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
5. How does rain affect the freshly placed concrete? (FC1)
Rain may affect the water cement ratio at top portion of freshly placed
concrete provided that the concrete is not properly protected from rain. To
substantially change the water-cement ratio of the concrete at the surface
of the slab, external energy must be supplied to the system such as
troweling passes with excess water on the concrete surface. The energy
supplied by the finishing operations pushes the excess water into the slab
surface creating a high water cement ratio in the near surface of the
concrete so that its strength and durability is reduced. Sometimes, the
damage to the concrete surface is apparent since the texture of the surface
is easily damaged after the initial curing period. When the surface strength
is affected, the long-term durability of the concrete may be reduced.
However, the concrete strength and durability below the surface would not
be affected.
6. Why is liquid nitrogen added to fresh concrete sometimes instead
of ice block/chilled water? (FC2)
Traditionally, chilled water and ice have been employed to reduce the
temperature of concrete mix in hot weather condition. Chilled water has a
limitation in its cooling potential. For instance, even if all mixing water has
been converted into chilled water, the temperature reduction achieved in
concrete mix is only about 2.7 o C. The complete/partial replacement of
mixing water with ice may be a better alternative in terms of cooling
potential because ice possesses power from heat of fusion. However, it
presents practical difficulty in ensuring homogeneous distribution of ice
within concrete mix and the complete melting of ice. Unmelted ice block
may be hidden in concrete mix and if it melts before concrete setting it
creates high water cement ratio locally. In case unmelted ice block melts
after concrete hardening, large voids would be formed which impairs the
concrete strength and durability.
Liquid nitrogen is supercooled and has a very high cooling potential.
Fresh concrete can be cooled inside a read-mix truck by injection of liquid
nitrogen. Liquid nitrogen is kept at a temperature of -196 o C in storage
tank. Once liquid nitrogen is added to fresh concrete mix, nitrogen in
liquid form changes to gaseous state under normal atmospheric pressure
in a very short time (e.g. milliseconds).
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