I. Principles
According to the geographical location and construction period requirements of this project, combined with the actual situation and characteristics of this project at present, the principles of winter construction are: seriously implementing the national technical and economic policies, achieving advanced technology, economic rationality, reducing energy consumption, preventing pollution, being safe and applicable, ensuring quality and shortening working hours.
Second, the basis
Code for Winter Construction of Building Engineering (JJ104-2011)
Code for Acceptance of Construction Quality of Concrete Structure Engineering (GB50204-20 10)
Engineering construction calculation manual, construction organization design and construction drawings.
Technical specification for application of concrete admixture (GB50119-2013)
Code for quality acceptance of underground waterproof engineering (GB 50208-201kloc-0/technical code for underground waterproof engineering (GB50 108-2008).
1.2.9 Atlas 10J30 1
Third, the winter construction characteristics
1, many quality accidents
Due to unfavorable construction conditions and environment, winter construction is a season with frequent engineering quality accidents, especially concrete projects.
2. The concealment and lag of quality accidents.
In winter construction, most quality problems are exposed in spring, which brings great difficulties to accident treatment. Light repair and heavy rework will not only bring losses to the project, but also affect the service life of the structure.
3, winter construction planning and preparation time is strong.
Due to the short preparation time and complex technical requirements, the construction will be rushed if the preparation is not sufficient, which is easy to cause quality accidents.
Four, the determination of winter construction period and construction method
Article 1.0.3 of the Code for Winter Construction of Building Engineering (JJ104-201) stipulates that "when the daily average outdoor temperature is stable below 5℃ for five consecutive days, winter construction should be started; When the outdoor daily average temperature is higher than 5 C for 5 consecutive days, the winter construction is terminated. " According to this regulation and the statistical data of the Central Meteorological Observatory, the date of early winter when the daily average temperature in Harbin is stable below 5℃ is1October 20th of that year to March 3rd of the following year1.
According to the actual situation of the concrete parts of this project and the winter construction on the construction site, and according to Article 6.3.2 of the Code for Winter Construction of Building Engineering (JJ104-201), when the outdoor minimum temperature is not lower than-15℃, the surface coefficient is 5m-. After concrete pouring, the exposed surface should be covered with waterproof materials such as plastic sheets, and covered with cold blankets, with straw mats or quilts for thermal insulation. The concrete construction of this project adopts comprehensive heat storage method for curing, and all sub-items and processes are carried out in the greenhouse, which is heated by coke oven and hot air curtain to ensure that the temperature in the greenhouse is higher than zero 10℃. If the temperature requirement cannot be met, the concrete project will stop construction.
3. 1 The concrete constructed in winter shall not be frozen until its strength reaches 50% of the design strength after being put into the mould.
3.2 The mixing and molding temperature of concrete should be above 10℃, but not higher than 35℃. All concrete should be mixed according to antifreeze -20℃. 3.3 When the precast beam slab concrete enters the formwork, the formwork temperature shall not be lower than 2℃. 3.4 Considering the construction technology and construction period, the concrete will reach the design strength within 10 days. Second, the winter application preparation
Verb (abbreviation of verb) preparation
(A), organizational preparation
1, be familiar with, study and review the construction drawings carefully, thoroughly understand the design intent, conduct a comprehensive investigation on the engineering projects entering the winter construction, master the real data, and provide the necessary theoretical basis for the winter construction.
2. Organize all managers to learn winter construction specifications and standards, and conscientiously implement technical measures and winter construction scheme disclosure.
3. Conduct training and disclosure for temperature measuring personnel. The main work contents are atmospheric temperature measurement and concrete curing temperature measurement, so as to achieve the authenticity of concrete temperature measurement and provide scientific basis for concrete curing.
4, disclosure of main materials, prepare heat preservation and heating materials.
5, set up a winter temperature measurement team, team members are composed of project department and construction group personnel, temperature measurement team led by the project department, construction group sent 2~3 people responsible for temperature measurement.
