Concrete heating temperature in winter snip. Warming up concrete in winter. Basics of winter concreting
25. Technology of production of concrete works in winter time
A feature and requirement for winter concreting is the creation of such a mode of laying and hardening of concrete, in which, by the time of freezing, it acquires the necessary strength, called critical. The limits of such strength are indicated in SNiP.
Methods for laying concrete in winter determined by the methods used to maintain it. In practice, both non-heated curing methods (the thermos method) and methods of artificial heating or heating of structures (electrical heat treatment of concrete, the use of heating formwork and coatings, heating with steam, hot air or in greenhouses) are used.
1 TO general practices curing accelerations include: the use of highly active cements; minimum W/C value; high frequency of raw materials; long mixing time; thorough compaction of the concrete mixture.
2. Application of antifreeze additives (sodium chloride in combination with calcium chloride, sodium nitrate, potash, etc.), providing hardening at low temperatures. This allows you to transport the mixture in non-insulated containers and lay it in the cold. The mixture with antifreeze additives is placed in structures and compacted in compliance with general rules concrete laying.
3. Heating of materials at the place of concrete preparation (thermos method): heating of raw materials with steam (in stacks in a warehouse, in intermediate bunkers, in service bunkers); insulated formwork (boards 40 mm thick and 1…2 roofing layers, double hollow formwork with a layer of sawdust, etc.); electric heating of the concrete mixture before laying in special tubs.
4. Heating of concrete at the place of laying in blocks: electric heating (surface and deep electrodes, in thermoactive formwork, electric heaters). Electrode heating of concrete is provided through electrodes located inside or on the surface of concrete. Adjacent or opposite electrodes are connected to wires of different phases, as a result of which an electric field arises between the electrodes in concrete, heating it. The current in reinforced structures is passed with a voltage of 50-120 V, and in non-reinforced structures - 127-380 V. When the current passes, the concrete heats up and within 1.5-2 days. acquires stripping strength; heating in greenhouses and tents (the air is heated inside the tent) is an effective and progressive way winter concreting; heating warm air from heaters; steam heating with special formwork.
26. Defects in concrete masonry and ways to eliminate it. Concrete care
Reasons for the appearance of defects in laying the concrete mix: non-compliance of the concrete mix with the requirements of GOST or the conditions of the laying unit (dimensions, reinforcement); violation of concrete laying technology.
Laying defects: shells, stratification of concrete, sagging, sponginess of the surface, hairline cracks. Sinks - voids in the block, not filled with concrete or filled with lean concrete (gravel without cement mortar). The reasons for their appearance are the arrival at the place of laying concrete containing gravel of unacceptable fineness in terms of the size of the block and the density of its reinforcement; due to the outflow of cement mortar through the cracks in the formwork and at the joints of the formwork; due to poor sealing. Most often they appear in difficult-to-work parts of the blocks. External shells are detected when demoulding, but inside the block they cannot be detected.
To eliminate internal sinks, grouting is used by injecting cement mortar with mortar pumps through holes made in concrete. The outer shells are scraped, the lean porous concrete is removed to sound concrete and sealed with concrete containing fine gravel.
The reasons for the separation of concrete are excessively long vibration during compaction, dropping it into a block with high altitude. The delamination defect cannot be eliminated. Placed concrete with such a defect must be removed and replaced.
Cement laitance flows and porous concrete surface appear at the junction between the concrete surface and the formwork as a result of leakage of cement laitance during compaction of the overlying layers of concrete and pinching of air bubbles. They are eliminated when preparing the surface of a building block for concreting an adjacent block.
Hair cracks in concrete appear as a result of its shrinkage and indicate the irrational composition of the concrete mixture (in particular, excess cement), oversized building blocks and high thermal stresses or bad care(rapid desiccation). This defect is irreparable.
The elimination of removable defects consists in cutting out low-quality concrete, cleaning the cut-out place from dirt, dust to sound concrete and preparing the surface in the same way as in a construction joint. Concrete newly laid in a defective place must be cared for in accordance with the rules set forth earlier until it reaches the desired strength.
Concrete maintenance is to protect it from mechanical damage, premature loads, to keep it moist, to remove excess heat from large blocks, to maintain positive temperatures in winter, to prevent premature removal of formwork. Without care and with poor care of hardening concrete, a sharp decrease in its strength is observed. Freshly laid concrete should be protected from walking and driving on it, as well as from shaking during the operation of construction machines, until the initial strength is obtained within 10 ... 12 hours.
In the first days after laying, it should be in a warm and humid environment. Best Temperature hardening 15...20°С. Therefore, at the stage of concrete care, it is watered, covered from the sun with straw mats, matting, tarpaulins.
Moisturize concrete from hoses with a scattered stream in the form of rain. This operation begins immediately after it has been established that cement particles will not be washed out of the set concrete when exposed to water.
Concrete is poured at air temperatures above 5 ° C, starting it at normal conditions after 10...12 hours, and in hot dry weather after 2...4 hours after laying and continuing for 3...14 days with an interval of 3 to 8 hours. Water consumption for irrigation is at least 6 l / m 2.
While the concrete is in the formwork, it is wetted. After stripping, moisten and protect the stripped surface. At temperatures below 5 ° C, watering is stopped and the concrete is covered with matting or tarpaulin.
Concrete care is greatly simplified when it is covered with moisture-proof films, painted in 1 ... 2 layers with one of the following materials: bitumen or tar emulsions, oil-bitumen solutions, ethinol varnish, synthetic rubber latex, etc. Film-forming materials are applied to the dried surface of the laid concrete. Consumption of materials from 300 to 700 g/m 2 . After the layer has dried, the surface of the concrete is covered for 20–25 days with a layer of sand 3–4 cm thick.
Coating with film-forming materials is allowed only in structural joints and on the uppermost exposed part of the concrete structure. In construction seams, painting is unacceptable.
When performing construction, it often becomes necessary to concrete foundations, reinforcement or other sections in winter season. In this case, it is necessary to prevent the water contained in the concrete from freezing. If this happens, the ice crystals will significantly reduce performance characteristics material and its strength.
