Page 227 - Handbook of Materials Failure Analysis

2 Material and Age Structure of Utility Networks 223

materialandstructuralsolutionstoconstructwatersupplyandwastewaterdisposalpipe-
lines. As construction of particular systems proceeded over many years, it is hard to
determine unequivocally the age structure of the networks. On the basis of general data
and historical information, one can state that water supply conduits that have been func-
tioning for over 50 years in many big cities are primarily made of gray cast iron or steel,
and less frequently of asbestos cement. In wastewater disposal systems, the highest per-
centage of conduits that are over 50 years old are networks made of stoneware and con-
crete, while a share of reinforced concrete appears in conduits that are 25-50 years old.
Ontheotherhand,inthemajorityofthesecities,conduitsthatare25-50yearsoldbelong
to the first generations of conduits made primarily of PVC and constitute 30-45% of the
networklength.Relatively,newconduitsinwastewaterdisposalsystems,thatare 15-25
yearsold,areconduitsmadeofPVCandPE.Materialanalysiscombinedwithagestruc-
ture analysis allows one to conclude that the service durability of materials used to con-
structwatersupplyandwastewaterdisposalnetworkssuggestsaconsiderablepartofthe
conduits is in need of replacement (Table 9.1).
The Conduit age is an important factor that influences system susceptibility to
failure, as it is correlated with conduit service durability. More vivid data illustrating
the dependence of the age and material type refer mainly to all the wastewater dis-
posal systems that were assembled as early as the nineteenth and twentieth centuries.
However, not all materials show this correlation.
In order to investigate the range of changes in the number of damages as a func-
tion of time, the analysis focused on variability of the number of damages suffered in
successive years of observation. The investigation took the type of conduit materials
into account. Results are presented in the diagram in Figure 9.6 and Table 9.2.
In the investigation period, the number of failure events tends to grow. Years
2010 and 2011 are exceptional, as considerable differences in the numbers of failures
were identified in these years. The highest number of damages (415) was identified
in 2010, while only 351 network operation disturbances were recorded the next year.
The above situation was primarily due to failures of cast iron conduits. One can also
notice that the number of failures tends to grow in the case of all conduits, irrespec-
tive of their material. However, the highest increase is observed in the case of

Table 9.1 Material Type and Suggested Service Durability as a Function
of System Type
Wastewater Disposal
Age (Years) Water Supply Network
60-100 [15,16] Steel Brick
60-120 [15,16] Gray cast iron
50 lat [17] Gray cast iron, steel, Stoneware, concrete, brick
asbestos cement
50-25 [17] PVC Reinforced concrete
25-15 [17] PE, nodular cast iron, PVC PVC, PE