It is important to monitor the condition of aging asbestos cement watermains
The use of asbestos cement (AC) material to manufacture pipes began in the early nineteenth century in Genoa, Italy. Dr. A. Mazza, the president of Eternit Pietra Atrificiale S.P.A of Genoa, established a factory in 1907 to produce AC roofing material under the process invented by L.Hatschek. In 1911, Dr. Mazza started experimenting with the manufacturing of AC pipes and in 1916 production began on a commercial basis.
AC pipes were first introduced in North America in 1929 when Johns-Manville Corporation installed an AC pipe manufacturing machine (Hu et al. 2013). Problems with rust formation in galvanized steel led to the exploration of other pipe material, including AC pipes, for use in potable watermains during the 1940s, ’50s and ’60s. However, due to health concerns associated with their manufacturing process, and the possible resale of asbestos fibres from deteriorated pipes, AC pipes were largely discontinued in North America and their use was stopped in the 1980s. Although they were discontinued in North America, a significant portion (an estimated 16% – 18%) of the water distribution system in North America is still made of them.
AC pipes can be classified into two categories, Type I and Type II. Type I contains 20% asbestos fibre and 80% Portland cement, cured under moist conditions. Type II steam autoclaved AC pipes differ from Type I pipes, in that 40% of the cement is replaced with silica that bonds with the free lime, producing a more stable product since it permits various hydrothermal reactions, in addition to the hydrolysis of cement (Bracken 2012; Hu et al. 2013). Most of the AC pipes in North America were installed after the transition to the Type II steam autoclaved AC pipe, which occurred in the 1940s. Type II pipes manufactured in North America were made using 15% – 20% of asbestos, 45% – 51% of Portland cement, 32% – 34% of quartz (which contains mainly silicon oxide) and contained less than 1% of free lime (Nebesar and Riley 1983).
AC Pipe Failure
The failure of AC water mains is influenced by a number of factors that can be grouped into three categories (Hu and Hubble, 2007):
• Physical characteristics of the pipes (e.g., pipe age, pipe size, manufacturing process); • The pipe location environment (e.g., climate, soil type and groundwater properties); and, • Operational characteristics (e.g., conveyed water quality and procedures for operation, maintenance, repair and replacement). The interaction of these factors determines the deterioration processes and modes of failure for AC watermains. Soft water with very low ion content (low carbonate and bicarbonate content) is aggressive to calcium hydroxide and results in the leaching of calcium hydroxide from AC pipe materials and a consequent reduction in mechanical strength. Key parameters for identifying aggressive water include low pH, low alkalinity, calcium hardness, and negative Langelier index.
Factors affecting external chemical attack are similar to those influencing internal attack (i.e., pH, alkalinity and sulphates contained in the soils or groundwater can damage AC pipe materials (Jarvis, 1998)). These chemical processes either leach out components of the cement material or penetrate the pipe wall to form products that weaken the cement matrix, reducing the structural integrity of the pipe, as well as affecting water quality. In order to understand in-service failures, it is necessary to have knowledge of the pipe stresses and any degradation of mechanical performance with time. Regardless of the source of the loading, for failure to occur, the stress level must exceed the strength of the pipe material. The relationship between the failure load of AC pipes and their in-service condition is defined in the combined load theory adopted by the ANSI/AWWA C401.
AC Pipe Assessment Methods
While the techniques available for metal pipe have advanced with the requirements of the oil and gas industries, cementitious pipes used in the water and wastewater industries have not benefited from similar advancements. Currently, two well-established non-destructive technologies (NDT) are used for assessing AC pipes: ground penetrating radar or pipe penetrating radar, and acoustic measurements.
In addition to the NDT, there are assessment methods for assessing AC pipelines. These include hardness, phenolphthalein, pipe crushing/ bursting, and petrographic tests. Tests are recommended on pipes extracted from a failed pipe segment. However, a planned coupon sampling program may be utilized to provide a better understanding of the system condition at a selected location that represents the entire sounding environment for the water system (e.g., similar soil conditions, and/or similar surface load conditions). NDT methods may be a preferable option, but none of the available assessment technologies can work in all conditions.
