One and One is One

Two lifts can become a single, strong layer.

By Tom Kuennen, Contributing Editor

With two-lift hot mix asphalt paving, it’s a case of 1+1=1.

As more durable hot mix asphalt pavement designs are explored in the United States, two-lift asphalt paving as it’s practiced in Europe offers potential for consideration.

At the same time, two-lift concrete paving also is winning converts.

Two-lift HMA paving describes the placement and compaction at-screed of an intermediate lift of asphalt, followed immediately by placement and compaction at-screed of the friction or surface course. Placement of two layers of hot mix asphalt – hot-on-hot, so to speak – provides a durable, near-monolithic layer of asphalt that resists traffic loads and pavement ills.

Instead of subsequent placement of a hot, flat friction course on top of a cold, flat intermediate layer – with only a messy tack coat to bond them – true hot-on-hot bonding, with aggregate interlock, is provided with the varieties of two-lift HMA paving.

The downside is the added cost. And for the contractor, placement can be complicated, with the use of a material transfer vehicle and one or two modified asphalt pavers with high-compaction screeds, according to its two current permutations from European manufacturers. Staging of multiple trucks with two different mixes, from two different plants, is the norm.

And while two-lift hot mix asphalt paving is uncommon, even in Europe where it is being popularized, on these shores it’s still worth a look because it may become part of our “toolbox” of asphalt paving techniques in the years to come.

That’s why Eurostyle two-lift asphalt paving was described as a “paving technique of the future” by National Asphalt Pavement Association Vice President-Research and Technology Dave Newcomb, P.E., Ph.D., at a workshop on asphalt paving for trade journal editors at the National Center for Asphalt Technology at Auburn University a few years ago.

Also, consider that two-lift paving already exists in other forms in the United States. For example, hot-in-place recycling as implemented by one recycling contractor recycles and rejuvenates the existing asphalt surface into an intermediate course, and on it immediately places a surface course of virgin hot mix asphalt. The result of this hot-on-hot technique is a near-monolithic pavement with aggregate interlock, but utilizing an intermediate layer of 100 percent recycled asphalt pavement (RAP).

Further, NovaChip surface treatment and its generic clones simultaneously spray hot polymer modified binder on swept pavement immediately ahead of a hot, open-graded asphalt mix, with immediate placement of a polymer modified open-graded mix, or immediate placement of aggregate, followed by an immediate second spray of polymer or rubber modified binder. Either way, the result is a durable, open-graded, thin surfacing that stands up to Interstate-level traffic.

For the portland cement concrete side, two-lift “wet-on-wet” PCC pavements already are actively promoted in the United States, with a number of demonstration placements each year.

Here’s a look at the status of two-lift HMA paving today:

InLine Pave from Vögele

A two-lift HMA placement technique that has achieved some degree of success in Europe, but no penetration in the United States, is the InLine Pave process from Joseph Vögele AG.

“InLine Pave is a cutting-edge process,” says Brodie Hutchins, general manager, Vögele America, Inc., Chambersburg, Pa. “It has not swept the world or Europe or Germany. But Vögele has sold several trains to European contractors, and some specifying agencies are studying the process, to fully understand the advantages.”