Fundamental rules
In order for winter concreting to be successful, and the quality of concrete not to deteriorate, it is necessary to adhere to several basic rules for conducting the process in the cold season:
- First of all, you should use special antifreeze additives, which will prevent freezing and increase its strength.
- In the absence of additives, the concrete mixture should be diluted only with heated water, and the prescribed methods should be used to ensure high quality structures.
- Machines that will transport concrete in the cold season must be insulated.
- The base for concrete before starting work must be thoroughly cleaned of dust and dirt and heated.
- Snow and ice should be removed from reinforcement and formwork to be used in the concreting process. If the reinforcement has a diameter of more than 25 mm or is made of a rolled profile, at an air temperature below -10 degrees it is heated until it acquires a positive temperature. The same operation must be carried out with large metal embedded parts.
- Concreting work must be carried out at an accelerated pace, continuously, to prevent cooling of the concrete layer laid in the first place.
- After pouring concrete, its entire surface must be insulated wooden shields or mats.
Compliance with these uncomplicated conditions will allow you to get high-quality concreting, maintaining strength and reliability.
Concrete curing methods
Modern construction uses several curing methods concrete mortar at sub-zero temperature, which should be considered sufficiently effective and cost-effective.
Winter concreting methods can be divided into 3 groups:
- the thermos method, based on the conservation of heat introduced into the concrete solution during its manufacture or before pouring into structures;
- electrical heating, carried out by contact, induction or infrared heaters after laying the solution;
- the use of special chemical antifreeze agents, with the help of which the effect of lowering the eutectic point of the water present in the mixture is achieved.
These methods, when concreting in winter, can be used individually or combined if necessary. The choice of the method used when carrying out construction work is influenced by such factors as the massiveness and type of structure, the composition and required strength of concrete, natural conditions at certain times of the year, the equipment of the construction site with one or another type of power equipment and some others.
So, for example, the thermos method is recommended when working with highly exothermic Portland fast-hardening cements. It is they that have the greatest heat release, providing a high heat content of the created structure. At the same time, the curing of the concrete solution on the basis of the method can be carried out in combination - "thermos with additives", where it occurs due to chemical accelerators, or according to the "hot thermos" method, where serious electrical power is required to heat concrete to high positive temperatures.
In contrast to the thermos method, artificial heating of the concrete mortar involves not only raising the temperature of the laid material to the maximum allowable, but also maintaining it for the time necessary for the concrete to gain a given strength. Usually the method artificial heating used when working with structures that have high level massiveness, where the specified strength cannot be obtained only when using the thermos method.
Antifreeze chemicals are added to concrete solutions in an amount of 3 to 16%, depending on the desired result and the mass of the mixture, and provide stable hardening of the material at negative temperature. As a rule, the choice of the type of additives depends on the type of construction, the amount of fittings used, the presence of stray currents and aggressive media, as well as the temperature at which the process takes place.
To date, the following agents are used as antifreeze additives:
- sodium nitrite;
- calcium chloride combined with sodium nitrite;
- calcium chloride combined with sodium chloride;
- calcium nitrate-nitrite in combination with urea;
- calcium nitrate in combination with urea;
- calcium nitrite-nitrate in combination with calcium chloride;
- nitrate-nitrite-calcium chloride in combination with urea;
- potash.
Besides, in modern construction in the cold season, the antifreeze additive sodium formate is often used, but its use is limited in prestressed structures with steel reinforcement intended for use in gas or aquatic environments with air humidity over 60%. It should be noted that the use of this additive is prohibited in the construction of structures with reactive silica or used in industrial plants that consume direct current.
It should be added that all chemical additives are strictly forbidden to be used during concreting. reinforced concrete structures electrified railways and industrial enterprises, where the occurrence of a stray electric current is observed.
Warm-up methods
All of the above methods have been successfully applied on extensive and well-equipped construction sites. Some of them require the organization of a rather costly additional equipment or equipment.
In conditions of small construction works foundation concreting country house, greenhouses or paving, not all of the proposed methods look appropriate. In this case, winter concreting may be accompanied by such actions as the construction of a temporary shelter at the work site, where the required area will be heated with a heat gun, or the use of PVC film and other warming materials.
Covering the concrete mixture is recommended in cold weather at temperatures from -3 to +3 degrees. PVC film and other insulation allow you to accumulate heat inside concrete structure, which leads to faster solidification and hardening of the solution.
If the air temperature reaches -5 to -15 degrees, experts recommend using electric or gas heat guns. They are set up as follows:
- on the wooden frame a layer of PVC film is strengthened, creating a reinforcement in the form of a tent;
- heat guns are installed in the tent.
The higher the temperature in the tent, the faster the concrete mixture will set, and, accordingly, the shorter the warm-up time.
As a rule, for the acquisition of primary strength by concrete, which allows further work, enough warming up for 1-3 days.
Guidelines
So, you need to work on laying concrete on your suburban area. What algorithm of actions should be chosen so that concreting in winter conditions is successful?
The first step is to purchase concrete. In addition, it is allowed independent production concrete mix. To prepare the M200 grade material, you will need:
- 3 parts of M500 cement (it is forbidden to use wet cement or having a solid state);
- 5 parts of sand (both quarry and washed sand are allowed; the use of sand with clay or other additives is strictly prohibited);
- 7 parts of crushed stone (it is recommended to use washed gravel crushed stone with fractions from 5 to 20 mm; the use of lime crushed stone, as well as pebbles and unwashed crushed stone is prohibited);
- water (should make up about 25% of the whole mixture).
To use concrete in the winter, chemical antifreeze elements and plasticizers can be added to it.
If the average daily temperature during the execution of work is not more than -5 degrees, the following steps must be taken:
- Carefully check all material used for the preparation of the concrete mixture - crushed stone, sand and water - for the absence of snow and ice and in without fail warm them up.
- Arrange a frame of lumber and cover it with insulating material, creating a tent.
- Check the tent for cracks through which cold air can enter.
- If the tent meets all the necessary requirements, you can connect heat gun or heat generator.
- should be carried out until it acquires a light white color. The mixture should be warm to the touch, indicating a setting and curing reaction. If the concrete has turned dark gray, this indicates that it has frozen and lost its characteristics. Such a solution must be gouged and the concreting work must be done again.
