2-3 MPa or less.
The minimum (critical) strength of concrete, which provides the necessary resistance to ice pressure and subsequently maintains the ability to harden at positive temperatures without significant deterioration in the properties of concrete, is given in Table. 10.1.

Table 10.1. The minimum (critical) strength of concrete that concrete must acquire by the time of freezing (available only when downloading full version books in Word doc format)

If all the ice and snow was not removed from the formwork before concreting, then shells and cavities appear in the concrete. An example is the construction of a boiler house in permafrost conditions.
The base of the boiler house was a monolithic reinforced concrete slab, into which the heads of piles sunk into the ground were embedded. A ventilated space was provided between the slab and the ground to isolate the ground from heat penetrating through the floor of the boiler room. Reinforcing bars were made from the top of the piles, around which ice formed, which was not removed before concreting. This ice melted in the summer and the base plate of the building turned out to be supported only by the reinforcement of the piles (Fig. 10.1). Reinforcing outlets from piles were deformed under the action of the weight of the entire building and the base slab received large uneven settlements.

Rice. 10.1. Scheme of states of the monolithic slab of the base of the boiler house (a - during concreting; b - after the ice remaining in the formwork has melted): 1 - monolithic slab; 2 - ice left in the formwork; 3 - pile reinforcement; 4 - pile (available only when downloading the full version of the book in Word doc format)

Non-compliance with the design of the strength of concrete and reinforcement of structures, as well as poor-quality welding of reinforcement outlets and crossing of rods affects the strength, crack resistance, and rigidity of monolithic structures as well as similar defects in precast concrete elements.
Slight corrosion of reinforcement does not affect the adhesion of reinforcement to concrete, and, consequently, the operation of the entire structure. If the reinforcement is corroded in such a way that the corrosion layer exfoliates from the reinforcement upon impact, then the adhesion of such reinforcement to concrete deteriorates. At the same time, along with a decrease in the bearing capacity of the elements due to a decrease in the cross section of the reinforcement due to corrosion, an increase in the deformability of the elements and a decrease in crack resistance are observed.
Early stripping of structures can lead to the complete unsuitability of the structure and even its collapse during stripping due to the fact that the concrete has not gained sufficient strength. The demoulding time is mainly determined by the temperature conditions and the formwork type. For example, the formwork of the side surfaces of walls, beams can be removed much earlier than the formwork of the lower surfaces of bending elements and the side surfaces of columns. The last formwork can be removed only when the strength of the structures is ensured from the effects of their own weight and the temporary load acting during the period construction works. According to N. N. Luknitsky, the removal of the formwork of slabs with a span of up to 2.5 m can be carried out not earlier than the concrete reaches 50% of the design strength, slabs with a span of more than 2.5 m and beams - 70%, long-span structures - 100%.
When stripping the vaulted structures, the circles at the castle must first be released, and then at the heels of the structure. First, free the manger at the heels, then the vault will rest on the circle in its castle part, and the vault is not designed for such work.
At present, monolithic reinforced concrete structures are widely used, especially in multi-storey housing construction.
Construction organizations, as a rule, do not have the appropriate formwork and rent it. Formwork rental is expensive, so builders minimize the turnaround time. Usually stripping is done two days after concrete is laid. At such a pace of erection of monolithic structures, a particularly thorough study of all stages of work is required: transporting the concrete mixture, laying concrete in the formwork, retaining moisture in concrete, heating concrete, insulating concrete, monitoring heating temperature and curing of concrete.
To reduce the negative impact of the concrete temperature difference, it is necessary to choose the minimum allowable temperature of concrete heating during stripping.
For vertical structures (walls), the concrete heating temperature can be recommended at 20°C, and for horizontal structures (floors) - 30°C. In the conditions of St. Petersburg, within two days, the average air temperature is 20 ° C and, moreover, 30 ° C does not happen. Therefore, concrete should be heated at any time of the year. Even in April and October, the author did not manage to see the heating of concrete at construction sites.
In winter, concrete floors should be insulated when heated by laying a layer of effective insulation over a polyethylene film. And in many cases this is not done. Therefore, floor slabs, concreted in winter, have a concrete strength from above 3-4 times less than from below.
When stripping in the middle of the floor slab section, a temporary support is left in the form of a rack or formwork section. Also, temporary supports should be installed before stripping strictly vertically on the floors, which is also often not observed.
Since the concrete strength of the walls during stripping does not reach the design value, it is necessary to make their intermediate calculation to determine the number of floors that can be built in winter.
There is a large shortage of instructive literature on monolithic reinforced concrete, which affects its quality.

The adhesion of formwork to concrete is affected by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the forming surface of the formwork. The adhesion value can reach several kg/cm 2 , which makes it difficult to remove the formwork, degrades the quality of the surface of the concrete product and leads to premature wear of the formwork panels.

Concrete adheres to wooden and steel formwork surfaces more strongly than to plastic ones due to the poor wettability of the latter.

Types of lubricants:

1) aqueous suspensions of powdered substances 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 is used from: CaSO 4 × 0.5H 2 O 0.6 ... 0.9 wt. h., lime dough 0.4 ... 0.6 wt.h., LST 0.8 ... 1.2 wt.h., water 4 ... 6 wt.h. These lubricants are washed away by the concrete mix, contaminate concrete surfaces, and therefore are rarely used;

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

3) lubricants - concrete setting retarders 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 the slowdown in the setting of concrete in the 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, are well retained on vertical surfaces and do not contaminate concrete.

Formwork installation .

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

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

1) racks must be installed on bases with a bearing area sufficient to protect the concreted structure from unacceptable subsidence;

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

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

4) the base for the formwork must be verified before it is installed.

Formwork and circles of reinforced concrete arches and vaults, as well as formwork of reinforced concrete beams with a span of more than 4 m, should be installed with a construction lift. The value of the building 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, a sliding clamp is put on the upper end of the rack. Runs are installed along the racks on the fork supports fixed at the upper end of the rack, on which the formwork panels are installed. Sliding crossbars also rest on the runs. They can also be supported directly on the walls, but in this case support nests must be made in the walls.

Before installing the collapsible formwork, beacons are set up, on which risks 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 brand so as to provide free access to any element.

It is necessary to lift shields, fights, racks and other elements, as well as to supply them to the workplace on the scaffold in packages using lifting mechanisms, and the fastening elements must be supplied and stored in special containers.

The formwork is assembled by a specialized unit, accepted by the master.

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

Formwork for structures with a height of more than 4 m is assembled in several tiers in height. The panels of the upper tiers rest on the lower tiers or are installed on support brackets installed in concrete after dismantling the formwork of the lower tiers.

When assembling the formwork of a curvilinear shape, special tubular contractions are used. After assembling the formwork, it is straightened by wedge tamping sequentially in diametrically opposite directions.

test questions

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

13. Reinforcement of 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 of 525 to 1900 MPa. Over the past 20 years, the volume of world production of rebar has increased by about 3 times and reached more than 90 million tons per year, which is about 10% of all steel products produced.

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

The average consumption of reinforcing steel per 1 m 3 of reinforced concrete in different countries of the 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 due to the transition to steel A500C 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 the assembly of individual elements, as well as for mounting and other fixtures. Steel consumption in the manufacture of reinforced concrete structures is about 40% of the total volume of metal used in construction. The share of bar reinforcement is 79.7% of the total volume, including: ordinary reinforcement - 24.7%, increased strength - 47.8%, high-strength - 7.2%; the share of wire reinforcement is 15.9%, including ordinary wire 10.1%, increased strength - 1.5%, hot-rolled - 1%, high-strength - 3.3%, the share of rolled products for embedded parts is 4.4%.

Reinforcement installed according to the calculation for the perception of stresses in the process of manufacturing, transportation, installation and operation of the structure is called working, and installed for structural and technological reasons - mounting. Working and mounting reinforcement is most often combined into reinforcing 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 work of the reinforced concrete structure under load.

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

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

The reinforcement of reinforced concrete structures is classified according to 4 criteria:

- According to the manufacturing technology, hot-rolled bar steel, supplied in bars or coils, depending on the diameter, and cold-drawn (made by drawing) wire steel are distinguished.

