Steel structure bridge engineering anti-corrosion coating construction technology.

Taking a bridge in Tianjin as an example, the bridge is a self-anchored fully welded steel structure bridge, the shape design is the first time in China, through the research on the key technology of bridge coating construction, to ensure the quality of bridge coating construction, so that the service life of the bridge is guaranteed.
(keywords) bridge project; steel bridge; coating; projectile (sand blasting) rust removal; quality inspection
1 the importance of steel structure coating engineering
With the development of national economic construction, the use of steel structure is becoming more and more extensive, the quality and service life of steel structure will attract more and more attention and attention. In the impact of steel structure life, coating anti-corrosion operation as a very important link in front of people

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A set of statistics from the China Steel Structure Association: According to statistics from developed countries, the annual economic losses caused by corrosion account for about 4% of GDP, while a survey of 10 enterprises in China's chemical system in 1980 showed that the economic losses caused by corrosion amounted to 3. 94%, 1982 Anshan Iron and Steel Company survey results, corrosion losses of about 100 million yuan.
of steel structure not only causes its own economic losses, but also directly affects the safety of the structure, so it is of far-reaching economic and social significance to do a good job in the anti-corrosion work of steel structure.
2. Engineering Overview
This project is a self-anchored suspension combination full welded steel structure bridge, the bridge uses Q345qD bridge-specific steel, the total bridge steel volume is about 4,654t, the coating engineering volume is about 3. 17m2. The exterior design coating of the project is divided into 3 layers, followed by primer, medium paint and finish. The interior design coating of the bridge is divided into 2 layers, i.e. primer, medium paint, design anti-corrosion age of more than 10 years (see Figure 1).
national key project "Bird's Nest", the use of high-strength steel consumption 4. 20,000 t, engineering anti-corrosion coating area of 28. 40,000m2, coated in 6 layers and designed for 25 years.
steel structure with its fast construction speed, large span and other advantages, is now widely used in industrial and civil construction, road transportation and other industries, improve the quality of anti-corrosion coating of steel structure has been urgent.
3. Coating Engineering Construction
The bridge coating project construction, according to the order of construction, is described in two parts: Part 1 for the steel structure processing plant coating, Part 2 for the site steel components after installation.
3. 1 Coating work in steel structure processing plant
Main construction content: (1) steel plate ejaculation (or sand blasting) after rust removal brush engineering paint; (2) steel components in the plant after processing and finishing the coating.
3. 1. 1 Steel plate ejaculation (or blasting) after rusting brush engineering paint
steel components processing and production takes a process of up to several months, so the purchased steel plate, first for projectile (or blasting) treatment, and then brush engineering paint to prevent corrosion in the processing and production works. To create the roughness of the steel plate, the plate is blasted or blasted to remove rust. Taking this engineering steel structure bridge coating operation as an example, the design surface blasting treatment to Sa2. 5, surface roughness 40 to 75 m, the specific anti-corrosion requirements as shown in Table 1.
according to the national rust removal standard GB82923-88, Sa2. 5 indicates that rust is sprayed or ejected very thoroughly, and that the steel surface is free of visible grease and dirt, oxides, rust, etc., and that any residual marks should be only slight spots of dot or stripes.
3. 1. 2 Sand blasting rust removal process
This project uses sand blasting method, the use of compressed air pressure, continuous use of quartz sand or steel sand impact steel component surface, the steel surface rust, oil and other debris clean up, revealing the steel nature of a rust removal method. This method is highly efficient, thoroughly rust-removal, and is a more advanced rust-removal process. The specific construction process for sand blasting and rust removal is as follows.
1) The construction environment selects the relative humidity ≤85%, controls the surface temperature of the steel, and is above the air dew point temperature of more than 3 degrees C.
2) Clean up the surface of the steel plate for dust, oil and moisture.
3) Abrasive selection generally chooses steel or iron pellets as abrasives. Steel balls as abrasive, although the price is high, but can be recycled many times, and rust removal effect is good, no dust pollution. In the selection of procurement abrasives, the choice of high density, toughness, a certain degree of granularity requirements of the particles; The surface of the abrasive shall not be oily, and the water content shall ≤1%; Choose the particle size of the abrasive, the specific choice of which, depending on the roughness of the engineering drawings, according to the roughness to choose the particle size of the abrasive.
