Jefferson County, WI, plans to use more foamed asphalt in road base reconstruction following its own research, and following field demonstrations by a major Wisconsin road contractor.
Jefferson County used foamed asphalt technology in an $800,000, 5-mile, Phase I reconstruction of County Highway N early in 1999.
“Foamed asphalt offers an option for road reconstruction,” said Jeff Haas, Jefferson County Highway Commissioner. “It has long-term benefits, with an anticipated, average cost savings of 20 to 25 percent over standard methods, when used where appropriate.”
A Wirtgen WR 2500 remixer equipped for foam asphalt was used in the application. To make a foamed asphalt-stabilized base, a high-surface-area asphalt froth is created by precisely injecting cold water into hot liquid asphalt as it enters the machine’s mixing chamber. There, air bubbles in the expanded liquid asphalt froth act as the carrier of liquid asphalt to fines in a reclaimed asphalt pavement (RAP) aggregate mix.
In less than 15 seconds the froth subsides and the dispersion of asphalt is achieved, eliminating time waiting for the “break” required when expensive asphalt emulsions are used. The technology also sidesteps use of costly cutback solvents. The liquid asphalt cement is pure, with nothing added to it to change its properties. That makes it more economical to use than emulsions, which are processed oil.
Jefferson County continued to use the process in 1999, Haas said. Phase II of the Highway N project was a 3.5-mile stretch that was reconstructed using foamed asphalt in the fall. Haas also anticipated using foamed asphalt on a 4.6-mile reconstruction of County Highway Y.
“We’re keeping an open mind to expanding the use of foamed asphalt and will be taking a very hard look at this year’s projects,” Haas said. “Hopefully we’ll have good end results that will allow us to use this method in the future.”
Payne & Dolan Inc., Waukesha, WI, was the prime contractor for the project.
Demonstrations
Reinforce Research
Haas and his crews are responsible for 837 kilometers (520 lane mi.) of county trunk highway, and under an outsourcing arrangement, 644 kilometers (400 mi.) of state trunk highway for the Wisconsin Department of Transportation, and municipal and township road maintenance on an as-needed basis.
The Jefferson County Highway Department employs some 84 workers and spends in excess of $11 million each year, excluding outsourced work for other agencies. Approximately 73,340 persons live in this southeastern Wisconsin county.
Jefferson County has been doing base recycling and stabilization for the past five years, but considered foamed asphalt stabilization only in early 1998, when reconstruction of County Highway N became possible under a State of Wisconsin local roads improvement program. “It was then that we looked at hot foamed asphalt injection as an option, instead of the straight pulverizing, followed by binder and surface mats,” Haas said.
That followed foamed asphalt demonstrations in Dodge County, to the immediate north. “We saw four different methods of base recycling,” Haas said. “The foamed process appears to provide a cost-effective method of reducing the amount of paving that has to be done, as well as reusing the existing pavement for the binder [base course] mat.”
Answer for Local Roads
Foamed asphalt is a viable answer to low-cost, in-situ recycling of local roads, observers maintain. Full-depth base recycling addresses the needs of small-budget township and county governments.
Previously, these governments only had the option of seal coating roads, or patching them. Eventually the roads would wear out. Full-depth recycling is an economical, viable product that a small-budget government can employ to bring its roads up to acceptable conditions. If the job is designed right, a base-recycled and stabilized road can stay in use without a wearing surface for up to a year, or even have a double seal coat placed on it as a wearing surface.
“N has the highest average daily traffic volume of any county road in the county, over 4,000 per day,” Haas said. Increasing truck traffic and farm loads — along with the general increase in auto traffic —were taking their toll on the two-lane blacktop.
“We were seeing alligator and reflective cracking from previous overlays,” Haas said. “There was a lot of thermal cracking in general. We knew a simple overlay would work for a while, but that the cracks would reflect through again. By going with this process — which pulverized, recycled and stabilized the top 4 inches of the existing pavement — we created a binder-type mat in-place, thereby eliminating the need for paving a new binder mat with virgin materials.”
Existing County Highway N was 6.4 meters (21 ft.) wide and pre-bid cores showed 22 centimeters (8.5 in.) of asphaltic concrete over 10 to 12.7 centimeters (4 to 5 in.) of crushed aggregate base course. The road was pulverized to a depth of 24 centimeters (9.5 in.). Following foam-asphalt base treatment in May it received a 2.5-centimeter (1-in.) leveling course of hot mix asphalt (HMA) and a 4-centimeters (1.5 in.) finish or riding course, on a 10-meter (32 ft.) width which included 1.2-meter (4 ft.) paved shoulders paved 6.4 centimeters (2.5 in.) deep.
Special Provisions Employed
The project was executed under bid documents containing special provisions and standards that must be met by the contractor. The lack of formal specs is not slowing Jefferson County’s activity with foamed asphalt.
“We’re going to take a serious look at doing this kind of work on a regular basis,” Haas said. “We will be doing core samples to analyze how the roadway is standing up over time. We’ll use those cores and results to see if modifications to the process will have to be done in the future.”
Traffic volumes and condition or composition of subgrades will determine whether the foamed asphalt injection method is indicated for a particular project, Haas said.
Foamed asphalt is not appropriate for all roads, he said. “One case would be a road that not only has deteriorated pavement, but marginal soils in the subgrade that can’t be stabilized by pulverizing alone,” Haas said. “If there are any horizontal or vertical curve realignments, or minimal shoulders, or drainage problems that need to be addressed, more extensive reconstruction may be required.”
Recycling, Contractors and Cash Flow
If full-depth reclamation and recycling represents less asphalt produced, moved and placed on behalf of a contractor — thus less profit — a paving contractor who expands into milling and recycling has a chance to reclaim some of that cash flow.
For example, a foamed asphalt project such as Jefferson County’s Highway N represents a loss of the binder or base course for the contractor, this despite the contractor’s placement of a surface course over the foamed asphalt-recycled road.
The ability to do the entire foamed asphalt project — in this application, using the WR 2500 — enables a contractor to recapture much of that income while providing a value for the government agency customer. This leveraging of construction funds through recycling may enable the customer to do more road recycling work in a given season.
With recycling, contractor and owner are using a material that’s already been mined, processed, bought and paid for by the government agency. This is becoming critical because virgin aggregate sites are getting harder and harder to find and put into operation. Asphalt recycling will continue to play a major role in roadbuilding.
(The preceding article appears courtesy of Wirtgen.)
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