Long-lasting anti-corrosion coating of steel bridges

In recent years, China's bridge construction has developed rapidly, steel bridge with its good earthquake resistance, light self-quality, short construction cycle and other advantages, has been more and more applications, especially across the river, across the sea, across the canyon in the large span bridge are used steel bridge. Surface corrosion, stress corrosion and fatigue corrosion are one of the important reasons why this super-large steel bridge component has the appearance defect, the life is reduced, and the working ability is lost. Steel corrosion not only brings huge economic losses, but also endangers the safe operation of the structure
the
. Therefore, the long-lasting anti-corrosion coating of steel bridge is very important in the construction of the project.
there are many factors affecting the long-term corrosion of steel bridges, and the angle of concern is different. Among them, the most concerned is the paint itself and construction quality management. This paper mainly discusses the impact of standard specifications, coating system design, coating materials, construction management, etc. on the quality of long-lasting anti-corrosion coating.
1
Revestate Of Steel Bridge Construction
The Highway Bureau of the Ministry of Transport has implemented the relevant requirements of the State Council's Opinions on resolving excess capacity in the steel industry to achieve relief from poverty and development (Guofa No. 6 of 2016). With supply-side structural reform to accelerate the construction of highway steel structure bridges, promote the transformation and upgrading of highway construction, put forward some opinions, the views mentioned: At present, China's steel capacity utilization rate of less than 70%, the steel industry to capacity, inventory situation is urgent, need to increase support from various industries. At present, the developed countries in the world have gradually developed to the stage of steel structure bridges, France, Japan, the United States, the proportion of steel structure bridges has reached 85%, 41%, 35%, compared with China's steel structure bridge applications are not wide enough, accounting for less than 1% of the total number of bridges.
, China has added about 28,000 bridges a year, which is an important area for the use of steel. It is also a rare opportunity to accelerate the development of steel bridges. In recent years, more and more large-scale steel bridges, to the company built steel bridges, for example, after the Hong Kong-Zhuhai-Macao Bridge, there are currently more than 100,000 t projects in production projects, as shown in Table 1.
Table 1.1 million t or more in the production of bridge projects
as can be seen from Table 1, to complete these projects with high quality, high-quality coating is an indispensable and important part of the whole bridge construction system engineering.
2
steel bridge common long-lasting heavy anti-corrosion system
steel bridge anti-corrosion standards in the anti-corrosion system is more, such as TB/T1527-2011 "railway steel bridge protection coating" and JT/T722-2008 "road bridge steel structure anti-corrosion coating technical conditions
The JT/T722-2008 system is divided in more detail according to the corrosion environment and the age of protection. The bridge built by the Company is usually a national key project, its requirements of anti-corrosion system are long-lasting. Therefore, the most commonly used is the most cost-effective system, the construction performance of the best one.
2.1
Fluorocarbon paint anti-corrosion system
In recent years, the company produced more than 10,000 t bridge coating programs are mostly using epoxy-rich zinc-rich primer and epoxy cloud iron intermediate paint and fluorocarbon paint (conventional standard system) long-lasting heavy anti-corrosion system, such as the Shanghai-Tong Yangtze River Bridge, Humen Yangtze River Bridge, Wuhu Yangtze River Bridge, Pingtan Strait Bridge, Hong Kong-Zhuhai-Macao Bridge and so on.
2.2
Polyurethane paint anti-corrosion system
In recent years, the company has built a foreign project, more use of inorganic / epoxy zinc-rich primer and epoxy cloud iron intermediate paint and polyurethane varnish heavy anti-corrosion system, such as Hong Kong Angzhou Bridge, Germany single-line (double-line) bridge, Venezuela Bridge, Bangladesh Padma Bridge and the United States VN Bridge.
3
factors and recommendations affecting the corrosion durability of steel bridges
The current design life of large steel bridges is generally 100 to 120a, steel bridges start to operate, some bridges in the environment can no longer carry out large-scale re-corrosion coating, so the bridge needs to implement long-lasting anti-corrosion coating in manufacturing construction. Technical standard specifications have a strong guiding significance for anti-corrosion coating, but the different degree of awareness of its understanding will be different, and with the development of new coating technology, if too dogic understanding of standard specifications may be counterproductive; One of the important factors, especially the design of innovative anti-corrosion support program, must fully demonstrate the reasonableness of the coating, new varieties or new coatings because there is no large-scale production application, lack of sufficient application experience, there will be a variety of problems, therefore, the coating itself is also one of the important factors affecting the corrosion resistance of steel bridges. In addition, construction management, construction process is also a direct impact on the steel bridge corrosion durability factors.
