Transportation Officials Weigh Pavement Preservation Options
This article originally appeared in Western Builder, February 2010.
The city of Watsonville, Calif., has adopted a three step preventive maintenance program for its asphalt road pavement system, replacing the costlier mill-and-fill method as the treatment of choice.
Dismayed by the shortage of a keying ingredient for its $10 million chip seal program in 2008, Minnesota transportation officials are evaluating the performance of an alternative product that was applied in a recent chip seal demonstration on Route 25 near Becker.
Held with the approval of Roger Olson, P.E., research operations engineer for the MnDOT Office of Materials, the demonstration involved the application of chip seal on a mile of the two-lane highway using two different polymer-modified asphalt emulsions. Lake Delton, Wis., contractor Scott Construction Inc. applied the chip seal using CRS-2L, an SBR-latex polymer-modified emulsion on the northbound lane, and CRS-2P, an SBS-polymer-modified emulsion on the southbound lane. Both asphalt products are cationic emulsions.
SBS, comprised of styrene-butadiene-styrene, is the only type of asphalt emulsion polymer modifier allowed in MnDOT specifications for chip seal. It has been successfully used for some years.
SBR, made of st yrene-butadiene-rubber, is a latex polymer modifier. It is currently prohibited by the state for use in chip seal asphalt emulsion. (State specs refer to chip seal as “Bituminous Seal Coat.”)
The aim of the demonstration co-sponsors – Henry G. Meigs LLC, asphalt emulsion manufacturer, and BASF Corporation’s construction polymers business, SBR manufacturer – is not to replace the SBS-modified emulsion but to prove that SBR-modified emulsion performs equally well and needn’t be excluded from the specs. If MnDOT approves the use of SBR polymer, then it would have two acceptable binder products for its chip seal applications, reducing its vulnerability to unexpected shortages.
This dependency impacted the state’s chip seal program in 2008, when, due to global market supply conditions, asphalt emulsion suppliers experienced a shortage of SBS.
The shortage forced the agency to pare back this portion of its ambitious pavement preservation program during 2008. Minnesota has roughly 13,500 miles of roadway, making its trunk highway system the fifth largest in the nation. Recognizing it is more cost-effective to apply preventive maintenance surface treatments to pavements still in good condition rather than wait until they’re in poor condition and requiring reconstruction, the agency is committed to pavement preservation.
Investing In Pavement Preservation
According to Erland “Luke” Lukanen, P.E., pavement preservation engineer for the Office of Materials and Road Research, the agency usually spends about $20 million per year on these treatments. They range from simple “clean and fill” crack repair to micro surfacing to chip seal to “thinmill and overlay.” Thin-mill and overlay is also employed as a “hold-over” when the existing pavement is in poor condition but they are unable to perform reconstruction at the time, said Lukanen.
“We’ll use micro surfacing on high-volume-traffic roads, to correct road profiles and fill wheel ruts, while chip seal is the most commonly used pavement preservation treatment,”
“Years ago we had a lot of windshield damage claims. We needed a fast-setting asphalt emulsion to retain stone and found a local supplier who had SBS polymermodified asphalt emulsion.”
MnDOT tried the polymer-modified emulsion and learned what other transportation agencies were beginning to discover: Polymer-modified emulsions are quick-setting and better at stone retention than non-polymer-modified emulsions because they form a thicker film of asphalt residue on the aggregate. For these reasons the polymer-modified emulsions are increasingly being used in a number of thin – usually less than one-inch thick – preventive maintenance surface treatments including chip seal, slurry seal and micro surfacing , all of which fall under the mantle of pavement preservation. Based on successful trials of the product, the Minnesota agency adopted the SBS-modified asphalt emulsion in its specifications for bituminous seal coat, commonly referred to as chip seal.
Alternatives Yield Benefits
Relying on a single type of polymer-modified asphalt emulsion served MnDOT well until the SBS shortage of 2008. In the wake of this, the agency was approached by representatives of Henry C. Meigs LLC and BASF Corporation who offered to sponsor a demonstration of side-by-side applications of chip seal employing SBS- and SBR-modified asphalt emulsions. Both companies feel acceptance of an SBR alternative would reduce MnDOT’s vulnerability to a shortage of one of the products and it would benefit all companies supplying such products.
This could be characterized as “a rising tide raises all boats” philosophy of doing business. In line with this, BASF Corporation supports the pavement preservation process, rather than urging the use of its SBR product to the exclusion of other products. Meigs LLC likewise does not promote the exclusive use of one product over another.
