For KDOT, the specification on air void content is 4%, plus or minus 1%. Air voids are calculated by determining the bulk specific gravity and theoretical maximum specific gravity—both lab tests on material sampled from behind the paver.
In-place density is typically determined using the nuclear density gauge. Density requirements in Kansas vary based on location within the asphalt pavement structure and the expected axle loadings. Typical density specs set a minimum of 91%.STRIVING TO BE THE BEST
Georgia, on the other hand, does not use statistical processes in QC/QA. “FHWA gave us a review in 2004 and told us that we're doing the right things, and the statistical piece was one of their recommendations,” said Georgene M. Geary, P.E., state materials and research engineer. “We're striving to get there.”
Geary said the state is working with its information technology department to develop statistical methods and formulas by which to evaluate materials test data. “Right now we mainly look at the limits in test data,” said Geary. “Our contractors do their own tests, and if our tests match theirs, then we use their tests in the pay decision.”
In Georgia, all asphalt plants that produce state mixes must be pre-qualified and placed on the approved list of plants. Producers must have quality plans, and laboratories must pass state inspections.
Contractors submit QC data through a computer program developed by the state, which verifies that the tests come from the right plant and that the right test data is in the correct place. “That's done by my technicians in the field,” said Geary. “We have 150 field people in seven districts, and I have specialty field people. We have 11 engineers whose specialty is asphalt, and the same way with concrete.”WAIT AND SEE
In Colorado, contractors are required to run their own QC tests, but they are not used for payment. The state has run pilot projects—one around 1997 and other around 2001—to evaluate the process of using contractor tests for asphalt payment, said Tim Aschenbrener, materials and geotechnical branch manager, Colorado DOT.
“It did not work well for us; it ended up in a claims situation,” said Aschenbrener. “The biggest problem was that we had disputes over the test results. The state showed different results than the contractor, so we used the state tests for payment. But interestingly, if we had used the contractor's tests, we would have paid less. The contractor wanted his test results to be used, which was odd. The contractor filed a claim that was settled for a fraction of what they claimed. It demonstrates the level of sophistication a contractor needs to have.
“We have taken a ‘wait-and-see' approach to the use of contractor tests for the pay decision,” said Aschenbrener. “We're very skeptical of it. There's a lot of potential for problems.”
He noted that FHWA is pressing for use of the PWL concept that Kansas employs, and said Colorado has used the PWL method on its own tests since the early 1990s. “We have some contractors who want to use their tests for acceptance, but they're aware of the potential problems and generally are accepting of our decision for now,” said Aschenbrener.