United States Department of Transportation
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Abstract:Portland cement is the most dominant material used in concrete pavements in the state of Nebraska. In order to improve performance, reduce cost, and advance sustainability, a percentage of the Portland cement is replaced with a recycled material known as fly ash. In recent years, Nebraska Department of Roads (NDOR) began noticing premature deterioration in many Portland cement concrete pavements (PCCP). A preliminary investigation into these pavements led NDOR to identify Class C fly ash used as a supplementary cementious material (SCM) in PCCP as one of the possible causes of the distress. As a result, NDOR changed their specifications banning the use of Class C fly ash in PCCP. This research project was conducted to investigate the cause of the PCCP deterioration and propose methods of mitigation while allowing the use of Class C fly ash. A thorough review of all relevant literature was conducted and potential mixes using Class C fly ash were identified. A testing program was established to determine which potential mixes meet the expected performance criteria. The first phase of testing was to assess the potential for Alkali Silica Reactivity (ASR) using ASTM C 1567. The testing was carried out on 14 potential mixes as well as the mix used in deteriorated PCCP and the reference mix currently used by NDOR in PCCP. Based on ASR testing results, four mixes were chosen to undergo overall performance testing, which includes strength and durability properties as well as fresh concrete properties. The testing comprised ASTM C666, C1202, C157, C403, C39, C78, and NDOR's wet-dry test. Testing results have indicated that three mixes have superior performance over the reference mix with 25% Class F fly ash as the only SCM. The three mixes were used in two field applications and specimens were taken for further laboratory testing to ensure their overall performance. The three proposed mixes have the same aggregate composition of the reference mix (70% 47B sand and gravel + 30% limestone), while containing different percentages of Class C fly ash and other SCM: 16% Class C fly ash + 20% Class F fly ash; 20% Class C fly ash + 20% Class F fly ash; and 15% Class C fly ash + 18% Class F fly ash + 15% Slag.
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