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

Weigh-batching: Weigh batching machines shall provide facilities for the accurate
control and measurement of the materials either singly or cumulatively and shall be
capable of immediate adjustment by operators in order to permit variations if
ordered by the Engineer. All weight dials shall be easily visible from the place at
which filling and emptying of the hoppers are controlled.
Addition of water and mixing:
Water: The addition of water to a mixer shall be controlled such that between five
and ten percent of the water enters the mixer before the cement and aggregate and a
further five to ten percent of water enters the mixer after the said materials have
been batched. The remainder of the water shall be added at a uniform rate with the
said materials. The water-measuring device shall also be readily adjustable so that
the quantity of water added to the mixer can, if necessary in the opinion of the
witnessing Engineer’s representative be varied. The natural moisture contents of
the aggregates shall be determined before the commencement of concreting or at
such intervals as may be necessary or as required by the Engineer. The Contractor
shall make due allowance for the water contained in the aggregate when
determining in consultation with the Engineer’s representative, the quantity of water
to be added to each mix, and shall adjust the amount of water added to each mix to
maintain consistently the approved water/cement ratio of the mixed concrete. All
important concrete shall be machine mixed to give complete coating of cement
mortar on each coarse aggregate particle and to produce uniform coloured concrete
with uniform distribution of materials. The mixer shall be run minimum
1½ minutes. In case, for a minor job, hand mixing is permitted by the Engineer, it
shall be done on smooth watertight platform not allowing the added water to flow
out. The fine aggregate shall be spread in uniform thickness layer over which
cement as required shall be placed and they shall be mixed thoroughly to give
dry mortar.
Water is then added gradually in required proportion, turning the mass, to give
desired consistency mortar. The required quantity of coarse aggregate is then placed
on mixing platform, wetted and mortar added. The entire mass is turned and
returned to give uniform concrete of required consistency. 5% additional cement
shall be used for hand mixed concrete.
Admixtures: Any admixtures approved by the Engineer, which may be used, shall
be measured separately in calibrated dispensers and shall be added to the mixture
together with the water.
GTS : General Technical Specification. Page 60