Theme: winter construction preparation, main technical quality control of winter construction, common quality problems of winter construction concrete and preventive measures.
2. Winter construction preparation
(1) technical preparation
1, construction technical scheme (measures) must be formulated on the premise of ensuring construction quality and production safety, which has certain technical reliability and economic rationality.
2. The construction technical scheme (measure) shall include the following contents: construction deployment (progress plan), construction procedures, construction methods, equipment and material deployment plan, technical training and labor plan for construction personnel (temperature measuring personnel and admixture mixing personnel), heat preservation materials and admixture materials plan, operation points, quality control points, inspection items, etc.
(2) Production preparation
Arrange the construction tasks and site preparations according to the established schedule, such as site water supply pipeline insulation and anti-freezing, mixer shed insulation, site leveling, temporary road setting, decoration engineering doors and windows sealing and insulation, etc.
(3) Resource preparation
According to the established plan, organize the preparation of employees' admixture materials, thermal insulation materials, construction instruments (thermometers) and labor protection articles.
Three. Main technical measures for winter construction
(1) Earthwork
1, the foundation earthwork should avoid winter as far as possible. If winter construction is needed, detailed construction scheme, reasonable construction scheme and feasible technical measures should be formulated, and construction management should be organized at the same time, so as to complete the construction in a short time.
2. The roads on the construction site should be kept clear, and necessary anti-skid measures should be taken for transport vehicles and driving roads (such as covering straw bags along the road).
3. When excavating earthwork on one side of adjacent buildings, measures should be taken to protect the foundation soil of old buildings from freezing damage. During construction, dig and fill as quickly as possible to prevent the foundation from freezing.
4. Drainage measures should be taken in the foundation pit groove to prevent water accumulation, which will cause the lower part of the earth wall to collapse due to repeated freeze-thaw cycles.
5. Necessary insulation measures should be taken at the bottom of the foundation pit after excavation, such as retaining mud or laying straw bags.
6. Before earthwork backfilling, the ice, snow and insulation materials at the bottom of the foundation pit should be cleaned up.
7. Outdoor foundation pit or pipe trench can be backfilled with soil containing frozen blocks, but the volume of frozen blocks shall not exceed 65,438+05% of the total volume of filled soil. Indoor pits, ditches and pipe trenches shall not be backfilled with soil containing frozen blocks.
8. When using artificial backfill, the thickness of each layer of soil shall not exceed 20cm, and the compaction thickness shall be 10- 15cm.
9. Backfilling shall be carried out continuously to prevent the foundation soil or filling layer from freezing.
(2) Pile foundation engineering
1, special construction scheme and technical measures shall be prepared before construction.
2. The distance from the axis of pile foundation should be increased appropriately to avoid the influence of frozen soil crust when piling. The number of leveling points should not be less than two.
3. The strength of precast pile concrete should reach 70% of the design strength standard value before hoisting, and 100% of the strength standard value before transportation and piling.
4. Pile piling should take into account the driving route and piling sequence of the pile driver, and the number of piles should not exceed 4.
(3) Reinforcement engineering
1. cold-drawn steel bar
(1) When steel bars are cold-drawn at negative temperature, stress control method or cold-drawing rate control method can be adopted. The cold drawing rate control method should not be adopted for the cold drawing of hot rolled steel bars that cannot distinguish batches.
(2) When steel bars are cold-drawn by controlling stress at negative temperature, the elongation decreases with the decrease of temperature. If the control stress is constant and the elongation is insufficient, the strength of steel bars can not meet the design requirements. Therefore, the control stress of negative temperature cold drawing should be higher than that of normal temperature cold drawing.
The maximum cold drawing rate shall meet the requirements in the following table:
Table 1 control stress and maximum cold drawing force of cold drawing
2. Negative temperature welding of steel bar
(1) The construction personnel engaged in steel bar welding construction must hold the welder's license before they can work at their posts.
(2) Flash butt welding, arc welding (batten welding, lap welding, groove welding) and electroslag pressure welding can be used for the welding construction of steel bars at negative temperature.
(3) The welding of steel bars should be arranged indoors as far as possible. If it must be done outdoors, the ambient temperature should not be too low. On snowy days, there should be some shelter measures. When welding uncooled joints, contact with ice and snow is strictly prohibited.
