Concrete Products

APR 2018

Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.

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Page 47 of 71

46 • April 2018 TECHNICAL TALK BY TOM KUENNEN The annual Transportation Research Board meeting in January is one of the highlights of the pavement and bridge research commu- nity. This year's event drew over 13,000 transportation engineers and specialists from the United States and other countries to Washing- ton, D.C. Delegates could audit 5,000-plus presentations in over 800 sessions addressing topics in transportation construction, including projects based on precast/prestressed concrete structures and meth- ods. Concrete Products was there and this month presents a report on new research findings in precast/prestressed research at TRB 2018. We looked at ready-mixed and cast-in-place concrete previously, see March 2018, pages 54-57. For more information, visit PRECAST PANELS EQUAL SUSTAINABLE INFRASTRUCTURE Precast/prestressed concrete panels for the repair and rehabilitation of existing pavements offer significant benefits to the environment, say David Merritt, The Transtec Group, Inc., Austin, and Sam Tyson, P.E., Federal Highway Administration, in their 2018 TRB paper, Sustainable Pavements with Precast Prestressed Concrete. Benefits include rapid con- struction during nighttime and other off-peak hours with minimum disruption to traffic, and high-quality, durable concrete pavement offering long-life performance with little or no maintenance, they note. "Precast prestressed concrete pavement (PPCP) applications on repair and rehabilitation projects throughout the United States during the last 10 years have firmly established its ability to deliver these important aspects of sustainability," Merritt and Tyson write. "Projects constructed in California and Texas have been in service for more than six and eight years, respectively, [and] required no maintenance to date, providing initial support for the competitive life cycle cost of the PPCP system that was anticipated during design and construction." In 1998, FHWA started a feasibility study examining PPCP as a method for expediting rehabilitation and reconstruction of existing concrete pavements. It produced a concept for such pavements which has since been implemented on projects throughout the U.S., and features sustainability benefits realized in the design, manufacture, construction and life cycle of PPCP. • Existing subgrades utilized. "A number of sustainability benefits are realized through the design of PPCP," the authors write. PPCP utilizes full-depth precast panels installed over the existing subbase and sub- grade whenever possible, they say. Full-depth panels eliminate the need for any additional paving operations on-site, like asphalt or concrete overlays. "By utilizing the existing subbase and subgrade whenever possible, the additional step of subgrade and/or subbase reconstruction and the new materials, construction processes, and disposal of existing materials associated with it are eliminated," Merritt and Tyson observe. Also, standardization of panels permits a much more efficient process for the fabricator, by allowing the panels to be mass-produced to the extent possible, they add, noting: "Standardized panels also facilitate efficient storage and shipping procedures since all panels of a particular type can be interchanged." •  Manufacturing offers additional environmental benefits. "Sup- plementary cementitious materials, such as fly ash recycled from coal combustion, are commonly used in precast concrete elements," Merritt and Tyson say. "PPCP projects completed to date have been produced using concrete mixtures containing up to 25 percent fly ash replace- ment of cement. Panels are produced in existing plants which produce a variety of precast elements. This eliminates the need to set up batch plants at or near the project site specifically for the paving concrete." "Established precast plants with diverse product lines also readily reuse formwork for various types of precast elements," they add. "Man- ufacturing of new formwork for PPCP panels is generally restricted to only the side forms, which in turn can be re-used for other precast elements. Finally, while all concrete batch plants generate a certain percentage of waste material, precast plants seek to minimize any waste and readily reuse waste that is generated. It has been estimated that approximately 2 percent of concrete generated at a precast plant is waste, but approximately 95 percent of this waste is beneficially reused elsewhere in the plant or separated back into constituent materials for use in fresh concrete." •  Sustainable construction. This is primarily realized through the reduction in traffic congestion during construction. "This benefits the contractor, owner agency, and the traveling public," Merritt and Tyson affirm. "PPCP permits construction to be completed during non-peak travel times, such as at night or over weekends, thereby greatly reduc- ing or even eliminating construction-related traffic congestion." •  Life cycle sustainability benefits of PPCP are realized through the durability of the finished product. PPCP is utilized as a long-lasting, low-maintenance product, and not as a temporary fix. Minimal mainte- nance over the lifespan of the pavement reduces environmental impacts. In November 2009, the Virginia Department of Transportation com- pleted construction of a PPCP project on Interstate 66 near Fairfax City. It demonstrated the precast pavement concept for overnight pavement reconstruction in order to reduce construction-related traffic congestion, but under high traffic volumes and shorter nightly closures. The existing pavement slab thickness was 9 in. in the three inside lanes and 11 in. in the outside 15-ft. lane. Although reconstruction with conventional cast-in-place concrete pavement would have likely required a slab thickness of 10 to 12 in., PPCP permitted the use of thinner panels so it was not necessary to excavate any underlying base or raise the pavement profile to accommodate the additional thickness. The precast panel thickness was specified as 8-3/4 in. The panels were produced on a long-line fabrication bed with up to three 27 ft. or five 12-ft. panels cast at a time. "In general, one set of panels was fabricated every day or every other day, with the exception of joint panels, which required additional form set-up time," Merritt and Tyson explain. "A concrete mixture with approximately 20 per- cent fly ash replacement of cement was used." Bar tendons were used for temporary post-tensioning of each sec- tion of panels. The bars provided the necessary clamping force for the joint epoxy and temporary post-tensioning force for the pavement to be opened to traffic each day. The two bars for each panel were coupled to the bars from the panels already in place, and tensioned simultaneously after the installation of every two panels. After the post-tensioning strands for each section (between joint panels) were installed and stressed, the stressing pockets where filled with a rapid-strength-gain concrete patch material. The post-ten- sioning tendons were then grouted, followed by grouting beneath the slabs using ports cast into each of the precast panels. Tie-bar slots were cut into the existing pavement at the beginning of the section to receive tie-bars from the first precast panel, and closure pours were required at the end of the section to transition back to the existing pavement. After all panels were installed, the pavement was diamond-ground for smoothness, and the expansion and longitudinal joints between lanes sealed. "The use of PPCP has clearly been demonstrated to be a practical solution for agencies needing to repair and rehabilitate existing pave- ments, especially in high-volume traffic situations where construction must be performed during nighttime and other off-peak hours after which all lanes must be returned to service for daily use," Merritt and Tyson conclude. "The precast concrete industry in the U.S. is fully capable of supplying this new market for precast products, and with the support of contractors and agencies that have utilized PPCP for repair and rehabilitation of high-volume roadways, it will quickly become a more commonly used solution." Continued on page 48 Precast/prestressed pavement, deck panels drive research at TRB

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