3.7.5
Belt Conveyors
Belt conveyors shall be designed and operated to assure a uniform flow of
concrete from mixer to final place of deposit without segregation of
ingredients or loss of mortar and shall be provided with positive means,
such as discharge baffle or hopper , for preventing segregation of the
concrete at the transfer points and the point of placing. Belt conveyors
shall be constructed such that the idler spacing shall not exceed 900 mm 36
inches. The belt speed shall be a minimum of 90 meters 300 feet per minute
and a maximum of 225 meters 750 feet per minute. If concrete is to be
placed through installed horizontal or sloping reinforcing bars, the
conveyor shall discharge concrete into a pipe or elephant truck that is
long enough to extend through the reinforcing bars.
3.7.6
Concrete Pumps
Concrete may be conveyed by positive displacement pump when approved. The
pumping equipment shall be piston or squeeze pressure type; pneumatic
placing equipment shall not be used. The pipeline shall be rigid steel
pipe or heavy-duty flexible hose. The inside diameter of the pipe shall be
at least 3 times the nominal maximum-size coarse aggregate in the concrete
mixture to be pumped but not less than 100 mm 4 inches. Aluminum pipe
shall not be used.
3.8
PLACING CONCRETE
Mixed concrete shall be discharged within 1-1/2 hours or before the mixer
drum has revolved 300 revolutions, whichever comes first after the
introduction of the mixing water to the cement and aggregates. When the
concrete temperature exceeds 30 degrees C 85 degrees F, the time shall be
reduced to 45 minutes. Concrete shall be placed within 15 minutes after it
has been discharged from the transporting unit. Concrete shall be handled
from mixer or transporting unit to forms in a continuous manner until the
approved unit of operation is completed. Adequate scaffolding, ramps and
walkways shall be provided so that personnel and equipment are not
supported by in-place reinforcement. Placing will not be permitted when
the sun, heat, wind, or limitations of facilities furnished by the
Contractor prevent proper consolidation, finishing and curing. Sufficient
placing capacity shall be provided so that concrete can be kept free of
cold joints.
3.8.1
Depositing Concrete
Concrete shall be deposited as close as possible to its final position in
the forms, and there shall be no vertical drop greater than 1.5 meters 5
feet except where suitable equipment is provided to prevent segregation and
where specifically authorized. Depositing of the concrete shall be so
regulated that it will be effectively consolidated in horizontal layers not
more than 300 mm 12 inches thick, except that all slabs shall be placed in
a single lift. Concrete to receive other construction shall be screeded to
the proper level. Concrete shall be deposited continuously in one layer or
in layers so that fresh concrete is deposited on in-place concrete that is
still plastic. Fresh concrete shall not be deposited on concrete that has
hardened sufficiently to cause formation of seams or planes of weakness
within the section. Concrete that has surface dried, partially hardened,
or contains foreign material shall not be used. When temporary spreaders
are used in the forms, the spreaders shall be removed as their service
becomes unnecessary. Concrete shall not be placed in slabs over columns
and walls until concrete in columns and walls has been in-place at least
SECTION 03300A
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