A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
For watermain pipe size less than 600mm, ductile iron is normally used
because internal welding for steel pipes below 600mm is difficult to be
carried out. Moreover, it requires only simple jointing details which allow for
faster rate of construction. For watermain pipe size above 600mm, steel
pipes are recommended because steel pipes are lighter than ductile iron
pipes for the same material strength and therefore the cost of steel pipes is
less than that of ductile iron pipes. In addition, in areas of difficult access
the lighter mild steel pipes pose an advantage over ductile iron pipes for
easy handling.
5. Why are some ductile iron pipes cement-mortar lined? (DIP1)
Cement mortar lining provides a high pH in the inner surface of ductile iron
pipes and it serves as a physical barrier to water to guard against corrosion
of iron from acidic water. Moreover, owing to smooth nature of cement
lining, it results in the provision of high flow.
For cement-mortar lined ductile iron pipes to convey water, water infiltrates
the pores of lining and releases some calcium hydrate. The freed calcium
hydrate form calcium carbonate with calcium bicarbonate in water so that it
serves to clog the pores of linings and avoid further permeation of water.
Moreover, iron also reacts with lime to precipitate iron hydroxide which also
seals the pores of linings. As such, the lining provides both chemical and
physical barrier to aggressive water.
6. In some countries like the United States, asphaltic seal-coat is used
in cement mortar lining of ductile iron pipes. Why? (DIP1)
The original intention of adding a thin asphaltic seal-coat on freshly placed
cement-mortar lining is to reduce water loss during hydration so as to
achieve better curing of the linings. In fact, it also helps hinder the leaching
of cement by corrosive water. Otherwise, leachates from cement linings
may cause a rise in pH in water.
However, asphaltic seal-coat is considered undesirable from environmental
point of view. The seal-coat material is solvent-based which contains
volatile organic compounds, which is an air pollutant.
7. What is the purpose of polyethylene encasement for ductile iron
pipes? (DIP2)
The provision of polyethylene encasement avoids the direct contact of
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