To that end, construction of PCC/PCC test sections took place over three days of paving, for a full-scale, 200-foot-long demonstration slab, and two full-scale, two-lane 500-foot-long sections along I-94 at MnROAD.
PCC on PCC sections to be constructed at MnROAD were designed to feature a 3-inch, high-quality exposed aggregate concrete (EAC) PCC lift over a 6-inch “low-cost” or recycled concrete aggregate (RCA) PCC lower lift. The term “low-cost” signifies that the PCC design was such that the lowest possible amount of cement and most inexpensive coarse aggregates were used by the contractor, and this reflects European practice.
The upper layer was specified to be placed between 15 and 90 minutes after the placement of the lower layer. This specification was in response to concerns of German and Austrian consultants to the R21 project and observations collected on a R21 scanning tour of European composite pavements.
“The general consensus among the research team was that the placement of the second lift – as soon after the first lift as possible – was important to eliminating problems that might be associated with the heterogeneity of the two concretes in the PCC/PCC pavement,” they state.
A Challenging Range of Problems
These problems include differential shrinkage, different rates of hydration, and the compound problem of bonding at the interface of the two PCCs. While the use of two pavers was an initial step to meeting this specification, there were other logistics that needed to be fulfilled to ensure the lifts were placed within a maximum of 90 minutes of one another, they say.
The R21 project included embedding thermocouples, moisture sensors, dynamic strain gauges, and vibrating wire strain gauges in the MnROAD sections. This effort involved considerable efforts in the installation, documentation and activation of these sensors before, during and after construction, the authors write.
Many members of the R21 project team had reservations over the use of alternatives to more conventional materials as constituents in the concrete for the lower lift of the PCC on PCC pavement, they say. These alternatives included the replacement of 50 percent of the coarse aggregate with the RCA, and the replacement of 60 percent of portland cement with a supplementary cementitious material (SCM).
While the paving operation planned to use two pavers in a manner similar to the European methods, the
MnROAD construction was to use one batching plant and not two. The use of one ready-mix plant was immediately recognized as a challenge to the project, both in terms of maintaining a consistent mix in alternating between batches and in terms of delivering both the upper and lower concrete mixes in a timely fashion.
“The R21 project elected to use an exposed aggregate concrete surfacing for the demonstration slab and mainline sections,” the authors write. “One initial problem that the project team resolved was the specified gradation curve for the PCC mix to be used for the upper lift. The challenge was achieving a gradation curve that attempted to meet the EAC standard observed by the R21 team in Europe, yet was within the means of the contractor and its ready-mix supplier.”
Paving at MnROAD began last April 28, with the construction of a 200-foot demonstration slab, and concluded on May 10 with the completion of 1,000 feet total of test sections along the mainline I-94 test area, say Tompkins, Vancura, Rao, Khazanovich and Darter. The two-lift paving used two GOMACO model GHP-2800 pavers and a material transfer device spaced between the two pavers, to place fresh mix for the upper lift.
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