Methods of laying concrete mixture. The quality of concrete in the structure. The height of free drop of concrete mixture when feeding it into reinforced structures. Reinforcement and concrete works

Home |GOSTs and SNiP |SNiP 3.03.01-87. Load-bearing and enclosing structures. Concrete works

Date: November 29, 2017

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The implementation of construction activities for the construction of any objects is inextricably linked with the production of concrete work. They are carried out when pouring screeds, erecting foundations, constructing blind areas, and constructing monolithic structures. According to the provisions of the current SNiP, concrete work is carried out with certain grades of concrete in accordance with the approved algorithm. This guarantees the strength and stability of the structures being built, as well as a long service life. Let's take a closer look at the main provisions of building codes.

Laying concrete mixtures.

2.8.
Before concreting, rock foundations, horizontal and inclined concrete surfaces of working joints must be cleaned of debris, dirt, oil, snow and ice, cement film, etc. Immediately before laying the concrete mixture, the cleaned surfaces must be washed with water and dried with a stream of air. 2.9. All structures and their elements that are covered during subsequent work (prepared structural foundations, reinforcement, embedded products, etc.), as well as the correct installation and fastening of the formwork and its supporting elements must be accepted in accordance with SNiP 3.01.01-85.

2.10. Concrete mixtures should be laid

into concrete structures in horizontal layers of equal thickness without breaks, with a consistent direction of laying in one direction in all layers.

2.11. When compacting the concrete mixture, it is not allowed to rest vibrators on reinforcement and embedded products, ties and other formwork fastening elements. The depth of immersion of the deep vibrator into the concrete mixture should ensure its deepening into the previously laid layer by 5 - 10 cm. The step of rearrangement of deep vibrators should not exceed one and a half radius of their action, surface vibrators should ensure that the vibrator platform overlaps the border of the already vibrated area by 100 mm.

2.12. Laying the next layer of concrete mixture

allowed until the concrete of the previous layer begins to set. The duration of the break between laying adjacent layers of concrete mixture without forming a working joint is established by the construction laboratory. The top level of the laid concrete mixture should be 50 - 70 mm below the top of the formwork panels.

2.13. The surface of the working joints made when laying the concrete mixture intermittently must be perpendicular to the axis of the columns and beams being concreted, the surface of the slabs and walls. Concreting may be resumed once the concrete reaches a strength of at least 1.5 MPa.

Working joints, in agreement with the design organization, may be installed during concreting:

columns - at the level of the top of the foundation, the bottom of purlins, beams and crane consoles, the top of crane beams, the bottom of column capitals;
large beams, monolithically connected to the slabs - 20 - 30 mm below the mark of the bottom surface of the slab, and if there are haunches in the slab - at the mark of the bottom of the haunch of the slab;
flat slabs - anywhere parallel to the smaller side of the slab;
ribbed floors - in a direction parallel to the secondary beams;
individual beams - within the middle third of the span of beams, in a direction parallel to the main beams (purlins) within the two middle quarters of the span of purlins and slabs;
arrays, arches, vaults, tanks, bunkers, hydraulic structures, bridges and other complex engineering structures and structures - in the places specified in the projects.

2.14. Requirements for laying and compacting concrete mixtures

are given in the table.
2. 2. SNiP requirements for laying and compacting concrete mixtures.

Parameter	Parameter value	Control (method, volume, type of registration)
SNiP 3.03.01-87 Concrete mixtures.
Strength of concrete base surfaces when cleared of cement film:	Not less, MPa:	Measuring according to GOST 10180-78, GOST 18105-86, GOST 22690.0-77, work log
water and air jet	0,3
mechanical wire brush	1,5
hydrosandblasting or mechanical cutter	5,0
2.Height of free dropping of concrete mixture into the formwork of structures:	No more, m:	Measuring, 2 times per shift, work log
columns	5,0
floors	1,0
walls	4,5
unreinforced structures	6,0
lightly reinforced underground structures in dry and cohesive soils	4,5
densely reinforced	3,0
3. Thickness of laid layers of concrete mixture:	Measuring, 2 times per shift, work log
when compacting the mixture with heavy suspended vertical vibrators		5-10 cm less than the length of the working part of the vibrator
when compacting the mixture with suspended vibrators located at an angle to the vertical (up to 30°)		No more than the vertical projection of the length of the working part of the vibrator
when compacting the mixture with manual deep vibrators		No more than 1.25 times the length of the working part of the vibrator
when compacting the mixture with surface vibrators in structures:		No more, see:
unreinforced		40
with single fittings		25
with double fittings		12

The quality of concrete in a structure largely depends on the correct placement of the concrete mixture during concreting. The mixture must adhere tightly to the formwork, reinforcement and embedded parts of the structure and completely (without any voids) fill the volume of the concreted part of the structure.

Laying a concrete mixture includes the following processes: supplying the concrete mixture to the concrete structure, distributing (leveling) and compacting it.