(4) Flash butt welding:
1) negative temperature flash butt welding, preheating flash welding or flash-preheating-flash welding process should be adopted. When the reinforced end face is relatively flat, preheating flash welding should be adopted; When the end face is uneven, the flash-preheating-flash welding process should be adopted.
2) Compared with normal temperature welding, corresponding measures should be taken, such as increasing the tensile length 10% to about 20%, increasing the contact pressure during preheating and increasing the preheating interval.
3) Welding parameters can be flexibly selected according to steel grade, diameter, welding temperature and welder's technical level.
(5) Arc welding
1) During welding, defects such as overheating, burns, biting meat and cracks must be prevented, and the eccentric stress state of the joint should be prevented structurally.
2) In order to prevent the temperature in the heat affected zone of the joint from rising suddenly, lath and lap arc welding should be carried out by layer temperature control. When the batten is welded, the batten and the main reinforcement are fixed by four-point tack welding. During lap welding, the tack weld shall be fixed at two points, and the tack weld shall be more than 20mm away from the lath or lap end.
3) During groove welding, the weld root, groove end face, steel bar and steel backing plate shall be well fused.
(6) Electroslag pressure welding
1) The welding current depends on the diameter of steel bar and the ambient temperature during welding.
2) The time for disassembling the connector kit should be extended by about 2 minutes; The slag shell of the joint should be extended for 5 minutes before slagging.
(4) Concrete engineering
1, basic requirements
Portland cement or ordinary portland cement should be selected for winter construction concrete. The cement label should not be less than 32.5, the cement dosage per cubic meter of concrete should not be less than 300kg, and the water cement ratio should not be greater than 0.6. Early strength agent should be added. If necessary, antifreeze should be added (determined according to temperature).
(2) In order to reduce the frost damage, the water consumption in the mixture ratio should be reduced as much as possible. The method is to control slump, add water reducer, and give priority to the use of high-efficiency water reducer.
(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.
(4) Uncooled concrete has high brittleness, so the structure should not bear impact load or dynamic load before cooling.
2, concrete mixing
(1) The aggregate used for mixing concrete must be clean and must not contain ice, snow, frozen blocks or substances that are easy to freeze and crack. When admixtures containing potassium ions and sodium ions are mixed, active aggregates should not be used. When conditions permit, sand and gravel should be screened and cleaned at a temperature above zero, and covered with plastic paper and tarpaulin.
(2) When mixing concrete mixed with admixture, if the admixture is powdery, its dosage can be directly sprinkled on the cement and put into use at the same time. If the admixture is liquid, it should be prepared into a solution with a specified concentration before use, and then the solution with a specified concentration should be prepared into a construction solution according to the use requirements. Each solution should be placed in a clearly marked container and must not be confused. The additive solution used in each shift should be prepared at one time.
(3) When the construction period is about 0℃, the early strength agent can be added to the concrete, and the dosage should meet the use requirements and specifications, and the simulation test should be done before joining to verify the relevant technical measures; For the concrete with the requirement of dismantle within a time limit, the design grade of concrete should be appropriately improved.
(4) When antifreeze is added to concrete, it is forbidden to use high-alumina cement.
(5) Strictly control the water-cement ratio of concrete, and the water in aggregate and admixture solution shall be deducted from the mixed water.
(6) When mixing concrete mixed with admixture, the mixing time should be 0.5 times of the mixing time at room temperature.
(7) The outlet temperature of concrete mixture should not be lower than 65438 00℃, and the inlet temperature should not be lower than 5℃.
3. Transportation and pouring of concrete
(1) The concrete mixing site should be as close as possible to the construction site, so as to reduce the heat loss during the transportation of materials, and at the same time, the containers used for transportation (including shape, size and insulation measures) should be selected correctly.
(2) Before concrete pouring, the snow, ice and garbage on the formwork and steel bars should be removed, especially at the joint of new and old concrete (such as the joint of beam and column).
(3) When commercial concrete is used, the performance of antifreeze added to commercial concrete should be understood before pouring, and corresponding anti-freezing and heat preservation measures should be taken.
(4) When concrete is poured in layers, the pouring layer shall not be lower than the temperature specified in the calculation or lower than 2℃ before it is covered by the upper concrete.
(5) For key projects or projects with continuous superstructure construction, effective measures should be taken to ensure the expected strength of concrete.
