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Asbestos-Cement (Transite) Pipe in Water Distribution Systems

Overview


Asbestos-cement (AC) pipe was used extensively in the mid-1900s in potable water distribution systems, particularly in the western United States. The Chrysotile Institute estimates AC pipe lifespan at 70 years, but actual service life depends largely on pipe condition and working environment. Because thousands of miles of AC pipe installed in distribution systems in the U.S. is nearing the end of its useful service life, AC pipe condition assessment and strategic replacement planning will need to be done in the coming decade.

Over time, AC pipe undergoes gradual degradation in the form of corrosion (i.e., internal calcium leaching due to conveyed water and/or external leaching due to groundwater). Such leaching leads to reduction in effective cross-section, which results in pipe softening and loss of mechanical strength. Accordingly, as the water distribution system ages, the number of AC pipe failures increases with time. In light of these risks, an AC pipe condition assessment is essential to determine the remaining useful service life and develop a suitable, proactive replacement plan for the distribution system. Exponent engineers and scientists assist water agencies in development of strategic and cost-effective AC pipe replacement plans, customized to the individual challenges of the distribution system.

The condition assessment and proactive replacement planning process consist of the following steps:

1. Collection of System Data

  • Identification of prevalent AC pipe failure mechanisms (beam failure, bursting under pressure, joint failure, etc.)
  • Analysis of historical AC pipe leak records with respect to geographic location in the distribution system, incorporating geographic information system (GIS)
  • Identification of factors affecting AC pipe failure propensity, which may include:
    • Pipe age
    • Pipe diameter
    • Pipe class
    • Pipe manufacturer
    • Internal/external water chemistry
    • Internal water pressure
    • Soil physical and chemical properties
    • Groundwater table elevation
    • Overburden
    • Climate

2. System-Wide Sampling, Condition Assessment and Laboratory Testing

  • Residual strength testing:
    • Crushing strength test (ASTM C 500)
    • Hydrostatic pressure test (ASTM C 500)
    • Flexural strength test (ASTM C 500)
    • Splitting tensile strength (ASTM C 496)
  • Degradation depth evaluation
    • Determination of calcium leaching depth
    • Scanning electron microscopy
    • Energy dispersive x-ray spectroscopy
    • Petrographic examination (ASTM C 856)
    • Matrix hardness test
  • Assessment of O-ring condition
    • Compression set test (ASTM D 395)
    • Hardness test (ASTM D 1415)
    • Fourier transform infrared spectroscopy (FTIR)

3. Development of Service Life Prediction Model

Depending on the quality of historical leak records, size of the distribution system and number of samples collected for laboratory testing, the following types of service life prediction models can be developed:

  • Historical leak rate based model:
    • Leak rate model based on influential factors (identified in Item 1)
    • Remaining service life determined by acceptable leak rate threshold
  • Pipe degradation/residual strength model:
    • Model predicting rate of pipe degradation and/or strength loss based on laboratory testing, pipe attributes, and operational environment characteristics
    • Failure threshold degradation depth/residual strength obtained from failed pipe samples
    • Remaining service life based on projection of degradation rate/strength loss to failure threshold

4. Development of Master Replacement Plan

  • Based on system-wide remaining service life prediction model
  • Incorporates hydraulic, operational and financial considerations, critical customers, seismic risk, optimal feasible replacement length and other factors.