Key words: cement mortar, spiral steel pipe
Municipal water supply pipeline, spiral steel pipe in addition to anti-corrosion, but also need internal anti-corrosion treatment. At that time, the most commonly used method of corrosion of cement mortar fabrics was to evenly coat the steel pipe wall with cement mortar and leave the pipeline for protection. The cement mortar in the coating can not only avoid the corrosion of the inner wall of the pipeline, prolong the service life of the pipeline, protect the water quality, adhere to or improve the water supply of the pipeline, but also have remarkable economic and social benefits.


The common cement mortar used for spiral steel pipe inside the coating is as follows:


(1) Requirement of Spiral Steel Tube Material
Cement should use more than 42.5 grade Portland cement, usually Portland cement or slag Portland cement. Sand should be consolidated, clean, classified natural sand, no impurities, its ambiguity is not more than 2%, the maximum particle size is not more than 1.2 mm, gradation should be determined according to construction technology, diameter, site construction conditions. Mixed water is necessary for cleanliness. It does not have the influence of strength of cement mortar such as soil, oil, acid and alkali, and durability of materials. Drinking water is usually used.


(2) Construction Conditions of Coating Spiral Steel Pipe
The construction of cement mortar fabrics should usually be completed under pipelines, test pressure and compaction as planned. In the process of fabric construction, the pipeline must be in a stable state. If the first layer of material is treated by the next layer of anti-corrosion, the steel pipe wall will become thinner and the stiffness will be small. During the construction and transportation, the soil will also undergo large deformation and damage the coating. Before fabric construction, the inner wall of pipeline must be cleaned to remove soft oxides, floating rust, dirt, grease, welding slag and other attachments; the protrusion of steel pipe wall should not exceed 1/3 of the coating thickness; the vertical deformation should not exceed the planning rules, and should not exceed 2% of the pipe diameter.


(3) Coating Construction of Spiral Steel Pipe
Cement mortar coatings can be constructed by mechanical spraying, manual decompression, tug or centrifugal prefabrication. If it is necessary to use prefabricated method for inner coating, the protective coating should be used in the transportation, installation and backfilling of spiral steel pipe. During construction, the main mixing cement mortar, the ratio of cement to sand is 1: (1-2), the slump of cement mortar is 60-80 mm, and the compressive strength of cement mortar should not be lower than 30 MPa. The inner coating of cement mortar by mechanical coating method has smooth surface, fine, uniform thickness and good effect. Mechanical spraying has been used in foreign countries since 1930s. Since 1960s, it has been carried out in Shanghai, Qingdao, Dalian and other cities in China. When using mechanical spraying method, elbows, tees, special pipes and gate valves near the pipe section can be dirty by hand, and smooth splitting parts can be connected with mechanical spraying. In addition, towels or centrifugal prefabrication methods are also widely used, which are not detailed here. No matter what construction method is adopted, the protection technology is the key link to ensure that the cement mortar coating does not crack and empty drum.  Therefore, cement mortar should plug the pipeline immediately after the coating is formed, so as not to form air convection; after the final condensation water wet protection, the protection time of Portland cement should not be less than 7 days, slag cement should not be less than 14 days; water should continue to block and adhere to wetness.


(4) Construction quality of coating on spiral steel pipe
The coating quality in cement mortar shall conform to the following requirements.
1) The crack width shall not be greater than 0.8mm, and shall not be longer than the length of the pipeline, and shall not be greater than 2.0m.
2) The thickness error and pitting corrosion of the coating and surface defects such as voids should conform to the regular depth, and the defect area of spiral steel tube should not be greater than 5 cm 2.
3) The flatness requires a 300 mm ruler, which is connected to the wall along the vertical axis. The gap between the measured coating surface and the ruler should be less than 2 mm.
4) The hollow area per M 2 shall not exceed 2, and each shall not exceed 100 cm2.
Tube joints are very important problems in pipe structures, including local stress and strain analysis, failure mechanism research, elastic-plastic analysis, design parameters and so on.