(6) Before grouting the holes and vertical joints of prestressed concrete members, the concrete in the pouring parts must be preheated and hot water mud, mortar or concrete should be used. After pouring, the strength at room temperature shall be kept not lower than 15Mpa.
(7) Concrete test blocks with the same curing conditions shall be set on site as the basis for form removal.
4, concrete curing
(1) The compressive strength of concrete poured in winter before curing at negative temperature should not be less than 40% of the design strength, and the concrete below C 10 should not be less than 5Mpa.
(2) Thermal insulation materials (straw bags and sacks) should be kept dry.
(3) When the formwork is externally insulated, except the foundation can be insulated with pouring, other structures can only be poured with concrete after the insulation material is solidified. Before pouring concrete, thermal insulation materials such as straw curtains and sacks can be hung on the surface of steel formwork and fixed.
(4) Thermal insulation materials should not be directly covered on the newly poured concrete layer. Plastic film can be covered first, and then thermal insulation materials such as straw bags and sacks can be covered. The laying thickness of thermal insulation material is: in general, one layer should be laid above 0℃; Spread two or three layers below 0℃; Mass concrete should be covered with insulation layer immediately after pouring and secondary compaction, and the thickness and material of insulation layer should be determined according to calculation.
(5) After form removal, the concrete should also be covered with thermal insulation materials in time to prevent cracks caused by the sudden drop of concrete surface temperature.
5, specimen lien
Concrete test blocks shall be manufactured, managed, maintained and submitted for inspection according to specifications and construction requirements. Sampling and sample retention shall meet the following requirements:
(1) When mixing 100 plates of concrete with the same mixture ratio and not exceeding 100m3, the sampling shall not be less than once.
(2) When the concrete with the same mixture ratio is less than 100 plate per shift, the sampling shall not be less than once.
(3) When a continuous casting exceeds 1000m3, the concrete with the same mixture ratio shall not be less than once every 200m3.
(4) For the concrete with the same mixture ratio, each layer shall be sampled at least once.
(5) For concrete structures with impermeability requirements, concrete specimens shall be randomly selected at the pouring site. The concrete of the same project and the same mixture ratio shall be sampled at least once, and the number of retained groups can be determined according to actual needs.
(6) At least one set of standard curing specimens shall be retained for each sampling, and the number of groups of specimens cured under the same conditions shall be determined according to actual needs.
(5) Masonry engineering
1, material requirements
The cement shall be (1) ordinary portland cement with the label of 32.5R, and there shall be no moisture caking in the cement.
(2) Before laying ordinary bricks, hollow bricks, small concrete hollow blocks and aerated concrete blocks, the surface dirt, ice and snow should be removed. Bricks and blocks frozen after immersion shall not be used.
(3) Thermal insulation and anti-freezing measures should be taken for lime paste. If it is frozen, it should be melted before use.
(4) Medium sand should be used for sand, and the silt content should meet the specification requirements. Sand should not contain ice or frozen blocks with a diameter greater than 1cm.
(5) The consistency of masonry mortar should be properly adjusted compared with normal temperature construction, and additives should be preferred to improve the consistency of mortar. At negative temperature, the consistency of mortar can be 1 ~ 3cm higher than that at normal temperature, but not more than 12cm, so as to ensure the bonding force between mortar and brick.
2, the construction method
(1) Masonry adopts "three-in-one masonry method". If the mortar is tiled, the length of the mortar should meet the temperature when the mortar is laid, so as not to be too low.
(2) It is forbidden to use frozen mortar for masonry.
(3) When the outdoor temperature is lower than +5℃, materials such as bricks and blocks should not be watered, and the mixing time of mortar should also be increased, which is generally 1.8 times of the mixing time at room temperature, about 2.5~3 minutes.
(4) In order to prevent heat loss of mortar during mixing, transportation and storage, the following methods can be adopted:
1) The mixing of mortar can be carried out in the heat preservation shed (the temperature in the shed is above 5℃), and the mortar should be used as it is mixed, and the storage time should not exceed 60 minutes, and secondary accumulation and transportation are not allowed.
2) The mixing place should be as close to the construction site as possible to shorten the transportation distance.
(5) The thickness of horizontal and vertical mortar joint of masonry should be 8~ 12mm, which should generally be controlled around 10 mm.
(6) Control the masonry height, and the daily masonry height shall not exceed1.80m..
