push a 71,000 GVW dump truck. The push-bar shall be pivotally mounted and shall
be equipped with steel rollers. The hopper shall have dual-bar feeders to
deliver material to the spreader screws at an even rate. Each side of each
feeder shall operate independently of the other side.
3.14 Compaction. Compaction shall be accomplished by tamping, electric
vibration, or hydraulic-actuated vibration, or hydraulic-mechanical actuated
3.14.1 Tamping. When tamping is employed, the machanism shall have a nominal
vertical stroke of 1/8 inch, and an operating frequency of not less than 1,500
strokes per minute at governed engine speed. Tamper bars shall be replaceable,
and extensions shall be provided to accommodate screed extensions.
3.14.2 Electric vibration. When electric vibration is employed, the screed
shall be vibrated by not less than four vibrators operating at a constant
vibration of 3,600 vibrations per minute at governed engine speed. The
vibrators shall be protected against the entrance of moisture and dust. The
intensity of vibration shall be controlled by rheostats. Screed extensions shall
receive the same vibration as the screed.
3.14.3 Hydraulic-actuated vibration. When hydraulic-actuated vibration is
employed, an oscillating screed and a vibrating compacting unit shall be
provided. The screed shall oscillate at not less than 600 strokes per minute at
governed engine speed with a stroke of not less than 1/4 inch. The compacting
unit shall vibrate at not less than 2,000 cycles per minute at governed engine
speed. Compacting-unit extensions equal in length to screed extensions shall
receive the same vibration as the compacting unit.
3.14.4 Hydraulic-mechanical actuated vibration. The screed shall be vibrated
by not less than four mechanical vibrators. Vibrators shall be mechanically
inter-locked and capable of control by the operator through a frequency range of
2,000 to 4,000 cycles per minute. Screed extensions shall recieve the same
vibration as the screed.
3.15 Spreader screws. The spreader screws shall distribute the bituminous
material evenly to the whole width of the compacting unit and across either half
of the compacting unit as independent operations. The spreader screws shall
operate in conjunction with the bar feeders and shall have automatic controls with
3.16 Screed. Provisions shall be made for heating the screed to prevent the
bitument from adhering. The screed shall be raised and lowered by power-operating
hydraulic hoist. A mechanical locking device that will hold the screed in the
raised position during traveling shall be provided. A steel platform with
antiskid surface shall be provided for the screed operator. The screed shall
incorporate adjustments to control the thickness of the pavement strip.
Sufficient adjustment shall be provided to lay pavement at any thickness between
1/2 inch and 6 inches. The screed shall be capable of providing negative crown to
not less than 1/2 inch and positive crown to not less than 2-1/2 inches of the
finished mat on a 10 foot width.