Concrete Work
Bonding Agent
Bonding Agent – EMACO 157 D Primer (Barra Emulsion 57
D)
Before concreting, Barra Emulsion was
applied to the existing concrete surface to have proper bonding of already
casted concrete and new casting member.
Properties
• Supply
form: Liquid
• Color:
Milky White
• pH
Value: approximately 9 – 10.5
• Application
Temperature: not below 5° C Toxicity:
non-toxic
Column Concreting
There were two types of concrete used in this site. They were
Ready Mix and Site mix. The difference between these two were, Ready Mix was
weight batched and Site Mix was volume batched. Ready mix concrete batching was
done in onsite batching plant. There are two onsite batching plants and backup
plant from Tokyo Super 10 km far from the site.
Surface
Preparation was done after concreting slab, ideally the concrete in the column
area was chipped and cleaned using pressurized
air from a compressor
to remove any loose
particles and grout
from the bonding
surface and reinforcement
bars are cleaned
to remove any
grout before erecting formwork. This gives a rough clean surface for the
concrete to bond.
Scaffolding was
arranged around the column for the workers to operated and checked to see if
they were braced properly to ensure safety.
In determining whether
scaffolding arrangement was
adequate, the best method is to have a
person from the concreting gang to
confirm that was
sufficiently safe for them to work on. Otherwise, there have been a few
instances where there have been lapses in scaffolding during concreting that
potentially caused injuries. Especially when working at upper levels, columns
in the edges of the building it was of utmost importance to eliminate any
safety hazards. All workers should properly wear safety belts during concreting
when working at heights.
G30 RM concrete
was used for columns. Concrete pouring was done using mobile crane, and
concrete buckets. (Figure 1.11). Concreting height was controlled under 1800 mm
to avoid segregation. Ordering concrete with a proper slump was important,
because if the mix was too stiff it will be unworkable and will require longer
compaction. This increases the chance of segregation at bottom of the column,
therefore, a proper mix should be used. Therefore, slump needs to be checked
after batching especially in column concreting to make sure it’s within the
required range.
Figure
1.11. Column Concreting
After the concrete has been placed,
it contain air voids. Concrete compaction was done for avoid the air voids in
fresh concrete. Poker vibrator was used for compacting concrete. It consists
short straight tube about 750 mm long and 25mm – 50mm diameter. The tube was
attached to an electric motor or petrol engine with flexible hose. Poker must
be insert to the concrete quickly and allow to penetrate to the bottom of the
layer as soon as possible under its’ own weight. The vibrator should let in
concrete around 15 seconds. Poker should be withdraw slowly and insert back
nearly about 300 mm away from its’ last position. The vibrator was kept away
75mm-100mm from formwork because formwork can be damaged. Also kept away from
reinforcement unless steel bars can be leave from correct position. Also
formwork was tapped using rubber hammer after concrete pouring to avoid the
concrete having hollow ditches.
Slab Concreting
The slab thickness of a floor is 150mm
& 200mm. Concrete levels are dropped on some areas in the floor. A slab is
concreted in 2 - 3 stages. This method was implemented in project planning to
improve progress, better utilize labor, and avoid bottlenecks in the
construction process. When planning the concreting process, arrangements are
made in advance according to the methods decided. If concrete was batched at
site two truck mixers were used to avoid the delay during batching concrete. If
a pump car was arranged, location of placing the pump car should be determined and
clear access provided in advance. If a concrete was stopped between grids, it
is done at 1/3rd or 2/3rd of the distance between the grids. The construction
joint between the two successive concretes was comparatively a weak plane
through the continuous slab. Therefore the position of the construction joint
was determined thus to give the least bending moment and shear force as
possible.
Surface Preparation of the formwork
was cleaned and blasted
with pressurized air
to remove any
saw dust and other
debris; this gives a smooth
finished surface to
the concrete. Formwork
panel joints and
stop board gaps
were covered with
masking tape and
sponge to prevent grout
leaking. Construction joints were chipped and cleaned to give rough
profile for better bonding. If
concreting was done
at night adequate
lighting needs to be
arranged and similar precautions (to that of column concreting) were taken on
scaffolding arrangements if required (for concreting beams at the edges of the
slab) and equipment used .
Concrete should
be poured uniformly as possible and formation of heaps should be avoided, if so
the concrete heap needs to be dispersed before using the vibrating poker.
Concrete was placed along the shorter span of the panel; this avoids formation
of cold joints since time between successive concrete pours was less. (Figure
1.12) Compacting concrete was done with a poker vibrator. The concrete was
leveled using gauge and troweled. An officer and a worker from the formwork
section should check grout leakage underneath slab and repaired immediately.
Figure 1.12. Slab Concreting
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