PACKAGE : I Bid Document VOLUME : II - Public Health Engineering

Vibration:
Vibrators shall be inserted into the uncompacted concrete vertically and at regular
intervals. Where the uncompacted concrete is in a layer above freshly compacted
concrete, the vibrator shall be allowed to penetrate vertically for about 75mm into
the previous freshly compacted layer. The vibrators shall not be allowed to come
into contact with the reinforcement of formwork nor shall they be withdrawn
quickly from the mass of concrete but shall be drawn back slowly while in motion
so as to leave no voids. Internal type vibrators shall not be placed in the concrete in
any arbitrary manner nor shall concrete be moved from one part of the work to
another by means of the vibrators. The vibrators shall have minimum 3600
(preferably 5000) impulses per minute.
Duration:
The duration of vibration shall be limited to that required to produce satisfactory
compaction of the concrete without causing segregation. Vibration shall on no
account be continued after the appearance of water or grout on the surface.
Hand compaction:
This shall be permitted exceptionally for small jobs by the Engineer. In such cases,
compaction shall be attained by means of rodding, tamping, ramming and slicing
with suitable tools. The thickness of concrete layers will also be suitably reduced
when hand compaction is resorted to.
7.11 Under water concreting.
No concrete shall be placed in water without the Engineer’s written permission,
which may only be granted if in his opinion it is not practicable to place the
concrete in the dry. Concrete shall not be placed in running water nor shall concrete
be allowed to fall through water. Any water entering the area where concrete is
being placed shall, at the contractor’s expense, be kept clear of the concreting
works. If under water concreting is permitted, the specified mix of concrete shall be
strengthened by increasing the cement content by atleast 10.0% and reducing the
water/cement ratio to no more than 0.45, and the placing shall be only through a
tiemie approved by the Engineer. The volume or mass of the coarse aggregate shall
not be less than 1 ½ times not more than twice that of the fine aggregate. The
material shall be so proportioned as to produce a concrete having a slump of not
less than 100mm and not more than 180mm.
GTS : General Technical Specification. Page 63