| 摘要 |
Sand grains with the dia. of 2.5mm or more are separated with a known screeh, and grains with less than 2.5mm dia. are screened by means of two-stage screens of different mesh sizes, the first stage screen having a 0.6mm mesh size and the other having 0.15mm mesh, positioned in inclination parallel to each other. The screens are oscillated in the rate of 2,900-3,600/m. Sands are screened into the groups of grains 2.5mm to 0.6mm, 0.6mm to 0.15mm and less than 0.15mm to be mixed into a composite aggregate of predetermined proportions of grains.
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