– According to the method of hardening, bar reinforcement can be hardened thermally and thermomechanically or in a cold state.

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

– According to the method of application, reinforcement is distinguished without prestressing and with prestressing.

Varieties of reinforcing steel. For reinforcing reinforced concrete structures, the following is used: bar steel that meets the requirements of the standards: hot-rolled bar - GOST 5781, the classes of this reinforcement are denoted by the letter A; rod thermomechanically strengthened - GOST 10884, classes are designated At; wire from 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 structures, the presence of prestressing, the conditions of manufacture, installation and operation. All types of domestic non-stressed reinforcement are well welded, but are produced especially for prestressed concrete structures and limited welded or non-welded types of reinforcement.

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

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

Bar fittings are produced with a diameter of 6 to 80 mm, fittings 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 bars or coils, the rest of the reinforcement is supplied only in bars with a length of 6 to 12 m, measured or random 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 of a periodic profile: class A-II reinforcement has two longitudinal ribs and transverse protrusions running along a three-start helix. With a reinforcement diameter of 6 mm, protrusions are allowed along a single-start helical line, and with a diameter of 8 mm - along a two-start helix. Reinforcement of class A-III and above also has two longitudinal ribs and transverse protrusions in the form of a herringbone. On the surface of the profile, including the surface of the ribs and protrusions, there should be no cracks, shells, rolling captivity and sunsets. In order to distinguish steels of class A-III and above, the end surfaces of the bars are painted in different colors or the steel is marked with convex marks applied during rolling.

At present, steel with a special screw profile is also being produced - europrofile (without longitudinal ribs, and transverse ribs in the form of a helix solid or intermittent), which makes it possible to screw on the rods of screw connecting elements - couplings, nuts. With their help, the reinforcement can be joined without the help of welding in any place and form temporary or permanent anchors.

Rice. 46. ​​Rod hot-rolled reinforcement of a periodic profile:

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

For the manufacture of reinforcement, carbon (mainly St3kp, St3ps, St3sp, St5ps, St5sp), low and medium alloy steels (10GT, 18G2S, 25G2S, 32G2Rps, 35GS, 80S, 20KhG2Ts, 23Kh2G2T, 22Kh2G2AYu, 22Kh2G2R, 20Kh2G2SR), by changing the carbon content and alloying elements are governed by the properties of steel. The weldability of reinforcing steels of all grades (except 80C) is ensured by the chemical composition and technology. Carbon equivalent value:

Seq = C + Mn/6 + Si /10

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

Rod thermomechanically hardened reinforcement is also divided into classes according to mechanical properties and performance 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). 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 sickle-shaped ribs, smooth reinforcement can be produced on request.

Reinforcing steel with a diameter of 10 mm or more is supplied in the form of bars of fixed length, welded steel can be supplied in bars of random length. Steel with a diameter of 6 and 8 mm is supplied in coils, delivery in coils of steel At400C, At500C, At600C with a diameter of 10 mm is allowed.

For welded reinforcing steel At400C carbon equivalent:

Seq = C + Mn/8 + Si /7

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

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

Rice. 47. Rod steel thermomechanically hardened with a periodic profile

a) - crescent profile with longitudinal ribs, b) - crescent 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 interval for smooth steel should be 245 MPa, for steel of a periodic profile - 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), also Vrp-1 class wire with a sickle-shaped profile is 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 indexes of the reinforcing wire class with the last designation correspond to the guaranteed value of the conditional yield strength of the wire in MPa with a confidence level of 0.95.

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

At present, 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 the Vr600 class. high-strength wire is manufactured with a normalized value of straightness and is not subject to straightening. The wire is considered straight if, with free laying of a segment with a length of at least 1.3 m, a segment with a base of 1 m and a height of not more than 9 cm is formed on the plane.

Tab. 3. Regulatory requirements for the mechanical properties of high-strength wire and reinforcing ropes

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 voltage = 0.7 in% of the initial stress.

Reinforcing ropes Manufactured from high tensile cold drawn wire. For the best use of the strength properties of the wire in the rope, the lay 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 - suites of 2 seven-wire ropes, K3x7, K3x19.

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

Hot-rolled bars of classes A-III, At-III, At-IVC and wire VR-I are used as non-tensioned working reinforcement. It is possible to use reinforcement A-II if the strength properties of reinforcement of higher classes are not fully used due to excessive deformations or crack opening.

For mounting loops of prefabricated elements, hot-rolled steel of class Ac-II grade 10GT and A-I marks VSt3sp2, VSt3ps2. If the installation of reinforced concrete structures takes place at a temperature below minus 40 0 ​​С, 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 plates.

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

In reinforced concrete structures intended for operation at low negative temperatures, the use of reinforcing steels subject to cold brittleness is not allowed: at an operating temperature below minus 30 0 C, steel of class A-II grade VSt5ps2 and class A-IV grade 80C 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 meshes and frames, cold-drawn wire of class VR-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 plasticity when exposed to dynamic, vibration, alternating loads,

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

- well welded by all types of welding,

– have good adhesion to concrete – have a clean surface, during transportation, storage, storage, measures must be taken to prevent steel from contamination and moisture. If necessary, the surface of the steel reinforcement should be cleaned mechanically,

– high-strength steel wire and ropes should be supplied in coils of large diameter, so that the rebar being unwound is straight, mechanical dressing this steel is not allowed,

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

Reinforcement blank .

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 separate straight or bent rods, meshes, flat or spatial frames, as well as the introduction of dispersed fibers into the concrete mixture. The reinforcement must be located exactly in the design position in the mass of concrete or outside the concrete contour with subsequent coating cement-sand mortar. The connections of steel reinforcement are mainly carried out by means of electric welding or twisting with knitting wire.

The scope of reinforcing works includes manufacturing, pre-assembly, installation into the formwork and fixing of the reinforcement. The main volume of fittings is manufactured centrally at specialized enterprises, the production of fittings under conditions construction site it is expedient to organize on mobile reinforcing stations. Manufacture of reinforcement includes operations: transportation, acceptance and storage of reinforcing steel, straightening, cleaning and cutting of reinforcement supplied in coils (except for high-strength wire and ropes that are not straightened), docking, cutting and bending of rods, welding of meshes and frames, if necessary - bending of meshes and frames, assembly of spatial frames and their transportation to the formwork.

Butt joints are carried out by crimping couplings in a cold state (and high-strength steels - at a temperature of 900 ... 1200 0 C) or by welding: contact butt, semi-automatic arc submerged arc welding, arc electrode or multi-electrode welding in inventory forms. With a rod diameter of 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 frameworks from bent meshes requires less labor, metal and electricity, provides high reliability and manufacturing accuracy.

Reinforcement is installed after checking the formwork, installation is carried out by specialized links. For the installation of a protective layer of concrete, gaskets made of concrete, plastic, metal are installed.

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

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.

a. Filling the formwork with concrete mix

For concreting structures in sliding formwork, concrete mixes are used on Portland cement grades of at least 400 with the onset of setting no earlier than 3 hours and the end of setting no later than 6 hours. Based on the cement test data, the speed of concreting and lifting of the sliding formwork should be determined.

The draft of the cone of the applied concrete mixture should be: when compacted with a vibrator 6-8 and manual compaction 8-10 cm, and W / C - no more than 0.5. The grain size of coarse aggregate should be no more than /6 of the smallest cross-sectional size of the concreted structure, and for densely reinforced structures - no more than 20 mm.

The thickness of walls and beams erected in sliding formwork, as a rule, should not be less than 150 mm (the weight of concrete should be greater than the friction forces), and the volume of concrete per 1 linear meter. m of their height should not exceed 60 x3.

Initially, the formwork is filled with concrete mixture in two or three layers to a height equal to half the formwork, for a period of not more than 3;6 hours. The second and third layers are laid only after the previous layer is laid along the entire perimeter of the formwork. Further filling of the formwork resumes only after the start of its lifting and ends no later than 6 hours later.