To supply concrete mixture to the structure, buckets and buckets in combination with various cranes, belt conveyors and concrete pavers, concrete pumps and pneumatic blowers, trunks and vibrating chutes, and vibrating chutes are used. In a number of cases, for example, during the construction of roads, airfield pavements, gentle slopes of canals and dams and other similar structures, concrete mixture delivered by vehicles is unloaded directly to the laying site without the use of machinery.

The height of free dumping of concrete mixture when feeding it into reinforced structures should not exceed 2 m, and when feeding it to the ceiling - 1 m, with the exception of columns without crossing reinforcement clamps with cross-sectional sides from 0.4 to 0.8 m, when the dumping height is the formwork reaches 5 m.

The permissible height for dropping concrete mixture into the formwork of unreinforced structures is established by the construction laboratory. This takes into account the preservation of the homogeneity and strength of the concrete, as well as the integrity of the base and formwork. The height of the drop should not exceed 6 m. When supplying concrete mixture from a higher height in places where it is impossible to lower the bucket with a crane, vibrating chutes, inclined trays, vertical trunks are used, and at a height of more than 10 m, vibration trunks with dampers are used.

The height of free dropping of concrete mixture on porous aggregates when laid in vertical formwork should not exceed 1.5 m, and when supplied to horizontal formwork - 0.7 m. Free dropping of concrete mixture from a greater height is allowed when introduced into the mixture additives that reduce its separation.

In a concreted structure, the mixture is distributed in horizontal layers of equal thickness, laid in one direction (95, a).

The thickness of the concrete mixture to be laid depends on the means of compaction. When using heavy suspended deep-well vibrators, the thickness of the laid layer is 5... 10 cm less than the length of the working part of the vibrator, if the vibrator is located vertically. When the vibrator is inclined (at an angle of up to 35° to the vertical), the thickness of the layer is taken equal to the projection of the working part of the vibrator onto the vertical.

When using manual deep vibrators, the thickness of the laid layer should not exceed 1.25 times the length of the working part of the vibrator.

When compacting a concrete mixture with surface vibrators, the layer thickness should not exceed 250 mm in structures unreinforced or reinforced with single reinforcement and 120 mm in structures with double reinforcement.

When compacting with external vibrators, the thickness of the concrete mixture layers is determined experimentally depending on the cross-section of the structure, the power of the vibrators, their spacing and the characteristics of the concrete mixture -

When distributing the mixture, it can only be thrown over in order to avoid separation in exceptional cases; double transfer is not allowed.

Each laid layer of concrete mixture is thoroughly compacted before laying the next one. The duration of laying the layer is limited by the time when the cement begins to set. Overlapping the previous layer with the next one must be done before the cement in the previous layer begins to set.

The time for laying and overlapping layers is determined by the laboratory. The time depends on the outside temperature, conditions and properties of the cement used. Approximately it is about 2 hours.

If the laying time of a layer exceeds the period established by the laboratory, then when vibrating the subsequent layer, the solidity of the concrete of the previous layer is disrupted, so concreting should be stopped. Concreting can be resumed only when the concrete reaches a compressive strength of at least 1.5 MPa. The moment when concrete reaches such strength is determined in the laboratory.

At the point of contact between the previously laid concrete mixture and the freshly laid one, a so-called working joint is formed. To ensure good adhesion of the previously laid mixture to the freshly laid one, the surface of the previously laid layer is left uneven (not smoothed) and cleaned of the cement film with a water or air jet after the cement has set.

In large areas, it is sometimes impossible to cover the previous layer of concrete before the cement in it begins to set. In this regard, on some construction sites the concrete mixture is laid in steps (95, b) with the simultaneous laying of two or three layers. When concreting in steps, there is no need to cover the layers over the entire area of the massif. In this case, a rigid concrete mixture is used and only the steps are covered.

Laying in steps is permitted provided that the detailed concreting technology is followed. This method is used when concreting hydraulic structures with long blocks with a length to width ratio of more than 2. In domestic construction, examples of concreting with blocks 70 m long and 15 m wide are known.

In hydraulic engineering, blocks of a large area are also concreted to their entire height in one horizontal layer up to 100 cm thick. In this case, the duration of laying the layer does not depend on the time when the cement begins to set. But between each laid layer and the previous one, a working seam is formed that requires processing.

When concreting structures, it is necessary to ensure that the position of the formwork, reinforcement and embedded parts remains unchanged. While the concrete mixture has not hardened, some displacements from the design position can be easily eliminated.

During concreting, it is necessary to systematically clean the reinforcement, formwork and embedded parts from adhering mortar, and also protect the concrete structure from rain. Concrete washed away by rain must be removed from the structure.

It is advisable to erect monolithic concrete and reinforced concrete structures without seams. But during the construction of large structures, it is completely impossible to fulfill this requirement, since cracks would form in monolithic structures under the influence of temperature fluctuations and uneven settlement. Therefore, large concrete and reinforced concrete structures are divided into sections using expansion joints.