(2) Production preparation
1, preparations for the construction site
(1) Remove the accumulated water in the site, make necessary repairs to the construction site, cut off the water flowing into the site, and take drainage measures to eliminate the site icing caused by water and steam used in site construction.
(2) After the snow on the construction site is cleared, it should not be placed near the stacking site of mechanical and electrical equipment and parts.
(3) Anti-skid measures should be taken for the pavement of transportation roads on the construction site to ensure smooth transportation.
(4) Ensure smooth fire control roads.
2, winter construction resources preparation
A stability of admixture materials and varieties
According to the varieties of additives selected in the winter construction scheme and the market supply situation, the formula, varieties and dosage of additives are finally put forward.
(1) admixture dosage plan. According to the parts and quantities of additives used in the project, the required dosage plan is calculated and provided to the material supply department.
(2) The second interview of admixture. For additives sold in the market, a second interview should be conducted in advance to ensure that their performance meets the technical requirements. For one-component admixture, the content of its effective components is determined.
3. Selection of heat insulation materials
Thermal insulation materials used in winter construction require good thermal insulation performance, reasonable price, local materials, and some materials require good fire resistance. Commonly used thermal insulation materials can be roughly divided into:
A. Covering the concrete surface for thermal insulation: plastic sheets with good gas insulation performance, cold blankets with good thermal insulation performance, straw curtains, quilts, etc. Be chosen. The straw curtain is flammable and easy to disperse, so it should be wrapped with glass fiber cloth before use. )
B. Insulation of foundation trench and foundation pit: choose cheap insulation materials such as straw curtain.
C. Thermal insulation of entrance, wind shield and greenhouse: quilts, cold blankets, plastic sheets and tarpaulins are generally used.
E. Waterproofing and thermal insulation of greenhouse, concrete curing and paving: using multiple charcoal stoves and coke. Bulk coal is not easy to hoist, so it should be packed in woven bags before transportation. )
Six, thermal insulation materials and equipment maintenance
According to the varieties, specifications, turnover times and quantity of thermal insulation materials selected by winter construction method, the annual planned consumption is calculated, and the plan and entry date are put forward to the material department.
1. Instruments and meters at various stages of construction shall be equipped with thermometers and thermometers.
2. Before the winter construction, inspect and maintain the working machinery, and conduct a comprehensive inspection of the machinery to ensure that it is in normal operation.
3. Strengthen the inspection of power supply system, transmission lines and self-generating equipment to eliminate potential safety hazards in construction.
4. The temporary water supply pipeline on the construction site shall be insulated and protected against freezing.
5, completes the winter construction of concrete, mortar and admixture test work, put forward the construction mixture ratio.
Seven, the construction temperature measurement method
1, the relevant provisions of the winter construction temperature measurement
A Temperature measurement range for winter construction: atmospheric temperature, raw material temperature such as cement, water, sand and stone, indoor temperature of concrete or curing shed, temperature of concrete or mortar when it is taken out of the tank and put into the mold, initial temperature and curing temperature of concrete after it is put into the mold, etc.
B. Responsibilities of temperature measuring personnel
(1) Record the daily atmospheric temperature and report it to the person in charge of the site;
(2) Record the temperature of concrete mixture, the temperature of concrete leaving the tank and the temperature of concrete entering the mould;
(3) Measurement of concrete curing temperature: arrange temperature measuring holes as required, and draw the distribution map and quantity of temperature measuring holes. Measure the initial temperature and atmospheric temperature of concrete curing as required, and control the initial temperature and time of concrete curing.
3. Preparations for temperature measurement in winter construction
Personnel preparation
Assign a special person to take charge of temperature measurement, and organize training and disclosure before starting temperature measurement.
Tool preparation
Thermometer: plug-in automatic temperature tester is used to measure liquid and sandstone materials. If you can't buy a thermometer, you can use it directly instead.
4, temperature measurement methods and requirements
According to the arrangement of temperature measuring points, the temperature measuring hole is drilled directly with a hand-held electric drill after the initial setting of concrete. Temperature measurement shall be carried out in numbered order and recorded in the temperature measurement record. See the table below for the arrangement of on-site temperature measurement.
