United States Department of Transportation
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Abstract:Supplemental cementitious materials (SCMs), such as fly ash, slag, silica fume, and metakaolin have become commonplace in Florida concrete, particularly for high durability or high-strength concrete. The Florida Department of Transportation (FDOT) currently allows fly ash to be used as a cement replacement at 18-50% levels and slag cement to be substituted at 50-70% when incorporated into binary blends for general use, mass concrete, or precast concrete products in extremely aggressive environments. Some precast concrete producers would like to use slag cement instead of fly ash. High volumes of slag cement in the concrete can reduce early-age strength gain. The research objectives of this study were to determine the maximum amount of slag cement that could obtain high enough compressive strength to detension prestressed concrete products in 18 hrs after concrete placement in FDOT class V and above mixtures, and the minimum amount of slag cement required to achieve comparable durability against chloride ingress to a control mixture containing 20% ASTM C618 Class F fly ash, and durable in a very severe sulfate attack environment. Compressive strength results showed that it is possible to achieve early-age strengths needed for precast concrete production, however some slags may require additional heat or accelerators to achieve such strengths. Accelerated curing of concrete gave bulk and surface resistivity results in between that measured for 56 and 91 days of curing at lab temperature. All mixtures tested for sulfate attack resistance that contained slag cement with a Type IL would qualify as high sulfate resistant cementitious systems. Based on the resistivity and sulfate attack test results, it is recommended that the minimum slag replacement level required for chloride durability in Class V, VI, and VII concrete be changed to allow slag cement replacement levels of 35% when used with Type IL cement.
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