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Paving Principles and Control Methods
How a Screed Works
11
P-32+ and P-32 Operator’s Manual P/ N: 1001547-01
(R). The key element to bear in mind is that the screed passing over the paving material compacts the material
to a certain degree. Variables in the resistance of the material to compactive forces causes changes in the
screed's angle of attack, which in turn affects mat thickness and therefore mat smoothness (Figure 21).
Figure 21: Reaction of Material Under Screed
Gradation Mix Characteristics
This aspect of the paving material varies according to the intended use of the material as abase course, binder
course or the final wearing surface. Normally, maximum aggregate size, ratio of aggregates, fines content and
most importantly, asphaltic binder content, is specified by the contracting agency.
Adherence to mix design specifications is usually the responsibility of the material supplier.
Segregation is a material deficiency caused by a separation of the larger aggregate sizes from the bulk of the
paving material.
This condition is encountered especially in mixes with relatively large maximum size aggregate (example: 1"
and larger, the so called “large-particle” mixes). When paving material is deposited in piles, as in an asphalt
plant silo, a haul truck, a paver hopper, on the ground in front of the paver in a windrow, or on the ground in
the auger chamber, segregation can and does frequently occur.
Segregation can also be the result of improper hopper dumping.
During normal operation, the vibration of the pavers hopper causes segregation. Therefore, dumping the hopper
after each truck should be avoided because material that has rolled to the outside of the hopper (the large
aggregate) fills the conveyors and auger chamber and result in a segregated area behind the paver with a
noticeable difference in surface texture.
I f it doesn't look right, it isn't right: Surface and texture irregularities indicate that the homogeneous
characteristics of the material in the mat have been interrupted, which usually results in bumpiness and
premature failure of the pavement in those areas.
Segregation can also be the result of excessively worn augers: “Center Streak” segregation can be caused by
worn “Kicker Paddles” at the center chain cause or near the outside auger bearings. In fact, center streak
segregation is frequently caused by incorrect arrangement of the auger segments adjacent to the auger chain
case. (Consult the Manufacturers Manual).
Reaction of Material
Under Screed (R)
R
The areas listed above where segregation can occur are all areas that the material may encounter
before being laid down as a mat. Therefore, these areas of segregation must be addressed prior to
any paving. The screed cannot rectify segregation during the paving process.
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