Temperature measurement project concrete curing temperature; Atmospheric temperature; Working environment temperature; The temperature of cement, water, sand and stone; Temperature of concrete and mortar out of the tank; Temperature of concrete entering mould.
Measurement times: 3 times day and night, 2 times per shift: 7: 00, 15: 00, 2 1: 00. When concrete and mortar are opened, the temperature measurement conditions are 4MPa before and 4MPa after. The temperature measurement frequency is 12 times day and night, 4 times day and 4 times night.
The temperature measurement time is every 2h (according to the concrete pouring time), and every 6h at 2: 00, 8: 00, 14: 00 and 20: 00. End time of on-site temperature measurement: the concrete reaches the critical strength, the temperature difference between the concrete surface temperature and the environment after formwork removal is ≤ 15℃, and the cooling rate of concrete is not more than 5℃/h, and the temperature is measured.
5. Temperature management
Under the guidance of technicians, the site foreman is responsible for the leadership work such as temperature measurement, heat preservation and additive addition. Every day, he should check the temperature measurement records, take measures in time when finding any abnormality, and report to the relevant leaders and technical leaders.
B, the project technical personnel should ask about temperature measurement, heat preservation and heating every day and the existing problems, report to the competent leader in time, and assist the site construction management personnel to solve the difficult problems in winter construction.
C, construction temperature measurement personnel in each layer or each section to stop temperature measurement, temperature measurement records, usually found that the problem should be promptly reported to the site management and technical personnel, so as to take immediate measures.
D the temperature measuring personnel should be on duty 24 hours a day, and a strict succession system should be implemented. The temperature measuring personnel shall fill in the temperature measuring records by division and item, and keep them properly.
E. temperature measurement records shall be filed by technicians for future reference.
Eight, greenhouse heating method
Because the heating in the shed is uninterrupted for 24 hours, and the temperature in the shed is not lower than 10℃, it is required to set up a fireman for every six furnaces, and two special personnel and a safety officer for each shift to look after the operation area and maintain good ventilation and smoke exhaust. Due to the slow rise of concrete strength, after the concrete pouring of the bottom plate and precast wallboard is completed, it should be covered with plastic sheets, and the bottom and periphery of the covered plastic sheets should be sealed tightly, and a cold blanket should be added on them. Add a layer of grass curtain or quilt on the cold blanket to achieve the effect of heat storage and maintenance. At the same time, a closed space of the greenhouse is formed above the pipe gallery to ensure the temperature in the greenhouse. In places where coke ovens cannot be placed, hot air curtain is used for heating, and the newly poured prefabricated wallboard is enclosed in it, so that the temperature in this area can meet the normal requirements and the rising speed of concrete strength can be improved.
Scope of greenhouse construction: from the beginning of cushion concrete construction to the end of external wall waterproof layer construction, so all construction sections of Hongtu Street common ditch project must be fully covered.
Nine, each type of work in winter construction matters needing attention
1 reinforcement works
1. 1 cold drawing and cold bending of steel bars
1. 1. 1 The cold drawing temperature of steel bars should not be lower than -20℃, and the stretching temperature of prestressed steel bars should not be lower than-15℃.
1. 1.2 the stress control method or the cold drawing rate control method can be used for cold drawing of steel bars at negative temperature. Stress control method should be adopted for prestressed tendons of prestressed concrete structures. The method of controlling cold drawing rate is not suitable for cold drawing of hot rolled steel bars that cannot be divided into batches.
The determination of 1.65438+ cold drawing rate should be the same as that of normal temperature construction.
1.65438+
1. 1.5 When the temperature is lower than -20℃, it is forbidden to conduct cold bending operation on low-alloy grade II and III steel bars, so as to avoid strengthening the bending point of steel bars and causing brittle fracture of steel bars.
1. 1.6 After cold drawing at negative temperature, the appearance quality of steel bars should be checked one by one, and there should be no cracks and local necking on their surfaces.
