Better Roads Staff
For the rigidified FRP tube arches described above, the SCC mix incorporates HRWRs to achieve enhanced flowability, and VMAs to achieve stability, eliminating aggregate segregation. The mix also includes set retarders (for stabilizing hydration), shrinkage reducing admixtures, and 0.375-inch pea gravel aggregate.
In a September, 2011 technical paper from the Illinois Center for Transportation, University of Illinois at Urbana-Champaign, Transfer and Development Links in Prestressed Self-Consolidating Concrete Bridge Box and I-Girders, authors Bassem Andrawes and Andrew Pozolo said the American precast industry has taken significant strides to adopt SCC in commercial projects, though concern about early-age bond behavior has limited the material’s application in prestressed members.
To explore the application of SCC in Illinois bridge construction, Illinois DOT and the Illinois Center for Transportation sponsored a three-phase study investigating the bond behavior of steel strands in pretensioned bridge box and I-girders. In the first phase, 56 pullout tests were conducted to compare the performance of seven-wire strands embedded in SCC to that of strands in conventionally consolidated concrete blocks.
In the second phase, transfer lengths of prestressing strands in two 28-foot SCC hollow box girders and two 48-foot SCC I-girders were determined experimentally. In the third phase, development lengths of strands in the four girders were determined through a series of iterative flexural tests.
They found that pullout test results at various ages showed strand performance in SCC to be comparable with strand performance in the conventionally consolidated concrete.
I-girders were found to perform adequately in both shear and flexure even when the embedment lengths were lower than the predicted development length values, which ranged from 73.9 to 81 inches. “With satisfactory pullout behavior and adequate transfer and development lengths, it is reasonable to conclude that the SCC mixture in this study had sufficient bond to prestressing strands,” the authors conclude.
Defending Use of Fly Ash
The Environmental Protection Agency has taken aim at coal combustion fly ash used in precast and cast-in-place concrete, a move that seriously concerns the people who design and build bridges.
Fly ash is the residue of the burning of pulverized coal in thermal power plants. The ash particles are collected mechanically or by electrostatic precipitators. Fly ash is a pozzolan, meaning it is a siliceous and aluminous material that, in the presence of water, will combine with an activator (lime, Portland cement or kiln dust) to produce a cementitious material, according to Fly Ash Facts for Highway Engineers, a publication of the FHWA and authored by the American Coal Ash Association (ACAA).
Fly ash use on federal-aid highway projects was encouraged by its classification as a “recovered” product under the federal Resource Conservation and Recovery Act (RCRA), which generally mandates use of fly ash in cement or concrete in construction projects using $10,000 or more of federal funds.
The pending EPA classification of fly ash as hazardous waste has the potential to disrupt this accepted use of fly ash in the production of high-performance concrete. But legislation protecting fly ash was approved this October by the U.S. House of Representatives, and at press time awaited action by the U.S. Senate Environment and Public Works Committee.
Five Democrat and five Republican senators have filed the bipartisan Coal Residuals Reuse and Management Act (S.1751), creating national disposal standards for coal ash while protecting the material from a hazardous waste designation.
S.1751 is patterned after the bill of the same name that passed the House of Representatives in mid-October, with 37 Democrats voting yes.
Sen. John Hoeven (R-N.D.) observed that states can manage the disposal of coal byproducts with good environmental stewardship while permitting beneficial uses like building bridges, roads and buildings that are stronger and less expensive.
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