Wednesday, June 15, 2011

RAP Testing

Sorry to all of my readers about being MIA the last couple of weeks.  The busy season is finally upon us and it seems to have hit like a tidal wave.  Getting back into the swing of things I wanted to talk about sampling and testing recycled asphalt pavement (RAP).

There are three types of RAP that your plant might have stockpiled on site at any given time.  The first type is plant reject.  This is the tonnage that you waste at plant start up, returned mix, or any other mix that you just didn't have use for.  This type of RAP is typically higher in oil content than the other types of RAP because it is fresh and usually hasn't had the opportunity to weather.  The second type of RAP is reclaimed asphalt.  These are the giant slabs and chunks of road that are delivered to your plant.  This type of RAP typically has higher processing costs than the other two types of RAP because it must be crushed down and screened in order to obtain a RAP product that you can use in your hot mix asphalt.  The third type of RAP is millings (or grindings).  This type of RAP is ground up from the road and processed into a much smaller and sometimes very consistent product before it even leaves the construction project to be shipped to your plant.  With a certain amount of care it is even possible for the grinder to process the millings into a product that is 3/8" minus and can be fed directly into your hot plant without additional processing.   

Both millings and grindings can be easily contaminated with other products such as base rock, metal, organics, and/or paving fabric, all of which can reek havoc on your equipment and/or finished end product.  This is why quality control starts as soon as the truck of RAP pulls into your plant.  Before allowing a truck with RAP product to dump on your stockpile, it is important to inspect the load.  If you see any of the things that I mentioned above, it would be wise of you to either reject the load or to have them dump it in another area besides your clean RAP stockpile.  If you accept contaminated loads as part of your business, I would suggest having at least two different stockpiles of RAP.  At a minimum you should have a clean RAP pile and a contaminated RAP pile.  In a perfect world, aka if you have the space, you would have a pile for your plant reject, a pile for clean grindings, a pile for clean reclaimed asphalt, and a pile for contaminated RAP of all sorts.  You could even go as far as having separate piles for materials you received from large jobs.  Material that was all removed at the same location has a high chance of originally all coming from the same plant and having a very consistent grading and/or oil content.  This could greatly help you later on in your testing and HMA production. 

Chances are that unless you had a really kindhearted contractor grinding your RAP, you will have to use additional processing to create a RAP product that you can add to your hot mix asphalt.  If you have smaller grindings or plant reject, you may only have to run the RAP over a screen to break up the chunks.  If you have larger pieces like in reclaimed asphalt you may have to crush it down first and then run it over a screen.  Whatever process you decide to use, you will eventually end up with what looks like a darker colored aggregate pile.  You can sample this pile just like you would any other aggregate pile.  However, be aware that in extremely cold, extremely hot, or wet weather your RAP pile might want to stick together.  This is normal but your loader might have to work the pile a little bit more to break the chunks apart.

As a rule of thumb it is recommended that you sample your RAP once every 1000 tons.  You should, at a minimum, run gradation and oil content tests in order to know for sure that the quality of what you're putting into your hot mix is not varying enough where you would have to make a plant change.  There are two recommended ways to test the oil content of your RAP samples.  The first is by the extraction method.  This is the most accurate for testing the oil content of RAP but is time consuming and is difficult to use as a quality control procedure during production.  What most companies do is take 3-5 samples around their RAP pile in various places and use the extraction method to determine the oil contents.  You then average these oil contents to determine a base oil content for comparing your ignition test results to.  The ignition oven is the second type of oil content test that is used for RAP.  The RAP sample is tested the same as a traditional HMA sample in the ignition oven.  However, since you cannot create a correction factor in the traditional way, you must assume one.  This is where the extraction average from before comes into play.  The test results from the ignition test are compared to the average extraction oil content to determine if the oil content is varying in production so that the asphalt plant operator can make a change to the virgin oil content if necessary. 

In regards to gradation, it is possible to run a gradation test on RAP in the same way that you would run it on a virgin aggregate.  However, know that this will leave asphalt binder residue on your equipment and may blind your smaller sieves.  More often a laboratory will use the aggregate leftover from the extraction test or ignition oven to perform a gradation. 

Caltrans test method LP-9 is a procedure for testing RAP for use in Hot Mix Asphalt and can be found at the Caltrans Lap Procedures Website

Does your plant test RAP differently?  Do you have any tips or tricks for sampling or testing RAP?

1 comment:

  1. We check our RAP on a daily basis because we have a job that always runs at 50% RAP. We also check our cold feeds from the asphalt bunkers prior to the start of production. As for the actual sampling of the material, I usually ask the loader operator to assist me by making a small pile for me to pull from. I have only been in the industry for about a year now and one of the most important things that I have found is that proper sampling and proper sample preparation techniques are crucial to quality control. Anyways, I enjoy reading the blog...keep up the good work.