the mat and too little results in an open texture in the
center. The trailing edge is what actually sets the crown.
Crown adjustments may be made independently or
simultaneously during the paving operation.
AUTOMATIC SCREED CONTROLS.— The
screed controls must be adjusted by the screed operator
as paving progresses. Automatic screed controls are
designed to adjust automatically to place a uniform mat
of the desired thickness, grade, and shape (fig. 16-45).
Types and Operating Principles.— Automatic
screed controls can be used in several different ways,
but all automatic screed control operations require a
reference system for the automatic system to follow.
This reference system can be the base on which the
asphalt hot mix is being placed, the lane next to the
material being placed, or a string line. When a string
line is used as a reference, the automatic control will
follow the height of the string line exactly, so the mat
conforms to it; therefore, placement of the string line (or
other reference system) must be precise.
Automatic screed controls can also follow traveling
reference systems. A traveling reference system, such
as a ski attached to a control arm, notes changes in base
contours and adjusts the screed automatically to
A string line or traveling reference system allows
the automatic control to adjust screed height as
necessary to maintain proper longitudinal (lengthwise)
grade of the pavement. Automatic screed controls use a
system, attached to a beam, running between the two
screed pull-arms to maintain proper transverse
(widthwise) grade.
A pendulum in the slope control housing moves side
to side with changes in the transverse grade of the
roadway, triggering necessary adjustments in the slope
control mechanism.
Automatic control systems have several advantages
over manually controlled screed systems. Some of the
advantages are as follows:
l Automatic controls compensate for changes in
grade and slope more quickly than a screed operator
could.
l Automatic controls help disassociate the screed
from the erratic vertical movement of the tractor unit.
. Automatic controls adjust the screed tow points
to enable the screed to follow a path parallel to the grade
and slope of the reference system, which may be
compensate.
different from the path plane of the tractor unit.
Figure 16-45.—Automatic screed reference system.
16-40
