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.
