CONCRETE (COMPACTION BY) VIBRATION
WHY?Cast-in-place concrete and concrete for prefabricated
elements is vibrated in order to compact it, ensure it flows in
between the rebar and throughout the formwork, filling all voids
and to reduce the quantity of entrapped air. Normal concrete does
not flow under its own weight!
Well compacted concrete is stronger. It is also more durable,
reacts less to outside corrosive factors. Well-compacted concrete
is homogeneous and has about 1-2% entrapped air. Its surface after
removing the formwork is smooth. It is also more impervious.
Strength decreases with 10% for 2% volume of voids and with 30% for
5% volume of voids per total volume.Although good quality concrete
is delivered (and we pay a lot for it), poor vibration can
significantly decrease the quality of the cast element.
WHAT?Concrete can be compacted in many ways. Using rods,
vibrators, vibrating tables, vacuum or centrifugal equipment.One of
the most common and best methods for compacting cast-in-place
concrete is using vibrators, which are mobile, require less manual
labor and if best practices are followed give good quality
results.Concrete vibration sets in motion the fresh concrete due to
the vibrations induced by the tools, causing the concrete to flow
and fill gaps, also eliminating entrapped air in the paste.
HOW?Concrete vibrators used depend on the size and shape of the
concreted element and on the concrete properties (workability, max.
aggregate size, etc.), also on the space between rebar and the
distance between them. The usual diameter of a vibrator is of 5 7
cm.The general rules for vibration are: Uniform vibration, in
conformity with the radius of action of the vibrator. Vibrators
should be placed at 1 radius of action of the vibrator between
them.
Fast insertion, slow removal (1 second for every 10 cm removed)
vibration between 5 30 seconds. Insert straight Vibrator has to
reach at least 10-15cm into the previously cast concrete layer Do
not touch rebar, it causes the bars to displace Do not push
concrete laterally with vibrator, it should flow because of the
vibration Vibrate until very little air escapes the concrete, until
surface is level and shiny, but no cement water is at the surface
(indicator of the segregation of concrete) Do not over-vibrate can
cause segregationCannot use very fluid (workable) concrete, because
normal concrete losses strength with fluidity due to more added
water. Admixtures increase the price of concrete, so thats not very
handy.PROBLEMS due to under-vibration: Excessive entrapped air
Honeycombing Sand streaks Cracking Cold jointsPROBLEMS due to
over-vibration: Segregation Sand streaks Form deflection