BID DOCUMENT - IDCO

(e)
No change in the specified mix will be permitted except with the authority of the Engineer.
1.07.1.1 MIXING
(a)
(b)
(c)
All concrete used in the works shall be mixed in mechanical batch mixer of approved pattern fitted with
power driven loading skip.
Each batch of concrete shall be thoroughly mixed for period of not less than 2 minutes to produce wellmixed
concrete of uniform consistency and appearance. The minimum mixing period specified is predicated
on proper control of the speed of rotation of the mixer and the introduction of the materials including water
into the mixer. The engineer reserves the right for the required uniformity of composition and consistency
within the batch and from batch to batch.
The leading skip the mixer drum its blades and the discharge chute shall be kept clean and free from
hardened mortar or concrete.
1.08 CONVEYING AND PLACING
Before placing concrete the sub grade should be properly prepared and the forms and reinforcement should
be erected. A moist sub grade is especially important to prevent too rapid extraction of water from the
concrete when pavement, floors and similar work are be cast in hot whether. Where the foundation is rock,
all those materials should be removed before the concrete is placed. When it is necessary to cut rock in
foundation, the surfaces in general should be vertical or horizontal and not sloping.
Where formworks are used they shall be clean, tight, adequately braced and constructed with materials that
will import the desired texture to the finished concrete. Care should be taken to see that sawdust nails and
other debris are removed form the space to the concerted. Forms should be moistened or oiled previous to
placing to facilitate free removal. Where they have been exposed to the sun it may be necessary to saturate
the wood thoroughly before concreting.
Reinforcing steel used for concreting should be cleaned and free of loose rust or mill scales. The concrete
shall be placed before the initial set so that it remains workable and can be compacted satisfactorily.
Additional water shall not be added to improve workability as it disturbs the water cement ratio.
Placing of concrete shall be done in a continued operation till the predetermined position of construction
joint is reached.
The maximum thickness of layer shall not be mire than 300mm even for wet consistencies.
Horizontal flow concrete shall be avoided as far as possible
Each subsequent layer shall be deposited while the previous layer is soft
In sloping member small quantities of concrete shall be deposited starting from the lower end of the slope.
In case of reinforced concrete work using shutter or formworks, the shutter and forms must be rendered
watertight and concrete shall not be deposited until the engineer in charge has inspected reinforcement,
forms and their support.
Before depositing fresh concrete over or against hardened concrete of construction joints the hardened
surface shamble roughened and cleared by wire brush or water jet with a pressure of 0.07kg/cm2 or by
sand blasting after the concrete has hardened sufficiently. The surface shall not be allowed to any between
jetting and placing on concrete. A thin layer of motor of the same perpetration as in the concrete shall be
applied just before placing fresh concrete.
1.09 VIBRATION
(a)
The concrete after placing shall be thoroughly compacted as it is placed to secure a dense structure close
bond with reinforcement and smooth surface by robbing, hand tamping or mechanical vibration as required
and directed by the engineer. Concreting between two walls shall be compacted by power driven vibrators
of the immersion type and concrete in slabs with no form work or upper surface shall be compacted by
power driven vibrators of the pan type or by vibrating screeds. The vibrators shall be of ample power
operating at not less than 6000 RPM and of kind approved by the engineer. They shall be operated by
workmen skilled in their use he shall be additional to the laborers employed for placing and tamping the
concrete.
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