Ultrathin Overlay Prolongs Desert Pavement

Ultrathin, gap-graded wearing cgurses bonded to the surface by a warm polymer modified membrane give Las Vegas Paving road work in the desert

In addition to enhancing friction, gap-graded and open-graded mixes provide an open surface texture to allow rain water to flow through the surface, reducing tire spray, eliminating ponding, and suppressing noise at the tire /pavement interface. They represent one version of a porous asphalt pavement.As such, UTACS are similar to a Caltrans7 bonded wearing course, 7/a gap- or open-graded, ultra-thin hot mix asphalt mixture applied over a thick polymer modified asphalt emulsion membrane,77 to quote the agency.When visited in October, Las Vegas Paving was applying a UTACS to busy Jones Ave. between Tropicana and Russell for the Clark County Department of Public Works.The completed systems so far total over 250kW, which includes a roof-mounted system at Pavers Shoes' state-of-the-art head office and distribution centre in York and the largest Solyndra system in the UK on the Clarence Dock in Leeds for Town Centre Securities and British Waterways.Right paver neededThe Super 1800-2 with SprayJet Module can also pave widths up to 19.7 feet.One eight-person crew handles the Super 1800-2 Sprayjet and UTACS paving, and Andy Kimball is paving superintendent. A material transfer vehicle is required by agency spec, although the benefits of an MTV in an ultra-thin overlay aren't as obvious as with thicker lifts.The combined systems will generate about 5,000 Mwh of electricity over the 25 year feed-in tariff period.Such bonded wearing courses are primarily used in high traffic areas as a surface treatment over hot mix asphalt or Portland cement concrete pavements. They are placed over structurally sound pavements as a maintenance treatment, but may also be used in new construction and rehabilitation projects as the final wearing course.The placement of emulsion spray is automated, and the system regulates percentages of emulsion applied, making it easy on the crew, Webster says."It's not unlike a prime or tack coat, but it provides much more coverage," Webster says. "And no one can run on it. The Super 1800-2 places the coat behind the wheels but ahead of the material, so cars or our haul vehicles can't drive over it and pick up the sticky, polymer modified emulsion.Bonded wearing coursesLas Vegas Paving acquired its Super 1800-2 Sprayjet in early 2010, and has been using it for UTACS ever since.On Jones Ave., warm polymer modified emulsion was being sprayed directly ahead of the screed via spray bars on each side of the Super 1800-2 Sprayjet at a rate of 0.13 gallons per square yard.This 2,700-ton job involved three lanes each way, including shoulders and turn pockets.With the exclusive SprayJet module mounted, a contractor or government agency can use the Super 1800-2 to place opengraded asphalt emulsions and other wearing courses. And with the optional module easily removed, the Super 1800-2 SprayJet can place conventional hot mix asphalt pavements. Its compact design makes it ideal for urban applications and low-cost transport.In a bonded wearing course like those specified by Caltrans, the polymer modified asphalt emulsion membrane seals the existing pavement while bonding the gap-graded or open-graded mix to the surface. The thicker nature of the membrane allows it to "wick" upwards into the mix, filling voids in the aggregate and creating an interlayer of high cohesion that does not delaminate or bleed, if applied correctly.An extra emulsion tank is available for contracts requiring very large rates of spread. The extra tank holds 1,320 gallons, so a total of 1,849 gallons of bitumen emulsion can be carried on board the paver. The paver carries the extra tank in its material hopper."We are the new kids on the block with this process,77 says Clark Webster, general superintendent, Las Vegas Paving. 77This is our first year paving UTACS, and we were concerned that interest would not last, but agencies are still interested."A total income of pounds 1.65m will be generated from the feed-in tariff and save the businesses about pounds 450,000 in electricity based on current energy prices.The Super 1800-2 SprayJet works with material transfer vehicles and truck interfaces. A stand-alone heating unit operated by diesel fuel is installed in the extra tank, keeping the emulsion at the desired temperature. A pump is part of the extra-tank unit, and its job is to circulate the emulsion to keep the material homogeneous."The emulsion membrane seals the existing surface and produces high binder content at the interface of the existing roadway surface and the gap- or open-graded mix, all in one pass."However, to correctly place UTACS or other bonded wearing courses, the right kind of paver is needed: One that has the ability to spray asphalt emulsion onto a pavement, and then immediately place a thin overlay on top. Las Vegas Paving found the Vogele Super 1800-2 with optional Sprayjet module fit the bill.

One eight-person crew handles the Super 1800-2 Sprayjet and UTACS paving, and Andy Kimball is paving superintendent. A material transfer vehicle is required by agency spec, although the benefits of an MTV in an ultra-thin overlay aren't as obvious as with thicker lifts.