By: Bill Rieken
At every stage in handling aggregate and asphalt, there is a
chance for material segregation to occur. At the quarry, improper stockpiling
techniques and loader handling can compromise spec product gradations. At the
hot-mix asphalt plant, again how the aggregate is stored and the use of
improperly designed self-relieving cold feed bins can increase the chances of
particle segregation. Then, after the asphalt is mixed, silo storage and truck
loading practices at the plant can further segregation problems.
En route to the site, the asphalt will undoubtedly develop
some degree of thermal segregation. Thermally segregated material makes the
asphalt more difficult to compact and affects mat densities, which will
ultimately affect mat life.
With numerous opportunities for segregation to occur even
before the asphalt reaches the jobsite, it does not seem quite fair that the
paving crew is now challenged to lay a high-quality, smooth mat void of
segregation. Fair or not, however, it is a fact of life.
Education and paving practices
Minimizing segregated material exiting the screed can be
accomplished in several ways before adding equipment. First and foremost, the
paving crews must be thoroughly educated in the correct paving techniques and
how to properly use each piece of equipment on the site. No amount of paving
equipment will end segregation unless the crew operates it correctly.
There are a number of quality paving practices that will
help to reduce segregation in the mix. Beginning at the plant, using the three-dump
method for loading trucks—dumping two batches at each truck end and a
final batch in the middle—will reduce larger particle run-off to the
sides and end of the truck, reducing particle segregation.
Once at the jobsite, the paving crew can help reduce
segregation by employing the following practices:
Controlled hopper wing cycling: The wings are where the
large, cooler stone tends to collect if not properly reintroduced back to the
mix. Regular cycling, where allowed by spec, will reduce large buildups of this
segregated material.
Keep the hopper full: Leave as much surge as possible
between truck exchanges and do not run the hopper empty. This will minimize
cyclical segregation by allowing hot, uniform material from the next truck to
blend with mix from the previous dump.
Keep a constant head of material at the spreading augers: A
consistent flow of material to the spreading augers will prevent them from
spinning too fast or too slow, which can cause longitudinal segregation.
Time the conveying and spreading systems: Ensure the ratio
pots or flow gates are set to deliver enough, but not too much, material to the
spread augers so they run continuously.
Correct lead crown setting and proper strike off adjustment:
Equipment fine-tuning issues will help eliminate longitudinal segregation.
Correct spread auger length: This will ensure the proper
amount of material conveyed to the screed end plates.
Longitudinal segregation
Also, before adding any equipment all paving contractors can
eliminate one of the most common forms of segregation—longitudinal. This
type of segregation is not a function of having the right equipment on the job.
Rather, it is a result of improper adjustments to the paver itself. By
fine-tuning paver components, longitudinal segregation issues occurring at the
center line, reversing augers, bearing supports and “end gates” can
be corrected.
Center line segregation occurs under the auger chain drive,
where larger material is allowed to dribble under the housing. Manufacturers
try to eliminate this problem by incorporating variations of reversing auger
designs to force material under the housing. However, without proper material
containment, larger stone will still dribble and collect in this area.
Cedarapids, Cedar Rapids, Iowa, incorporates baffles to the
screed chain drive housing to provide such containment. The baffles direct
material to the reversing auger, providing uniform material flow under the
housing.
Allowing the screed’s spreading augers to operate too
fast or slow also will result in longitudinal segregation. If allowed to rotate
too fast, longitudinal stripes will occur in line with the reversing augers.
Opening the paver’s feed gates or adjusting the delivery ratio pots,
associated with the use of remix pavers, to allow more asphalt flow to the
spreading augers will eliminate this form of segregation.
Conversely if the spread augers are allowed to spin at too
low a rate, larger stone will drop and collect at the bearing support. Lowering
the feed gates or decreasing feed proportion of the ratio pots will reduce the
flow of asphalt to the augers, which in turn allows them to spin faster and
eliminate segregation at the bearing support.
Finally, segregation found at the screed extensions on rear
extended screeds is a result of not using auger extensions. Without using
augers and retaining plates in the extensions, larger material cascades to the
screed extensions, resulting in end-gate longitudinal segregation. By simply employing
the use of available augers and adding material retaining plates to the
tractor, contractors can eliminate this problem.
Equipment essentials
Fine-tuning equipment will prevent longitudinal segregation,
and proper paving techniques help minimize occurrences of material and thermal
segregation. However, when it comes to eliminating cyclical and other forms of
segregation, contractors will need to turn to additional paving equipment.
Some DOT specifications are so strict that the traditional
slat conveyor and end-dump truck method cannot be used to pass spec. This is
due to the fact that, no matter which brand of paver selected, the combination
of a slat delivery system and dump truck will leave a cold streak down the
center of the screed, which will not meet some state temperature specs.
When looking to add equipment, quite possibly the most
critical component a paving contractor can employ to reduce segregation is the
hopper insert. These inserts channel asphalt directly into the paver’s
conveyor system— whether traditional slat or counter-rotating
augers—creating a live bottom action that reduces drop segregation. Their
design promotes a natural reblending of material and prevents larger, cooler
material buildup in the hopper wings. Hopper inserts also create extra surge
capacity to aid in the continuous paving process.
However, as with so many other things, too much of a
positive can turn into a negative. Contractors must pick the right size insert
for the paver. An insert which is too large will reintroduce drop segregation
to the paving process. Material will pool off to the sides of an insert which
is too large and will only flow into the conveyor as the asphalt level drops.
This introduces cooler, larger material back into the mix, potentially resulting
in random patch segregation. A visual inspection of material flow through the
insert will let you know if it is the correct size.
Some of the most popular pieces of equipment in the battle
against segregation are material transfer devices (MTDs) and material transfer
vehicles (MTVs), both of which help establish a continuous paving process. MTDs
attach to and are guided by the paver. These machines do not require an
additional operator to control and can be used in conjunction with either
windrowing or dump-truck paving techniques. Some, like the Cedarapids’
MS-3 (Circle 920) and MS-4 (Circle 921) mat smoothness machines, will have
shock-absorbing push rollers to eliminate truck bumping and screed settling
when using end-dump trucks. Some MTDs have the ability to reblend the material
and many will have a low discharge height, avoiding potential material
resegregation in the hopper.
MTVs, on the other hand, do not attach to the paver and
require an additional operator. Some MTVs offer on-board reblending
capabilities, which aid in the reduction of thermal and material segregation
before discharging into the hopper. These machines typically have a large surge
capacity, but this storage capability cools the asphalt approximately
10ºF, so the plant will have to compensate for this temperature loss. MTVs
are characterized by a high discharge point into the paver, which could lead to
some material resegregation in the hopper.
Some contractors working with particularly difficult mixes
prone to segregation, such as stone-matrix asphalts, are finding success in
reducing segregated material by using the remix anti-segregation system pavers.
The remix paver offers reblending capabilities at the last stage in the paving
process by replacing traditional slat conveyors in the hopper with two sets of
counter-rotating augers.
About The Author: Rieken is a paver application specialist for Cedarapids, Cedar Rapids, Iowa.
