A More Efficient Material Transfer

Mat smoothness

Mat smoothness machines were also designed around the concept of allowing contractors to take advantage of continuous paving benefits. The first designs appeared in the late 1980s, about the same time as the first MTVs. They were developed for contractors who did not use belly-dump trucks and for sites where there is not enough room to efficiently use the longer trucks.

The mat smoothness machine can be considered a type of hybrid between a windrow machine and an MTV. It attaches to the paver like a pick-up machine. Therefore, it eliminates the need for an additional operator and offers about the same low operating, maintenance and fuel costs. "Operating costs for an MTD are dramatically lower than an MTV," says Rieken. Also its compact design, similar to the windrow machine, means the mat smoothness machine is easily transported without permits from site to site.

Like an MTV, mat smoothness machines have a receiving hopper, allowing end-dump trucks to be used with this continuous paving process. By employing large-diameter, ribbon-style augers, these machines help to solve truck-end segregation issues by reblending the material before it is conveyed to the paver's hopper. Shock-absorbing push rollers help the paver to pick up the truck without bumping, which results in smooth mats. High transfer rates reaching 1,200 tons per hour allow the truck to quickly unload and make way for the next truck, allowing the paving train to move at a steady pace.

Since this type of machine is connected to the paver, truck operators must be educated not to back into the mat smoothness machine. Rather, similar to dumping directly into the paver's hopper, the truck should wait for the train to receive it, so the shock-absorbing push rollers can smoothly pick up the truck.

Although mat smoothness machines offer the additional surge capacity for continuous paving, they do not offer as much storage as with MTVs. Additionally, there are some state specifications that require "non-contact" paving, which a mat smoothness machine will not meet.

Transfer vehicles

The first MTVs were also developed in the late 1980s, which corresponded with a change in mix designs to a larger aggregate. "Large aggregate mixtures were used at the turn of the century under the Warren Brothers patent. A lawsuit in Kansas in the 1920s basically said that mixes with smaller maximum aggregate size did not infringe on the patent, so producers went to smaller aggregate and got comfortable with it," explains David Newcomb, vice president, research and technology for NAPA.

However, as traffic volume and wheel loads increased, the smaller aggregate did not adequately withstand the punishment. "Larger size aggregate mixtures had a resurgence in the late 1980s, mostly as heavy-duty type mixes," he adds. The change to a larger aggregate increased the occurrences of segregation, especially between truck exchanges.

MTVs were initially created solely for the purpose of continuous paving. "The first MTVs featured large storage bins to allow extra paving time in between truck exchanges," mentions Rieken. These MTV designs did not feature reblending capabilities.

It was not until the mid 1990s — coinciding with the advent of the Superpave System — that MTVs were equipped with some type of reblending system. During this same time period, studies were also being conducted on the affects of material segregation. These studies also concluded that thermal segregation can be just as detrimental to the life of the road.

By offering reblending capabilities, MTVs can deliver a more homogeneous mix to the paver to improve mat quality. These machines can better handle extreme segregation issues than windrow and mat smoothness machines. This is due to the vehicles' auger reblending systems, which remix the material prior to delivering it to the paver's hopper.

MTVs allow for offset paving, which comes in handy for special applications like paving superelevated bank turns on racetracks. They also offer non-contact paving, which is required by some states specifications.