Expansion joints are filled with gaskets or covered with bitumen dowels (sealing barrier) for watertightness (in hydraulic structures).

The structure or its sections between expansion joints are temporarily divided by additional joints into smaller parts that are concreted without interruption, called blocks or concreting sections. Breaking down into blocks is required both to reduce shrinkage and temperature deformations of concrete associated with heat generation during setting and hardening of cement, and because of the limitation of the area of the concreted area necessary for timely overlap of layers during concreting. Such seams are called construction or shrinkage seams.

Since most structures have to be concreted intermittently (for example, for installing formwork and reinforcement), working seams are formed in places where concreting is interrupted. They are combined with construction and shrinkage. Therefore, the distance between construction joints is set in the project taking into account the conditions of work on the basis of technical and economic calculations.

To speed up and reduce the cost of construction, it is advisable to take the block sizes in plan as large as possible, and therefore, the distances between construction and working seams as large as possible, since this reduces the volume of formwork and preparatory work on the structure.

CURTINING AND CARE OF CONCRETE

2.15. During the initial period of hardening, concrete must be protected from precipitation or moisture loss, and subsequently maintain temperature and humidity conditions to create conditions that ensure an increase in its strength.

2.16. Concrete care measures

, the order and timing of their implementation, control over their implementation and the timing of dismantling of structures should be established by the PPR.

2.17. Movement of people on concreted structures and installation of formwork on overlying structures is allowed after the concrete reaches a strength of at least 1.5 MPa.

TESTING OF CONCRETE DURING ACCEPTANCE OF STRUCTURES

2.18. Strength, frost resistance, density, water resistance, deformability, as well as other indicators established by the project, should be determined in accordance with the requirements of current state standards.

INTRODUCTION

Winter concreting includes work performed when the average daily outside air temperature is below 5°C and the minimum daily temperature is below 0°C. It is believed that winter concreting can be carried out at air temperatures down to minus 40°C. In practice, winter concreting has been mastered down to temperatures of minus 15-20°C.

To ensure that concrete gains the required strength, special measures are taken to prepare and carry out concrete work in winter.

For winter concreting, special concretes with chemical antifreeze and plasticizing additives are used.

When performing work, freshly laid concrete is heated in various ways using steam, heated water or electricity.

Freshly laid concrete is protected from heat loss (thermos method) by covering it with various insulation materials (mats, blankets, panels).

Special measures, in particular for the insulation of working bodies and concrete pipes, are carried out when preparing machines and technological equipment for winter concreting.

The main requirement when performing winter concreting is to create favorable conditions for concrete to acquire the required design strength in a short time.

Massive monolithic structures (base slabs and blocks) with cooling surface module M

p from 2 to 4 are concreted using the thermos method using quick-hardening cements, hardening accelerators and anti-frost and plasticizing additives.

Structures (columns, blocks, walls) with a cooling surface module of 4-6 are concreted using the thermos method using preheating of the concrete mixture, heating wires and heating formwork.

Relatively thin-walled structures (partitions, ceilings, walls) with a cooling surface module of 6-12 are concreted using the methods mentioned above using heating wires, thermoactive flexible coatings (TAGC), and heating flat elements (HEP).

This document discusses the method of winter concreting using heating wires. This method has a number of advantages compared to heating with steam, hot water, and infrared irradiation. The effectiveness of the method increases in combination with the other measures and techniques of winter concreting mentioned above: the use of high-quality concrete with chemical additives, insulation materials, preparation of machines and technological equipment.

The use of heating wires makes it possible to erect buildings and structures that are no different in strength from those erected in the summer.

This document contains methodological recommendations and examples that allow you to select work methods (modes, techniques) and materials for winter concreting for a specific construction project, taking into account local conditions and the characteristics of the construction organization. The choice of method of work and materials is made at the stage of developing a work project (technological maps), agreed with the customer and approved in the prescribed manner.

This document is necessary not only for the development of the above-mentioned technological documentation, but can be useful when licensing a construction organization (company) to carry out this type of work, when certifying a quality management system, when certifying the quality of winter concreting,

The document is based on research work carried out at TsNIIOMTP and other institutes of the construction industry, as well as a generalization of the experience of winter concreting of Russian construction organizations.

When developing the document, regulatory and methodological documents were used, the main ones of which are given in section 2.

Production of concrete work at subzero temperatures

Excerpts from SNiP related to concrete work in winter: transportation, laying concrete mix, how to pour concrete in winter at subzero temperatures.

SNiP. PRODUCTION OF CONCRETE WORK AT NEGATIVE AIR TEMPERATURES

2.53. These rules are followed during the period of concrete work when the expected average daily outside air temperature is below 5 °C and the minimum daily temperature is below 0 °C.

