Applications & Innovations
| January 11, 2012 |
Previous Concrete Overlays: Wet-on-Dry Works
By Tina Grady Barbaccia
In the recently-released three-year study, An Integrated Study of Pervious Concrete Mixture Design for Wearing Course Applications, mix designs and procedures for portland cement pervious concrete (PCPC) overlays for highway applications were successfully developed and field- tested in Minnesota. The pervious concrete overlay was constructed at the Minnesota Road Research Project Low-Volume Road facility (MnROAD), a cold regional pavement test track near Albertville, Minn., to determine the durability of pervious concrete as an overlay. The study, the largest and most comprehensive to date on PCPC, was conducted through Iowa State University and the National Concrete Pavement Technology Center, and authored by Vernon R. Schaefer, John T. Kevern and Kejin Wang.
The results of the tests conducted show that a pervious concrete overlay can be designed, constructed, operated and maintained. A pervious concrete overlay has several inherent advantages, including reduced splash and spray and reduced hydroplaning potential, as well as being a very quiet pavement. The good performance of this overlay in a particularly harsh freeze-thaw climate, Minnesota, shows pervious concrete is durable and can be successfully used in freeze-thaw climates with truck traffic and heavy snow plowing.
The overlay, which was nominally 4 inches thick with formed joints over the original skewed joints, was constructed in October, 2008, over concrete originally placed in July, 1993. Condition surveys of the overlay were conducted in 2009, 2010 and 2011, and according to the report, the primary distress to the overlay pavement was joint deterioration. With a minor amount of cracking, the joint deterioration is believed to be the result of the method of joint placement; saw cutting the joints would have resulted in less deterioration. The joint deterioration increased each year and is likely due to snowplow effects. The flow characteristics have been measured each year, with high infiltration results and good consistency from year to year. Operations during rain events indicate that the pervious overlay quickly removes rainwater from the pavement surface and that the water migrates lateral to the side of the pavement, indicating pervious concrete is a successful tool for mitigating splash and spray as well as reducing hydroplaning difficulties.
Wet-on-dry overlays are the standard for traditional concrete overlays, says Kevern, Ph.D., LEEP AP, who developed an effective wet-on-dry mix design for pervious concrete overlays. “Pervious is normally pretty dry. There is not a lot of extra paste so it took a bit of research to produce good bonding characteristics.” In terms of performance, “the pervious concrete overlay mix isn’t being designed to be structural pavement, but we’re developing a mix strong enough to have good bonding characteristics.”
This was the first time it had been tried for a pervious concrete overlay, he says, and it also was the first wet-on-dry pervious concrete overlay in the United States. Previously, it had been tested in Belgium, The Netherlands, Japan and Australia. “This placement is wet-on-dry, which is much more difficult than wet-on-wet. Wet-on-wet has been constructed in various locations and we know it works but, is complicated and expensive to construct. Wet-on-dry would be much cheaper but, until now no one has successfully built one.”
The project was designed to place the overlay with a slipform paver, but because of scheduling, it was placed using semi-mechanized placement. The next step is to fully mechanize placement of the pervious concrete overlay as slipform. “The next phase is to get it on an open facility with high-speed traffic,” Kevern says, although the test track undergoes the rigors of a fair amount of traffic and plowing, which is a good durability test, “We ultimately need to get it in service in a small installation before this moves to widespread use.”
If the use of these kinds of pervious concrete overlays moves to widespread use, they may have a bright future in noise reduction, particularly in urban areas. “This has half the noise generation of normal concrete,” Kevern points out. “This is where the overlay would shine – in urban areas – where there are noise considerations.” However, pervious is a filter, so an area where there is a heavy amount of dirt being tracked on and off the pavement would not be an appropriate application site for this type of overlay.
“This was an integrated study,” Kevern says. At the time we put this proposal together, we looked at all the downfalls of pervious concrete. The way the report is organized answers all of those questions.” The testing also confirmed that curing under plastic is a viable technique. “We proved that curing under plastic is a good option,” Kevern says. “The industry had been doing it, and it was hard to improve on.”
One of the big questions that was posed during the study and testing was whether air entrainment was needed. “We found that, ‘Yes,’ we need that air,” Kevern says. “The workability of pervious previously was not able to be quantified, but based on the asphalt compaction techniques we developed, we have found a way to develop workability. We developed a mix design that was strong and highly permeable. The testing in the lab found that it was equal or better than regular, good-quality concrete … and is freeze-thaw durable.” Study co-author Vern Schaefer, Ph.D., P.E., professor of civil engineering at Iowa State University, says that when developing the freeze-thaw pervious concrete, a mix design was used that put in air voids of about 20 percent. “This allows water to run through it quite easily,” Schaeffer says. “Until about five years ago, when the pavement froze it all went to hell. Now we have a freeze-thaw concrete mixture that, quite simply, works.”
Schaefer says there has been a great deal of skepticism about pervious concrete pavement overlays. The applications started out being used in parking lots. “People said it wouldn’t work on highways, but we proved it could be used on highway applications – and work,” Schaefer says. “There is still some concern about how it will hold up against truck traffic, but it is subjected to truck traffic in the current test applications and it suffers some of the damage of any overlay. The big breakthrough was the successful use of wet-on-dry applications, and it is working beyond our dreams.” v
For a downloadable PDF of An Integrated Study of Pervious Concrete Mixture Design for Wearing Course Applications, go to http://www.rmc-foundation.org/images/PvC%20Wearing%20Course%20Apps%20Final%20Report%2010-11.pdf.
PLUS: For a look at how a concrete overlay came to the rescue of an aging, secondary, farm-to-market road in Iowa, go to our digital edition at betterroads.com