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Material Transfer Devices & Vehicles are new to the Indian road construction industry but are very popular in developed countries for large paving projects. These machines can help reduce materials segregation issues and also allow a more even heat profile in the mat, for more homogeneous compaction.

Establishing a continuous paving process with paver alone will deliver a smoother mat, but it takes a great deal of planning and coordination between plant production, trucking needs, and the paving train's speed. Sometimes it's difficult to continuously lay material with a paver alone, as truck exchanges are slow and hopper capacity is limited. It's not recommended for the paving crew to run the hopper dry as this can lead to truck-end segregation. So this limits the surge capacity of the hopper and the distance that can be paved before the next truck arrives.

A number of machines are available to help bridge this gap and deliver more paving time between truck exchanges. Material transfer devices (MTDs), like windrow and mat smoothness machines, and material transfer vehicles (MTVs) deliver the additional surge capacity to help contractors better establish a continuous paving process to produce a higher quality, smoother mat. Continuous paving using MTD's or MTV's minimizes truck waiting time at the paving site and may minimize aggregate segregation and temperature differentials.

Selecting the right machine for the paving train depends on a number of factors from production goals and mix designs to truck availability.

One of the most mature technologies for material transfer is the windrow machine, also known as a pick-up machine. The first windrow machines were developed in the 1970s, and their design has remained relatively the same over the years. It's simple, low-cost design allows contractors to pave more tons in a single day.

With this machine, the material is laid in windrows in front of the paver by belly-dump trucks. The windrow machine attaches to and is maneuvered by the paver as it transfers material from the ground to the paver's hopper. As long as the crew has a steady stream of trucks, a windrow machine will deliver considerably higher production than paving with a paver alone.

There are some challenges associated with this process that crews must be aware of to be successful. Using experienced paving crews is helpful, and a good dump man is critical in windrow paving. The right size and amount of material must be laid in the window, or it could easily starve the hopper or overflow the paver with the material.

Crews also have to be cognizant of potential temperature differentials from the truck exchange. These differentials can make the mat more difficult to compact to specified densities. Overlapping the windrows will help prevent temperature differential issues.

Since the material is laid on the ground in front of the paver, ground and ambient temperatures as well as the weather are also factors that can limit when the crew can pave with this process. Additionally, the required trucks are more specialized for this process than end-dump trucks, and not all markets will have an abundance of belly-dump trucks available.

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. Also, its compact design, similar to the windrow machine, means the mat smoothness machine is easily transported 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 re-blending 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 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.

Although mat smoothness machines offer the additional surge capacity for continuous paving, they do not offer as much storage as with MTVs.

MTV's 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. These MTV designs did not feature reblending capabilities. Later development in MTVs incorporated some type of reblending system.

MTV's with reblending capacities are able to 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.

MTV's allow for offset paving, which comes in handy for special applications like paving superelevated bank turns. They also offer non-contact paving. These vehicles also offer large storage capacities to give contractors the necessary material surge to pave continuously at higher speeds, allowing more tons to be laid. Upper and lower paver hopper inserts can be used in conjunction with MTVs, offering more mixed storage for the paving train.

Material transfer devices and vehicles have allowed contractors to make great strides in improving mat quality while increasing the amount of mix being laid daily. These machines combat segregation and establish continuous paving, which results in smoother mats