Determination of wall thickness and condition of asbestos cement pipes in sewer rising mains using Surface Penetrating Radar

Asbestos pipes are commonly used for water and sewage pressure mains. There are very few non-destructive ways of assessing the wall thickness and condition of these Asbestos Cement (AC) pipes. Traditionally, their examination was undertaken by tapping to remove a physical specimen (a 'coupon') of the pipe. This procedure is time consuming, costly and risky. Alternatively, the mains have to be shut down to allow access by CCTV cameras. Advances in Surface Penetrating Radar (SPR) technology in recent years now means that defects in thin walled pipes can now be detected non-destructively from the outside. Selected sewage pipes of different ages, diameters and thicknesses are examined in situ using SPR. In addition, some samples of damaged pipes were examined separately. The main mechanisms for pipe deterioration are from chemical attack at the pipe obvert, and scouring at the pipe invert. The results demonstrate that SPR is an effective method for detecting areas of pipe deterioration and delamination and can detect scour depths as small as 4 mm. The SPR images clearly show a change in the shape and amplitude of the reflected signal from the inner face of the pipe walls and also an apparent increase in material thickness due to retention of water within the pipe wall material in areas affected by chemical attack. Variations are also evident in the signal reflection from the interior of the pipe wall between that at the pipe obvert and that of the side walls. These variations may be attributable to chemical attack and material changes occurring in the AC material. Variations were also observed in the shape and amplitude of reflected signal from the inner face of the pipe invert that are attributed to material loss or scouring at the invert. The SPR image also clearly recorded fluctuating levels of fluid within the pipes during the data collection. Although this makes analysis more of a challenge due to the effect on the reflected signal, this information is very important in locating areas in which pipe failures are more likely to occur through chemical attack and pipe pressure surges. Information on the size of the air pocket present within the pipe and size of the fluctuations can be determined. The signal velocities variations can be detected if the wall thickness is known. Such variations may relate to the moisture content which has been absorbed by the pipe material from the surrounding moist soil and from the pipe contents. The signal velocities determined are less accurate for very thin pipes because of the uncertainty in measuring the thickness interface in the signal. It is evident from the results of cut pipe sections that the SPR provides much better resolution of the inside surface of the pipe when the pipes are empty. It may be possible to drain the pipes for a short duration to allow the pipes to be scanned in the empty state. This will also eliminate the problem of fluctuating water levels obscuring defects in the pipe obverts. SPR imaging is a quick and effective method which is now available for non-destructively evaluating AC pipes.