Until the formwork is filled with concrete mixture to its full height, it is lifted at a speed of 60-70 mm/h.

b. The process of compacting the mixture

After the initial filling of the formwork to its full height, with its further lifting, the concrete mixture is laid continuously in layers up to 200 mm thick in thin walls (up to 200 mm) and no more than 250 mm in other structures. Laying a new layer is carried out only after the completion of laying the previous layer before its setting begins.

During the concreting process, the upper level of the mixture to be laid must be more than 50 mm below the top of the formwork panels.

The concrete mixture is compacted with rod vibrators with a flexible shaft or manually - with screws. The diameter of the vibrator tip should be 35 mm for wall thicknesses up to 200 mm and 50 mm for thicker walls.

During the compaction of the mixture, it is recommended to raise and lower the vibrator by 50-100 mm within the layer being laid, while the tip of the vibrator should not rest against the formwork or reinforcement, and should not reach the previously laid setting layer of concrete.

The rate of laying the concrete mixture and lifting the formwork should exclude the possibility of adhesion of the laid concrete to the formwork and ensure the strength of the concrete coming out of the formwork, sufficient to maintain the shape of the structure and at the same time make it easy to rub the traces of the formwork on its surface with a grater.

c. Breaks in concreting

The intervals between formwork lifts should not exceed 8 minutes when using vibrators and 10 minutes when manually compacting the concrete mixture. The rate of formwork lifting at an outside air temperature of +15, +20 ° C and the use of Portland cement M 500 reaches 150-200 mm per hour.

In the process of concreting walls in a sliding formwork, there may be "failures" of concrete: the formwork carries with it a part of the weak concrete of the wall, as a result, shells are formed, reinforcement is exposed. The main causes of "failures" are as follows: contamination of the formwork; non-observance of the formwork taper; large breaks during concreting.

In cases of a forced break in concreting, measures should be taken to prevent adhesion of the laid concrete to the formwork; the formwork is slowly raised until there is a visible gap between the formwork and the concrete, or it is periodically raised and lowered within one step of the jack (“step in place”). When resuming concreting, it is necessary to clean the formwork, remove the cement film from the concrete surface and rinse them with water.

In the process of concreting, traces of formwork movement and small shells on outer surface concreted buildings and inside silos, bunkers and rooms, immediately after the concrete leaves the formwork, they are rubbed with a 1: 2 cement mortar.

d. Mix supply

Mats or tarpaulins are attached to the lower edges of the formwork to protect fresh concrete from drying out (hypothermia) and in summer it is regularly watered with water using an annular pipeline.

Window and door blocks in buildings and structures are installed in place during the movement of the formwork, for which they are pre-prepared (antiseptic, sheathed with roofing paper) in accordance with the requirements of the project. To reduce the gaps between the formwork walls and the block box to 10 mm, rails are sewn to the box, which are subsequently removed. The reinforcement around the block is installed in accordance with the project.

Laying of concrete near the installed blocks is carried out simultaneously from two sides. After the formwork rises above the installed blocks, the temporary rails are removed.

Tower cranes, mine hoists, self-elevating cranes are used to supply concrete mix, reinforcement, jacking rods and other goods to the formwork.

Concrete pumps and pneumatic blowers are also used to supply the mixture. Upon completion of the erection of the structure, the sliding formwork and all structures and equipment fixed on it are dismantled in a manner in which, after the removal of individual parts, the stability and safety of the remaining elements are ensured.

The channels in the concrete formed by the movement of the protective tubes must be carefully sealed after the removal of the jacking rods.

e. Prefabricated floors

During the construction of structures in winter conditions, concrete is heated in specially constructed greenhouses above the working floor and on external scaffolds using steam or electric heaters or infrared radiation.

Slabs of multi-storey floors, flights of stairs and landings are concreted using additional inventory formwork or assembled from prefabricated elements. In the latter case, during the construction of a building or structure, the need for alterations and additional devices in the sliding formwork is eliminated.

Prefabricated ceilings can be mounted with a tower crane after the walls have been erected in a "well" to the entire height of the building. In this case, the slabs are supported on special inventory, removable brackets fixed on the walls slightly below a number of small openings in the wall. Reinforcing bars are passed through the openings, butted with outlets from floor slabs. Docking of external walls with floor slabs is carried out with the help of grooves in the walls. This technology ensures the continuity of concreting, fast and high-quality construction of walls.

Monolithic ceilings can be concreted after the walls of the building are erected with a “well”. Inventory formwork panels and supporting devices (metal telescopic racks and sliding crossbars) are transferred from floor to floor by a tower crane or manually.

Monolithic slabs can also be concreted using drop formwork mounted on a special platform. This method is especially effective if concrete pumps or pneumatic blowers are used to supply the concrete mixture.

f. Floor concreting

Concreting of floors with a lag of 1-2 floors from the concrete of walls, the process of erecting buildings is complicated by the need for frequent stops when lifting the sliding formwork.

The method of combined cyclic concreting of walls and ceilings is that the concreting of walls in a sliding formwork stops each time at the level of the next ceiling. The empty formwork of the walls is brought out above this mark so that between the bottom of the sliding formwork and the mark of the bottom of the slab there is a gap equal to the thickness of the future slab. At the same time, the formwork panels of the outer walls, as well as the formwork that forms the inner surface of the elevator shafts and other cells that do not have overlaps, are made larger in height than the panels of the rest of the formwork. Concreting of floors is carried out on panel or sectional formwork with the working floor panels removed after stopping and aligning the sliding formwork.

The construction of buildings and structures with a height of 40-50 m in monolithic reinforced concrete using the sliding formwork method, according to the main technical and economic indicators, is at the level of construction from prefabricated reinforced concrete structures, and the construction of high-rise civil buildings has a number of advantages: reduction of construction time; reduction of labor intensity and estimated cost of construction, including by reducing specific capital investments in the base of the construction industry; increasing the reliability, durability and rigidity of structures due to solidity and the absence of joints, which is especially valuable in construction in seismic areas, in mine workings and subsiding soils.

g. Construction of high-rise structures

In recent years, a new method for erecting high-rise structures from monolithic reinforced concrete in a sliding formwork of a rodless system, consisting of hydraulic or pneumatic support-lifting devices, has been developed and implemented in our country, providing reliable support by compressing the erected part of the walls with special grips and creating friction support forces.

On the basis of the proposals of the Donetsk PromstroyNIIproekt, a pilot production sample of a mobile formwork was created, consisting of two (lower and upper) support-lifting sections of walking action with support on the walls of the structure under construction, electromechanical worm-and-screw lifts, sliding formwork forms and frames for fastening. With the help of this formwork, the tower supports of the transport galleries of the blast-furnace ore warehouse were erected at the construction site of the Zaporozhye iron ore plant.

The erected tower supports have an outer diameter of 6 m and a height of 14 m, the thickness of the walls is 300 mm. The construction of one tower was carried out by a team of five people. The average speed of concreting reached 0.3 m/h with the machine speed of raising the formwork in the process of laying and compacting the concrete mix 0.6. m/h At the same time, the lower section of the lifting device rested on concrete of 10-12 hour strength. A step of lifting sections of 2 m made it possible to conduct continuous concreting for 6-6.5 hours.

h. Climbing formwork

Climbing formwork is used in the construction of structures of variable cross-section in height, including chimneys, hyperbolic cooling towers, television towers and other tall objects. The main element of this formwork is a mine hoist with a working platform, to which a set of adjustable external and internal formwork is attached.

The design of the lift allows you to periodically increase it from above or grow it from below. After each cycle of installation of formwork panels, reinforcement and laying of the concrete mixture, the next lifting of the working platform and rearrangement of the formwork is carried out.

The formwork of chimneys up to 320 m high consists of external and internal panels, bearing rings, a framing (support) frame, radial movement mechanisms, a working platform, suspended scaffolding, as well as a post mine lift with a lifting head, assembled from 2.5-meter tubular sections and equipped with a cargo cage and a passenger-and-freight elevator.

The lifting head mounted on a lift with a lifting capacity of 25 and 50 tons, when the formwork is moved to the next tier, rises at a speed of up to 3 mm / s. The working step of formwork lifting is 2.5 m.

i. Pipe shaft concreting

The formwork consists of two shells - outer and inner, which are assembled from panels made of 2 mm thick sheet steel, bolted together.