2.54. The preparation of the concrete mixture should be carried out in heated concrete mixing plants, using heated water, thawed or heated aggregates, ensuring the production of a concrete mixture with a temperature not lower than that required by calculation. It is allowed to use unheated dry aggregates that do not contain ice on the grains and frozen lumps. In this case, the duration of mixing the concrete mixture should be increased by at least 25% compared to summer conditions.

2.55. Methods and means of transportation must ensure that the temperature of the concrete mixture does not decrease below that required by calculation.

2.56. The condition of the base on which the concrete mixture is laid, as well as the temperature of the base and the method of laying must exclude the possibility of the mixture freezing in the area of contact with the base. When curing concrete in a structure using the thermos method, when preheating the concrete mixture, as well as when using concrete with antifreeze additives, it is allowed to lay the mixture on an unheated, non-heaving base or old concrete, if, according to calculations, freezing will not occur in the contact zone during the estimated period of curing the concrete.

At air temperatures below minus 10 °C, concreting of densely reinforced structures with reinforcement with a diameter greater than 24 mm, reinforcement made of rigid rolled sections or with large metal embedded parts should be carried out with preliminary heating of the metal to a positive temperature or local vibration of the mixture in the reinforcement and formwork areas, with the exception of cases of laying preheated concrete mixtures (at a mixture temperature above 45 ° C). The duration of vibration of the concrete mixture should be increased by at least 25% compared to summer conditions.

2.57. When concreting elements of frame and frame structures in structures with rigid coupling of nodes (supports), the need to create gaps in the spans depending on the heat treatment temperature, taking into account the resulting temperature stresses, should be agreed upon with the design organization. Unformed surfaces of structures should be covered with steam and heat insulating materials immediately after concreting is completed.

Reinforcement outlets of concrete structures must be covered or insulated to a height (length) of at least 0.5 m.

2.58. Before laying the concrete (mortar) mixture, the surfaces of the joint cavities of precast reinforced concrete elements must be cleared of snow and ice.

2.59. Concreting of structures on permafrost soils should be carried out in accordance with SNiP II-18-76.

Acceleration of concrete hardening when concreting monolithic bored piles and embedding bored piles should be achieved by introducing complex antifreeze additives into the concrete mixture that do not reduce the freezing strength of concrete with permafrost soil.

2.60. The choice of concrete curing method for winter concreting of monolithic structures should be made in accordance with the recommended Appendix 9.

2.61. The strength of concrete should be monitored, as a rule, by testing samples made at the site where the concrete mixture is laid. Samples stored in the cold must be kept for 2-4 hours at a temperature of 15-20 °C before testing.

It is allowed to control the strength by the temperature of the concrete during its curing.

2.62. The requirements for work at subzero air temperatures are set out in the table. 6

6. Requirements for the production of concrete work at subzero temperatures.

Parameter	Parameter value	Control (method, volume, type of registration)
Pour concrete at sub-zero temperatures.
1. Strength of concrete of monolithic and prefabricated monolithic structures at the moment of freezing:	Measuring according to GOST 18105-86, work log
for concrete without antifreeze additives:
structures operating inside buildings, foundations for equipment not subject to dynamic influences, underground structures	Not less than 5 MPa
structures exposed to atmospheric influences during operation, for the class:	Not less, % of design strength:
B7.5-B10	50
B12.5-B25	40
B30 and above	30
structures subject to alternating freezing and thawing in a water-saturated state at the end of curing or located in the seasonal thawing zone of permafrost soils, subject to the introduction of air-entraining or gas-forming surfactants into the concrete	70
in prestressed structures	80
for concrete with antifreeze additives	By the time the concrete has cooled to the temperature for which the amount of additives is designed, at least 20% of the design strength
2. Loading of structures with the design load is allowed after the concrete reaches strength	At least 100% design	—
3. Temperature of water and concrete mixture at the outlet of the mixer, prepared:	Measuring, 2 times per shift, work log
on Portland cement, slag Portland cement, pozzolanic Portland cement of grades below M600	Water no more than 70 °C, mixtures no more than 35 °C
on quick-hardening Portland cement and Portland cement grade M600 and higher	Water no more than 60°C, mixture no more than 30°C
on aluminous Portland cement	Water no more than 40 C, mixtures no more than 25 ° C
Temperature of the concrete mixture placed in the formwork at the beginning of curing or heat treatment:	Measuring, in places determined by the PPR, work log
with the thermos method	Set by calculation, but not lower than 5°C
with antifreeze additives	Not less than 5 C above the freezing point of the mixing solution
during heat treatment	Not lower than 0 °C
5. Temperature during curing and heat treatment for concrete at:	Determined by calculation, but not higher, °C:	During heat treatment - every 2 hours during the period of temperature rise or on the first day. In the next three days and without heat treatment - at least 2 times per shift. The rest of the holding period - once a day
Portland cement	80
slag Portland cement	90
6. Rate of temperature rise during heat treatment of concrete:	Measuring, every 2 hours, work log
for structures with surface modulus:	No more than, °C/h:
up to 4	5
from 5 to 10	10
St. 10	15
for joints	20
7. Concrete cooling rate at the end of heat treatment for structures with surface modulus:	Measuring, work log
up to 4	Determined by calculation
from 5 to 10	No more than 5°C/h
St. 10	No more than 10°C/h
8. The temperature difference between the outer layers of concrete and air during stripping with a reinforcement coefficient of up to 1%, up to 3% and more than 3% should be, respectively, for structures with a surface modulus:	Same
from 2 to 5	No more than 20, 30, 40 °C
St. 5	No more than 30, 40, 50 °C

