“What is nice about the ME-PDG is the fact that you input the same traffic data and environmental data, and you really have three main working components of the ME-PDG,” Donahue says. “You have the traffic data analysis and the integrated climatic model which simulates what’s happening to the pavement profile over the design life.

“The third component of the ME-PDG is the mechanistic analysis. For asphalt, the inputs are the mix components, the volumetrics, the binder grades and so forth. For concrete, the inputs are factors such as cement content, strength of the concrete, dowel bars and the like. Then the ME-PDG uses modeling software and transfer functions to predict levels of distresses in the designed pavement.

“For asphalt, we want to make sure that the level of rutting or cracking does not exceed a certain level over the design life,” says Donahue. “And for concrete, we want to make sure that faulting and cracking don’t exceed a certain level at a certain time.”

Alternate bidding for rehabilitations

For major rehab jobs, MoDOT uses fixed designs. Pavements get either an 8-inch unbonded concrete overlay or a 12-inch asphalt overlay on rubblized concrete. Those two designs are bid as alternates. “If we actually designed each of those we could possibly increase or decrease the thickness a little bit, depending on what traffic or the environment presents. But we don’t do that,” says Donahue.

“Part of the reason is, we have kind of a tenuous balance with each industry when comparing those designs,” he says. “So we didn’t want to tinker with the rehab side too much.” Those thicknesses were derived using the 1993 AASHTO

Design Guide combined with best practices data from other states.

MoDOT has also done some alternate bidding by proposing only asphalt overlays on asphalt or composite pavements. If the state wants to effectively do a 3.75-inch overlay on such a pavement, that would normally consist of two layers of asphalt: a 2-inch binder course and a 1.75-inch wearing surface.

There are three fairly equivalent ways to do that. You can require a leveling course, then place the two lifts of asphalt. “Or, we may allow them to mill off 2 inches and place 2 inches back, followed by the 1.75-inch wearing course,” says Donahue. “The third possibility would be to do hot in-place recycling the surface and then place a 1.75-inch overlay on that. So there you have three different scenarios that may play to the strength of one contractor or another. From our point of view, we’re getting 3.75 inches of new, or new-quality material.

“We also have done some conventional asphalt versus concrete bidding with thinner overlays,” says Donahue. “We did a couple of projects where we had a 5-inch unbonded concrete overlay bid against a 5.75-inch asphalt overlay. In one case it went to concrete and another time it went to asphalt. The way it pans out depends heavily on the volatility of asphalt prices at the time.”

For projects of less than 14,000 square yards of discontinuous paving, or less than 7,500 continuous square yards, MoDOT does not apply the life cycle cost adjustment factor. “If it’s less than that we’ll just call it optional bidding and not apply the adjustment factor,” says Donahue. “The contractor just picks one material or the other.”

Missouri estimates that an asphalt pavement will need a mill-and-fill rehab in year 20.

MoDOT has three different base designs that affect the overall thickness of the pavement structure. One is an 18-inch shot rock base, with fragments up to 12 inches in length. The material must be approved by a state geologist. Usually that material is available on a job site that includes unclassified excavation, especially in the southern half of the state. On top of the shot rock, MoDOT specifies a cap stone, or finer crushed aggregate, to give the paver a flat platform.

Another base is a 4-inch crushed stone base, usually used in the northern half of the state, which generally has no rock available on site. The third base, for interstates or high-volume freeways, consists of two layers. The top is a 4-inch open-graded permeable treated base. It’s stabilized with asphalt or cement. Under that is a 4-inch crushed stone base that serves as a filter layer.