The outer formwork of the chimneys consists of rectangular and trapezoidal panels 2.5 m high. The combination of these panels will make it possible to obtain a cone-shaped surface of the pipe.

The outer formwork is suspended from the bearing ring, which, when the pipe perimeter is reduced, is replaced with a new one of smaller diameter.

For the convenience of laying concrete, the inner formwork is assembled from panels measuring 1250x550 mm.

Pipe shaft concreting: work organization scheme; development of the outer climbing formwork of the conical chimney; rectangular panels; trapezoidal panels; c - panel of the inner shell of the formwork; covered canopy; protective cover; mine lift; lining platform; clip; work site; distributing bunker; ladle of a cargo cage; lifting head; passenger-and-freight elevator; hoist; cargo cage; Cathead; strip overlay; lugs made of strip steel; steel strips; steel sheet 2 mm thick.

To give rigidity to the panels, overlays are welded to their upper and lower edges, with the help of which the panels are assembled in height. From the outer side of the shields, lugs are welded into which reinforcing bars 10-14 mm are laid, forming a series of elastic horizontal rings.

j. Construction of cooling tower shells

Shields are installed in two (sometimes three) tiers. The formwork of the second tier is installed after the concrete is placed in the formwork of the first tier. After 8-12 hours after concrete is placed in the second tier, the outer formwork is removed and installed in the next highest position. After installing the reinforcement of the third tier, the lower tier of the inner formwork is removed and rearranged higher. Then the cycle repeats. Reinforcement is installed by individual rods manually.

The concrete mixture is fed by a bucket of a cargo stand into a receiving hopper located on the working site, then into the movable hopper of the concrete paver and from there - along the trunk into the formwork. The concrete mixture is compacted with deep vibrators with a flexible shaft.

The speed of concreting the shafts of chimneys at an outdoor temperature of 15-20 ° C reaches 1-1.5 m / day.

The construction of cooling tower shells is carried out using a unit, which is a lattice (increasable) tower, on the rotary head of which rotating booms are mounted, to which climbing formwork shields are attached, as well as working cradles.

The concrete mixture is fed to the upper platform of the cradle in a vibrating bucket by a hoist moving along the boom. Concreting is carried out in tiers by analogy with concreting chimneys.

2. Methods of concreting structures

a. Concreting in sliding formwork

Special methods of concreting structures. Sliding formwork concreting is used in the construction of chimney walls, working towers of elevators and silos, shaft headframes, water towers, as well as frames of multi-storey buildings. Structural elements of buildings and structures erected in sliding formwork must be vertical, which is dictated by the main feature of sliding formwork.

The method of concreting monolithic reinforced concrete buildings and structures in sliding formwork is a highly organized and complex-mechanized, flow-speed construction process. Formwork installation, reinforcement, laying and compaction of the concrete mixture, concrete stripping are carried out in combination and continuously in the process of formwork lifting (SNiP N1-B.1-70).

Sliding formwork includes: formwork panels, jacking frames, a working floor with a canopy along the outer contour of the formwork, suspended platforms, formwork lifting equipment.

Formwork panels are made inventory 1100-1200 mm high from the following materials: steel sheet not less than 1.5 mm thick; planed wooden boards with a thickness of at least 22 mm; waterproof plywood 8 mm thick; baked plywood 7 mm thick or fiberglass 3 mm thick. In some cases, wood-metal shields are made, in which the frame is made of rolled steel profiles, and the sheathing is made of planed boards or plywood. Circles for fixing formwork panels, as a rule, are made of rolled steel profiles.

b. Erection of non-standard structures

Metal formwork panels are used in the construction of a number of structures of the same type (silos, chimneys, tanks), when the side walls perceive the high pressure of the freshly laid concrete mixture and, in addition, multiple turnover of the formwork panels is ensured.

Wooden and wood-metal shields have less rigidity and turnover, but at the same time, lower cost compared to metal ones. They are used in the construction of residential and civil buildings, where the wall thickness does not exceed 200 mm, as well as in dry and hot climates to protect concrete from overheating.

Promising are formwork panels made of waterproof plywood and fiberglass. They are durable and lighter than shields made of other materials, but still more expensive than them.

For the construction of non-standard structures, non-inventory wooden formwork is used. By design, sliding formwork inventory boards are used in two types: large-block and small-block.

In large-block shields, metal circles are rigidly fastened to the skin. These shields are strong, durable and relatively easy to assemble.

In small-block shields, only metal circles are rigidly connected to each other, forming the frame of the walls, and the formwork panels are hung on the circles without fastening to each other.

3. Concreting of bases and floors

a. Concrete preparation

Concrete floors and bases (preparations) are widely used in industrial and civil buildings.

Concrete preparations are arranged mainly in one-story industrial workshops for cement and asphalt floors, floors made of cast-iron slabs, end wooden blocks and other types of floors with a thickness of 100-300 mm on prepared and leveled soil. For concrete bases, rigid concrete mixes of grades 100, 200 and 300 are usually used.

Concrete and cement-sand floor coverings are made up to 40 mm thick from concrete or mortar according to preparation. In multi-storey buildings, reinforced concrete floors usually serve as the foundation.

The scope of work on the installation of single-layer concrete floors in one-story buildings includes: preparation of soil bases; installation of lighthouse boards; reception, leveling of the concrete mixture; surface grouting or ironing.

Prior to the start of the concrete preparation, all underground work on the installation of foundations, channels, tunnels, etc. must be completed, backfilling of the sinuses of the pits, leveling and compaction of the soil must be completed.

Soil preparation. With dense soils, the concrete mixture is laid directly on the planned soil. Bulk and disturbed soils in the foundations must be compacted mechanically. In places inaccessible to compacting mechanisms, the thickness of the soil layer compacted by manual rammers should not exceed 0.1 m.

b. Floor concreting techniques

Soils subject to significant settlement are replaced or strengthened. In the latter case, the concrete preparation is reinforced with mesh.

A layer of crushed stone or gravel 60-150 mm thick is rammed or rolled into the base surface of weak soils before laying concrete preparation on it. Before installing floors on water-saturated clay, loamy and dusty soils, it is necessary to lower the groundwater level and dry the base until the design bearing capacity is restored. On heaving soils, the installation of floors should be carried out in compliance with the instructions of the project.

Planning and compacting soil with an admixture of frozen soil, as well as with snow and ice, is prohibited. It is also not allowed to install concrete floors on frozen soils.

Techniques for concreting floors and foundations. Before concreting, beacon boards are installed along the level so that their upper edge is at the level of the surface of the concrete preparation (Fig. 14, a). The distance between the boards depends on the length of the vibrating rail and is usually 3-4 m. Lighthouse boards are fixed with wooden stakes driven into the ground. Floors and bases are concreted in strips through one, starting from the places most remote from the passage.

c. Concreting preparations

Intermediate strips are concreted after the concrete of adjacent strips has hardened. Before concreting the intermediate lanes, the lighthouse boards are removed. The length of the strips is taken as large as possible. The layer of concrete mixture in preparation before its leveling and compaction should exceed the level of lighthouse boards by 2-3 cm.

The concrete mixture is compacted with a vibrating rail, which is a metal beam (channel, I-beam), on which one or two electric motors from a surface vibrator are mounted.

When concreting preparations and floor coverings, each vibrated section must be covered with a vibrating screed, respectively, by 150 mm and half of its width.

Techniques for concreting floors and bases: scheme for concreting the base under the floors; hand tool for smoothing concrete surfaces; laid base; foundation preparation; stakes; side formwork; scraper with a rubber band to remove laitance; trowel; trowel; ironing board; rubber band.

Depending on the conditions of work, the laying of the concrete mixture by concrete pavers into the bases is carried out in two ways: “away from you”, when the unit moves behind the concreting front, and the concrete in the area of ​​the unit’s action has time to gain the strength necessary for its movement, and “on itself”, when the mechanism moves ahead of the concreting front, since the concrete does not have time to gain the necessary strength.

d. Production of concrete mix

The first method is preferable, since it creates a wide front of work to prepare the foundation. With the second method preparatory work advance the laying of the concrete mix by one plot, the length of which is equal to the radius of the mechanism.

