Nondestructive Evaluation of Pre-stressed Concrete Cylinder Pipe by Resonance Acoustic Spectroscopy: Theoretical and Modelling Considerations.

摘要

The theoretical basis for adapting the technique of resonance acoustic spectroscopy (RAS) for the purposes of inspecting buried, in-service water mains composed of prestressed concrete cylinder pipe (PCCP, lined-type) is presented in this thesis. The proposed nondestructive evaluation (NDE) technique is sensitive to degradation of the outermost layer (a protective mortar coating) of PCCP which occurs over only part of the pipe's circumference. The frequency spectrum of healthy pipes is evaluated through a sequence of experimental measurements and finite element modelling studies (modal analyses). Several simplifying assumptions about the vibratory response of PCCP are gleaned from the modal analysis of healthy pipes, facilitating the development of a model of buried, water filled sections of pipe subject to varying degrees of damage. The model treats the pipe as a multilayered cylinder in plane strain condition and is solved via a transfer matrix (T-Matrix) method which has been extended by the author to accommodate non-axisymmetric elastic parameters (to represent pipe damage) and coupling to media external to the pipe (to represent coupling to soil and water). The model is then used to assess the effects of coupling to the surrounding soil, as well as the effects of increasing levels of mortar damage. From the results of these investigations, a novel damage metric called the asymmetry index is defined. This metric is appropriate for assessing damage to the protective mortar coating, and is based on the splitting of degenerate flexural modes which occurs when the axial symmetry of a pipe section is perturbed. Finally, a procedure for collecting and processing the resonant spectra from pipes in the field is presented. This procedure allows for the asymmetry indices associated with different vibrational modes to be computed at various axial locations along the pipeline thus providing a map of mortar damage along the water main. The primary advantage of the proposed technique is that it does not require precise knowledge of the pipe's dimensions or elastic parameters. In addition, the technique is applicable for any type of surrounding soil.