Uncertainties associated with laser profiling of concrete sewer pipes for the quantification of the interior geometry

Abstract

Structural strength and hydraulic capacity are two essential parameters in the assessment of the need for sewer rehabilitation. Especially concrete pipes suffer from loss of wall thickness due to biochemical corrosion and, consequently, a decreasing structural strength along with an increase of hydraulic roughness. Unfortunately, routinely used visual inspection methods do not allow a quantification of the internal pipe geometry which would enable not only detecting but also quantifying the progress of biochemical corrosion. Advances in laser technology and digital cameras theoretically allow a cost-effective application of laser profilers to measure the interior geometry of sewer pipes. An analysis of associated uncertainties revealed that the position and alignment of the laser are the main source of measurement errors. A full-scale laboratory set-up demonstrated, based on tests on a new and an 89 years old corroded sewer pipe, that laser scanning is indeed capable of measuring the interior geometry accurately enough to determine wall thickness losses for corroded pipes, provided that the position and alignment of the laser and camera are accounted for. The obtained accuracy, however, was not enough to quantify the hydraulic roughness.