There are many ways to prevent steel structure from rusting, and the following methods are usually adopted.
1. The steel structure is made of alloy steel which is not easy to rust.
2, chemical oxide layer protection method
3. Adopt metal coating protection method.
4. Non-metallic coating protection law
In the later maintenance process, nonmetallic coating protection methods are especially commonly used. The surface of the component is protected by paint and plastic to prevent it from contacting with the surrounding corrosive medium, so as to achieve the purpose of anticorrosion. This method has the advantages of good effect, low price, many kinds of coatings, wide selection range, strong applicability, no restrictions on the shape and size of the component, the ability to form a thin film of any shape on the surface of the component, firm adhesion, and convenient use. You can also give the component a beautiful color.
Requirements for Daily Maintenance of Coatings For maintenance personnel, the daily maintenance of steel structures should first be the maintenance of surface coatings of components. The quality of coating maintenance directly affects the service life of steel structure. Therefore, to do a good job in daily maintenance, we should start with the following points:
1, the surface of steel structure must be kept clean and dry, and the places where steel structure is easy to accumulate dust (such as steel column feet and joint plates) should be cleaned regularly.
2, regularly check the steel structure protective coating in good condition, there are the following situations should be timely maintenance:
(1) It was found that 90% of the coating surface lost luster.
(2) The rough, weathered and cracked surface of the coating reaches 25%;
(3) The coating film bulges, and the slightly corroded area of the component reaches 40%;
3. The steel structure affected by high temperature and high temperature should be protected by protective plates to protect the coating from high temperature damage.
4, try to avoid contact with corrosive substances, components have been in contact with should be cleaned up in time.
Repair and update of coating 1. Rust removal on the surface of components For steel structure engineering that has been used for a period of time, there will inevitably be some attachments on its surface, such as rust, dirt, dust, etc. Old paint film, etc. Before painting the steel structure surface, if these attachments are not completely removed, they can be temporarily covered after painting. However, due to its isolation effect, the adhesion between the coating and the component matrix is seriously reduced, and the paint film will fall off prematurely, which eventually leads to the decline of the corrosion resistance of the surface coating and cannot play its due protective role. Therefore, before the component surface is painted, the attachments on the steel structure surface should be thoroughly cleaned.
In the construction of steel structure maintenance project, surface cleaning mainly includes removing the old paint film. In the process of derusting, due to the limitation of construction conditions, generally adopted methods mainly include:
1, manual derusting
In this method, steel tools such as scraper, shovel, hammer and wire brush are used to remove dirt manually, and emery cloth, sandpaper and grinding wheel are used to polish it manually, so that the surface of the component is basically free of oil, rust and burr. This method is often used in steel structure maintenance engineering because of its convenience, simple equipment, low labor cost and no limitation of construction site conditions and scale. Its main disadvantages are poor working conditions, low working efficiency, incomplete rust removal and difficult quality assurance. Therefore, when South Korea adopts this method to remove rust, managers should emphasize the quality requirements.
2, mechanical derusting
In order to improve derusting quality and work efficiency. In order to improve the working conditions of construction workers, a large number of small pneumatic or electric equipment are used for rust removal of steel structures. The main derusting methods for using this equipment are:
(1) Angle grinder: This small pneumatic device is mainly used for cleaning flat places. It can use sandpaper, grinding wheel and wire brush as needed.
(2) Needle beam rust remover: This small pneumatic equipment is generally equipped with 30~40 needle beams, which can be adjusted according to different working faces, and is mainly suitable for curved, narrow, uneven and crevice places.
(3) Single-ended cold air gun: This small pneumatic device is also called zinc. It uses the impact of shovel head to remove rust. The diameter of shovel head is generally 25~40mm, and it can impact 1000 ~ 6000 times per minute, which is suitable for narrow places.
3, sandblasting derusting
In projects where construction and maintenance can be stopped, sand blasting can be used to remove rust. The rust on the top surface of steel structure members is cleaned by sandblasting machine, revealing the true colors of metal. A better sandblasting machine can automatically screen out the sprayed fine powder of yellow sand, iron sand or iron shot to prevent dust from flying and reduce the impact on the health of operators. This method can completely remove rust. The efficiency is also high, and it has been widely used in developed countries, which is a relatively advanced rust removal method.