1.2 steel bar connection
1.2. 1 Steel processing shed shall be built on the construction site, with heat preservation and windproof measures. Steel welding should be carried out in the processing shed as far as possible, and it must be carried out outdoors, and the ambient temperature should not be lower than -20℃. When the wind force exceeds level 3, snow and wind prevention measures should be taken to make the working environment temperature meet the requirements, so as to reduce the temperature difference of welding parts. When welding, control the temperature of each layer of weld or increase the current to reduce the welding speed. After welding, it is forbidden to touch the ice and snow immediately. After steel bars are processed, they should be placed, covered and marked according to the classification code of the used parts.
1.2.2 negative temperature electroslag pressure welding, when the reinforced end face is relatively normal, preheating should be adopted; When the end face is uneven, it should be leveled before preheating and welding.
1.2.3 defects such as overheating, burns and cracks must be prevented during negative temperature arc welding of steel bars, and the eccentric stress state of joints should be prevented in structure.
1.2.4 Before the rebar welding in winter, the trial welding must be carried out according to the construction conditions, and the welding can be formally carried out only after passing the test. 2.2.5 According to the design requirements, the tertiary rebar with a diameter greater than 25mm is connected by straight thread. When mechanical connection is used, the temperature has little influence on the processing of steel bars, so it can be constructed in winter.
2, concrete engineering
2. 1 concrete mixing
2. 1. 1 Requirements for concrete raw materials for winter construction
(1) Portland cement and ordinary Portland cement are the preferred cements. Attention should be paid to the influence of admixture materials on frost resistance and impermeability of concrete. The cement label should not be less than 42.5, the minimum cement dosage of concrete should not be less than 300kg/m3, and the water cement ratio should not be greater than 0.6.
(2) The aggregate used in concrete must be clean, and it must not contain frozen objects such as ice and snow and minerals that are easy to freeze and crack. In concrete mixed with antifreeze containing potassium and sodium ions, active substances should not be mixed into aggregate to avoid alkali-aggregate reaction.
(3) The concrete pouring project in winter should be selected reasonably according to the construction method; (4) The common drinking tap water can be mixed with clean natural water;
2. 1.2 admixture
(1) Selection of additives; In winter construction, from the structural type, nature and construction site of this project;
(2) testing of additives; The technical indexes of all additives in winter construction must reach the corresponding quality;
(3) Admixture management; The admixture configuration and mixing used in the winter construction of mixing concrete and mortar should be specially designed; When using additives, always determine the concentration and pay attention to adding.
(3) In the winter concrete pouring project, various additives should be selected reasonably according to the construction method, and attention should be paid to the corrosive effect of chlorine-containing additives on steel bars. Chlorine-free antifreeze should be used appropriately, and chlorine-containing admixture should be used for non-load-bearing structural concrete. Chlorine salt content shall not exceed 1% of cement weight, and chlorine salt content in plain concrete shall not exceed 3% of cement weight.
(4) Generally, drinking tap water and clean natural water can be used for mixing concrete, but sewage, industrial wastewater, acidic water with low PH value and water with sulfate content (calculated by SO4) exceeding about 1% of water weight shall not be used for concrete. In order to reduce the frost damage, the water consumption in the mixture ratio should be reduced to a minimum. The method is to control slump, add water reducing agent and give priority to high efficiency water reducing agent.
2. 1.2 admixture
Selection of (1) additives
In winter construction, the admixture should be selected according to the structure type, nature, construction site and the purpose of using the admixture. In the selection, we should consider: improving the workability of concrete or mortar, reducing water consumption, improving the quality of mixture and improving the early strength of concrete; Reduce the freezing point of the mixture and accelerate the hydration of cement at low or negative temperature; Promote the early and middle strength growth, reduce drying shrinkage, and improve the freeze-thaw resistance; Under the condition of ensuring the quality, improve the turnover speed of formwork, shorten the construction period, shorten or cancel the heating maintenance and reduce the cost; When choosing additives, we should pay attention to its influence on the later strength of concrete, the corrosion of steel bars and the impact on the environment, such as concrete additives containing ammonia; Try not to use slag cement with low hydration heat in winter construction.