4) When compressed air is treated for sand blasting and rust removal, the compressed air used must separate the oil and moisture, otherwise the oil and moisture will be attached to the steel surface during the blasting process, which will not only affect the adhesion of the coating, but also destroy the uniformity and tightness of the coating.
5) The amount of blasting engineering is determined according to the size of the paint, the principle is that the engineering paint must be painted within 24h after blasting.
6) Clean the surface of the rusted steel after sand blasting, clean it with brushes, rags, etc., or blow clean the residual abrasives with clean compressed air.
7) Roughness inspection roughness testing generally using professional instrument testing, such as RT200, R1020-type roughness detection electronic instrument, such as carrying convenient small testing equipment or professional equipment for testing.
3. 1. 3 After sand blasting rust brush engineering paint
paint is for steel structure processing and production process corrosion. Construction experience believes that this link is very important, must be in accordance with the requirements of the specification of serious brushing operations. Engineering paint as the steel structure of the first "barrier", if the first "barrier" no quality assurance, until the completion of each rod parts processing, a long time, sun and rain, most of the project paint peeling or rust spots, so the only solution is to take artificial grinding rust removal, and then paint. This not only causes financial and labor costs, but also delays in construction time, so it is particularly important to ensure the quality of construction of engineering paint.
Brushing engineering paint work notes: (1) sand blasting or particle rust removal, roughness must be in accordance with the specification requirements after sampling, inspection to meet the design requirements of roughness, 24h internal brushing engineering paint;
3. 1. 4 Factory steel components after processing and production is completed coating
after the steel plate raw material processing brush on the engineering paint, the next process is the production of steel components, to be finished after the steel components processing and finishing the paint.
To this steel structure bridge as an example, the steel components mainly include steel box beams and main trusses, in the steel box beams and main truss rod parts of the internal brush primer and medium paint, box beam and main truss rod outside only brush primer and medium paint, paint until the completion of the site installation group.
the steel structure engineering components coating primer selection zinc-rich primer, the paint film rust-proof ability, with cathode protection and can penetrate into the welding place, can withstand solvents, and weather resistance in the sun stable. Medium paint selects thick film type epoxy medium paint.
3. 1. 5 Factory steel components coating work
factory processing and production of each rod parts, in turn, the primer, medium paint painting brushing work, the specific work is as follows.
1) Steel structure rod parts processing and production processing paint parts, the use of artificial electric grinding wheel small area secondary rust removal, rust removal grade requirements for St3.
2) In the factory processing steel component weld position, uneven place by hand electric polishing, you can use the corresponding putty paste as a flat bottom, dry after the use of sand cloth flat, and then brush the primer.
3) Partly processed rod parts, engineering paint intact and rust-free parts, just clean the surface part of the oil, dust, etc. , you can paint the primer.
4) The choice of paint should be made in accordance with the design and specification requirements.
5) Paint method is basically using brush coating method and manual roll coating method, brush primer using a good elastic hard brush, so that you can increase the adhesion of the paint film, brush the last past paint can use a good elastic soft brush, so that the effect of the brush surface smooth, flat. Some small rod parts of the interior has no operating space, before processing and assembly paint, but in the welding assembly process, because part of the welding using single-sided welding, double-sided molding technology, so the internal weld position of the original brushed paint film damage must be repaired, so the use of air spray method, spraying with a spray gun often have to spray several times to achieve the required paint film thickness, this method is easy to waste paint, suitable for confined space.
3. 1. 6 Key precautions for coating operations
In order to ensure the quality of coating operations, in the process of coating operations, to grasp the key links, key operations, the specific precautions are as follows.
1) The paint construction environment cannot be coated when the air humidity >85% or when the surface of the component is exposed.
2) The temperature of the coating operation is suitable at 5 to 38 degrees C, and when the temperature is below 5 degrees C, in principle, the painting should be stopped unless the corresponding low temperature coating material is used.
3) When the temperature ≥ 40 degrees C, the coating should be stopped. Because the temperature of the ≥ is 40 degrees C, bubbles are creating when the paint is painted on the surface of the steel plate, thus reducing the adhesion of the paint film.
4) When a coating enters the field, it must be determined by the coating performance of the professional department, and the appearance transparency, color, cortability, tentacles, viscosity, fineness, density, solid content and other indicators must be qualified before they can be put into use.