4. Remove rust with pickling paste.
Special pickling paste for rust removal can be purchased in the market. The application method is to coat the pickling paste on the surface of the component to be placed, and its thickness is about 1~2mm. After soaking and rolling for a proper time, peel off a small piece of pickling paste to check the rust removal. If the surface of the part is metallic, peel off the pickling paste, rinse it with water, and thoroughly remove the residual acid. Except for some special cases, this rust removal method has been rarely used at present.
Second, different measures can be taken to clean the old paint mold on the surface of the component according to the different situations of the paint film. Specific measures are:
1. If the old paint film is solid and complete, and the surface of components is well adhered, impurities on the surface of the old paint film can be removed with soapy water or dilute alkaline water, washed and dried with clear water, and then painted after polishing.
2. If most of the old paint film is well attached to the parts, and some parts need to be removed, it should be cleaned according to the above method, and it should go through putty, polishing, touch-up paint and other processes to ensure that it is flat and consistent with the old paint film and has the same color.
3. If most of the old paint film has been damaged and fallen off, it needs to be completely removed. The removal method is as follows:
1) flame spraying method: the paint film is melted by the torch flame, and then scraped off immediately with a shovel. This method is generally used to clean old paint films in a small area.
2) Alkaline water cleaning method: brush for 3-4 times with dilute solution made of lime and soda ash or 5%~ 10% (mass fraction) sodium hydroxide solution. Wrinkle and peel off the old paint film, then scrape it off with a shovel and wash it with clean water.
3) Paint remover brushing method: Brush the old paint film with paint remover sold in the market. After about half an hour, the old paint film will swell and wrinkle. Wipe off the old paint film with a shovel and a wire brush to remove other impurities on the surface of the component.
4) Paint stripping paste method: Apply paint stripping paste to 2-3 layers on the surface of the old paint film for about 2-3 hours, and the paint film will be damaged, then remove it with a shovel and rinse it with water. If the old paint film is too thick, in order to shorten the soaking time, an appropriate number of openings can be broken in the old paint film with a knife before the paint remover is applied.
4, paint construction methods and matters needing attention
Painting construction method: There are two common methods in maintenance engineering, namely brush painting method and spray painting method.
Precautions:
1) Before painting, the base shall be thoroughly cleaned, kept dry, and primed as soon as possible within 8 hours.
2) When painting the primer, the appropriate painting method should be selected according to the area.
3) When the top coat is painted, it should be painted in the colors and varieties specified in the design requirements, and the painting method is the same as that of the primer.
4) Brush the environment
5) Diluent should be used correctly.
Common defects, causes and preventive methods in painting construction
The temperature resistance of steel subjected to conventional fire protection treatment is poor, and many properties change with the rise and fall of temperature. When the temperature reaches 430-540℃, the yield point, tensile strength and elastic modulus of steel will drop sharply and lose its bearing capacity. It is necessary to maintain the steel structure with refractory materials. It has not been treated with fire retardant paint or fire retardant paint before. The fire resistance limit of a building depends on the fire resistance limit of building components, and its bearing capacity should last for a certain time when a fire occurs, so that people can evacuate safely, rescue materials and put out the fire.
The fire prevention measures are as follows: Therefore, the exposed steel members should be painted with fire prevention paint, and the specific requirements are: the fire resistance time of steel beams is 1.5h, and the fire resistance time of steel columns is 2.5h to meet the requirements of building codes.
The damage caused by regular monitoring of steel structure corrosion is not only manifested in the thinning of the effective section of the component, but also in the "rust pit" on the surface of the component. The former reduces the bearing capacity of members and the overall bearing capacity of steel structures, especially for thin-walled steel and light-weight steel structures. The latter causes the phenomenon of "stress concentration" in steel structure, and brittle fracture may suddenly occur when the steel structure is subjected to impact load or alternating load. However, there is no sign of deformation when this phenomenon occurs, so it is not easy to find and prevent it in advance. Therefore, it is very important to monitor the stress, deformation and cracks of steel structures and main components.
The main factor causing excessive deformation of steel structure (1) is that the actual dynamic load of steel structure exceeds the maximum allowable load in steel structure engineering design (commonly known as "overload").
(2) Long-term impact of dynamic load.
