Concrete Products

MAR 2016

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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58 • March 2016 TECHNICAL TALK BY TOM KUENNEN At TRB an innovative process was described that accelerates slab replacement in concrete pavement rehab projects, increasing produc- tivity while reducing construction time. The process involves the use a system of temporary and reusable precast panels, and self-consol- idating concrete (SCC) mix, write Jamshid Armaghani, Ph. D., P.E., Global Sustainable Solutions, LLC; Kamal Tawfiq, Ph. D., P.E. and Ste- ven Squillacote, FAMU-FSU College of Engineering, Tallahassee; and Michael Bergin, P.E., Florida Department of Transportation, Gaines- ville, in Temporary Precast Panels and Self-Consolidating Concrete for Accelerated Slab Replacement. "Slab replacement is the main activity in most concrete pavement rehabilitation projects," they write. "Cracked slabs are replaced par- tially or fully with new concrete as part of the pavement rehabilita- tion project to extend service life of the pavement." Due to the short construction window, high early strength is specified for the concrete for the replacement slabs, they add. That's because the construction window is limited to only a few hours, during which the lanes are closed to accomplish this and other tasks. "In urban streets and/or highly congested roads and highways, the lane closure time for slab replacement may range from eight to 10 hours, and mostly, during nighttime," Armaghani, Tawfiq, Squillacote and Bergin say. "The short construction window limits the contrac- tor's production rate for slab replacements." In response, precast concrete pavements have been used in reha- bilitation projects as permanent replacements or overlays for long continuous sections of concrete pavements, or in isolated individual or group slabs, they write. The technology includes precast post-ten- sioned slabs for continuous sections, or precast reinforced panels for applications in isolated individual or consecutive slabs. But there is much room to expand usage of precast slabs for highway construction. Likewise, self-consolidating concrete (SCC) has not seen wide use in pavement construction, although it's used in structural work like bridge foundations. To facilitate use of precast panel tech- nology in pavements, the researchers simulated how panels could be combined with SCC to build pavements under traffic. "The research objective was to speed up construction of replacement slabs, increase contractor productivity during lane closure periods, and shorten the time for maintenance of traffic and overall construction," the authors observe. "Other potential benefits from the research include mitiga- tion of premature cracking and cost savings." An SCC mix was developed to satisfy high workability for rapid discharge and casting, and meet high early strength of 2,200 psi required by the Florida DOT for lane opening. Several trial mixes were prepared and tested for workability and strength to arrive at the final SCC mix design. Field demonstration of the system of temporary precast panels and SCC mix for slab replacement was conducted in a test pit at a dedicated test track in Green Cove Springs, south of Jacksonville. A 4,000 psi concrete mix was prepared in a commercial batch plant and cast around the test pit to simulate an existing concrete pavement around a removed distressed area—the replacement slab pit—of a pavement. Two precast panels were used in the evaluations. The panel dimensions were 6- x 12-ft. x 8-in. Each panel had two reinforcing layers; in the first, #4 bars were used for reinforcement, and #5 bars in the second. The reinforcing bars were spaced at 12 in. on centers in both directions. Also, four lift anchors were embedded in each slab and were fastened to the reinforcement. The planar dimensions of the two panels were approximately 3 in, shorter than the pit dimensions, which would create a 1.5-in. gap between each panel and the surrounding concrete. The perimeter of each panel was grooved and fitted with 1.5-in. backer rod foam to fill the gap between the adjoining sides of the panels as well as the gaps with surrounding concrete. A 60,000-lb. concrete pump truck was used to load the precast panels. After completing the load testing, the precast panels were left in place at the test pit in test track for a period of four weeks. A day prior to casting the replacement slab, the two panels were removed by the same excavator used in the original installation. The removal of both panels took less than 10 minutes. The SCC mix then was placed in the test pit. "It was not possible to achieve a smooth surface finish because of the lack of proper tools and professional finishers at the site as the mix started to settle and set rapidly," the authors say. "However, at the job site, with the availability of skilled labor and proper finishing equipment, the slab would have been cast and finished in less than 10 minutes. This rate will contribute to a much higher productivity compared to conventional concrete mixes." The authors conclude: • Due to robust design, the temporary re-usable precast concrete panels were highly durable during installation, and very stable in the slab replacement pit under heavy loading and breaking force; • The SCC mix, of Type I/II cement, 57 grade aggregate, silica sand, low w/c as well as a combination of HRWR, workability retainer and accelerator met the high workability requirements for rapid discharge and shorter casting time, and exceeded the 2,200 psi strength required by the Florida DOT for lane opening; and, • The SCC mix retained its high slump flow for almost 60 minutes without significant loss of workability. This makes it very adapt- able to batch plant mixing, and to truck mixer transport and dis- charge without segregation. REUSABLE PRECAST PAVEMENT PANELS PAIR WITH SCC FOR REHAB Top, typical pavement precast panel design for Florida experiment; above, SCC mix delivery for test track replacement slab pit. GRAPHIC, PHOTO: Armaghani, Tawfq, Squillacote and Bergin

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