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Rejectable Air Voids (October, 2018)

Question:

We have recently completed a paving contract where a number of air void results are either borderline or rejectable based on Table 9 of OPSS.MUNI 310.

 Under 310.08.05 asphalt with rejectable air voids is covered, but there is no recourse for asphalt with multiple consecutive borderline air void results.

 Is there a reason borderline air voids are not dealt with in the same was as borderline gradation and AC in 310.08.04, where “A total of three borderline test results for the same attributes representing up to 5,000 tonnes of HMA production shall result in the work being deemed rejectable”?

Response:

This is one of the grey areas in the specification that are difficult to solve. The approach used in OPSS.MUNI 301 dated Nov 2017 is based on years of experience. On one hand we want to have the asphalt mixes as good as possible, but on the other hand there are so many variabilities during asphalt mix production that sometimes we have to live with borderline values. Air voids are one of the most difficult parameters to control. The plant operator has control over how much of each type of aggregate he uses and how much asphalt cement he adds to the mix. But he does not know too well what air voids he will get. So, this is why the requirements for air voids are not as strict as for AC content, for instance. Although, a few agencies develop special provisions where they state that three borderline air voids values will be deemed as rejectable. Of course, we don’t want air voids to be in the rejectable zone.

The best solution, in my opinion, is to:

  1. Have a good mix design with air voids at or very close to the target value of 4.0%; and
  2. The Contractor should run a trial batch before starting full production of the mix. During the trial batch the air voids should be checked and they should be as close to the mix design value as possible. Some fine tuning of the mix is typically required. Please keep in mind that the mix design is done at the lab while the real materials are used at the plant. By running a trial batch most surprises during production can be avoided or at least minimized.