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1. Importance of steel structure coating project
With the development of national economic construction, the use of steel structures is becoming more and more widespread, and the quality and service life of steel structures will attract more and more attention and attention. In the influence of the service life of the steel structure, the coating anti-corrosion operation is placed in front of people as a very important link.
A set of data from the China Steel Structure Association: According to statistics from developed countries, the annual economic loss due to corrosion accounts for about 4% of GDP, and the results of a survey of 10 affiliated companies by China ’s chemical system in 1980 showed that The economic loss due to corrosion is equivalent to 3.94% of the total output value. In 1982, Anshan Iron and Steel Company surveyed that the corrosion loss was about 100 million yuan.
The corrosion of the steel structure not only causes its own economic losses, but also directly affects the safety of the structure. Therefore, it is of far-reaching economic and social significance to do a good job in preventing corrosion of the steel structure.
2. Project Overview
This project is a self-anchored truss crane combination fully welded steel structure bridge. The bridge uses Q345qD special steel for the bridge. The steel amount of the whole bridge is about 4654t, and the coating engineering amount is about 3.17m2. The external design coating of the project is divided into 3 layers, followed by the bottom Paint, medium paint, top coat. The internal design coating of the bridge is divided into two layers, namely primer and middle paint, and the design anti-corrosion period is more than 10 years (see Figure 1).
The national key project "Bird's Nest" uses 42,000 tons of high-strength steel, the anti-corrosion coating area of ​​the project reaches 284,000 m2, the coating is divided into 6 layers, and the design anti-corrosion period is 25 years.
Due to its advantages of fast construction speed and large span, steel structure has been widely used in industries such as civil construction, highway transportation and other industries. Improving the anti-corrosion coating quality of steel structure is an urgent task.
3. Painting engineering construction
The construction of the bridge coating project will be described in two parts according to the construction sequence: Part 1 is the painting in the steel structure processing plant, and Part 2 is the painting after the installation of the steel components on site.
3.1 Painting operations in steel structure processing plants
Main construction contents: â‘ After projecting (or sand blasting) the steel plate, remove the rust and paint it with engineering paint; â‘¡ The painting after the production of steel components in the factory is completed.
3.1.1 After the steel plate is blasted (or sandblasted), it is painted with engineering paint after derusting
The processing of steel components requires a period of time, which can last for several months. Therefore, the purchased steel sheet should be processed by blasting (or sand blasting) and then painted with engineering paint to prevent corrosion in the manufacturing process. In order to create the roughness of the steel plate, the steel plate is treated by blasting or sand blasting. Taking the steel structure bridge painting work of this project as an example, the design surface is blasted to Sa2.5, and the surface roughness is 40 to 75 μm. The specific anti-corrosion requirements are shown in Table 1.
Table 1 Bridge anticorrosion requirements
According to the national standard for derusting GB82923-88, Sa2.5 indicates that the derusting is very thoroughly sprayed or projected, and there is no visible grease and dirt, oxide scale, rust and other attachments on the surface of the steel. Any remaining traces should be only dots or stripes Slight stains.
3.1.2 Sand blasting and derusting process
This project adopts sand blasting and derusting method, using the pressure of compressed air to continuously strike the surface of steel members with quartz sand or steel sand to clean up the rust, oil stains and other debris on the surface of the steel, revealing the true nature of the steel. method. This method has high efficiency and complete rust removal, and is a more advanced rust removal process. The specific construction process of sand blasting and rust removal is as follows.
1) For the construction environment, choose relative humidity ≤85%, control the surface temperature of the steel, and it should be more than 3 ℃ higher than the air dew point temperature.
2) Cleaning the surface of the steel plate Clean the dust, oil and water on the surface of the steel plate.
3) Abrasive selection Generally choose steel shot or iron shot as abrasive. Using steel shot as abrasive, although the price is high, it can be used repeatedly for many times, and it has good rust removal effect and no dust pollution. When choosing abrasives, choose granular materials with high density, strong toughness, and certain particle size requirements; the surface of the abrasives must not have oil stains, and the water content is ≤1%; the particle size of the abrasives, which one to choose, depends on the engineering design drawings For the roughness, select the particle size of the abrasive according to the roughness.
4) Compressed air treatment When sandblasting and rust removal, the compressed air used must separate oil and water, otherwise oil and water will be attached to the steel surface during the sandblasting process, which not only affects the adhesion of the coating, but also destroys the coating Uniformity and compactness.
5) The amount of sandblasting and blasting works is determined according to the strength of the paint. The principle is that the engineering paint must be painted within 24 hours after sandblasting.
