Three sides of pavement preservation
Mn/DOT's experience with chip seal
At the ninth annual Minnesota Pavement Conference, many speakers followed a time-honored tradition of adding a little spice to their presentations. In a session on pavement preservation, Jerry Geib, the pavement preventive maintenance engineer at Mn/DOT's Office of Materials, began with a joke—of sorts. He held up a pavement core and said, "So I walked into a bar, put my core on the table, and said, 'Give me a beer —and one for the road.'" Before too many groans were heard from the audience, Geib acknowledged that it was a really bad joke. But he said he hoped people would take a close look at the core he was holding up because it was a good example of what he came to talk about: successful seal coating.
He then referred the audience to a paper titled "Chip Seal Program Excellence in the United States," which had been presented by Douglas Gransberg at the 2005 Transportation Research Board conference. Geib said Gransberg discovered a commonality in successful seal coating projects: Everyone who reported excellent performance used a one-stone-thick layer.
He said his own research bears out Gransberg's findings. On rural trunk highways, the best seal coats use rock that is very consistent in size: FA-3 modified—that is, slightly small FA-3 with very little that will pass through a number 4 sieve—laid down at about 17 to 18 pounds per square yard. He emphasized the importance of uniformity in both size and shape. "The larger single-size stone makes construction easier and you get a heavier application of binder when all your chips are the same size; it's like gluing down little dice. Those cubical chips stand up very well to turning traffic."
Binder application rate plotted against ADT, with three curves for various levels of pavement deterioration.
He discussed the use of FA-2 rock on trunk highways. He characterized the smaller FA-2 as more of a sand seal. On city streets where "somebody's going to be backing out of a driveway right after the chips have gone down," he said, "the FA-2 works well since there will be less pick-up."
If seal coating is done early enough in the life of a pavement, Geib said, about three-tenths of a gallon of emulsion should be the right amount. "If we wait until that road really becomes aged and oxidized, we might need four-tenths of gallon—a 33 percent increase. He said the best approach is to lay down a couple hundred feet or so, roll it, and then check for pick-up. "If you're getting a lot of pick-up, drop it down two-hundredths of a gallon. That may not seem like a big difference, but if you figure the percent difference, it really is. You can make adjustments throughout the day as your pavement changes."
Geib recommended CRS-2P (polymer-modified) emulsion. "With the CRS-2P, we do a hard sweep and turn it back to traffic at the end of the night. With CRS-2, it really takes that emulsion two weeks to develop the chip-grabbing strength that CRS-2P gets in a day."
He also said Mn/DOT fog seals all chip seals at the time of construction with CSS-1H emulsion, diluted 1:1 with water. "Depending on your rock size, from six-hundredths to a tenth of a gallon—and we want good, clean rock, zero to 1 percent pass in the 200 sieve with no clay material. If you get just a little clay in there, the emulsion will grab the clay, not the chips."
Geib also emphasized the importance of properly adjusted and managed equipment. "The end nozzle has to be spraying so we're getting our three- to four-tenths of a gallon right at the edge. With the CRS-2P, you've got to get that rock on within one minute. The chip spreader and rollers need to be right behind the sprayer." He recommended three rollers going no more than five miles per hour. "If the rollers start going too fast, they turn over those chips and get the black side up and then we're going to get a lot more pick-up. You also need to sweep no later than the next morning. It really is all these details that make the difference."
Geib's advice is based on a whole lot of seal coating done by Mn/DOT. He said the agency's work in the past year program included about 2,900 lane miles, which amounts to about 10 percent of Mn/DOT's trunk highway system. "What I'd like to see is that any agency would do a little more than this—so you complete the entire system every seven or eight years." At that rate, aging due to oxidation will be significantly slowed down, "and by doing that, you can hopefully slow down the thermal cracking, which is what really turns pavement maintenance into a much more expensive proposition."
As additional resources, Geib recommended:
- Minnesota Sealcoat Handbook (1999-07)
- The Sealcoat Design Program
- A maintenance technical advisory guide from the California Department of Transportation (CALTRANS)
In-progress FHWA fog seal study
Larry Scofield, of the Arizona Highway Department, reported preliminary results of a Federal Highway Administration study of fog seal. Not to be outdone by Jerry Geib, Scofield also started his presentation off with a bang—of a sort. He showed a mid-air photo of a jet plane creating a visible sonic boom. "If you took this photo a second sooner or later, you wouldn't see anything. Imagine the fun that guy had trying to get this picture!" He compared the photographer's challenge with the problem of achieving optimal timing in pavement preservation.
With pavement maintenance, Scofield said, we are mostly in a reactive mode: we fix it after it breaks. The goal of the FHWA study is to move to a predictive mode, where we know what to do and when to do it because we can predict when a problem will occur. To do that, he said, "we need to figure out what to measure, how to measure it, and what range of values it should or should not have. But we still don't even know which things we want to predict! We need to be concerned with a fog seal's performance—its durability and how well it protects against oxidation. But also, safety is a huge issue with fog seals—so we're dealing with competing issues."
On the performance side, Scofield said, the study is looking at chemical issues, such as the compatibility of mix components and the breakdown of asphalt as it oxidizes. The study also is looking at rheological issues—the stiffness of the material. This is being done with destructive testing in the lab and with seismic analysis, which may allow a prediction of when to apply fog seal to prevent cracking.
To deal with safety, the study looks at pavement friction. One method being evaluated is a dynamic friction tester that factors in vehicle speed. The device has small pads that spin on the surface of the pavement. "It may be that a particular treatment won't work on an interstate highway where the posted speed is 75 mph," Scofield said, "but it may work well on a lower-volume road where friction is not an issue. So one size does not fit all."
In-progress crack-sealing study
Imad Al-Qadi, of the University of Illinois, discussed an ongoing study of crack sealing being conducted by a consortium of 13 states and 13 Canadian agencies. He said the major goal is to develop performance-based specifications for hot-poured crack sealants using parameters such as viscosity, stiffness, creep, fracture, and bond strength.
Al-Qadi said the investigators want to develop consistent procedures that can be used by all agencies and methods that incorporate the equipment that most agencies already have on hand. They will identify tests for three factors: long- and short-term aging, low-temperature performance, and adhesion.
A challenge to developing consistent standards for sealants is their inherent inconsistency. Al-Qadi showed that there is significant segregation of the filler in sealants as they are received from the manufacturer. He showed a typical disc of sealant that was found to contain 17 percent filler at the bottom but only 1.8 percent filler at the top. He also showed that sealants vary considerably in consistency from lot to lot.
—Richard Kronick