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.
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