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
difficult for water to re-enter into the reduced space.
19. What is the mechanism of plastic settlement in fresh concrete?
Within a few hours after the placing of fresh concrete, plastic concrete may
experience cracking owing to the occurrence of plastic shrinkage and
plastic settlement. The cause of plastic settlement is related to bleeding of
fresh concrete. Bleeding refers to the migration of water to the top of
concrete and the movement of solid particles to the bottom of fresh
concrete. The expulsion of water during bleeding results in the reduction of
the volume of fresh concrete. This induces a downward movement of wet
concrete. If such movement is hindered by the presence of obstacles like
steel reinforcement, cracks will be formed.
20. What are the differences in the behavior and properties of
recycled-aggregate concrete when compared to normal-aggregate
concrete? (CM4)
Higher porosity of recycled aggregate compared to natural aggregate leads
to a higher absorption. Moreover, recycled aggregate has lower specific
gravity than natural aggregate and will make concrete with higher drying
shrinkage and creep. Such differences become more significant when
there is an increasing amount of recycled fine aggregates. Recycled
aggregates also contain more chloride than normal aggregates which may
affect the durability of concrete. Moreover, excessive recycled fine
aggregates can also generate a harsh concrete mix with low workability.
21. Why are recycled aggregates and recycled concrete aggregates
not suitable for “High strength concrete”? (CM4)
Not all aggregates are suitable for producing “High strength concrete”
because the strength of aggregates may control the ultimate strength of
concrete once they are crushed apart before the failure of cement paste.
Recycled aggregates and recycled concrete aggregates are normally not
recommended in producing “High strength concrete”. Owing to their
intrinsic effect of reduced compressive strength, it requires increased
cement content to counterbalance this effect in normal concrete situation.
However, in the case of “High strength concrete”, the very high cement
level has already been adopted which offers little scope for further increase
in cement content.
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