Lesson 6/Learning Event 4
Example: Bitumen 6.0% (total mix)
Mineral filler 5.0% (aggregate in total mix)
100 - 6 = 94% (aggregate in total mix)
100 - 5 = 95% (dried aggregate in total aggregate)
0.94 x 0.95 = 0.893 (or) 89.3% dried aggregate (total mix).
Or if Mineral Filer is Added:
NOTE: It should be remembered that the preceding example is calculated for maximum
capacity at 100% efficiency. During a sustained operation, due operating efficiency, the
actual plant output will be somewhat lower than this example.
In some instances it will be found that the screening capacity will be somewhat lower
than the dryer capacity. This condition normally occurs with a low aggregate moisture content
or when an extremely high percentage of fine aggregate is required for the mix. When this
condition occurs, substitute the screening capacity for the dryer capacity in the preceding
example.
Step 6. Determine the Feeding Rate.
The percent required from each gradation unit bin and the percent of mineral filler to be
fed by the fines feeder are expressed as a percent of the total aggregate. To determine the
percent of the total required from each component, multiply the percent of total aggregate from
each component by the percent of aggregate in the total mix.
Refer to above. The percentages listed in the second column of this example are the
percentages that were determined by screen selection. (Refer to Figure 20). However, these
percentages are of the total aggregate in the mix. The aggregate makes up only 94% of the total
mix. Therefore, by multiplying these percentages by 0.94, the percent of the total mix required
from each feeding component is determined. As a check for accuracy, add the percent of total
mix required from the four gradation unit bins and the percent of total mix required from the
fines feeder. The sum of these figures must equal the percent of aggregate in total mix (in this
case, 94%).
The percent of total mix required from each component is then multiplied by the plant
output. In Step 4, the plant output was calculated. In this example, 154.5 TPH will be the plant
output. Refer to above. The figures listed in the right column are the required amounts of
material from the appropriate component. Notice that these figures are in either lbs/rev or in
TPH. Again, as a check for accuracy, add these figures. The sum must equal the plant output.
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