PAINTSQUARE BLOG

Bridge parapet walls pose a special challenge, engineering professor Richard Miller explains: The concrete walls are relatively thin in comparison with many other structural members, and in cooler climates, they’re hit with road salt mercilessly. The result is often spalling and deterioration—and that’s what the Ohio Department of Transportation contracted Miller and two co-authors to study.

University of Cincinnati

Richard Miller, professor of engineering at the University of Cincinnati, is studying how to correct deterioration of  bridge parapet walls using coatings.

Miller, who studied at Northwestern University and teaches at the University of Cincinnati, is a concrete expert, currently serving on the board of the Precast/Prestressed Concrete Institute. Along with Oklahoma State professor Norbert Delatte and Cincinnati graduate student Sarah Mullins, Miller undertook research to determine what the best way to address spalling on parapet walls is, and whether a particular sealant or coating might be preferable to others in order to prevent further damage. The team published an initial paper last year.

Miller spoke with PaintSquare News about the research, which continues this winter with field testing, and which so far has indicated that perhaps polyurea coatings could be the key to preventing parapet problems.

PaintSquare: What special concerns come with deteriorating parapet walls?

Miller: The problem is that if they begin to deteriorate, and pieces fall off of them, these pieces can fall into traffic and hurt someone. So what ODOT has been doing—and a lot of people have been doing—has been just putting an epoxy coating on them when they’re new. But that epoxy coating doesn’t always seem to work: With some of the bridges, within a year or two it starts to flake off or wear off. [ODOT was] having problems where the coating would start to come off and the bridge would start to deteriorate, and what they have to do, then, is chip the bad concrete off the bridge.

Up until now, they haven’t done anything to the chipped surface, which means more stuff can get in there and it starts to deteriorate even more. What they were basically looking at is: Is there something they could easily put onto the surface that would kind of seal it up and protect it against all of the salt and the other things that may try to deteriorate it?

How did you approach this project?

One of the things we found is that in general there’s not a lot of long-term data on the traditional coatings—nobody’s going to give you the money to study something for 30 years. Most of the [previously published] studies were about four or five years. But Norb Delatte knows a guy by the name of Jim Davidson, a professor at the University of Birmingham-Alabama. We had seen this on television: The "Mythbusters" had taken bedliner—and of course polyureas are one of the products used for bedliners—and they painted wood and concrete block walls with them, then they set bombs off next to them. And oddly enough, this bedliner actually protected these stupid walls against getting blown up by bombs! What Dr. Davidson has done is actually to do this scientifically. And what he has shown is that polyureas can actually act like reinforcing [agents] on these brick walls. He was working with the Air Force.

Images courtesy ODOT unless otherwise noted

If deteriorating parapet walls aren't treated with some sort of sealer, then can deteriorate further, leading to rebar corrosion.

Our thought is: Polyureas, and actually polyaspartics, are used for concrete floor coatings, industrially—these things are pretty rugged. And one of the problems [with parapet walls] is that the state workers will go in there, they’ll clean up all the bad concrete that’s falling off, but there might be stuff below the surface that’s still having a problem, and then it starts to come loose later. And we were thinking maybe if you coat it with a thick enough coat of this polyaspartic, maybe if it starts to come loose, it won’t fall off into traffic until you can get out there and do something about it.

What we’re doing now is, Oklahoma State University is working with these coatings. They’re putting them on concrete and trying to see: How do they react to freeze-thaw? Do they actually hold pieces of concrete together under stress? Things of that nature. And we went out, got five different [polyaspartic coatings] from five different manufacturers, painted them on some parapets, and we’re going to see how they survive the winter.

What’s the timeline on the work?

We’ll report back in the spring, and that will kind of end our involvement with the project, then District 6—basically surrounding the Columbus area in the middle of Ohio—they’re going to monitor what goes on with these polyaspartics going forward.

What did you find in regard to surface preparation of these surfaces?

What was interesting about this is: Several of the manufacturers said that the surface moisture content had to be less than 3 percent. And we’ve got a little gauge that measures that, and we found that even after we pressure-washed off the surfaces of these things, the workers were able to take a propane torch—like they use for pre-heating surfaces when they put asphalt patches down—and they were able to put that on there for just a couple minutes and bring it down under 3 percent.

Parapet walls are relatively thin in comparison with many other structural members, and in cooler climates, they’re hit with road salt mercilessly; the result is often spalling and deterioration.

We also had been doing some hydrodemolition, and we found that by the next morning, the surface was plenty dry. And in fact, one manufacturer even recommended wetting down the surface before application. So we really didn’t find surface preparation to be all that problematic.

Another thing is: With my schedule getting a little crazy, we didn’t get out to [apply the coatings] until late October, early November. We did some of this in some pretty cold weather, and it seemed to work fine. They seem to have a very large [application] temperature range.

Assuming you find success with these coatings in the next round of testing, do you think this practice would take hold quickly, or are there drawbacks to worry about?

The main problem you run into is: We have 50 different states, and each and every state has their own set of rules. It’s a lot about how you get through some of the state processes. One is just the time it takes to get on the “approved” list. The only other thing we’re seeing is, if you actually use a spray-on polyurea, you have to have all this protective [clothing]. The polyaspartics can be painted on, but it’s labor-intensive. That would be a little bit of a drawback, especially if you’re working with a really rough surface, which is what you get when you chip off concrete. It’s a matter of the labor time on this.

But we’re still in the evaluation stage, so it’s way too early to tell if it’s going to work or not work. Sometimes things look really good in the beginning, then they don’t work so well later on; you never know about these things. While the results are promising at the beginning, we have a long way to go before we can make a final verdict here.

ABOUT THE THE BLOGGER PaintSquare Staff In this occasional blog series, PaintSquare staff writers speak with industry experts on life in the coatings industry, and the advice they'd like to share with others based on their years of experience. SEE ALL CONTENT FROM THIS CONTRIBUTOR