Arguably, the material transfer device (MTD) is the most important piece of ancillary equipment that can significantly impact the efficiency of the paving process. Attaching to and being maneuvered by the paver, the MTD will help a contractor match the production at the plant to the paving train, deliver a more uniform product to the paver and improve trucking efficiencies.
"Contractors will use MTDs, like the Cedarapids MS-4, mat smoothness machine to establish a continuous paving process, increase production and reduce both material and thermal segregation," comments Bill Rieken, paver applications specialist for Terex Roadbuilding's Cedarapids, Inc.
A primary reason for the boost in productivity, some contractors claiming upwards to a 40 percent increase, is the much more efficient utilization of delivery trucks. Without the MTD, contractors hold the truck to the paver much longer. When the truck is empty, the paver hopper is virtually empty as well, allowing the paver to move only a short distance before running out of mix. This establishes a stop-and-go paving process because the paver has to wait for the next truck to replenish the surge of mix in the hopper.
By incorporating an MTD into the paving train, the contractor establishes a continuous paving process. The surge capacity built within the hopper of the MTD feeds the paver's hopper, so the delivery truck is not tied to the paver as long as when paving without the MTD. The goal when using an MTD is to leave the paver hopper full in between truck exchanges, so the train can continue paving while picking up the next delivery truck.
According to Rieken, "When continuously paving, the contractor can pave at a slower speed to match the production at the plant. Without a transfer device, the paver has to pave faster in order to make up time for the truck exchange and match the speed of the mix coming from the plant. This results in a stop-start paving process." Even though paving at a slower speed, the contractor can lay more asphalt during a shift, since the paving is constantly moving.
A faster exchange
Dumping material into the MTD's hopper permits a faster truck exchange than when dumping directly into the paver's hopper as Carroll & Carroll, a Savannah, GA-based paving contractor, recently discovered. The company added a Cedarapids MS-4 MTD to its paving train during a $3.2-million airport runway paving application at Hunter Army Airfield.
Immediately the crew realized a significant increase in trucking exchange efficiency. "With the MS-4 our truck exchange time dropped to 1 1/2 minutes, which is excellent," says Ken Pate, vice president of Carroll & Carroll. The transfer device's 7-ton receiving hopper capacity continued to feed material to the paver during truck exchanges.
By establishing a continuous paving process, Carroll & Carroll also increased its production. Operating two, 12-hour shifts the contractor was able to lay up to 4,000 tons of asphalt per shift. The crew maxed out production at 8,072 tons in a single 24-hour period. The MTD played an important role in allowing the contractor to finish the 10,146-foot-long by 200-foot-wide runway a day ahead of schedule.
By shortening the truck exchange cycle, contractors increase the trucking efficiency, thereby using fewer trucks to deliver mix to the jobsite as Eagle River, WI-based paving contractor, Pitlik & Wick, discovered. "We can eliminate about a half a truck by using the MTD. On those jobs where we are in between the number of trucks needed to meet production, we can go with the lower number," claims Brian Pitlik, vice president of Pitlik & Wick.
The company added an MS-4 MTD to its paving train to finish a 35,000-ton mill and resurface project on Highway 51 in Woodruff, WI. Even with inclement weather, efficiencies gained by using the MTD allowed Pitlik & Wick to meet and maintain planned production, and by lowering the number of trucks necessary, adding the MTD saved the company money on transportation costs.
Improving mat quality
A continuous paving process also reduces segregation and improves mat quality. "Deviations in ride often occur when the paving speed stops. Optimum uniformity is dependent upon a consistent paving speed," says Rieken.
In the stop-start paving process associated without using the MTD, the screed has a chance to "settle" into the fresh mat while waiting for the next truck. This affects final ride quality of the mat and makes it more difficult to meet the smoothness incentives built into some contracts.
Additionally, contractors will, at times, notice segregated mix coming from the end of the load delivered by the truck. This "load-end" segregation is visible in the mat and cyclically gives evidence of each time the paver has to stop and wait for the next truck. This lack of mix uniformity makes it much more difficult for the roller to meet spec densities in this section of the mat, affecting mat quality.
The paving crew from Iron Mountain, MI contractor, Bacco Construction, noticed these signs of "load-end" segregation on a 75,000-ton U.S. Highway 2 paving application. Before adding an MTD to the paving train, they could see some segregated material in the Superpave 5E3 mix design at the points where the paver had to stop and wait for the next truck.
According to John Fortier, president of Bacco Construction, "We rented the MS-4 for the additional hopper surge capacity to eliminate 'load-end' segregation and to establish continuous paving." After adding the MTD to the paving train, the crew reported that the signs of "load-end" segregation had disappeared. "The transfer device gave us a smoother, higher quality mat with a more uniform aggregate mixture throughout (the mat)," continues Fortier.
Re-blending the material
A re-blending action to the asphalt mixture is accomplished through a variety of methods, depending on the model of MTD used. With the Cedarapids MS-4, large-pitch augers located in the machine's receiving hopper re-blends and channels the material to its delivery elevator.
An additional step of material re-blending also occurs when the contractor adds an insert to the paver's hopper. The size of the insert increases the hopper's surge capacity while the shape channels asphalt directly the paver's delivery system — whether traditional slat conveyor or Remix auger — eliminating the opportunity for the larger aggregate to pool in the hopper wings.
"It's crucial to include a hopper insert when using a transfer device. The live bottom action associated with an insert creates a natural re-blending of the asphalt and increases surge capacity to aid in the continuous paving process," comments Rieken. According to Fortier, the re-blending action also reduces thermal segregation, which makes the asphalt mat much easier to compact to spec densities.
Whether looking to improve trucking efficiencies, increase production or lay a higher quality, more uniform mat, paving contractors should consider the addition of a material transfer device to the paving train.