6) Clean the steel surface after sand blasting and derusting, use brushes, rags or other tools to clean it, or blow off the residual abrasive with clean compressed air.
7) Roughness inspection Roughness inspection generally uses professional instruments for testing, such as RT200, R1020 type roughness testing electronics, and other portable small-scale testing instruments or professional equipment for testing.
3.1.3 Apply engineering paint after sandblasting and derusting
The engineering paint is applied for anticorrosion in the process of manufacturing the steel structure. Construction experience believes that this link is very important, and the brushing operation must be carried out carefully in accordance with the specifications. Engineering paint serves as the first "barrier" of the steel structure. If the first "barrier" has no quality guarantee, after the processing of each rod is completed, it will take a long time, the sun will rain, most of the engineering paint will peel off or rust, so the only solution is The only way is to take manual polishing to remove rust, and then repaint. This not only causes financial and labor costs, but also delays the construction period, so it is particularly important to ensure the construction quality of the engineering paint.
Matters needing attention in the application of engineering paint: â‘ After sandblasting or shot blasting to remove rust, the roughness must be selected at random according to the requirements of the specification. After the inspection meets the design requirements of roughness, the engineering paint shall be painted within 24 hours; The material and the environment of use are selected according to the design requirements, and the use of quality-assured paint is used; â‘¢The finished rod parts must be protected by cover and other protective measures.
3.1.4 Painting after finishing the production of steel components in the factory
After the steel plate material treatment is processed and the engineering paint is applied, the next process, that is, the manufacture of the steel member is performed, and the coating paint is sequentially applied after the steel member is processed and manufactured.
Taking this steel structure bridge as an example, the steel components mainly include steel box girder and main truss. Primer and intermediate paint are painted inside the steel box girder and main truss bar. The middle paint and topcoat will not be applied until the on-site installation and assembly are completed.
The coating primer of the steel structure engineering component is zinc-rich primer, which has strong anti-rust ability, has cathodic protection and can penetrate into the welding place, is resistant to solvents, and has stable weather resistance under sunlight. The middle paint chooses the thick film type epoxy middle paint.
3.1.5 Painting operations of steel components in the factory
After processing and manufacturing all the rods in the factory, the primer and the middle paint are applied in sequence, and the specific operations are as follows.
1) In the process of manufacturing steel structure rods, the parts where paint is removed, the manual electric grinding wheel is used for small area secondary rust removal, and the rust removal level requirement is St3.
2) After welding the position of the welded steel parts processed in the factory, the uneven parts can be polished by hand, and the corresponding putty paste can be used as a flat bottom layer. After drying, use the emery cloth to smooth it, and then apply the primer.
3) The partially processed rods, the parts where the engineering paint is intact and free of rust spots, only need to clean the surface part of the oil, dust, etc., you can apply the primer.
4) The choice of paint should be based on design and specification requirements.
5) The brushing method basically adopts the brushing method and the manual roller coating method. The brush primer uses a hard brush with good elasticity, which can increase the adhesion of the paint film. Brush, the effect of this brush is smooth and smooth. For some small rods, there is no operation space inside, brush the paint before processing and assembly, but in the welding and assembly process, because part of the welding adopts single-sided welding and double-sided molding technology, the internal weld seam position was originally brushed If the paint film is damaged, it must be replaced. Therefore, the air spray method is used. Spraying with a spray gun often needs to be sprayed several times to achieve the required film thickness. This method is easy to waste paint and is suitable for confined spaces.
3.1.6 Important matters needing attention in painting operations
In order to ensure the quality of painting operations, in the process of painting operations, we must grasp the key links and carry out key operations. The specific precautions are as follows.
1) In the construction environment of painting, when the air humidity is greater than 85% or there is condensation on the surface of the component, the coating operation cannot be carried out.
2) The temperature of the coating operation is preferably 5 ~ 38 ℃, when the temperature is lower than 5 ℃, in principle, paint should be stopped, unless the corresponding low-temperature coating material is selected for application.
3) When the temperature is ≥40 ℃, the coating operation should be stopped. Because the temperature of the component is ≥40 ° C, bubbles will be generated when the steel plate is painted with paint, thereby reducing the adhesion of the paint film.
4) When the paint enters the field, it must be tested by the paint performance of the professional department, and the indicators of transparency, color, skinning, thixotropy, viscosity, fineness, density, solid content and other indicators must be qualified before they can be put into use.
5) On-site preparation between thinner, curing agent, main paint, base material, etc., must be carried out on-site preparation according to the best mix ratio obtained from the test, and fully stirred evenly, and the color and viscosity should be uniform before they can be applied.
