accommodate their use.
3.1.5.4
Belt Conveyors
Belt conveyors shall be designed and operated to assure a uniform flow of
concrete from mixer or delivery truck to final place of deposit without
segregation of ingredients or loss of mortar and shall be provided with
positive means for preventing segregation of the concrete or loss of mortar
at the transfer point(s) and the point of placing. The idler spacing shall
not exceed 900 mm 36 inches. Belt speed shall be a minimum of 90 m 300 feet
per minute and a maximum of 230 m 750 feet per minute. Belt width shall
be a minimum of 600 mm 24 inches if the NMSA is 150 mm 6 inches and shall
be a minimum of 400 mm 16 inches if the NMSA is 75 mm 3 inches or less.
The NMSA required in mixture proportions furnished by the Government will
not be changed to accommodate the belt width.
3.1.5.5
Pump Placement
Concrete may be conveyed by positive-displacement pump when approved. Pump
placement will be approved only for areas where placement by bucket or
conveyor is difficult or impractical. The pumping equipment shall be
piston or squeeze-pressure type. The pipeline shall be rigid-steel pipe or
heavy-duty flexible hose. Aluminum pipe shall not be used. The inside
diameter of the pipe shall be at least 3 times the nominal maximum size of
the coarse aggregate in the concrete to be pumped but not less than 100 mm
4 inches.
3.2
PREPARATION FOR PLACING
3.2.1
Vibrators
An adequate number of vibrators shall be on hand to meet placing
requirements, and spare vibrators shall be available to maintain production
in the event of breakdown. There shall be adequate air pressure available
for air vibrators and adequate voltage for electric vibrators. Vibrators
of the proper size, frequency, and amplitude shall be used for the type of
work being performed in conformance with the following requirements:
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NOTE: The following table to be used with SI units.
**************************************************************************
HEAD DIAMETER
AMPLITUDE
APPLICATION
MILLIMETERS
VPM
MILLIMETERS
Thin walls, beams, etc. 32
-
64
9,000
- 13,500
0.5
-
1.0
General construction
50
-
88
8,000
- 12,000
0.6
-
1.2
Heavy sections
75
-
150
7,000
- 10,500
0.75
-
1.5
Mass concrete
125
-
175
5,500
- 8,500
1.0
-
2.0
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NOTE: The following table to be used with IP units.
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SECTION 03700
Page 55
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