Every year since 1914, members of the transportation industry have traveled to West Lafayette, Indiana for the Purdue Road School.
This year’s event was attended by more than 3,000 attendees from a variety of transportation fields. The Wednesday luncheon featured a fireside chat between Purdue President Mitch Daniels and Ford CEO Jim Hackett. Over 160 technical sessions were offered to share project success stories, emerging technologies, and other industry information.
One such session was co-led by Asphalt Materials, Inc. research engineer Andrew Eicher and Hendricks County Highway Department superintendent Curt Higginbotham. Their topic dealt with one of the toughest challenges in asphalt paving: longitudinal joint failure.
“Somebody told me once: you start having trouble with your asphalt the day you laid it,” Higginbotham said. “There’s a lot of things that could go wrong when you lay asphalt. You could make it at the plant wrong. You could haul it wrong. You could lay it wrong. You could compact it wrong.”
But even if the entire project is executed perfectly, asphalt pavements will always have one particular weakness. “On your longitudinal joints, you’re still always going to have a problem where there’s a low-density area.” Higginbotham said.
There are many methods for addressing joint failure. It’s possible to wait for the road to crack and then reseal it. Joint adhesives, asphalt zippers, and J-Band® are also solutions that combat joint failure.
For his roads, Higginbotham has chosen rapid penetrating emulsion (RPE). A key part of what makes RPE effective is its penetrating, air-void-filling capability. Unlike a crack seal that simply covers the surface of a crack, RPE soaks into the pavement itself and decreases the permeability of the surface to which it’s applied.
The RPE works so well as a moisture barrier that Higginbotham has considered using it as a full-width treatment for his pavements. “I’m not to that position yet, but I’m getting close to it,” he said. “We have some areas that we’re experimenting with that- we have done some full width spraying with the RPE and I’m kind of watching it to see what it does.”
AMI’s Andrew Eicher, who gave the second half of the presentation, went on to explain how RPE works to improve the life of the road.
RPE is specially designed to “penetrate the interconnected voids in the pavement,” according to Eicher. RPE’s unique chemistry helps it create water resistance quickly while maintaining texture.
How can we be certain that RPE really works to fill voids? In addition to the normal specification for an emulsion, AMI has developed two tests to quantify penetration for RPE. The first test measures how much of an RPE sample can pass through a 500-mesh sieve. This sieve is so fine that water can’t pass through it, but RPE is able to penetrate through.
Another test compared a dilute SS-1H emulsion to RPE by applying both to the same slab, side by side. The RPE penetrated much faster than the SS-1H solution, and the asphalt slab absorbed more RPE by volume.
In the near future, AMI engineers will be able to directly measure RPE penetration using digital imaging.
To test water permeability, AMI engineers tried to shoot compressed air through two core samples. One core was treated with RPE and one was not. A considerable amount of air passed through the untreated core, while almost no air was able to pass through the RPE core.
The importance of permeability ultimately comes down to air voids. Reducing voids is absolutely critical to improving pavement life. “The higher your voids are, the more life you’re going to lose on your pavement,” Eicher said. “If your voids are at 7%, you’re going to get 100% of a 15-year life. When that goes down to 10% voids, you’re getting 82% of your life… Bottom line is: the joint’s going to fail first because of these high-void [areas].”
There have even been some use of RPE in places like automobile race tracks. While J-Band has also been used on race tracks to improve the joints from the bottom up, RPE was applied to treat the entire track surface from the top down. Specifically, the goal of applying RPE was to fill hairline cracks and seal the surface. GreensBroom helped with the job to ensure a perfectly uniform application.
“When you have high-speed traffic going over 200 miles per hour out there, we don’t want to take any risks.” Eicher said. “It turned out really well… in a relatively short amount of time we had cars running full speed on a race track, and the people at the track were saying there were no issues with friction. Some actually commented that the friction was better, and it was like a new pavement.”
Other applications of RPE are currently in development. AMI has done some field testing of using RPE on cold mix (comparable to a fog seal).
Rumble strips are another niche were RPE could potentially prove useful. Because they are cut in high-void areas, it’s prudent to seal rumble strips well after they’re made.
While RPE is not approved for INDOT use yet, AMI is working on special provisions for it and hopes to have those submitted soon.
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Rapid Penetrating Emulsion (RPE) Frequently Asked Questions (FAQ)
Q: What does RPE stand for?
A: RPE stands for Rapid Penetrating Emulsion. Its name refers to the speed and quantity of asphalt that is able to penetrate the pavement, especially in comparison to treatments like a dilute SS-1H.
Q: What sets RPE apart from other emulsions that perform similar tasks?
A: RPE is specially formulated to penetrate quickly and thoroughly. Its reduced surface tension allows it to absorb into our pavement fast so you can get back on the road sooner. RPE reduces air voids, lowers permeability, fills small cracks, and adds a nice dark color to your pavements.
Q: What can RPE be used for?
A: RPE can be used on centerline joints or as a full-width pavement treatment to increase pavement life. There are other potential applications that are still in the process of being researched or approved (RPE used on rumble strips, used as a tack coat, etc.).
Q: How is RPE applied?
A: Very similarly to other emulsions: with a distributor, usually one equipped with fog nozzles. The exact rate of application varies based on permeability, texture, amount of air voids, and age of the pavement. The applicators go in with an estimate of what their rate should be and fine-tune it on site to ensure a good application. RPE can be applied as soon as the pavement has cooled. It will work on hot pavement, but you might see some flashing of the moisture if the pavement is too hot.
Q: What should I do if I’m interested in learning more about RPE, or about potentially using it on my own projects?
A: Contact Dave Henderson, our Director of Specialty Products, at email@example.com