In unheated premises in concrete preparation, every two strips arrange longitudinal and after 9-12 m along the length of the strips, transverse temperature-shrinkage seams, which break the area to be concreted into separate slabs with dimensions of 6X9-9X12 m.

Longitudinal seams are made by installing planed boards coated with hot bitumen, or boards wrapped with roofing paper. After the concrete has set, the boards are removed and the seams are filled with bitumen. Seams are also arranged by coating with bitumen a layer of 1.5-2.0 mm of the side faces of the strips before laying the concrete mixture in adjacent spaces.

For the formation of transverse expansion joints(half-joints) use metal strips 60-180 wide and 5-7 mm thick, which, during the concreting process, are laid in preparation for 73 of their width and then removed after 30-40 minutes. The resulting recesses after the final hardening of the concrete are cleaned and filled with grade III bitumen or cement mortar.

e. Surface of concrete bases

In places where there is a break in the concreting of foundations and floors, it is not allowed to install a vibrating screed at the edge of the laid layer, as this will cause slipping and delamination of the concrete mixture. Therefore, at the end of the work shift, in places of the planned break in concreting, a partition of boards is installed and the last portion of the concrete mixture is leveled and vibrated along it.

The surface of concrete bases before laying on it continuous floor coverings on a cement binder or from piece materials on a cement-sand mortar must be cleaned of debris and cement film.

In the early age of concrete, mechanical steel brushes are used for this purpose. With a high strength of concrete, with the help of a pneumatic tool, grooves with a depth of 5-8 mm are applied to its surface every 30-50 mm. This makes it possible to obtain a rough surface of the underlying layer and to ensure its better adhesion to the upper layer.

Concrete or cement-sand floor coverings consist of a 20-40 mm layer of concrete or mortar and are concreted similarly to preparation in strips 2-3 m wide through one.

Before concreting the coating, beacon wooden slats or metal frame corners are fixed on the surface of the concrete base. The concrete mixture is compacted with vibrating screeds, and the concrete surface is leveled with a wooden slat moved across the strip.

f. cement milk

Cement laitance, which has come to the surface during compaction of concrete bases and floor coverings, is removed using a scraper with a rubber band.

For small volumes of work, the surface of the concrete floor is finally finished with an ironing board or a tarpaulin rubberized tape, the length of which should be 1-1.5 m longer than the width of the concreted strip. The ends of the tape are attached to the rollers that serve as handles, the width of the tape is 300-400 mm. The compacted concrete mixture is smoothed 25-30 minutes after laying. When the tape is moved alternately across and along the strip, the protruding thin film of water is removed from the concrete surface and the concrete floor is pre-smoothed. The final leveling of the surface is carried out after 15-20 minutes with shorter movements of the tape.

To give the concrete floor high abrasion resistance, its surface is treated with a metal trowel approximately 30 minutes after the final leveling, exposing crushed stone grains. If high abrasion resistance is not required, then a cement mortar floor is laid on the concrete preparation.

If it is necessary to immediately install a two-layer floor, first the bottom layer is laid between the lighthouse boards and compacted with a site vibrator or an obliquely installed vibrating rail, then with a break of no more than 1.5-2 hours (for better connection of the lower layer with the upper one), a clean floor is made.

e. Iron surface of concrete

For large volumes of work, the surface of a clean concrete floor in initial period hardening is rubbed with a SO-64 (or OM-700) machine, consisting of a trowel disc with a diameter of 600 mm, an electric motor and a control handle. Rotating at a speed of 140 rpm, the trowel disc levels and smoothes the concrete floor surface. The productivity of the machine is 30 m2/h.

Ironing of the concrete surface is used to give the floor an increased density. It lies in the fact that dry and sifted cement is rubbed into the surface of wet concrete until an even sheen appears on it. Dry concrete surfaces are moistened with water before ironing. Ironing can be done manually using steel trowels or with a CO-64 trowel.

A variety of concrete floors are mosaic, made from a mixture that includes: white or colored Portland cement, marble, granite or basalt chips and mineral dye. A mosaic layer 1.5-2 cm thick is laid, as a rule, on an underlying layer of cement mortar of approximately the same thickness. The limitation of single-color fields and the implementation of the patterns provided for by the project is carried out with the help of strips-veins made of glass, copper or brass, embedded in the underlying layer of the solution. These strips are placed in such a way that their upper ribs serve as beacons when laying and leveling the mosaic layer.

Finishing the surfaces of mosaic floors electrical machines after hardening of concrete (after 2-3 or more days). After the first grinding, the flaws found on the floor surface are puttied with a colored cement-sand mortar. Then the floor is sanded with finer abrasives, treated with polishing powders and glossed with a buffing machine.

4. Concreting of columns

a. Formwork for rectangular columns

Columns as an element of the frame of buildings and structures are rectangular, polygonal and circular. The height of the columns reaches 6-8 m or more.

The formwork of rectangular columns is a box of two pairs of panels (wooden, metal or combined). The lateral pressure of the concrete mixture is perceived by the clamps that compress the box. Clamps are made of inventory metal with a large turnover of formwork and wooden - with a small number of revolutions. Holes in the straps of the metal clamp for fastening wedges allow them to be used for columns of various sections. To clean the box, a temporary hole is made in the lower part of one of the shields. Block forms are also used for concreting columns.

Typical unified shields and formwork panels are attached to the reinforcing blocks with tie bolts and pulled together with tie rods. The formwork of low columns is fixed in two mutually perpendicular directions with inclined jointing (braces). With a column height of more than 6 m, the formwork boxes are attached to specially arranged scaffolding.

After installing the formwork of the column, holes of 500x500 mm in size and work platforms for concrete work are arranged every 2-3 m in height. The formwork of high columns can only be mounted on three sides, and on the fourth side it can be built up during the concreting process.

b. Column concreting

For round columns, special metal block forms are made.

Compliance with the thickness of the protective layer in the columns is ensured by special cement gaskets, which, before concreting, are attached to the reinforcement bars with a knitting wire embedded in the gaskets during their manufacture.

Concreting of columns with transverse dimensions from 400 to 800 mm in the absence of crossing clamps is carried out from above without interruption in sections up to 5 m high. Columns with sides of a section of less than 400 mm and columns of any section with crossing clamps, which contribute to the separation of the concrete mixture when it falls, are concreted from the side plots with a height of no more than 2 m.

Column formwork: assembled box; inventory metal collar; wooden clamp on wedges; detail of a wooden clamp knot; box; inventory metal clamp; wedges fastening clamps; frame for column formwork; cleaning hole door; cover shields; holes for wedges embedded shields; hard plates.

With a higher height of sections of columns concreted without working joints, it is necessary to arrange breaks for the concrete mixture to settle. The duration of the break should be at least 40 minutes and not more than 2 hours.

c. Frame structures

In cases where the columns are part of the frame structure and above them, there are beams or girders with thick reinforcement, it is allowed to first concrete the columns, and then, after the installation of the reinforcement, the beams and girders.

The lower part of the formwork of the columns when concreting them from above is recommended to be initially filled to a height of 100-200 mm with a cement mortar of the composition 1: 2-1 = 3 to prevent the accumulation of coarse aggregate without mortar at the base of the column. When a portion of the concrete mixture is dropped from above, large aggregate particles are embedded in this solution, forming a mixture of normal composition.

The concrete mixture in the columns is compacted by internal vibrators with a flexible or rigid shaft. Compaction with external vibrators attached to the formwork of small-section columns is less effective and is practically not used.

In order to avoid the formation of shells during the concreting of columns (especially corners), it is very useful to tap with a wooden mallet from the outside at or slightly below the concrete layer being laid.

Concreting of columns in accordance with SNiP III-B.1-70 is carried out to the full height without working joints. It is allowed to arrange working joints: at the level of the top of the foundation, at the bottom of the girders and beams or crane consoles and the top of the crane beams.

d. Concreting of frame structures

In columns of beamless ceilings, it is allowed to arrange seams either at the very bottom of the columns, or at the bottom of the capitals. The capitals are concreted simultaneously with the floor slab.