Source: SNiP 3.03.01-87
stroyremkom.ru

Features of concrete work production according to SNiP

When constructing any construction project, it is impossible to do without performing concrete work, be it installing a screed, blind area or pouring a foundation. This type of work involves laying a concrete mass - an artificial building material obtained from a mixture of cement, filler and water. The brand and type of cement and filler used to prepare the solution determine the purpose and scope of its application. For example, mixtures made on the basis of pozzolanic Portland cement are used in the construction of structures whose operation takes place in conditions of high humidity.

SNiP for concrete work - main provisions and structure of the standard

The building codes and regulations, approved in 1987 and registered under number 3.03.01, are the main regulatory document that regulates the requirements for the production of concrete work. For example, according to the document, concreting should be carried out from pre-sifted components, which are dosed by weight. The rules require that the components be introduced into the solution in a strict order and mixed for a certain time.

The general structure of the rules is quite voluminous and covers a range of issues:

requirements for materials used for solutions;
recommendations for dosing components when mixing;
methods of laying concrete on various types of foundations;
features of surface protection and care of hardening concrete;
testing methodology for the hardened mass at the acceptance stage;
specifics of concreting at different temperatures;
requirements for concrete reinforcement and formwork construction;
methods for quality control of concrete-based structures.

When constructing any construction project, it is impossible to do without concrete work.

When developing a work project, it reflects all types of activities that must be carried out in accordance with the requirements of building codes. Deviations from the provisions regulated by the rules reduce the quality of concrete measures and affect the safety of structures and durability. Let's consider the main provisions of the main sections of the regulatory document.

Types of concrete and types of work on its laying

Based on its properties, concrete can be divided into:

prestressing - concrete that contains expanding cement or an additive that ensures expansion of the concrete during the hardening process;
rapid-hardening concrete gains strength in a short period of time;
highly functional concrete;
decorative concrete is obtained by coloring, texturing, polishing, engraving, embossing and other methods to achieve certain aesthetic properties;
draining concrete, which contains only coarse aggregate (the content of fine aggregate is minimized or absent altogether).

Also, according to a combination of characteristics, concrete can be classified into: heavy, fine-grained, light, cellular, silicate, heat-resistant and chemically resistant.

When constructing concrete and reinforced concrete structures, a number of interrelated processes are performed in accordance with SNiP:

work on the manufacture and installation of formwork, stripping;
reinforcement work, which consists of the manufacture and installation of reinforcement structures in the position specified by the project;
concrete work, including the preparation of a concrete mixture, its transportation (in the case of preparing the mixture at a site other than the work site), supplying the solution to the installation site, directly laying the concrete and compacting it, as well as maintaining and caring for the concrete during its hardening.

Each type of work, according to SNiP, has a number of features. For example, transportation of a ready-made concrete mixture should include measures to protect the concrete from exposure to precipitation and sunlight, delamination, and in winter also from freezing. And the preparation of a concrete mixture of a certain brand must be carried out in strict adherence to technology that ensures workability.

The concrete mixture is laid in horizontal layers over the entire area of the concreted area. In this case, all layers must have the same thickness, and work is carried out continuously in one direction with careful compaction.

In cases where conventional concreting methods are unsuitable or uneconomical, special ones are used: casting, separate concreting, underwater concreting, concreting in low temperatures or hot climates.

When laying the mixture by casting, superplasticizers - additives based on naphthalene sulfonic acid or melamine resin, which increase the mobility of the mixture. When carrying out work using this method, there is no need to distribute and vibrate the mixture, which reduces cement consumption.

When using separate concreting, coarse aggregate is first placed in the formwork, and then a cement-sand mortar is placed to fill all the voids. This method is used when concreting in conditions of abundant groundwater.

Underwater concreting is used in construction, repair and restoration work of underwater parts of structures. There are the following concreting methods: using a vertically moving pipe, laying in bags, using the rising solution method, laying in bunkers.

When building in winter, it is necessary to create such a regime so that by the time the concrete freezes, it acquires critical strength. Such work is carried out using non-heating methods and using artificial heating. The first type includes the thermos method and the addition of antifreeze additives. Artificial heating includes electrical heat treatment of concrete, heating with hot air, steam, and the use of heating formwork or greenhouses.

