Force V will decrease at this point and be balanced by
3. To reduce mat thickness, tilt the screed plate so
that less material crowds under the screed plate.
4. The amount and condition of the material,
leaving the auger, can change the equilibrium of the four
forces. Excessive flow of material increases force H. A
cold, stiff mix will increase H and to some extent V. An
excessively hot, fluid mix decreases H and V. Stopping
and starting the paver also cause changes in equilibrium
among the forces. The key to controlling the action of
the screed is to maintain in a uniform manner those
forces acting on the screed.
5. The secret of good paver operations is a balance
of the forces and uniformity to maintain that balance.
When balance and uniformity are attained, the screed
path follows the paver in a plane parallel to that of the
pivot point. As the paver goes up over an irregularity,
the pivot point rises and the screed begins to rise also.
However, because the screed reacts to changes in
elevation more slowly than the pivot point does, the
screed rises very little and thereby maintains the plane
of the surface of the mat over the irregularity, and the
impact of the irregularity is reduced. The same is not
true of long irregularities (longer than several lengths of
the paver). Grade line irregularities of this type should
be corrected before placing surface courses with the
6. Screeds, equipped with tamping bars and
vibratory mechanisms, are designed to strike off and
then compact the mixture slightly, as it is placed. The
two purposes for this screed action are that it achieves
maximum leveling of the mat surface and ensures that
minimum distortion of the mat surface occurs with
TAMPING BAR TYPES OF SCREEDS
Tamping bar types of screed compactors compact the
mix, strike off the excess thickness, and tuck the material
under the screed plate for leveling. As figure 16-41
shows, the tamper bar has two faces:
. A beveled face on the front that compacts the
material, as the screed is pulled forward.
l A horizontal face that imparts some compaction,
but primarily strikes off excess material, so the screed
can ride smoothly over the mat being laid.
The adjustment that limits the range of downward
travel of the tamping bar is the single most important
adjustment affecting the appearance of the finished mat.
At the bottom of its stroke, the horizontal face should
extend one sixty-fourth of an inch (about the thickness
of a fingernail) below the level of the screed plate.
When the bar extends down too far, mix builds upon the
screed face. This buildup scuffs the surface of the mix
being placed and also causes the tamping bar to lift the
screed slightly on each downward stroke, and this often
causes rippling of the mat surface.
When the horizontal face of the tamping bar is
adjusted too high (either by poor adjustment or due to
wear of the bottom of the horizontal face), the bar does
not strike off excess mix from the mat. Consequently,
the screed plate begins to strike off the material, and this
results in surface pitting of the mix being placed
because the leading edge of the screed plate drags the
larger aggregates forward. For this reason, the tamper
bar should always be checked before operating the
paver, and it should be adjusted if necessary. Before the
tamper bar approaches knife-edge thinness, it should be
VIBRATING TYPES OF SCREEDS. The
operation of vibratory screeds is similar to that of
tamping screeds, except that the compactive force is
generated either by electric vibrators, rotating shafts
with eccentric weights, or hydraulic motors (fig. 16-43).
On some pavers both the frequency (number of
vibrations per minute) and the amplitude (range of
motion) of the vibrators can be adjusted. In others, the
frequency remains constant and only the amplitude can
Frequency and amplitude must be set
according to the type of paver, the thickness of the mat,
the speed of the paver, and the characteristics of the
mixture being placed.
Once set, the frequency and
amplitude do not normally need adjustment until the mat
thickness or mix characteristics change.
Figure 16-43.-Vibratory type of screed.