What to do if the re-concreting process is not possible? In this case, carefully cover the structure with PVC film. This will keep the top layer of concrete intact during frosts and thaws. Perhaps in the spring the concrete will be able to continue the hydration process. Of course, its strength will become as low as possible, but doing this is better than just leaving the structure in the rain and snow.
Concrete is a very popular building material today, for the manufacture of which components such as cement, water, aggregate and water are used. But it's one thing when you pour concrete in the summer, because the warm season favorably affects the process of curing. What happens in winter? At severe frosts the set of strength characteristics is terminated, and this is highly undesirable. In this case, it is necessary to apply a number of measures that will allow the concrete to warm up. To do this, you need to know all the features of the concrete flow chart for the winter period and actual ways warming up.
Technological map and methods of heating concrete
Warm up with a welding machine
This heating method involves the use of the following materials:
- pieces of reinforcement;
- incandescent lamps and a thermometer for measuring temperature.
The process of installing pieces of fittings is carried out in parallel with the circuit, with adjacent and straight wires, between which a pouring lamp is mounted. It is thanks to her that it will be possible to make voltage measurements.
Use a thermometer to measure temperatures. In terms of time, this process takes a long time, about 2 months. At the same time, for the entire heating process, it is necessary to protect the structure from the influence of cold and water. Apply heating welding machine suitable for small volumes of concrete and excellent weather conditions.
infrared method
The meaning of this method is that equipment is being installed, the operation of which is performed in the infrared range. As a result, it is possible to convert radiation into heat. Exactly thermal energy embedded in the material.
Infrared heating of the concrete mixture is an electromagnetic oscillation, in which the wave propagation speed will be 2.98 * 108 m / s and a wavelength of 0.76-1, 000 microns. Very often, tubes made of quartz and metal are used as a generator.
Main Feature presented technology is the possibility of power supply from conventional alternating current. At infrared heating concrete power parameter may vary. It depends on the required heating temperature.
Thanks to the rays, energy can penetrate into deeper layers. To achieve the required efficiency, the heating process must be carried out smoothly and gradually. It is forbidden to work at high power levels here, otherwise upper layer will have high temperature, which will eventually lead to a loss of strength. It is necessary to use this method in cases where it is necessary to heat up thin layers of the structure, as well as prepare a solution to speed up the coupling time.
What are the pros and cons of a house made of aerated concrete, indicated in this
Induction method
To implement this method, it is necessary to use alternating current energy, which will be converted into thermal energy in the formwork or reinforcement made of steel.
After the converted thermal energy will be distributed to the material. It is advisable to use the induction heating method when heating reinforced concrete frame structures. It can be crossbars, beams, columns.
If you use induction heating of concrete according to external surfaces formwork, then it is necessary to install successive turns, which are isolated from inductors and wire, and the number and pitch are determined by calculation. Taking into account the results obtained, it is possible to produce templates with grooves.
When the inductor has been installed, it is possible to heat the reinforcing cage or joint. This is done in order to remove frost before concreting takes place. Now exposed formwork and structure surfaces can be covered with thermal insulation material. Only after the arrangement of the wells can you start direct work.
When the mixture reaches the required temperature, the heating procedure is stopped. Make sure that the experimental indicators differ from the calculated ones by at least 5 degrees. The cooling rate can keep its limits of 5-15 C/h.
Application of transformers
To increase the temperature regime in concrete, you can use such an inexpensive and simple method as heating wire PNSV.
The design of this cable includes two elements:
- solid core round shape made of steel;
- insulation, for which PVC plastic or polyethylene can be used.
If you need to heat a mixture of 40-80 m3, then it will be enough to install only one transformer substation. This method is used when the outside air temperature has reached -30 degrees. It is advisable to use transformers for heating monolithic structures. For 1 m of weight, a wire of 60 m will suffice.
Which manufacturers of autoclaved aerated concrete exist are indicated in this
Such a manipulation is performed according to the following instructions:
- A heating wire is laid inside the concrete. It is connected to the station or transformer terminals.
- With help electric current the mass begins to gain temperature, as a result of which it manages to solidify.
- because the material has excellent properties conductivity of thermal energy, heat with high speed begins to move throughout the array.
Table 1 - Characteristics of wires of the PNSV brand
| 1 | AC voltage, V | 380 |
| 2 | Cable section length for voltage 220 V: | |
| – PNSV1.0 mm, m | 80 | |
| – PNSV1.2 mm, m | 110 | |
| – PNSV1.4 mm, m | 140 | |
| 3 | Specific heat dissipation power of the cable: | |
| – for reinforced installations, W/r.m. | 30-35 | |
| – for non-reinforced installations, W/r.m. | 35-40 | |
| 4 | Recommended supply voltage, V | 55-100 |
| 5 | Average core resistance value: | |
| – PNSV1.2 mm, Ohm/m | 0,15 | |
| – PNSV1.4 mm, Ohm/m | 0,10 | |
| 6 | Method parameters: | |
| – Specific power, kW/m3 | 1,5-2,5 | |
| – Wire consumption, lm/m3 | 50-60 | |
| – Cycle of thermal curing of structures, days | 2-3 |
The heating wire, which is laid inside the concrete, should heat the structure up to 80 degrees. Electrical heating occurs with the help of transformer substations KPT TO-80. Such an installation is characterized by the presence of several stages of low voltage. This makes it possible to adjust the power heating cables, as well as adjust it according to the changed air temperature.
Cable use
The use of this heating option does not require high costs electricity and accessories.
The whole process proceeds as follows:
- The cable is being installed concrete base before pouring the solution.
- Fix everything using fasteners.
- Be careful during cable installation and operation to avoid damage to its surface.
- Connect the cable to the low voltage electrical cabinet.
Antifreeze additives
With the addition of antifreeze additives, concrete is able to withstand the most aggressive atmospheric precipitation. The components included in such a mixture can be very different, but the role of the main one is assigned to antifreeze. It is a liquid that does not allow water to freeze.
If it is necessary to cock structures made of reinforced concrete, then the mixture should contain sodium nitrite and sodium format. Main Feature antifreeze mixtures remains the preservation of anti-corrosion and physico-chemical properties at low temperatures. 
