Moisture Sensitivity of Asphalt Pavements (2003)

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71 TOPIC 2 Questions and Answers DALLAS LITTLE Texas A&M University, Speaker Q1—Alan James, Akzo Nobel As far as I understand the presentation, the asphalt contains good and bad players as far as the adhesion is concerned. Carboxylic acids are bad players and nitrogen compounds are good players. Can the surface energy measurements distinguish between good and bad players or do both contribute to the surface energy numbers? A—Dallas Little In a sense surface energy measurements can differentiate, but perhaps not with the precision you imply in your question. Surface energy can differentiate between acid–base and Lifshitz–Van der Waals interactions, for example, but they cannot directly differentiate between, say, carboxylic acids and carbonyls or between carboxylic acids and nitrogen compounds. But the fact is that surface energy can give us a “global” sense of bond energy, and this is perhaps where we need to begin. Although I did not present it in this paper, we look at the impact of surface energy on moisture damage as sort of a two-step process. In the first part, the adhesive bond formed between the asphalt and the aggregate protects against damage. We obviously want this value to be high. In the second step, the bond energy or Gibbs free energy between the asphalt and aggregate demonstrates a preference of the aggregate to bond with water rather than asphalt. This free energy value turns out to be negative, which indicates a reduction in energy as water replaces asphalt at the aggregate surface, and hence this is a favored process. If the absolute value of this number is large, then the rate of debonding will be high; if it is smaller, then the rate of damage due to debonding will be lower. Therefore, we seek a large bond energy directly between the asphalt and aggregate (impeding bond interruption in the first place) and a less negative value of bond energy between the asphalt and aggregate in the presence of water (slowing the rate of debonding if water gets to the interface). Q2—John Harvey, University of California at Davis With the long-term chemical and pH changes that could occur in the field over a period of a few years, could benefits of treatment diminish? Most treatment benefits have been laboratory tested with accelerated tests lasting several weeks. Are you aware of any long-term testing confirming the benefits identified with short-term tests? A—Ray Robertson, Western Research Institute To answer your question, John, we have some field sites that several different states have very kindly put in for us where the comparison is among asphalts that are used. In other words, the principal variable at each site is the asphalt. We are looking at differences in the long-term performance characteristics of those asphalts. That, to me, is the gold standard measuring stick. While I’m up, can I make a comment on what was asked over here on surface energy measurements? You really don’t want to measure surface energy of individual components. The

72 Moisture Sensitivity of Asphalt Pavements: A National Seminar real advantage of the method is to measure the positive and negative effects together. For any of you who weren’t around the day after TRB, we had a pretty lively discussion on that subject for a while. Really, the surface energy measurement is to get the summation of the effects of all of the components of the asphalt. Can I make one more comment? To continue with what you have heard quoted here on carboxylic acids, one of the things we are going to have to watch more carefully is what happens with aging of asphalts, what kinds of new materials are formed. Again, to pick on Dr. Ken Thomas, he has identified some components from aging that cause substantially greater moisture sensitivity than carboxylic acids. How one treats these materials to “get them out of the way” is a subject we will probably discuss quite a bit more tomorrow. A—Dallas Little I really like the comment, Ray. I use the term global; you use the term summation effect. But the fact is we need something to simplify what we are looking at because, otherwise, the complexity of the interaction can be overpowering. So, if you can come up with a tool, even though it may be somewhat limited because you can’t differentiate among the species, if it gives you this overall summation or global effect then it is valuable. It is kind of an unbiased measure of the bonding propensity between the asphalt mastic and the aggregate. That is what we are really looking at and see promise in. Q3—Gayle King, Koch Pavement Solutions Dallas, I am intrigued by your compression test, which applied multiple loads to a submerged specimen. I assume you are trying to create the pore pressures that we once hoped to simulate with the environmental conditioning system, but maybe missed. Best guess! Are you creating higher pore pressures than observed with the ECS, and how might it compare with the Hamburg WTD or other mechanical wheel-tracking devices? Any feel for whether you are creating the same kind of damage that the Texas Department of Transportation has seen in problem mixes where static immersion tests did not pick up moisture problems? A—Dallas Little Gayle, that’s a very, very insightful question. I would have to say that, unfortunately, we haven’t measured the pore pressures in the experiment. We brought the system to about 85% saturation pretty much without confinement prior to running the repeated low compression experiment. So we really don’t have a handle on what those pore pressures are, but that is something that should be done. That is a very important part of the puzzle. Q4—Barry Baughman, Ultra Pave Dr. Little, have you looked at using polymeric aggregate treatments to protect the aggregate from the moisture while improving the adhesion to the asphalt? A—Dallas Little No, we haven’t looked at it. Our research to this point has looked at basic aggregates, and I use the term “basic” to refer to natural, or unaltered, aggregate. We looked at just the basic aggregates and the bitumen. However, we do have a study under way with the International Center for Aggregate Research that is looking at different coating or modification effects on the aggregate to see how they might affect surface energy and how that might affect the resistance to damage.