(2) Testing of additives
The technical indexes of all additives in winter construction must meet the corresponding quality standards, and should have product certificates. When there is doubt about the performance of the admixture that has entered the site, it must be tested again and can be used only after it is confirmed to be qualified. The inspection contents of admixture components include: composition, content, purity, concentration, etc. The dosage of commonly used additives can generally be used according to relevant regulations. In case of special circumstances, the dosage of additives should be determined through experiments according to the structure type, use requirements, climatic conditions and maintenance methods.
(3) Admixture management
In winter construction, the special person shall be responsible for the configuration and addition of additives for concrete and mortar mixing, and make records carefully. Admixture solution should be prepared into standard concentration in advance, and then prepared into solution according to the use requirements. All kinds of additives should be put in containers with obvious signs and should not be confused. Each batch of solution can meet at least one day's usage.
When using additives, the concentration should be determined frequently, and the mixing should be strengthened to keep the concentration uniform.
2. 1.3 concrete mixing
(1) In winter, reasonable feeding sequence should be made for concrete mixing, so as to obtain good workability of concrete and uniform humidity of mixture, which is beneficial to strength development.
(2) Generally, aggregate and powdery admixture are put in first, and then hot water is put in after dry mixing. After stirring for a certain period of time, when the water temperature drops to about 40℃, put the cement into it and stir it evenly. Pay attention to avoid false setting of cement when mixing.
(3) The mixing time of concrete should be 50% longer than normal temperature, and comply with relevant regulations.
(4) After concrete mixing, it shall be transported to the watering place in time. During transportation, attention should be paid to prevent concrete heat loss, surface freezing, concrete segregation, cement mortar loss and slump change. In the case of long transportation distance and multiple transshipment, strengthen the thermal insulation coverage of transportation tools. Ensure that the concrete pouring temperature is about 65438 00℃, at least not lower than 5℃. When the temperature of concrete entering the mold is lower than 5℃ by thermal calculation, the mixing water and aggregate should be heated. See the table below for the heating temperature.
Heating thermometer
Ordinary Portland cement with cement type number less than 525 #, ordinary Portland cement with slag Portland cement number less than 525 #, ordinary Portland cement at 80℃, 60℃, 60℃ and 40℃ (5) can be used first by heating with water, sand and stone; When there is no hot water source, 1~2 prefabricated vertical hot water boiler can be installed on site to provide hot water, and the coal consumption can be estimated with reference to 200kg/ 1 prefabricated boiler h, or an electric heater can be used, and sand can be heated according to the relevant instructions in mortar.
The concrete mixing station shall be produced in strict accordance with the notice of mixture ratio issued by the laboratory, and the mixture ratio shall not be modified without authorization. Rinse the mixer with hot water 10min before stirring, and the stirring time is 1.5 times of that at room temperature. When mixing, the feeding sequence is gravel → sand → water → cement and additives → additives. During the production period, assign full-time personnel to be responsible for the unloading bin and remove frozen sand and gravel. Deduct the moisture brought by aggregate and antifreeze from the mixed water, strictly control the maximum replacement value of fly ash, and ensure that the water-cement ratio is not greater than 0.6. The temperature of mixed water must be measured at any time. The water temperature should be controlled at 50 10℃, and the sand temperature should be controlled at 20 ~ 40℃, so as to ensure that the cement is not in direct contact with water with the temperature above 80℃, and the slump of concrete is less than 200mm.
3.2 Concrete Pouring 3.2. 1 Preparation before pouring
(1) The tank body of the tanker transporting concrete is wrapped with thermal canvas and other measures to reduce the temperature loss, which requires reasonable organization, rapid transportation and rapid pouring into the mold to ensure that the temperature of concrete entering the mold is not lower than 10℃.
(2) The pumped concrete in pipeline shall be wrapped with thermal insulation materials to ensure that the concrete will not suffer from surface freezing, concrete segregation, cement mortar loss and slump loss during transportation.
concrete pouring
(1) can be constructed by conventional thermal storage methods.