When erecting ready-mixed concrete, the production of curbs, it is necessary to use a mixture that contains calcium chloride. This component allows you to achieve a fast curing speed, resistance to low temperature regime.
The ideal antifreeze additive remains Chemical substance like potash. It dissolves very quickly in water, and there is no corrosion. If you use potash when heating concrete in winter, you will be able to save on building materials.
If you use antifreeze additives, it is very important to adhere to all safety standards. For example, you should not use concrete with such components when the structure is under tension, monolithic chimneys are being erected.
SNiP
All installation and construction activities must be carried out in accordance with established norms. The process of concreting in winter is no exception. Heating up the concrete structure low temperatures ah air occur according to the following documents:
- SNiP 3.03.01-87 - Bearing and enclosing structures
- SNiP 3.06.04-91 - Bridges and pipes
On the video - warming up concrete in winter, technological map:
Despite the fact that the presented documentation only indirectly touches on the topic related to the heating of concrete, it contains certain sections in which there is a technology for pouring concrete mortar in the frosty season.
Timing
When calculating the heating of concrete, it is necessary to take into account such factors as the type of structure, total area heating, volume of concrete and electric power.
During heating work with concrete, it is worth developing a technological map. It will contain all the values of laboratory observations, as well as the warm-up time and the hardening time of the material. 
The calculation of concrete heating begins with the choice of a scheme. For example, most often choose a four-stage. The first stage involves the curing of the material. After that, the temperature indicators are increased to a specific value, heating and cooling are carried out, the duration of exposure before the start of the event is approximately 1-3 hours at low temperature conditions. After this, you can proceed to the calculation of heating, which is directly dependent on the speed and final temperature.
Throughout the process, it is worth monitoring the temperature, noting all the results with an increase in 30-60 minutes, and when cooling down, control is carried out 1 time per shift. If the mode is violated, it is necessary to maintain all parameters by turning off the current and increasing the voltage. In this case, the actual indicators and those obtained during the calculation may not coincide. After that, a graph of the dependence of time on strength is built, where they denote required value heating time and temperature, and then find the required strength value.
The concrete heating process is very important events, without which the concrete structure will simply cease to gain strength during frost, as a result of which this will lead to a decrease in the grade and further destruction. It is not difficult to carry out all these activities, it is enough just to determine which of the presented ones suits you best.
Methodological documentation in construction
ZAO TsNIIOMTP
WINTER CONCRETING
WITH THE APPLICATION OF HEATING WIRES
MDS 12-48.2009
Moscow 2009
This methodological document contains information about winter concreting with the use of heating wires: technical requirements for heating wires and power electrical equipment, methodological provisions for calculating and selecting the parameters of the concrete heat treatment mode, recommendations for organizing work, rules and techniques for performing technological operations, norms and evaluation procedures the quality of work. Examples of concreting of typical structural elements of a building are given: columns, walls and ceilings.
The information contained in the document can be used to draw up technological documents for winter concreting: projects for the production of works, technological maps, technical regulations etc.
The methodological document is intended for design and construction organizations and construction specialists involved in the production of concrete work in winter conditions.
The methodological document was developed by employees of CJSC "TsNIIOMTP" - candidates of tech. Sciences V.P. Volodin and Yu.A. Korytov.
INTRODUCTION
Winter concreting includes work performed at an average daily outdoor temperature below 5°C and a minimum daily temperature 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 is mastered to a temperature of minus 15-20°C.
For concrete to gain the required strength, special measures are taken to prepare and produce concrete work in winter.
For winter concreting, special concretes with chemical antifreeze and plasticizing additives are used.
During work, freshly laid concrete is heated different ways using steam, heated water or electricity.
Freshly laid concrete is protected from heat loss (thermos method), covering various heaters(mats, bedspreads, panels).
Special measures, in particular for the insulation of working bodies and concrete pipelines, are carried out in the preparation of machines and technological equipment for winter concreting.
The main requirement for winter concreting is to create favorable conditions for the concrete to acquire the required design strength in a short time.
Massive monolithic structures ( foundation slabs and blocks) with cooling surface module M n from 2 to 4 are concreted using the thermos method using fast-hardening cements, hardening accelerators and antifreeze 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, floors, walls) with a cooling surface modulus of 6-12 are concreted using the methods mentioned above using heating wires, thermosetting flexible coatings (TAGP), heating flat elements (HEP).
This document discusses the method of winter concreting using heating wires. This method has a number of advantages in comparison with heating with water vapor, hot water, infrared irradiation. The effectiveness of the method is increased in combination with other measures and methods of winter concreting mentioned above: the use of high-quality concrete with chemical additives, insulation, preparation of machines and technological equipment.
The use of heating wires makes it possible to erect buildings and structures that do not differ in their strength from those erected in the summer.
This document contains guidelines and examples that allow you to select methods of work (modes, techniques) and materials for winter concreting for a particular construction site, taking into account local conditions and features of the construction organization. The choice of the method of work and materials is made at the stage of development of the project for the production of works (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 may be useful in licensing construction organization(companies) for the performance of this type of work, when certifying the quality management system, when certifying the quality of winter concreting,
The document is based on research work carried out at TsNIIOMTP and other institutions of the construction industry, as well as a generalization of the experience of winter concreting of Russian construction organizations.
When developing the document, normative and methodological documents were used, the main of which are given in Section 2.
1 AREA OF USE
The document applies to winter concreting using monolithic reinforced concrete heating wires. building structures(slabs, walls, ceilings, columns, etc.) with a cooling surface modulus of 4-10, in the construction and repair of residential, public and industrial buildings and structures.
Winter concreting using heating wires is carried out at an ambient temperature, as a rule, down to minus 20°C.
The document is used for the development of projects for the production of works (technological maps), for the certification of monolithic reinforced concrete structures and for the licensing of organizations that perform winter concreting.
The application of the document helps to ensure the design strength of monolithic reinforced concrete structures erected in winter conditions.
2 REGULATORY AND METHODOLOGICAL DOCUMENTS
|
Thermal insulation materials |
Heat transfer coefficient To, W/( m 2 ° С ), at wind speed, m/s |
||
|
Polyfoam (PVC) 120 mm thick |
|||
|
Pine sawdust 100 mm thick |
|||
|
Mineral wool slabs, mm: |
|||
|
Slag layer thickness 150 mm |
|||
|
Boards wooden thickness, mm: |
|||
4.3.2 As a heater for open concrete surfaces in addition to those given in table 5, expanded clay, perlite, covelite slabs, peat slabs, reeds and other heat-insulating materials are also used.