The surface of the working joints, arranged when laying the concrete mixture intermittently, must be perpendicular to the axis of the columns to be concreted.

Concreting of frame structures should be carried out with a break between the placement of the concrete mixture in the columns (racks) and crossbars of the frames. Working seams are arranged a few centimeters below or above the junction of the frame crossbar to the rack.

Walls (including partitions) are of constant and variable cross-section, vertical and inclined, in terms of round, curvilinear, polygonal and straight.

When concreting walls and partitions, the following types of formwork are used: standard unified panels and panels of collapsible-climbing formwork, block-forms, rolling climbing-climbing, sliding-climbing and sliding formwork.

The collapsible small-panel formwork is installed in two steps: first, on one side, to the entire height of the wall or partition, and after installing the reinforcement, on the other. If the wall thickness is more than 250 mm, the formwork of the second side is installed with special inventory.

They are installed on the entire height of the wall, otherwise - in tiers in the process of concreting. In the formwork installed to the entire height of the wall, holes are provided for supplying the concrete mixture through them into the structure.

5. Concrete walls

a. Design wall thickness

Wall formwork up to 6 m high is mounted from mobile platforms or light scaffolds. At higher altitudes, forests are arranged. The formwork of the walls is fastened with struts or braces, tie bolts or wire ties.

To comply with the design thickness of the walls, concrete or wooden spacers are installed in the places where the screeds pass. The latter are removed during the concreting process.

Collapsible large-block formwork is installed in tiers in the process of concreting the walls. This allows you to limit yourself to a set of formwork of only two tiers. All works of the full cycle of concreting walls in this formwork are carried out in the following sequence: first, scaffolding (scaffolding) is installed or increased, then the working seam of concreting is processed and reinforcement is installed, after which the formwork is rearranged from the lower tier to the upper one. The concreting cycle of one tier ends with the laying and compaction of the concrete mixture and the subsequent curing of the concrete in the formwork.

Block form for formwork: fixing clamp No. 1; reinforced concrete tape; bedding; screw jack; formwork block; fencing element for the 1st tier of concreting; formwork panel; fixing clamp No. 2; working floor; fencing element for the 2nd tier of concreting; inventory insert; sliding rack; double wooden wedge.

b. Block formwork

Formwork block forms are used when concreting walls of considerable height and length, i.e., when their repeated use is ensured. The block-form of the design of the Kharkovorgtekhstroy trust consists of blocks, panels, additional and fasteners.

The rigidity of the blocks is ensured by horizontal braces and supporting trusses, which also serve as scaffolds. For the installation, alignment and dismantling of the formwork, the supporting trusses are equipped with jacking devices. The dimensions of ordinary blocks are 3X8,3X2 and 1.5x3 m.

Rolling formwork designed by Donetsk PromstroyNIIproekt: trolley; Column; beam; shield lifting winch; formwork board; clamps; stairs; sliders; clamping device; flooring; fencing; bunker.

The deck of blocks, panels and extensions is assembled from small-sized shields made of 45X45x5 mm corners and 3 mm thick sheet steel. In the ribs of the shield frame there are holes with a diameter of 13 mm for attaching the shields to each other.

The assembled formwork blocks, if necessary, can be disassembled into separate panels. The block form of the formwork is rearranged by tiers during the concreting process. When concreting walls of constant and variable cross-section, rolling formwork is used (including horizontally moved on skids).

c. wall construction

Concreting of structures can be carried out in tiers with continuous or cyclic movement of the formwork, as well as by grips to the entire height of the wall. Rolling formwork designed by the Donetsk PromstroyNIIproekt consists of two metal panels 6-8 m long and 1.3 m high. The frame of the panels is made of an angle, and the deck is made of sheet steel 6 mm thick. Formwork size 6700X X 5400X3900 mm, weight 800 kg. With the help of special devices - sliders - the shields are attached to the guide columns of the portal.

The columns of the portal at the bottom rest on the trolley, and at the top they are connected by a beam, which allows you to spread the columns to the required width (up to 600 mm). The movement of the shields perpendicular to the surface of the structure being concreted is carried out by a screw device, and the lifting is carried out on cables through fixed blocks attached to connecting beams. Moving the formwork along the concreted wall is carried out with the help of double-sided winches.

The construction of walls in sliding and climbing formwork is discussed below, among the special construction methods.

When concreting walls, the height of sections erected without interruption should not exceed 3 m, and for walls less than 15 cm thick - 2 m.

d. Concrete supply

With a higher height of wall sections concreted without working joints, it is necessary to arrange breaks lasting at least 40 minutes, but not more than 2 hours to settling the concrete mixture and preventing the formation of sedimentary cracks.

If there is a window or door opening in the wall to be concreted, the concreting should be interrupted at the level of the upper edge of the opening or, if possible, a working seam should be arranged in this place. Otherwise, sedimentary cracks form near the corners of the mold. When supplying a concrete mixture from a height of more than 2 m, link trunks are used.

The lower part of the wall formwork during concreting from above is first filled with a layer of cement mortar of composition 112-1: 3 in order to avoid the formation of porous concrete at the base of the walls with accumulation of coarse aggregate.

When concreting the walls of tanks for storing liquids, the concrete mixture should be laid continuously over the entire height in layers with a thickness of not more than 0.8 of the length of the working part of the vibrators. In exceptional cases, the formed working joints must be very carefully processed before concreting.

The walls of large tanks are allowed to be concreted in vertical sections, followed by processing and filling with concrete mixture of vertical working joints. The joints of the walls and the bottom of the tanks are made in accordance with the working drawings.

6. Concreting beams, slabs, vaults

a. Concreting of ribbed slabs

Concreting beams, slabs, vaults, arches and tunnels. Beams and slabs, ceilings are usually concreted in collapsible formwork from standard unified panels and panels. Beams and girders are also concreted in block forms.

The formwork of the ribbed floor is made of small-piece wooden panels supported by wood-metal sliding racks at a height of up to 6 m and specially arranged scaffolding at a height of more than 6 m.

The formwork of the beam is made of three shields, one of which serves as the bottom, and the other two - as side railings of the surfaces. The side panels of the formwork are fixed at the bottom with pressure boards sewn to the head of the rack, and at the top - with the formwork of the slab.

Concreting of ribbed slabs: general form scaffolding and ribbed formwork; the location of the working seams when concreting ribbed slabs in a direction parallel to the secondary beams; the same, the main beams; beam formwork; slab formwork; circled; run formwork; column formwork; sliding racks; pressure boards; stands; frieze boards; slab formwork boards; circled; circumferential boards; side shields; bottom: rack head; working position of the seam (arrows show the direction of concreting).

b. Beamless slab formwork

The boards of the formwork flooring of the slab are laid with an edge on the circles of the boards, which, in turn, rest on the boards under the circles, nailed to the seam strips of the side beams and supported by supports.

To fix the circles and side panels, frieze boards are laid along the perimeter of the slab, which also facilitates the stripping of the slab. With a beam height of more than 500 mm, the side panels of the formwork are additionally reinforced with wire ties and temporary braces.

The distance between the racks and circles is determined by calculation. The supporting racks are unfastened in mutually perpendicular directions with inventory strands or braces.

Beamless slab formwork consists of column formwork, capitals and slabs. The formwork of the slab consists of two types of panels laid in circles between frieze boards sewn onto the tops of the racks. To support the circles, paired runs are arranged from boards resting on racks. The shields of the capitals rest on the formwork of the columns on one side, and are supported by circles along the outer contour.

When mounting the suspended formwork of floor slabs along precast concrete or metal beams, metal suspension loops are arranged, laid out along the beams with a given step. These loops are used to install the circumferential boards, on which the circumferential boards and boards of the slab formwork rest.

c. protective layer

Concreting of ceilings (beams, purlins and slabs) is usually carried out simultaneously. Beams, arches and similar structures with a height of more than 800 mm are concreted separately from the slabs, arranging working seams 2-3 cm below the level of the lower surface, and if there are haunches in the slab, at the level of the bottom of the slab haunch (SNiP Sh-V.1-70 ).

