Mix laydown involves everything used to place the
delivered HMA on the desired surface at the desired thickness. The asphalt
paver is the principal machine involved in mix laydown and is assisted by the
material transfer vehicle (MTV) in some instances.
The asphalt paver is a self-propelled formless laydown machine with a floating
screed (see Figure 4). HMA is loaded in the front, carried to the rear by a set
of flight feeders (conveyor belts), spread out by a set of augers, then leveled
and compacted by a screed.This set of
functions can be divided into two main systems:
Tractor. The tractor contains the material feed system, which accepts
the HMA at the front of the paver, moves it to the rear and spreads it out to
the desired width in preparation for screed leveling and compaction.
Screed. The most critical feature of the paver is the self-leveling
screed unit, which determines the profile of the HMA being placed (Roberts et
al., 1996). The screed takes the head
of HMA from the material delivery system, strikes it off at the correct
thickness and provides initial mat compaction. Figure 5 shows screed
components and the six basic forces that act upon the screed to determine its
height and, thus, pavement thickness.
Since the screed is free floating it will slide across the
HMA at an angle and height that will place these six forces in equilibrium.
When any one of these forces is changed, the screed angle and elevation will
change (which will change the mat thickness) to bring these forces back into
equilibrium. Therefore, changing the following paver characteristics will
affect these forces, and thus mat thickness, in the described manner:
Paver speed. If a paver speeds up and all other
forces on the screed remain constant, the screed angle decreases to restore
equilibrium, which decreases mat thickness (think of what happens to the ski
angle of a water skier as boat speed increases).
Material head. If the material head
increases (either due to an increase in material feed rate or a reduction in
paver speed), screed angle will increase to restore equilibrium, which increases
mat thickness.
Tow point elevation. As the tow point rises in
elevation, the screed angle increases, resulting in a thicker mat. As a
rule-of-thumb, a 1-inch movement in tow point elevation translates to about a
0.125 inch movement in the screed's leading edge. Without
automatic
screed control, tow point elevation will change as tractor elevation changes
due to roughness in the surface over which it drives. Locating the screed tow point near the middle of the tractor significantly
reduces the transmission of small elevation changes in the front and rear of the
tractor to the screed. Because the screed elevation responds
slowly to changes in screed angle, the paver naturally places a thinner mat over
high points in the existing surface and a thicker mat over low points in the
existing surface (TRB, 2000).
Screed angle can also be adjusted manually by using a thickness control screw or depth crank.
Screed angle adjustments do not immediately change mat thickness but rather
require a finite amount of time and tow distance to take effect. Figure
6 shows that it typically takes five tow lengths (the length between the tow
point and the screed) after a desired level is input for a screed to arrive at
the new level. Because of this screed reaction time, a screed operator who
constantly adjusts screed level to produce a desired mat thickness will actually
produce an excessively wavy, unsmooth pavement.
Figure 6: Screed Reaction
to a Manual Decrease in Screed Angle
(after TRB, 2000)
Since it is not practical to manually control tow point elevation, pavers usually operate using an automatic
screed control, which controls tow point elevation using a reference other than
the tractor body. Since these references assist in controlling HMA
pavement grade, they are called “grade reference systems” and are listed below
(Roberts et al., 1996):
Erected stringline. This consists of
stringline erected to specified elevations that are independent of
existing ground elevation. Most
often this is done using a survey crew and a detailed elevation/grade
plan. Although the stringline
method provides the correct elevation (to within surveying and
erecting tolerances), stringlines are fragile and easily broken, knocked
over or inadvertently misaligned. Lasers can be used to overcome the difficulties associated with
stringlines because they do not require any fragile material near the
pavement construction area. Lasers
can establish multiple elevation or grade planes even in dusty or
high-electronic and light-noise areas and are therefore sometimes used to
construct near-constant elevation airport runways. Even the laser method becomes quite
complicated, however, when frequent pavement grade changes are required.
Mobile
reference.This consists of a
reference system that travels with the paver such as a long beam or tube attached to the paver
(called a "contact" device since it actually touches the road - see
Figures 7 and 8) or an ultrasonic device
(called a "non-contact" device since it relies on ultrasonic pulses and not
physical contact to determine road elevation).The mobile reference system averages
the effect of deviations in the existing pavement surface over a distance
greater that the wheelbase of the tractor unit. Minimum ski length
for a contact device is
normally about 25 ft. with typical ski lengths being on the order of
40 to 60 ft. (Asphalt Institute, 2001).
Joint
matching shoe.This usually
consists of a small shoe or ski attached to the paver that slides on an
existing surface (such as a curb) near the paver. Ultra sonic sensors accomplish the same task
without touching the existing surface by using sound pulses to determine
elevation. This type of grade control results in
the paver duplicating the reference surface on which the shoe or ski is
placed or ultra sonic sensor is aimed.
Figures
7 and 8: Automatic Grade Control Using a
Mobile Reference Beam
Material transfer
vehicles (MTVs) are used to assist the paver in accepting HMA. Most pavers are
equipped to receive HMA directly from end dump or
live bottom trucks, however in certain situations it can be necessary or advantageous to use
an MTV. Paving using bottom dump trucks and windrows requires a windrow
elevator MTV (see Figure 9 and Video 1), while other MTVs (see Figures 10, 11,
12, and Video 2) are used to provide additional
surge volume, which is advantageous because it allows the paver to operate
continuously without stopping, minimizes truck waiting time at the paving site
and may minimize aggregate segregation and
temperature differentials.