To insulate formwork panels, poured thermal insulation based on, for example, polyurethane foam and phenolic plastic can be used.
The same thermal insulation materials are used for shelter metal frame formwork and ribs, which are, as you know, "cold bridges".
4.4 Truck-mounted concrete pump and concrete pipeline
4.4.1 The preparation of the working bodies of the concrete pump (bunker, other units) and the concrete pipeline consists, first of all, in warming them with heat-insulating materials. Insulation should be such that the heat loss of the concrete mixture during its loading into the bunker, transportation and laying in the formwork is minimal and ensures the temperature of the mixture specified by the project during laying.
The hopper of the concrete pump truck is regularly cleaned and protected from snow and wind.
In some cases (for example, at outdoor temperatures down to minus 5°C, when concreting secondary structures), the concrete pump can be used without winter training, that is, in the summer version.
4.4.2 Preparation for winter of other organs, components and assemblies of the concrete pump truck is carried out during the seasonal Maintenance, which includes scheduled operations for the replacement of oils and working fluids, adjustment and other operations to ensure the smooth operation of the concrete pump truck in winter.
4.4.3 Before starting the operation of the concrete pump truck (transportation and laying of the concrete mixture), the concrete pipeline is heated with warm air, steam or hot water.
The bunker of the concrete pump and the concrete pipeline after work is cleaned warm water. The water remaining after cleaning is completely removed.
4.4.4 At the initial moment of operation of the truck-mounted concrete pump, the temperature of the starting solution and the concrete mixture that filled the concrete pipeline must be at least 30 ° C.
The temperature of the concrete mixture during the laying process must correspond to the temperature specified by the project.
With an insulated concrete pipeline, an unintentional stop of the concrete pump truck is allowed for up to 30 minutes. With a longer stop, it is necessary to remove the concrete mixture from the concrete pipeline.
5 CONCRETE HEAT TREATMENT TECHNOLOGY
5.1 Before starting work on laying the heating wires, as a rule, formwork and reinforcement work must be completed. In some cases, it is advisable to lay the heating wires simultaneously with the reinforcing and formwork work.
As part of winter concreting, the following preparatory and basic work is performed.
Carry out preparatory work on the organization of the workplace and equipping it with means of labor and technological equipment, to create safe conditions labor. Arrange a workplace fencing, carry out an alarm and lighting. Power equipment is installed on a flat solid platform and along the grip - electrical wiring sections. The heating wires are connected to the electrical wiring sections, and the sections to the transformer.
The main works of winter concreting (heat treatment of concrete) are carried out after the completion of concrete laying. Exposed concrete surfaces are covered with a waterproofing film, heat-insulating material and voltage is applied to the heating wires.
The rate of cooling of concrete is usually assumed to be 2.0-3.0°C/h.
5.3 To ensure, at a given ambient temperature and wind speed, a given mode of heat treatment of a reinforced concrete structure, characterized by a surface modulus, a class of concrete with a known cement consumption, a temperature of the concrete laid in the formwork, the electrical parameters of concrete heating are determined by the parameters of the existing formwork and insulation, wires and power equipment: heat transfer coefficient, specific heating power of the concrete structure, linear electrical load, wire pitch and length.
5.4 Heat transfer coefficientKdetermined by (including using linear interpolation or extrapolation) or by the formula
where
α λ \u003d 2.1 - 3.2 W / (m 2 ° C) - coefficient of heat transfer from the formwork by radiation;
δ i = 0.015 - 0.1 m - thickness of the layer of heat-insulating material;
λ i \u003d 0.02 - 0.8 W / (m 2 ° C) - thermal conductivity coefficient of the heat-insulating material;
α to \u003d 20.0 - 43.0 W / (m 2 ° С) - heat transfer coefficient by convection:
at wind speed up to 5 m/s α k \u003d 20.0 W / / (m 2 ° C),
at 10 m/s α k \u003d 30.0 W / (m 2 ° C),
at 15 m/s α k \u003d 43.0 W / (m 2 ° C).
Calculation examples To shown in .
5.5 Specific heating power of the concrete structure R ud is determined by the ratio of the total power R heating to the heated area of the concrete structure. The specific power required to heat concrete to a given temperature is determined. Specific power depends on the difference between the heating temperature of the concrete and the outside air ∆T, °С, massiveness of the heated structure, characterized by the modulus of the cooled surface M n, from the heat transfer coefficientKand cement content in the concrete mix C.
Theoretically, the difference between the heating temperature of concrete and outdoor air ∆T, °С, can range from minus 40 to plus 80, i.e. 120°С; in practice, it ranges from minus 20 to plus 50, that is, 70 ° C. Cooled surface module has practical value in the range from 4 to 10 m -1 ; this range includes typical foundation slabs, columns, floors, walls and ceilings. The heat transfer coefficient, depending on the type of heat-insulating materials used, as well as the thickness and designs of heaters, wind speed, varies over a wide range: from 0.2 to 6.0 W / (m 2 ° C); for insulated formwork panels, it does not exceed 3.0 W / (m 2 ° C). Since concrete hardening is an exothermic process, the more cement, the less is required. electric power for concrete heating. So, with an increase in the cement content in the winter concrete mixture by a factor of two (from 200 to 400 kg/m3), the required specific heating power is reduced, all other things being equal, from 960 to 600 W/m2, that is, by 37%. The dependence of the specific heating power of concrete on the considered parameters was established experimentally and presented in the form of a nomogram (Fig. 1).
5.6 with a diameter of a steel current-carrying core of 0.6-3.0 mm is specified experimentally from the interval: for reinforced structures 30-35 W / m, for unreinforced structures 35-40 W / m. With a linear electrical load of more than 40 W / m, the temperature of the wire exceeds 100 ° C, which leads to structural failures in concrete and a decrease in its strength. In addition, the electrical insulation of the wire may be broken and a short circuit to the fittings and embedded parts may occur.
