High Asphalt Content Solutions

Last week I told you about what might be going on if you have asphalt contents that go back and forth from high to low and back again. This time I'm going to go over some reasons your asphalt contents might be consistently higher than they should be.

Again, for the purposes of this post I am assuming that you are using an ignition oven to test for asphalt content.


For verifying the lab, try these sources:

1. Is the technician calculating the results correctly? (technician error) 
2. Are you using the correct correction factor? (technician error) 
3. Is the correction factor current? Has the rock source changed in such a way that the correction factor should be adjusted? 

One way to double check your AC content results in the laboratory is to run additional tests in the lab.  The Rice test, bulk specific gravity of compacted HMA, AC content by Nuclear Oven, AC content by extraction, stabilometer, and calculating the sample's volumetrics can all verify your varying oil contents on their own or when compared to the charts in the mix design.


If you can't find an issue in your lab move on to the plant. 
Here are some sources of problems at a drum plant:

1. Are you using the correct mix formula?  (plant operator error)
2. Does you the aggregate moisture in your computer match the actual aggregate moisture?  If the plant is set lower than the actual moisture the computer will think that you are putting in more rock than you actual are and add more asphalt than you need per your mix formula.   One way to check this is testing the feed aggregates for their moistures and comparing the combined moisture content to what is entered in the computer.  You can also typically mitigate this in the laboratory by running a CT 370 test on every oil content you run.  However, I caution you with depending too much on CT 370 when running rubber mixes as CT 370 may not be effective with crumb rubber in the mix.
3. Are your aggregate belt scales weighing correctly?  If your scale(s) are reading high then you will have higher asphalt contents because the computer thinks that you are putting in more rock so it puts in more oil per your mix formula.
4. Is your belt scale binding?  You can see the same problems as in #3.
5. Is your asphalt meter calibrated correctly?  If it is reading high you will have the same issues as in #3.

Here are some sources of problems at a batch plant:

1. Is your weigh pot emptying completely with each batch?  If it is not, you may have some rich batches and some dry batches.  You should check your scales to make sure that they are weighing correctly.  Batch plants are typically very accurate in how much oil you are putting into the mix. This is because there is not as much risk as in a drum plant in how it is mixed because each batch is mixed on a much smaller scale.  One benefit to a batch plant is that most automated systems can give you a readout of the exact weight of each bin of material and asphalt that was added to the batch.  These reports can be extremely useful in seeing trends in your asphalt weights.
2. Is your weight pot calibrated correctly?  If it is reading low you will have a higher oil content.   
3. Is the aggregate fully coated with asphalt?  If not, it may be due to the oil injectors not operating properly.  There could be an electrical issue or leak.  Your technician may be consistently sampling from the rich side of batches, showing a consistently high asphalt content.  
4. Is the mix gradation cohesive or are some parts fine and some parts rocky?  Take a look at a truck of hot mix and be sure to dig into the bed a little bit.  If the pugmill is not mixing fully, the top half of the batch (the part you can see) could still look cohesive even though the bottom half may not look like the top.  Fine samples have more asphalt than coarse samples because the fines will absorb and hold onto more asphalt.  Improper mixing could be due to missing pugmill tips, poor paddle alignment, or you may just need to increase your mixing time. 
5. Is there and automation error?  Many batch plant automation systems have complicated equations that are used to determine how much aggregate and/or asphalt is added to a batch.  One of these equations is "free-fall compensation" which calculates gravity into the weight of the aggregates and/or asphalt in an effort to reduce the amount of time it takes to make a batch.  Instead of waiting until the scale meets its actual target value it will calculate the free fall compensation and drop the material into the pugmill a little bit early.  It is rare but possible that your computer is not calculating this correctly and if you run out of options you can always call the manufacturer and find out if it may be the cause of your high asphalt contents. 

There are sure to be several other reasons that you may be showing a consistently high asphalt content at your plant.  The key is to have an open mind and make sure that you keep the finger pointing to a minimum so that your employees feel comfortable letting you know what is wrong when they see it.  


Keep your eyes peeled for our next post on consistently low asphalt contents.


If you have any additional solutions post them on our discussion forum!  Trouble-Shooting