(3) The influence of natural disasters (such as earthquakes) or uneven settlement of foundation.
(4) Improper use and maintenance or mechanical damage cause the members in the steel structure engineering to break and quit the work, which leads to unbalanced stress on the steel structure and abnormal deformation.
Inspection and treatment of engineering deformation of steel structure If the steel structure is deformed too much in the use stage, it means that the bearing capacity or stability of the steel structure can no longer meet the needs of use. At this time, the owner should pay enough attention and quickly organize relevant people in the industry to analyze the causes of deformation. Put forward the treatment plan and implement it immediately to prevent the steel structure engineering from causing greater damage.
1, test method
When checking the deformation of steel structure engineering, it is generally necessary to visually check whether there are abnormal deformation phenomena in the whole steel structure engineering and components, such as excessive bending deformation of slender bars and webs. Deformation of connecting plate, etc. Parts with abnormal deformation by visual inspection shall be further inspected. The main contents and methods of inspection are:
(1) During the appearance inspection of beams and trusses in steel structure engineering, when it is found that the deflection of the lower chord of the truss is too large, the plane of the truss is distorted, the roof is partially uneven, and the decoration such as ceiling and painting in the room is cracked, it can be considered that the deformation of the beams and trusses is abnormal, and it is necessary to tighten the two ends of the numerical support or truss chord with thin wires to measure the deformation data in the vertical direction (deflection) and the horizontal direction. If it is a rough measurement, otherwise, multi-point deformation data should be taken along the length direction. If necessary, draw a bitmap of the axis variables of the structure.
(2) Steel wire hammer and theodolite can be used to check the deformation of steel structure columns. During inspection, the deformation data should be measured in two vertical directions to determine the inclination or deflection degree of the column, and if necessary, the axial displacement diagram of the column should be drawn according to the measured data.
(3) For small plate members such as connecting plates and webs, the ruler method can be used for comparative measurement.
(4) For the deformation inspection of slender bars with bending deformation as the main factor, the bar can be stretched at two key points with thin iron wire to obtain deformation data.
(5) It should be noted that the overall shape data of steel structure engineering is determined with reference to its design standard installation position. In order to ensure that the deformation data of steel structure engineering can be measured in the use stage, it is necessary to know the original position deviation of steel structure engineering during installation.
For inspection and all kinds of deformation data, records should be arranged for future reference. For steel structures with large deformation, corresponding marks should be made in relevant parts to create convenient conditions for future maintenance.
2. Technical treatment of steel structure engineering deformation
(1) In general, when the measured deformation data of steel structure exceeds the following standards, it can be considered that there is abnormal deformation in the structure, and the steel structure engineering needs necessary technical treatment.
1) The maximum deflection of roof truss without traveling crane shall not exceed L/200(L is the span of beam); The vertical deformation of the roof truss of a building with a bridge crane should not cause the lowest point of the lower chord to invade the minimum gap required to be reserved between the bridge crane and the roof truss, and the maximum deflection of the crane beam should not exceed L/500(L is the span of the beam).
2) The maximum deflection of roof slats shall not exceed L/ 150(L is the free length of slats). Otherwise, the roof will be uneven and leak.
3) The maximum deflection of the compressed chord within the free length (that is, between two adjacent points) shall not exceed L/ 1000(L is the free length) and shall not be greater than 10 mm. ..
4) The maximum deflection of tension members within the free length shall not exceed L/ 100(L is the free length).
5) For plate members (such as connecting plates), the deflection value within the range of 1m should not exceed1.5 mm.
(2) Treatment methods. For structures with abnormal deformation, rectification and reinforcement can be carried out according to specific conditions. In order to ensure safety, technical treatment (such as removing live load) should be carried out when the structure is unloaded or partially unloaded.
1) For bars with little deformation, it can be corrected with wrench or straightener.
2) For plates or bars with dead bending deformation, jack can be used for correction, and oxyacetylene flame can be used for correction when conditions permit.
3) When steel structure engineering (such as inclined column and twisted roof truss) is deformed as a whole, reasonable reinforcement measures should be taken according to the deformation reasons besides timely correcting the deformation.
