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The Minnesota Department of Transportation has ramped up to schedule a test bridge deck project on the US Hwy. 75 overhead on I-94 in Moorhead this September. According to Steve Babcock, Concrete Engineering Specialist with MNDOT in Maplewood, "The high performance materials will first be installed in the tops of the bridge abutments for a test run. Then, with the information learned, the whole deck will be poured." He cautioned, "We have done a hand-full of projects like this around the state, not all with success. Two bridges have had to go through deck replacement due to clumping of the silica fume. We hope we will get it right on this project." Mr. Babcock was asked why they thought this mix was better than their current conventional mixes with the Minnesota standard thin deck overlay process. He noted the following, "We actually are not certain about the advantages; the jury is still out. But the possibilities inherant in the low permeabilities of this mix make the test case efforts worthwhile. Low permeability should yield long life". Roy Sander, Area Operations Manager at Aggregate Industries reports that the MNDOT has requested that they come up with a suitable mix design. He says, "We have received approval on a mix design using four bins which meet the 8-18 percent criteria for a Shilstone mix design. We will use two sizes of rock (3/4" minus & pea) and two of sand (coarse & regular) to attain the mix specifications. Plant No. 1(Moorhead) will be committed to the project, temporarily adjusting materials in the aggregate bins for the needs of this project." Sanders notes that they are accustom to multiple bin mixes because of the progressive demands for mix performance in Minnesota DOT projects. Steve Skyberg, Mix Design Specialist with Aggregate Industries, notes that total cementitious will target 611 pounds per cubic yard; 75% portland cement, 20% flyash and 5% micro-silica for 370 c.y. pour. He mentioned that admixtures rates will likely be unpredicable due to the silica fume, so will be added to the mixer at the bridge deck site. While high compressive strength is not a goal for this project, Mr. Skyberg thought the mix will attain 5000 psi compressive strength, well in excess of the 4000 psi design. Mr. Babcock confirmed the likelihood of high strengths; maybe even pushing 6000 psi. Concrete containing silica fume has a history of successful use, especially in the Maritime Provinces of Canada. There, bridges across salt water channels and tunnels have been built using silica fume to attain the desired 75 year design life. Low permeability of the concrete is the desirable trait when sea water and moist freezing environments is a problem. Traditionally, these projects have used silica fume at 8 to 9% of cementitiious with compressive strengths yielding 8000 to 10,000 psi capabilities. Concrete with silica fume addition is known for some unusual finishing traits. Of primary concern is the total lack of bleed water. Without the bleedwater at the slab surface, finishing procedures must be implemented immediately after placement. Also, probability of shrinkage cracking is high without the presence of bleedwater. Therefore, evaporation reducers are normally applied almost immediately, followed by the curing procedure. Fogging of the newly placed slab surface has also been recommended in some cases to minimize shrinkage cracks because of the extreme sensitivity of these mixes. Silica fume is available in three forms here in North Dakota. One is intermixed within the powder cement through a process normally accomplished at the cement terminal. Another is in a liquid solution marketed by the barrel or by the truck load. The third is a dry form package in 25 pound water soluable bags. Silica fume is remarkable for its light density. It is reported that a cement truck load (dry) of it would only weigh 8 tons. In particle size, it is comparable to cigarette smoke. Except for the new water soluable bag capabilities, this is the reason it has traditionally been handled in a mixture rather than as an individual product. This small size is probably also the reason it works so well in the concrete matrix, filling in voids between the larger particle sizes of the cement and fly ash. For further information, contact the North Dakota Ready Mix & Concrete Products Association office at 701-255-7250. David C. Sethre, P.E., the author of this article, is the Marketing Director for the North Dakota Ready Mix & Concrete Products Association. |