Concrete work: existing types, requirements for quality and materials according to SNiP

Types of concrete and types of work on its laying

Based on its properties, concrete can be divided into:

prestressing - concrete that contains expanding cement or an additive that ensures expansion of the concrete during the hardening process;
rapid-hardening concrete gains strength in a short period of time;
highly functional concrete;
decorative concrete is obtained by coloring, texturing, polishing, engraving, embossing and other methods to achieve certain aesthetic properties;
draining concrete, which contains only coarse aggregate (the content of fine aggregate is minimized or absent altogether).

Also, according to a combination of characteristics, concrete can be classified into: heavy, fine-grained, light, cellular, silicate, heat-resistant and chemically resistant.

When constructing concrete and reinforced concrete structures, a number of interrelated processes are performed in accordance with SNiP:

work on the manufacture and installation of formwork, stripping;
reinforcement work, which consists of the manufacture and installation of reinforcement structures in the position specified by the project;
concrete work, including the preparation of a concrete mixture, its transportation (in the case of preparing the mixture at a site other than the work site), supplying the solution to the installation site, directly laying the concrete and compacting it, as well as maintaining and caring for the concrete during its hardening.

The concrete mixture is laid in horizontal layers over the entire area of the concreted area. In this case, all layers must have the same thickness, and work is carried out continuously in one direction with careful compaction.

When laying the mixture by casting, superplasticizers are added to concrete - additives based on naphthalene sulfonic acid or melamine resin, which increase the mobility of the mixture. When carrying out work using this method, there is no need to distribute and vibrate the mixture, which reduces cement consumption.

Underwater concreting is used in construction, repair and restoration work of underwater parts of structures. There are the following concreting methods: using a vertically moving pipe, laying in bags, using the rising solution method, laying in bunkers.

When building in winter, it is necessary to create such a regime so that by the time the concrete freezes, it acquires critical strength. Such work is carried out using non-heating methods and using artificial heating. The first type includes the thermos method and the addition of antifreeze additives. Artificial heating includes electrical heat treatment of concrete, heating with hot air, steam, and the use of heating formwork or greenhouses.

Types of concrete work

Existing types of concrete work

The following requirements are imposed on finished reinforced concrete and concrete structures of all types in accordance with SNiP:

operational safety;
serviceability;
durability;
additional requirements specified in the design documentation.

The quality of the finished structure depends on the quality of the materials used and compliance with technological regulations at all stages of construction. In order to ensure that SNiP requirements are met, control is carried out at all stages of concreting:

acceptance/storage of building materials;
execution and installation of reinforcement structures;
production and installation of formwork;
preparatory measures for the base and formwork surfaces for concreting;
the process of preparing and transporting concrete mixture;
work on laying and compacting the mortar, as well as caring for it during hardening.

During the preparation of a concrete mixture, the accuracy of the dosage of components, the duration of the mixing process, as well as the density and plasticity of the mixture are checked. During transportation, the mixture should not separate, set or lose mobility.

At the reinforcement stage, in addition to the quality of the reinforcing bars, the quality of welding connections and the correct placement of reinforcement in the structure are checked. When constructing formwork, special attention is required to the correctness of its installation, the tightness of butt joints, as well as the location of the formwork relative to the reinforcement structure.

Before directly laying the mortar, the quality of the lubricant and the cleanliness of the formwork surfaces must be checked; during the laying process, the height from which the mixture is dropped, the duration and uniformity of compaction are strictly observed. The presence of voids and delamination is strictly unacceptable.

Concrete work carried out in winter requires special control measures. During their production, the absence of ice when feeding unheated aggregates into the concrete mixer, the temperature of the supplied water, the concentration of salts and the temperature of the solution at the outlet of the mixer are checked.

Documents regulating concrete work

The main documents regulating the performance of concrete work are construction norms and rules (SNiP), which determine the list and requirements for the materials used, the sequence of concrete work and the requirements for them.

Thus, according to SNiP 3.03.01−87, the use of a natural mixture of gravel and sand that is not dispersed into fractions is prohibited for preparing a concrete mixture. To ensure that a high-quality solution is prepared, the components are dosed by weight and not by volume. The SNiP for concrete work clearly defines the order of laying the components for preparing various types of solutions and the duration of their mixing.

The SNiP document also regulates the use of various types of concrete (heat-resistant, alkali- and acid-resistant, etc.) and methods of its installation, including work in conditions of high and low temperatures, as well as methods of transportation, control, acceptance and requirements for finished concrete structures .

All activities for performing concrete work must be reflected in the work execution plan (WPP).

Failure to comply with regulated standards and SNiP requirements for the quality of materials and performance of work, as well as deviations from the project, leads to a deterioration in the quality of the work performed and, accordingly, the safety and service life of concrete structures.

plita.guru

Types of concrete work

Requirements for the quality of concreting

The following requirements are imposed on finished reinforced concrete and concrete structures of all types in accordance with SNiP:

operational safety;
serviceability;
durability;
additional requirements specified in the design documentation.

acceptance/storage of building materials;
execution and installation of reinforcement structures;
production and installation of formwork;
preparatory measures for the base and formwork surfaces for concreting;
the process of preparing and transporting concrete mixture;
work on laying and compacting the mortar, as well as caring for it during hardening.