Each has unique capabilities and advantages when compared to other condition assessment methods. Therefore, selecting technology for assessing watermains becomes an optimal process that relies on a set of criteria that each pipe owner can develop to assess their AC pipe inventory. As for the AC pipe scanner (ACPS) and ePulse inspection tools, the main difference is that the ACPS tool is an invasive inspection tool that requires the pipe to be out of service. The ePulse is an external correlator concept that inspects the pipe while it is still in service. Both tools measure undegraded thickness. However, the ePulse measures minimum average between two sensors, and the ACPS scans thickness along the entire pipe segment, in addition to detecting voids.
AC Pipe Condition Grading
Based on the physical condition of assets, decision makers can properly manage their assets and select suitable intervention actions that aim at maintaining, rehabilitating or replacing them. The condition grading system is a valuable method used by asset management specialists to assign a numerical number from 1 to 5, based on a predefined program rating system. While the standard practice of condition grading for the gravity pipelines industry is based on the National Association of Sewer Services Companies (NASSCO) methodology, the pressurized pipeline grading system does not have a similar standardized approach. Rajani and Makar (2000) developed a methodology to estimate the remaining service life of grey cast iron mains. It considered the residual resistance capacity of grey cast iron pipelines, anticipated corrosion rates, and the measurement of corrosion pits to predict the remaining factor of safety (FoS). Despite this significant contribution, the present research is dedicated to evaluating grey cast iron only.
The methodology to use residual FoS was presented as a method that can be used by decision makers to describe pressurized pipe conditions using a numerical number (residual FoS). When calculating the residual FoS of AC watermain pipelines, imposed loads as per the ANSI/AWWA C401, along with the measured wall thickness information obtained via NDT assessment methods, must be considered. The analysis is presented via a nondimensional condition grading curve generated for AC watermains that uses a condition rating system from 1 to 5, based on the residual FoS methodology. Since the rating is based on the residual FoS, each score from 1 to 5 is associated with a recommended intervention action based on the Guidance Manual for Managing Long Term Performance of AC Pipe (2013).
One study used an acoustic based inspection tool to measure the average remaining wall thickness of 150 mm AC pipe segment, class 150, located at the municipality of Thames Centre, Ontario, and constructed in 1974. The inspection tool provided a calculated average remaining wall thickness of 11.5 mm along the pipe segment, which is a 32% change in wall thickness from the original pipe thickness (16.8 mm). The maximum measured working pressure was approximately 60 psi. The calculated vertical load, combined soil and surface load assuming trench condition with a narrow trench with a width length of 2D when D is the pipe diameter, 1.5 m earth soil cover depth and 19 KN/m3 soil density is approximately 1050 N/m. Residual tensile strength based on the Hu et al. model for a 44-year-old pipe is 17.9 Mpa. The internal pressure and external load for failure is 2.7 Mpa and 0.024 Mpa respectively, assuming a bedding factor equal to 1.5 for non-gravel beds and other soil surrounds. The calculated residual FoS due to internal load is 0.16, and 0.26 due to external load.
Based on the nondimensional condition grading curve, the pipe condition grade is 2, which is associated with an intervention action of non-structural rehabilitation methodology based on the Guidance Manual for Managing Long Term Performance of AC Pipe (2013). Although the acoustic inspection of the AC pipe showed 32% degradation, the analysis of pipe physical condition and imposed loads showed an acceptable condition (grade 2). Relying only on thickness measurement may mean condemning pipe prematurely when it still has substantial strength that will allow additional service life. As long as there are no additional and/or new loads imposed on the pipe, periodic inspections using NDT assessment technology can be determined via estimating the time for the pipe stress status to reach condition grade 3 via linear deterioration estimation.
Rabia Mady is with CIMA+.
Acknowledgement The author would like to thank the Municipality of Thames Centre and especially Carlos Reyes, CISEC, P.Eng., director of environmental service at the Municipality of Thames Centre for providing the inspection results.
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