According to Dan F. Drew, president of Portage, Wis.-based Henry G. Meigs LLC, the company makes and provides all types of paving asphalts and emulsions. It has no favorites. “We’re not contractors, we’re manufacturers. We want to market both 2P (SBS) and 2L (SBR), and to have MnDOT take a look at the performance of 2L. We have a long-term interest in the whole industry,” Drew said. He believes that if the agency approves a second type of polymer-modified product it will initiate a free market effect, i.e., it will introduce more competition and lead to more bids and lower prices for pavement preservation jobs, and ultimately, more pavement preservation projects.
But approval hinges on the successful performance of the latex polymermodified asphalt emulsion applied during the demonstration and its subsequent 2010 spring evaluation.
Two Polymers, One Set Of Rules
Scott Construction crews installed the chip seal on a one-mile strip of Minnesota Route 25 near Becker. They used the same equipment and the same application rates and rolling sequences for the northbound and southbound lanes, following MnDOT specifications.
Equipment consisted of a 3,500-gallon Etnyre asphalt distributor truck, an Etnyre QUAD ChipSpreader, and three pneumatic rollers – Hyster, HYPAC and Roscoe. The contractor utilized two John Deere tractors equipped with brooms and blowers to sweep the road. On hand for the demonstration were key MnDOT personnel Erland Lukanen, pavement preservation engineer ; Thomas J. “Tom” Wood, research project super visor ; and Patty Johnson, transportation specialist.
Dan Drew was also in attendance as were Randy Scott, executive vice president of Scott Construction Inc., and Arlis Kadrmas, technical development leader for BASF Corporation’s asphalt polymers dispersions for construction.
SBR Chip Sealing
The project began with the chip sealing of the northbound lane using CRS-2L, the SBR modified asphalt emulsion. Traffic was shifted to the southbound lane and the crew swept the northbound lane. At 8:20 a.m., with a pavement temperature of 82 degrees Fahrenheit according to Kadrmas, the crew began chip sealing the northbound lane. The Etnyre distributor applied CRS-2L at the specified rate of .35 to .36 gallons per sq. yds. Within one minute the aggregate, a crushed granite supplied by Martin Marietta Materials of which 100 percent passed a 38-inch sieve, was broadcast by the ChipSpreader at the rate of 14 pounds per square yard over the sprayed emulsion.
Initial rolling took place within two minutes following aggregate placement, with the three rollers making at least three passes over the aggregate. Rollers were aligned to ensure the entire 12-foot lane width was covered in one pass of the three machines.
According to Kadrmas, the lane was completed at 9:20 a.m. with the pavement temperature at the end of the mile recorded at 87 degrees.
While MnDOT specs call for sweeping surplus aggregate from the road on the same day of seal coat construction, the crew swept the lane within two hours of completion.
SBS Chip Sealing
Traffic was switched to the completed lane and crews began chip sealing the southbound lane at 10:40 a.m., utilizing the SBS modified CRS-2P. The pavement temperature at this time was 97 degrees. Following the exact same procedures as used on the northbound lane, the crew completed the southbound lane at 11:15 a.m., with the final pavement temperature registering 101 degrees. Sweeping of this lane was performed after approximately 20 minutes. On the next day, Scott Construction applied a fog seal of CSS-1H diluted at approximately .10 gallons per sq. yd. This was applied not only over the two 12-foot travel lanes that were chip sealed, but on the two eight-foot existing hot mix asphalt shoulders as well. While fog sealing the next day was included as part of the demonstration protocol, the agency’s seal coat specifications do not require that fog seal be done the next day, according to Patty Johnson, but it is option of the contractor. The specifications do require a minimum one-day cure on the seal coat prior to fogging.
Johnson, together with Tom Wood and Luke Lukanen, will inspect the one-mile demonstration strip in April 2010. She said they’ll look for snowplow damage and will even speak to MnDOT snow plow operators to get their observations and opinions. This will help determine the fate of the proposal to add the SBR modified CRS-2L asphalt emulsion February 2010 Page 5 Arlis Kadrmas, BASF Corp.; and MnDOT personnel Patty Johnson, Thomas Wood and Erland Lukanen, closely monitor chip seal process.
In commenting on this year’s demonstration and subsequent evaluation, Tom Wood said, “My goal is to write a fair (chip seal ) specification so that contractors can make a fair profit and we get a good product.”
The 2008 demonstration went well and the evaluation in spring 2009 was favorable, leading to the approval of the latex polymer product in the state’s micro surfacing program.