A Closer Look at Prevailing Civil Engineering Practice - European ...

A Closer Look at Prevailing Civil Engineering Practice, What, Why and How
Vincent T. H. CHU
pipes become aged so that some may break down, it then become a great
hazard. When someone takes a little asbestos which are distributed to
other parts of the body so that no single parts of the body have excessive
amount of asbestos, the risk should be theoretically on the low side. The
harmful effect of asbestos is its ability to accumulate in human body. The
microscopic fibers lodged in tissues can act like time bombs and cause
cancer years later. Since asbestos exposure is cumulative, adults have
three or four decades to develop cancer after exposure while youngsters
have six or seven.
However, according to the findings of WHO, asbestos fibres are too large
to be absorbed during the digestion. Therefore, the chance of significant
transmission of asbestos fibres would seem to be low. Some evidence
suggests that high density asbestos-cement products pose no detectable
risk to the public because asbestos fibres are carcinogenic only when
inhaled but not ingested.
10. In road opening, it is sometimes noted that asbestos cement
pipes are broken up into pieces. Why
Asbestos cement is mainly a mixture of cement and asbestos fibres with
density greater than 1000kg/m 3 . It contains about 10% asbestos fibres and
it is a light grey hard material. The fibres are tightly held in cement mixture
and they shall be discharged if asbestos cement undergoes significant
disturbance such as drilling and sawing.
However, one of the important characteristics of asbestos cement is the
relative brittleness. As such, asbestos cement pipes can be broken easily
when falling at height or driven over by heavy vehicles. Hence, it is not
uncommon that asbestos cement pipes are observed to break up into
pieces in inspection pits.
11. What is the difference between “linear characteristic” and “equal
percentage characteristic” in controlling butterfly valves
For “linear characteristic”, the flow rate is directly proportional to the
amount of travel of butterfly disk. For example, at 25% open of butterfly
disk, the flow rate of 25% of maximum flow.
For “equal percentage characteristic”, equal increment of opening of
butterfly disk leads to equal percentage change in flow rate. For example,
when butterfly disc open from 30% to 40%, it generates a change in flow
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