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
Air test has the advantage that the test itself is simple and faster to be
carried out. It does not require the disposal of significant quantities of water
used in the test which is a mandatory requirement for water test. However,
in case leakage exists in the constructed segment of gravity pipelines, the
position of leakage can hardly be located in air test. Moreover, the rate of
water leakage cannot be determined from air tests. In addition, air test is
readily affected by atmospheric condition because air has a relatively high
coefficient of thermal expansion. The test is also influenced by the moisture
condition of the test pipelines because it affects the passage of air through
the pipelines.
For water test, though it is comparatively slow, it can detect the location of
water leakage. However, the leakage rate results from water test may not
truly reflect its actual leakage because pipeline materials like concrete and
clay are porous and would absorb water during the test.
5. What is the purpose of carrying out water absorption test for
precast concrete pipes? (T2)
Cement will mix with more water than is required to eventually combine
during hydration of cement paste. As such, some voids will be left behind
after the hydration process which affects the strength and durability of
concrete. With the presence of air voids in concrete, it is vulnerable to
penetration and attack by aggressive chemicals. Good quality concrete is
characterized by having minimal voids left by excess water and therefore,
water absorption test for precast concrete pipes is adopted for checking the
quality of concrete in terms of density and imperviousness.
6. Which types of soils are unsuitable for testing under sand
replacement test? (T3)
Any soils that can be excavated with handtools is suitable provided that the
void or pore openings in the soil mass are small enough to prevent the
calibrated sand used in the test from getting into the natural voids.
Moreover, the soils being tested should have sufficient cohesion or particle
interlocking to maintain side stability during excavation of the test pit.
Furthermore, it should also be firm enough not to deform or slough due to
the pressures exerted in digging the hole and pouring the sand.
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