Moisture Sensitivity of Asphalt Pavements (2003)

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223 TOPIC 6 Questions and Answers L. ALLEN COOLEY, JR. National Center for Asphalt Technology, Speaker Q1—Steve Healow, Federal Highway Administration, California Division I have a question on your infrared images. They look like they were from Washington State and I find them compelling. I wonder if you can elaborate on the first slide and the third slide. The first slide was where you had the heterogeneous mat; there was thermal segregation all over the place, whereas on the third slide, there was no thermal segregation. It looked pretty homogeneous for the entire mat. What was different about those two processes? Was it the same contractor and was his level of effort different between those two images? What was the contractor doing in the first image to maximize his thermal segregation and what was done in the third image to minimize thermal segregation? A—Allen Cooley I don’t know a good answer. Those slides were from a NAPA training presentation. My guess, knowing a little bit about what’s going on with the thermal imaging, is that the first slide was a worst-case scenario, long haul distance, no material transfer vehicle, and so forth—all those types of things. And the last slide was probably where the material transfer vehicle that remixed the mix was used. That way you are getting a more uniform temperature within the mat. Q2—Carl Monismith, University of California, Berkeley Allen, I noted that neither John D’Angelo’s presentation nor yours included anything about the actual mixing process, the mix production. It seems to me that this could be a problem also. Again, I’ll show my age. I grew up in an era when batch plants were generally the way to produce hot mixes; moisture content in batch mixes was controlled to less than 1⁄2%. I am wondering if part of our problem, at times, comes about because there may be less control of the moisture in the aggregate at the time of production in the widely used drum plants. This certainly could lead to moisture sensitivity problems. Thus, I would hope that people might discuss this in the breakout session concerned with production. A—Allen Cooley I agree 100% and that’s a little bit my fault, because when I saw the title of John’s presentation, I thought he was going to cover it, and he probably thought I was going to cover it. In our paper, we do discuss moisture contents of the aggregates. We know if you leave the moisture in the aggregate, there is a higher potential for the displacement of the film, which can lead to moisture damage. That is contained in our paper. Q3—Dave Newcomb, National Asphalt Pavement Association On your distribution of air voids on projects, it is really like you said, disheartening to see such low densities, and low density is a precursor to distresses. Has anybody followed up to actually quantify how many of those pavements or what the condition of those pavements actually are?

224 Moisture Sensitivity of Asphalt Pavements: A National Seminar A—Allen Cooley We are in the process of doing that. As part of NCHRP 9-9 (1), which is the research study to evaluate the design gyration levels within Superpave, we are actually going back and coring each one of the 40 projects. Obviously, we got density at the time of construction, but we are also obtaining cores after 3 months, 6 months, 12 months, and 24 months. At 24 months, we are doing a performance evaluation. There’s talk with the project panel that we may also obtain 4- year cores to take a look at densification, which is what we need for the design gyration level stuff. But at that time period, if it’s extended to the fourth year as well, we will do another performance evaluation and have that type of information. Q4—Roger Smith, Consultant We often hear the term “first line of defense” applied to density and achieving density in our mat. I think that maybe that’s the second line of defense once you’re out there paving. One thing I’ve seen from my experience, especially in private work and especially at city/county level, is overlay work done without regard to reestablishing cross slope on the surface of the pavement so the water drains off the pavement. I think it’s very important for the agencies, whether it be the state or the local agencies, as part of their overlay design and project, to really ensure that they are taking that opportunity of the overlay to reestablish cross slope. A—Allen Cooley That’s a very good comment, thank you. Any more questions? Q5—Gerry Huber, Heritage Research Group A couple of comments, Allen. The one thought that went through my mind when you showed the tapered longitudinal joint, I thought right back to your comments about permeability. I know one of the concerns of some of the folks who are using those is getting density into them and that you end up with a 12-inch-wide strip that’s very low on density. Just a comment. A—Allen Cooley I wasn’t pushing that particular method. What I was doing was saying there’s new technology. I think you’re absolutely right. There is some low-density stuff. Besides the notched wedge, there are different materials that you can put on the joint to try and limit some of the permeability issues and stuff like that. I was solely saying there are some new technologies but we need to go farther as well. Q6—Gerry Huber, Heritage Research Group That’s pretty much what I thought you were saying. I just thought I’d raise it as a point for the audience. Then the last thing is dealing with the pictures of the thermal segregation. One of the things I’ve trained myself to do whenever I see those pictures is immediately look at the scale on the side of the picture, because the tighter the range on the scale, the more blotchy it ends up looking. The botchy slide has a bottom end of 200 degrees and the other slide has a bottom end of 80 degrees. That may be the reason why the second and third slides may have ended up disguising some of the blotchiness, or the first one enhanced it either way. A—Allen Cooley May have.

St. Martin, Cooley, and Hainin 225 Q7—Mansour Solaimanian, Pennsylvania State University Good presentation, Allen. I think what you talked about in terms of permeability is very important. What you mentioned was that you could have basically the same air void and different nominal maximum sizes and get different permeabilities. I think that is probably one of the reasons why we cannot find a good match between our laboratory results and pavement performance. We always emphasize that you must get your air void level within the given range in the laboratory and conduct your partial vacuum saturation and you get 50% to 60% or whatever saturation level for the same air void level for two different mixes. We then conduct the test and we come up with either pass or fail. Now we put those mixes in the field and we see totally different behavior even at the same level of voids, because they have different void sizes and permeabilities. So I think any test method that works with the laboratory air voids in terms of establishing criteria should really look at permeability as one of the factors that contributes. That is one of the things that we are going to look at in Phase 2 of NCHRP 9-34. Rather than just emphasizing that you should get a specific air void level and do your test, maybe we should say because of different void structures, you should look at the permeability and go from there. A—Allen Cooley I agree wholeheartedly. Just as another comment on that, Kevin Hall had a paper a couple of years ago. He came up with a test to evaluate the air void pathway within samples. If I remember right, when he looked at lab samples, basically what he found is all of the flow is coming out the side of the sample. When you looked at field samples, all the flow was going through the samples. What we’re seeing is the density gradient within the lab-compacted samples. That’s another piece of the puzzle along with the permeability. We need to take a look at both of them at the same time when we look at a laboratory moisture damage test.

TOPIC 7 Field Experiences