(7) Before calling it a day, the vertical mortar joint on the top surface should be filled, and the masonry surface should be covered with thermal insulation materials (such as straw bags and plastic films).
(8) Increase the number of on-site test blocks, and maintain them under the same conditions at the site to test the actual strength of on-site masonry mortar.
(6) Decoration works
1, decoration and plastering works
(1) Exterior wall painting shall be carried out at room temperature first. If exterior wall painting is needed when the lowest temperature is lower than 0, scaffolding shall be hung with double-layer straw curtains for wind protection, and water-mixed mortar mixed with salt shall be used. When the temperature is between 0℃ and -3℃ (which refers to the expected lowest temperature within three days), 2% (calculated by weight percentage of water) shall be added.
(2) When the daytime temperature is close to 0℃ and the nighttime temperature may reach below 0℃, the working surface should be narrowed and painting should not be arranged as far as possible.
(3) Before indoor painting, doors and windows should be closed to provide glass.
(4) When the temperature is below 0℃, if indoor decoration is necessary, with the approval of the branch leaders, indoor heating can be adopted, and the honeycomb coal stove can be used to keep the indoor temperature above 0℃ to prevent the decoration from being frozen in a large area.
2. Winter construction of paint and coating engineering
(1) In winter, the construction of paint and coating works should be carried out under heating conditions, and the indoor temperature should be kept in balance without sudden changes. Indoor relative humidity shall not be greater than 80% to prevent condensation. The ambient temperature shall not be lower than +5℃ when painting oily paint, and not lower than +3℃ when painting waterborne paint, and shall be controlled in combination with the temperature specified in the product manual. - 10.
(2) In winter, when painting works, the temperature should not change drastically. After the construction, it shall be maintained for at least two days and nights until the oil film and coating are dry.
3. Winter construction of other decoration projects
(1) glass structure
The glass transported from cold areas to warm areas should be cut after its temperature has eased, and the installation of door and window glass should be carried out at positive temperature.
(2) Winter construction of facing (brick) project
When facing (brick) is constructed in winter, the mortar temperature should not be lower than +5℃. Below +5℃, the construction quality can not be guaranteed, and the operation is more difficult. Effective anti-freezing measures should be taken before the mortar hardens in construction.
(7) Ground engineering
1. During the construction of indoor ground leveling layer and surface course, doors and windows openings shall be covered and insulated to ensure that the indoor temperature is above 5℃. It is predicted that when the outdoor temperature is around 0℃ for three consecutive days, cement mortar should be mixed with 1~2% salt solution, and reliable anti-freezing and heat preservation measures should be taken.
2. Portland cement and ordinary portland cement should be used in winter cement mortar, and the water consumption should be reduced as much as possible, and the fineness of mortar should be less than 5cm.
3. For large-area concrete ground construction with tamping and plastering, all preparations should be made before construction, and the pouring and vibrating speed should be appropriately accelerated during construction (on the premise of ensuring the internal compactness of concrete), and the pouring thickness should be slightly higher than the design thickness. After vibration leveling, plastic film should be laid in time and covered with thermal insulation materials.
(8) Roof engineering
1, roofing construction in winter should be carried out in windless and sunny weather, and make full use of sunshine conditions to improve the surface temperature. The movable windproof device shall be installed on the windward side.
2. Before the construction of each floor of the roof, the snow, ice and sundries on the base should be cleaned up. The materials used should not contain frozen ice and snow.
3. The whole insulation layer and plate insulation layer bonded with asphalt shall be constructed at a temperature not lower than-10℃, and the whole insulation layer and plate insulation layer bonded with cement, lime or emulsified asphalt shall be constructed at a temperature not lower than 5℃. If the temperature is lower than the above requirements, heat preservation and anti-freezing measures should be taken.
4. When the leveling layer is cement mortar, the strength grade of mortar should not be lower than M5.
5. When coiled material is used for waterproof layer, hot melt method or cold bonding method can be used for construction. The temperature of hot-melt method is not lower than-10℃, and that of cold-bonding method is not lower than -5℃. When coating is used as waterproof layer, flux coating must be used, and the construction temperature shall not be lower than -5℃.
6. Waterproof works should be constructed by units with good construction quality and reputation, and operators should hold relevant certificates.
