Analysis on the problems and Countermeasures in th

2022-08-16
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Analysis of the problems and Countermeasures in the application of fire retardant coatings for steel structures

with the vigorous development of the construction industry, the form of building structures is changing with each passing day. Steel structures have been widely valued because of their high strength, light weight, good seismic resistance, fast construction, low cost of building foundation, less occupied area of structure and high degree of industrialization. Although the steel structure is non combustible, it is very easy to conduct heat. When its temperature reaches above 540 ℃, its mechanical properties decline rapidly, while when the temperature reaches 600 ℃, the strength of steel is almost zero. When the test results show that the fire resistance limit of bare steel is usually only min. According to the requirements of China's relevant building fire protection design codes, the fire resistance limit of building columns, beams, floors and roof load-bearing components is 0 0h. On May 5th, 1998, a fire broke out in Beijing Yuquanying Huandao Furniture City, and the fire prevention of steel structures failed to meet the standard. All 13000 M2 light steel structures collapsed, causing a direct economic loss of 20.87 million yuan; After the "September 11 incident", which shocked the world, experts believed that the plane did not crash the building, but because the plane destroyed the fire-proof coating on the steel structure of the building when it hit the building, and exploded and caught fire, exposing the steel structure to the raging fire. After more than an hour, the structure softened, lost strength, and collapsed. Therefore, the steel structure must be protected against fire, so as to improve its fire endurance

there are various fire prevention measures for steel structures. In the past, methods such as casting concrete shell, setting fire barrier with fire bricks, wrapping with non combustible materials such as gypsum board, or filling water inside hollow pipes for cooling have been used. At present, the method of spraying fire-proof coating on steel structures is more used to protect steel structures. Because this measure has good fire and heat insulation performance, the construction is not limited by the geometric shape of the structure, generally there is no need to add auxiliary facilities, it is easy to implement, and it has a certain aesthetic and decorative effect, which has been widely used. However, there are some problems in the selection, durability and construction quality of fire retardant coatings for steel structures, which also bring some difficulties to the fire prevention supervision

I. selection of fire retardant coatings for steel structures

(I) classification of fire retardant coatings for steel structures

at present, according to the classification of GB "technical conditions for the use of fire retardant coatings for steel structures with great influence on strength due to faulty water addition", fire retardant coatings for steel structures can be divided into ultra-thin, thin coated and thick types according to their coating thickness and performance characteristics. The coating thickness of ultra-thin steel structure fire retardant coating (CB type) is not more than 3mm, and the coating thickness of thin coated steel structure fire retardant coating (B type) is 3~7mm. These two types of coatings can absorb heat, expand and foam in case of fire, and form a foam like carbonized insulation layer, so as to prevent heat transmission to the steel structure, delay the temperature rise of the steel structure, and play a fire protection role. These two kinds of coatings are usually called intumescent fire retardant coatings for steel structures. The coating thickness of thick steel structure fire-proof coating (Class H) is 7~45mm, which presents granular surface, with small density, and the coating does not foam when heated. Relying on its low thermal conductivity, it can delay the temperature rise of steel structure and play a role in fire protection. The fire resistance limit can reach 0.5~3.0h. This coating is usually called steel structure fire-proof and heat insulation coating. The two kinds of coatings have different performance characteristics and are suitable for different occasions. All steel structure fire retardant coatings are divided into indoor and outdoor use types

(II) problems in the selection of fire retardant coatings for steel structures

the main problems in the selection of fire retardant coatings for steel structures are as follows: first, ultra-thin fire retardant coatings for steel structures have become a hot spot in the research and development of fire retardant coatings for steel structures in China. Ultra thin fire retardant coatings for steel structures are generally solvent based systems. Compared with thick and thin coating fire retardant coatings for steel structures, ultra-thin fire retardant coatings for steel structures have better decorative properties, The coating is thinner, and the amount used in the project is greatly reduced, thus reducing the total cost of the project. It is a variety vigorously promoted in the market at present. However, the phenomenon of one-sided pursuit of thinner and thinner coatings and one-sided promotion of ultra-thin coatings have appeared in the fire protection engineering of steel structures, which is extremely detrimental to providing practical and reliable security for buildings. Because the ultra-thin steel structure fire retardant coating mainly depends on the physical and chemical reaction of the organic components in the formula, any factor affecting the carbonization and expansion properties of the coating will affect its final fire resistance performance, and it is difficult to ensure that it can provide more than 2.0h fire protection for components after a long time of coating

second, designers often simply indicate the protective measures of fire-proof coating for steel structures, and cannot specify in detail which type of fire-proof coating for steel structures is selected. In the construction, they simply consider the price, beauty and other issues, resulting in improper selection of fire-proof coating. Third, the requirements of the building environment and the nature of use are often ignored when selecting the type. When the open-air steel structure is exposed to the sun and rain, or when the upper part of the top steel structure of high-rise buildings adopts the transparent plate, the sun is exposed to the sun, and the environmental conditions are relatively harsh, the outdoor steel structure fireproof coating should be selected. Fourth, the paint whose technical performance only meets the indoor requirements is used outdoors

