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Abstract
Accurate determination of asphalt content and aggregate gradation is critical for controlling quality of asphalt mixtures during construction. Most state specifications require quantitative evaluation of the asphalt content of mixes as a criterion for acceptance. The ignition oven test procedure specified in AASHTO T 308 (Standard method of test for determining the asphalt binder content of Hot Mix Asphalt [HMA] by the ignition method) is required or allowed by most state DOTs for determining the asphalt content and aggregate gradation of asphalt mixtures. The ignition oven test specified in AASHTO T 308 procedure requires the determination of asphalt content correction factors for each asphalt mix and for each ignition oven used. However, in some instances when numerous asphalt mix designs and several ignition ovens are available, correction factors (CFs) are shared between ignition units, even when that practice is not allowed by the standard. There is a need to identify the consequences of sharing correction factors between units/mixes and also to identify testing parameters that affect the measured CFs. By identifying possible causes of variation, the test procedure could be adjusted to make the CFs more consistent between ignition ovens. Also, it should be possible to reduce the amount of difference in CFs between all types of equipment. A study was conducted to assess the variability of ignition oven CFs for different ignition oven unit brands and mixes to better understand the implications of sharing CFs. Twenty-three laboratories used various brands of ovens to test four mixes containing aggregates with varying CFs. The results indicated that CFs were significantly different for the different mixes even when the same unit brand was used. The within-lab and between-lab precision developed in this study suggests that different precision statements are necessary for aggregates with high breakdown potential and that the current precision included in AASHTO T 308 was likely developed for low weight loss aggregate making it unacceptable to use for aggregates with higher CFs. It was also found that the addition of lime caused no significant difference in the measured asphalt content when the CF was correctly measured. However, it was cautioned that a change in amount of lime during mixture production would affect the measured asphalt content. In addition, ways to minimise variability in asphalt CF were evaluated. It was determined that ignition tests conducted at lower temperature (427°C) proved to be effective in reducing the variability in measured asphalt content since the lower temperature reduced the asphalt correction factors for asphalt mixes.
Acknowledgements
The authors would like to thank Dr Edward Harrigan and the project panel for their guidance throughout the project.
Disclosure statement
No potential conflict of interest was reported by the authors.
Discussion
The work by Kowalski referenced here was done in our lab and I worked on it. I want to share a little of what we learned. Karol Kowalski put thermistors in the top and bottom baskets and found that when the asphalt ignited, the mix in the top basket was getting overheated. So, one of the recommendations he made, aside from lowering the temperature, was to reduce the sample size and only put the sample in the bottom basket. After we did the work in the lab with laboratory-prepared mixes, we went out in the field and got real mixes, brought them back, and tried that technique, and it seemed to work. So, all of that is included in the final report on the project. But what I found since then in talking to some of the contractors who tried to use our method was it didn’t seem to work very well when you took fresh mix that was still hot and ran it. And I have no explanation for that. But as far as I know, the contractors are not using our method and the Indiana DOT hasn’t adopted it because of that problem when testing fresh mix rather than mix that’s cooled and then been reheated and put in the ignition oven. So, if anybody has any suggestions on what may be causing that, I’m really open to it. Because it looked like it was going to work and then when they tried it in the field, it didn’t.
So, what was the issue that they were having?
I was told that the residue after they thought the ignition oven had completed was kind of gummy and it hadn’t actually burned out all of the asphalt. I believe that’s the case. I know Pete Capon is here and maybe his folks tried that technique. But that’s my understanding.
Yeah, I’m not aware of that. This is just a portion of the study. We were trying to put just a little portion of that study in our paper, but we’ve run several tests in the lab and I’m not aware of any issues that they had. That’s the reason for our recommendation. We could burn all the asphalt without really significant change in the time it took …
Were you using fresh mix that was still hot or was it cooled and then …
Well, I guess probably cooled down some.
That’s the only thing I can think of that’s different
Brian probably has something to say.
I’m going to speak for my experience in Florida. An agency that has a PWL spec that uses contractor data for acceptance and the agency is verifying that data, there is a very good incentive to improve the test method to reduce the variability. One of the most accurate tests that we’ve found in Florida is asphalt content by ignition. You must do a correction factor on each specific mix. To tighten up the test procedure you should eliminate alternatives such as different oven types or different temperatures if in fact you're not going to be embarrassed as an agency when you verify the contractor’s data. Variability must be as low as possible. We have found that that is the best incentive to try and improve on any test procedure is if you have a PWL spec where the agency is verifying the contractor’s data.
Thank you. I guess the main issue … We are looking at ignition. As I mentioned in my presentation, if you have an aggregate that has a low correction factor, the procedure that we have right now works pretty well. But when you are dealing with these high-lows there is no way you can actually fall within that precision. That’s the reason you have to look for alternatives to improve it.
Florida’s native aggregate is a very soft limestone has very high correction factors, and we seem to be not having that problem.
Going back somewhat to Becky’s and your conversation, I think you and I had spoken in some of the quality assurance work we’ve done in contractors’ labs. We’ll run samples that are split out at the same time but run when one will be cooler and one warmer, and we have seen some bias in that type of procedure. I think one possibility that might be affecting what you see is if you have a room temperature sample some of the volatiles are likely driven off before the sample comes up to temperature. And so, you're doing more of a pyrolysis. There is another NCAT report that we did when we worked there where we used the Tempyrox oven, which is used to clean glassware, which basically limits the oxygen. And so, I think that may be part of the influence is that you’re just losing more of the volatiles. If you're already at 300 degrees, you have less temperature that you need to increase before the sample ignites and burns.
Thank you, Brian.
I very much enjoyed your presentation; very practical. I just wanted to share a little experience that we’ve had, particularly with correction factors being specific to the oven. In British Columbia, we did a correction factor at our office in the exact lab in the exact oven we were using. We then took it up to a mountain pass, a higher elevation, and we got completely different numbers. We realized the numbers were wrong, we redid the correction factor, and found that it was significantly different even in the same oven because of the relatively significant change in elevation. So, a little bit more word of caution with respect to how important it is to be specific to the correction factor.
So, were you using the same oven that you moved to that other location or just the same brand? Because what we also saw even if you're using the same brand …
No, it was the same oven in a mobile laboratory. We just thought it would be more convenient to do it at the office and then we took it up to the mountain pass and the results were completely different.
Okay. Thank you for your comment.