In order to prevent sedimentary cracks, the concreting of beams and slabs monolithically connected with columns and walls should be carried out 1-2 hours after the concreting of these columns and walls.

The concrete mixture is placed in beams and girders in horizontal layers, followed by compaction with flexible or rigid shaft vibrators - in strong or weakly reinforced beams. The concrete mixture is placed into the floor slabs along the lighthouse rails, which are installed on the formwork with the help of linings in rows of 1.5-2 m. After concreting, the rails are removed, and the resulting depressions are smoothed out. With double reinforcement of floor slabs, the leveling and compaction of the concrete mixture is carried out from the adjustable deck so as not to bend the upper reinforcement.

Floor slabs are concreted in the direction of secondary beams. The protective layer in slabs, beams and girders is formed with the help of special gaskets from cement mortar or clamps. As the structures are being concreted, the reinforcement is slightly shaken with metal hooks, making sure that a protective layer of the required thickness forms under the reinforcement.

d. Floor concreting

Concrete mixture in slabs up to 250 mm thick with single reinforcement and up to 120 mm thick with double reinforcement is compacted by surface vibrators, in slabs of greater thickness - deep.

Working joints when concreting flat joints can be arranged anywhere parallel to the smaller side of the slab. In ribbed slabs, when concreting parallel to the direction of the main beams, the working seam should be arranged within two middle quarters of the span of the run and slabs, and when concreting parallel to secondary beams, as well as individual beams, within the middle third of the beam span.

The surface of working joints in beams and slabs must be perpendicular to the direction of concreting. Therefore, in the planned places for a break in the concreting of the slabs, boards are installed on the edge, and in the beams - shields with holes for reinforcement.

Expansion joints in the ceilings are arranged on the consoles of the columns or by installing paired columns, ensuring free movement in the joint of the beams in the horizontal plane along the metal base sheet.

When concreting floors in multi-storey frame buildings, receiving platforms are arranged at the level of each floor, and conveyors and vibration chutes are installed inside the building to supply the concrete mixture after it has been lifted by a crane to the place of installation.

e. Vaults and arches

In the process of concreting coatings, ceilings and individual beams, it is not allowed to load them with concentrated loads exceeding the allowable ones specified in the project for the production of works.

Vaults and arches of small length are concreted in a collapsible small-piece or large-panel formwork supported by racks. For concreting arches and arches of great length, inventory rolling formwork mounted on a trolley is used. On the lower part of the formwork, lifting and lowering circles are installed, carrying a two-layer sheathing, consisting of boards laid with a gap of 10 mm, and waterproof plywood. The gap between the boards reduces the risk of formwork jamming in the vault when it swells. Raising and lowering the circles is carried out with the help of hoists and blocks, and the entire formwork moves along the rails with the help of a winch.

Vaults and arches of a small span should be concreted without: breaks simultaneously from both sides of the supports (heels) to the middle of the vault (castle), which ensures the preservation of the design form of the formwork. If there is a danger of formwork bulging at the vault lock during the concreting of the side parts, it is temporarily loaded.

Rolling formwork of the vault-shell: cross section; lengthwise cut; tightening the arch-diaphragm; retractable racks; hand hoists.

7. The process of concreting complex structures

a. Massive arches and vaults

Arches of great length are divided along the length into limited areas of concreting by working seams located perpendicular to the generatrix of the arch. Concrete is laid in limited areas in the same way as in vaults of short length, i.e. symmetrically from the heels to the castle.

Massive arches and vaults with a span of more than 15 m are concreted in strips parallel to the longitudinal axis of the vault. The laying of the concrete mixture in strips is also carried out symmetrically on both sides from the heels to the vault lock.

The gaps between the strips and sections of arches of great length are left approximately 300-500 mm wide and concreted with a rigid concrete mixture 5-7 days after the concreting of the strips and sections is completed, i.e. when the main concrete laying occurs.

With steep arches, the sections at the supports are concreted in a double-sided formwork, and the second (upper) formwork is installed with separate panels along the concreting.

The concrete mixture is compacted in massive arches and vaults with internal vibrators with a flexible or rigid shaft, depending on the degree of reinforcement, in thin-walled vaults - with surface vibrators. The tightening of vaults and arches with tensioners should be concreted after tightening these devices and turning the coatings around. Rigid puffs without tensioning devices are allowed to be concreted simultaneously with the concreting of the coating.

b. Tunnels and pipes

Tunnels and pipes are concreted in open trenches and underground in collapsible and retractable mobile formwork. The movable wooden formwork of the curvilinear walk-through tunnel with a cross section of up to 3 m consists of shields in the form of curved circles, sheathed with planed boards, waterproof plywood or sheet steel on a boardwalk. Racks supporting the working flooring are sewn to the circles of the outer shields. The internal formwork consists of two shields, the bottom of which rests on paired wedges, and the top is bolted in the vault lock.

The outer and inner formwork are connected to each other by tie bolts. The length of the shields is usually taken equal to 3 m, the weight of the formwork reaches 1.5 tons. The outer and inner formwork is moved using a winch along wooden rails. The outer formwork can also be moved to a new location by a crane. Rolling timber formwork, designed by Eng. V. B. Duba for concreting tunnels and rectangular collectors consists of sections 3.2 m long.

The internal formwork section consists of four U-shaped steel frames sheathed with planed boards, plywood or sheet steel. Each frame consists of two side posts and two: semi-crossbars connected to each other by three hinges. The outer frames of the formwork section have in the middle one sliding rack made of pipes, pulled together by screw jacks. The frames are supported by means of middle racks and retractable horizontal beams on a trolley moving along a rail track.

c. Vaults of tunnel structures

The outer formwork section consists of five frames with braces and detachable crossbars. Racks of frames from the inside are sheathed with boards. The outer formwork is fastened with inner bolts passed through removable girders. The formwork allows concreting tunnels with a width of 2100-2800 mm and a height of 1800-2200 mm: The mass of one formwork section reaches 3 tons.

The outer formwork is usually moved by crane. When stripping the formwork, the tie bolts are removed, the joints of the crossbars are disconnected: the frames of the outer formwork, after which the formwork is removed. To remove the inner formwork with the help of jacking devices available in the extreme racks, half-bars with ceiling shields are lowered.

Concreting of tunnels is carried out, as a rule, in two stages: first the bottom, and then the walls and ceilings (vault) of the tunnel.

The vaults of tunnel structures are concreted simultaneously on both sides from the heels to the castle with radial layers. The castle is concreted in inclined layers along the vault, while the formwork is laid as the concrete is poured in short sections - from circle to circle.

In powerful vaults of tunnel structures, arranged working seams should be radial. The desired direction of the surfaces of the seams is ensured by the installation of formwork: shields. Before concreting the castle, the cement film from the surface: the concrete must be removed.

d. Tunnel finishes

Tunnel finishes are advisable to be concreted in parallel with the tunneling, since in this case the total time for constructing the tunnel is reduced. However, with small cross-sectional dimensions of the tunnel, due to cramped conditions, the finish is erected at the end of the tunneling of the entire tunnel or individual sections between intermediate faces.

Tunnel lining is concreted either continuously over the entire cross section of the working, or in parts in the following sequence: tunnel tray, vault and walls, or vice versa.

For the formwork, the concrete mixture is fed from the end or through hatches in the formwork using concrete pumps or pneumatic blowers. In the side walls and the tunnel tray, the concrete mixture can also be fed by tipping trolleys using distribution chutes.

The concrete mixture is compacted layer by layer with deep vibrators through the windows in the formwork or with external vibrators attached to the formwork.

If the walls of the tunnel finish are concreted after the vault (the “supported vault” method), then before concreting, the formwork from the lower surface of the vault feet is removed and the surface is thoroughly cleaned. The walls are concreted in horizontal layers with the simultaneous build-up of the formwork to a mark less than the mark of the bottom of the heel of the vault by up to 400 mm. The space between the fifth arch and the adjoining wall is filled with a rigid concrete mixture and carefully compacted. Previously, pipes are laid at the junction for the subsequent injection of cement mortar.

