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
3. How does concrete cover enhance fire resistance? (CS1)
In the event of exposing the concrete structures to a fire, a temperature
gradient is established across the cross section of concrete structures. For
shallow covers, the steel reinforcement inside the structures rises in
temperature. Generally speaking, steel loses about half of its strength
when temperature rises to about 550 o C. Gradually, the steel loses strength
and this leads to considerable deflections and even structural failure in the
worst scenario. Hence, adequate cover should be provided for reinforced
concrete structure as a means to delay the rise in temperature in steel
reinforcement.
4. In designing concrete structures, normally maximum aggregate
sizes are adopted with ranges from 10mm to 20mm. Does an increase
of maximum aggregate size benefit the structures?
To answer this question, let’s consider an example of a cube. The surface
area to volume ratio of a cube is 6/b where b is the length of the cube. This
implies that the surface area to volume ratio decreases with an increase in
volume. Therefore, when the size of maximum aggregate is increased, the
surface area to be wetted by water per unit volume is reduced.
Consequently, the water requirement of the concrete mixes is reduced
accordingly so that the water/cement ratio can be lowered, resulting in a
rise in concrete strength.
However, an increase of aggregate size is also accompanied by the effect
of reduced contact areas and discontinuities created by these larger sized
particles. In general, for maximum aggregate sizes below 40mm, the effect
of lower water requirement can offset the disadvantages brought about by
discontinuities as suggested by Longman Scientific and Technical (1987).
5. What the preferable size of cover blocks? (CS1)
The purpose of cover blocks are:
i. Maintain the required cover.
ii. Prevent steel bars from getting exposed to the atmosphere so
that steel corrosion may result.
iii. Place and fix reinforcement based on design drawings.
As cover blocks after concreting shall form part of concrete structure, it is
preferably that the cover blocks shall possess similar strength to the
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