The True Causes of AC Pipe Failures—According to the Data

Very often, condition assessments of asbestos cement (AC) pipe will produce contradictory results. A phenolphthalein stain test may indicate advanced material degradation, but ring crushing tests will produce strength values exceeding original specifications. The stain-test data will exhibit considerable variability, with far-from-uniform degradation, even on the inside of pipes, where the environment is presumed to be relatively uniform. Samples taken from broken pipes will often appear un-degraded, yet something produced the break. This paper discusses findings from an exhaustive statistical and physical condition analysis of a large urban water utility, where 24 years of break data and 96 samples were analyzed to guide decisions regarding 1100 miles of AC pipe. Among the findings are: (1) the role of carbonation in AC pipe degradation; (2) the influence of external loading, particularly the shrink-swell potential of soils in causing breaks; and (3) how break history predicts future breaks for both young and old mains. This paper also addresses the efficacy of acoustic-velocity tests for assessing the integrity of AC pipes. The method appears promising, avoiding the need to extract samples from live mains. This paper is based on a recently published water research foundation report 4480, “Development of an Effective AC Pipe Management Strategy for Utilities.”