5.7 The pitch and length of the wires must create such a density of their laying, which ensures the necessary uniformity of heating of the concrete in the structure.
Wire pitch bdetermined by the formula
The length of the wires, depending on the linear electrical load, the diameter of the wires (current-carrying core) and the operating voltage, can be roughly determined from the nomogram in Fig. 2 and refined in terms of the shape and dimensions of the structure.
The wire pitch is selected from the range of 50-150 mm. For structures in contact with the ground, a pitch of 150-200 mm can be taken. At the joints of the elements, in the sauces for columns and equipment, in local terminations, the wire pitch is reduced to 25-70 mm.
The length of the wires must be a multiple of the height of the walls, columns, foundations and the width of the floors.
Examples of determining the pitch and length of wires are given in.
Between straight lines 2 and 4 heat transfer coefficientsK, W / (m 2 ° C), we draw a visually straight line equal to 3.6 W / (m 2 ° C).
∆ T= 60°С with ordinate M n = 8.0 m -1 module surface of the column. From this point we draw a horizontal line until it intersects with the mentioned straight line, equal toK\u003d 3.6 W / (m 2 ° C).
C\u003d 350 kg / m 3.
The projection of the obtained point onto the ordinate of the specific heating power of the wire indicates R beats \u003d 320 W / m 2.
Heating wire pitch (b) is determined by
b= 1/(R beat / R+1) \u003d 1 / (320/33 + 1) \u003d 0.09 \u003d 0.1 m,
where R\u003d 33 W / m - specific load on the wire from the recommended interval R= 30-35 W/m for reinforced structures.
Wire length L, required for winding according to the scheme, G on the reinforcing cage with a step of 10 cm, is
L = 2(BUT + B)With/ b\u003d 2 (0.5 + 0.5) 7.5 / 0.1 \u003d 150 m.
dd= 1.2 mm.
R\u003d 33 W / m we draw the ordinate to the point of intersection with the curve, then from this point horizontally we find the point of intersection with the curvedU, C. The projections of the intersection points on the ordinate of the length of the heater allow you to choose the length of the heaterl, m. The closest value is the length of the heater 25 m at operating voltageU= 55 V. Thus, 6 heaters of 25 m each are laid on the cooling surfaces of the column.
The specific consumption of the wire (per 1 m 3 of concrete) will be 150.0 / 1.87 ≈ 80.0 m.
The mode of heat treatment of concrete will be determined taking into account the recommendations and provided that the strength of concrete is at least 70%R 28 . The duration of heating at a heating rate of 4.0 ° C / h is at least 6 hours, isothermal exposure at +40 ° C according to the schedule (see) - 60 hours and cooling to zero at a cooling rate of 2.0 ° C / h - not less than 20 hours
Similar calculations were performed at air temperatures of -10 and -15°C.
The main parameters of the heat treatment of concrete in the column are summarized in the following table 6.
Table 6
|
Air temperature, °С |
Specific heating power R beat, W / m 2 |
Heater step b, mm |
Wire diameter d, mm |
Heater length, m |
Voltage U, AT |
6.2 Wall
Concreting (concrete class B15, cement consumption 350 kg / m 3) walls with dimensions A´ AT ´ C (3000 ´ 500 ´ 6000 mm) is produced in an inventory steel formwork with a board size of 2000´ 1000 mm, insulated mineral wool boards 60 mm thick. Heating wire PNSV 1 is provided for heat treatment of concrete´ 1.4 and transformer substation type KTPTO-80-86 VI
The temperature of the concrete mixture laid in the formwork is +5°С;
The average outdoor temperature during the day is -15°С;
Wind speed 3 m/s;
The temperature of isothermal curing of concrete is +45°С.
It is assumed that heat losses through the upper and lower surfaces of the wall are insignificant (the upper open surface is securely covered with heat-insulating material) and therefore are not taken into account.
Wall cooling surface module M n equals
M n = F/ V\u003d 39.0 / 9.0 \u003d 4.3 m -1.
Heat transfer coefficient To formwork is determined by the formula (1)
K = 1/(1/ α λ + å δ i / λ i + 1/ α to ) \u003d 1 / (1 / 2.8 + 0.06 / 0.6 + 1/25) \u003d 2.0 W / (m 2 ° C),
where
α λ
δ i = 0.06 m - thickness of the layer of heat-insulating material;
λ i \u003d 0.6 W / (m 2 · ° С) - thermal conductivity of the heat-insulating material;
α to \u003d 25.0 W / (m 2 · ° С) - heat transfer coefficient by convection at a wind speed of 3 m / s.
Find the temperature difference between heated concrete and outside air ∆ T, which is
∆ T\u003d 45 - (-15) \u003d 60 ° C.
R beats are determined by the nomogram fig. one.
Finding the point of intersection of the line ∆ T= 60°С with ordinate M n = 4.3 m -1 module of the wall surface. From this point we draw a horizontal line until it intersects with the straight line of the heat transfer coefficient equal toK\u003d 2.0 W / (m 2 ° C).
We lower the perpendicular from this point to the straight line of cement consumption C\u003d 350 kg / m 3.
R beats \u003d 250 W / m 2.
Heating wire pitchbdetermined by formula (2)
b= 1/(R beat / R + 1) = 1/(250/34 + 1) = 0.12 m,
where d= 1.1-1.4 of the recommended range R= 30-35 W/m for reinforced structures.
Wire length Lrequired for winding according to the scheme of Fig. 3, in on the reinforcing cage with a step of 12 cm, is
L = 2BUT(With + AT)/ b\u003d 2 3 (6 + 0.5) / 0.12 ≈ 324 m.
From a point on the abscissa of the specific load Rd= 1.4 mm. We lower the perpendicular from this point to the operating voltage curvesU, V. The projections of the intersection points on the ordinate of the heater length make it possible to select the heater length. The closest value is a heater length of 27 m at operating voltageU= 58 V. Thus, 12 heaters of 27 m each are laid on the cooling surfaces of the wall.
The specific consumption of the wire (per 1 m 3 of concrete) will be 324.0 / 9.0 \u003d 36.0 m.
