Transportation officials attending the Midwestern Pavement Preservation Partnership (MPPP) annual meeting observed a maintenance surface treatment demonstration on a busy Minnesota highway as the state took another look at the use of asphalt emulsion modified with currently barred synthetic rubber.

As part of the Partnership's 2008 meeting, held in Minneapolis on September 8-11, DOT officials from 10 states were bused to Highway 55 in nearby suburban Plymouth to see contractor Vance Brothers micro surface a 1-mile stretch of the highway using a cationic asphalt emulsion modified with SBR (Styrene Butadiene Rubber) latex. The synthetic rubber modified product has been excluded from Minnesota's Department of Transportation specifications for micro surfacing since 2006 after a single application using it hadn't performed well. Prior to that incident, Mn/DOT had always used natural rubber latex modified emulsion.

The implications of such a ban are important because the state's trunk highway system consists of some 11,000 miles of roadway, ranking this as the fifth largest in the nation. And since Mn/DOT is seen in the region as being in the forefront of highway maintenance and construction practices, several neighboring states have followed suit and barred the product in favor of emulsion modified with natural rubber latex for micro surfacing applications.

To counter what it considers an unfortunate perception of SBR based on a single performance, BASF, the Chemical Company's construction polymers group, which supplies SBR to asphalt emulsion manufacturers, sought and received permission from Mn/DOT to sponsor a demonstration of micro surfacing using CSS-1H SBR latex modified asphalt emulsion. Furthermore, the demonstration was scheduled to take place on a two-lane section of heavily-traveled Highway 55, with 29,000 vehicles per day, during MPPP's three-day meeting in Minneapolis.

Vance Brothers, an 85-year-old family-owned business with headquarters in Kansas City, MO, not only manufactures and distributes a variety of products, including liquid asphalts, hot mix asphalt and pavement sealers, but also applies surface treatments and installs other products as well. When a local emulsion supplier and a local surface treatment contractor could not be found to provide SBR modified emulsion and install it as part of the demonstration, Vance Brothers agreed to take on the BASF sponsored demo.

It was a challenging project on many levels, according to Chris Lubbers, BASF Corporation's senior technical service engineer from construction polymers, who originally approached Mn/DOT with the proposal.

“Vance Brothers had to provide and install a micro surfacing product containing SBR latex modified asphalt emulsion that meets the Type II mix specifications of the International Slurry Surfacing Association (ISSA), and also meet additional Mn/DOT requirements,” said Lubbers. “For example, Mn/DOT requires much of its micro surfacing work to be done at night to minimize traffic disruption, and the mix has to be able to meet a one-hour cure time requirement.”

Another hurdle for the Kansas City-based contractor/supplier to overcome was getting a work crew and tankers of polymer modified asphalt emulsion to the demonstration site approximately 400 miles away, while the window for performing the micro surfacing in time for MPPP's planned bus tour was rapidly closing.

Adding difficulty was the need to complete half of the demonstration project at night with the remainder completed the following day. Because the event was taking place at the closing of Mn/DOT's paving season, the emulsion would have to perform well not only at nighttime ambient temperatures around 50 degrees, but also at an anticipated high of 75 degrees the next day when the MPPP bus full of public works officials would cast discerning eyes on the operation.

“Vance had to tweak the emulsion formula. It absolutely had to work both day and night,” said Lubbers. “And it did.”

Erland “Luke” Lukanen, P.E., veteran pavement preservation engineer in Mn/DOT's Office of Materials, observed that the demonstration was not scheduled at the most favorable time of the year. He and Lubbers had co-authored a slide presentation to the MPPP meeting outlining the chemical and physical characteristics of polymer-modified asphalt emulsions, together with a history of Mn/DOT's use of micro surfacing and some of their requirements. Among the points of the presentation: The treatment has to be accomplished in ambient temperatures above 50 degrees, and work must be completed before September 15.

Lukanen explained that micro surfacing is used extensively in Minnesota to preserve pavements on roadways experiencing 10,000 ADT vehicle counts and more with most of these occurring in the Metro District. This is the largest of Minnesota's eight DOT districts, encompassing seven counties and the state's largest cities, Minneapolis and St. Paul. (Almost half of the state's 4.9 million people live in those two cities.) He added that in addition to preserving road surfaces, micro surfacing is employed widely for filling ruts, improving pavement friction and increasing the quality of ride.

The use of chip seal is another surface treatment used extensively throughout the state to preserve pavements, especially in rural districts where traffic counts are lower, Lukanen noted. Funds for road maintenance are distributed to each district, and they decide how to spend the money, he said.

“The DOT's pavement management program produces a list of road candidates for maintenance work, but the list is always too big for available funds. So each district has to pare down the list and select roads that really need the attention now,” said Lukanen.

As pavement preservation engineer for Mn/DOT, Lukanen was keenly interested in the performance of SBR latex-modified asphalt emulsion in the micro surfacing demonstration and watched closely as Vance Brothers applied the treatment during both the evening and morning operations.

Micro surfacing is used to protect structurally sound pavement from further distress and oxidation. A cold-mix material, it consists of mineral aggregate, mineral fillers – usually portland cement – water, and a latex polymer-modified asphalt emulsion. The latex polymer binds the asphalt, mineral aggregate and fines together. Micro surfacing can be spread in variable thicknesses, for example, to fill wheel ruts and provide leveling, and is designed to be a quick-traffic system, which means it is ready to accept traffic within an hour. No rolling is required since vehicle traffic accomplishes that. Usually a tack coat is unnecessary before micro surfacing unless the existing pavement is concrete.

For the demonstration, a crew of nine – including Vance's materials lab manager, Stan Fronckewicz – plus asphalt emulsion tanker and micro surfacing machines drove some 400 miles from Kansas City to Plymouth in nine hours straight, according to Tim Harrawood, the company's southern region general manager. On arriving at the staging area, one of Mn/DOT's truck stations, the crew calibrated its two truck-mounted Akzo Nobel micro surfacing machines for the job. Each machine carried about 10 tons of aggregate, of which some 97 percent had to pass a number four sieve, plus up to six bags of portland cement, 700 gallons of water and 700 gallons of asphalt. Crushed granite was provided by Martin Marietta of St. Cloud, MN, while global manufacturer Holcim's portland cement was used.

On the second day, the crew completed micro surfacing a test strip in the morning and another in the evening (a Mn/DOT requirement). That same evening the crew went to the Highway 55 job site and completed one lane of the mile-long demonstration project. The next morning, they returned and completed the second 12-foot lane as some 40 DOT officials from the MPPP tour bus looked on.

During the operation, Vance's machines applied micro surfacing at the rate of 20 pounds per square yard, producing a half-inch-thick surface. As one machine applied micro surfacing, the other returned to the staging area to replenish its materials, ensuring continuity of the application.

Mn/DOT's Lukanen said he was satisfied with the operation, but reserved final judgment pending the results of monitoring by two Mn/DOT personnel, Paul Nolan and Patty Johnson, transportation specialists, over the winter and early spring months.

“How it performs in winter is crucial,” said Lukanen.