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
material which possesses no binding ability by itself. When it is in finely
divided form, they can react with calcium hydroxide in the presence of
moisture to form compounds with cementing properties.
During cement hydration with water, calcium hydroxide is formed which is
non-cementitious in nature. However, when pulverized fly ash is added to
calcium hydroxide, they react to produce calcium silicate hydrates which is
highly cementitious. This results in improved concrete strength. This
explains how pulverized fly ash can act as cement replacement.
13. What are the functions of coating on concrete?
In designing protective coating on concrete structures, stoppage of water
ingress through the coating is normally required. Since chloride ions often
diffuse into concrete in solution and cause deterioration of concrete
structures, the prevention of water transmission into the coating certainly
helps to protect the concrete structure. However, if water gets behind the
coating from some means and becomes trapped, its effect may not be
desirable. Firstly, vapour pressure would be developed behind the surface
treatment and this leads to the loss of adhesion and the eventual peeling
off of the coating. Moreover, the water creates a suitable environment for
mould growth on concrete surface.
In fact, the surface treatment should be so selected that it is impermeable
to liquid water but it is permeable to water vapour. This “breathing” function
enhances the concrete to lose moisture through evaporation and reject the
uptake of water during wet periods.
14. What is the significance of Flakiness Index and Elongation Index?
Flakiness Index is the percentage by weight of particles in it, whose least
dimension (i.e. thickness) is less than three-fifths of its mean dimension.
Elongation Index is the percentage by weight of particles in it, whose
largest dimension (i.e. length) is greater than one and four-fifths times its
mean dimension.
Flaky and elongated particles may have adverse effects on concrete and
bituminous mix. For instance, flaky and elongated particles tend to lower
the workability of concrete mix which may impair the long-term durability.
For bituminous mix, flaky particles are liable to break up and disintegrate
during the pavement rolling process.
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