| April 01, 2010 |
Inside alternate bidding
Missouri Department of Transportation formula creates competition and saves money, too.
By Daniel Brown, Contributing Editor
This definitely keeps people on their toes,” says John Donahue, P.E., construction and materials liaison engineer with the Missouri DOT. “We are getting more bidders per job, it stimulates more competition, and it has saved the state millions of dollars.
He’s talking about alternate bidding, which pits two or more equivalent designs against one another in a competitive environment. In Missouri, MoDOT designs equivalent new pavements in concrete and asphalt – both for 45-year design lives – and takes bids on both for the same project. The low bidder gets the work, whether it’s asphalt or concrete.
Missouri has awarded more than $2 billion in construction using alternate bidding and leads the nation in the practice. Other states, including Louisiana and Ohio, also use alternate bidding, but not to the extent that Missouri has.
MoDOT figures that an asphalt pavement will need milling and filling of the driving lanes at year 20, and another mill-and-fill across driving lanes and shoulders at year 33. For concrete, MoDOT assumes a major rehabilitation at age 25. That means replacing 1.5 percent of the panels and diamond grinding the entire surface.
For comparison purposes, the state adds a life cycle cost adjustment (LCCA) factor to the asphalt bid. The adjustment factor is calculated by taking the present value of asphalt’s two mill-and-fill jobs and subtracting the cost of the concrete repairs. That number, the LCCA, represents the future cost of the repairs as discounted to the present using the current discount rate of the federal Office of Management and Budget. And the future cost of the repairs is estimated using current unit prices for asphalt, cement and aggregates.
If the asphalt construction bid plus the LCCA is the low bid, then asphalt gets the job. But if the asphalt bid plus the LCCA exceeds the low concrete bid, then concrete gets the job. That has happened only four times over more than 160 projects in six years, Donahue says.
“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.”
— John Donahue, P.E., construction and materials liaison engineer, Missouri DOT.
Input from both industries
MoDOT’s decision to use alternate bidding came from meetings with pavement team members. The team consisted of MoDOT personnel, representatives of both the concrete and asphalt industries, and personnel from the Federal Highway Administration.
“We were looking for a common platform, rather than having two separate design programs that handled asphalt and concrete, and then trying to determine if they were equitable,” says Donahue. MoDOT settled on using AASHTO’s Mechanistic-Empirical Pavement Design Guide (ME-PDG). It uses the same traffic and environmental data for both asphalt and concrete pavements.
The decision to use the ME-PDG went hand-in-hand with MoDOT’s move to alternate bidding. Regarding the ME-PDG, “There was obviously a lot of discussion about what stress thresholds would be acceptable levels for designing in both asphalt and concrete,” Donahue recalls.
Eventually MoDOT decided to consider just two distresses for asphalt and two distresses for concrete. For asphalt, the two were rutting in the HMA layers and fatigue cracking. On the concrete side, MoDOT would look at faulting and fatigue cracking – the percentage of slabs cracked.
“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.”
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.
If the situation calls for the 18-inch rock base and the grading contract is separate from the paving contract, then the grading contractor places the rock, but not the cap stone, which is done in the paving contract.
“In those cases the designer will set the profile for the thicker pavement, because you can’t shape rock base,” says Donahue. “You can’t blade that off.” So if an asphalt contractor gets the project and his design is thicker, at 10 inches, he simply places the cap material and paves with asphalt.
But if a concrete contractor gets the job, he has to place the cap material plus 2 inches of stone and then pave the 8 inches of concrete. By doing that, either profile reaches the same elevation. “If the award goes to concrete, it’s up to the concrete contractor to add the 2 inches of crushed stone,” says Donahue. “It’s on them.”
For the other two bases – the permeable treated base on the crushed stone, or the 4 inches of crushed stone – the material must be imported. So that material is included in the paving contract.
“So because the paving contractor is placing the base and the pavement, we set the subgrade elevation for the thinner pavement,” says Donahue. “If it goes concrete, then that contractor just places the base on the finished subgrade, then paves it. But if an asphalt contractor gets the project, he has to blade off the subgrade, which is doable, because it’s a fine-graded material. Then the asphalt contractor cuts the subgrade down by whatever the difference in the pavement thicknesses is and places his base on top of that.”
Naturally, if grading and paving are both included in one contract, the winning contractor will control the subgrade elevation because he is committed to one pavement type or the other. v
How They Do It In LA
For several years now, Louisiana has done an alternate design-alternate bid (ADAB) process on projects where the difference in life-cycle costs between concrete and asphalt is less than 25 percent. “If the difference is greater than 25 percent, then we only bid it in the less-expensive material,” says Jeff Lambert, pavement design engineer with the Louisiana Department of Transportation and Development.
The state’s ADAB model adds a factor “C” that represents costs for future rehabilitations and user delays associated with a contractor’s base bid, called “A”. The “B” component is time-based bidding that may also include an incentive for early completion. The state transportation department calculates the “C” component and awards the contract to the company with the low sum of A + B + C.
For asphalt, the “C” component includes two mill-and-fill rehabs over 40 years. Similarly for concrete, two rehabs are calculated into the life cycle cost over 40 years. At 20 years, joints are cleaned and resealed, and some patching and shot blasting is included. Additional patching and surface retexturing is figured in at year 30, says Lambert.
“We’re sure that both industries accept the fundamental principles of it,” says Lambert. “They appear to be happy with it; they’re still bidding on our projects. It has increased the number of bidders.” He says Louisiana uses the ADAB model on most larger new construction and reconstruction projects.
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