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. Is curing compound suitable for all concrete? (C1)
For concrete structures with low water-cement ratio (i.e. less than 0.4), it
may not be suitable to use curing compound for curing. When hydration
takes place, the relative humidity of interior concrete drops which leads to
self-desiccation and drying-out. With no external supply of water, the
cement paste can self-desiccate in such an extent that the hydration
process stops. As such, curing compounds may not be sufficient to retain
enough water in the concrete. In this case, wet curing is a better choice
which serves to provide an external source of water.
10. What is the difference between curing compound and sealing
compound? (C1)
Curing compound is primarily used for reducing the loss of moisture from
freshly-placed concrete and it is applied once after concrete finishing is
completed. Sealing compounds is adopted to retard the entrance of
damaging materials into concrete and they are normally applied after the
concrete is placed for 28 days. The harmful substances include water,
deicing solutions and carbon dioxide which eventually cause freeze-thaw
damage, steel corrosion and acid attack respectively.
11. Curing time in summer is less than that in winter. Why?
While concrete sets, it gains hardness and strength as the process of
hydration slowly spreads the entire body of material. Curing should be
allowed to continue for several days before subjecting the new concrete to
significant stress. The period of curing depends on the temperature
because the rate of all chemical reactions is dependent on temperature.
Therefore, in summer the rate of reaction (hydration) is faster so that a
shorter curing time is required. On the contrary, in winter the rate of
reaction (hydration) is slower so that a longer curing time is required.
12. Why does plastic sheet cause discolouration to freshly placed
concrete? (C2)
Plastic sheets are commonly used in curing to prevent moisture loss from
concrete surface. However, it is not uncommon that discolouration occurs
on the concrete surface. When plastic sheeting is spread over concrete
surface and in direct contact with concrete, it tends to leave colour streaks
on concrete surface. The problem of discolouration becomes even worse
108