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
accumulate and grow in these tiny cracks and form bonding so that the
cracks are sealed. Since the first documented discovery of autogenous
healing by the French Academy of Science in 1836, there have been
numerous previous proofs that cracks are sealed up naturally by
autogenous healing. Because of its self-sealing property, designers
normally limit crack width to 0.2mm for water retaining structures.
3. What is the purpose of setting indirect tensile strength in
water-retaining structures? (DWRS2)
The crack width formation is dependent on the early tensile strength of
concrete. The principle of critical steel ratio also applies in this situation.
The amount of reinforcement required to control early thermal and
shrinkage movement is determined by the capability of reinforcement to
induce cracks on concrete structures. If an upper limit is set on the early
tensile strength of immature concrete, then a range of tiny cracks would be
formed by failing in concrete tension [4]. However, if the strength of
reinforcement is lower than immature concrete, then the subsequent
yielding of reinforcement will produce isolated and wide cracks which are
undesirable for water-retaining structures. Therefore, in order to control the
formation of such wide crack widths, the concrete mix is specified to have a
tensile strength (normally measured by Brazilian test) at 7 days not
exceeding a certain value (e.g. 2.8N/mm 2 for potable water).
4. What is the importance of critical steel ratio in calculating thermal
reinforcement? (DWRS3)
The fulfillment of critical steel ratio means that in construction joints or
planes of weakness of concrete structure, steel reinforcement will not yield
and concrete fails in tension first. This is important in ensuring formation of
more cracks by failure of concrete in tension, otherwise failure in steel
reinforcement would produce a few wide cracks which is undesirable.
5. In the design of service reservoirs, horizontal reinforcement in
walls of reservoirs is placed at the outer layer. Why?
Since service reservoirs are designed as water-retaining structures with
stringent requirement of crack width control, the design of reinforcement of
service reservoirs is under the control of serviceability limit state. For the
walls of service reservoirs, contraction and expansion of concrete are more
significant in the horizontal direction of walls because of their relatively long
lengths when compared with heights. In this connection, in order to
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