(III) correctly select the fire-proof coating for steel structures

1. Expansion type fire-proof coating should not be used outdoors

intumescent fire retardant coating was first used for wood structures. Due to the poor outdoor weather resistance, easy aging and peeling, the reliability of expansion in humid and corrosive environments, as well as the dense smoke and toxic gas produced by resin decomposition in the process of high-temperature flame barbecue foaming, it is not suitable to use intumescent fire retardant coating in tunnels, outdoors and buildings where people often stay without special measures. Projects in nuclear power, electric power, petrochemical and chemical industries should mainly be based on thick fire retardant coatings

fail to meet the UL standard requirements for modified polypropylene materials for front and rear bumper bodies

2. Carefully select thin coated steel structure fire retardant coating, and strictly control the selection of ultra-thin steel structure fire retardant coating

it is necessary to strictly control the use of all steel structure buildings, especially steel columns, steel beams, steel floors and steel evacuation stairs in large public and civil buildings. Steel columns, steel floors, steel evacuation stairs and other major building components should give priority to the use of steel sheet concrete and other fire protection materials. If it is really difficult, thick steel structure fire retardant coating can be used for protection, thin steel structure fire retardant coating should be carefully selected, and the selection of ultra-thin steel structure fire retardant coating should be strictly controlled. Thin coated and ultra-thin fire retardant coatings for steel structures of high-rise buildings and multi-storey steel structure plants are not suitable

3. Fire retardant coating shall be selected according to the building location and fire resistance limit requirements

hidden steel structures in buildings have low requirements on the appearance quality of the coating, and thick steel structure fire retardant coating shall be used as far as possible. For exposed steel frames, steel roof trusses and roof load-bearing structures, when the fire resistance limit specified in the specification is less than 1.5h, thin coated or ultra-thin steel structure fire retardant coating can be selected. When the fire resistance limit is more than 2.0h, thick steel structure fire retardant coating should be selected

II. Durability and use management of fire retardant coatings for steel structures

(I) durability of fire retardant coatings for steel structures

no matter what kind of fire retardant coating, the fire resistance limit given in its test report is the test result before its use, and after several years of application of fire retardant coating, whether the fire resistance performance of fire retardant coating remains the same? At present, there is a lack of scientific evaluation on the service life of various types of fire retardant coatings in the engineering environment, and the fire retardant coating manufacturers cannot provide the service life and replacement cycle in different service environments in the product description. The use of fire retardant coatings for steel structures is based on the attitude of "once and for all". The research on the durability of fire retardant coatings for steel structures has become an urgent work. It is necessary to formulate evaluation standards and methods for the durability of fire retardant coatings, so as to scientifically determine the cycle or years of replacement or maintenance of fire retardant coatings. In addition, the maintenance of fire retardant coating is not a simple re brushing. Can the re brushing on the old fire retardant coating base meet the technical performance requirements? How to overcome the construction difficulty of refreshing the fire retardant coating, especially for concealed works? These are topics that need further study

(II) use and management of fire retardant coatings for steel structures

there are many manufacturers of fire retardant coatings for steel structures in China, and there is no unified technical standard. There are few varieties of ultra-thin fire retardant coatings for steel structures and outdoor fire retardant coatings for steel structures with excellent performance, so they should be used with great caution

the paint model must be approved by the design unit, construction unit, construction unit and fire department. Fire retardant coating should also have long-term functionality with the long-term use of buildings. Therefore, after the construction and acceptance of fire retardant coating, attention should also be paid to maintenance and management. First, avoid being damaged by other operations or accidental impact, wear, rain and pollution. Regularly observe the integrity of the coating, and the coating shall not fall off. Second, for intumescent fire-proof coatings, the foaming performance of fire-proof coatings should be regularly measured by prefabricated test pieces during construction. When the foaming performance of fire-proof coatings is lost, corresponding measures should be taken

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