Nondestructive Condition Assessment of Concrete Pipes

Sonic/ultrasonic measurements provide a comprehensive evaluation of PCCP (Prestressed Concrete Cylinder Pipes) condition. The results of sonic/ultrasonic tests are used to: 1) determine the strength of the concrete core and mortar coating; 2) determine the presence of delamination in the core; 3) identify where the pipe is out of compression and not acting as the designed composite structure; 4) identify where the mortar coating is debonded or delaminated at the wire wrap; and 5) identify areas of weakened core concrete due to loss of prestressing or micro cracking. These results are used to identify distressed pipe to be designated for repair or replacement. In addition the data will benchmark areas that are questionable and to be examined on a periodic basis.

Stress wave measurements in the sonic/ultrasonic frequency range are used to determine the direct transmission velocity of the compressional and shear (or Rayleigh) waves as well as reflected compressional waves from the back of the pipe core (examine the core and mortar condition). The transmission velocity values (shown schematically below) determine the elastic deformational characteristics of the concrete, including Young's, shear, and bulk moduli, Poisson's Ratio, and calculated strength values. Lower velocities are associated with cracking in the concrete, which is related to the stress history of the pipe including poor handling in installation.

The reflected signals resonate between the walls at a frequency that is related to the thickness and compressional wave velocity of the concrete liner. Since the longitudinal velocity is measured directly and average thickness of the pipes are known, the average strength of the concrete core as well as the presence of delamination are determined from measured resonant frequency. Where delamination has occurred in the core or between the core and coating, the velocities in the thickness direction is substantially reduced and the wall thickness resonant frequency is non-existent. Lower resonant frequencies can be attributed to construction differences or to weakening of the core concrete due to a loss of prestressing or micro cracking. Construction differences, a double wire wrapping for example, lowers the resonant frequency because the wall is thicker.

Sonic/ultrasonic direct and resonant frequency measurements are obtained using a projectile impact energy source, a hand-held four sensor array. Measurements can be made on the inside of the pipe or on the exterior of the pipe (Picture below).

An initial wire break and consequent damage to the integrity of the pipe can be readily identified by the absence of the wall resonant frequency. This wall resonant frequency is created by a continuous reflection (echo) of the sonic wave from wall front to back (see below). When the time traces are displayed in the frequency domain, this wall thickness frequency spectral peak is readily seen. An intermediate delamination such as separation of the metal membrane from the concrete (even though slight) is also readily seen. This damage can be at the very early stage of wire severance. If the wire has pulled away from the severed point, then the damage is spread circumferentially; when the wire has not moved from its severance point because the covering mortar integrity has held it in place, then the degradation of the resonant frequency will be evident only in close proximity to the wire break.

In some cases the wire break gives rise to a "drumhead" frequency caused by the combination of delamination and cracks. While the presence of the drumhead may be audible sometimes to the human ear, it is always detectable to sonic/ultrasonic instrumentation. When the wire is severed over a longer distance (multiple breaks for several inches), then the concrete loses compression and its velocity lowers which is indicative of a more severe problem. Finally at some latter stage when the velocity has dropped by 20% or more the drumhead may become audible to the human ear (hollow) when the pipe is sounded.

Unlike other methods such as sounding, acoustical emission and magnetics that may detect pipe with broken wires, NDT's sonic/ultrasonic provides an assessment of the overall performance/condition of each pipe section by detecting anomalies in the concrete that are indicative of conditions that may lead to future failure. Such defects as delaminated coating, improperly wrapped pipe, and overloaded pipe are identified.

Because the NDT data are repeatable, measurements in time can be compared to determine if pipe has deteriorated and if so, develop a rate towards failure. These data provide a pipeline owner with information to develop a beneficial cost program for repair.

For additional information call NDT Corporation at 508-754-0417 or check our web site www.ndtcorporation.com.