Workmanship for visual concrete
Good workmanship is essential to achieve a
quality concrete finish. The workforce should understand what is
required and its importance.
All tools should be clean before use and all
concrete handling equipment such as concrete skips, chutes and
pumps should be wet before use. As concrete is placed air gets
entrapped in the mix. Unless using self compacting concrete (SCC)
this air should (for all concrete, not just visual concrete) be
removed by compacting the concrete, usually with a vibrating poker.
For visual concrete not using SCC, a high frequency constant
amplitude electronic internal vibrating poker should be used. The
poker should be the appropriate size for the element and be fitted
with a rubber cone to prevent damage to the formwork.
For vertical elements the concrete should be
placed into position in layers not more than 500mm deep along the
full length of the section. If the layers are any deeper it
restricts the air from escaping during compaction. The concrete
should be placed where it is required and not moved by means of the
vibrating poker. The poker should be immersed in the concrete
sufficiently long to ensure full compaction and withdrawn slowly to
avoid trapping air against the formwork. The next layer of concrete
should be placed as soon as possible to avoid variations in colour
between the concrete layers. The concrete should be placed at the
rate of 2-3m/hour to avoid cold joints, pour planes and concrete
splashes hardening on the upper form face. The concrete should be
re-vibrated at the top of the pour after 1 to 2 hours to eliminate
In horizontal elements the concrete should be
placed in a systematic way. A continuous and uninterrupted supply
of concrete will ensure that cold joints do not occur. For thin
horizontal layers of concrete such as a floor slab the internal
poker vibrator should be inserted at a slight angle and drawn
horizontally through the concrete. This ensures better compaction
than inserting the vibrator vertically. The position of
construction joints should be agreed with the designer.
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