Moisture Sensitivity of Asphalt Pavements (2003)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

202 TOPIC 5 Questions and Answers JOHN D’ANGELO FHWA, Speaker Q1—Gayle King, Koch Pavement Solutions John, excellent presentation! You’ve addressed a lot of important issues. Your point regarding inappropriate application of chip seals to seal moisture into the pavement brings back some particularly unpleasant learning experiences from the past. More importantly, you’ve really captured some of the pressing binder chemistry issues we don’t handle well as an industry. I’ve followed four projects in which surface mixes failed due to stripping within 8 months, and each was caused by specific binder chemistry problems. In each case, good performance was achievable when other binders of similar grade were substituted for the problem material. Although each failure was related to binder chemistry, the causes were different. One was an acid/amine compatibility problem, one was a crude source problem, one was caused by the addition of absurdly high concentrations of emulsifiers, and the fourth is still under investigation. Three of these projects were built using the current AASHTO T283 or agency equivalents thereof. One was CDOT’s project on Copper Mountain, with more recent failures about 2 years ago in Oklahoma and last year in Nebraska. In all cases, Hamburg wheel tracking indicated disintegrator mixes, and the binder always showed some signs of reemulsification. To make an emulsion, one needs asphalt, an emulsifier (chemical salt, surface active clay), water, heat, and mechanical energy. In mixes, mechanical energy creates pore pressure, and the resulting shear stresses cause the binder to strip/emulsify. No mechanical energy, no emulsion. We are consistently missing such stripping mechanisms with T283. Wet wheel-tracking tests can predict these problems. For example, when clay acts as the asphalt emulsifier, Hamburg tends to cause much more damage than might be presumed from static immersion tests. Aschenbrener’s and Kandahl’s Hamburg/methylene blue studies emphasize this point. I apologize for the long comment, but I believe we are missing critical stripping mechanisms by relying on laboratory tests that do not create damage caused by pore pressure. A—John D’Angelo Thank you, Gayle. I’ll have to sort of agree with you. I agree that the existing AASHTO T283 doesn’t have any mechanical action per se and it doesn’t cause the problem where you can get reemulsification of the asphalt. Also, the pore pressures cause some of the separations of asphalt from the aggregate even if it’s not emulsification. I think it’s critical to have the mechanical action. I’m not a big fan of the Hamburg, though, because I think it’s too severe a test. If it passes with the Hamburg test, you’ve probably got a pretty indestructible mix. I’ll grant you that one, but it might be a little too severe. That’s why I’m holding great hopes for the NCHRP 9-34 procedure. Q2—Bill Bailey, Rock Binders I’ve always wondered over the years—you had an excellent slide here, by the way—why the ratios of the tensile strength numbers being high didn’t really relate to the control, on the control

