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
(i) It stains the surface of hardened concrete.
(ii) Excess oils have nowhere to escape and find its way inside the
cement paste and form holes subsequently. The oils bead up
because of its incompatibility with water in chemical nature.
(iii) Higher cost is associated with increased usage of form oils.
(iv) In a relatively short time essentially most diesel oil evaporates so
that it creates environmental problem.
3. How can permeable formwork improve the quality of concrete?
Permeable formwork serves as a filter that allows excess water and
trapped air to escape from concrete surface. During compaction by
vibrators, the fluid movement through permeable formwork drives out air
and water, leaving behind a denser and stronger concrete. The movement
of water results in a decrease in water in fresh concrete and fines cement
particles from interior concrete shall be carried towards the formwork.
Hence, it lowers the water cement ratio at concrete surface and enhances
a higher strength near concrete surface.
In traditional formwork, during concrete compaction process the vibration
tends to force water to the surface of concrete mass owing to the
rearrangement of solid particles in concrete. As such, the concrete in
concrete/formwork interface possess more water than the interior concrete.
Consequently, the higher water cement ratio at concrete surface would
lower the surface strength. Permeable formwork functions and solves this
problem by allowing excess water to pass through.
4. What is the relation of pouring rate and temperature with concrete
pressure on formwork? (F2)
Freshly placed concrete exerts pressure on formwork during the placing
operation. It is influenced by the rate of placing and the air temperature.
For instance, if the concrete pouring rate is too slow, setting of concrete
starts to take place. As a result, the concrete at the bottom of the formwork
sets prior to the placing of fresh concrete at the top and the maximum
pressure will be reduced.
Temperature affects the rate of hydration of concrete. The higher the air
temperature is, the higher will be the rate of hydration reaction.
Consequently, fresh concrete tends to set at a faster rate. The pressure
exerted on formwork decreases with an increase in temperature. For this
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