6) When brushing the first layer of primer, the brushing direction should be the same, and the stubble should be neat. Dip and brush shortly when painting, to prevent the brush from running too much with paint. After the first brush of the unified brush is dried, the second brush is applied. The second brushing direction is perpendicular to the first brushing direction, so that the thickness of the paint film is uniform.
7) Epoxy paint layered paint brush, each layer should be applied before the previous layer is solidly dried, and the primer must be qualified after inspection before the next process can be carried out.
8) The steel components processed and welded in the factory can be painted with primer only after the welding seam inspection, polishing, descaling, welding scars, dust and other treatments. When painting in the factory, each steel member will leave a width of 20cm at the installation and docking position of the two rods without painting, which is convenient for on-site welding.
9) After painting the primer, the quality and film thickness must be checked, and the interval is long enough, and the intermediate paint must be painted after the primer is completely dry (generally, the interval is 6 ~ 8h). After painting the paint, if it encounters strong wind or rain within 4h, it should be covered to prevent the surface from being contaminated with dust and moisture, so as not to affect the adhesion of the coating.
10) Special paint film detector is used for paint surface detection, and simpler and practical manual detection methods can also be used. Here is a simple and effective method that the author summarized at the construction site. Use a small dry wooden board or a small plastic board to press the small cotton ball against the surface of the painted film, and then remove the small wooden board. If the cotton ball can Free fall, and there is no trace of fiber or cotton ball on the paint film, and there is no loss of light. It can be considered that the paint film has been actually dried.
3.2 Painting after completion of installation of steel components on site
The on-site steel structure installation phase coating operations mainly include: the external topcoating operation of the truss rods and box beams, the assembly of the 3-layer bottom, middle and topcoating positions of the butt welds, and the knocking off of the paint during the installation of the steel components on the site Make up parts.
1) The top paint operation of the upper truss and the outside of the box girder. Because the primer and intermediate paint of the steel members are completed in the steel structure processing plant, and the painting work is completed after the on-site rods are docked, if the painted film is not damaged, only Clean the dust, moisture and other surfaces on the surface of the rod, then you can perform the topcoat operation.
2) The painting of the weld at the butt joint of the steel member. The painting operation here can only be carried out after passing the weld inspection. Before painting, it is necessary to polish the welding seam to remove welding slag, oxide scale, welding scars, dust, burrs, oil stains, etc. Some uneven areas can be filled with putty paste, and then brushed on the bottom, middle and surface paint.
3) Paint the parts where some of the rods are knocked off due to transportation and site installation. Here, small areas of manual rust removal and polishing are required before painting. The rust removal method uses manual electric grinding wheels for small area rust removal. The rust removal level is St3. After rust removal, the surface is cleaned, and the bottom, middle and top coats are added in sequence. .
4. Steel structure coating quality inspection
4.1 Inspection before painting
1) The rust removal on the steel surface must reach the rust removal level specified in the design, and the primer should be applied within the specified time.
2) The paint entering the factory shall have a product qualification certificate, and be re-inspected and tested for the paint mix ratio in accordance with the paint product acceptance rules.
3) The coating environment should meet the requirements stipulated in the code.
4.2 Inspection of painting process
1) During the painting operation, when the construction viscosity of the paint becomes thick, the operator cannot adjust it without permission, and it should be adjusted by a designated person.
2) In the layered coating operation, use a paint film thickness meter to measure the thickness of each layer of coating.
3) In the layered coating operation, each paint film is not allowed to bite the bottom, peel off, miss the coating, blistering and other phenomena.
4.3 Inspection after painting
1) Inspection of coating appearance The appearance of the coating should be uniform, smooth, plump, shiny, without defects such as biting, peeling, cracks, pinholes, etc.
2) The thickness of the coating is checked by a dry film thickness gauge, and the quality acceptance is carried out in accordance with the acceptance specifications and design requirements of the steel structure coating.
5 Conclusion
Steel structure coating protection uses the coating of paint to isolate the coated steel rod from the environment, so as to achieve the purpose of anti-corrosion and extend the service life of the coated object. Therefore, the quality of the coating is a key factor that affects the protection of the coating. The quality of the coating is not only related to the quality of the coating itself, but also related to the rust removal quality of the steel member surface, the thickness of the paint film, and the coating process. Therefore, only by controlling the entire process of coating and corrosion protection from steel structure processing to on-site installation can we ensure the quality and service life of the steel structure project, thereby reducing the cost of anti-corrosion maintenance in the later period and improving product safety and stability.
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