D’Angelo and Anderson 203 being 800 tensile strength and then the ratio failing but the failing tensile strength would be 800 or higher because the original in that failing sample would be higher. Nobody’s ever really addressed that. I’m just a kind of dumb old country boy who doesn’t understand a lot of this so I appreciate your letting me be here. I do understand mathematically that if you take the square feet of a ton of mix and measure it for the area, 5 microns will cover that substantially. But anyway, I’d like answers to the other questions. A—John D’Angelo In the development of the tensile strengths ratio, that was one of the things that was a relatively easy test, the indirect tensile strength of a mix. You can do it with some very simple equipment and you get a result. One of the best ways to look at the response of the material between an unconditioned and a conditioned was to evaluate the ratio between the strengths. The next issue is how do you then add the effect of the overall total strength of the mixture. To do this, it becomes a lot more complicated. Some states have put minimum strength requirements on the mix, so that if you don’t get a minimum strength, you won’t pass the test. No one has figured out how to really address that issue of total strength, unless you go into some of the other criteria. You’re looking at things like modulus, which then makes the test much more difficult to run. Q3—Dick Root, Root Pavement Technology Just a quick comment. When we start fooling around with allowing a reasonable level of air voids to work with and you start taking absolute tensile strengths, then if you did have 6% air voids versus 8%, you had noncomparable results. So we ignored absolute tensile strength and looked at the ratios for that very reason. Q4—Bob Humer, Asphalt Institute First of all, John, thank you very much for an excellent paper. I really appreciate your stressing some of the very basic points as a first line of defense against moisture sensitivity. If we can’t do those basic things right, then maybe after that there is some chemical stuff we can look at. Especially stressing good mix design, good compaction, and proper lift thickness to get compaction. One of your slides shows the minimum lift thickness and a maximum lift thickness. The minimum we’ve talked about, so we don’t have to argue about the three times the nominal maximum aggregate size. But you have a maximum lift thickness there of five times the nominal maximum aggregate size. Where does that come from, and why is there such a maximum limit, other than for compaction energy? Why would there be such a tight limit on the maximum lift thickness? A—John D’Angelo On the maximum of five times the nominal aggregate size, that’s typically the relationship for coarser-graded mixes. What you have there is when there is a lot more of the stone or the stone content as a larger percentage of the materials, you have to be careful. When the lift thicknesses for coarse-graded mixes get a little bit too high, they’ll have a tendency to push around some. It’s almost like pessimum voids with permeability. If you get too much lift thickness in these very coarse-graded mixes, they’ll have a tendency to shove around significantly and even uncompact themselves, so you have to be careful with that. Again, it depends on the type of mixtures you have. I know the French typically use a dense-graded mix in a lot of the work that they do. They actually go up to seven times the normal maximum aggregate size. However, they don’t usually use these very coarse-graded mixes. The contractors won’t want to work in France,

204 Moisture Sensitivity of Asphalt Pavements: A National Seminar because I guarantee you, as contractors, you’d hate it. The French will tell you exactly the lift thickness, the type of roller you’ll have, how many passes to make to compact the mix, and then they’ll hold you to the compaction. Q5—Don Goss, Valero Thank you, John, for your presentation. I thought you included a lot of good information. I just have a couple comments to make. One, earlier in the presentation you mentioned asphalt as the bottom of the barrel of the crude, and you implied that maybe it was waste material, and that hurt, John! A—John D’Angelo I would never really consider it waste. Q6—Don Goss, Valero Just for the record, there are other uses for the material—as a base oil in marine fuel, of course, in roofing products, and as coker feed, which would create a higher fuel yield from the barrel of crude. So, I just wanted to make that comment, and say that with the development of a lot of the recent tests that reflect the fundamental engineering properties of the binder, many of us who produce asphalt consider it no longer as a by-product but as an engineered product in its own right, and we think it’s a very valuable material. I guess on a little bit more serious note, with respect to testing the binder with everything in it, I think you make a good point in respect to grading the binder. With respect to reflecting moisture sensitivity, I just want to comment that I think it is important that we test the mix because the binder may not contain everything that’s going to impact moisture sensitivity. Thank you for allowing me to comment. A—John D’Angelo I agree with you wholeheartedly. Basically, I tried to stress through the whole presentation that you want to test all the products, and more important than anything it’s the final product, the hot- mix asphalt on the roads, that’s key to test. You’ve got to look at the details of the components, but most important of all, you have to look at how they go together and what the product looks like as it’s going to be used. Q7—Dale Rand, Texas Department of Transportation Two quick comments. We’ve seen over the last couple of years a problem with the TSR, particularly with the polymer-modified asphalts. For example, you can have a wet strength that is 150 psi and yet the mix still fails the tensile strength ratio. This has been a big problem and a big frustration. I know from the industry side and from the TxDOT side trying to get tests that pass when we are at the same time pushing the use of more and more polymer-modified asphalts in the applications for high-traffic areas. When you take a test that’s got 25% variability and you start adding all these polymers and lime to it and all these other additives, it’s been a big frustration for us. So for whatever it’s worth, we made the decision never to run that test again and we had zero opposition from industry or TxDOT. We were waiting for somebody to say, “Wait, you are doing the wrong thing.” The other comment I wanted to make was on your concern about the Hamburg being too severe a test. I’d go back to what you said about one size does not fit all. With the Hamburg and what we are doing now, one criterion does not fit all also.