During the preparation of a concrete mixture, the accuracy of the dosage of components, the duration of the mixing process, as well as the density and plasticity of the mixture are checked. During transportation, the mixture should not separate, set or lose mobility.

At the reinforcement , in addition to the quality of the reinforcing bars, the quality of welding connections and the correct placement of reinforcement in the structure are checked. When constructing formwork, special attention is required to the correctness of its installation, the tightness of butt joints, as well as the location of the formwork relative to the reinforcement structure.

Before directly laying the mortar , the quality of the lubricant and the cleanliness of the formwork surfaces must be checked; during the laying process, the height from which the mixture is dropped, the duration and uniformity of compaction are strictly observed. The presence of voids and delamination is strictly unacceptable.

Concreting walls and partitions

Walls and partitions in collapsible formwork are concreted without interruption in sections no more than 3 m high.

When supplying concrete mixture from a height of more than 2 m, link trunks are used. Thin walls and partitions less than 15 cm thick, where it is impossible to use trunks, are concreted in tiers up to 2 m high, while on one side the formwork is erected to the entire height at once. Reinforcement is attached to this formwork. The second side of the formwork is erected to the height of one tier after concreting of the previous tier is completed. The concrete mixture is compacted using internal or external vibrators. Concreting is resumed on the next highest section of the wall or partition only after the construction of the working seam.

If it is necessary to concrete sections of walls and partitions with a height of more than 3 m without working joints, it is necessary to arrange breaks in work to allow the concrete mixture to settle. The duration of breaks should be at least 40 minutes and no more than 2 hours.

When concreting walls from above, the lower part of the formwork is first filled to a height of 10-20 cm with a cement mortar of composition 1:2 - 1:3 in order to avoid the formation of porous concrete with an accumulation of coarse aggregate in this part of the wall.

When concreting the walls of liquid storage tanks, it is necessary to continuously lay the concrete mixture to the entire height in layers no more than 0.8 times the length of the working part of the vibrator. In exceptional (emergency) cases, it is allowed to construct a working seam followed by careful treatment of its surface. The joints between the walls and the bottom of the tanks are made in the places provided for by the design.

In large tanks, the circumference is divided into sections by vertical seams and concreted sectionally, but it is better if such tanks are concreted continuously along the entire circumference.

To make the surfaces of the bottoms and walls of tanks more waterproof, iron plating is used.

The walls in the sliding (movable) formwork begin to be concreted, filling the form with concrete mixture to half its height in two or three layers with compaction with vibrators. Laying two (three) layers of concrete mixture along the entire perimeter should take no more than 3.5 hours. Then the formwork is torn off and raise (continuously) at a speed of 30-60 cm/h until the formwork is filled with concrete mixture to its entire height.

Subsequently, the concrete mixture is placed in the mold continuously in layers of 20-25 cm, not reaching the top by 5 cm. Typically, the layers of the concrete mixture to be laid are taken in height equal to the distance between the horizontal rows of reinforcement, but not more than 25 cm. The next highest layer is started lay only after finishing laying the previous one at a given height along the entire perimeter of the formwork.

To prepare the concrete mixture, cement is used with the start of setting no earlier than 3 hours and the end of setting no later than 6 hours. The water-cement ratio should be no more than 0.5 for areas with a harsh climate and 0.55 for other areas.

The grain size of coarse aggregate should be no more than 1/6 of the smallest cross-sectional size of the structure being concreted, and for densely reinforced structures - no more than 20 mm.

The concrete mixture is fed into movable forms using buckets or vibrating feeders. When filling the corners of the molds, shovels and buckets are used.

The concrete mixture is compacted with a vibrator with a flexible shaft or manually bayoneted with screws (metal rods). To avoid damage to the underlying layers of concrete, do not rest the vibrating tip against the formwork or reinforcement.

The rate of laying the concrete mixture is determined by the most advantageous operating speed for lifting the forms, eliminating the possibility of the laid concrete adhesion to the formwork and damage to the concrete upon exiting the forms. At this speed, the concrete released from the formwork is hard to the touch, but marks from the formwork panels on it are easily smoothed out. Its compressive strength is approximately 8-10 kg/cm2. The intervals between formwork lifts should not exceed 8 minutes when compacting with vibrators and 10 minutes when compacting manually.

With sliding formwork, breaks in concreting should not be allowed for more than 2-3 hours. For longer breaks, it is necessary to continue slowly lifting the forms until a visible gap appears between the concrete and the walls of the formwork. Before resuming concreting, the surface of the hardened concrete in the joint must be processed according to the rules set out in the Preparatory work section.

