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setting of the cement, thereby creating a more porous matrix with higher permeability. The same effect occurred with the dry-mix shotcrete. The dry-mix shotcrete with 3% accelerator shows a slightly higher K value than that of dry-mix shotcrete without accelerator. Tortuosity Tortuosity is a measure of the extent to which a chemical species must deviate from a direct route when diffusing from Point A to Point B through the pore network of concrete.11 A low diffusion coefficient (and hence a low rate of diffusion) is achieved when the total volume fraction of porosity is low, its constrictivity is low, and its tortuosity is high. The constrictivity is a measure of the extent to which changes in the width of pores along their length hinder the diffusion of chemical species. Tortuosity in this study was defined based on the moisture diffusion model and the ionic migration model.10 The higher the number that comes out of the analysis, the higher the tortuosity, which means it is more difficult for a chemical species to deviate from a direct route when diffusing from Point A to Point B through the pore network of concrete. Therefore, a low diffusion rate is related to high tortuosity. To make comparisons simple, the tortuosity values are plotted in Fig. 10. Tortuosity test results clearly show that mixtures with cement only have the lowest tortuosity, mixtures with fly ash have higher tortuosity, and mixtures with silica fume have the highest tortuosity. This is consistent with the beneficial effects of adding SCMs into concrete and shotcrete—that is, SCMs react with Ca(OH)2 to reduce the porosity of the concrete matrix and therefore reduce the diffusion rate of chemical species. Tortuosity test results also show that tortuosity ranges between 20 and 120 (unitless) for cast concrete, cast wet-mix shotcrete, shot wet-mix shotcrete, and dry-mix shotcrete. This shows that the extent for chemical ions to deviate from one point to another point in the matrix network is not adversely affected by the shotcrete process. Although there are exceptions, both the wet-mix and dry-mix shotcrete processes typically produce a matrix network that has tortuosity as high as or higher than that of cast-in-place concrete. FURTHER RESEARCH To further evaluate the properties of shotcrete, it is recommended to conduct research on different mixtures, including mixtures with ground-granulated blast-furnace slag and limestone. CONCLUSIONS Test results for BA and VPV correlate well with the CoD. BA and VPV results also correlate well with parameters such as permeability (K values) and tortuosity. More specifically: 1. All transport properties show, consistently, that the porosity decreases from mixtures with cement only, to fly ash, to silica fume. This is consistent with the general understanding of the effect of supplementary cementitious materials, which involves Fig 8—Example of drying test results: liquid contribution and vapor contribution are obtained by measuring mass loss of 50 mm samples and 10 mm samples separately. (Note: 1 g = 0.035 oz; 1 mm = 0.039 mm.) Fig. 9—Permeability from water absorption test. SAhCotIc Mreatet e• rSiaulms mJoeur r2n0a1l6/M ay-June 2016 38 435


2016SummerShotcreteEMag
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