Material transfer vehicles (MTVs) and devices (MTDs) are arguably the contractor’s best allies in the battle against segregation. However, there are key design differences, so contractors must carefully review the equipment options to ensure they are getting the right machine for the job.
Early MTV designs offered large storage capacities and some reblending capabilities. “The intent of those first designs was to allow crews to pave continuously to eliminate truck-end segregation,” explains Rieken.
While improving mat consistency, MTV size posed several challenges at the jobsite. High capacity material storage meant that mix temperature could drop by up to 10°F during the truck to MTV to paver exchange.
A high center of gravity led to high material discharge points from the conveyor, which opens the door to reintroducing segregation when loading the paver’s hopper.
These heavy machines also resulted in empty wheel loads of 75 psi, which led to parking problems, made crossing bridges cumbersome and limited MTV usage.
A newer generation of MTVs builds on the continuous paving benefits of previous designs, offers improved reblending capabilities and lowers ground contact pressures. “Some states require a limit of 10°F temperature differential across the mat, which calls for more aggressive remixing of the asphalt,” says Rieken.
New innovations of variable-pitch, counter-rotating auger sets thoroughly reblend 100% of the mix to virtually eliminate thermal and material segregation.
Low machine center of gravity lowers the conveyor’s discharge point to lessen the chance of drop segregation. Also, track drives reduce fully loaded ground contact pressure to less than 20 psi, eliminating many of the issues associated with wheeled MTVs.
Low psi MTVs usher in the ability to be used on base lifts for reducing material segregation. Fear of subbase rutting with high psi wheeled machines has led many state specifications to limit MTV usage to the top paving lifts.
“This is counterintuitive and counterproductive,” says Rieken. “Base mixes are made with larger stone sizes that are more prone to segregate, so we should be using MTVs on the base lifts. We have customers using the CR662RM RoadMix to pave base lifts, but it cannot be used as an MTV by spec.”
Similar to MTVs, MTDs establish continuous paving and can be used in conjunction with either dump truck or windrow paving.
Unlike MTVs, these machines attach to and are guided by the paver, eliminating the need for an additional operator. Some have shock-absorbing push rollers to eliminate truck “bumping” and screed settling. Several designs will also reblend material and have a low discharge height to eliminate drop segregation.
For contractors working with mixes prone to particle segregation such as Stone Matrix Asphalt (SMA) but don’t want to use a transfer vehicle or device, there are alternatives. Replacing the traditional slat conveyor with two sets of two counter-rotating augers, these anti-segregation pavers offer the same aggressive reblending capabilities as the new generation MTVs with the added benefit of doing so at the last stage of paving. This combats material as well as thermal segregation.
Segregation busting advancements
Without adding specialized equipment to the paving train, a properly sized hopper insert goes a long way in reducing segregated mix. Inserts channel asphalt directly into the paver’s conveyor system – whether traditional slat or Remix counter rotating augers – to create a live bottom action that reduces particle segregation.
Offering extra surge capacity to aid in continuous paving, the hopper insert design promotes a natural reblending of material and prevents larger, cooler material from building up in the hopper’s wings.
Rieken cautions contractors not to think that bigger is better. “Insert size,” he says, “must be matched to the paver hopper, or material will pool off to the sides, which will flow into the conveyor as the asphalt level drops. This results in random patch segregation.”
At the screed, consistency is paramount in reducing longitudinal segregation. Operators must monitor that the ratio pots or flow gates are set to deliver enough, but not too much, material to the spread augers, so they run continuously. If the augers are allowed to spin too slowly, larger aggregate drops off at the outboard bearing support, which results in longitudinal segregation.