Concrete Compaction - Methods |
Concrete
should be placed and compacted immediately after mixing. The concrete should be
placed within 30 to 40 minutes to prevent the danger of concrete getting its
initial set, before laying the concrete, the shuttering should be cleaned of
all of dust or debris. Crude oil or grease etc is usually applied to the
shuttering before concreting to prevent the shuttering absorbing the water from
the concrete or getting stuck to it. In placing the concrete, care should be
taken to see that it should not be thrown from heights. Concrete should be laid
in layers 15 to 30 cm (6” to 12”) in thickness and each layer should be
properly compacted before laying the next one.
Compaction
of concrete should be proceed immediately after placing. The function of
compaction of concrete is to expel the air bubbles in the mass and make it
impermeable in addition to its securing the desired strength. The concrete mass
should be consolidated or compacted till the cream of the cement starts
appearing on the surface. Over compaction may lead to segregation of concrete
while-under-compaction may leave air voids in concrete and results in honey
combing. Compaction may be done by hand or mechanical device.
Hand
compaction : The hand
compaction may be done by rodding, tamping or hammering. Tamping is usually
adopted for compacting concrete for slabs or other such surfaces. Rodding is
done for thin vertical members. Hammering is done for massive plain concrete
works and for compacting an almost dry concrete the surface is beaten with
heavy flat bottom rammers till the thin film of mortar start appearing on the
surface.
Mechanical
compaction: Mechanical
compaction is done by the use of vibrators. Vibrators are of three types 1. Internal 2. External 3. Surface. Internal vibrators are commonly used in large
works for flat surface compaction. In this the vibrator is immersed in the full
depth of concrete layer. The vibrator should be kept in one position for about
3 minutes and then removed and placed another position. External vibrators
are placed against the form work and are only adopted for thin section of
members or in places where internal vibrators cannot be used with ease. Surface vibrators are generally employed in
concrete road construction. Compaction of concrete by use of vibrators permits
the use of stiff concrete mix of high strength and ensure better compaction
than that obtained by the method of hand compaction
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