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
The number of construction joints in concrete structures should be
minimized. If construction joints are necessary to facilitate construction, it is
normally aligned perpendicular to the direction of the member. For beams
and slabs, construction joints are preferably located at about one-third of
the span length. The choice of this location is based on the consideration of
low bending moment anticipated with relatively low shear force [10].
However, location of one-third span is not applicable to simply supported
beams and slabs because this location is expected to have considerable
shear forces and bending moment when subjected to design loads.
Sometimes, engineers may tend to select the end supports as locations for
construction joints just to simplify construction.
13. What are the potential problems for retardation of fresh concrete?
Retardation of fresh concrete has several advantages as follows:
(i) A rapid hydration process results in loss in concrete strength because
the concrete will have a poorer structure with a higher gel/space ratio
compared with the concrete with a lower hydration rate.
(ii) During the hydration process, a substantial heat of hydration will be
generated. If the hydration process is carried out too swiftly, it will cause
a rapid rise in temperature and results in considerable early thermal
movement in concrete.
(iii) In hot weather concreting, the loss of workability is substantial. In order
to ensure sufficient compaction of fresh concrete, it is necessary to
extend the time for fresh concrete to remain plastic.
14. What are the major problems in using pumping for concreting
works? (PC1)
In pumping operation, the force exerted by pumps must overcome the
friction between concrete and the pumping pipes, the weight of concrete
and the pressure head when placing concrete above the pumps. In fact, as
only water is pumpable, it is the water in the concrete that transfers the
pressure.
The main problems associated with pumping are the effect of segregation
and bleeding. To rectify these adverse effects, the proportion of cement is
increased to enhance the cohesion in order to reduce segregation and
bleeding. On the other hand, a proper selection of aggregate grading helps
to improve the pumpability of concrete.
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