3.1 standard factor
3.1.1 standard lag
due to the implementation of standards or specifications every 5 to 10a only revised, and anti-corrosion coatings and process updates faster, coupled with the increasingly stringent environmental regulations, the standard will inevitably appear part of the content can not meet the existing coating technology and environmental requirements. Therefore, in the use of standards can not be orderly, too dogman, otherwise in the actual construction can not guarantee the duration and can not guarantee the quality.
the standard is after all a relatively strong technical documents, there is no certain coating related knowledge of people only by literal meaning more difficult to accurately grasp the content of it, from time to time the bid will be very necessary. For example: TB/T1527-2011 "Railway Steel Bridge Protection Coating", special epoxy zinc-rich primer dry film thickness of 40 m / road, epoxy cloud iron intermediate paint dry film thickness of 40 m / road, these two requirements are similar, are coating dry film thickness requirements, which do not match the current high solids low VOC content requirements. Current coatings are usually sprayed with a dry film thickness of about 80 m without thinners or a small amount of thinner. If strictly in accordance with the standard, special epoxy zinc-rich paint should be added a large number of thinners to reduce the viscosity of the coating, to ensure that the thickness of a primer sprayed is about 40 m. Because thinners do not participate in the chemical reactions of coatings, they all evaporate into the atmosphere to pollute the environment. Environmental regulations and standards have no practical meaning for VOC restrictions.
In addition, the friction surface of the use of arc aluminum spray process, due to the construction process produced by a large number of noise, arc, metal dust on people and the environment is very good, so the bridge steel structure corrosion spray aluminum construction is usually night work. However, if the coating plant is close to the residential area, the noise during the evening coating construction work is very loud, which will also affect the normal life of the residents in the vicinity.
3.1.2 Lack of understanding of the standard
Design anti-corrosion coating system in reference to TB/T1527-2011 "Rail Bridge Protection Coating", often ignore the friction surface of the application conditions for the use of arc aluminum coating. The standard clearly states that "for the production or storage of welding beams in the area south of the Yellow River, it is advisable to use inorganized zinc-rich anti-rust anti-slip coating spray friction surface." "This is due to the high humidity and porous aluminum coating in the area south of the Yellow River, where the surface of the aluminum coating is prone to rust, as shown in Figures 1 and 2. After full test and verification, the surface of aluminum coating rust during the guarantee period does not affect the anti-slip coefficient, only affects the appearance. When this happens to the actual bridge rod parts, most owners or supervisors can not accept the appearance of rust and beauty, require rectification, not only labor-seeking and even affect anti-slip performance.
Figure 1 storage less than 1 month aluminum coating
Figure 2 storage 5 months (during the rain) aluminum coating
standard in the long-lasting anti-corrosion coating in the core position, it is equivalent to the industry's "regulations", it makes the processes, quality requirements, construction methods, such as "law-based", standard guidance is very important. To this end, when recommending standard revisions, the health and safety of the person should be the first to be concerned. If there are alternative new coatings, you should not hesitate to remove unsealing processes or materials from the standard. At the same time, the new version of the standard should be published with a number of bid fairs to help downstream related units correctly understand and use the bridge anti-corrosion standards. When the standard update lags behind, the bridge construction management personnel should use the development of the vision to interpret the standard and norms.
3.2
Coating matching system design factors
the bridge standard can choose more anti-corrosion supporting system, the designer can be based on the corrosion environment and expected corrosion life of the steel bridge, the correct choice of the matching coating matching system. However, the innovative design of anti-corrosion support scheme, if not fully demonstrated its rationality, will most likely lead to waste of resources, environmental pollution and quality hazards. As in table 2, there are four design systems that are not appropriate, as follows:
(1) Inorge rich primer thickness is designed to be 20 m.
(2) inorganic zinc-rich primer sprayed with a minimum of 60 m, if the design of 20 m, can only add more diluent, but a spray thickness of at least 40 m, which not only increases paint consumption but also aggravates environmental pollution.
roughness of the aluminum coating with an electric arc (3) arc requires Rz40 to 60 m.
roughness required for an electric arc spray aluminum coating should be Rz60 to 100 m. Because the coating is attached to the substate mainly by mechanical binding, the rough surface can increase its contact area and adhere more firmly. Lower surface roughness reduces the layer adhesion between the aluminum coating and the substation, and the aluminum coating falls off easily.
(4) high-strength bolt friction face bit using arc spray aluminum and inorganical zinc-rich workshop primer coating.
high-strength bolt friction surface in the bridge standard provides a stable anti-slip coefficient requirements, that is, at the factory anti-slip coefficient of 0.55 and above, set up to reach 0.45 and above (not greater than 6 months). At present, only the arc spray aluminum layer and inorganical zinc-rich anti-slip coating 2 coatings can meet the requirements, these two coatings must be used independently, that is, the coating for a separate arc spray aluminum layer or inorganical zinc-rich anti-slip coating, is by no means a composite coating. On the other hand, inorganical zinc-rich workshop primer is not inorganical zinc-rich anti-slip coating, does not have a high anti-slip performance, this part with the wrong coating will directly affect bridge safety.