The surface of walls concreted in sliding formwork is rubbed down immediately after the concrete leaves the molds, using special scaffolding suspended from the forms. Concrete is rubbed with steel trowels without adding mortar, only slightly moistening it with water using a brush. At the same time, sinks are sealed and concreting defects are corrected. In windy weather and high air temperatures (30°C and above), the formwork panels are painted white, and the concrete below the panels is covered with aprons, which are continuously moistened.

Concrete science
Manufacturing technology of prefabricated reinforced concrete structures and parts
Preparation of concrete mixtures
Production of mortar mixtures
Transporting concrete mixture
Preparation of reinforcement
Formwork
Preparing molds, forming concrete and curing products
Reinforcement and forming of prestressed products
Features of the production of various types of concrete and reinforced concrete products
Concreting of various structures
Massifs and foundations
Underlayment, floor and road surfaces
Walls and partitions
Columns
Ceilings and individual beams
Arches and vaults
Tunnel linings
Shotcrete works
Underwater concreting
Concrete work in winter conditions
Production of prefabricated structures and parts from lightweight concrete
Production of prefabricated products from dense silicate concrete and concrete with clinker-free binder
Production of concrete and reinforced concrete products at landfills
General safety rules and fire prevention measures at a construction site

Documents regulating concrete work

Thus, according to SNiP 3.03.01−87, the use of a natural mixture of gravel and sand that is not dispersed into fractions is prohibited for preparing a concrete mixture. To ensure that a high-quality solution is prepared, the components are dosed by weight and not by volume. The SNiP for concrete work clearly defines the order of laying the components for preparing various types of solutions and the duration of their mixing.

All activities for performing concrete work must be reflected in plan (WPP).

Failure to comply with regulated standards and SNiP requirements for the quality of materials and performance of work, as well as deviations from the project, leads to a deterioration in the quality of the work performed and, accordingly, the safety and service life of concrete structures.

Concrete works.

2.1. The selection of cements for preparing concrete mixtures should be made in accordance with these rules (recommended Appendix 6) and GOST 23464-79. Acceptance of cements should be carried out in accordance with GOST 22236-85, transportation and storage of cements - in accordance with GOST 22237-85 and SNiP 3.09.01-85.

2.2. Fillers for concrete are used fractionated and washed. It is prohibited to use a natural mixture of sand and gravel without sifting into fractions (mandatory appendix 7). When choosing aggregates for concrete, materials from local raw materials should be used predominantly. To obtain the required technological properties of concrete mixtures and operational properties of concrete, chemical additives or their complexes should be used in accordance with mandatory Appendix 7 and recommended Appendix 8.

CONCRETE MIXTURES

2.3. Dosing of concrete mixture components should be done by weight. It is allowed to dose additives introduced into the concrete mixture in the form of aqueous solutions by volume of water. The ratio of components is determined for each batch of cement and aggregates when preparing concrete of the required strength and mobility. The dosage of components should be adjusted during the preparation of the concrete mixture, taking into account data from monitoring indicators of cement properties, humidity, granulometry of aggregates and strength control.

2.4. The order of loading components and the duration of mixing the concrete mixture must be established for specific materials and conditions of the concrete mixing equipment used by assessing the mobility, uniformity and strength of concrete in a specific batch. When introducing pieces of fibrous materials (fibers), it is necessary to provide a method for their introduction so that they do not form lumps and inhomogeneities.

When preparing a concrete mixture using separate technology, the following procedure must be observed:

water, part of the sand, finely ground mineral filler (if used) and cement are dosed into a running high-speed mixer, where everything is mixed;
the resulting mixture is fed into a concrete mixer, pre-loaded with the rest of the aggregates and water, and everything is mixed again.

2.5. Transportation and supply of concrete mixtures should be carried out using specialized means that ensure the preservation of the specified properties of the concrete mixture. It is prohibited to add water at the site of laying the concrete mixture to increase its mobility.

2.6. The composition of the concrete mixture, preparation, acceptance rules, control methods and transportation must comply with GOST 7473-85.

2.7. Requirements for the composition, preparation and transportation of concrete mixtures are given in table. 1.

1. SNiP requirements for concrete

Parameter	Parameter value	Control (method, volume, type of registration)
SNiP 3.03.01-87 Concrete work.
1. Number of fractions of coarse aggregate at grain size, mm:	Measuring according to GOST 10260-82, work log
up to 40	At least two
St. 40	At least three
2. Largest aggregate size for:
reinforced concrete structures	No more than 2/3 of the smallest distance between reinforcement bars
slabs	No more than 1/2 the thickness of the slab
thin-walled structures	No more than 1/3-1/2 of the thickness of the product
when pumping with a concrete pump:	No more than 0.33 internal diameter of the pipeline
including grains of the largest size, flakie and needle-shaped	No more than 15% by weight
when pumping through concrete pipelines, the content of sand with a particle size of less than, mm:	Measuring according to GOST 8736-85, work log
0,14	5 — 7 %
0,3	15 — 20 %

Source: SNiP 3.03.01-87
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