The adhesion of concrete to formwork reaches several kgf/cm2. This makes it difficult to remove the formwork, degrades the quality of concrete surfaces and leads to premature wear of the formwork panels.

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

Adhesion (adhesion) is understood as the connection between the surfaces of two dissimilar or liquid contacting bodies due to molecular forces. During 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 the laying period. In addition, in the process of vibroconsolidation 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, untreated steel and fiberglass are well wetted and the adhesion of concrete to them is quite large, concrete adheres slightly to poorly wetted (hydrophobic) getinax and textolite.

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

When applied to the surface of the oil film, it hydrophobizes, which sharply reduces adhesion.

Shrinkage has a negative effect on adhesion and, consequently, on adhesion. The greater the shrinkage value in the butt layers of concrete, the more likely the appearance of shrinkage cracks in the contact zone, which weaken the adhesion. Under the cohesion in the contact pair formwork - concrete should be understood as the tensile strength of the butt layers of concrete.

The surface roughness of the formwork increases its adhesion to the concrete. This is because a rough surface has a larger actual contact area than a smooth one.

The high-foam formwork material also increases adhesion, since cement mortar, penetrating into the pores, prn vibrocompaction forms points of reliable connection.

When removing the formwork, there can be three options for separation. In the first variant, the adhesion is very small, and the 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 adhesion and cohesion in terms of their values ​​are approximately the same. The formwork is torn off partly along the plane of contact between the concrete and the formwork, partly along the concrete itself (mixed or combined separation).

With adhesive detachment, the formwork is easily removed, its surface remains clean, and the concrete surface has 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 setting retarders; combined lubricants.

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

More often than others, a lime-gyp-coBVio suspension is used to lubricate the formwork, which is prepared from semi-aqueous gypsum (0.6-0.9 weight "hours), lime dough (0.4-0.6 weight hours), sulfite- alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).

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

The most common hydrophobizing lubricants are based on mineral oils, EKS emulsol or salts of fatty acids (soaps). After they are applied to the surface of the formwork, a hydrophobic film is formed from a number of oriented molecules (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.

In the third group of lubricants, the properties of concrete to set slowly in thin butt layers are used. 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, in which the setting is slowed down.

Most effective combined lubricants, which use the properties of the forming surfaces in combination with the delay in the setting of concrete in thin butt layers. Such lubricants are prepared in the form of so-called inverse emulsions. In some of them, in addition to water repellents and setting retarders, plasticizing additives are introduced: sulfite-yeast vinasse (SDB), soap naft or TsNIPS additive. These substances, when vibrocompacted, 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, the components are poured into the mixer tank and the mixer is turned on.

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

Emulsion lubricants are stable and do not delaminate within 7-10 days. Their use completely eliminates the adhesion of concrete to the formwork; they are well kept on the forming surface and do not pollute the b “! Gon.

These lubricants can be applied 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 of certain factors on the formwork.

For metal shields, SE-3 enamel is recommended as an anti-adhesion coating, which includes epoxy resin (4-7 parts by weight), methyl polysiloxane oil (1-2 parts by weight), lead litharge (2-4 parts by weight). ) and polyethylenepolyamine (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 a 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 plank and plywood formwork TsNIIOMTP developed a coating based on phenol-formaldehyde. 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 withstands up to 35 cycles without repair.

Despite the rather high cost (0.8-1.2 rubles/m2), anti-adhesive protective coatings are more profitable than lubricants due to their repeated turnover.

It is advisable to use shields, the decks of which are made of 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 value of adhesion of concrete to the formwork reaches several kgf/cm 2 . This makes it difficult to remove the formwork, degrades the quality of concrete surfaces and leads to premature wear of the formwork panels.
The adhesion of concrete to the formwork is affected by the adhesion and cohesion of concrete, its shrinkage, roughness and porosity of the forming surface of the formwork.
Adhesion (adhesion) is understood as the connection between the surfaces of two dissimilar or liquid contacting bodies due to molecular forces. During the period of contact between concrete and formwork, favorable conditions to show 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 vibroconsolidation 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. Kc values ​​for different types formwork are equal: small-panel - 0.15, wooden - 0.35, steel - 0.40, large-panel (panels of small panels) - 0.25, large-panel - 0.30, volume-adjustable - 0.45, for block - forms - 0.55.
Wood, plywood, untreated steel and fiberglass are well wetted and the adhesion of concrete to them is quite large, concrete adheres slightly to poorly wetted (hydrophobic) getinax and textolite.
The contact angle of polished steel is greater than that of untreated steel. However, the adhesion of concrete to ground steel is only slightly reduced. This is explained by the fact that at the border of concrete and well-treated 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 the concrete. This is because a rough surface has a larger actual contact area than a smooth one.
The highly porous formwork material also increases adhesion, since the cement mortar, penetrating into the pores, forms points of reliable connection during vibration compaction. When removing the formwork, there can be three options for separation. In the first variant, the adhesion is very small, and the cohesion is quite large.
In this case, the formwork is torn off exactly along the contact plane. Another option is adhesion more than cohesion. In this case, the formwork is torn off along the adhesive material (concrete).
The third option - adhesion and cohesion are approximately the same in their values. The formwork is torn off partly along the plane of contact between the concrete and the formwork, partly along the concrete itself (mixed or combined separation).
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.
Lubricants for formwork, depending on their composition, principle of operation and performance properties, can be divided into four groups: aqueous suspensions; hydrophobic lubricants; lubricants - concrete setting retarders; combined lubricants.
Aqueous suspensions of powdered substances inert to concrete are a simple and cheap, but not always effective means for eliminating concrete adhesion 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 protective film is formed on the forming surface of the formwork, which prevents concrete from sticking.
More often than others, a lime-gypsum suspension is used to lubricate the formwork, which is prepared from semi-aqueous gypsum (0.6-0.9 wt. H.), Lime dough (0.4-0.6 wt. H.), Sulfite-alcohol stillage (0.8-1.2 parts by weight) and water (4-6 parts by weight).
Suspension lubricants are erased by the concrete mixture during vibrocompaction and contaminate concrete surfaces, as a result of which they are rarely used.
The most common hydrophobizing lubricants based on mineral oils, EKS emulsol 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 to set slowly in thin butt layers are used. 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.
The most effective are combined lubricants that use the properties of the forming surfaces in combination with the slowdown in the setting of concrete in thin butt layers. Such lubricants are prepared in the form of so-called inverse emulsions. In some of them, in addition to water repellents and setting retarders, plasticizing additives are introduced: sulfite-yeast vinasse (SDB), soap naft or TsNIPS additive. These substances, when vibrocompacted, plasticize the concrete in the butt layers and reduce its surface porosity.
ESO-GISI lubricants are prepared in ultrasonic hydrodynamic mixers, in which mechanical mixing of the components is combined with ultrasonic mixing. To do this, the components are poured into the mixer tank and the mixer is turned on.
The ultrasonic mixing plant consists of a circulation pump, suction and pressure pipelines, a junction box and three ultrasonic hydrodynamic vibrators - ultrasonic whistles with resonant wedges. The liquid supplied by the pump under an overpressure of 3.5-5 kgf/cm2 flows out at high speed from the vibrator nozzle and hits the wedge-shaped plate. In this case, the plate begins to vibrate at a frequency of 25-30 kHz. As a result, zones of intense ultrasonic mixing are formed in the liquid with simultaneous division of the components into tiny droplets. The duration of mixing is 3-5 minutes.
Emulsion lubricants are stable, they do not delaminate 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 concrete.
These lubricants can be applied 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 of certain factors on the formwork. In some cases, lubricants cannot be used. So, when concreting in a sliding or climbing formwork, it is forbidden to use such lubricants because they get into the concrete and reduce its quality.
Anti-adhesive protective coatings based on polymers give a good effect. They are applied to the forming surfaces of the shields during their manufacture, and they withstand 20-35 cycles without reapplication and repair.
A coating based on phenol formaldehyde 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 withstands up to 35 cycles without repair.
Despite the rather high cost, anti-adhesive protective coatings are more profitable than lubricants due to their repeated turnover.
It is advisable to use shields, the decks of which are made of 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.