Inspection method and repair of cracks in steel structure The whole and components of steel structure engineering should not appear obvious deformation under normal working conditions, not to mention cracks or other mechanical damage, otherwise additional stress will appear due to excessive deformation, or the bearing capacity of components will be weakened due to cracks and other mechanical damage. In severe cases, it will damage members and endanger the overall safety of steel structures.
1, crack inspection method
The main methods to check the cracks in steel structures are observation and knocking.
(1) observation method refers to observing the paint surface of parts with a magnifying glass about 10 times. When linear rust marks are found on the paint surface, or there are thin and straight cracks on the paint surface, the surrounding paint film bulges and there is rust powder inside, it can be preliminarily concluded that there are cracks here, and the paint film here should be removed for further detailed investigation.
(2) The tapping method refers to tapping parts and parts with a wooden hammer covered with rubber. If it is found that the sound is not brittle, the sound transmission is uneven, and there is a sudden interruption, it can be concluded that the components are cracked.
If there are uncertain signs of cracks, physical flaw detection methods (such as X-ray) can be used for further inspection. If you don't have this condition, you can drop oil here for inspection, and you can judge whether there is crack rotation here from the shape of oil diffusion. When the oil spreads symmetrically in an arc, it means that there is no crack here. When the oil stains spread in a straight line, it means that cracks have been formed here.
2, crack repair steps
(1) First, drill a round hole with the diameter equal to the thickness of the steel plate at both ends of the crack, so that the crack tip falls into the hole to prevent the crack from spreading further.
(2) The gap between two boreholes shall be welded. When welding, the crack edge can be cut into different types of grooves by gas cutting according to the thickness of the component to ensure the welding quality. When the thickness is less than 6mm, the shape of 1 is adopted (i.e. no groove); When the thickness is more than 6mm but less than 14mm, the V-shaped groove is adopted; When the thickness is greater than 14mm, an X-groove is required.
(3) After heating the metal around the crack to 200_C, weld the crack with E43 (low carbon steel plate) or E55 (manganese steel plate) covered electrode.
(4) When the cracks are large and have a great influence on the strength of members, the metal cover plate should be reinforced by high-strength bolts except welding cracks.
Regular inspection and maintenance of other diseases of steel structure In the daily management and maintenance of steel structure engineering, attention should be paid not only to the inspection of corrosion diseases, but also to the following aspects:
(1) Whether there are cracks, looseness, fractures, etc. at joints such as welds, bolts and rivets.
(2) Whether members such as bars, webs and connecting plates have excessive local deformation and damage.
(3) Whether the deformation of the whole structure is abnormal or beyond the normal deformation range.
In order to find the above diseases and abnormalities in time. In order to avoid serious consequences, the owner must carefully check the steel structure regularly. While mastering its development and change, find out the causes and abnormal phenomena of the disease, and if necessary, through correct theoretical analysis, get the influence degree on the strength, stiffness and stability of steel structure, and take reasonable measures to deal with it.
Inspection and treatment methods of weld defects In the inspection of weld, attention should be paid to whether cracks appear in the weld during the use stage, and at the same time, defects left over from weld design and construction should be found.
1, common inspection methods of weld defects
(1) During the appearance inspection, the sundries on the weld should be removed first, and the appearance quality of the weld should be observed with a magnifying glass (5~20 times). In addition to the requirement that the weld must be free of defects, it is also required that the weld should have a good appearance. A good weld appearance should have a fine surface. There are no creases, gaps and incomplete welding grooves, and the connection with the base material is smooth.
(2) Borehole inspection This is a destructive weld inspection method. Inspect the welds of important parts from the outside. In order to further confirm, the drilling method can be used to check whether the weld has defects such as porosity, slag inclusion and incomplete penetration. After inspection, these holes can be filled with the same covered electrode as the original weld.
(3) Nitric alcohol etching method is generally used to check cracks that are not easy to observe. The method is to clean up the suspicious place thoroughly, polish it, rinse it with acetone or benzene, and drip the nitric acid alcohol solution with the content of about 8% for erosion. If there is a crack in the weld, a brown hook will be displayed.
Ultrasonic, X-ray and R-ray inspection shall be carried out on the main welds of important components. Ultrasonic, X-ray and R-ray inspection methods shall be used to check whether the whip marks are defective, and X-ray and R-ray negatives shall be taken for analysis and inspection when necessary. The quality of welds inspected by this method is the most reliable, and it is recommended that qualified housing management departments use this method as much as possible.
