Paving in today's climate is a balancing act. Contractors have a desire to lay high tonnages of material per day. Yet with current mix designs like Superpave and SMA (Stone Matrix Asphalt), there's a tipping point between paving speed and quality. Whether dictated by spec or company pride or policy, sometimes the paving crew must slow down to deliver the desired mat quality.
For contractors who want to lay more tons of material in a day, there is a right way and a wrong way to deliver high mat quality. High speeds using only a paver can create a hurry-up and wait paving sequence as the paver runs out of asphalt before the next truck arrives. "This makes it difficult to maintain a constant head of material at the screed, so smoothness suffers," says Mark Hunt, general manager of Terex Roadbuilding's Asphalt Group.
Establishing a continuous paving process 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," says Bill Rieken, paver application specialist for Terex Roadbuilding. "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.
Selecting the right machine for the paving train depends on a number of factors from production goals and mix designs to truck availability and state specifications.
Pick it up!
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. Its simple, low-cost design allows contractors to pave more tons in a single day.
With this machine, 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," says Rieken.
However it's not all about production when using a pick-up machine. "Contractors have told me the use of belly-dump trucks with the windrow paving process saves them about $1 per ton compared to using end-dump trucks," he adds.
Windrow paving is practiced more west of the Mississippi and in western Canada. It is also popular in areas with a shortage of trucks. Because belly-dump trucks carry more material than end-dump trucks and can quickly drop their load, this process uses fewer trucks.
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 windrow, or it could easily starve the hopper or overflow the paver with 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 spec 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.