(2) For mass concrete, slag portland cement should still be used as cement, and admixture with water reducing and retarding effects should be selected. Thermal insulation method should be adopted and covered with thermal insulation materials to prevent the invasion of cold air and reduce the temperature difference between inside and outside of concrete. During construction, temperature measurement should be strengthened, and the difference between internal temperature and surface temperature of concrete and the difference between surface temperature and ambient temperature should not exceed 25℃.
(3) When transporting concrete, the storage and transportation time should be shortened as much as possible. When the transportation time is prolonged due to failure, such as when the temperature of concrete in the bucket drops to +2℃, it is not allowed to enter the mold.
(4) Concrete construction should be carried out in the daytime with high temperature, which is beneficial to temperature control and personnel operation. (5) When the monolithic concrete with large thickness is poured in layers, temporary measures shall be taken in time to cover the concrete temperature of the pouring layer before covering a layer of concrete, so as to ensure that the surface temperature of the concrete is not lower than 2oC.
(6) The layered pouring thickness of concrete shall not be less than 20cm. Because it is winter construction, the vibration should be faster, and the vibration time should not be delayed arbitrarily.
3.2.3, concrete pouring and curing
1) After concrete pouring, in order to prevent the heat and moisture from losing too quickly and the wind and snow from attacking, the temperature of concrete entering the mold should be above 10℃, and it should be covered in time after pouring, and the temperature in the shed should be above 10℃. Concrete should be covered with heat storage and insulation to resist the sudden drop of temperature and improve the early strength of concrete.
3 concrete curing:
Cover the concrete with a layer of plastic cloth and cold-proof felt, and add a straw curtain or quilt to the cold-proof felt for heat preservation and maintenance. Water conservation is prohibited in winter construction.
3.3, concrete quality control and inspection
Winter concrete construction shall meet the following requirements in addition to the current national standard Code for Construction Acceptance of Concrete Structure Engineering (GB50204) and other local standards:
3.4 Design of concrete test block
According to the specification, at least two groups of specimens with the same curing conditions should be retained at room temperature. One group is used to measure the strength of concrete before freezing, that is, the critical strength, and the other group is used to measure the 28d strength. The concrete specimen shall be made of the pouring structure mixture at the pouring site. Before the pressure test, the specimen should be parked indoors at normal temperature, and then the pressure test should be carried out after thawing. The parking time is 4~ 12h.
3.5 form removal
3.5. 1 The dismantling time of concrete formwork should be determined according to the structural characteristics, natural temperature and concrete strength, and it is generally appropriate to dismantle it slowly.
3.5.2 Dismantle the formwork, and the concrete strength must also meet the requirements. The formwork removal strength of concrete by heat storage method is generally controlled at 4N/mm2, and it is about 48h at-5℃-8℃ and 72h at-8℃-12℃. After ripping, continue to maintain.
3.5.3 After the concrete is cooled to 5℃, the formwork and insulation layer can be removed. When the temperature difference between concrete and the outside is greater than 20℃, the concrete surface after formwork removal shall be temporarily covered to allow it to cool slowly.
3.5.4 In the process of formwork removal, if freezing damage is found to the concrete, the removal shall be suspended, and the removal can be continued only after being treated. 3.5.5 The formwork of the stressed member shall be removed when the member meets the design and specification requirements.
X. Quality assurance measures
1 temperature measurement shall be carried out by designated personnel, and the temperature measurement records shall be recorded in detail and filed.
2. Control the minimum temperature of concrete entering the mold ≥ 10℃, ensure that the temperature of concrete leaving the tank is greater than 15℃, and finally ensure that the temperature of concrete entering the mold meets the requirements.
3. According to the on-site temperature measurement, complete the supplementary heat preservation scheme in time and provide it to the production department in time to ensure that the concrete is not frozen.
If the site conditions and facilities can't meet the requirements of winter construction, waterproof construction and concrete pouring are not allowed. Winter concrete pouring must be reported to the technical, quality and safety departments for approval.
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