Some vibratory screeds require a pre-strike-off unit.
This unit is a rounded moldboard that controls the
amount of mix passing under the screed.
SCREED CONTROLS (ADJUSTMENTS).
In operating the screed, two types of controls are
l Control of the thickness of the mat
l Control of the crown, formed in the mat for
Both functions are regulated by controls built into
the paver (fig. 16-44).
It is important to understand that, when the paver
is operating, control adjustments, made to the screed,
take time to go into effect. For example, when a
thickness control screw is adjusted to change the
thickness of the mat, the paver is likely to move a
distance of several feet before the change is
completed, and the mat is produced in the new
thickness. For this reason, it is necessity that a screed
operator know the effective delay involved in making
adjustments to a particular screed unit and be able to
anticipate adjustments accordingly. Furthermore, it is
important that after such adjustment of the thickness
controls, the paver be allowed to travel far enough for
the correction to be completed before another
adjustment is made.
Excessive adjustment or
overcontrol of the thickness controls is one of the
principal contributors to poor pavement smoothness.
The condition of the screed unit is important when
a high-quality mat is to be placed. To ensure the
screed control linkage is snug, the operator should
check the wear points. Also, the screed plates should
be checked regularly for signs of wear, such as pitting
and warping. The plates should always be properly
adjusted before paving begins. Both the leading and
trailing edges of the screed have a crown adjustment.
The leading edge should always have slightly more
crown than the trailing edge. This provides a smooth
flow of material under the screed. Too much lead
crown results in an open texture along the edges of
Figure 16-44.-Mat thickness and crown controls.