Strength and Durability Characteristics of a 70% Ground Granulated Blast Furnace Slag (GGBFS) Concrete Mix

David Newton Richardson, Missouri University of Science and Technology

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For a bridge pier and abutment mass concrete project, three mixes were studied: an ordinary portland cement (OPC) mix (Type I PC) and two 70% by weight ground granulated blast furnace slag (GGBFS) mixes (Type II Low Heat PC). One of the slag mixes contained a high range water reducer (HRWR). Tests for compressive strength, freezethaw durability, rapid chloride permeability, and salt scaling were conducted on field samples. Results showed that the strengths of the slag mixes were lower than the OPC mix at all ages up to one year, although the use of HRWR did improve the strength somewhat. Freeze-thaw durability and salt scaling resistance of both slag mixes were inferior compared to the OPC control. However, under optimized wet plus dry curing periods, slag mix durability did approach that of the OPC mix. Chloride permeability of both slag mixes was significantly lower than the OPC mix. Analysis of past studies indicates that, although slag replacements up to 80% have been used successfully, in order for slag mixes to reach strength parity with OPC mixes, the optimum replacement of PC by slag usually falls between 40 and 60%, depending on the nature and proportions of the materials. Laboratory mixes were made which reflected variables of age, PC type, w/cm, and total cementitious material content. It was concluded that although the optimum slag proportion for strength was 50%, slag replacement levels of up to 70% could be used to achieve moderate strength levels. Strength parity with zero slag mixes is possible with 70 % slag under proper conditions, which include sufficient activity of the slag-PC system.