Equipment World Digital Magazine
Issue link: http://read.dmtmag.com/i/634023
F or smooth, long-lasting asphalt pavements, the experts agree: eliminate aggregate and thermal segregation in the mix, keep the paver moving, and isolate the paver from the haul truck. There's one machine that does it all: the material transfer vehicle (MTV). While not specified for use in all states, MTVs are being used on asphalt paving projects through- out the country, even in states like California, which is evaluating their use, but doesn't require them for most mixes. Material transfer vehicles were developed to eliminate contact be- tween truck and paver, resulting in smoother mats. Every time a truck bumps a paver during a load, a dis- continuity in the mat may form. Es- sentially, MTVs provide noncontact paving between truck and paver, along with surge capacity to allow the paving process to continue dur- ing truck exchanges. In doing so, a full-function MTV will independently store, remix and deliver mixes from haul truck to the paver. Some states don't require MTVs on shorter-length jobs. For example, Connecticut requires an MTV only when placing an HMA surface course that is greater than 5,000 feet in length and 28 feet in width. When hauled from job to job, or used within a job with bridges, weight can pose a problem with large MTVs. Connecticut demands that a load factor rating analysis be obtained for all structures that the MTV may cross during paving. "The MTV must be a self-pro- pelled vehicle specifically designed for the purpose of delivering the HMA mixture from the delivery truck to the paver," the state says. Contractors must detail what make and model MTV will be used, the axle weights and spacing for each piece of paving equipment, plus provide a drawing showing the axle spacing. They also must ensure that the MTV has the capability to remix the HMA mix. New-design, lighter- weight MTVs may ward off these requirements. Fighting mix segregation In addition to isolating the paver from haul truck, via their remixing capability, MTVs also fight both material and thermal segregation, leading to a more consistent mix when placed as pavement. Full-capability MTVs remix or agitate the mix as it is lifted, con- veyed or stored in the mobile ma- terial transfer vehicle. The remix- ing can be active (using paddles within surge hoppers or ahead of conveyors) or passive, as the mix moves or is dropped within the MTV. This remixing provides a more even temperature within the mix – or reblending of aggre- gate – that leads to more durable pavements. Aggregate segregation can also develop at the plant, at the silos with the way the trucks are loaded and in the paver. It's recommend- ed that the asphalt plant and driv- ers follow the three-drop system, that is, a drop in the front of the truck body, one in the rear and a final drop in the middle. Segrega- tion also may occur when crews let the paver hopper run too low. According to the new National Cooperative Highway Research Program synthesis, NCHRP 477, Methods and Practices on Reduc- tion and Elimination of Asphalt Mix Segregation, end-of-truck seg- regation is a type of material segre- gation that can result in coarse mat finishes. End-of-truck material segregation is also known as truck-to-truck and truckload-to-truckload segregation and can be reduced by fully raising the truck bed before opening the back gate, according to the report. "When the gate is finally opened, the mix is discharged from the truck bed in mass into the paver hopper. This process rapidly fills the paver hopper and prevents the coarser particles from collecting in the paver wings," the report says. To fight mix segregation, New Jersey DOT requires the MTV to have a remixing system to con- tinuously blend the HMA before placement. Remixing may occur within the MTV or in the paver hopper. The Garden State also demands the MTV be equipped with a high- capacity truck unloading system to receive mixtures; a storage system capable of holding at least 15 tons of HMA; and a discharge con- veyor capable of slewing to either side to deliver HMA to the pav- ing spreader while allowing the MTV to operate from an adjacent lane. A paver hopper insert with a minimum capacity of 14 tons in the hopper of conventional paving equipment is also required. EquipmentWorld.com | February 2016 55 What is mix segregation? S egregation in a mix is two-fold; it can be large stone separating from small stone [material segregation], or temperature variations within the truck body [thermal segre- gation]. On hauls, measurable discrepancies may occur between the temperature of the mix deep inside the truck body, compared to that around the edges of the body and on top of the mix. And when mix is dropped into the truck bed, or when mix is transported, move- ment during transport can cause larger aggregate particles to roll to the sides of the load and away from the finer particles.