Better Roads Staff
“One obvious tactic is reducing the amount of Portland cement in the mix,” says Wathne. “We’ve been substituting industrial waste products like fly ash and steel slag for decades. A lot of our work now is aimed at stimulating wider adoption of these products by communicating to agencies what kind of substitutions are possible for various applications.”
When fly ash or slag resources are located near a project, the substitution of either product can substantially reduce concrete’s carbon footprint and its cost while enhancing or preserving its pavement qualities.
Like the asphalt industry, concrete interests work to convince agencies with prescriptive specifications for mix content to think more in terms of performance specifications. Wathne notes, for example, that the average DOT probably allows fly ash concentrations of 20 to 25 percent of the cementitious material, while others allow less, and others, like Delaware and Florida, use concentrations of 50 to 70 percent.
Wathne’s group is also trying to reduce costs and enhance sustainability by encouraging mix optimization and reduced pavement thickness.
Reductions in pavement thickness are expected to come from widespread adoption of AASHTO’s newest edition of the Mechanical Empirical Pavement Design Guide (MEPDG).
Mix optimization focuses on creating a combination of materials that allows the engineer to minimize the amount of cement in the mix. “The biggest factor here is aggregate gradation,” notes Wathne. “The goal is to find sizes and shapes that pack together in an optimal way, so you can use less ‘paste’ and thereby save on cost and carbon.”
Reductions in pavement thickness are expected to come from widespread adoption of AASHTO’s newest edition of the Mechanical Empirical Pavement Design Guide (MEPDG), the software version of which is known as DARWin-ME. The concrete paving industry, like the asphalt industry, hails the potential this pavement design discipline has for eliminating “over-designed” pavements and the waste they represent in unneeded materials, energy and cost.
Of course, cautions Wathne, some of those overdesigned pavements performed extraordinarily well when estimates of traffic loads turned out to be dramatically low, so improved practices in forecasting traffic loads is an essential part of the goal of efficient design.
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