Lesson 5/Learning Event 2
Estimate Percentage by Weight of Aggregates
The next step in Trial No. 1 is to estimate the percentages by weight of each of the aggregates to
be combined to meet the "desired" gradation. The "desired" line indicates that 4 - 8 percent is
desired passing the #200 sieve. A study of the gradation of the materials shows that the LSD has
90 percent pausing and the FRB has 10 percent passing the #200 sieve. If 3 percent of LSD were
used, the percent passing the #200 sieve for this material would be 2.7 percent (3% x 90% =
2.7%). If 10 percent of FRB was used, the percent passing of this material through the #200
sieve would be 1 percent (10% x 10% = 1%), making a total for these two materials in the final
blend of 3.7 percent.
Since the fine material has 3 percent passing the #200 sieve, it is evident additional material will
pass the #200 sieve from the fine aggregate, making the total passing the #200 sieve near the
desired 4 - 8 percent. This would require in the neighborhood of 50 percent of fine material
(50% x 3% = 1.5%). This 1.5 percent plus the 3.7 percent from the other two materials would
give a total of 5.2 percent. However, all other requirements for the various sieve sizes must also
be met. For the first trial, try percentages of 3 percent LSD, 10 percent FRB, 50 percent fine,
and the balance of 37 percent coarse aggregate. Enter these percentages in the proper spaces
under column "Percent Used" for Trial No. 1 as shown in the chart.
Multiply the Percentages
The next step is to multiply all percentages of materials passing each size sieve by those
percentage figures. Enter the results in the proper spaces on the chart. The percentages
calculated passing each sieve should then be totaled and entered on the "Blend" line.
Check the Blend
Check the blend against the "Desired." The blend percentages passing sieves of 3/8-inch through
#200 are less than the "Desired." The blend percentage passing the 1/2- and 3/4-inch through
#200 sieve are correct, indicating that the selection of percentages of FRB and LSD is about
correct. This leaves the selection of obtaining additional material passing the 3/8-inch through
#100 sieves to the fine river bar sand.
Trial No. 1
In Trial No. 2, as shown in the chart, the percentages chosen are 27 percent coarse, 20 percent
FRB (an increase of 10 percent for FRB and a decrease of 10 percent for coarse), and the same
percentages as before for the FA and LSD. The resulting blend using these percentages is shown
in Figure 12. When compared with the requirements of "Desired," it is very near the ideal. It is
now possible to state in trial no. 3 that our blend is 28/50/19/3,
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