(4) Special attention: Dynamic load, alternating load and tensile force can cause the defective weld to crack rapidly, resulting in serious consequences. Therefore, for steel structure engineering under dynamic load, the tension area on the component should be strictly checked to prevent omission.
2. Treatment methods of welding defects
No matter which method is used for inspection, corresponding measures should be taken to deal with the defects in the weld.
(1) For weld cracking, in order to analyze the nature of cracks, it is necessary to find out all cracks produced in the use stage, conduct comprehensive treatment, and completely eliminate the disease. It belongs to the place where cracks left by construction can be directly repaired.
(2) For the defects in weld design, such as insufficient weld size or weld leg size, the reasonable size should be redesigned after accurate theoretical calculation. If necessary, the weld can be constructed on the specimen under the same welding conditions as the structure, and then the mechanical test with the same stress as the steel structure engineering can be carried out to confirm the reasonable weld and weld leg size.
(3) If the weld has defects such as incomplete penetration, slag inclusion and air holes, it should be re-welded.
(4) The repair welding shall be carried out when the weld has bite and arc pit.
(5) Re-welding at the flash point should be completely eliminated.
Inspection and Maintenance of Bolt-rivet Connection For newly-built steel structures, except welded joints, bolt connection is widely used. Because the current high-strength bolt connection mode is very mature, compared with rivet connection mode, its operation method has the advantages of simplicity, quickness, low labor intensity and convenient maintenance and replacement. At present, except for special occasions, rivet connections are often replaced by high-strength bolts. For the owner, the steel structure project managed and maintained by himself is not necessarily new, and the owner should learn to check and maintain the bolt and rivet connection at the same time.
In the inspection of bolts and rivets, attention should be paid to whether the bolts and rivets are sheared and loosened when they are subjected to and used, and the key inspection part is the part subjected to alternating load and dynamic load. In the process of inspection, we should also take into account the defects left over from design and construction.
Materials used to check the connection between bolts and rivets are: 10 times magnifying glass, 0.3KG hammer, feeler gauge, wrench (or self-operated wrench), etc.
1, test method
(1) Bolts are generally inspected by visual inspection, hammering and wrench test. Mainly check whether the bolt is loose and whether the bolt rod is broken. For bolts under dynamic load, the nuts should be removed regularly, and whether there are micro-cracks on the bolt rod should be carefully checked with a magnifying glass. When necessary, physical flaw detection methods such as X-ray can be used for inspection to eliminate hidden dangers for a long time.
(2) For cracked or broken bolts, find out the cause of damage, make detailed records and replace them in time. Tighten loose nuts during inspection. If it is a high-strength screw to be tightened, tighten the bolt to the specified torque with an indicating wrench according to the type (friction or bearing type) and strength grade requirements of the first screw.
(3) During rivet inspection, one hand can be close to the nail head, and the other hand can hammer from the side of the nail head. If the nail head is jumping, it means that the rivet is loose and needs to be replaced, as well as the rivet with rotten head, missing edge or crack. When replacing, high-strength bolts can be used instead, and the bolt diameter must be determined according to the principle of equal strength to ensure that the bearing capacity of steel structure engineering will not be affected after replacing rivets.
In the actual inspection, to correctly judge whether the rivet is loose or sheared too much requires not only the inspectors to have some practical experience, but also a high sense of responsibility. For important structures, it is generally required to re-examine at least once to prevent major omissions.
2. Repair and treatment of bolt and rivet connection
Generally, it is not unloaded. In order to avoid overloading other bolts and rivets, replace the bolts one by one; When replacing rivets, if the total number of a group of stressed rivets does not exceed 10, they should be replaced one by one. If it exceeds 10, in order to improve work efficiency, the number of rivets that can be replaced at the same time is 10% of the total number of rivets in a group. A set of rivets refers to:
(1) rivets between nodes of truss composite members.
(2) Flange rivets within the length of each meter of the flange of the flexural member.
(3) Fix the rivets of a single component on the connecting plate.
(4) Riveting on one side of the splicing seam on the splicing cover plate.
If the bolts and rivets in steel structure engineering are damaged to a great extent and need to be replaced in large quantities, they should be repaired in no-load state in order to ensure safety.