The mode of heat treatment of concrete will be determined taking into account the recommendations of section 5.2 and provided that the concrete strength is at least 70%R 28 . The duration of heating at a heating rate of 4.0°C/h is at least 10 hours, isothermal holding at +45°C according to the graph in Fig. 7 - 48 h and cooling to zero at a cooling rate of 2.0 ° C / h - at least 22 h.
Similar calculations were performed at air temperatures of -10 and -20°C.
Table 7
|
air temperature ha, °С |
Specific heating power R beat, W / m 2 |
Heater step b, mm |
Wire diameter d, mm |
Heater length, m |
Voltage U, in |
The main parameters of the heat treatment of concrete in the wall are summarized in the following table 7.
6.3 Overlap
Concreting (concrete class B25, cement consumption 400 kg / m 3) floors with dimensions A´ AT ´ C (6000 ´ 6000 ´ 200 mm) is made in a formwork of laminated plywood 21 mm thick. The open floor surface is insulated with mineral wool boards 80 mm thick, thermosetting flexible coatings (TAGP) or flat heating elements (HEP).
Heating wire PNSV 1 is provided for heat treatment of concrete´ 1,2 and transformer substation type KTPTO-80-86.
Concreting conditions are as follows:
The temperature of the concrete mixture laid in the formwork is +10°С;
Temperature of isothermal curing of concrete +45°С;
Outside air temperature: -16°C during the day, -20°C at night;
Wind speed 1.5 m/s.
Determining the parameters of the heat treatment mode of concrete is carried out in the following sequence.
It is assumed that heat losses through the open upper surface of the ceiling are insignificant (reliably covered with heat-insulating material) and therefore are not taken into account.
Floor Cooling Surface Module M n is equal to
M n = F/ V\u003d 40.8 / 7.2 ≈ 6.0 m -1.
Heat transfer coefficientKformwork made of laminated plywood is determined by the formula (1)
K = 1/(1/ α λ + å δ i / λ i + 1/ α to ) \u003d 1 / (1 / 2.8 + 0.021 / 0.4 + 1/20) \u003d 2.2 W / (m 2 ° C),
where
α λ \u003d 2.8 W / (m 2 ° C) - coefficient of heat transfer from the formwork by radiation;
δ i = 0.021 m - thickness of laminated plywood;
λ i \u003d 0.4 W / (m 2 · ° С) - thermal conductivity coefficient of laminated plywood;
α to \u003d 20.0 W / (m 2 · ° С) - heat transfer coefficient by convection at a wind speed of 1.5 m / s.
Finding the temperature difference ∆ T heated concrete and average temperature outside air during the day (equal to -18°C), which is
∆ T\u003d 45 - (-18) \u003d 63 ° C.
Required specific heating power of concrete R UD is determined by the nomogram.
Finding the point of intersection of the line ∆ T= 63°С with ordinate M n = 6.0 m -1 module of the floor surface. From this point we draw a horizontal line until it intersects with the straight line of the heat transfer coefficient equal toK\u003d 2.2 W / (m 2 ° C).
We lower the perpendicular from this point to the straight line of cement consumption C\u003d 400 kg / m 3.
The projection of the obtained point onto the ordinate of the specific heating power indicates R beats \u003d 300 W / m 2.
Heating wire pitchb determined by
b = 1/( P beat / R + 1 \u003d 1 / (300/34 + 1) \u003d 0.10 m,
where d= 1.1-1.4 from the recommended range p = 30-35 W/m for reinforced structures.
Wire length Lrequired for laying in the lower level of reinforcement according to the scheme, b with a step of 10 cm, is
L = B(A/b + 1) + BUT\u003d 6 (6 / 0.1 + 1) + 6 ≈ 372 m.
Between curves 1.4 and 1.1 mm wire diameterddraw a visual curve equal tod= 1.2 mm.
From a point on the abscissa of the specific load R\u003d 34 W / m we draw the ordinate to the point of intersection with the curve, then from this point horizontally we find the point of intersection with the curved= 1.2 mm. We lower the perpendicular from this point to the operating voltage curvesU, AT . The projections of the intersection points onto the ordinate of the heater length make it possible to select the heater length. The closest value is a heater length of 25 m at operating voltageU= 55 V. Thus, 15 heaters of 25 m each fit into the ceiling.
The specific consumption of the wire (per 1 m 3 of concrete) will be 372.0 / 7.2 ≈ 52.0 m.
The mode of heat treatment of concrete will be determined taking into account the recommendations and provided that the strength of concrete is at least 70%R 28 . The duration of heating at a heating rate of 4.0°C/h is at least 9 hours, isothermal holding at +45°C according to the schedule is 48 hours, and cooling to zero at a cooling rate of 2.0°C/h is at least 22 hours.
Similar calculations were performed at an air temperature of -10°C.
The main parameters of the heat treatment of concrete in the ceiling are summarized in the following table 8.
