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
9. What is the significance of isolation joints? (J1)
Isolation joints isolate slabs or concrete structure from other parts of
structure. The presence of isolation joints allows independent vertical or
horizontal movement between adjoining parts of the structure. Otherwise,
the structure may experience cracking owing to the restrained movement
caused by directional connection between adjoining concrete structures.
10. Is waterstop necessary in construction joints of water-retaining
structures? (J1)
Construction joints are introduced during construction and they are not
intended for movement to take place. If the concrete panel on each side of
the construction joint has been designed to have maximum crack width of
0.2mm, theoretically this joint would also behave in the same way as its
adjoining material. Hence, the design of this part of water-retaining
structure such as concrete and steel are identical on both sides of panel.
As such, BS8007 makes provision that it is not necessary to incorporate
waterstops in properly constructed construction joints based on the above
reasoning.
11. What is the purpose of critical steel ratio in concrete structures?
The purpose of critical steel ratio is to control the cracking pattern by
having concrete failing in tension first. If steel reinforcement yields first
before the limit of concrete tensile strength is reached, then wide and few
cracks would be formed. In the calculation of critical steel ratio, the
thickness of the whole concrete section is adopted for analysis. However, if
the concrete section exceeds 500mm in thickness, only the outer 250mm
concrete has to be considered in calculating minimum reinforcement to
control thermal and shrinkage cracks [36]. It is because experimental
works showed that for concrete section greater than 500mm, the outer
250mm on each face could be regarded as surface zone while the
remaining could be regarded as core. The minimum reinforcement to
control cracking should therefore be calculated based on a total maximum
thickness of 500mm.
12. What is the crack pattern induced by hydration due to internal
restraint?
Let’s take a circular column as an example to illustrate this.
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