2015. . No. 2(68) - Jun 2015, pp. 31-34
Inspection of Concrete Structures with Complex Fillers. Method Using the Natural Oscillations Localized at a Single Point
Various fillers are used at pouring of mass concrete; fillers differ both in sizes and physical and mechanical parameters. Fillers particle sizes are commensurable with ultrasonic wave length at low frequency concrete testing. It leads to maximal ultrasound attenuation and makes this method of testing impossible for application.
Method. The article presents the nondestructive method of concrete testing based on local natural frequency generation perpendicularly to the outer surface of the structure; natural oscillations are generated by a hammer blow. After a series of blows a standing wave may appear in the concrete wall; its frequency may be measured with a natural frequency meter. The wall thickness corresponds to a half-wave length; a sound velocity can be calculated on the basis of measured frequency, strength parameters of the concrete (its grade) can be determined with special techniques. In case there are cracks, the blow energy is consumed for their development and a standing wave does not appear.
The presented method does not depend on a filler size. The process of local natural frequency generation is analyzed as well as the processes which limit the generation. The value of natural bending oscillations deflection is supposed to serve as the limitation criterion of method usage.
Results. The concrete walls of water intake structure have been inspected; the inspection results have enabled the concrete discontinuity zone to be detected. The zone has been located in the area of water freezing within concrete thickness.
Discussion. The concrete of hydraulic structures is usually non-homogenous. It is necessary to gain some practice in order to apply this method successively as it depends on such a subjective factor as a blow; nevertheless the good practice is quite easily reachable. The method can also be applied for revealing internal cracks within brickwork and between bricks and mortar. The method however reveals strength degradation in general without determination of the number of cracks and their sizes.
Keywords: concrete, nondestructive testing, complicated filler, local oscillation, wall inspection
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