Compacting Concrete

WHAT IS COMPACTION Compaction is done by shaking,
or vibrating, the concrete which liquefies it, allowing the
trapped air to rise out.
The concrete settles, filling all the space in the forms.
WHEN TO COMPACT Compaction must be done
as concrete is placed, while it is still plastic. Never
let concrete dry-out and stiffen because it will be
too hard to compact.
WHY COMPACT Properly compacted concrete is
more dense, strong and durable. Off-form finishes
will also be better.
EXTERNAL VIBRATION
Screeding Screeding levels and compacts thin concrete slabs and the top layers of
thicker slabs. A screed board will not compact the concrete very well. Mechanical vibration
or hand rodding is required to provide adequate compaction.
The Mechanical Screed Concrete is screeded TWICE.
The first screed levels the concrete roughly and compacts it.
The second screed levels and compacts the concrete more.
The screed is pulled along the top of the forms by two
workers.
Always keep a small amount, or surcharge, of
concrete, in front of both beams of the screed to
avoid holes forming in the surface. If a hollow
develops, the screed will not compact the concrete.
The mechanical screed compacts the concrete as it vibrates.



INTERNAL VIBRATION Internal vibration is
done with a mechanical vibrator or poker
vibrator. The POKER is put into concrete and
vibrates it from the inside.
Method
Make sure there are enough workers so some
can compact while others continue to place.
Put the poker into the concrete QUICKLY. Take the poker out very SLOWLY otherwise a hole,
or weak spot, may be left in the concrete.
The SIZE of the poker determines how much concrete is vibrated at one time.
The area vibrated at one time is called the
RADIUS OF ACTION. This can be seen by over
what radius air bubbles rise to the surface.
The radius of action will be greater with a
LARGER poker and more-workable concrete.
Always compact in a definite pattern so the radius
of action overlaps and covers the whole area of
the concrete.
The poker should be long enough to reach
and enter into the layers of concrete under
the one being compacted.



PRECAUTIONS Taking the poker out TOO QUICKLY will leave a hole in the concrete.
To close the hole, vibrate near the hole and take the poker out VERY SLOWLY.
NEVER touch the form face with the
poker as it can damage the formwork
and the concrete.
NEVER touch the reinforcement with
the poker.
NEVER spread or move concrete sideways with
the poker, always use a shovel.
NEVER leave the poker running when not in use.
HOW LONG TO COMPACT For concrete of average workability (ie slump of 80 mm)
with a poker size between 25–75 mm, concrete should usually be vibrated for between
5 and 15 seconds.
It is worse to UNDER-VIBRATE than to OVER-VIBRATE concrete.