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    A Dynamic Segregation Test Method for Heavyweight Concrete: Density Variation Method
    (Amer Soc Testing Materials, 2020) Gokce, H. Suleyman; Andic-Cakir, O.
    Several segregation evaluation methods have been developed for concrete; however, none of them have been specially focused on heavyweight concrete, which has more of a tendency toward segregation because of differences in density of paste matrix to heavy aggregate. Thus, for heavyweight concrete, the segregation may be determined by measuring the difference density in a vertical direction. In this study, a simple reliable method, i.e., the density variation method, that considers the variation in vertical layers of the concrete is modeled to assess dynamic segregation of heavyweight concrete. In addition, 35 heavyweight concrete mixtures with high consistency were designed with various mixing parameters, and the results were compared with the model findings. The suggested test method was also compared with a conventional method (column segregation test by wet-sieving), and an equivalent segregation limit was determined. A strong linear relation (R-2 : 0.86) was found between the density variation method and column segregation test, and a segregation coefficient limit for segregation of heavyweight concrete is recommended as 1.7 %.
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    Natural radioactivity of barite concrete shields containing commonly used supplementary materials
    (Elsevier Sci Ltd, 2020) Gokce, H. S.; Ozturk, B. Canbaz; Cam, N. F.; Andic-Cakir, O.
    The recycling of hazardous materials within new composites has a sustainable importance as it contributes on the reduction of high radionuclide concentrations. In this study, the effect of cementitious materials i.e. viscosity modifier, silica fume and fly ash on the Ra-226, Th-232 and K-40 activity levels of barite concrete shields was researched by using different mixing compositions in terms of binder content, w/b ratio and the amount of supplementary cementitious materials. In comparison to cement, silica fume and fly ash used in the production of concrete shields were found to have significantly higher activity levels of K-40, as well as Ra-226, Th-232 and K-40, respectively. The Ra-226, Th-232 and K-40 activity levels of concrete shields range between 2.2 and 20.7 Bq kg(-1); 2.6 and 7.0 Bq kg(-1); 51.5 and 89.3 Bq kg(-1), respectively. These activity values were found similar or less than those of building materials in the world. These satisfactory results are mostly caused by the barite source which has lower radioactivity levels. The concrete mixture details used in the study caused significant variation in the natural radioactivity of barite concrete shields. The introduction of silica fume which has high K-40 activity levels and fly ash with high Ra-226, Th-232 and K-40 activity levels compared to cement and barite aggregate, significantly increased the radiological hazard parameters, although all results were found to be below the limits recommended by international reports. In conclusion. Ra-226 activity levels of the concrete shields were significantly increased (up to 8.4 times) by the variation of mix design parameters used in comparison with concrete mixtures that have the least Ra-226 activity level. The Th-232 and K-40 activity levels were as well increased in relatively less amounts (1.7 and 0.73 times higher, respectively). (C) 2019 Elsevier Ltd. All rights reserved.