g PSUdioxide causes carbonation of concrete i.e., loss of alkalinity of concrete or loss of the protective
power that concrete gives to the reinforcement or other steel embedded in it. Once the carbonation
depth exceeds the thickness of concrete cover to the embedded steel, steel becomes vulnerable
to the attack of moisture. This expedites rusting of steel as the protective concrete cover
remains no longer alkaline in nature.
Voids also reduce the contact between embedded steel and concrete. This results in loss of
bond strength of reinforced concrete member and thus the member loses strength. Voids such
as honeycombs and blowholes on the exposed surface produce visual blemish. Concrete
surface is not good to look with all such blemishes. Concrete with smooth and perfect, surface
finish not only looks good but is also stronger and more durable.
Compaction is achieved by imparting external work over the concrete to overcome the
internal friction between the particles forming the concrete, between concrete and reinforcement
and between concrete and forms and by reducing the air voids to a minimum. The compaction
of concrete can be achieved by the following methods.
r2g
This method of compaction is used for small and unimportant jobs. However, this method is
extremely useful for thin elements such as slabs, and for members with congested reinforcements.
This method can be used for mixes with any workability except for very fluid or very plastic
mix. Hand compaction is achieved by rodding ramming, or tamping.
Rodding is done with the help of 16 mm diameter, 2 m long steel rod to pack the concrete
between the reinforcement, sharp corners and edges. Rodding is done continuously during
concreting. Ramming is permitted only for unreinforced concrete constructions. The roof and
floor slabs are usually tamped for achieving compaction. The tampers are 100 × 100 mm in
section and about 1 m long. Tamping bars not only compact the concrete but also level the top
surface. The limitation of this method is that a large water-cement ratio is required for full
compaction.
g
2 2
This is the most common and widely used method of compacting concrete for any structural
element. The vibrations imparted to the fresh concrete reduce the internal friction between the
particles of concrete by setting the particles in motion and thus produce a dense and compact
mass. On vibration, the concrete mix gets fluidized and the internal friction between the
aggregate particles reduces, resulting in entrapped air to rise to the surface. On losing entrapped
air the concrete gets denser. Vibrations do not affect the strength of concrete but concrete of higher
strength and better quality can be made with lesser water and given cement content. Therefore,
with vibrations stiff concrete with low water cement ratio can also be well compacted. For full
compaction, vibration can be considered to be sufficient when the air bubbles cease to appear and
sufficient mortar appears to close the surface interstices and facilitate easy finishing operation.
Vibration helps entrapped air to escape first from between the coarse aggregate particles and later
from the mortar. When vibration continues some more entrapped air from the mortar is driven
out. However, during this second phase, concrete does not show any movement but it is in this
phase that maximum entraped air is driven out and that is the time when most of the consolidation
takes place. Plastic mixes need less time of vibration than harsh or dry mixes. The various types
of vibrators in use are needle, formwork, table or platform, and surface vibrators.