Nondestructive Evaluation of Concrete Pipe

An evaluation of 6.5 miles of underground large diameter Prestressed Concrete Cylinder Pipe (PCCP) has been successfully completed using continuous sonic/ultrasonic velocity and resonant frequency (pulse-echo) measurements.

Failures of PCCP usually occur as a result of broken prestressed wire. The wire failure may be due to any of a number of factors such as environmental corrosion, embrittlement, as well as construction and installation procedures. As the number of wire severances increase, the area of cracked (relieved) concrete increases along with the potential for a pipe failure. An accurate determination of the extent of weakened concrete is key to identifying PCCP sections that need repair, replacement or continued monitoring.

Weakened concrete conditions are identified by a loss or change in the pipe wall resonant frequency; these are quantified by their (lower) shear and compressional wave velocity values. The extent and magnitude of these conditions is used to identify potential deficient pipe sections.

The results of this survey distinguished individual pipe sections that are good and undamaged from sections that require immediate or future repairs; many of the pipes that had some damage are candidates for future monitoring. The sensitivity of the sonic/ultrasonic NDT results are such that less than four and sometimes even a single wire severance can be identified. The sonic/ultrasonic nondestructive testing of the WSSC 96" pipe demonstrated much more good pipe exists than had been anticipated; this saved tens of millions of repair dollars.

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