(5) add 100 m of fluorocarbon paint if exposed directly to air.
this error does not involve safety hazards, but it wastes paint, which is the most expensive in a composite coating system, resulting in unnecessary cost increases. The fluorocarbon finish of 100 m can be changed to 70 to 80 m.
Table 2 Design Coating System
Note: The exposed part of the oblique puller sleeve should be coated according to the inner surface of the steel anchor beam, and if exposed directly to the air, add a layer of fluorocarbon paint of 100 m.
addition, there is an occasional ergoncation design, such as the requirements of steel girder bridge box rod pieces (up and down string rod, abdominal rod, etc.) internal coating. Whether domestic or foreign such bridge rods are used inside the air-tight anti-corrosion. The box-type rod part of the steel girder bridge is different from the inside of the steel-beam bridge, the general girder box-shaped rod is completely enclosed structure, and the internal space is small. And the steel box beam bridge internal space is larger, each coating section is not closed space, people and equipment can enter and exit the interior. The inner surface coating system of the bridge listed in the standard also refers to the steel box beams rather than the steel girder bridges. If the inner part of the box rod of the girder bridge requires coating, the inside of the slotted structure can only be coated before the box rod is welded, and then the cover plate is welded. However, this kind of operation is not conducive to quality assurance, one is because the structure welding, sand blasting, spraying these three processes can not be operated in the same plant, groove type is also an unstable structure, lifting is more likely to cause component deformation; Therefore, the internal coating of the closed space of the girder bridge rod is meaningless at all.
, the design of a reasonable coating supporting system is a very important part of the system, if the coating supporting system has problems and hidden dangers, and then careful scientific management are in vain. It is suggested that system innovation should first consider the rationality of theory, the necessity of implementation, then carry out objective experiments, and finally consider the operability of the field. In addition, it is recommended that the design drawings should be reviewed for the coating system should also have the corresponding professional audit, to avoid the personal cognitive errors caused by waste of resources or buried from the source of coating quality and safety hazards.
3.3
construction management (owner, supervisor, general package) factors
scientific, strict coating construction process management is very important. But the actual construction management does not speak of science only talk about strict, overemphasized the appearance effect, excessive repair will be the bridge later coating quality buried hidden dangers.
common cases are as follows:
(1) surface defects sometimes produced during the rolling of steel plates, some of which are acceptable within standard tolerances. However, in order to pursue the appearance effect usually requires the steel plate putty filling defects. If the 300 m pit is filled, coupled with a normal coating of nearly 300 m thick, the overall coating thickness of the local surface is at least 600 m, the coating is too thick, the stress is large, it is easy to crack off.
(2) inorganic zinc-rich anti-slip coating surface to produce white zinc salt, in order to ensure the appearance of the effect, the coating surface is required to clean.
anti-corrosion of zinc-rich paint is to protect steel through the chemical reaction of metal zinc. Zinc is a lively metal, and the air is easily oxidized to produce zinc salts. Therefore, the appearance of white material on the surface of the zinc-rich coating is the normal reaction product zinc salt, can block shield corrosion further, although the appearance is not beautiful, but does not affect the anti-slip coefficient, do not need to be treated. If cleaned, improper operation by the workers will affect the anti-slip coefficient and anti-corrosion performance of the coating.
(3) high-strength bolt hole inner wall requires paint paint paint, can not rust. This position at home and abroad coating methods are the same, do not carry out special brushing, if the brush to paint it, the inner wall edge will inevitably accumulate paint drops, installation due to the presence of raised paint drops, will affect the stitching plate between the close. Therefore, the correct approach is to spray the friction surface on both sides of the steel plate, the use of spray gun angle will also be more or less covered to the inner wall, occasional local appearance is not covered place, do not use a brush to repaint. If the coating and installation interval is long, the inner wall will sometimes rust before installation, but after the high-strength bolts are screwed, no air will enter the interior and corrosion will stop.
(4) primer partial re-painting method only accept spraying, not allowed to brush, for fear of affecting the appearance of the effect.
if the thickness difference is less than 20 m, the primer should be partially patched by brushing. Because when air is sprayed, even with more diluent, it is at least 40 m and the coating range is expanded. Zinc-rich coating should not be too thick, especially high zinc content of coatings, easy to crack. For zinc-rich primers, the coating is not as thick as possible and patched