5) On-site deployment between diluents, curing agents, main paints, substations, etc., must be based on the best match ratio obtained by the test for on-site deployment, and fully stirred evenly, color, viscosity to achieve uniformity before painting.
6) When brushing the 1st layer primer, the brush direction should be the same and the wiring should be neat. Brushing should be diligent, short brush, to prevent the brush with too much paint and fall. After the unified brush 1st dry, then brush 2nd time, 2nd brush direction and 1st brush direction perpendicular, so that the thickness of the paint film is uniform.
7) Epoxy paint layered paint brush, each layer should be coated before the previous layer of solid work, the primer after coating must pass the inspection before the next process.
8) The steel components that are welded in the factory can only be painted after welding, polishing, peeling, welding scars, dust, etc. In the factory coating work, the steel components will be installed in the two rods docking position set aside 20cm width without painting, convenient on-site welding.
9) After brushing the primer must be quality and paint film thickness inspection, and the interval is long enough, you must wait until the paint is completely dry (general interval of 6 to 8h) before you can brush the middle paint. After painting, in 4h in the event of high winds or rain, should be covered to prevent surface contamination of dust and water vapor, so as not to affect the adhesion of the coating.
10) paint surface detection using a special paint film detector for testing, you can also use a simpler and more practical manual detection method. In this introduction to the author in the construction site summed up a simple and effective method, with dry small planks or small plastic plates to coated paint film surface pressure, and then remove the small wood, if the cotton ball can fall freely, and paint film non-stick fiber or cotton ball traces, and no loss of light phenomenon, can be considered that the paint film has actually dried.
3. 2 On-site steel components after installation
on-site steel structure installation phase coating operations mainly include: truss rod parts and box beam exterior paint work, assembly butt weld position of the 3 layers of bottom, medium, paint work, and steel components in the process of transport to the site installation to knock off the paint part of the paint.
1) The upper truss and the outside of the box beam paint work because in the steel structure processing plant to complete the steel components of the primer, medium paint, the site rod parts after docking after the coating work, if the brushed paint film has not been damaged, just the rod surface dust, moisture and other surface clean, you can carry out paint work.
2) The coating of the steel component at the weld at the docking position must pass the weld test before it can be carried out. Before painting, also need to be welded, to weld slag, oxidizing skin, welding scars, dust, burrs, oil, etc. , part of the uneven place can be filled with putty paste, and then brush the bottom, medium and finish.
3) Part of the rod part due to transport lifting and on-site installation of lifting knock off the paint part of the paint brush. Here before painting needs to be artificial small area rust removal polishing, rust removal method using artificial electric grinding wheel small area rust removal, rust removal grade requirements for St3. After rust removal, the surface is cleaned, and the bottom, medium and finish paint are added in turn.
4 steel structure coating quality inspection
4. 1 Pre-coating inspection
1) Steel surface rust removal must reach the designed rust removal level, and within the specified time to paint the primer.
2) the paint into the factory should have product certification, and in accordance with the acceptance rules of the paint products for re-inspection and coating matching test.
3) The coating environment should comply with the requirements set out in the specification.
4. 2 Inspection of the coating process
1) In the coating operation, the operator cannot adjust the construction viscosity of the coating without authorization, and shall be adjusted by the designated person.
2) In layered coating operations, the thickness of each coating is determined using a paint film thickness meter.
3) In layered coating operations, each paint film is not allowed to bite, peel, leak, foam and other phenomena.
4. 3 After coating inspection
1) coating appearance inspection coating appearance should be uniform, flat, plump, shiny, no bottom, peeling, cracks, pinholes and other defects.
2) Coating thickness inspection is determined by dry film thickness meter, and in accordance with the steel structure coating acceptance specifications and design requirements, quality acceptance.
5 Conclusion
Steel structure coating protection using coating coating to isolate the coated rod parts from the environment, so as to achieve the purpose of corrosion prevention, extend the life of the coated objects. Therefore, the quality of the coating is the key factor affecting the coating protection, and 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 component surface, the thickness of the paint film, the coating process and so on. Therefore, only the steel structure from the processing and production to the site installation of the entire coating anti-corrosion control, in order to ensure the quality of steel structure engineering.