Little and Jones 73 Q5—Robin Graves, Vulcan Materials Co. Looking at the difference between a lime mitigation situation, adding lime to the aggregate versus to the bitumen. Have you looked at the solubility of calcium hydroxide in the bitumen system and do you know how soluble the calcium is and whether there are any pH changes in that system? A—Dallas Little I have not done that, Robin. That is a good question. You know, this thing about adding lime to the bitumen is intriguing because over the years we have looked at adding it, not as an antistrip, but considering it as a filler to the bitumen. Today I showed you one slide where the amount of damage that an asphalt sample can handle before failure is strongly affected by the filler. This is because the filler acts to mitigate the damage by absorbing energy, redirecting microcracks, crack pinning, and all those mechanisms. Over the years, we’ve found that the impact of hydrated lime as filler is bitumen-dependent: it works better in some than it does in others. We have referred, in past publications, to lime as an interactive filler with some bitumens while it acts as an inert filler with others. This is probably because the surface of the lime forms an interactive layer or buffer region because of absorption of polars in some bitumens. For example, hydrated lime in SHRP asphalt AAD is much more effective than it is when mixed with AAM. Didier Lesueur with LCPC in Nantes, France, and I presented a paper in 1999 (Effect of Hydrated Lime on Rheology, Fracture, and Aging of Bitumen, Transportation Research Record: Journal of the Transportation Research Board, No. 1661, pp. 93–105) concerning this interactive effect. This goes all the way back to the work of Claine Petersen, Hank Plancher, and others who state that this interactive effect is due to functionalities such as carboxylic acids interacting with the surface of the lime. So all that is really interesting, but we have not at this point in time looked at the ionization potential of the lime within the asphalt, and that would be something we would need to do. Q6—Joe Button, Texas Transportation Institute Dallas, would you answer this question in the short term and the long term? Do you see the surface energy measurement process as a specification test in the future? A—Dallas Little Yes, I do. I think it will be a specification test. There is a project under negotiation right now where we are trying to look at it as a specification test, and I certainly see the potential for doing that. I see the potential for shortening the time period for the surface energy measurement on the aggregate. We can then use the bond energy between the aggregate and bitumen as a basis to specify aggregates and bitumens on the basis of compatibility with one another. The short answer is yes; the long answer is we’ve got a little bit of work to do to get there. We also have to keep in mind that it’s not just surface energy that affects the response of the asphalt mixtures. Other factors do as well. Mixture properties such as mixture compliance, the time effects on compliance, and so forth affect the ability of the mixture to resist damage. The surface energy characteristics can also help us define crack potential. Not just bonding potential; they can help us define the potential of the mixture to crack. Dick Schapery in 1974 developed a viscoelastic fracture model, which says essentially that the energy you put into the system is balanced by the surface energy that is created on crack surfaces as they develop. So, there is a fundamental relationship between surface energy and crack growth and crack healing, as Schapery predicted

74 Moisture Sensitivity of Asphalt Pavements: A National Seminar in the mid-1970s. If you can develop a mixture that is resistant to the propagation of cracks and enhances the healing of microcracks, then certainly you are reducing the damage potential. Systems that crack more and have more crack damage have a greater propensity for moisture damage because the moisture can migrate into the damaged crack areas. So surface energy is not just related to the bonding effect; it is also related to the propagation of damage through microcracking. Q7—Roger Smith, Consultant We’ve heard that certain fine fillers, such as hydrated lime, can be a benefit. I’d like to hear your thoughts on the general effect of high dust (P 200) on moisture sensitivity. A—Dallas Little I think I have tried to limit the discussion to what would be the appropriate amount of filler in the constraints of the overall mix design. If the appropriate amount of filler is present to pin the cracks, absorb some of the energy, and make the mixture more resistant to damage, then you are in good shape. Obviously, you can abuse that and add too much, and then you get into a whole line of other problems. You could develop a mix that is too dry, a mix that is so dry that you alter the adhesive bond between the mastic and the aggregate surface, and then you go down another route that might cause more damage than good for sure. So you know, asphalt is a very humbling material. It is a material that keeps us all in check and often surprised. Q8—Bill Maupin, Virginia Transportation Research Council Dallas, have you looked at time dependency effect on bond strength? In other words, could you initially have a strong bond that may become weak over time with certain asphalt–aggregate additive systems? A—Dallas Little I think you could, Bill, and we have not looked at that. I think that some research indicates that if you have some environmentally induced shifts in pH and so forth, certain types of additives or certain systems make a difference. We haven’t looked at that, but it is certainly something important to look at.

TOPIC 3 Test Methods to Predict Moisture Sensitivity of Hot-Mix Asphalt Pavements