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What is Revibration?
Revibration is vibration of concrete which has already
been placed & compacted. It is also, sometimes, referred to as “delayed
vibration”.
Is
Revibration beneficial?
Revibration is beneficial provided the already placed
& compacted concrete can regain plastic state under revibration. Generally
speaking, Concrete will benefit from revibration provided the concrete is
sufficiently plastic to permit the running vibrator to sink of its own weight.
The major benefit of revibration is the improved bond
between successive lifts of concrete as trapped water gets removed during
revibration. Revibration, if done at appropriate time, also results in
improvement of compressive strength. However, if revibration is done too late,
concrete may get damaged.
What happens
during Revibration?
Revibrating concrete momentarily liquifies the
concrete again. The primary chemical process that occurs in the first 2 hours
after concrete is placed is the formation of calcium hydroxide, which typically
makes up 15 to 25 percent of ordinary portland cement concrete. The other major
product of hydration is calcium silicate hydrate, which usually makes up about
50 percent of ordinary portland cement concrete and gives the concrete its
hardness and durability.
Formation of calcium silicate hydrate begins in
earnest only after several hours have elapsed.
Somewhere in that process, the concrete reaches initial set. After
initial set, formation of the more brittle, weaker calcium hydroxide continues
but falls behind the calcium silicate hydrate formation, which accelerates
dramatically between initial set and final set.
When revibration occurs after the initial set, it
breaks down some of the calcium hydroxide that has already been formed. That
allows freshly placed concrete adjacent to the revibrated concrete to join with
it, rather than introducing a construction joint, and it again becomes a
monolithic concrete structure.
When to
Revibrate?
Concrete can be successfully revibrated upto about 4
hours from the time of mixing. The revibration results in increase in
compressive strength by about 3 to 14%. This increase is measured between two
cases (i.e one without revibration & the other with revibration) with same
total period of vibration. The improvement is more prominent at early ages
& for concretes liable to heavy bleeding (where expelling of trapped water
during revibration results in increased strength).
Why
Revibration is not widely used?
Revibration involves extra cost since it is an extra
step in the production of concrete. Also if revibration is done too late,
concrete may get damaged.
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