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
Wet-mix method involves the pumping of ready mixed concrete to the
nozzle. Compressed air is introduced at the nozzle to impel the mixture
onto the receiving surface. The wet-mix method is more favorable for large
volume placements. Rebound is less than in the dry-mix method. The
nozzleman does not need to be concerned with the control on water
addition.
18. What is the difference between carbonation and carbon dioxide
attack? (D1)
For carbon dioxide attack, carbon dioxide dissolves in water to form a weak
acid called carbonic acid. It would dissolve the cement matrix. However,
the amount of carbon dioxide from the atmosphere is usually not sufficient
to cause harm to concrete structures until additional source of carbon
dioxide is available (e.g. decaying vegetable matter).
Carbonation is the process of converting alkaline hydroxides in concrete to
carbonates by reaction with carbon dioxide. The significance of
carbonation lies in the reduction of pH of pore water in concrete structure
from 12-13 to 8-9 so that it drops the protection to steel reinforcement. The
process takes place at concrete surface and spreads inwards. The passive
nature play an important role in steel corrosion as it prevents corrosion
even in the presence of water and oxygen. This passive nature is derived
from a stable and thin layer of iron oxide formed at the surface of steel
reinforcement. However, if the pH of concrete is dropped, this passive
oxide layer becomes unstable and corrosion may start once water and
oxygen supply is available.
Stress would be generated owing to the occurrence of shrinkage. For
instance, concrete at surface tends to dry more rapidly than the interior,
leading to differential shrinkage. This results in the formation of internal
balancing forces in which there is tension on concrete surface with
compression in the interior concrete. About 90% of ultimate concrete strain
occurs in the first year.
When the water is allowed to absorb water, only part of shrinkage is
reversible. Water loss from gel pore with C-S-H and capillary pres can be
readily replenished. However, water loss from the space between C-S-H
layers cannot be easily replaced with water. Once water is lost from the
space between C-S-H layers at the first place, the bond between the layers
becomes stronger and the layers would get closer together, thus making it
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