Table 8
|
Air temperature, °С |
Specific heating power R beat, W / m 2 |
Heater step b, mm |
Wire diameter d, mm |
Heater length, m |
Voltage U, AT |
|
The quality of winter concreting should ensure the design strength of monolithic concrete and reinforced concrete structures. General requirements to quality control of concrete are set out in SNiP 12-01-2004 and SNiP 3.03.01-87. The quality of winter concreting depends on the implementation preparatory work, the selected mode of heat treatment and quality control of work. Before starting the main work, it is necessary to check the operability of the equipment and the automation system, the absence of damage to the wires, and the reliability of the insulation. The heat treatment mode must be checked and, if necessary, adjusted according to the results of laboratory tests of concrete samples. Before laying wires and concreting, the quality of snow and ice removal from the base, reinforcement and formwork is checked. In the first hours of concrete heating and at least twice a day, the current and voltage in the mains are measured. Monitoring the operation of equipment, inspecting wires, cables and electrical connections in order to detect damage, sparks, etc. produced constantly. The insulation resistance of the heaters must be at least 1.0 MΩ when cold and 0.5 MΩ when hot. After concreting, the compliance with the project and the reliability of covering open concrete surfaces with waterproofing and heat-insulating materials are checked. During the heating process, the temperature of the concrete is measured at least every two hours. At least twice a shift, temperature sensor readings are taken to plot the temperature of heating, curing and cooling of concrete. The control of concrete strength development is carried out according to the temperature regime of the most critical or less heated sections of the structure.. Labor safety in construction. Part 2. Construction production; and GOST 12.4.059-89. Concrete works with electric heating should be carried out, as a rule, during daylight hours. Construction site, work site, workplace at night, they must be illuminated in accordance with the requirements of GOST 12.1.046-85 “SSBT. Construction. Lighting standards for construction sites. When supplying and compacting the concrete mixture, the formwork and supporting structures should be carefully inspected, the installation of racks and struts should be checked for reliability. When compacting the concrete mixture with electric vibrators, it is not allowed to move the vibrator by current-carrying hoses, and during breaks in work and when moving from one place to another, the electric vibrators must be turned off. The operation of the truck-mounted concrete pump and mixer truck must be carried out in accordance with the instructions of the manufacturers, set out in the operating instructions. Unite steel pipes concrete pipeline with rubber-fabric hoses is necessary with the help of bolted inventory clamps. It is necessary to ensure that the hoses with a moving concrete mixture do not have kinks. Before flushing the concrete pipeline, unauthorized persons (workers not participating in this work) must be removed at a distance of at least 10 m. Under the boom of a concrete pump, any work is prohibited. The work area of the concrete pump truck must have a fence, warning signs must be posted in front of the work area that meet the requirements of GOST R 12.4.026-2001. Below are the basic safety rules for the production of electrically heated concrete. Concrete electric heating workers must be equipped with rubber boots (dielectric galoshes) and rubber gloves. The heating wires are connected to the network after the power is turned off. In places of the fence, you should hang red bulbs that light up when voltage is applied to the wires. Reinforcement in the formwork, embedded parts, as well as metal non-current-carrying parts of the equipment are grounded by attaching to them neutral wire power cable. When using a ground loop, before turning on the voltage, measure the resistance of the loop, which should be no more than 4 ohms. Near the transformer, switchboards and knife switches are laid wooden decking covered with dielectric carpets. Do not apply operating voltage to heating wires if they are not in concrete, but in air, if they have mechanical damage or are not securely connected to cables. It is allowed, subject to the above rules, to lay and compact concrete with the wires not disconnected, if the operating voltage does not exceed 60 V and there are no wires that could be damaged in the area of the internal vibrator. Wire heaters should not be connected to a mains voltage higher than 220 V. Electrical work during winter concreting is carried out by specially trained electricians, carried out under the guidance and supervision of an engineering and technical worker appointed by order of the organization. |
Basics of winter concreting
Concrete work at an average daily outdoor temperature below 5°C and a minimum daily temperature below 0°C is carried out according to special rules established for work in winter conditions (SNiP III-15-76).
In winter conditions, the main task is to prevent premature freezing of the laid concrete. It is necessary that concrete maintain a positive temperature (above 00) during placement and curing until its strength reaches a certain value, called "critical" strength.
For structures subjected immediately after curing to alternate freezing and thawing, the critical strength of concrete, regardless of its class, must be at least 70%. and in prestressed structures - at least 80% of the design strength.
For structures that are immediately subjected to the design pressure of water (tanks, retaining walls) immediately after the end of aging, as well as structures to which special requirements in terms of frost resistance and water resistance, the critical strength must be at least 100% of the design strength.
For massive structures special purpose(dams, supports, bridges, etc.), the conditions and terms of permissible freezing of concrete are established in the project. The requirements listed above are due to the fact that concrete at a negative temperature (below 0 ° C) does not harden, since the water in it turns into ice and the physico-chemical processes of interaction between cement and mixing water practically stop. However, when the frozen concrete thaws, the hardening processes resume, and if freezing occurs not earlier than reaching its critical strength, then the concrete will subsequently acquire the specified (design) strength. If the concrete is allowed to freeze earlier, then there will be a partially irreversible loss of strength (mainly due to a violation of the adhesion between the coarse aggregate and the cement slurry).
The loss of strength will be the greater, the younger the concrete was at the time of freezing (for example, concrete on Portland cement, reaching strength on the 28th day and frozen a day after laying, irrevocably loses up to half of its strength). Concrete, frozen when it reaches the above values of critical strength, must be kept after thawing under conditions that ensure that it obtains design strength until the design load is loaded on the structure.
By the time the bearing formwork of concrete and reinforced concrete structures is removed, it is required that the strength of concrete be 50 ... 100% of the design strength. Such structures after stripping can in many cases be subjected to low temperatures without harm to them, but in each specific case it is still necessary to compare stripping and critical strength. In those cases when, from the conditions of multiple turnover of the formwork, the last one (for example, side panels foundation formwork, under-columns, walls, etc.) are removed before the achievement of critical strength by concrete, the stripped surfaces should be temporarily covered.
The same has to be done in those cases when the temperature difference between the surface of concrete and the outside air exceeds the following values: 20CC - for structures with a surface modulus of 2 to 5 and; W ° C - for structures with a surface module of 5 and more. Otherwise, during rapid cooling, temperature cracks form on the surface of the bean.
Stripping of structures is carried out at a positive temperature of concrete; in no case should the formwork be allowed to freeze to concrete.
For hardening in winter conditions of concrete prepared with ordinary water (without introducing into it chemical additives salts, lowering the freezing point of the salt solution formed in this case), it is necessary, first of all, that the mixture is laid in the formwork warm and all its components have a positive temperature. It is impossible, for example, to lay a concrete mix prepared on frozen sand and gravel into the formwork. When such a mixture is heated after laying, the moisture contained in the frozen state in sand and gravel will thaw and occupy a smaller volume (it is known that water increases during freezing and, conversely, ice decreases in volume by about 10% during thawing). "13 this results in loose , porous, and consequently, low-strength concrete.
Therefore, in winter, the concrete mixture is prepared on heated water; aggregates (sand, crushed stone) are also heated or thawed to a positive temperature. An exception may be allowed for dry crushed stone or gravel that does not contain frost on grains and frozen clods (moisture content is not higher than 1 ... 1.5%). Such a filler can be loaded into the mixer unheated, provided that the concrete mixture will have a given positive temperature after leaving the mixer. The cement is not heated, because when mixed with water and aggregates, it quickly takes on a positive temperature.
Transportation and laying of the concrete mixture is carried out quickly so that its temperature in the formwork is positive.