The object of detection is corrugated pipes on the top and bottom of cast-in-situ box girder of a bridge under construction. The bellows are made of plastic with a diameter of 10 cm and 19 steel cables.
Detection Purpose and Detection Method
Testing purposes: The purpose of this test is to understand the compactness of bellows grouting and to provide a basis for judging the quality of bellows grouting.
Detection methods: Two methods, GPR and TD-BWG, are used in this detection, which can improve the scientificity and reliability of the detection.
The ground penetrating radar instrument is equipped with 1.6GHZ antenna. The radar wave velocity in concrete is 12 cm/ns and the wavelength is 7.5 cm at 1.6 GHz. The minimum 2.5 cm thick space can be distinguished according to scattering theory. A continuous scanning acquisition method controlled by a ranging wheel is adopted. The concrete surface is arranged along the side of the bellows, and the detection depth is 25 cm.
Acoustic scattering method is developed by Beijing Tongdu Engineering Geophysical Prospecting Company. The acquisition station is Summit II TDEG, equipped with 32 channels of 20 kHz high frequency geophone towline. The wavelength of elastic wave in concrete is 20 cm, and the thickness of space can be distinguished by scattering theory. Scattering tracing method is used to detect the location and size of grouting defects, which is not limited by the material of the channel.
The principles of acoustic scattering method and radar method are different, and the physical parameters of detection have different meanings. Radar mainly reflects the change of electromagnetic parameters of medium and is sensitive to metal and water. It is more sensitive to the air area and less sensitive to the small air area. Due to the large attenuation, a slight deviation from the survey line may not be detected. Sound wave is sensitive to the difference of mechanical properties, especially to the shallow area. For large scale and serious void defect, the results are the same, but for small size defect, there are some differences. The combination of the two is beneficial for identifying the main defects. The results of this test are detailed in tables 2, 3 and 6 - 17.
Through the inspection of corrugated pipes in 20 sections of a bridge, it is found that the first defect (the length of defect is greater than 1 meter) is 1 meter in one place, the second defect (the length of defect is greater than 0.5 meters less than 1 meter) is 1.7 meters in three places, the third defect (less than 0.5 meters) is 1.2 meters in three places, the total length of defect is 3.9 meters, the total length of detection is 80 meters, and the proportion of defect length to detection length is 4.87%.
Because of the influence of concrete thickness and steel mesh, the detection accuracy of some small defects is low, which belongs to the normal situation. However, the larger defects are reflected in the results of radar method and acoustic scattering method, which are more consistent, which also shows that the two detection methods are scientific and reliable. It is suggested that the defects above 0.5 meters should be disposed of.
Appearance inspection test:
With the naked eye, the inner wall can be illuminated by a light source.
Dimension measurement methods:
The inner diameter and wall thickness of plastic bellows are measured according to GB/T 8806.
Ring stiffness detection method:
According to GB/T 9647, the first descending velocity of the upper pressure plate is (5 + 1) mm/min. When the deformation of the inner diameter in the vertical direction of the specimen is 3% of the original inner diameter, the load on the specimen is recorded.
Detection methods of local transverse loads:
The length of the sample is 1100 mm, and a point is taken at the trough of the middle part of the sample. The transverse load F is applied to the cylindrical head with the end of R=6 mm. The required load value is 800N within 30 seconds, and the surface of the pipe is observed after 2 minutes of loading. After 5 minutes of unloading, the deformation of the outer diameter of the plastic bellows is measured at the loading place.
Flexibility test method:
By making a plug gauge, when the sample is bent to the final end position and remains bent for 2 minutes, observe whether the plug gauge can pass through the bellows smoothly.
Test methods for impact resistance:
The test temperature is (0 +1). The drop weight and impact height are shown in Table 4. The test method was carried out according to GB/T 14152.
When plastic bellows are tested for the above six indicators, one of the indicators is unqualified, then double samples should be taken from the plastic bellows products to make samples, and the unqualified items in the indicators should be re-examined. All the unqualified items are qualified, and the batch of plastic bellows products should be judged as qualified batch. If one of the test results is still unqualified, the batch of plastic bellows products should be judged as unqualified.