D’Angelo and Anderson 205 You really have to look at it based on the PG grade of the asphalt. Anyway, I just wanted to comment on it. A—John D’Angelo Thank you, Dale. I’m not trying to attack the Hamburg. I used tensile strength for the slides because that’s the data we have for the most part. Though I’m not a fan of the Hamburg test, I don’t think that the TSR is the answer, but it’s what’s being used today predominantly because we have a lot of data on it. The ultimate test will have some kind of mechanical action. You’ve elected to use the Hamburg and work with it. That test has its problems, too; that’s why I want to continue to look for something new. Of course, my thing is that I’m sort of into research and technology transfer. I’m always looking for something new; nothing is good enough for me. I always have to find something new. Otherwise, I’d be out of a job or I’d be bored. Q8—Gayle King, Koch Pavement Solutions Run your 1.2% acids through the Hamburg. I predict you won’t like the results. A—John D’Angelo I’m not saying they are good or bad. Go ahead, Tim. Q9—Tim Aschenbrener, Colorado Department of Transportation I had a question regarding the pavement design, and I didn’t see it covered in this area, but I think it’s really critical. I was wondering if you could make a few comments on the importance of an aggregate base course. A—John D’Angelo Are you talking about just the general graded aggregate base? Q10—Tim Aschenbrener, Colorado Department of Transportation Yes. Our asphalt industry conducted a survey of the 10 best-performing asphalt pavements in Colorado and came up with a series of lessons learned. One of the common features in all those pavements was the existence of an aggregate base course between the subgrade and the asphalt pavement. In areas where we constructed full depth asphalt on the subgrade, we continuously found severe moisture damage at that interface. Where aggregate base course existed, it did not. So when repair is needed to the full depth asphalt, it is extremely expensive. So I think one area that is critical in the pavement design is to ensure that good-quality aggregate base course is in place. A—John D’Angelo There has been a lot of discussion on specifically base type and moisture damage. Should the pavement be full depth asphalt with a black base or a thinner asphalt layer with an aggregate base? To address the problem with moisture, an asphalt permeable base that’s the drainage layer to make sure you don’t have the moisture that’s being brought up from the subgrade, which causes significant problems, was developed. That’s one of the approaches taken to address that problem. Then again, even with aggregate bases, you run into problems with drainage. You have to be sure you have a good drainage layer to get that water out of that base or it can cause significant problems, either full depth asphalt or aggregate. There are different ways to tackle any one of these issues. I don’t know if the issue is if it’s good to have an aggregate base. I think

206 Moisture Sensitivity of Asphalt Pavements: A National Seminar it’s better to not have a lot of moisture sitting in a layer that has high stresses, and if you are at the bottom of that asphalt layer, that’s where the stresses start to develop. Probably some of the aggregate bases are reasonable in making sure that layer doesn’t stay saturated continuously, I would guess. Q11—Bob Rea, Nebraska Department of Roads We use a lot of liquid antistrips and plan on using them a lot more in the future also, but we also agree with your concern that early on we saw a lot of the tensile strength ratios get much tighter with the liquid antistrips, but at the same time they were lowering the tensile strength of the mix. Just wondered if there are some threshold values that one would look at for a minimum tensile strength and then use a ratio from there or anything like that. A—John D’Angelo Well, there is not really a minimum ratio. I think Jim Anagnos sort of talked about that. Originally, a lot of these liquid antistrips would soften the asphalt and you would get better ratios, but they actually softened the asphalt, and that is part of what caused the problem with lower tensile strengths. That’s why if you are going to use liquid antistrips, it is critical that you test the binder for the binder properties to make sure it meets specs with the liquid antistrip in it, to make sure you didn’t soften the binder. He showed several slides earlier where the newer materials don’t do that anymore, but that’s based on a limited study. I would imagine there are some suppliers out there that are supplying things that will cause problems. To avoid that kind of problem involves more than just setting a minimum value for a tensile strength ratio. It is to make sure that binder you are testing to meet a certain stiffness value has the amine in it to make sure it’s really not reducing that strength.

TOPIC 6 Production and Construction Issues for Moisture Sensitivity of Hot-Mix Asphalt Pavements