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
allowed to continue for several days before subjecting the new concrete to
significant stress. The period of curing depends on the temperature
because the rate of all chemical reactions is dependent on temperature.
Therefore, in summer the rate of reaction (hydration) is faster so that a
shorter curing time is required. On the contrary, in winter the rate of
reaction (hydration) is slower so that a longer curing time is required.
9. How does rain affect the freshly placed concrete
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.
10. What is the purpose of uniform rate of application of water (i.e. 2m
depth in 24 hours) in watertightness test of water retaining
structures
In watertightness test of water retaining structures, it normally requires the
filling of water at a uniform rate or letting the water pool stand alone for
some time before actual measurement is carried out. The reason for such
provision in watertightness test is to allow sufficient time for water
absorption to take place in concrete. After allowing sufficient time for water
absorption to occur, the subsequent measurement of a fall in water level is
deemed to be caused by water leakage instead of water absorption
provided that adjustment has been made to other external effects such as
evaporation.
11. Is curing compound suitable for all concrete
For concrete structures with low water-cement ratio (i.e. less than 0.4), it
may not be suitable to use curing compound for curing. When hydration
takes place, the relative humidity of interior concrete drops which leads to
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