4. Common quality defects in concrete construction in winter and preventive measures
(1). Corrosion of steel bars and concrete cracks
Due to the oxidation corrosion of steel bars accompanied by volume expansion, concrete cracks along the direction of main reinforcement or stirrup. Secondly, poor cement stability, high water-cement ratio of concrete, low early strength and rapid water loss will also cause cracking. The moisture in concrete moves from the edge to the center, and the pressure will also cause axial cracks.
Preventive measures: 1. Strictly control the dosage of chlorine salt. According to the Code for Construction and Acceptance of Reinforced Concrete Engineering, the content of chloride salt shall not exceed 65,438+0% of the cement weight, so the basic steel bars will not rust.
2. When a limited amount of chlorine salt is added, it should be fully dissolved or stirred evenly to prevent local reinforcement corrosion caused by deflection. 3. Controlling the cement quality and water-cement ratio of concrete mixture, increasing its density and preventing water transfer can effectively prevent concrete cracks.
(2) Structural evacuation and water transfer
Concrete with water transfer and structural evacuation has the characteristics of ice crystals and khaki surface, fragile mortar aggregate bonding and dumb sound. At the same time, due to the pressure difference, temperature difference and humidity difference inside the concrete, water moves from the edge to the center, resulting in cracks.
Preventive measures: 1. The composite admixture composed of antifreeze, water reducing agent, early strength agent and air entraining agent is properly added to reduce the water cement ratio, and the structural compactness is improved by repeated vibrating and pressurized vibrating.
2. It is effective to "freeze" the concrete surface by using the wet characteristics of concrete.
(3) The surface is gray.
The so-called "surface sanding" is characterized by the separation of mortar from coarse aggregate, sand on the surface and bare aggregate. Mainly due to the high water-cement ratio of concrete mixture, serious segregation and bleeding, poor cohesiveness and water retention, low curing temperature, the hydration of cement tends to stop, and the concrete moisture quickly precipitates, leading to the surface graying.
Preventive measures: strictly control the water-cement ratio, extend the mixing time of concrete mixture, and cover the surface with plastic film to keep water.
(4). Crystalline corrosion-frost on concrete surface
After the concrete hardens, some admixture solutions penetrate into the concrete surface through capillary action, and the water on the concrete surface gradually evaporates, which will also affect the combination of concrete and decorative layer.
Preventive measures:
1. Properly control the dosage of admixture (preferably no more than 5~7% of cement weight).
2. After the admixture is fully dissolved, appropriately extend the mixing time.
3. Immediately after concrete pouring, cover its surface with 1~2 layers of film plastic. Prevent concrete moisture from moving out.
Verb (abbreviation of verb) Safety and fire prevention in winter construction
(1) Anti-skid measures should be taken in winter construction.
(2) It is forbidden to use open flame for heating in the construction site and temporary work shed. Specific fire safety measures should be formulated and the responsibility should be implemented to people.
(3) Electrical equipment and switch box should have protective covers, live wires should be arranged overhead, and wire winding should be thoroughly checked to ensure good insulation effect.
(4) When there is snow and ice accumulation on the scaffold, it should be cleaned before construction. The inclined springboard should be nailed with anti-slip strips or covered with straw bags, and the frame body should be checked for looseness and sinking at any time for timely treatment.
(5) There should be protective shed or other isolation measures around the stairs, elevators and derricks at the entrances and exits of the upper and lower interchange operations.
(6) Safety belts must be used when working at heights, safety helmets must be worn when entering the construction site, and reserved holes in the floor must be covered with cover plates. It is not allowed to cover it with reeds and straw bags to prevent it from slipping. Protect and guard external scaffolding in winter, and it is forbidden to throw it down at high altitude.
(7) Vibrating machines such as rammers and grinders and wires of frequently moving machines and tools shall not be dragged on the ground or immersed in water. Overhead insulation should be good.
(8) The temporary water pipes on the construction site should be buried underground or wrapped with thermal insulation materials such as straw bags and coated with paper reinforcement. The water in the water tank should be drained before work.
(9) Straw bags, straw curtains and other thermal insulation materials shall not be piled up in the open air, so as to avoid losing the thermal insulation effect due to moisture.
(10) Flammable, explosive and toxic substances on site shall be kept by special personnel and properly placed. Open flame operation shall be subject to the examination and approval system of hot work permit, and necessary safety and fire protection articles shall be configured.
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